Molecular simulation evaluation method and device for drag reduction effect of CO2 nanobubbles at coal substrate interface

Abstract

The application discloses a kind of coal-based bottom interface CO2 Nanobubble drag reduction effect molecular simulation evaluation method and device, it is related to molecular dynamics simulation technique field, comprising: according to composite molecular dynamics simulation system carries out molecular dynamics production simulation, collects trajectory data in production simulation process, obtains the overall velocity profile and overall density profile of system along coal-based bottom normal direction;Based on overall velocity profile, set near-wall fitting interval above coal-based bottom surface and linearly fit the velocity distribution in interval, obtain apparent slip length;Based on trajectory data, intercept two-dimensional section containing CO2 Nanobubble center region, obtain local velocity field distribution in section;According to overall velocity profile, overall density profile, apparent slip length and local velocity field distribution, evaluate the influence law of different scale CO2 Nanobubble on water-gas system slip drag reduction.This provides more scientific evaluation and calculation method for micro-nano interface flow drag reduction regulation.

Description

Technical Field

[0001] The embodiments of the present invention relate to the field of molecular dynamics simulation technology, and in particular to a molecular simulation evaluation method and apparatus for the drag reduction effect of CO2 nanobubbles at a coal substrate interface. Background Technology

[0002] Engineering practices such as coal seam water injection, micro / nano bubble water lubrication, CO2 geological utilization, and enhanced coalbed methane extraction all involve water phase transport, gas enrichment, and momentum transfer processes at the solid-liquid-gas three-phase interface near the coal seam. Accurately describing the flow behavior of the water phase near the coal surface is crucial for revealing the permeability enhancement and drag reduction mechanisms of coal seams and optimizing injection and production process parameters.

[0003] In macroscopic flow systems, engineering calculations typically employ no-slip boundary conditions, assuming zero tangential velocity of the fluid at the solid wall. However, existing research indicates that when the flow scale decreases to the micro-nano range, the wall exhibits hydrophobic properties, or gaseous patches exist at the solid-liquid interface, the liquid may exhibit a finite tangential velocity at the solid wall. In such cases, slip velocity or slip length needs to be introduced to correct the boundary response. Interfacial slip can alter shear dissipation and effective flow resistance near the wall, providing a crucial theoretical foundation for understanding the mechanisms of fluid transport and drag reduction within coal-bearing nanopores.

[0004] At the solid-liquid interface, nanobubbles are considered one of the typical candidate mechanisms for apparent slip. The basic principle is that nanobubbles attached to the solid wall can displace liquid molecules in the near-wall region, forming a low-density, low-viscosity gas phase region between the wall and the mainstream liquid. This reduces the direct contact between water molecules and the solid wall, lowering interfacial frictional resistance. However, in real-world conditions, nanobubbles are not ideal planar cushion structures. Their curvature, protrusion height, contact line morphology, and lateral dimensions can all cause streamline bending of the mainstream fluid as it bypasses the bubble, potentially generating additional pressure drag. Existing theoretical analyses indicate that when the protrusion of the interfacial bubble exceeds a certain critical value, the drag reduction benefits from low-friction contact may be offset by the flow resistance caused by the bubble's geometry; the bubble may even transform from a drag-reducing element into a drag-increasing obstacle.

[0005] Coal substrates possess unique molecular structural features, with their aromatic skeletons, aliphatic side chains, oxygen-containing functional groups, pore heterogeneity, and atomic-level surface roughness collectively forming a complex solid-liquid-gas interfacial environment. Simultaneously, CO2 molecules exhibit a strong tendency for interfacial adsorption and the ability to accumulate on the coal surface. Micro- and nano-CO2 bubble water has been experimentally proven to alter the wetting behavior, pore structure, and surface chemical state of coal. However, current research primarily focuses on macroscopic characterization of wetting properties and multi-scale pore structure analysis. Direct molecular-scale simulation evidence and systematic analytical methods are still lacking regarding the microscopic mechanisms by which individual interfacial CO2 nanobubbles on a real coal molecular substrate alter the velocity field, density field, and slip boundary conditions of the aqueous phase, particularly the competitive relationship between local slip-promoting effects and overall additional resistance as the bubble size increases. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface. By constructing a coal substrate-water-CO2 composite molecular dynamics system and applying a constant external driving force to the water phase, the overall velocity and density profiles of the water phase along the normal direction of the coal substrate under CO2 nanobubble conditions of different scales are obtained. The apparent slip length is obtained by linear extrapolation of the near-wall region velocity, and the local velocity field distribution is analyzed by extracting a two-dimensional cross-section of the bubble center region. This enables the molecular-scale characterization of the competitive relationship between the low-friction gas phase coverage effect induced by CO2 nanobubbles and the flow drag effect caused by the bubble protruding interface, providing a direct analytical means for studying the drag reduction mechanism of gas-liquid interface slip in coal-based nanopores.

[0007] In a first aspect, embodiments of the present invention provide a molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at a coal substrate interface, comprising:

[0008] Molecular dynamics production simulation is performed based on a pre-constructed composite molecular dynamics simulation system, wherein the composite molecular dynamics simulation system includes a coal substrate, a water layer located above the coal substrate, and CO2 nanobubbles aggregated at the coal-water interface.

[0009] In molecular dynamics production simulations, an external driving force is applied to the water layer in a constant direction, causing the water phase to flow in that constant direction.

[0010] The trajectory data at each moment during the molecular dynamics production simulation is collected, and the overall velocity profile and overall density profile of the composite molecular dynamics simulation system along the normal direction of the coal substrate are obtained based on the trajectory data.

[0011] Based on the overall velocity profile, a near-wall fitting interval is set above the coal substrate surface, and the velocity distribution within the near-wall fitting interval is linearly fitted to obtain the apparent slip length at the coal substrate surface.

[0012] Based on the trajectory data, a two-dimensional cross-section containing the central region of CO2 nanobubbles is extracted, and the local velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section is obtained.

[0013] Based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field distribution, the influence of CO2 nanobubbles of different sizes on slip drag reduction in the water-gas system is evaluated.

[0014] In a preferred embodiment, the overall velocity profile and overall density profile of the system along the normal direction of the coal base are obtained based on the trajectory data, including:

[0015] The trajectory data is preprocessed to remove non-steady-state trajectories of a preset duration during the initial stage of production simulation, thereby obtaining steady-state trajectory data;

[0016] The water molecules and coal substrate in the steady-state trajectory data are mapped to a uniform periodic cut, preserving the signed distance of the water molecules relative to the coal substrate surface;

[0017] Based on the signed distance, the grid is divided along the normal direction of the coal substrate at a preset box interval. The water phase velocity data and the number of water molecules in each grid are statistically analyzed to obtain the overall velocity profile and the overall density profile.

[0018] In a preferred embodiment, based on the signed distance, a grid is divided along the normal direction of the coal substrate at a preset bin interval. The water phase velocity data and water molecule quantity within each grid are statistically analyzed to obtain an overall velocity profile and an overall density profile, including:

[0019] Based on the molecular complete trajectory in the steady-state trajectory data, the velocity data of each water molecule is obtained by performing finite difference through the continuous displacement of oxygen atoms in the water molecule.

[0020] Using the signed distance as the normal coordinate, the velocity data and spatial position of each water molecule are assigned to their respective grids;

[0021] The average value of the velocity components of all water molecules along the driving force direction in each grid is calculated and used as the average velocity at the corresponding normal position of each grid. The overall velocity profile is constructed based on the distribution of the average velocity at the corresponding normal position of each grid along the normal direction.

[0022] Calculate the ratio of the number of water molecules to the grid volume in each grid, and use it as the number density or mass density at the corresponding normal position of each grid. Based on the distribution of the number density or mass density at the corresponding normal position of each grid along the normal direction, construct the overall density profile.

[0023] In a preferred embodiment, based on the overall velocity profile, a near-wall fitting interval is defined above the coal substrate surface. The velocity distribution within the near-wall fitting interval is linearly fitted to obtain the apparent slip length at the coal substrate surface, including:

[0024] A preset reference position of the upper surface of the coal substrate in the normal direction is obtained as the wall position, and a near-wall fitting interval is set above the wall position and within a preset range of the linear velocity distribution region;

[0025] Linear regression is performed on the overall velocity profile within the near-wall fitting interval to fit a linear relationship between velocity and normal distance, and the near-wall shear rate is determined based on the slope of the linear relationship.

[0026] Based on the linear relationship, determine the corresponding velocity value at the wall position, which is taken as the apparent slip velocity at the wall.

[0027] The apparent slip length is calculated based on the ratio of the apparent slip velocity to the near-wall shear rate.

[0028] In a preferred embodiment, a two-dimensional cross-section containing the central region of CO2 nanobubbles is extracted based on the trajectory data, and the velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section is obtained, including:

[0029] The position of the CO2 aggregate centroid under periodic boundary conditions in each frame of trajectory data is determined and used as the reference center of the tangent.

[0030] Using the aforementioned cross-sectional reference center as a reference, a two-dimensional cross-section is cut that is perpendicular to the coal substrate surface and parallel to the direction of the driving force.

[0031] The two-dimensional cross-section is divided into spatial grids, and the velocity components of water molecules along the driving force direction in each grid are counted. The average value of all velocity components in the grid is calculated as the local velocity value corresponding to each grid.

[0032] Based on the local velocity values ​​corresponding to each grid, the local velocity field distribution within the two-dimensional sectional region is constructed.

[0033] As a preferred embodiment, based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field distribution, the influence of CO2 nanobubbles of different sizes on drag reduction in the water-gas system is evaluated, including:

[0034] The average velocity and near-wall density of the outer water layer were determined for bubble systems of different scales and a CO2-free control system, and the changes in the average velocity and near-wall density of the outer water layer of each bubble system relative to the control system were calculated.

[0035] Compare the apparent slip lengths of each bubble system with the control system to determine the difference in apparent slip lengths of each bubble system relative to the control system;

[0036] Based on the difference between the local velocity field of the bubble system and the velocity field at the same location in the control system, the local acceleration zone upstream of the bubble and the local deceleration / recirculation zone downstream of the bubble are determined.

[0037] Based on the changes in the average velocity of the external water layer, the changes in near-wall density, the difference in apparent slip length, and the spatial distribution of local acceleration and deceleration / return zones, the competitive relationship between the low-friction gas phase coverage contribution and the prominent interface shape resistance contribution of CO2 nanobubbles is determined.

[0038] Based on the aforementioned competitive relationship, the influence of bubble size on slip reduction efficiency is determined;

[0039] In a preferred embodiment, based on the average velocity change of the external water layer, the near-wall density change, the apparent slip length difference, and the spatial distribution of local acceleration and deceleration / return zones, the competitive relationship between the low-friction gas phase coverage contribution and the prominent interface shape resistance contribution of CO2 nanobubbles is determined, including:

[0040] The contribution of low-friction gas phase coverage is determined based on the near-wall density change.

[0041] The contribution of the protruding interface shape drag is determined based on the average velocity change of the external water layer and the spatial distribution of local acceleration and deceleration / backflow zones.

[0042] Based on the combined characteristics of the near-wall density change and the average velocity change of the external water layer, the primary and secondary relationship between the contribution of the low-friction gas phase cover and the contribution of the protruding interface shape drag is determined.

[0043] If the near-wall density change is lower than that of the control system and the average velocity change of the outer water layer is lower than that of the control system, then the contribution of the protruding interface shape resistance is determined to be greater than the contribution of the low-friction gas phase coverage.

[0044] As a preferred embodiment, the non-monotonic influence of bubble size on slip drag reduction efficiency is determined based on the aforementioned competitive relationship, including:

[0045] Compare the competing relationship between the low-friction gas phase coverage contribution and the prominent interface shape resistance contribution in bubble systems of different scales;

[0046] If a larger bubble system exhibits a greater decrease in near-wall density and a greater decrease in average velocity of the outer water layer compared to a smaller bubble system, and also has a lower apparent slip length, then it can be determined that increasing the bubble size within this scale range has a negative impact on slip drag reduction efficiency.

[0047] As a preferred embodiment, determining the non-monotonic influence of bubble size on slip drag reduction efficiency based on the aforementioned competitive relationship further includes:

[0048] If the apparent slip length of the bubble system approaches zero or becomes positive relative to the control system, but the average velocity of the external water layer is still lower than that of the control system, then it is determined that the contribution of the low-friction gas phase coverage of the bubbles in this scale range has not been converted into global drag reduction.

[0049] Secondly, embodiments of the present invention also provide a molecular simulation evaluation device for the drag reduction effect of CO2 nanobubbles at the coal-water interface, used to perform the molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface as described in any of the above claims, the device comprising:

[0050] The production simulation module is used to perform molecular dynamics production simulation based on a pre-constructed composite molecular dynamics simulation system, wherein the composite molecular dynamics simulation system includes a coal substrate, a water layer located above the coal substrate, and CO2 nanobubbles aggregated at the coal-water interface; in the molecular dynamics production simulation, an external driving force along a constant direction is applied to the water layer to drive the water phase to flow along the constant direction;

[0051] The trajectory acquisition and profile acquisition module is used to acquire trajectory data at each moment during the molecular dynamics production simulation, and to acquire the overall velocity profile and overall density profile of the composite molecular dynamics simulation system along the normal direction of the coal substrate based on the trajectory data.

[0052] The slip length calculation module is used to set a near-wall fitting interval above the coal substrate surface based on the overall velocity profile, and to perform linear fitting on the velocity distribution within the near-wall fitting interval to obtain the apparent slip length at the coal substrate surface.

[0053] The local flow field acquisition module is used to extract a two-dimensional cross-section containing the central region of CO2 nanobubbles based on the trajectory data, and to obtain the local velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section.

[0054] The drag reduction impact assessment module is used to evaluate the impact of CO2 nanobubbles of different sizes on drag reduction in the water-gas system based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field distribution.

[0055] Thirdly, embodiments of the present invention also provide an electronic device, the electronic device comprising:

[0056] One or more processors;

[0057] Storage device for storing one or more programs;

[0058] When the one or more programs are executed by the one or more processors, the one or more processors implement a molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at a coal substrate interface, as described in any embodiment of the present invention.

[0059] Fourthly, embodiments of the present invention also provide a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements a molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at a coal substrate interface as described in any embodiment of the present invention.

[0060] In summary, the present invention achieves the following beneficial effects:

[0061] This invention establishes, for the first time, a molecular dynamics evaluation method for drag reduction of CO2 nanobubbles on a real coal substrate. By systematically comparing the velocity field, density field, and slip boundary of bubble systems of different scales with a CO2-free control system, it reveals for the first time the anomalous physical mechanism of drag reduction failure of CO2 nanobubbles at the coal substrate interface, where form drag dominates. Furthermore, this invention establishes a multi-index comprehensive evaluation framework based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field. Through a dual-window linear fitting strategy, it effectively separates the apparent slip dominated by near-wall shear from the overall effective slip considering the influence of bubble flow, distinguishing the decoupling phenomenon between the local boundary effect in the near-wall region and the global drag reduction effect of the external water layer. This provides a more scientific evaluation standard and calculation method for drag reduction control of micro- and nano-scale interfacial flows. Attached Figure Description

[0062] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings. The drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0063] Figure 1 This is a flowchart of a molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at a coal substrate interface, provided in an embodiment of the present invention.

[0064] Figure 2 This is a time series diagram of the evolution of the center position and radius of rotation of CO2 nanobubbles provided in an embodiment of the present invention, wherein... Figure 2(a) is a diagram showing the evolution of the center position of the large bubble. Figure 2 (b) is a diagram showing the evolution of the center position of the small bubbles. Figure 2 (c) is a time series plot of the bubble's radius of rotation;

[0065] Figure 3 These are overall water velocity and water density profiles of the three systems provided in the embodiments of the present invention in the region above the coal substrate, wherein... Figure 3 (a) is a cross-sectional view of the overall water velocity. Figure 3 (b) is a cross-sectional view of the overall water density;

[0066] Figure 4 This is a comparison diagram of the linear extrapolation of velocity and apparent slip length of the three systems provided in the embodiments of the present invention within the near-wall fitting interval. Figure 4 (a) is a schematic diagram of linear extrapolation of the launching velocities of the three systems. Figure 4 (b) is a schematic diagram of the apparent slip length in the three systems;

[0067] Figure 5 This is a comparison diagram of the water phase velocity field in the x-direction on the XZ section at the bubble center of the three systems provided in the embodiments of the present invention. Figure 5 (a) is a schematic diagram of the velocity of the cross-section of the small bubble. Figure 5 (b) is a schematic diagram of the velocity of the large bubble cross section; Figure 5 (c) is a schematic diagram of the velocity of the bubble-free cross section;

[0068] Figure 6 This is a graph showing the lateral weighted average water velocity distribution along the normal direction on the XZ section of the three systems provided in this embodiment of the invention.

[0069] Figure 7 These are velocity difference field diagrams of the three systems provided in the embodiments of the present invention on the XZ cross plane, wherein, Figure 7 (a) is a schematic diagram of the velocity difference between small bubbles and no bubbles. Figure 7 (b) is a schematic diagram of the velocity difference between large bubbles and no bubbles; Figure 7 (c) is a schematic diagram of the velocity difference between large and small bubbles;

[0070] Figure 8 This is a schematic diagram illustrating the competitive mechanism for CO2 nanobubble size regulation of coal-water interface flow provided in an embodiment of the present invention, wherein... Figure 8 (a) is a schematic diagram illustrating the competition between low-friction gas phase coverage and interfacial resistance in a small bubble system. Figure 8 (b) is a schematic diagram of the competition between low-friction gas phase coverage and interface resistance in a large bubble system;

[0071] Figure 9This is a schematic diagram of the structure of a molecular simulation evaluation device for the drag reduction effect of CO2 nanobubbles at a coal substrate interface provided in an embodiment of the present invention;

[0072] Figure 10 This is a schematic diagram of the structure of an electronic device provided in an embodiment of the present invention. Detailed Implementation

[0073] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0074] Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flowcharts. Although the flowcharts describe operations (or steps) as being processed sequentially, many of these operations (or steps) may be performed in parallel, concurrently, or simultaneously. Furthermore, the order of the operations may be rearranged. The process may be terminated when its operation is completed, but may also have additional steps not included in the figures. The process may correspond to a method, function, procedure, subroutine, subroutine, etc.

[0075] Example 1

[0076] like Figure 1 As shown in the flowchart of Embodiment 1 of the present invention, a molecular simulation evaluation method 100 for the drag reduction effect of CO2 nanobubbles at the coal substrate interface is provided. The method 100 specifically includes the following steps:

[0077] Step S110: Perform molecular dynamics production simulation based on a pre-constructed composite molecular dynamics simulation system, wherein the composite molecular dynamics simulation system includes a coal substrate, a water layer located above the coal substrate, and CO2 nanobubbles aggregated at the coal-water interface.

[0078] As a preferred embodiment, in order to elucidate the scale response of CO2 nanobubbles at the coal substrate interface to the slip and drag reduction of the water-gas system, the present invention uses a non-equilibrium molecular dynamics method to construct a compressed coal substrate-water-CO2 composite system. The composite system includes a coal substrate obtained by compression using a coal macromolecule model, a water layer placed above the coal substrate, and CO2 nanobubbles aggregated at the interface between the coal substrate and the water layer.

[0079] Specifically, constructing a compressed coal-based water-CO2 composite system includes the following steps:

[0080] First, a coal matrix model is constructed. In a preferred embodiment, the coal matrix consists of representative coal macromolecules C106 H 64 The N2O5 was randomly stacked and compressed vertically and laterally. Each coal molecule contains 177 atoms, and the system contains 110 coal molecules, for a total of 19,470 coal substrate atoms. After compression, the substrate retains atomic-level roughness and internal porosity, which can more realistically reflect the complex molecular structure of coal. In the subsequent slip simulation stage, the coal substrate was frozen as a whole as a static wall.

[0081] Next, CO2 nanobubbles were constructed at the coal-water interface. A predetermined number of CO2 molecules were placed near the coal-water interface, causing them to spontaneously aggregate and form nanobubbles during the simulation.

[0082] To investigate the effects of CO2 nanobubbles of different sizes, this invention constructed three comparative systems: a small bubble system containing 37 CO2 molecules; a large bubble system containing 111 CO2 molecules; and a CO2-free control system containing no CO2 molecules.

[0083] Furthermore, to stabilize the bubble morphology in the simulation, small bubbles and large bubbles were respectively held in harmonic flat-bottomed wells with radii of 1.00 nm and 1.50 nm. However, the restoring force was only applied after the CO2 carbon atoms had crossed the preset flat-bottomed well to avoid over-constraint. The composition and bubble holding conditions of the three systems are shown in Table 1.

[0084] Table 1

[0085]

[0086] Step S120: In the molecular dynamics production simulation, an external driving force along a constant direction is applied to the water layer to drive the water phase to flow along the constant direction.

[0087] As a preferred embodiment, all production simulations in this invention were performed using the GROMACS2020.6-MODIFIED software package. The system was first minimized to reach a stable configuration, and then equilibrated under NVT (constant number, volume, and temperature) tracking to bring the system temperature to a preset value.

[0088] During the production stage, a time step of 0.001 picoseconds was used, and the system operated at 298.15 K for 10 ns (nanoseconds). Temperature coupling was achieved using a V-rescale thermostat with a time constant of 0.5 picoseconds. The aqueous phase (SOL) and CO2 were individually temperature controlled to ensure that each component reached thermal equilibrium.

[0089] Pressure coupling was disabled during the simulation to maintain a constant simulation chamber volume; simultaneously, the coal substrate was completely frozen in three directions as a static wall, and the aqueous phase (SOL) was subjected to a stress of 0.0005 nm·ps along the positive X-axis. -2 Driven by the constant acceleration of the liquid phase, the CO2 molecules are not directly subjected to external forces; their motion is driven only by the drag of water molecules and the interaction with the coal surface, in order to simulate the physical scenario of the gas phase passively migrating with the liquid phase under real conditions.

[0090] Nonbonded interactions were handled using the PME (Particle Mesh Ewald) method to address long-range electrostatic interactions. Lennard-Jones interactions and the real-space Coulomb cutoff radius were both set to 1.0 nm, and dispersion correction was enabled to compensate for the dispersion energy loss introduced by the cutoff. All hydrogen-containing chemical bonds were constrained using the LINCS (Linear Constraint Solver) algorithm with a constraint order of 4 to allow for a larger time step.

[0091] The trajectory data was output in XTC format, one frame every 5 ps. To eliminate initial non-steady-state effects, the trajectory data in the first 1 ns was discarded, and 1801 frames in the 1 ns-10 ns interval of the production simulation were used for statistical analysis. The detailed parameters of the production simulation process are shown in Table 2 below:

[0092] Table 2

[0093]

[0094] Step S130: Collect trajectory data at each moment during the molecular dynamics production simulation, and obtain the overall velocity profile and overall density profile of the composite molecular dynamics simulation system along the normal direction of the coal substrate based on the trajectory data.

[0095] In a preferred embodiment, this step specifically includes the following sub-steps:

[0096] Data preprocessing: The trajectory data is preprocessed to remove non-steady-state trajectories with a preset duration (e.g., 1 ns) at the beginning of the production simulation, so as to obtain steady-state trajectory data;

[0097] Coordinate mapping: The water molecules and coal substrate in the steady-state trajectory data are mapped to a unified periodic cut, while preserving the signed distance of water molecules relative to the coal substrate surface. Specifically, the position analysis uses the molecular complete (no-jump) trajectory. Since trajectory centering would cause the coal substrate and water layer to cross the periodic boundary in the Z direction, this invention does not directly bin the bounding coordinates. Instead, in each frame, a continuous coal substrate is reconstructed based on the maximum periodic gap of the coal atom Z coordinate, and its upper surface is uniformly mapped to a nominal position, such as 3.3 nm. This avoids the water layer breaking at the periodic boundary and completely preserves the water molecule information in the pores below the coal surface, avoiding the loss of the velocity and density contribution of pore water due to simple truncation.

[0098] Statistical binning: Based on the signed distance, the grid is divided along the normal direction of the coal base at a preset binning interval. The water phase velocity data and water molecule quantity in each grid are statistically analyzed to obtain the overall velocity profile and the overall density profile.

[0099] For example, the overall profile is divided into bins at 0.10 nm intervals within the range of 0 nm to 9.0 nm. To avoid the first adsorption layer immediately adjacent to the wall, i.e., the water molecule layer in the range of approximately 0.3 nm above the wall, where density oscillations and velocity fluctuations caused by strong solid-liquid interactions directly dominate the regional averaging results, this invention defines the following three characteristic water layers for subsequent comparative analysis:

[0100] Near-wall region: defined as 3.6nm-5.3nm, characterizing near-wall flow behavior that is directly affected by the wall and significantly disturbed by bubbles;

[0101] The middle water layer is defined as 5.3 nm to 8.3 nm, representing the transitional region far from the wall and tending towards mainstream behavior.

[0102] External water layer: defined as 3.6nm-8.8nm, a comprehensive index characterizing the overall water phase transport capacity.

[0103] In a preferred embodiment, based on the signed distance, a grid is divided along the normal direction of the coal substrate at a preset bin interval. The water phase velocity data and water molecule quantity in each grid are statistically analyzed to obtain the overall velocity profile and the overall density profile. The specific steps include:

[0104] First, based on the molecular complete trajectory in the steady-state trajectory data, the velocity data of each water molecule is obtained by performing finite difference through the continuous displacement of oxygen atoms in the water molecule;

[0105] Next, using the signed distance as the normal coordinate, the velocity data and spatial position of each water molecule are assigned to their respective grids; then, the average value of the velocity components of all water molecules in each grid along the driving force direction is calculated as the average velocity at the corresponding normal position of each grid, and the overall velocity profile is constructed based on the distribution of the average velocity at the corresponding normal position of each grid along the normal direction.

[0106] Secondly, the ratio of the number of water molecules to the grid volume within each grid is calculated to obtain the number density or mass density at the corresponding normal position of each grid. This number density is then converted to mass density by multiplying by the molar mass of water molecules and dividing by Avogadro's constant. The overall density profile is constructed based on the distribution of the number density or mass density along the normal direction at the corresponding normal position of each grid. In this invention, to facilitate comparison with macroscopic continuous medium theory, both the velocity and density profiles are time averages within a statistical time window, i.e., 1ns–10ns, a total of 1801 frames.

[0107] Figure 3 These are the overall aqueous phase velocity profiles and mass density profiles of the three systems (small bubbles, large bubbles, and no CO2) provided in the embodiments of the present invention in the region above the coal substrate. Figure 3 As can be clearly seen, the velocity profiles of all three systems exhibit a pattern of low velocity near the coal surface, acceleration in the middle water layer, and a velocity decline near the upper free interface. The density profile, however, shows a rapid transition around 3.3 nm on the coal surface, with distinct stratified peaks. Comparing the three systems reveals that, compared to the bubble-free system, neither small nor large bubbles increased the average transport velocity of the entire external water layer; instead, they exhibited varying degrees of overall deceleration.

[0108] Step S140: Based on the overall velocity profile, a near-wall fitting interval is set above the coal substrate surface, and the velocity distribution within the near-wall fitting interval is linearly fitted to obtain the apparent slip length at the coal substrate surface.

[0109] Specifically, the pre-defined reference position of the upper surface of the coal substrate in the normal direction is obtained as the wall position. For example, 3.3 nm, and a near-wall fitting interval is set within a preset range above the wall position and within the linear velocity distribution region. In the first preferred embodiment, a uniform near-wall fitting window of 3.6 nm-4.5 nm is used, which is located 0.3 nm-1.2 nm above the wall and mainly reflects the boundary effect dominated by near-wall shear.

[0110] Next, linear regression is performed on the overall velocity profile within the near-wall fitting interval to fit the velocity. With normal distance A linear relationship of change, for example:

[0111] ;

[0112] in, B is the slope of the fitted line, and B is the intercept, based on the slope of this linear relationship. Determine the near-wall shear rate Specifically, the shear rate is the gradient of the velocity along the normal direction, i.e. .

[0113] Then, based on the linear relationship, its position on the wall is determined. The corresponding velocity value at the point is used as the apparent slip velocity at the wall surface. According to the Navier linear extrapolation form:

[0114] ;

[0115] in, Wall position The extrapolated velocity at a certain point has the physical meaning of: if the linear velocity distribution in the near-wall region extends towards the wall... At that position, the tangential velocity possessed by the water phase. When When , it indicates that there is positive slip at the wall, that is, water molecules have a finite positive velocity at the wall; when When, it indicates no-slip boundary conditions; when When the value is 0, it indicates negative slip, meaning there is a reverse velocity or increased apparent viscosity at the wall surface.

[0116] Finally, the apparent slip length is calculated based on the ratio of the apparent slip velocity to the near-wall shear rate.

[0117] ;

[0118] Apparent slip length The physical meaning is: to equate the sliding velocity at the wall surface to the zero velocity at a virtual location inside the wall surface. This is the distance from the virtual zero-velocity surface to the actual wall surface. This indicates positive slip, with the zero-velocity surface below the wall. This indicates negative slip, with the zero-velocity surface above the wall. This corresponds to the no-slip condition.

[0119] To ensure the reliability of the fitting results, the goodness of fit for each system was tested. In this embodiment, the linear regression determination coefficients for the three systems in the near-wall region of 3.6 nm to 4.5 nm were used. The values ​​are 0.980 for small bubbles, 0.904 for large bubbles, and 0.982 for the CO2-free system, respectively. This indicates that the near-wall velocity profile has excellent linearity in this range, which meets the premise assumption of linear extrapolation.

[0120] Figure 4 Table 3 shows a comparison of the linear extrapolation of velocity and apparent slip length for the three systems provided in this embodiment of the invention within the near-wall fitting interval. Table 3 illustrates the slip parameters when the wall position is set at 3.3 nm and the near-wall fitting interval is within the range of 3.6 nm to 4.5 nm above the coal surface. Within this window, the apparent slip lengths for small bubbles, large bubbles, and the CO2-free system are -0.1429 nm, -0.0047 nm, and -0.0148 nm, respectively. Small bubbles exhibit the most significant negative slip, i.e., enhanced apparent viscosity, while the slip length of large bubbles is close to zero.

[0121] Table 3

[0122]

[0123] To further examine the effect of the fitting window, in the second preferred embodiment, the fitting window is extended from 3.6nm-4.5nm to 3.6nm-5.5nm to fully include the top of the large bubble and the velocity recovery region above it in the analysis. Under the extended window, the apparent slip lengths of the small bubble, the large bubble, and the CO2-free system become 0.1111nm, 0.0166nm, and 0.1230nm, respectively, all turning positive. Although the sign changes, the effective slip length of the large bubble is still significantly lower than that of the small bubble and the CO2-free system.

[0124] It should be noted that the change in the sign of the slip length after the fitting window is extended from the near-wall region to a region further away from the wall does not indicate a change in the near-wall slip characteristics themselves, but rather reflects the different physical meanings represented by the two windows: the slip length obtained by the near-wall window (3.6nm-4.5nm) is mainly determined by the adsorbed water layer adjacent to the wall and the interfacial friction characteristics, reflecting the local boundary effect; while the extended window (3.6nm-5.5nm) covers the bubble tip and the velocity recovery zone above it, and the extrapolated parameters obtained are not just simple boundary extrapolations of the near-wall shear zone, but include the overall effective slip parameters of the flow disturbance around the bubble tip and the velocity recovery process above. This dual-window fitting strategy is one of the key innovations of this invention, which can distinguish between the local boundary effect in the near-wall region and the global drag reduction effect of the external water layer, avoiding the misjudgment of directly equating local slip characteristics with overall drag reduction efficiency under a single fitting window.

[0125] The results from both windows show that while large bubbles can reduce negative slip in the near-wall region and increase the apparent slip length from -0.1429 to -0.0047, their prominent interface introduces stronger flow around and form drag. The effective slip length of 0.0166 nm under the extended window is still significantly lower than that of small bubbles (0.1111 nm) and the CO2-free system (0.1230 nm), indicating that large bubbles have the weakest overall effective slip performance, and increasing their size does not bring about an overall drag reduction benefit.

[0126] Step S150: Based on the trajectory data, extract a two-dimensional cross-section containing the central region of CO2 nanobubbles, and obtain the local velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section.

[0127] In a preferred embodiment, firstly, the position of the CO2 aggregate centroid under periodic boundary conditions in each frame of trajectory data is determined as the cross-sectional reference center. Since the lateral positions of the two bubble systems differ in the simulation box, to ensure consistency in the cross-sectional cut-off positions for different systems, this invention first calculates the mass-weighted centroid of the CO2 aggregate in each frame of trajectory, and uses the X and Y coordinates of this centroid as the cross-sectional reference center. For the system without a CO2 control, since there are no CO2 aggregates, the average center of the small bubble system over the entire statistical interval is used as the fixed reference position. In this embodiment, the average centers of the small and large bubbles differ by only 0.177 nm in the XY plane, which is lower than the preset warning value of 0.30 nm, indicating that the lateral positions of the cross-sections of the two systems are highly comparable.

[0128] Using the reference center of the cut surface as a reference, a two-dimensional cut surface is taken that is perpendicular to the coal substrate surface and parallel to the driving force direction (X direction). The thickness of the cut surface is preferably set to 0.5 nm. This thickness ensures that there are enough samples in the cut surface to obtain the statistically convergent velocity field, while maintaining a sufficiently high spatial resolution to resolve the flow field details near the bubbles.

[0129] The two-dimensional cross-section is finely divided into spatial grids. For example, the grid range is set to extend 3.0 nm from the left to the right of the cross-section center in the X direction, and from approximately 2.8 nm below the coal surface to 9.0 nm above it in the Z direction. The number of grids is set to 100 × 100, meaning each grid has a size of approximately 0.06 nm × 0.06 nm. The velocity components of water molecules along the driving force direction within each grid are statistically analyzed, and the average value of all velocity components within that grid is calculated as the local velocity value corresponding to each grid.

[0130] It is important to note that, during the statistical process, to ensure the statistical reliability of the obtained velocity field, this invention sets a sample size threshold: if the cumulative number of water molecules in a certain grid is less than 50, the grid is marked as a data missing region and represented in white in the velocity field map. This threshold ensures the statistical significance of the displayed velocity values ​​and avoids random errors caused by insufficient samples dominating the velocity field. After completing the above statistics, the local velocity field distribution within the two-dimensional cross-sectional region is constructed based on the local velocity values ​​corresponding to each grid.

[0131] Furthermore, the present invention further performs a lateral weighted average processing on the velocity field of the XZ section, that is, at each normal position... At that point, the velocity values ​​of all grids in the X direction are weighted and averaged according to the number of water molecule samples in that grid to obtain a one-dimensional velocity distribution along the normal direction. This process can eliminate local random disturbances and extract statistically representative average velocity features along the normal direction.

[0132] To intuitively compare the velocity field differences between different systems, this invention further constructs a velocity difference field:

[0133] ;

[0134] in, The value represents the average velocity component of water molecules along the X direction (driving force direction) at a certain grid point in a two-dimensional cross-section of a system containing CO2 nanobubbles. This velocity value is obtained by averaging the velocities of all water molecules in the grid within a statistical window of 1ns-10ns. This represents the average velocity component along the X direction of water molecules at the same spatial location within a two-dimensional cross-section in the CO2-free control system. This velocity value was also obtained by time averaging within a statistical window of 1 ns–10 ns, serving as a benchmark reference value for measuring bubble disturbance. This indicates that the velocity of the bubble system in the X direction is higher than that of the control system in a local region, i.e., there is local acceleration. This indicates that the velocity of the bubble system is lower than that of the control system, meaning there is local deceleration or backflow. The construction of the difference field makes the spatial distribution of velocity increments or deficits caused by bubble disturbances readily apparent.

[0135] Figure 5This is a comparison diagram of the velocity field of the aqueous phase in the X direction on the XZ section of the bubble center of the three systems provided in this embodiment of the invention. The white areas in the diagram represent grids with no water samples or a cumulative sample count below a threshold. It can be seen that both bubble systems exhibit anhydrous cavities consistent with the bubble contours. A local positive velocity band forms upstream (left side) and a negative velocity band forms downstream (right side), exhibiting a dipole-type disturbance of upstream acceleration and downstream backflow. The low-water sample cavity of the larger bubble extends to a higher position and forms a wider negative velocity band downstream, indicating a larger disturbance range.

[0136] Figure 6 The graph shows the transverse weighted average water velocity distribution along the normal direction on the XZ section of the three systems provided in this embodiment of the invention. From the perspective of statistical averaging, it confirms the characteristics of the local velocity field. In the outer water layer, the weighted average velocities of the small bubble, large bubble and bubble-free systems are 1.363 nm / ns, 1.364 nm / ns and 1.531 nm / ns, respectively, further confirming that the presence of bubbles does not bring about a net velocity increase.

[0137] Figure 7 These are velocity difference field diagrams of the three systems provided in the embodiments of the present invention on the XZ cross plane, wherein, This indicates that the former system has a higher local velocity. This indicates a lower level. The difference fields between small bubbles and no CO2, and between large bubbles and no CO2, both show velocity perturbations with opposite signs on both sides of the bubble. The difference field between large bubbles and small bubbles indicates that the increase in scale mainly amplifies the asymmetric momentum reconstruction near the interface, while having a smaller impact on the upper part of the water layer. This trend is consistent with the analysis results of the overall velocity profile and density profile.

[0138] Step S160: Based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field distribution, evaluate the influence of CO2 nanobubbles of different sizes on the slip drag reduction of the water-gas system.

[0139] As a preferred embodiment, this step reveals the drag reduction failure mechanism of CO2 nanobubbles at the coal substrate interface through comprehensive analysis of multi-dimensional indicators. Specifically, it includes:

[0140] First, the average velocity and near-wall density of the outer water layer for bubble systems of different scales and the CO2-free control system were determined, and the changes in the average velocity and near-wall density of the outer water layer for each bubble system relative to the control system were calculated.

[0141] Specifically, the external water layer was defined as the interval from 3.6 nm to 8.8 nm above the coal surface, encompassing the complete mainstream water layer from the top of the near-wall region to the free interface. The near-wall region was defined as the interval from 3.6 nm to 5.3 nm above the coal surface, which is the region where the coal substrate interface effect has the most significant impact on the velocity and density distribution of the water phase. The average velocity and average density of each system in the near-wall region and external water layer were calculated, where velocity was the statistical average of the velocity components of water molecules along the driving force direction within each grid, and density was the mass density obtained by averaging the ratio of the number of water molecules to the grid volume within each grid over time. Then, the above indicators for each bubble system were normalized and compared with the CO2-free control system to obtain the percentage change in velocity and density.

[0142] For example, the data in Table 4 show that in the outer water layer, the average velocity of the small bubble system was 1.532 nm per nanosecond, a decrease of 4.53% compared to the CO2-free control system; the average velocity of the large bubble system was 1.427 nm per nanosecond, a decrease of 11.10% compared to the CO2-free control system, a significantly greater decrease than that of the small bubble system. In the near-wall region, the density of large bubbles decreased by 12.01% compared to the control system, while that of small bubbles decreased by only 2.35%, indicating that large bubbles have a stronger drainage capacity, but at the same time, the velocity decrease in the outer water layer was more severe.

[0143] Table 4

[0144]

[0145] Next, the apparent slip lengths of each bubble system and the control system were compared to determine the difference in apparent slip length between each bubble system and the control system. In this embodiment, under a unified fitting window, the apparent slip length of the small bubble system was -0.1429 nm, the CO2-free system was -0.0148 nm, and the large bubble system was -0.0047 nm. That is, the apparent slip length of the large bubble increased by only about 0.010 nm compared to the CO2-free system, indicating a very limited improvement. When the fitting window was extended to 5.5 nm above the coal surface (i.e., covering the top of the large bubble and its velocity recovery zone), the apparent slip lengths of the small bubble, large bubble, and CO2-free systems became 0.1111 nm, 0.0166 nm, and 0.1230 nm, respectively, with the large bubble still being the lowest among the three.

[0146] Then, based on the difference between the local velocity field of the bubble system and the velocity field at the same location in the control system, the local acceleration zone upstream and the local deceleration / recirculation zone downstream of the bubble are determined. For example... Figure 7 As shown, the difference field indicates that there is a bubble upstream. Local acceleration zone, downstream exists The two local deceleration regions have opposite signs and are spatially symmetrically distributed. The appearance of this dipole-shaped perturbation structure indicates that although the presence of bubbles locally accelerates the water phase flow upstream, it also causes an equal amount of momentum loss downstream. The local acceleration and local stagnation cancel each other out after being averaged laterally. Therefore, the local acceleration upstream cannot be equated to the global drag reduction of the system.

[0147] Finally, based on the changes in the average velocity of the external water layer, the changes in near-wall density, the difference in apparent slip length, and the spatial distribution of the local acceleration zone and the local deceleration / recirculation zone, the competitive relationship between the low-friction gas phase coverage contribution and the protruding interface drag contribution of CO2 nanobubbles is determined; and based on the competitive relationship, the influence of bubble size on slip drag reduction efficiency is determined.

[0148] The competition between the low-friction gas phase coverage contribution and the protruding interface shape resistance contribution of CO2 nanobubbles is determined based on the average velocity change of the external water layer, the near-wall density change, the apparent slip length difference, and the spatial distribution of local acceleration and deceleration / recirculation zones. This includes:

[0149] Based on the near-wall density change, the contribution of low-friction gas phase coverage is determined. A decrease in near-wall density means that the bubbles displace the water molecules that originally occupied that area, forming a low-density, low-viscosity gas phase region between the wall and the main flow. This reduces the direct contact and energy exchange between water molecules and the coal surface, theoretically helping to reduce interfacial friction. In this embodiment, the near-wall density reduction of 12.01% for large bubbles is significantly greater than that of small bubbles (2.35%), indicating that large bubbles have a stronger potential for low-friction coverage.

[0150] Based on the changes in the average velocity of the external water layer and the spatial distribution of local acceleration and deceleration / recirculation zones, the shape drag contribution of the protruding interface is determined. Bubbles protruding from the coal surface form a curved gas-liquid interface, forcing the mainstream fluid to undergo streamline bending and local acceleration as it bypasses the bubbles. This creates a wake zone and recirculation zone downstream of the bubbles, forming additional pressure drag. This shape drag contribution is not directly reflected in the near-wall density change, but rather manifests in the reduction of the average velocity of the external water layer and the existence and extent of the downstream deceleration / recirculation zone.

[0151] Based on the combined characteristics of the near-wall density change and the average velocity change of the external water layer, the primary and secondary relationship between the contribution of low-friction gas phase coverage and the contribution of protruding interface shape drag is determined. If the near-wall density change is lower than that of the control system (indicating the presence of low-friction gas phase coverage) and the average velocity change of the external water layer is also lower than that of the control system (indicating global flow obstruction), then the contribution of protruding interface shape drag is determined to be greater than that of low-friction gas phase coverage, meaning that the presence of bubbles generally increases drag rather than reduces it. In this embodiment, the near-wall density of large bubbles decreased by 12.01%, while the average velocity of the external water layer decreased by 11.10%; the near-wall density of small bubbles decreased by 2.35%, while the average velocity of the external water layer decreased by 4.53%. Both bubble systems meet the above determination criteria, indicating that under the current simulation conditions, shape drag contribution dominates the bubble flow effect, and the potential benefits of low-friction coverage have not been converted into global net drag reduction.

[0152] Figure 8 This is a schematic diagram of the competitive mechanism for CO2 nanobubble size regulation of coal-water interface flow provided in an embodiment of the present invention, and Figure 8 (a) is a schematic diagram illustrating the competition between low-friction gas phase coverage and interfacial resistance in a small bubble system. Figure 8 (b) is a schematic diagram illustrating the competition between low-friction gas phase coverage and interfacial resistance in a large bubble system. Figure 8 (a) and Figure 8 As can be seen from the comparison in (b), this invention reveals for the first time that the essential role of CO2 nanobubbles at the coal substrate interface is the competition between low-friction gas phase patches and protruding interface shape drag. Although increasing bubble size expands the drainage area, it also enhances flow around the surface and shape drag, resulting in no improvement in overall drag reduction efficiency, or even a decrease. Based on this competitive relationship, the non-monotonic influence of bubble size on slip drag reduction efficiency can be determined: if a larger bubble system has a greater decrease in near-wall density and a greater decrease in the average velocity of the external water layer compared to a smaller bubble system, and a lower apparent slip length, then it is determined that within this scale range, increasing bubble size has a negative impact on slip drag reduction efficiency. Furthermore, if the apparent slip length of the bubble system approaches zero or becomes positive relative to the control system, but the average velocity of the external water layer is still lower than that of the control system, then it is also determined that its low-friction contribution has not been converted into global drag reduction.

[0153] Based on the above embodiments, the present invention achieves the following beneficial effects:

[0154] (1) A molecular dynamics evaluation method for drag reduction of CO2 nanobubbles on real coal substrate was established for the first time. By constructing a coal substrate obtained by stacking and compressing a real coal macromolecule model, the model gap in the study of coal-water-CO2 three-phase interface of existing molecular simulation methods was filled. By constructing a coal substrate obtained by stacking and compressing a real coal macromolecule model, the anomalous physical mechanism of drag reduction failure of CO2 nanobubbles at the coal substrate interface was revealed.

[0155] (2) This invention establishes an evaluation framework for judging the competition relationship between low friction contribution and form drag contribution through multi-dimensional joint analysis of overall velocity profile, overall density profile, apparent slip length and local velocity field. It can distinguish the decoupling phenomenon between local boundary effect in near-wall region and global drag reduction effect of external water layer, and provides a more scientific evaluation standard for drag reduction control of micro-nano scale interface flow.

[0156] (3) This invention constructs CO2 nanobubbles of different sizes at the coal-water interface and performs linear fitting of the same velocity profile in the near-wall region and the extended region through a dual-window method, effectively separating the apparent slip dominated by near-wall shear from the overall effective slip considering the influence of bubble flow. This overcomes the technical difficulty of objectively evaluating the boundary slip characteristics under bubble interference by using a single fitting window.

[0157] (4) This invention provides a theoretical basis and calculation support for coal seam water injection and CO2 geological utilization. Engineering practices such as coal seam water injection, micro-nano bubble water injection, CO2 geological utilization and enhanced coalbed methane mining all involve water phase transport and gas enrichment near the coal base. The evaluation method provided by this invention can predict the actual impact of CO2 nanobubbles on water phase transport efficiency at different scales, guide the optimization selection of bubble scale in engineering, and avoid the decrease in water injection efficiency caused by blindly pursuing nanobubble humidification.

[0158] Example 2

[0159] To verify the effectiveness of the method of the present invention, this embodiment performed statistical analysis on the simulated trajectories of the above three systems, and the results are as follows:

[0160] (1) Verification of bubble stability: Within the statistical window of 1ns-10ns, both small and large bubbles remained stably at the coal-water interface, such as Figure 2 As shown ( Figure 2 (a) is a diagram showing the evolution of the center position of the large bubble. Figure 2 (b) is a diagram showing the evolution of the center position of the small bubbles. Figure 2(c) is a time series diagram of bubble gyration radius. The gyration radii of small bubbles and large bubbles are 0.771±0.020nm and 1.060±0.013nm, respectively. The coefficient of variation of large bubbles is only 1.25%, and its normal broadening of 0.524±0.033nm is smaller than its lateral broadening of 0.640±0.031nm and 0.660±0.029nm, indicating that it maintains a stable interface attachment morphology of lateral broadening and normal flattening.

[0161] (2) Drainage Scale Analysis: Increased bubble size significantly expanded the interfacial drainage range. Using the low water sample cavity criterion, the cumulative number of water samples in the bubble system grid was less than 20% of that in the CO2-free control system grid at the same location, and the cumulative number of samples in the control system was not less than 50. The cavity area of ​​the large bubble sample was 3.415 nm. 2 It is approximately 1.105 nm in size of a small bubble. 2 It is 3.09 times larger and extends to a higher mainstream water layer, while the small bubble cavity only extends to about 4.26 nm at most.

[0162] (3) Velocity and density response: such as Figure 3 As shown, Figure 3 (a) is a cross-sectional view of the overall water velocity. Figure 3 (b) is the overall water density profile. With a coal wall position of 3.3 nm as the baseline, the average velocity of the outer water layer of small and large bubbles is 4.53% and 11.10% lower than that of the CO2-free control system, respectively. The near-wall density of large bubbles is reduced by 12.01%, which is significantly higher than that of small bubbles by 2.35%. The dual decrease in velocity and density indicates that, under the current conditions, although local gas phase coverage can reduce near-wall density, it does not translate into global net drag reduction. Instead, the flow field disturbance caused by bubble morphology leads to a decrease in overall transport efficiency.

[0163] (4) Comparison of slip lengths: combined with Figure 4 As shown, Figure 4 (a) is a schematic diagram of linear extrapolation of the launching velocities of the three systems. Figure 4(b) shows the apparent slip lengths for the three systems. Within a unified fitting window of 3.6 nm–4.5 nm, the apparent slip lengths for small bubbles, large bubbles, and no bubbles are -0.1429 nm, -0.0047 nm, and -0.0148 nm, respectively. When the fitting window is expanded to 5.5 nm and covers the top of the large bubble, the apparent slip lengths for the three systems become 0.1111 nm, 0.0166 nm, and 0.1230 nm, respectively. This dual-window fitting result indicates that the slip length obtained by the expanded window no longer only reflects the boundary characteristics of the near-wall linear shear zone, but also includes the overall effective extrapolation parameters of the bubble top perturbation and the velocity recovery process above. The slip length of the large bubble (0.0166 nm) in the expanded window is significantly lower than that of the small bubble (0.1111 nm) and the CO2-free system (0.1230 nm), indicating that although the large bubble weakens the near-wall negative slip, its larger interface protrusion scale introduces stronger flow around and local form drag, thereby reducing the overall effective slip performance.

[0164] (5) Local flow field characteristics: such as Figures 5 to 7 As shown, the local velocity field in the XZ profile exhibits a dipole structure of upstream acceleration and downstream recirculation, and the perturbation scale expands with increasing bubble size. Specifically, Figure 5 (a) is a schematic diagram of the velocity of the cross-section of the small bubble. Figure 5 (b) is a schematic diagram of the cross-sectional velocity of the large bubble. Figure 5 (c) is a schematic diagram of the velocity of the bubble-free cross section. As can be seen, Figure 5 The width and extension distance of the downstream negative velocity band of medium and large bubbles are significantly greater than those of small bubbles; Figure 6 The horizontal weighted average velocity further confirmed from a quantitative perspective that neither of the two bubble systems brought about a net velocity increase; Figure 7 The difference field (e.g.) Figure 7 (a) The speed difference between small bubbles and no bubbles Figure 7 (b) The velocity difference between large bubbles and no bubbles Figure 7 (c) The velocity difference between large and small bubbles reveals that the increase in scale mainly enhances the asymmetric momentum reconstruction near the interface. The above multi-figure joint analysis confirms that the essential role of CO2 nanobubbles at the coal substrate interface is the competition between low-friction gas phase patches and protruding interface drag, and that increasing bubble size does not necessarily improve drag reduction efficiency.

[0165] In summary, the technical solution of this invention achieves a systematic and high-precision molecular simulation evaluation of the drag reduction effect of CO2 nanobubbles at the coal-water interface. This method not only distinguishes the decoupling phenomenon between local boundary effects near the wall and the global drag reduction effect of the external water layer, but also reveals for the first time the drag reduction failure mechanism caused by CO2 nanobubbles being dominated by shape drag on the specific complex surface of the coal substrate. This invention provides direct theoretical basis and computational support for the optimal selection of bubble size in engineering practices such as coal seam water injection and lubrication, CO2 geological utilization, and enhanced coalbed methane extraction, helping to avoid the decline in water injection efficiency caused by blindly pursuing nanobubble lubrication.

[0166] Example 3

[0167] Figure 9 This is a schematic diagram of the structure of a molecular simulation evaluation device for the drag reduction effect of CO2 nanobubbles at the coal substrate interface, provided in Embodiment 3 of the present invention. Figure 9 As shown, the device includes:

[0168] The production simulation module 910 is used to perform molecular dynamics production simulation based on a pre-constructed composite molecular dynamics simulation system, wherein the composite molecular dynamics simulation system includes a coal substrate, a water layer above the coal substrate, and CO2 nanobubbles aggregated at the coal-water interface; in the molecular dynamics production simulation, an external driving force along a constant direction is applied to the water layer to drive the water phase to flow along the constant direction.

[0169] The trajectory acquisition and profile acquisition module 920 is used to acquire trajectory data at each moment during the molecular dynamics production simulation, and to acquire the overall velocity profile and overall density profile of the composite molecular dynamics simulation system along the normal direction of the coal substrate based on the trajectory data.

[0170] The slip length calculation module 930 is used to set a near-wall fitting interval above the coal base surface based on the overall velocity profile, and to perform linear fitting on the velocity distribution within the near-wall fitting interval to obtain the apparent slip length at the coal base surface.

[0171] The local flow field acquisition module 940 is used to extract a two-dimensional cross-section containing the central region of CO2 nanobubbles based on the trajectory data, and to obtain the local velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section.

[0172] The drag reduction impact assessment module 950 is used to evaluate the influence of CO2 nanobubbles of different sizes on drag reduction of the water-gas system based on the overall velocity profile, overall density profile, apparent slip length and local velocity field distribution.

[0173] The molecular simulation evaluation device for the drag reduction effect of CO2 nanobubbles at the coal substrate interface provided in this embodiment of the invention can execute the molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface provided in any of the embodiments of the invention above. It has the corresponding functions and beneficial effects of executing the molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface. For detailed process, please refer to the relevant operation of the molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface in the foregoing embodiments.

[0174] Example 4

[0175] Figure 10 This is a schematic diagram of the structure of an electronic device provided in Embodiment 4 of the present invention. The electronic device is intended to represent various forms of digital computers, and may also represent various forms of mobile devices. The components shown herein, their connections and relationships, and their functions are merely examples and are not intended to limit the implementation of the invention described and / or claimed herein.

[0176] like Figure 10 As shown, the electronic device includes at least one processor 11 and a memory, such as a read-only memory (ROM) 12 or a random access memory (RAM) 13, communicatively connected to the at least one processor 11. The memory stores computer programs executable by the at least one processor. The processor 11 can perform various appropriate actions and processes based on the computer program stored in the ROM 12 or loaded from storage unit 18 into the RAM 13. The RAM 13 can also store various programs and data required for the operation of the electronic device 10. The processor 11, ROM 12, and RAM 13 are interconnected via a bus 14. An input / output (I / O) interface 15 is also connected to the bus 14.

[0177] Multiple components in the electronic device are connected to the I / O interface 15, including: an input unit 16, such as a keyboard, mouse, etc.; an output unit 17, such as various types of displays, speakers, etc.; a storage unit 18, such as a disk, optical disk, etc.; and a communication unit 19, such as a network card, modem, wireless transceiver, etc. The communication unit 19 allows the electronic device to exchange information / data with other devices through computer networks such as the Internet and / or various telecommunications networks.

[0178] Processor 11 can be a variety of general-purpose and / or special-purpose processing components with processing and computing capabilities. Some examples of processor 11 include, but are not limited to, central processing units (CPUs), graphics processing units (GPUs), various special-purpose artificial intelligence (AI) computing chips, various processors running machine learning model algorithms, digital signal processors (DSPs), and any suitable processor, controller, microcontroller, etc. Processor 11 performs a molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at a coal substrate interface, as described above.

[0179] It should be understood that the various forms of processes shown above can be used, with steps reordered, added, or deleted. For example, the steps described in this invention can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this invention can be achieved, and this is not limited herein.

[0180] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at a coal substrate interface, characterized in that, include: Molecular dynamics production simulation is performed based on a pre-constructed composite molecular dynamics simulation system, wherein the composite molecular dynamics simulation system includes a coal substrate, a water layer located above the coal substrate, and CO2 nanobubbles aggregated at the coal-water interface. In molecular dynamics production simulations, an external driving force is applied to the water layer in a constant direction, causing the water phase to flow in that constant direction. The trajectory data at each moment during the molecular dynamics production simulation is collected, and the overall velocity profile and overall density profile of the composite molecular dynamics simulation system along the normal direction of the coal substrate are obtained based on the trajectory data. Based on the overall velocity profile, a near-wall fitting interval is set above the coal substrate surface, and the velocity distribution within the near-wall fitting interval is linearly fitted to obtain the apparent slip length at the coal substrate surface. Based on the trajectory data, a two-dimensional cross-section containing the central region of CO2 nanobubbles is extracted, and the local velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section is obtained. Based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field distribution, the influence of CO2 nanobubbles of different sizes on drag reduction in the water-gas system is evaluated, including: The average velocity and near-wall density of the outer water layer were determined for bubble systems of different scales and a CO2-free control system, and the changes in the average velocity and near-wall density of the outer water layer of each bubble system relative to the control system were calculated. Compare the apparent slip lengths of each bubble system with the control system to determine the difference in apparent slip lengths of each bubble system relative to the control system; Based on the difference between the local velocity field of the bubble system and the velocity field at the same location in the control system, the local acceleration zone upstream of the bubble and the local deceleration / recirculation zone downstream of the bubble are determined. Based on the changes in the average velocity of the external water layer, the changes in near-wall density, the difference in apparent slip length, and the spatial distribution of local acceleration and deceleration / return zones, the competitive relationship between the low-friction gas phase coverage contribution and the prominent interface shape resistance contribution of CO2 nanobubbles is determined. Based on the aforementioned competitive relationship, the influence of bubble size on slip reduction efficiency is determined.

2. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 1, characterized in that, Based on the trajectory data, the overall velocity profile and overall density profile of the system along the normal direction of the coal base are obtained, including: The trajectory data is preprocessed to remove non-steady-state trajectories of a preset duration during the initial stage of production simulation, thereby obtaining steady-state trajectory data; The water molecules and coal substrate in the steady-state trajectory data are mapped to a uniform periodic cut, preserving the signed distance of the water molecules relative to the coal substrate surface; Based on the signed distance, the grid is divided along the normal direction of the coal substrate at a preset box interval. The water phase velocity data and the number of water molecules in each grid are statistically analyzed to obtain the overall velocity profile and the overall density profile.

3. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 2, characterized in that, Based on the signed distance, the coal substrate is divided into grids at preset bin intervals along the normal direction. The water phase velocity data and water molecule quantity within each grid are statistically analyzed to obtain an overall velocity profile and an overall density profile, including: Based on the molecular complete trajectory in the steady-state trajectory data, the velocity data of each water molecule is obtained by performing finite difference through the continuous displacement of oxygen atoms in the water molecule. Using the signed distance as the normal coordinate, the velocity data and spatial position of each water molecule are assigned to their respective grids; The average value of the velocity components of all water molecules along the driving force direction in each grid is calculated and used as the average velocity at the corresponding normal position of each grid. The overall velocity profile is constructed based on the distribution of the average velocity at the corresponding normal position of each grid along the normal direction. Calculate the ratio of the number of water molecules to the grid volume in each grid, and use it as the number density or mass density at the corresponding normal position of each grid. Based on the distribution of the number density or mass density at the corresponding normal position of each grid along the normal direction, construct the overall density profile.

4. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 1, characterized in that, Based on the overall velocity profile, a near-wall fitting interval is defined above the coal substrate surface. The velocity distribution within this near-wall fitting interval is linearly fitted to obtain the apparent slip length at the coal substrate surface, including: A preset reference position of the upper surface of the coal substrate in the normal direction is obtained as the wall position, and a near-wall fitting interval is set above the wall position and within a preset range of the linear velocity distribution region; Linear regression is performed on the overall velocity profile within the near-wall fitting interval to fit a linear relationship between velocity and normal distance, and the near-wall shear rate is determined based on the slope of the linear relationship. Based on the linear relationship, determine the corresponding velocity value at the wall position, which is taken as the apparent slip velocity at the wall. The apparent slip length is calculated based on the ratio of the apparent slip velocity to the near-wall shear rate.

5. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 1, characterized in that, Based on the trajectory data, a two-dimensional cross-section containing the central region of CO2 nanobubbles is extracted, and the velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section is obtained, including: The position of the CO2 aggregate centroid under periodic boundary conditions in each frame of trajectory data is determined and used as the reference center of the tangent. Using the aforementioned cross-sectional reference center as a reference, a two-dimensional cross-section is cut that is perpendicular to the coal substrate surface and parallel to the direction of the driving force. The two-dimensional cross-section is divided into spatial grids, and the velocity components of water molecules along the driving force direction in each grid are counted. The average value of all velocity components in the grid is calculated as the local velocity value corresponding to each grid. Based on the local velocity values ​​corresponding to each grid, the local velocity field distribution within the two-dimensional sectional region is constructed.

6. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 1, characterized in that, Based on the changes in average velocity of the external water layer, near-wall density, apparent slip length difference, and spatial distribution of local acceleration and deceleration / recirculation zones, the competitive relationship between the low-friction gas phase coverage contribution and the prominent interface shape resistance contribution of CO2 nanobubbles is determined, including: The contribution of low-friction gas phase coverage is determined based on the near-wall density change. The contribution of the protruding interface shape drag is determined based on the average velocity change of the external water layer and the spatial distribution of local acceleration and deceleration / backflow zones. Based on the combined characteristics of the near-wall density change and the average velocity change of the external water layer, the primary and secondary relationship between the contribution of the low-friction gas phase cover and the contribution of the protruding interface shape drag is determined. If the near-wall density change is lower than that of the control system and the average velocity change of the outer water layer is lower than that of the control system, then the contribution of the protruding interface shape resistance is determined to be greater than the contribution of the low-friction gas phase coverage.

7. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 6, characterized in that, Based on the aforementioned competitive relationship, the non-monotonic influence of bubble size on slip drag reduction efficiency is determined, including: Compare the competing relationship between the low-friction gas phase coverage contribution and the prominent interface shape resistance contribution in bubble systems of different scales; If a larger bubble system exhibits a greater decrease in near-wall density and a greater decrease in average velocity of the outer water layer compared to a smaller bubble system, and also has a lower apparent slip length, then it can be determined that increasing the bubble size within this scale range has a negative impact on slip drag reduction efficiency.

8. The molecular simulation evaluation method for the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to claim 6, characterized in that, Determining the non-monotonic influence of bubble size on slip drag reduction efficiency based on the aforementioned competitive relationship also includes: If the apparent slip length of the bubble system approaches zero or becomes positive relative to the control system, but the average velocity of the external water layer is still lower than that of the control system, then it is determined that the contribution of the low-friction gas phase coverage of the bubbles in this scale range has not been converted into global drag reduction.

9. A molecular simulation evaluation device for the drag reduction effect of CO2 nanobubbles at a coal substrate interface, characterized in that, A molecular simulation evaluation method for performing the drag reduction effect of CO2 nanobubbles at the coal substrate interface according to any one of claims 1 to 8, the apparatus comprising: The production simulation module is used to perform molecular dynamics production simulation based on a pre-constructed composite molecular dynamics simulation system, wherein the composite molecular dynamics simulation system includes a coal substrate, a water layer located above the coal substrate, and CO2 nanobubbles aggregated at the coal-water interface; in the molecular dynamics production simulation, an external driving force along a constant direction is applied to the water layer to drive the water phase to flow along the constant direction; The trajectory acquisition and profile acquisition module is used to acquire trajectory data at each moment during the molecular dynamics production simulation, and to acquire the overall velocity profile and overall density profile of the composite molecular dynamics simulation system along the normal direction of the coal substrate based on the trajectory data. The slip length calculation module is used to set a near-wall fitting interval above the coal substrate surface based on the overall velocity profile, and to perform linear fitting on the velocity distribution within the near-wall fitting interval to obtain the apparent slip length at the coal substrate surface. The local flow field acquisition module is used to extract a two-dimensional cross-section containing the central region of CO2 nanobubbles based on the trajectory data, and to obtain the local velocity field distribution of the water phase along the driving force direction within the two-dimensional cross-section. The drag reduction impact assessment module is used to evaluate the impact of CO2 nanobubbles of different sizes on drag reduction in the water-gas system based on the overall velocity profile, overall density profile, apparent slip length, and local velocity field distribution.

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