Non-spherical particle and flow field bidirectional coupling gas-solid two-phase flow numerical simulation method

Abstract

The invention discloses a gas-solid two-phase flow numerical simulation method for bidirectional coupling of non-spherical particles and a flow field, and belongs to the technical field of biomass fuel utilization, and the method comprises the following steps: constructing a flow field model in Fluent, and correspondingly setting an initial boundary condition of the flow field; setting initialization conditions of non-spherical particles in the UDF; establishing an inertial coordinate system and a particle coordinate system, and coupling the two coordinate systems; and obtaining a translational motion table / rotational motion characterization equation, determining the position and direction of the particles by solving the two equations, correcting flow field parameters by using the positions and direction of the particles, and iteratively calculating particle motion parameters to realize bidirectional coupling of the non-spherical particles and the flow field. According to the numerical simulation method, through Fluent-UDF numerical simulation, the interaction between the non-spherical particles and the flow field can be accurately simulated, the accuracy of gas-solid two-phase flow numerical simulation is guaranteed, understanding of the complex process of direct combustion coupling of biomass particles of a coal-fired boiler is deepened, and theoretical support is provided for optimization of combustor design and the combustion process.

Description

Technical Field

[0001] The invention belongs to the technical field of biomass fuel utilization, and in particular relates to a gas-solid two-phase flow numerical simulation method for bidirectional coupling of non-spherical particles and flow fields. Background Art

[0002] The use of biomass in coal-fired boilers is one of the effective ways to reduce carbon emissions and recycle biomass solid waste, and has been increasingly widely studied and applied. Since biomass particles usually have a large particle size and present irregular non-spherical geometric characteristics, their dynamic characteristics in the gas-solid two-phase flow of coal-fired boilers are significantly different from those of conventional spherical pulverized coal fuel particles, making it difficult to accurately predict and simulate their dynamic behavior in the gas-solid two-phase flow.

[0003] Compared with spherical particles, the gas-solid two-phase flow dynamics of non-spherical particles are more complex. Especially in suspension combustion technology, their asymmetric shape, the drag, lift and torque generated by the rotation effect will have a significant impact on the motion trajectory during the combustion process.

[0004] At present, the simulation of the dynamic behavior of particles in the flow field can be achieved by using the CFD (Computational Fluid Dynamics) modeling method, which is usually completed in conjunction with the commercial numerical software Fluent. It has the advantages of efficient numerical model setting, free user-defined model setting, good model prediction performance and fast result display, and is widely used in various numerical simulation scenarios.

[0005] However, the existing numerical simulation methods are usually applicable to the numerical simulation of spherical particles, and there are still some limitations and challenges in the modeling of non-spherical particles. Among them, the particles are regarded as spherical by default in the discrete phase model (DPM) of Fluent, which oversimplifies the calculation and influence of forces such as drag, lift, and torque. Since the gas-solid flow behavior of non-spherical particles (such as biomass particles with an average equivalent diameter of 2~3mm) is more complex, this will lead to deviations in the results of simulated particle trajectories and residence time. Moreover, for real biomass particles, the traditional drag model (such as the Stokes drag model) used in DPM cannot accurately describe its dynamic behavior. The main reason is that the drag coefficient, lift coefficient and torque coefficient of non-spherical particles are not only related to the Reynolds number, but also affected by shape parameters (such as aspect ratio) and direction angle, which are not fully reflected in the existing default model of Fluent, resulting in the existing numerical simulation methods being difficult to directly apply to the simulation of the dynamic behavior of non-spherical particles. In addition, in the existing numerical simulation studies on the combustion of biomass in coal-fired boilers, most of them adopted simplified particle dynamics models. Especially in the treatment of particle shape, a spherical assumption is usually used, ignoring the rotation effect of particles, resulting in a large deviation between the simulation results and the actual situation. Summary of the invention

[0006] In response to one or more of the above-mentioned defects or improvement needs in the prior art, the present invention provides a gas-solid two-phase flow numerical simulation method with bidirectional coupling between non-spherical particles and flow field, which can effectively simulate the dynamic state of non-spherical particles in gas-solid two-phase flow, accurately capture the complex behavior of non-spherical particles, fully consider the rotation effect of non-spherical particles in the flow field, and greatly improve the numerical simulation accuracy of non-spherical particles.

[0007] To achieve the above object, the present invention provides a gas-solid two-phase flow numerical simulation method with bidirectional coupling between non-spherical particles and flow field, which includes the following process: (1) Set the flow field model and initial boundary conditions and solve the NS equations; (2) Set the initialization conditions of non-spherical particles and determine the physical parameters and initial motion parameters of the particles; (3) Establish an inertial coordinate system for the flow field and a particle coordinate system for non-spherical particles, and couple the two coordinate systems to obtain the Euler angle ; (4) The motion control translation equation of non-spherical particles is established, and the non-spherical particles are regarded as discrete phases. The following translation characterization equation is obtained:

[0008] In the formula, , , , represent the undisturbed velocity, mass, volume, and density of the particle, respectively; , , , , They are the drag force, lift force, virtual mass force, pressure gradient force and gravity on the particle, among which the drag force and lift force are determined by the direction angle of the particle. θ Decide; (5) The rotational equation for controlling the motion of non-spherical particles is established, and the rotational motion characterization equation is obtained by considering the triaxial torque of non-spherical particles:

[0009] In the formula, , and represents the components of the particle angular velocity, moment of inertia, and torque in the directions of the three particle axes; and for cylindrical particles, , ; (6) Solving the translational characterization equation and the rotational characterization equation, iteratively calculating the flow field and the motion parameters of the non-spherical particles, and updating the position and direction of the non-spherical particles; (7) Feeding back the particle motion parameters obtained by iterative calculation into the flow field model, and correcting the simulation calculation of the flow field based on the particle motion parameters of the non-spherical particles; thereafter, iteratively performing the simulation calculation of the motion process of the non-spherical particles based on the corrected flow field parameters; (8) The calculation process in (3) to (7) is repeated until the particle reaches the outlet boundary and the calculation converges.

[0010] As a further improvement of the present invention, when coupling the two coordinate systems in process (3), quaternions are introduced to describe the direction in the particle coordinate system, that is, , and through an orthogonal transformation matrix A From the inertial coordinate system , The particle coordinate system is obtained and ;

[0011]

[0012]

[0013] In the formula, , are the particle angular velocity in the inertial coordinate system and particle coordinate system, respectively; , is the torque in the inertial coordinate system and the particle coordinate system.

[0014] As a further improvement of the present invention, the quaternion is obtained by using the Euler angle To calculate, and get the following equation; and the quaternion is continuously updated over time and normalized at each time node, that is, ;

[0015] .

[0016] As a further improvement of the present invention, in the translation characterization equation, the forces acting on the non-spherical particles are calculated by the following formula:

[0017]

[0018]

[0019]

[0020] In the formula, is the projection area perpendicular to the direction of motion during the particle falling process, and ; , are the undisturbed velocity and density of the airflow fluid, respectively; is the gravitational acceleration of the particle; , and are the drag coefficient, lift coefficient and torque coefficient of the particle at the corresponding angles respectively.

[0021] As a further improvement of the present invention, corresponding to the drag coefficient , lift coefficient and torque coefficient The calculation is done by using the open source software OpenFOAM to meet the following conditions: 0.1≤ aspect ratio Ar ≤0.9, 0°≤direction angle θ ≤90°, 1≤Reynolds number Re The cylindrical particles with a diameter of ≤2000 are directly simulated under body-fitting grid to obtain the OpenFOAM simulation values.

[0022] As a further improvement of the present invention, the drag coefficient The calculation is done using the following relationship:

[0023]

[0024]

[0025]

[0026]

[0027]

[0028] In the formula, is the particle angle in the flow field The drag coefficient at ; is the drag coefficient when the particle angle in the flow field is 0° or 90°; ( i =2,3,4,5) are fitting coefficients, and by introducing the relevant fitting coefficients Calculated, and the relevant fitting coefficient Numerical selection was performed by OpenFOAM simulation.

[0029] As a further improvement of the present invention, the lift coefficient The calculation is done using the following relationship:

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037] In the formula, is the particle angle in the flow field The lift coefficient at ; To define the parameters, the corresponding formulas and numerical simulation parameters are calculated respectively; It is a numerical simulation parameter, which is selected through OpenFOAM simulation value.

[0038] As a further improvement of the present invention, in process (5), the torque components in each direction of the particle rotational motion are The calculation is performed by the following formula:

[0039]

[0040]

[0041]

[0042]

[0043]

[0044] In the formula, are the stress change rate at the particle position, the fluid angular velocity, and the diagonal drag tensor relative to the particle angular velocity and strain rate; is the particle angle in the flow field The torque coefficient at .

[0045] As a further improvement of the present invention, the torque coefficient Calculate using the following formula:

[0046]

[0047]

[0048]

[0049]

[0050]

[0051]

[0052] In the formula, are defined parameters, which are calculated by corresponding formulas and numerical simulation parameters respectively; It is a numerical simulation parameter, which is selected through OpenFOAM simulation value.

[0053] As a further improvement of the present invention, the movement process of the continuous phase gas flow field is continuously run in the Fluent module, and the movement process of the non-spherical particles is continuously iteratively run in the UDF module; the initial conditions of the gas field required in the movement process of the non-spherical particles are provided by the Fluent module to the UDF module, and after the particle movement in the UDF module is iteratively calculated for many times, the UDF module passes the position and direction data of the particles to the Fluent module to correct the operating parameters of the airflow field, and then the corrected airflow field parameters are passed to the UDF module as the gas field initialization conditions for a new round of iterative calculations.

[0054] The above-mentioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0055] In general, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects: (1) The present invention provides a method for numerically simulating gas-solid two-phase flow with bidirectional coupling between non-spherical particles and flow field, which comprises the following steps: constructing a flow field model in Fluent and setting the initial boundary conditions of the flow field accordingly; setting the initialization conditions of the non-spherical particles in UDF; establishing an inertial coordinate system and a particle coordinate system, and coupling the two coordinate systems; obtaining a translational motion characterization equation and a rotational motion characterization equation, and determining the position and direction of the particle by solving the two equations, and then by coupling the particle motion parameters with the flow field model, correcting the flow field parameters and iteratively calculating the particle motion parameters, thereby realizing bidirectional coupling between the non-spherical particles and the flow field, improving the accuracy of the simulation of the motion process of the non-spherical particles in the flow field, providing a basis for the real feedback of the motion process of the non-spherical particles in the gas field, and providing theoretical support for the application of non-spherical biomass particles.

[0056] (2) The numerical simulation method of gas-solid two-phase flow with bidirectional coupling between non-spherical particles and flow field in the present invention, by optimally designing the conversion method of the two coordinate systems and the calculation method of the various forces acting on the non-spherical particles, cooperates with the direct numerical simulation of OpenFOAM, fully guarantees the calculation of the drag coefficient, lift coefficient, torque coefficient and corresponding forces acting on the non-spherical particles, and fully considers the coupling of the translational motion and rotational motion of the non-spherical particles in the gas field, so that the simulation calculation of the motion process of the non-spherical particles is closer to the actual situation, thereby improving the accuracy and reliability of the simulation calculation results.

[0057] (3) The numerical simulation method of gas-solid two-phase flow with bidirectional coupling between non-spherical particles and flow field in the present invention can accurately simulate the interaction between non-spherical particles and flow field through Fluent-UDF numerical simulation. It can not only deal with the translational and rotational motion of non-spherical particles, but also fully consider the interaction between gas and solid phases. It has significant advantages in dealing with large particle size and irregular shape of biomass particles, and can effectively deepen the understanding of the complex process of direct combustion coupling of non-spherical biomass particles in coal-fired boilers, and provide theoretical support for optimizing burner design and combustion process. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without paying creative work.

[0059] Figure 1 It is an operation logic flow chart of the numerical simulation method of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field in an embodiment of the present invention; Figure 2~Figure 3 is a schematic diagram of an inertial coordinate system and a particle coordinate system constructed for non-spherical particles in a numerical simulation method according to an embodiment of the present invention; Figure 4 Schematic diagram of the motion trajectory of non-spherical particles under different models in a specific embodiment of the present invention; Figure 5 It is a comparative schematic diagram of the drag coefficient and the particle velocity under different models in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0061] In the description of the present invention, it should be understood that, unless otherwise expressly specified and limited, the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0062] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0063] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0064] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0065] Specifically, in a preferred embodiment of the present invention, a method for numerical simulation of gas-solid two-phase flow with bidirectional coupling between non-spherical particles and flow field is provided. The method aims to realize bidirectional coupling calculation between non-spherical particles and fluid by considering the coupling effects of various forces such as drag, lift, torque, virtual mass, pressure gradient force, etc. of non-spherical particles in gas-solid two-phase flow. While considering the effect of flow field on particles, the influence of particle motion on flow field is also fully considered, thus solving the translation and rotation coupling problem of non-spherical particles, accurately simulating the motion process of non-spherical particles, and improving the accuracy of simulation.

[0066] See also Figure 1 In a preferred embodiment, the numerical simulation method of gas-solid two-phase flow with bidirectional coupling between non-spherical particles and flow field includes the following process: (1) Set the flow field model and initial boundary conditions, and solve the NS equations (Navier-Stokes equations); In actual operation, the commercial numerical software Fluent is used to initialize the gas phase flow field and set the fluid velocity. , pressure, temperature and other important parameters.

[0067] Meanwhile, the NS equations can be solved by using the standard method described in any computational fluid dynamics code, which is a relatively mature existing technology and will not be described in detail here.

[0068] (2) Set the initialization conditions of non-spherical particles and determine the physical parameters and initial motion parameters of the particles; In actual setting, it is preferred to determine the particle initialization conditions and related parameters in a UDF (user-defined function), and the determined parameters preferably include the number of particles, initial position, initial velocity of non-spherical particles. , shape parameters (length of the major axis of the particle and bottom diameter ), aspect ratio , the angle between the fluid direction and the long axis of the particle , equivalent diameter and initial conditions of motion (mass flow rate).

[0069] In a preferred embodiment, the non-spherical particles are preferably cylindrical biomass particles.

[0070] (3) Establishing an inertial coordinate system for the flow field and a particle coordinate system for non-spherical particles in the flow field; In a preferred embodiment, the motion of the non-spherical particles in the flow field is a coupling of translational motion and rotational motion, and the two motions are described using different coordinate systems. The translational motion can be regarded as a motion process relative to the global coordinate system of the flow field, and the rotational motion can be regarded as a motion process relative to the particle's own coordinate system.

[0071] In a preferred embodiment, in order to characterize the translational motion of particles, a global coordinate system is established for the flow field, i.e. Figure 2 The inertial coordinate system shown in .

[0072] At the same time, a particle coordinate system is established for the particles At this time, it is preferred that the central axis of the particle is the Z' axis, and the two mutually perpendicular radial directions are the X' axis and the Y' axis, such as Figure 2 as shown in .

[0073] By coupling the inertial coordinate system and the particle coordinate system, the Euler angle is obtained ,like Figure 3 as shown in .

[0074] Furthermore, in the preferred embodiment, quaternions are introduced to describe the direction in the particle coordinate system, that is, . Through an orthogonal transformation matrix A From the inertial coordinate system , The particle coordinate system is obtained and , and ensure that singularities that occur when determining the rate of change of the Euler angles are avoided, and the elements of the transformation matrix represent the direction cosines of the particle axes in the inertial coordinate system. Specifically:

[0075]

[0076]

[0077] In the formula, , are the particle angular velocity in the inertial coordinate system and particle coordinate system, respectively; , is the torque in the inertial coordinate system and the particle coordinate system.

[0078] Furthermore, the quaternion in the preferred embodiment is represented by the Euler angle to perform calculations and update over time. In actual operation, quaternions need to be normalized at each time node, that is, , to prevent the accumulation of numerical errors. Specifically,

[0079]

[0080] In the formula, are the particle angular velocities in the three particle axis directions in the particle coordinate system.

[0081] (4) Establish the motion control translation equation of non-spherical particles, regard the non-spherical particles as discrete phases, and solve the particle motion control equation based on Euler equation and Newton's second law:

[0082] In the formula, , , , represent the undisturbed velocity, mass, volume, and density of the particle, respectively; , , , , They are the drag force, lift force, virtual mass force, pressure gradient force and gravity on the particle, among which the drag force and lift force are determined by the direction angle of the particle. θ Decide.

[0083] In actual operation, the particle motion control equations are written through UDF (user-defined function), and all particles in the model calculation domain are tracked in the Lagrangian coordinate system. By fully considering the drag, lift, virtual mass force, pressure gradient force and gravity given to the particles by the fluid, the force state of non-spherical particles in the flow field is characterized, and the calculation is combined with the local velocity and orientation of the fluid and particles.

[0084] Preferably, in this model, the thermophoretic force, Brownian force, Magnus rotational lifting force due to the rotation relative to the long axis of the particle, and Saffman shear lifting force are considered to have little effect on the non-spherical particles (biomass particles) in the gas-solid flow under the co-firing of coal-fired boilers, so they are ignored in the motion control equation.

[0085] Furthermore, in the above particle motion control equation, the forces acting on the particles are preferably calculated by the following formula:

[0086]

[0087]

[0088]

[0089] In the formula, is the projection area perpendicular to the direction of motion during the particle falling process, and ; , are the undisturbed velocity and density of the airflow fluid, respectively; is the gravitational acceleration of the particle; , and are the drag coefficient, lift coefficient and torque coefficient of the particle at the corresponding angles respectively.

[0090] Furthermore, in a preferred embodiment, the following conditions are preferably satisfied by the open source software OpenFOAM: 0.1≤ aspect ratio Ar ≤0.9, 0°≤direction angle θ ≤90°, 1≤Reynolds number Re ≤2000 cylindrical particles were subjected to direct numerical simulation (DNS) under body-fitting grid, and the OpenFOAM simulation values ​​were obtained and the drag coefficient was fitted. , lift coefficient and torque coefficient The calculation relationship of .

[0091] Specifically, the drag coefficient in the preferred embodiment is The calculation is done using the following relationship:

[0092]

[0093]

[0094]

[0095]

[0096]

[0097] In the formula, is the particle angle in the flow field The drag coefficient at ; is the drag coefficient when the particle angle in the flow field is 0° or 90°; ( i =2,3,4,5) are fitting coefficients, and by introducing the relevant fitting coefficients Calculated, and the relevant fitting coefficient The numerical values ​​are selected through OpenFOAM simulation, as shown in Table 1 below; it should be noted that the data in Table 1 are obtained through the aforementioned numerical simulation DNS. These are known quantities that can be directly selected during calculations.

[0098] Table 1 C D The correlation coefficients used in

[0099] Furthermore, the lift coefficient in the preferred embodiment is The calculation is done using the following relationship:

[0100]

[0101]

[0102]

[0103]

[0104]

[0105]

[0106]

[0107] In the formula, is the particle angle in the flow field The lift coefficient at ; To define the parameters, the corresponding formulas and numerical simulation parameters are calculated respectively; is a numerical simulation parameter, which is selected through OpenFOAM simulation values, and the selected values ​​are shown in Table 2 below; Table 2 C L The correlation coefficients used in

[0108] By calculating the drag coefficient and lift coefficient of particles at different angles, the drag and lift of non-spherical particles can be completed. Then, by deforming the translation equation controlling the motion of non-spherical particles, the following translation motion characterization equation can be obtained:

[0109] By calculating and selecting the parameters in the translation characterization equation, the translation state of non-spherical particles at different time nodes can be fed back in real time.

[0110] (5) Establish the rotational equations controlling the motion of non-spherical particles, and obtain the rotational motion characterization equations by considering the triaxial torque of non-spherical particles; The rotational motion of non-spherical particles can be regarded as a process in which the torque exerted by the fluid on the particle particles in the three axial directions of the particle particles in the particle coordinate system causes the particles to rotate around the main axis of the particle at an angular velocity; that is, the pitch torque caused by aerodynamic forces in the airflow field of non-spherical particles, the flow resistance torque caused by the relative rotation between the particles and the local fluid flow around the particles, and the torque caused purely by flow separation damping are considered separately.

[0111]

[0112] In the formula, , and Represents the components of particle angular velocity, moment of inertia and torque in the three particle axis directions; for cylindrical particles, the moment of inertia is further preferably expressed as , .

[0113] Since spherical particles have the same orientation, their rotational motion is usually ignored in the Lagrangian DPM model. The importance of torque cannot be ignored. Therefore, in a preferred embodiment, the pitch torque caused by aerodynamic force of non-spherical particles in the airflow field, the flow resistance torque caused by the relative rotation between the particles and the local fluid flow around the particles, and the torque caused purely by flow separation damping are considered separately.

[0114] Furthermore, for the torque components in each direction of the particle rotational motion The calculation is preferably performed by the following formula:

[0115]

[0116]

[0117]

[0118]

[0119]

[0120]

[0121] In the formula, are the stress change rate at the particle position, the fluid angular velocity, and the diagonal drag tensor relative to the particle angular velocity and strain rate; is the particle angle in the flow field The torque coefficient at , is calculated by the following formula:

[0122]

[0123]

[0124]

[0125]

[0126]

[0127]

[0128] In the formula, are defined parameters, which are calculated by corresponding formulas and numerical simulation parameters respectively; is a numerical simulation parameter, which is selected through OpenFOAM simulation values, and the selected values ​​are shown in Table 3 below; Table 3 C T The correlation coefficients used in

[0129] By setting the above formula and selecting parameters, the non-spherical particle motion control rotation equation in the preferred embodiment can be deformed to obtain the following rotation motion characterization equation:

[0130] (6) Solving the translational characterization equation and the rotational characterization equation, iteratively calculating the flow field and the motion parameters of the non-spherical particles, and updating the position and direction of the non-spherical particles; Based on the acquisition and calculation of the translational motion characterization equation and the rotational motion characterization equation, the translational motion and rotational motion of the non-spherical particles are characterized respectively. By coupling the translational motion and rotational motion of the non-spherical particles, the motion trajectory of the non-spherical particles in the airflow field can be accurately characterized, and the position and direction of the particles can be updated in real time.

[0131] (7) The particle motion process is coupled with the fluid motion process, and the particle motion parameters iteratively calculated in the UDF model are fed back to the fluid calculation in Fluent. Considering the feedback effect of the particles on the flow field, the simulation calculation of the flow field is corrected in real time based on the influence of the non-spherical particle motion on the flow field. After that, the simulation calculation of the non-spherical particle motion process is iteratively performed based on the corrected flow field parameters. (8) The calculation process in (3) to (7) is repeated until the particle reaches the outlet boundary and the calculation converges.

[0132] Compared with the existing CFD-DPM model simulation, the simulation calculation method in the preferred embodiment not only considers the effect of the flow field on the particles, but also fully considers the feedback effect of the particles on the flow field, realizing the transformation from one-way coupling to two-way coupling. At the same time, when calculating the movement of non-spherical particles in the flow field, it can not only accurately process the translational motion of non-spherical particles, but also solve the coupling problem of their rotational motion. By introducing the calculation of lift, pressure gradient force, virtual mass force, etc. into the motion equation, and considering the complex mechanical effects (such as torque changes) generated by the rotation of particles in the fluid, the coupling of translational motion and rotational motion during the movement of non-spherical particles is fully guaranteed, thereby realizing a more accurate flow simulation of non-spherical particles.

[0133] Obviously, the "bidirectional coupling" simulation calculation method in the preferred embodiment not only makes the simulation results more accurate in the particle motion trajectory, but also improves the simulation accuracy of the fluid field, especially for the simulation in application scenarios with high particle density, which fully meets the motion process simulation of non-spherical particles under complex motion.

[0134] When the simulation model is actually running, the model for fluid motion and the model for particle motion are run separately and are related to each other. The running process is as follows: Figure 1 as shown in .

[0135] Among them, the movement process of the continuous phase gas flow field is continuously run in the Fluent module, and the movement process of the non-spherical particles is continuously iterated in the UDF module. The initial conditions of the gas field required in the movement process of the non-spherical particles are provided to the UDF module by the Fluent module. After the particle movement in the UDF module is iterated and calculated for many times (for example, after 20 iterations), the UDF module passes the position and direction data of the particles to the Fluent module to correct the operation parameters of the airflow field. The corrected airflow field parameters are then passed to the UDF module as the gas field initialization conditions for a new round of iterative calculations. The above process is repeated until the particles move to the outlet boundary and the entire simulation calculation process converges.

[0136] More specifically, the operation process and use effect of the above-mentioned simulation calculation method are further described through the following specific embodiments: In this embodiment, the simulation is an example of the movement and combustion process of large cylindrical biomass particles in a 660MW hedge boiler of a power plant.

[0137] In actual simulation, the three-dimensional modeling of the counter-fired boiler is first carried out, and the mesh is divided. In the preferred embodiment, the size of the boiler body is 68.7 m×37.402 m×19.4192 m (height×width×depth); the burners are arranged in a front and rear wall counter-fire arrangement, and a total of 24 swirl burners are arranged in three layers on the spiral water-cooled walls of the front and rear walls of the furnace, with 4 on each layer of the front and rear walls. Except for the bottom layer of the rear wall, which is 4 plasma pulverized coal burners, the remaining layers are all low NO x Swirl pulverized coal burner; one layer of burnout air is arranged above the swirl pulverized coal burner on the front and rear walls, with two side burnout air nozzles and four burnout air nozzles on each layer. In order to achieve the co-combustion of straw biomass, the original coal-fired boiler is now modified, and two layers of swirl biomass burners are arranged on the left and right walls of the boiler, one on each layer, for a total of four biomass burners.

[0138] It should be noted that the modeling and meshing of the hedge boiler can be completed using mature existing technologies, which are not the focus of this embodiment and will not be elaborated here.

[0139] At the same time, in order to better demonstrate the adaptability and effectiveness of the numerical simulation method in the preferred embodiment in calculating the dynamic characteristics of non-spherical particles, in this specific embodiment, the movement and combustion process of cylindrical biomass large particles in a coal-fired boiler is simulated. Coal powder is sprayed into the furnace from the coal powder burner nozzles on the front and rear walls of the hedge boiler, and biomass particles enter from the biomass burner nozzles on the left and right walls of the boiler, and the specific parameter settings are shown in Table 4.

[0140] Table 4 Calculation parameters in specific embodiments

[0141] After completing the modeling, the mathematical model, boundary conditions, etc. are set up and iterative calculations are performed.

[0142] First, only the flow (continuity equation, momentum conservation equation) and turbulence equations are selected for cold calculation. After the fully developed flow field is obtained, other equations are selected for hot calculation. The hot calculation can obtain the distribution characteristics of furnace temperature, combustion components, etc. Among them, the discrete items of coal powder particles are set through the built-in model, while the biomass particles are simulated through user-defined function UDF coupling. The specific method of coupling calculation is as follows: (1) Based on FLUENT, numerical simulation of biomass particles (non-spherical particles / long cylindrical particles) mixed with coal-fired boilers was carried out to obtain the flow field information of the entire furnace; (2) Use UDF to read the flow field information data, including kinematic viscosity, flow field velocity, density, etc., and use it to calculate the trajectory of biomass particles; Specifically, DEFINE_DPM_INIT (existing program code) is used to set the cylindrical particle geometry (for example, the length and bottom diameter are 1mm and 4mm respectively, the aspect ratio is 0.25, and the particle shape parameters correspond to the part of the particles that accounts for the largest proportion of the mixed biomass particles), the incident direction, speed, particle size and other initialization conditions; further, DEFINE_DPM_DRAG and DEFINE_DPM_BODY_FORCE (existing program code) are used to calculate the gravity, drag, lift, pressure gradient force, and virtual mass force of the particles, and solve the motion control translation equation to complete the calculation of the translation motion characterization equation; thereafter, DEFINE_DPM_SCALAR_UPDATE (existing program code) is used to describe the rotational motion caused by the torque coefficient and the viscous torque of the fluid on the particles, and the discrete item parameters of the entire particle are updated; (3) The results of the particle trajectory calculation are fed back to the FLUENT module to update the position and direction of the particles, and the coupling iteration of the FLUENT gas continuous phase and the UDF biomass particle discrete terms is performed to update the information data of the flow field in the furnace. The operation is repeated until the numerical simulation of the biomass combustion in a coal-fired boiler converges.

[0143] Furthermore, the simulation results of the cylindrical aerodynamic model (CAM) in the specific embodiment are compared with the simulation results of the existing spherical drag model (SDM) and shape factor drag model (SFDM) of DPM. Figure 4As shown in the figure, (a) is the simulation result under the SDM model; (b) is the simulation result under the SFDM model; (c) is the simulation result under the CAM model.

[0144] according to Figure 4 From the results shown in the figure, it can be seen that the simulation results under the three models are significantly different. Since factors such as the resistance, lift and torque of rotational motion of non-spherical large particles are not introduced, the calculation results of the SFDM model and the SDM model at the same starting position show that the movement trajectories of more biomass particles fall into the cold ash hopper. This situation is particularly serious in the SDM model. This simulation calculation result will further affect the process of thermochemical conversion of particles and is inconsistent with the actual situation that there is no obvious change in the fly ash at the bottom of the furnace before and after the biomass is mixed in the power plant.

[0145] Further, in Figure 5 In this paper, the aerodynamic characteristic parameter information of the same biomass particle under different models is compared, and the drag coefficient and velocity of the particle at the upper and lower biomass particle nozzles are compared respectively. Figure 5 In the figure, (a) is the comparison of the drag coefficient of particles at the upper biomass nozzle; (b) is the comparison of the velocity of particles at the upper biomass nozzle; (c) is the comparison of the drag coefficient of particles at the lower biomass nozzle; (d) is the comparison of the velocity of particles at the lower biomass nozzle.

[0146] according to Figure 5 From the comparison results in Figure 3, it can be seen that by accurately considering the shape of cylindrical particles in the aerodynamic model (CAM), the drag coefficients of the upper and lower biomass particle nozzles are increased by 22.18% and 20.91% respectively compared with the data under the shape factor drag model (SFDM).

[0147] Moreover, the CAM model simultaneously considers the joint effects of non-spherical lift, virtual mass force, pressure gradient force, gravity and torque, which significantly increases the average velocity of the lower biomass particles compared to the spherical model (SDM), and more accurately predicts its complex motion state (because 70.84% ​​of the particles in the spherical model are landed on the cold ash hopper, so this type of particles is selected for comparison). The reason for the above results is mainly because the biomass particle drag coefficient calculated by the cylindrical particle aerodynamic model is greater than the spherical drag coefficient model at the same time. Comprehensive consideration of more forces jointly causes the increase in the average velocity of the particles and the reduction in residence time, resulting in a significant difference in the motion trajectory of the biomass particles, which is more in line with the actual motion situation.

[0148] In summary, for the numerical simulation method in this specific embodiment, it can realize the movement solution of particles in the process of gas-solid (air, coal powder and biomass particles) two-phase mixing in a complex and chaotic furnace combustion environment. Based on the design of the numerical simulation method, the coupling effect of the translation and rotational motion of non-spherical particles is fully considered. Compared with the simulation calculation method of the conventional SDM model and SFDM model, it can more accurately complete the simulation calculation of the dynamic parameters of non-spherical particles in gas-solid two-phase flow, improve the accuracy of numerical simulation of non-spherical particles, and provide a reliable basis and support for the subsequent combustion system design and operation optimization.

[0149] The gas-solid two-phase flow numerical simulation method of non-spherical particles and flow field bidirectional coupling in the present invention can accurately simulate the interaction between non-spherical particles and flow field through Fluent-UDF numerical simulation. It can not only deal with the translational and rotational motion of non-spherical particles, but also fully consider the interaction between gas and solid two phases. It has significant advantages in dealing with large particle size and irregular shape of biomass particles, and can effectively deepen the understanding of the complex process of direct combustion coupling of non-spherical biomass particles in coal-fired boilers, and provide theoretical support for optimizing burner design and combustion process.

[0150] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A numerical simulation method for gas-solid two-phase flow with bidirectional coupling between non-spherical particles and flow field, characterized in that: The process includes the following: (1) Set the flow field model and initial boundary conditions and solve the NS equations; (2) Set the initialization conditions of non-spherical particles and determine the physical parameters and initial motion parameters of the particles; (3) Establish an inertial coordinate system for the flow field and a particle coordinate system for non-spherical particles, and couple the two coordinate systems to obtain the Euler angle ; (4) The motion control translation equation of non-spherical particles is established. The non-spherical particles are regarded as discrete phases, and the following translation motion characterization equation is obtained: In the formula, , , , represent the undisturbed velocity, mass, volume, and density of the particle, respectively; , , , , They are the drag force, lift force, virtual mass force, pressure gradient force and gravity on the particle, among which the drag force and lift force are determined by the direction angle of the particle. θ Decide; (5) The rotational equation for controlling the motion of non-spherical particles is established, and the rotational motion characterization equation is obtained by considering the triaxial torque of non-spherical particles: In the formula, , and represents the components of the particle angular velocity, moment of inertia, and torque in the directions of the three particle axes; and for cylindrical particles, , ; (6) Solving the translational characterization equation and the rotational characterization equation, iteratively calculating the flow field and the motion parameters of the non-spherical particles, and updating the position and direction of the non-spherical particles; (7) Feeding back the particle motion parameters obtained by iterative calculation into the flow field model, and correcting the simulation calculation of the flow field based on the particle motion parameters of the non-spherical particles; thereafter, iteratively performing the simulation calculation of the motion process of the non-spherical particles based on the corrected flow field parameters; (8) The calculation process in (3) to (7) is repeated until the particle reaches the outlet boundary and the calculation converges.

2. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to claim 1 is characterized in that: When coupling the two coordinate systems in process (3), quaternions are introduced to describe the direction in the particle coordinate system, namely: , and through an orthogonal transformation matrix A From the inertial coordinate system , The particle coordinate system is obtained and ; In the formula, , are the particle angular velocity in the inertial coordinate system and particle coordinate system, respectively; , is the torque in the inertial coordinate system and the particle coordinate system.

3. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to claim 2 is characterized in that: The quaternion is expressed by Euler angles To calculate, and get the following equation; and the quaternion is continuously updated over time and normalized at each time node, that is, ; 。 4. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to any one of claims 1 to 3, characterized in that: In the translation characterization equation, the forces acting on non-spherical particles are calculated by the following formula: In the formula, is the projection area perpendicular to the direction of motion during the particle falling process, and ; , are the undisturbed velocity and density of the airflow fluid, respectively; is the gravitational acceleration of the particle; , and are the drag coefficient, lift coefficient and torque coefficient of the particle at the corresponding angles respectively.

5. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to claim 4 is characterized in that: Corresponding to the drag coefficient , lift coefficient and torque coefficient The calculation is done by using the open source software OpenFOAM to meet the following conditions: 0.1≤ aspect ratio Ar ≤0.9, 0°≤direction angle θ ≤90°, 1≤Reynolds number Re The cylindrical particles with a diameter of ≤2000 are directly simulated under body-fitting grid to obtain the OpenFOAM simulation values.

6. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to claim 5, characterized in that: The drag coefficient The calculation is done using the following relationship: In the formula, is the particle angle in the flow field The drag coefficient at ; is the drag coefficient when the particle angle in the flow field is 0° or 90°; ( i =2,3,4,5) are fitting coefficients, and by introducing the relevant fitting coefficients Calculated, and the relevant fitting coefficient Numerical selection was performed by OpenFOAM simulation.

7. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to claim 6, characterized in that: The lift coefficient The calculation is done using the following relationship: In the formula, is the particle angle in the flow field The lift coefficient at ; To define the parameters, the corresponding formulas and numerical simulation parameters are calculated respectively; It is a numerical simulation parameter, which is selected through OpenFOAM simulation value.

8. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to any one of claims 5 to 7, characterized in that: In process (5), the torque components in each direction of the particle rotational motion are The calculation is performed by the following formula: In the formula, are the stress change rate at the particle position, the fluid angular velocity, and the diagonal drag tensor relative to the particle angular velocity and strain rate; is the particle angle in the flow field The torque coefficient at .

9. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to claim 8, characterized in that: The torque coefficient Calculate using the following formula: In the formula, are defined parameters, which are calculated by corresponding formulas and numerical simulation parameters respectively; It is a numerical simulation parameter, which is selected through OpenFOAM simulation value.

10. The method for numerical simulation of gas-solid two-phase flow with bidirectional coupling of non-spherical particles and flow field according to any one of claims 1 to 3, 5 to 7, and 9, characterized in that: The movement process of the continuous phase gas flow field is continuously run in the Fluent module, and the movement process of the non-spherical particles is continuously and iteratively run in the UDF module; the initial conditions of the gas field required in the movement process of the non-spherical particles are provided to the UDF module by the Fluent module, and after multiple iterative calculations of the particle movement in the UDF module, the UDF module passes the position and direction data of the particles to the Fluent module to correct the operating parameters of the airflow field, and then the corrected airflow field parameters are passed to the UDF module as the gas field initialization conditions for a new round of iterative calculations.

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