Calculation method of plate heat exchanger

Abstract

The invention relates to the technical field of plate heat exchange, in particular to a calculation method of a plate heat exchanger, which comprises the following steps: S1, converting pressure and temperature data into pressure and enthalpy values, setting boundary conditions, determining a hot side and a cold side of the heat exchanger according to the boundary conditions, and setting non-boundary values by segmentally applying energy conservation to obtain geometric parameters and empirical constants; s2, obtaining physical property parameters of the fluid by using a two-dimensional grid linear interpolation method, and obtaining the physical property parameters of the fluid by taking the enthalpy value and the pressure as coordinates; and S3, determining the effective area required by each section, and obtaining a local heat transfer coefficient and a pressure drop when the volume requirement is met under the fluid physical property parameters and the channel flow velocity. According to the method, the problem that the calculation speed of a calculation program is low when the performance of the brazing sheet heat exchanger is evaluated under different working conditions is solved through unique grid division in the plate heat exchanger and rapid calling of fluid physical properties, the calculation error value is effectively reduced, an accurate check result is obtained within ten iteration steps, and the calculation cost is saved.

Description

Technical Field

[0001] The present invention relates to the technical field of plate heat exchange, and particularly to a calculation method for a plate heat exchanger. Background Art

[0002] A heat exchanger is an indispensable device in the chemical industry, energy, and many other industrial fields, used to transfer heat between two or more fluids. With the progress of industrial technology and the increasing requirements for energy efficiency, the design and performance optimization of heat exchangers have become increasingly important. However, heat exchangers encounter various problems during actual operation, such as low heat transfer efficiency, large pressure loss, and easy fouling. These problems lead to a decline in equipment performance and even failures. Therefore, it is very necessary to conduct a check calculation on the heat exchanger to ensure its stable and efficient operation under design conditions.

[0003] During the heat exchange process, the physical properties of the fluid change inside the heat exchanger with the change of local temperature, pressure, and other parameters. In the calculation of the heat exchanger, in order to obtain better local calculation results, it is essential to reasonably discretize the heat exchanger. Currently, the calculation of heat exchangers is basically based on geometric dimensions to discretize the heat exchanger one-dimensionally or two-dimensionally, and then according to the input parameters of the heat exchanger, the parameters of each discrete point are calculated in turn using the principles of heat transfer and fluid mechanics. This results in a slow calculation process for the heat exchanger. And because the heat exchanger is discretized according to its geometric structure, the designed calculation program is also limited by the geometric structure of the heat exchanger.

[0004] Discretized calculation is crucial for better analyzing local conditions in heat exchanger calculation. When calculating the required area and pressure drop of the heat exchanger, since the thermophysical properties of the fluid change significantly with local temperature, pressure, internal temperature pinch point, etc., errors will be introduced when using the parametric method for calculation. The current check calculations of heat exchangers are basically based on geometric shapes to discretize the heat exchanger, which results in the designed program using this method being inapplicable to heat exchangers with different geometric shapes. At the same time, traditional calculation methods are based on the inlet values of the heat exchanger and calculate in the pressure-temperature domain according to the relevant knowledge of heat transfer and fluid mechanics, and transfer the data to the following discrete points in sequence. This method has a slow process of obtaining the physical properties of the refrigerant and affects the efficiency. To address the above problems, a general new discretization method is adopted. This discretization method is not limited to the geometric conditions of the heat exchanger, calculates using a one-dimensional scheme in the pressure-enthalpy value domain, and has an optimized design during the calculation process, effectively reducing the calculation error value, and obtaining an accurate check result within ten iterations, saving the calculation cost. Summary of the Invention

[0005] The present invention provides a calculation method for a plate heat exchanger, which is used to solve the defects that in the prior art, when evaluating the performance of a brazed plate heat exchanger under different working conditions, the calculation program has a slow calculation speed and is affected by the geometric factors of the heat exchanger.

[0006] The present invention provides a calculation method for a plate heat exchanger, including:

[0007] S1: Convert pressure and temperature data into pressure and enthalpy values, set boundary conditions, allocate flow rate and heat load, determine the hot side and cold side of the heat exchanger according to the boundary conditions, and apply the law of conservation of energy in segments to set non-boundary values, so as to obtain geometric parameters and empirical constants.

[0008] S2: Use the finite grid interpolation method to construct a two-dimensional grid and interpolate to obtain the physical property parameters of the fluid, and use enthalpy value and pressure as coordinates to obtain the physical property parameters of the fluid.

[0009] S3: Determine the effective area required for each segment, meet the capacity requirements under the physical property parameters of the fluid and the channel flow velocity, and obtain the local heat transfer coefficient and pressure drop.

[0010] The present invention provides a calculation method for a plate heat exchanger. In step S1, the steps of obtaining the empirical correlation constants for single-phase and two-phase heat transfer include:

[0011] S11: Calculate pressure and enthalpy values, set pressure and enthalpy values as boundary conditions, allocate the mass flow rate of fluid elements, and determine the hot side and cold side of the heat exchanger according to the inlet conditions.

[0012] S12: Apply the energy conservation equation in each segment, determine the pressure and enthalpy values of non-boundary nodes, and use the iterative method to ensure that the pressure and enthalpy values of all nodes meet the convergence conditions.

[0013] S13: Obtain the relevant parameters of the heat exchanger model and obtain the empirical correlation constants for single-phase and two-phase heat transfer.

[0014] The present invention provides a calculation method for a plate heat exchanger, including: In step S2, the specific steps of using the two-dimensional grid linear interpolation method to obtain the physical property parameters of the fluid are:

[0015] S21: Collect the physical property parameters of the refrigerant within the range of pressure and enthalpy values.

[0016] S22: Use the finite grid interpolation method to construct a two-dimensional grid and interpolate according to pressure and enthalpy values. Create grid points on pressure and enthalpy values respectively according to the step size of the two-dimensional grid.

[0017] S23: Obtain the physical property parameters through the function of the saturation pressure, obtain the corresponding relationship between the saturation pressure and the saturation temperature for interpolation calculation.

[0018] S24: Verify the interpolation result to ensure that the error is less than the preset error threshold.

[0019] S25: Calculate the total heat exchanger area based on the sectional area, evaluate the flow distribution using an empirical model and calculate the area ratio, and output the verification result.

[0020] The present invention provides a calculation method for a plate heat exchanger, including: In step S22, the specific steps of constructing a two-dimensional grid and interpolating are as follows:

[0021] S221: Divide the grid area into subcritical and supercritical regions according to the thermodynamic properties of the refrigerant.

[0022] S222: At each node, obtain and save density, viscosity, thermal conductivity, specific heat capacity, and entropy.

[0023] S223: Use the linear interpolation method to check the accuracy of the physical property parameters at different state points. In the region with low accuracy, continue to refine the grid until the deviation of the physical property parameters is less than the preset error threshold.

[0024] S224: Use the bisection method to find the physical property parameters of the fluid at the determined node.

[0025] The present invention provides a calculation method for a plate heat exchanger, including: In step S221, further discretize at different enthalpy values, sort the enthalpy values of the subcooled state, saturated state, and superheated state and put them into a vector. In the subcritical and supercritical regions, use pressure and enthalpy values to form a finite element grid. The elements in the grid include line elements with two nodes, triangular elements with three nodes, and quadrilateral elements with four nodes.

[0026] The present invention provides a calculation method for a plate heat exchanger, and step S3 includes:

[0027] S31: Based on fluid parameters, flow rate, and heat load, preliminarily estimate the sectional area, distribute the fluid element pressure and enthalpy values, and set the length scale.

[0028] S32: Compare the node pressure and enthalpy values to determine the fluid phase state, calculate the physical property formula for the fluid in the micro two-phase flow state, and combine the heat transfer coefficient and the wall thermal resistance to determine the total sectional heat transfer coefficient.

[0029] S33: Divide the section into subsections again, and distribute the parameters according to the heat exchange ratio. Determine the length scale of each subsection by the ratio of the single-phase section capacity to the phase change section capacity in the subsection.

[0030] S34: Check the pressure and enthalpy value distributions, and after ensuring no errors, iteratively update the wall temperature until convergence.

[0031] The present invention provides a calculation method for a plate heat exchanger, including: in step S3, calculating the area required for each segment to meet the allocated heat load, and the formula is expressed as:

[0032]

[0033] In the formula, A requird,n is the area required to meet the allocated heat load, Q n is the heat to be transferred, U n is the local heat transfer coefficient, △T n is the average temperature of the fluid within the element.

[0034] The present invention provides a calculation method for a plate heat exchanger, including: the physical property calculation formula in the state of fluid micro two-phase flow is expressed as:

[0035]

[0036] In the formula, φ fg is the average fluid property of the given fluid, x is the gas phase fraction, φ g is the fluid property of the given fluid in the gaseous state, φ f is the fluid property of the given fluid in the liquid state.

[0037] The present invention provides a calculation method for a plate heat exchanger, including: in step S33, the formula for determining the length scale of each sub-segment by the ratio of the single-phase segment capacity to the phase change segment capacity in the sub-segment is expressed as:

[0038] h n =[h SP ·Q SP +h TP ·Q TP / Q n

[0039] △P n =[△P SP ·Q SP +△P TP ·Q TP / Q n

[0040] In the formula, h n is the total convective heat transfer coefficient of the micro-element segment, h sp is the single-phase convective heat transfer coefficient, Q sp is the heat transfer amount in the single-phase state, h TP is the two-phase convective heat transfer coefficient, Q TP is the heat transfer amount in the two-phase state, Qn is the total heat transfer amount of the micro-element segment, △Pn is the total pressure drop of the micro-element segment, P SP is the single-phase pressure drop, P TP is the two-phase pressure drop.

[0041] The present invention provides a calculation method for a plate heat exchanger, including: in step S34, the formula for calculating the temperature of the wall node through the energy conservation equation is expressed as:

[0042]

[0043] In the formula, Twall,1 is the temperature at wall node 1, Un is the local heat transfer coefficient, is the average temperature of micro-element segment 1. is the average temperature of micro-element segment 2. h E1 is the convective heat transfer coefficient of micro-element segment 1. h E2 is the convective heat transfer coefficient of micro-element segment 2.

[0044] The calculation method for a plate heat exchanger provided by the present invention, through a unique grid division and a method of quickly calling fluid physical properties in the plate heat exchanger, solves the problems that the calculation program has a slow calculation speed and is affected by the geometric factors of the heat exchanger when evaluating the performance of the brazed plate heat exchanger under different working conditions, and achieves that the grid division method has universality. The calculation memory requirement is smaller, and the calculation speed is faster. It effectively reduces the calculation error value, and an accurate verification result can be obtained within ten iterations, saving the calculation cost.

[0045] The finite grid difference method simplifies the complex internal flow and heat transfer problems in the heat exchanger into a series of numerical calculation problems at a series of nodes by discretizing the continuous calculation region. Through fine grid division, more flow and heat transfer details can be captured, thereby improving the accuracy of the calculation results. The structure of the plate heat exchanger is usually relatively complex, including multiple channels and plates. The finite grid difference method can flexibly handle this complex geometry by adjusting the grid size and shape to adapt to the actual structure of the heat exchanger. Through reasonable grid division, while ensuring the calculation accuracy, the number of calculation nodes can be reduced, thereby reducing the consumption of calculation resources. This is of great significance for large-scale and high-precision numerical simulations.

[0046] By quickly calling fluid physical property data, the waiting time in the calculation process can be reduced, and the overall calculation efficiency can be improved. The accuracy of fluid physical property data has an important impact on the simulation results. Quickly calling accurate fluid physical property data can ensure the reliability of the simulation results and provide strong support for the design and optimization of the heat exchanger. In the heat exchanger numerical simulation software, the function of quickly calling fluid physical property data is usually closely integrated with modules such as grid division, flow and heat transfer calculation, etc. This integrated design facilitates the overall optimization of the heat exchanger and improves the efficiency and accuracy of the simulation. Brief Description of the Drawings

[0047] To more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0048] Figure 1 is a schematic flow chart of a calculation method for a plate heat exchanger provided by an embodiment of the present invention;

[0049] Figure 2 is a topological structure of a countercurrent heat transfer discrete scheme for a calculation method of a plate heat exchanger provided by an embodiment of the present invention;

[0050] Figure 3 is a finite element mesh formed by pressure and enthalpy in the subcritical and supercritical regions for a calculation method of a plate heat exchanger provided by an embodiment of the present invention;

[0051] Figure 4 is the single-phase region and the phase change region of a calculation method of a plate heat exchanger provided by an embodiment of the present invention. Detailed implementation manners

[0052] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the present invention with reference to the drawings in the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments in the present invention belong to the scope of protection of the present invention.

[0053] The following will describe Figures 1-4 a new heat exchanger checking calculation method of the present invention.

[0054] As Figures 1-2 shown, the discrete method of the present invention divides the heat exchanger into a finite number of segments with equal heat loads, and each segment represents the side surface, wall nodes, and local wall elements. Each fluid node in the figure represents the local pressure and enthalpy values, and the temperature, dryness, saturation temperature, etc. of the node are determined from the pressure and enthalpy values. During the process of calculating the enthalpy value, the influence of the pressure drop is temporarily not considered. Fluid element: within a segment, the fluid element represents a local side surface of the heat exchanger and extends between two fluid nodes. The parameters included in the fluid element are: local channel mass flow rate and velocity, total mass flow rate of the side surface, local heat transfer coefficient, length of the segment, local pressure drop, and average fluid properties within the segment. Wall nodes and elements: the wall nodes represent the local wall temperature, and the wall elements extend between two wall nodes, representing local thermal resistance, fouling factor, and other parameters.

[0055] The calculation steps for pressure drop and heat transfer coefficient are as follows:

[0056] S1: Convert the pressure and temperature data into pressure and enthalpy values, set the boundary conditions, use the energy balance equation to determine the nodal values, and obtain the geometric parameters.

[0057] S11: Receive the pressure and temperature data provided by the user, use the property database to convert the temperature and pressure data into pressure and enthalpy values. Apply the energy conservation equation to calculate the flow rate and enthalpy of the fluid, set the calculated flow rate and enthalpy as the boundary conditions for the inlet and outlet nodes. According to the topological structure and flow distribution principle, allocate the channel mass flow rate for the fluid elements, and determine the hot and cold sides of the heat exchanger based on the initial input of the inlet and outlet conditions on both sides (the flow rate and enthalpy of the fluid).

[0058] S12: Apply the energy conservation equation in each segment to determine the pressure and enthalpy values of the non-boundary nodes. Use the iterative method to ensure that the pressure and enthalpy values of all nodes meet the convergence conditions. Take the average temperature on each side as the trial temperature for all the wall nodes on the opposite side. Based on the trial temperature and the heat exchanger model parameters (such as hydraulic diameter, flow length, etc.), preliminarily evaluate the heat transfer performance.

[0059] S13: Obtain the relevant parameters of the heat exchanger model from the product manual or database, including hydraulic diameter, effective heat transfer area, plate thickness, etc., and obtain the empirical correlation constants for single-phase and two-phase heat transfer / pressure drop. Allocate the obtained parameters to the appropriate objects in the discretization scheme, and optimize and adjust the parameters according to the heat transfer performance evaluation results to improve the efficiency of the heat exchanger.

[0060] S2: Use the method of two-dimensional grid linear interpolation to obtain the physical property parameters of the fluid to ensure accuracy and calculation efficiency.

[0061] S21: Use the REFPROP software to collect the physical property parameter data for the required refrigerant within a wide range of pressure and enthalpy values, such as density, viscosity, thermal conductivity, specific heat capacity, etc. Ensure that the data covers the subcooled, superheated, supercritical, and saturated states. Sort the collected data according to pressure and enthalpy values and store it in an easily accessible format.

[0062] Determine the grid step sizes for pressure and enthalpy values according to the data range and accuracy requirements, and construct a two-dimensional grid.

[0063] S22: As Figure 3As shown, a two-dimensional grid is constructed and interpolated. According to the grid step size determined in step S31, grid points are created for pressure and enthalpy values respectively. For each grid point, REFPROP is used to obtain the corresponding physical property parameter values, and a complete two-dimensional physical property parameter table is constructed. When calculating the physical property parameters at a specific pressure and enthalpy value, first find the grid cell where the point is located. Using the linear interpolation method, calculate the physical property parameters of the target point according to the physical property parameter values of the four vertices (or two adjacent vertices, depending on the interpolation direction) of the grid cell.

[0064] S221: According to the thermodynamic properties of the refrigerant, it is divided into subcritical (pressure below the critical pressure) and supercritical (pressure above the critical pressure) regions. In the pressure-enthalpy domain, the saturation region (i.e., the state where liquid and vapor coexist) is divided into a series of pressure steps. These steps are recorded in a vector for subsequent indexing, generating independent regions respectively while maintaining continuity at the domain interface. Divide the saturation region into a series of pressure steps. These pressure steps are put into a vector for indexing.

[0065] The saturation line represents the enthalpy value when the liquid state reaches saturation at a given pressure. The dew point line represents the enthalpy value when the vapor state reaches saturation at a given pressure. Due to the characteristics of the dew point curve, there will be duplicate enthalpy values, and duplicate items need to be removed to ensure the smoothness of the search.

[0066] The subcooled region and the superheated region respectively represent the regions where the liquid and vapor states are below the saturation line. These regions are further discretized at different enthalpy values to more accurately describe their physical property parameters.

[0067] Sort the enthalpy values of the subcooled state, saturated state, and superheated state and put them into a vector for quick lookup by indexing later.

[0068] In the subcritical and supercritical regions, a finite element grid is formed using pressure and enthalpy values. The elements in the grid include two-node line elements (representing the saturation line), three-node triangular elements (close to the evaporation point and dew point regions), and four-node quadrilateral elements (in the overall subcritical and supercritical regions).

[0069] S222: At each node, obtain and save properties such as density, viscosity, thermal conductivity, specific heat capacity, entropy, etc. These properties are stored in a separate data structure. The node property set is linked to the pressure and enthalpy vectors through an index variable for quick lookup and access when needed. Handling missing values: In the case of missing values in the standard data source, use Lagrange interpolation to obtain properties from nearby nodes to ensure the integrity and accuracy of the data.

[0070] S223: Use the linear interpolation method to check the accuracy of physical property parameters at different state points. In the region with lower precision, refine the grid until the deviation of the physical property parameters is less than 1%. Serialize and compress the physical property parameters of each refrigerant to reduce memory occupancy. The average size of each refrigerant is about 200K.

[0071] S224: When the pressure and enthalpy values of a certain node are known, use the bisection method to quickly and continuously find other physical property parameters of the fluid at this node. Compared with directly calling NIST REFPROP, this method has greatly improved the calculation speed. NIST REFPROP is a widely used thermodynamic property database and calculation software, but its calculation speed is relatively slow. By pre-computing and storing key data points and their interpolation results, the calculation efficiency is significantly improved.

[0072] S23: For the saturated state, obtain the physical property parameters through a function of the saturation pressure. Use REFPROP to obtain the corresponding relationship between the saturation pressure and the saturation temperature for interpolation calculation. Near the critical pressure, due to the drastic change of physical property parameters, data loss or inaccuracy may occur. Use the finite difference method to approximately calculate or smooth the missing data.

[0073] S24: Use REFPROP as a benchmark to verify the interpolation results and ensure that the error is less than 1%. According to the verification results, adjust the grid step size or the interpolation method to further improve the accuracy. Evaluate the calculation speed of the interpolation method and compare it with directly using REFPROP. Optimize the data structure and algorithm, and the calculation speed should be 100 times faster than directly calling REFPROP.

[0074] S3: It is necessary to determine the effective area required for each segment to ensure that the capacity requirement is met under the given fluid physical property parameters and channel flow rate.

[0075] S31: Receive fluid physical property parameters, channel flow rate, heat load, etc. Based on the capacity requirement and fluid parameters, initially estimate the effective area required for each segment. In the topological structure, assign initial pressure and enthalpy values to each fluid element and set the length scale.

[0076] S32: By comparing the average pressure and enthalpy values of the inlet and outlet nodes, determine the average temperature of the fluid within the element. Compare this average temperature with the saturation temperature at the given pressure and the local wall temperature, and then judge the phase state of the fluid. According to the local flow rate, steam quality, and average fluid properties, combine with the continuous empirical formula to calculate the physical property formula for the fluid in the micro two-phase flow state.

[0077] The physical property calculation formula for the fluid in the micro two-phase flow state is expressed as:

[0078]

[0079] In the formula, φ fg is the average fluid property of the given fluid, x is the gas phase fraction, and φ g is the fluid property of the given fluid in the gaseous state, and φ f is the fluid property of the given fluid in the liquid state.

[0080] Apply the pressure drop equation to each fluid element, calculate the local pressure drop using the equivalent flow length, and update the pressure distribution in the discretization scheme. Update the missing enthalpy values and the enthalpy values of non-boundary nodes according to the calculated pressure drop. For each segment, determine the total heat transfer coefficient through the local heat transfer coefficient and the wall thermal resistance.

[0081] Calculate the area required for each segment to meet the assigned heat load, and the formula is expressed as:

[0082]

[0083] In the formula, A requird,n is the area required to meet the assigned heat load, Q n is the heat to be transferred, U n is the local heat transfer coefficient, and △T n is the average temperature of the fluid within the element.

[0084] S33: As Figure 4 shown, to meet the given heat load, the total area required for the heat exchanger will be the sum of the areas required for each segment. The length scale used to calculate the pressure drop of each fluid element is set as the ratio between the local area demand and the total area demand. When the saturation temperature of the corresponding state point of the fluid appears inside the segment, special treatment measures need to be taken. This phenomenon usually occurs in the initial stage of processes such as superheating, subcooling, or condensation. The segments involved in these processes will be further subdivided into single-phase regions and phase change regions.

[0085] To ensure the continuity and stability of the result output while reducing the number of segments corresponding to the overall required stable result, each fluid segment needs to be further subdivided. Inside each segment, check whether the temperature range contains the saturation temperature. If the saturation temperature exists, further subdivide the segment into two sub-segments to accurately determine the parameters of the single-phase and phase change processes. The local pressure drop and the average heat transfer coefficient of the two sub-segments are allocated according to the heat transfer ratio of the sub-segment units. The formula for determining the length scale of each sub-segment through the ratio of the single-phase segment capacity to the phase change segment capacity in the sub-segment is expressed as:

[0086] h n =[h SP ·Q SP +h TP ·Q TP / Q n

[0087] △P n =[△P SP ·Q SP +△P TP ·Q TP / Q n

[0088] In the formula, h n is the total convective heat transfer coefficient of the micro-element section, h sp is the single-phase convective heat transfer coefficient, with the unit of W / (㎡·k), Qsp is the heat transfer quantity in the single-phase state, with the unit of W; h TP is the two-phase convective heat transfer coefficient, QTp is the heat transfer quantity in the two-phase state, and Qn is the total heat transfer quantity of the micro-element section. △Pn is the total pressure drop of the micro-element section, PSP is the single-phase pressure drop, and PTP is the two-phase pressure drop.

[0089] During the calculation process of this step, it is necessary to verify the calculated pressure and enthalpy value distributions. If it is found that the refrigerant-side pressure drop is too large, resulting in a disordered temperature distribution, the calculation shall be stopped and a warning shall be returned.

[0090] S34: Verify the calculated pressure and enthalpy value distributions to ensure that the refrigerant-side pressure drop will not cause a disordered temperature distribution. Within each section, the temperature of the wall nodes is iteratively updated through the energy conservation equation until convergence.

[0091] The formula for calculating the temperature of the wall nodes through the energy conservation equation is expressed as:

[0092]

[0093] In the formula, Twall,1 is the temperature at wall node 1, Un is the local heat transfer coefficient, is the average temperature of micro-element section 1; is the average temperature of micro-element section 2; h E1 is the convective heat transfer coefficient of micro-element section 1; h E2 is the convective heat transfer coefficient of micro-element section 2.

[0094] Calculate the root mean square difference between the temperatures of all wall nodes and the temperature of the previous iteration. When the error is less than 10 -3 , the iteration terminates, and usually convergence is achieved within ten iterations.

[0095] S35: Calculate the total area required for the heat exchanger based on the area required for each segment. The one-dimensional program used in the calculation cannot accurately predict the fluid distribution in the main port pipe, and the performance of the heat exchanger deteriorates as the number of plates increases. Therefore, an empirical model is used to evaluate the non-uniformity of the flow distribution, and the ratio of the total heat transfer area required to the available effective heat transfer area is calculated. Output the verification calculation results including channel velocity, port velocity, port pressure drop, channel temperature distribution, wall temperature distribution, heat transfer coefficient, etc.

[0096] The present invention provides a new method for verifying the calculation of a heat exchanger. A new discrete calculation method is adopted, and based on the principles of fluid mechanics and heat transfer, the heat exchanger is verified and calculated. The method of the present invention takes into account various factors such as the temperature, pressure, and flow velocity of the fluid, and evaluates the heat transfer area and pressure drop of the heat exchanger. Using the calculation method of the present invention, important parameters such as the heat transfer efficiency, temperature field distribution, and heat transfer area of the heat exchanger can be calculated more quickly. In addition, this method is not affected by the geometric shape of the heat exchanger. When the geometric structure of the heat exchanger changes, it is not necessary to re-discretize the heat exchanger.

[0097] In a heat exchanger, two parameters are particularly important - pressure drop and heat transfer coefficient. During the operation of the heat exchanger, there will be processes of condensation and evaporation phase change, which pose a challenge to the design and verification of the heat exchanger. Different from the traditional temperature-pressure calculation form, the present invention proposes a new finite grid interpolation method with universality in heat exchanger calculation based on pressure and enthalpy. The input parameters of the calculation program include heat exchanger geometric parameters, effective heat transfer area, flow arrangement form, heat transfer medium, heat load, and pressure drop limit. The calculation result is the ratio of the available area of the heat exchanger to the required heat transfer area under the working condition and the estimated actual pressure drop value of all fluid flow channels. It is judged whether the selected heat exchanger meets the requirements of the rated working condition through the calculation results.

[0098] A calculation method for a plate heat exchanger provided by the present invention solves the problems of slow calculation speed of the calculation program and being affected by the geometric factors of the heat exchanger when evaluating the performance of a brazed plate heat exchanger under different working conditions through a unique grid division in the plate heat exchanger and a method of quickly calling fluid physical properties.

[0099] The finite grid interpolation method simplifies the complex internal flow and heat transfer problems in heat exchangers into numerical calculation problems at a series of nodes by discretizing the continuous calculation region. Through fine grid division, more flow and heat transfer details can be captured, thereby improving the accuracy of the calculation results. The structure of plate heat exchangers is usually relatively complex, including multiple channels and plates. The finite grid interpolation method can flexibly handle such complex geometries by adjusting the grid size and shape to adapt to the actual structure of the heat exchanger. Through reasonable grid division, while ensuring the calculation accuracy, the number of calculation nodes can be reduced, thereby reducing the consumption of computing resources. This is of great significance for large-scale and high-precision numerical simulations.

[0100] Through the description of the above embodiments, those skilled in the art clearly understand that each embodiment can be implemented by means of software plus a necessary general hardware platform, and of course also by hardware. Based on this understanding, the essence of the above technical solution, or the part that contributes to the prior art, is embodied in the form of a software product. This computer software product is stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions for causing a computer device (a personal computer, a server, or a network device, etc.) to execute the methods described in each embodiment or some parts of the embodiments.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A calculation method for a plate heat exchanger, characterized in that: include: S1: converting pressure and temperature data into pressure and enthalpy values, setting boundary conditions, allocating flow and heat load, determining the hot side and cold side of the heat exchanger according to the boundary conditions, applying energy conservation in sections to set non-boundary values, and obtaining geometric parameters and empirical constants; S2: Use the finite grid difference method to construct a two-dimensional grid and interpolate to obtain the physical properties of the fluid. Take enthalpy and pressure as coordinates to obtain the physical properties of the fluid; S3: Determine the effective area required for each segment to meet the capacity requirements under the fluid physical parameters and channel flow rate, and obtain the local heat transfer coefficient and pressure drop.

2. A calculation method for a plate heat exchanger according to claim 1, characterized in that: In step S1, the steps of obtaining the single-phase and two-phase heat transfer empirical correlation constants include: S11: Calculate the pressure and enthalpy values, set the pressure and enthalpy values ​​as boundary conditions, allocate the fluid element mass flow rate, and determine the hot side and cold side of the heat exchanger according to the inlet conditions; S12: Apply the energy conservation equation in each segment to determine the pressure and enthalpy values ​​of non-boundary nodes, and use an iterative method to ensure that the pressure and enthalpy values ​​of all nodes meet the convergence conditions; S13: Obtain relevant parameters of the heat exchanger model and obtain empirical correlation constants of single-phase and two-phase heat transfer.

3. The calculation method of a plate heat exchanger according to claim 1, characterized in that: In step S2, the specific steps of using the two-dimensional grid linear interpolation method to obtain the physical parameters of the fluid are: S21: Collect refrigerant physical parameters within the pressure and enthalpy range; S22: constructing a two-dimensional grid and interpolating the pressure and enthalpy values ​​using a finite grid difference method, and creating grid points on the pressure and enthalpy values ​​respectively according to the step size of the two-dimensional grid; S23: obtaining physical property parameters through the function of saturation pressure, and obtaining the corresponding relationship between saturation pressure and saturation temperature for interpolation calculation; S24: verifying the interpolation result to ensure that the error is less than a preset error threshold; S25: Calculate the total area of ​​the heat exchanger based on the segmented area, use the empirical model to evaluate the flow distribution and calculate the area ratio, and output the verification results.

4. The calculation method of a plate heat exchanger according to claim 3, characterized in that: In step S22, the specific steps of constructing a two-dimensional grid and interpolating using the finite grid interpolation method are as follows: S221: Divide the grid area into subcritical and supercritical regions according to the thermodynamic properties of the refrigerant; S222: At each node, obtain and save density, viscosity, thermal conductivity, specific heat capacity, and entropy; S223: using a linear interpolation method to check the accuracy of the physical property parameters at different state points, and continuing to refine the grid in areas with lower accuracy until the deviation of the physical property parameters is less than a preset error threshold; S224: Use the dichotomy method to find the physical parameters of the fluid at the determined nodes.

5. A calculation method for a plate heat exchanger according to claim 4, characterized in that: In step S221, it includes: further discretizing at different enthalpy values, sorting the enthalpy values ​​of the supercooled state, the saturated state, and the superheated state and putting them into a vector, and in the subcritical and supercritical regions, using pressure and enthalpy values ​​to form a finite element mesh; the elements in the mesh include two-node line elements, three-node triangle elements, and four-node quadrilateral elements.

6. The calculation method of a plate heat exchanger according to claim 1, characterized in that: In step S3, the specific steps of obtaining the local heat transfer coefficient and the pressure drop include: S31: Based on fluid parameters, flow rate and heat load, preliminarily estimate the segment area, assign fluid element pressure and enthalpy values, and set the length scale; S32: Compare the node pressure and enthalpy values ​​to determine the fluid phase state, calculate the physical property formula of the fluid in the micro two-phase flow state, and determine the segmented total heat transfer coefficient by combining the heat transfer coefficient and wall thermal resistance; S33: the segment is further divided into sub-segments, and parameters are allocated according to the heat exchange ratio, and the length scale of each sub-segment is determined by the ratio of the single-phase segment capacity to the phase change segment capacity in the sub-segment; S34: Check the pressure and enthalpy distributions, and iteratively update the wall temperature until convergence after ensuring they are correct.

7. The calculation method of a plate heat exchanger according to claim 1, characterized in that: In step S3, the area required for each segment to meet the assigned heat load is calculated, and the formula is expressed as: In the formula, A requird,n The area required to meet the assigned heat load, Q n is the heat to be transferred, U n is the local heat transfer coefficient, △T n is the average temperature of the fluid within the element.

8. The calculation method of a plate heat exchanger according to claim 6, characterized in that: In step S32, the physical property calculation formula of the fluid in the micro two-phase flow state is expressed as: In the formula, φ fg is the average fluid property of a given fluid, x is the gas phase fraction, φ g is the fluid property of a given fluid in the gaseous state, φ f is the fluid property of a given fluid in the liquid state.

9. The calculation method of a plate heat exchanger according to claim 6, characterized in that: In step S33, the formula for determining the length scale of each sub-segment by the ratio of the single-phase segment capacity to the phase change segment capacity in the sub-segment is expressed as: h n =[h SP ·Q SP +h TP ·Q TP ] / Q n △P n =[△P SP ·Q SP +△P TP ·Q TP ] / Q n In the formula, h n is the total convective heat transfer coefficient of the microelement segment, h sp is the single phase convection heat transfer coefficient, Q sp is the heat transfer in single-phase state, h TP is the convection heat transfer coefficient between the two sides, Q TP is the heat transfer in the two-phase state, Qn is the total heat transfer of the micro-element segment, △Pn is the total pressure drop of the micro-element segment, P SP is the single-phase voltage drop, P TP is the two-phase voltage drop.

10. A calculation method for a plate heat exchanger according to claim 6, characterized in that: In step S34, the temperature of the wall node is calculated by the energy conservation equation as follows: Where Twall,1 is the temperature at the wall node 1, Un is the local heat transfer coefficient, is the average temperature of microelement segment 1; is the average temperature of microelement segment 2; h E1 is the convective heat transfer coefficient of microelement segment 1; h E2 is the convective heat transfer coefficient of infinitesimal segment 2.

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