Method for constructing order reduction model of gas-liquid two-phase heat exchanger

Abstract

The invention discloses a construction method of a reduced-order model of a gas-liquid two-phase heat exchanger. Each fluid channel of each layer of heat exchanger is divided into more than one control volume unit along the fluid flowing direction; constructing a prediction model for each unit by using a mass and energy conservation equation; discretizing the prediction model by adopting a finite volume method, and performing order reduction solution on the prediction model based on an experience convective heat transfer coefficient and a friction factor after discretization so as to obtain the inner wall surface temperature, the fluid temperature and the unit pressure drop of a single unit; and the temperature and the unit pressure drop of each unit are solved through iteration, so that the overall flow field temperature and pressure distribution of the heat exchanger is deduced. The method has the characteristics of low calculation cost, good precision control and high calculation stability.

Description

Technical Field

[0001] This invention belongs to the field of fully coupled flow-heat transfer three-dimensional modeling technology for order reduction of two-phase heat exchangers, specifically involving a method for constructing an order reduction model of a gas-liquid two-phase heat exchanger. Background Technology

[0002] In the fields of new energy and aerospace, heat exchangers have a series of special requirements, such as achieving high heat flux density and lightweight design within a limited space; high reliability and high heat dissipation efficiency; stable operation under multiple operating conditions; and low computational cost to enable rapid iterative design.

[0003] Traditional heat exchanger analysis models lack universal analytical formulas, and finite element / finite volume (FEM) solutions require extensive boundary inputs and incur significant computational costs. Existing 3D models often simplify the flow-heat transfer coupling to weak or unidirectional coupling, failing to simultaneously account for the bidirectional effects of the flow and heat fields, and balancing numerical stability with computational efficiency is challenging. Currently mature NTU methods are mostly based on laminar or ideal turbulent conditions, failing to accurately reflect localized heat transfer losses caused by uneven flow distribution. Furthermore, their boundary conditions are limited to isothermal or constant heat flux boundaries, leading to significantly increased errors when dealing with complex boundary geometries and multi-segment temperature fields. Summary of the Invention

[0004] The purpose of this invention is to provide a method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger. This invention features low computational cost, good accuracy control, and high computational stability.

[0005] The technical solution of this invention is: a method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, wherein each fluid channel of each heat exchanger layer is divided into one or more control volume units along the fluid flow direction; a prediction model is constructed for each unit using mass and energy conservation equations; the prediction model is discretized using the finite volume method, and after discretization, the prediction model is solved by reducing its order based on empirical convection heat transfer coefficients and friction factors, thereby obtaining the inner wall temperature, fluid temperature, and unit pressure drop of a single unit; by iteratively solving the temperature and pressure drop of each unit, the overall flow field temperature and pressure distribution of the heat exchanger is derived.

[0006] In the aforementioned method for constructing a reduced-order model for a gas-liquid two-phase heat exchanger, the prediction model is as follows:

[0007]

[0008] in, h The convective heat transfer coefficient is... This represents the contact area between the fluid and the solid. and These are the wall temperature and the fluid temperature, respectively. , and These represent the total heat exchange of the unit, the heat exchange within the unit, and the heat exchange between units, respectively. Q m For inbound traffic, c p For the specific heat capacity at constant pressure, Δ T f The temperature difference between the inlet and outlet of the unit.

[0009] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, the calculation formulas for the heat transfer within a unit and the heat transfer between units are as follows:

[0010]

[0011] in and These are the thermal conductivity of the solid and the contact wall area between the hot and cold fluids, respectively. This is a relaxation factor used to ensure the stability of the computational iteration; q up The heat flux density of the unit absorbs heat. q down The heat flux density of the unit that releases heat; V solid For the volume of the solid region, the differential term is discretized using the finite difference method to obtain the iterative formula for the temperature of the inner wall of the unit fluid:

[0012] in and These are the inlet and outlet fluid temperatures of the unit, respectively. The total heat transfer of the unit and the outlet fluid temperature are then obtained through iteration.

[0013] in L cold , L hot These represent the flow lengths of the cold and hot fluids within the unit, respectively. The unit pressure drop is obtained based on the Darcy-Weisbach equation and the correlation iteration of the friction factor, thus yielding the overall temperature and pressure fields.

[0014] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, the convective heat transfer coefficient... h With unit voltage drop Δ P The calculation formulas are as follows:

[0015]

[0016] in, Nu For Nusselt numbers, f As the friction factor, k l The thermal conductivity of the fluid, Where W is the element feature length, H is the element width, H is the element height, and L is the flow length. For fluid velocity, For fluid density; When the fluid is in a single-phase region, the empirical formulas for the Nusselt number and the friction factor are as follows:

[0017]

[0018]

[0019]

[0020] in, x For the fluid flow distance, rr Roughness; Re is the Reynolds number, Pr is the Prandtl number; When the fluid is in a two-phase region, the empirical formulas for the Nusselt number and the friction factor are as follows:

[0021]

[0022] in, r Dryness.

[0023] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, the following reduced-order assumptions are made before solving the problem: the temperature of the inner wall surface of a single unit is consistent, and the fluid temperature is uniformly distributed at the inlet and outlet sections of a single unit.

[0024] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, the method for solving the reduced-order problem is as follows: S1. Calculate the Prandtl number, Reynolds number, and inlet flow rate of the fluid within the calculation unit; S2. Based on the relationship between the inlet enthalpy of the fluid in the unit and the enthalpy of the saturated liquid and saturated vapor, determine whether the unit is a superheated unit, a subcooled unit, or a two-phase unit; S3. Calculate the Nusselt number of the cell based on cell type. Nu With friction factor f Then, the convective heat transfer coefficient and unit pressure drop are calculated; S4. Calculate the inner wall temperature and fluid temperature based on the convective heat transfer coefficient, Prandtl number, Reynolds number, and inlet flow rate.

[0025] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, in step S1, the Prandtl number... Pr Reynolds number Re Entry traffic Q m Calculated as follows:

[0026]

[0027]

[0028] in, The feature length of the unit. , , , and These are the specific heat capacity at constant pressure, thermal conductivity, flow velocity, density, and viscosity of the fluid, respectively. Where W is the cross-sectional area of ​​the fluid channel, H is the element width, and H is the element height.

[0029] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, in step S2, when the inlet enthalpy of the fluid within the unit is between the enthalpy of saturated liquid and the enthalpy of saturated vapor, the unit type is a two-phase unit. The calculation method for S3 in this case is as follows: For the evaporator, a phase change process occurs in the two-phase unit. Assuming that the entire unit is a two-phase region, the maximum heat exchange between the refrigerant and the hot-side fluid is calculated. The outlet enthalpy of the refrigerant is compared with the enthalpy of the saturated vapor. If it is less than or equal to the enthalpy of the saturated vapor, the assumption is valid. At this time, the entire unit is a two-phase region, and the dryness of the unit outlet is between 0 and 1. The Nusselt number and friction factor are calculated using empirical formulas for the fluid in the two-phase region. If the outlet enthalpy of the refrigerant is greater than the enthalpy of saturated vapor, the empirical formulas for the single-phase and two-phase regions of the fluid are used to calculate the Nusselt number and friction factor in the single-phase region, and the Nusselt number and friction factor in the two-phase region, respectively. The two Nusselt numbers are weighted by the inlet and outlet enthalpies and the enthalpy of saturated vapor to obtain the mixed Nusselt number, and the two friction factors are weighted to obtain the mixed friction factor.

[0030] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, for the condenser, the two-phase unit undergoes a phase change process. It is assumed that the entire unit is a two-phase region. The maximum heat exchange between the refrigerant and the cold-side fluid is calculated. The outlet enthalpy of the refrigerant is compared with that of the saturated liquid. If the refrigerant is greater than or equal to the enthalpy of the liquid, the assumption is valid. At this time, the entire unit is a two-phase region, and the dryness of the unit outlet is between 0 and 1. The Nusselt number and friction factor are calculated using empirical formulas for the fluid in the two-phase region. If the outlet enthalpy of the refrigerant is less than the enthalpy of the saturated liquid, the empirical formulas for the single-phase and two-phase regions of the fluid are used to calculate the Nusselt number and friction factor in the single-phase region, and the Nusselt number and friction factor in the two-phase region, respectively. The two Nusselt numbers are weighted by the inlet and outlet enthalpies and the enthalpy of the saturated liquid to obtain the mixed Nusselt number, and the two friction factors are weighted to obtain the mixed friction factor.

[0031] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, for the evaporator, in step S2, when the inlet enthalpy of the fluid within the unit is lower than the enthalpy of the saturated liquid, the unit type is a subcooled unit. In this case, the calculation method for S3 is as follows: If the enthalpy at the unit outlet is greater than the enthalpy of the saturated liquid, the enthalpy of the saturated liquid is used as the dividing line for calculation. The Nusselt number and friction factor before the dividing line are calculated according to the empirical formula for the fluid in the single-phase region, and the Nusselt number and friction factor after the dividing line are calculated according to the empirical formula for the fluid in the two-phase region.

[0032] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, for the evaporator, in step S2, when the inlet enthalpy of the fluid in the unit is equal to or higher than the enthalpy of saturated steam, the unit type is a superheated unit. In this case, in step S3, the Nusselt number and friction factor are calculated according to the empirical formula for fluid in the single-phase region.

[0033] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, for the condenser, in step S2, when the inlet enthalpy of the fluid within the unit is higher than the saturated steam enthalpy, the unit type is a superheated unit. In this case, the calculation method for S3 is as follows: If the unit outlet enthalpy is less than the saturated steam enthalpy, the saturated steam enthalpy is used as the dividing line for calculation. The Nusselt number and friction factor before the dividing line are calculated according to the empirical formula for fluid in the single-phase region, and the Nusselt number and friction factor after the dividing line are calculated according to the empirical formula for fluid in the two-phase region.

[0034] In the aforementioned method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, for the condenser, in step S2, when the inlet enthalpy of the fluid in the unit is equal to or less than the enthalpy of the saturated liquid, the unit type is a subcooled unit. In this case, in step S3, the Nusselt number and friction factor are calculated according to the empirical formula for the fluid being in the single-phase region.

[0035] The advantages of this invention are: It uses a piecewise discretization method and an equivalent order-reduction model to obtain a predictive model for the heat exchanger; it performs discretization and order reduction processing on the predictive model based on the momentum conservation equation and the energy conservation equation; the order reduction assumptions include uniform inner wall temperature of a single unit and uniform fluid temperature distribution at the inlet and outlet sections of a single unit; based on this, it employs the finite volume method to predict the heat exchanger's performance in terms of heat transfer rate, dividing the heat exchanger into several segments along the channels, with each microchannel being divided into small control volume units. This method achieves non-uniform flow distribution within different channels of the heat exchanger by redistributing the Reynolds number calculation for different flow channels.

[0036] This invention can couple the outlet flow field of each layer of the heat exchanger with the inlet flow field of the next layer, thereby achieving integrated connectivity of the heat exchanger.

[0037] This invention is based on a discrete prediction model using the finite volume method, and then combines it with empirical formulas for the Nusselt number and friction factor, significantly reducing the computational workload and cost. The computation time using this invention is in the minutes range, while traditional CFD methods require several days.

[0038] This invention allows for free judgment and entry into the calculation process of single-phase and phase change heats by comparing enthalpy values, with good accuracy control.

[0039] This invention ensures the stability of computational iterations by introducing a relaxation factor.

[0040] In the two-phase region, the enthalpy increment within the unit is calculated by the total heat exchange and flow rate, thereby obtaining the enthalpy distribution field within the two-phase region. Based on this method, combined with the index of the refrigerant performance parameter table, the temperature field and pressure field distribution of the refrigerant under the corresponding pressure and enthalpy can be obtained.

[0041] This invention provides a weighted average of the single-phase and two-phase heat transfer coefficients in units where two-phase and single-phase regions coexist. This is achieved by weighting the inlet and outlet enthalpies with the enthalpy of saturated vapor / liquid and obtaining the mixed Nusselt number and friction factor. This method enables more accurate prediction of two-phase phase heat transfer characteristics.

[0042] In summary, this invention provides a reduced-order calculation model for the heat transfer performance of a gas-liquid two-phase radiator, characterized by low computational cost, good accuracy control, and high computational stability, enabling large-scale analysis and calculation. Attached Figure Description

[0043] Figure 1 A discrete schematic diagram of the small control volume units in a plate heat exchanger; Figure 2 This forms the basic logical framework of the model of this invention; Figure 3 A schematic diagram of a method for determining the heat transfer state within a computational unit. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] The features and illustrative embodiments of various aspects of the present invention will now be described in detail. Numerous specific details are set forth in the following detailed description to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without requiring some of these specific details. The following description of embodiments is merely intended to provide a better understanding of the invention by illustrating examples of the invention. The invention is by no means limited to any specific setups and methods set forth below, but covers any improvements, substitutions, and modifications to structures, methods, and devices without departing from the spirit of the invention. Well-known structures and techniques are not shown in the drawings and the following description to avoid unnecessarily obscuring the invention.

[0046] It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other, and the various embodiments can be referenced and cited in each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0047] Example 1. A method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, taking an evaporator as an example, see [link to example]. Figures 1-3 , Figure 1 In this paper, we take the use of an equivalent reduced-order model to predict the heat transfer rate and unit pressure drop of a plate heat exchanger as an example. The plate heat exchanger is discretized into 100 control volume unit microchannels along the x-direction for cold airflow and the y-direction for hot airflow. Each small unit is formed by multiple layers of fluid channels, as shown in units 56 to 59. The first cold channel consists of units 1, 11, 21 to 91, the second channel consists of units 2, 12, 22 to 92, and so on. The first hot channel consists of units 1 to 10, the second channel consists of units 11 to 20, and so on. In addition to having the same discretization between layers, the size of all units within a single layer is independent. Based on the size of the flow channels, the mass and energy conservation equations of a single unit in the plate heat exchanger are determined. Specifically, the construction method is as follows: 1. First, input the characteristic parameters such as the fin structure and geometry of the heat exchanger, and give the flow and thermodynamic boundary conditions and fluid type at the inlet and outlet. Based on the refrigerant temperature and material property table, this process determines the inlet enthalpy, saturated liquid enthalpy, and saturated vapor enthalpy of the refrigerant. At the same time, based on the characteristic parameters such as the fin structure and geometry of the plate heat exchanger, under the given inlet and outlet flow boundary conditions, calculate the Prandtl number, Reynolds number, and inlet flow rate of the refrigerant.

[0048]

[0049]

[0050]

[0051] in The channel feature length, , , , and These are the specific heat capacity at constant pressure, thermal conductivity, flow velocity, density, and viscosity of the fluid, respectively. Let be the cross-sectional area of ​​the fluid channel.

[0052] 2. Determine if the refrigerant inlet enthalpy is between the liquid and saturated vapor enthalpies. If it is, proceed to step 4. If it is not: if it is greater than the saturated vapor enthalpy, the unit is a superheated unit, and the whole process is a single-phase process; proceed to step 3. If it is less than the saturated liquid enthalpy, assume the unit is a subcooled unit. Determine if the refrigerant outlet enthalpy is less than the saturated liquid enthalpy. If it is not greater, the assumption is valid, and the whole process is a single-phase process; proceed to step 3. If the refrigerant outlet enthalpy is greater than the saturated liquid enthalpy, use the saturated liquid enthalpy as the dividing line for calculation. The temperature field before the dividing line is calculated using step 3, and the variables after the dividing line are calculated using step 4.

[0053] 3. At this point, no phase transition has occurred in this unit. The Nusselt number and friction factor are calculated using the empirical formula for rectangular fins proposed by Gnielinski in the single-phase region and the empirical formula provided by Churchill, respectively. Fluid flow distance For roughness, typically taken as 0.0001, the empirical formula for the unidirectional region is as follows:

[0054]

[0055]

[0056]

[0057] Based on this, the corresponding equivalent convective heat transfer coefficient and unit pressure drop can be obtained as follows:

[0058]

[0059] Heat is transferred from the hot fluid to the finned tube wall via convection and conduction, and then from the finned wall to the cold fluid. In this method, it is assumed that the fluid inlet velocity and temperature within the unit are uniform, the wall surface temperature is consistent, and the surface is smooth. Therefore, the heat exchange rate can be calculated based on the fluid heat transfer within the heat exchanger. Taking the unit fluid and unit solid as the research objects respectively, the energy conservation equations for the unit fluid and solid can be obtained:

[0060]

[0061] in This represents the contact area between the fluid and the solid. and These are the wall temperature and the fluid temperature, respectively. , and These represent the total heat exchange, the intra-layer heat exchange, and the inter-layer heat exchange, respectively. The corresponding intra-layer and inter-layer heat exchange can be obtained according to Fourier's law of thermal conductivity.

[0062]

[0063]

[0064] in and These are the thermal conductivity of the solid and the contact wall area between the hot and cold fluids, respectively. This is a relaxation factor used to ensure the stability of the computational iteration. q up The heat flux density of the unit absorbs heat. q down Let the heat flux density be the heat released by the element; then, by discretizing the differential term using the finite difference method, an iterative scheme for the temperature of the inner wall surface of the element fluid can be obtained.

[0065] in and These are the inlet and outlet fluid temperatures of the refrigerant, respectively. The total heat transfer of the unit and the outlet fluid temperature of the refrigerant can then be iteratively obtained.

[0066]

[0067] The heat flux density between the two layers is Based on the Darcy-Weisbach equation and the correlation of friction coefficient, the unit pressure drop can be obtained iteratively, thus obtaining the overall temperature field and pressure field, and proceeding to step 7.

[0068] 4. This invention is illustrated by appendix. Figure 3 Determine the heat transfer state within the calculation unit. If the unit is determined to be a two-phase region, it is assumed that a vaporization phase change process has occurred within the unit. Assuming the entire section is a two-phase region, calculate the maximum heat exchange between the refrigerant and the air, and determine whether the outlet enthalpy of the refrigerant is less than the saturated vapor enthalpy. If it is less, the assumption is valid, and the entire section is a two-phase region with an outlet dryness fraction between 0 and 1. Proceed to step 5. If the outlet enthalpy of the refrigerant is greater than the saturated vapor enthalpy, proceed to step 6.

[0069] 5. At this point, the entire region is a two-phase zone, with the outlet dryness fraction between 0 and 1. The corresponding unit dryness fraction is calculated using the enthalpy value. The Nusselt number and friction factor are fitted using empirical formulas for the two-phase zone to obtain the equivalent convective heat transfer coefficient and unit pressure drop under two-phase conditions. The empirical formulas are as follows:

[0070]

[0071] in, r Dryness.

[0072] Subsequently, based on this, the corresponding unit wall temperature and unit heat exchange are calculated by combining the wall temperature calculation method in step 3. The enthalpy increment in the unit is calculated by the total heat exchange and flow rate, thereby obtaining the enthalpy distribution field in the two-phase interval. By indexing the refrigerant performance parameter table, the refrigerant temperature field and pressure field distribution under the corresponding pressure and enthalpy are obtained and the process proceeds to step 7.

[0073]

[0074] 6. At this point, the unit involves both two-phase and single-phase sections. The corresponding unit dryness fraction is calculated using enthalpy values. Empirical formulas for both two-phase and single-phase conditions are used to fit and obtain the two-phase and single-phase heat transfer coefficients. The two heat transfer coefficients are then weighted using inlet and outlet enthalpies and saturated steam enthalpies to obtain the mixed convective heat transfer coefficient. The friction factor is calculated similarly. The corresponding unit wall temperature and unit heat transfer are calculated using the wall temperature calculation method from step 3. Based on the heat transfer balance equation, the proportion of the two-phase region within the unit is calculated, and the superheated zone length fraction is also calculated, thus obtaining the overall temperature and pressure fields, and proceeding to step 7.

[0075] 7. Determine whether the heat exchange is balanced and whether the wall temperature is stabilizing. If balance has not been reached, return to step 2. If balance has been reached, output the temperature field and pressure field obtained by iteration.

[0076] In some embodiments, the outlet flow field of each heat exchanger unit is coupled with the inlet flow field and enthalpy of the next unit to achieve integrated connectivity of the heat exchanger; by fitting the temperature-enthalpy relationship function under specific refrigerant and pressure, table lookup is avoided and computational efficiency is improved; by using the non-uniform distribution of the inlet Reynolds number, the non-uniform distribution of flow rate in different channels is achieved; by adding a relaxation factor to the interlayer heat transfer, the overall stability of the model is greatly improved.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger, characterized in that, Each fluid channel of each heat exchanger layer is divided into more than one control volume unit along the fluid flow direction; a prediction model is constructed for each unit using mass and energy conservation equations; The finite volume method is used to discretize the prediction model. After discretization, the prediction model is solved by order reduction based on the empirical convection heat transfer coefficient and friction factor to obtain the inner wall temperature, fluid temperature and pressure drop of a single unit. By iteratively solving the temperature and pressure drop of each unit, the overall flow field temperature and pressure distribution of the heat exchanger is derived.

2. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 1, characterized in that, The prediction model is as follows: in, h The convective heat transfer coefficient is... This represents the contact area between the fluid and the solid. and These are the wall temperature and the fluid temperature, respectively. , and These represent the total heat exchange of the unit, the heat exchange within the unit, and the heat exchange between units, respectively. Q m For inbound traffic, c p For the specific heat capacity at constant pressure, Δ T f The temperature difference between the inlet and outlet of the unit.

3. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 2, characterized in that, The formulas for calculating the heat exchange within a unit and the heat exchange between units are as follows: in and These are the thermal conductivity of the solid and the contact wall area between the hot and cold fluids, respectively. It is a relaxation factor; q up The heat flux density of the unit absorbs heat. q down The heat flux density of the unit that releases heat; V solid For the volume of the solid region, the differential term is discretized using the finite difference method to obtain the iterative formula for the temperature of the inner wall of the unit fluid: in and These are the inlet and outlet fluid temperatures of the unit, respectively. The total heat transfer of the unit and the outlet fluid temperature are then obtained through iteration. in L cold , L hot These represent the flow lengths of the cold and hot fluids within the unit, respectively. The unit pressure drop is obtained based on the Darcy-Weisbach equation and the correlation iteration of the friction factor, thus yielding the overall temperature and pressure fields.

4. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 2, characterized in that, convective heat transfer coefficient h With unit voltage drop Δ P The calculation formulas are as follows: in, Nu For Nusselt numbers, f As the friction factor, k l The thermal conductivity of the fluid, Where W is the element feature length, H is the element width, H is the element height, and L is the flow length. For fluid velocity, For fluid density; When the fluid is in a single-phase region, the empirical formulas for the Nusselt number and the friction factor are as follows: in, x For the fluid flow distance, rr Roughness; Re is the Reynolds number, Pr is the Prandtl number; When the fluid is in a two-phase region, the empirical formulas for the Nusselt number and the friction factor are as follows: in, r Dryness.

5. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 1, characterized in that, Before reducing the order of the solution, the following assumptions are made: the temperature of the inner wall of a single element is uniform, and the fluid temperature is uniformly distributed at the inlet and outlet sections of a single element.

6. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 4, characterized in that, The method for reducing the order is as follows: S1. Calculate the Prandtl number, Reynolds number, and inlet flow rate of the fluid within the calculation unit; S2. Based on the relationship between the inlet enthalpy of the fluid in the unit and the enthalpy of the saturated liquid and saturated vapor, determine whether the unit is a superheated unit, a subcooled unit, or a two-phase unit; S3. Calculate the Nusselt number of the cell based on cell type. Nu With friction factor f Then, the convective heat transfer coefficient and unit pressure drop are calculated; S4. Calculate the inner wall temperature and fluid temperature based on the convective heat transfer coefficient, Prandtl number, Reynolds number, and inlet flow rate.

7. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 6, characterized in that, In step S1, the Prandtl number Pr Reynolds number Re Entry traffic Q m Calculated as follows: in, The feature length of the unit. , , , and These are the fluid's specific heat capacity at constant pressure, thermal conductivity, flow velocity, density, and viscosity, respectively. Where W is the cross-sectional area of ​​the fluid channel, W is the element width, and H is the element height.

8. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 6, characterized in that, In step S2, when the inlet enthalpy of the fluid within the unit is between the enthalpy of saturated liquid and the enthalpy of saturated vapor, the unit type is a two-phase unit. In this case, the calculation method for S3 is as follows: For the evaporator, a phase change process occurs in the two-phase unit. Assuming that the entire unit is a two-phase region, the maximum heat exchange between the refrigerant and the hot-side fluid is calculated. The outlet enthalpy of the refrigerant is compared with the enthalpy of the saturated vapor. If it is less than or equal to the enthalpy of the saturated vapor, the assumption is valid. At this time, the entire unit is a two-phase region, and the dryness of the unit outlet is between 0 and 1. The Nusselt number and friction factor are calculated using empirical formulas for the fluid in the two-phase region. If the outlet enthalpy of the refrigerant is greater than the enthalpy of saturated vapor, the empirical formulas for the single-phase and two-phase regions of the fluid are used to calculate the Nusselt number and friction factor in the single-phase region, and the Nusselt number and friction factor in the two-phase region, respectively. The two Nusselt numbers are weighted by the inlet and outlet enthalpies and the enthalpy of saturated vapor to obtain the mixed Nusselt number, and the two friction factors are weighted to obtain the mixed friction factor.

9. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 8, characterized in that, For the condenser, a phase change process occurs in the two-phase unit. Assuming that the entire unit is a two-phase region, the maximum heat exchange between the refrigerant and the cold-side fluid is calculated. The outlet enthalpy of the refrigerant is compared with that of the saturated liquid. If the refrigerant is greater than or equal to the enthalpy of the liquid, the assumption is valid. At this time, the entire unit is a two-phase region, and the dryness of the unit outlet is between 0 and 1. The Nusselt number and friction factor are calculated using empirical formulas for the fluid in the two-phase region. If the outlet enthalpy of the refrigerant is less than the enthalpy of the saturated liquid, the empirical formulas for the single-phase and two-phase regions of the fluid are used to calculate the Nusselt number and friction factor in the single-phase region, and the Nusselt number and friction factor in the two-phase region, respectively. The two Nusselt numbers are weighted by the inlet and outlet enthalpies and the enthalpy of the saturated liquid to obtain the mixed Nusselt number, and the two friction factors are weighted to obtain the mixed friction factor.

10. The method for constructing a reduced-order model of a gas-liquid two-phase heat exchanger according to claim 6, characterized in that, For the evaporator, in step S2, when the inlet enthalpy of the fluid in the unit is lower than the enthalpy of the saturated liquid, the unit type is a subcooled unit. In this case, the calculation method for S3 is as follows: If the enthalpy at the unit outlet is greater than the enthalpy of the saturated liquid, the enthalpy of the saturated liquid is used as the dividing line for calculation. The Nusselt number and friction factor before the dividing line are calculated according to the empirical formula for the fluid in the single-phase region, and the Nusselt number and friction factor after the dividing line are calculated according to the empirical formula for the fluid in the two-phase region.

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