Method for simulating steam jet surge condensation heat and mass transfer process based on staged source item reconstruction

Abstract

The invention discloses a method for simulating a steam jet surge condensation heat and mass transfer process based on staged source item reconstruction. Based on the thought of staged source item reconstruction, a phase change heat transfer energy source item in a two-fluid control equation is subjected to staged reconstruction, and interface heat transfer coefficient definition methods of different stages are designed, so that transient solution of a complex nonlinear phase change heat transfer process is realized. According to the method, the gas-liquid two-phase unsteady-state heat and mass transfer process of the steam jet condensation system under various operation conditions under the low mass flow working condition is comprehensively considered, the surge condensation behavior and the pressure oscillation characteristic of steam can be accurately predicted, and the method has guiding significance for fine design of pressure relief condensation equipment such as a jet pipe and a bubbler.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal hydraulics, and in particular relates to a method for simulating a steam jet surge condensation heat and mass transfer process based on staged source term reconstruction. Background Art

[0002] Steam jet condensation has efficient mixing and heat exchange capabilities and has been widely used in the fields of chemical industry, power and nuclear industry. Steam jet condensation in advanced nuclear power plants is a key technology to ensure reactor pressure safety. It uses high-speed jets from jet tubes or bubblers to spray superheated steam generated by accidents into supercooled water for direct condensation, thereby achieving rapid system depressurization. However, the surge phenomenon that occurs in the later stage of steam jet condensation operation is very likely to cause equipment resonance, which is an engineering problem that must be addressed in the system structure design. Therefore, clarifying the regional distribution characteristics of pressure oscillation of surge condensation has become one of the key points in the design and development of pressure relief condensation equipment. In recent years, domestic and foreign scholars and engineers have carried out a lot of research and empirical research on surge condensation, but the research on pressure oscillation remains at the observation and hypothesis level, and there are few cases in which the existing theoretical systems and technical achievements are directly applied to engineering projects. Considering the limitations of engineering prototype experimental conditions, the numerical calculation method is used to simulate the heat and mass transfer process of steam surge condensation, and then the steam condensation behavior is predicted, and the pressure oscillation mechanism of surge is revealed, which has higher engineering application value. However, in the numerical simulation work for the prediction of pressure oscillation characteristics of steam surge condensation, there are still a series of challenges. These include: (1) the lack of a robust numerical method to solve the strongly nonlinear phase change heat transfer problem accompanied by interface motion; (2) the calculation of phase change heat transfer at the interface requires the definition of the heat transfer coefficient on both the gas and liquid sides, and there is currently a lack of an effective method to define the heat transfer coefficient on the gas side. Summary of the invention

[0003] In view of the shortcomings of the current numerical simulation methods for steam jet condensation, this paper proposes a method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction. This method can realize the transient solution of complex nonlinear phase change heat transfer processes, and accurately predict the steam condensation behavior and pressure oscillation characteristics of surge condensation conditions, which has guiding significance for the refined design of pressure relief condensation equipment such as jet tubes and bubblers.

[0004] To achieve the above object, the technical solution adopted by the present invention is:

[0005] A method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction includes the following steps:

[0006] Step 1: According to the real structure of the steam jet condensation system, a physical model of the surge condensation of superheated steam and subcooled water through the jet tube is constructed, and the fluid calculation domain is meshed to realize the discretization of the physical space;

[0007] Step 2, using the mass flow rate and upstream temperature of the superheated steam as the inlet boundary conditions of the fluid calculation domain, and using the ambient pressure on the surface of the pool as the outlet boundary condition;

[0008] Step 3, using the k-ε model to describe the turbulence effect of the control equation, establishing mass, momentum and energy conservation equations for the gas phase and liquid phase based on the Euler-Euler two-fluid model (the Euler-Euler two-fluid model is a control equation that describes the flow and heat transfer relationship established for the fluid calculation domain), and solving the transient physical quantity distribution of the current flow field according to the boundary conditions, initial conditions or the data obtained by the iterative calculation in the previous step;

[0009] Step 4: Using the method of phased source term reconstruction, the nonlinear phase change heat transfer process is decomposed into two stages: heat transfer without phase change and heat transfer with phase change. Different heat transfer coefficient definition methods are designed according to the numerical calculation requirements of each stage to achieve mathematical model reconstruction, and the energy equation source term is calculated based on the physical quantities obtained in step 3.

[0010] Step 5, adding a temperature auxiliary model to the steam condensation rate source term of the continuity equation, and calculating the continuity equation source term according to the physical quantity obtained in step 3;

[0011] Step 6, explicitly relax the source term and update the source term of the control equation;

[0012] Step 7, update the number of iteration steps according to the change of time step;

[0013] Step 8, iteratively calculate the numerical model constructed by steps 3 to 7, record the flow field information after stable convergence, obtain the pressure oscillation characteristics of the measuring points at different time steps and the global physical quantity distribution of the flow field, and finally clarify the steam condensation behavior inside and outside the jet tube and the transient change process of temperature and pressure during the steam jet surge condensation process.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1) Based on the idea of ​​staged source reconstruction, the complex nonlinear phase change heat transfer problem accompanied by interface motion is modeled in sections, and a more rigorous heat transfer coefficient definition method is designed to address the accuracy and stability issues of numerical calculations. An explicit relaxation numerical technique that adjusts with the number of iterations is introduced to achieve high-fidelity transient numerical calculations of the steam surge condensation process.

[0016] 2) The steam condensation behavior, pressure oscillation characteristics and their changing laws in the steam surge condensation process were effectively predicted, and the surge pressure oscillation mechanism was clarified. The calculation results can guide the refined design of jet condensation equipment such as jet tubes and water storage tanks, which is of great significance to the actual operation of the project and the maintenance of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The present invention is a flow chart of a method for simulating the steam jet surge condensation heat and mass transfer process based on staged source term reconstruction.

[0018] Figure 2 It is a physical model of the surge condensation of superheated steam and subcooled water through the jet tube.

[0019] Figure 3 Meshing of the computational domain for steam jet surge condensation fluid.

[0020] Figure 4 Verification of the mesh independence of the average surge frequency solution and the average surge pressure peak solution.

[0021] Figure 5 The transient change process of pressure calculated by coupling 6 interface area density models with heat transfer models.

[0022] Figure 6 The transient changes of steam condensation behavior under six working conditions calculated by coupling the specific interface area density model with the heat transfer model. DETAILED DESCRIPTION

[0023] The present invention is described in detail below with reference to the accompanying drawings and taking a three-dimensional jet condensation simulation under low steam mass flux as an example:

[0024] like Figure 1 As shown, the numerical method of simulating the transient heat and mass transfer process of steam surge condensation based on staged source term reconstruction is suitable for the pressure oscillation characteristic analysis of the steam jet condensation system under low mass flow conditions, and mainly includes the following steps:

[0025] Step 1: According to the actual structure of the steam jet condensation system, a physical model of the surge condensation of superheated steam and subcooled water through the jet tube is constructed, and then the fluid calculation domain is gridded to realize the discretization of the physical space.

[0026] Steam jet condensation system Figure 2 As shown. Superheated steam of a certain mass flux is injected into the water tank through an adiabatic jet tube with a certain immersion depth, and condenses in direct contact with supercooled water in the area near the nozzle. The fluid calculation domain can be divided into a two-phase calculation domain consisting of the inside of the tube and the vicinity of the nozzle, and a stable supercooled water calculation domain far away from the nozzle where no phase interaction occurs. Based on the distribution of the fluid calculation domain and the motion characteristics of the gas-liquid interface, a three-dimensional structured grid is generated, and the grids inside the jet tube and near the nozzle are encrypted. The grid division results are shown in Figure 3 By comparing the transient pressure oscillation results calculated by different grids within a specific time range, the grid independence of the average surge frequency solution and the average surge pressure peak solution is verified. The results are shown in Figure 4As shown in , if the average surge frequency and the average surge pressure peak are both grid-independent solutions, the grid with the minimum number of grids that meets this requirement is used for calculation, otherwise the grid is further refined. Figure 4 It can be seen that when the number of grids is higher than 75600, further mesh encryption has little effect on the key numerical solution. Therefore, this set of grids is selected to simulate the transient heat and mass transfer process of steam surge condensation.

[0027] Step 2, combined with the actual operating conditions of the steam low mass flow jet condensation, the mass flow rate and upstream temperature of the superheated steam are used as the inlet boundary conditions of the fluid calculation domain, and the ambient pressure on the surface of the pool is used as the outlet boundary condition. The initial temperature of the supercooled water area is determined according to the supercooled water temperature of the operating conditions, and the temperature is set as the outlet reflux temperature to maintain the fluid temperature in the non-two-phase flow affected area. That is, the actual pool temperature during the actual operation of the steam jet condensation system is used as the liquid phase temperature for numerical calculation and applied to the initialization settings of the outlet boundary conditions and flow field information.

[0028] In this embodiment, the temperature of the superheated steam is selected to be 378.15K, and the steam mass flow rate is 10-50 kg·m -2 ·s -1 As the inlet boundary condition; the outlet pressure is the ambient pressure, and the return temperature is the subcooled water temperature of the current working condition; the setting range of the subcooled water temperature, i.e. the initial temperature, is 20 to 60°C.

[0029] Step 3, comprehensively considering the high turbulence of the jet, the compressibility and viscous dissipation of steam, and the non-equilibrium of heat and mass transfer between phases, the mass, momentum and energy conservation equations are established for the gas-liquid two-phase based on the Euler-Euler Two-fluid model, and the k-ε model is used to describe the turbulence effect of the control equation. According to the boundary conditions, initial conditions or the data obtained from the previous iterative calculation, the transient physical quantity distribution of the current flow field is solved.

[0030] Among them, the Euler-Euler two-fluid model is a control equation that describes the flow and heat transfer relationship established for the fluid calculation domain of the physical model. The complete two-fluid model consists of a liquid model and a steam model. The liquid model has a similar structure to the steam model, and the source term transfer direction is opposite (the source term of the steam model is subscripted as wg, and the source term of the liquid model is subscripted as gw).

[0031] Taking the steam model as an example, the ensemble average conservation equations for the phase change of single-component two-phase fluid flow and heat transfer in the fluid calculation domain are as follows:

[0032]

[0033]

[0034] Where t is time, α g is the gas phase volume fraction, ρ g is the gas phase density, represents the Nabla operator, is the gas phase velocity vector, is the mass transfer rate from liquid to gas, is the liquid phase velocity vector, p is the pressure shared by the two phases, is the gas phase stress-strain tensor, is the drag force, h g is the specific enthalpy of the gas phase, λ eff,g is the effective thermal conductivity of the gas phase, Q wg is the rate of heat transfer from the liquid phase to the vapor phase.

[0035] According to the above control equations, the separation algorithm of ANSYS FLUENT solver is used to solve the control equations. First, the momentum equation and the continuity equation are solved in turn to update the pressure field and the velocity field; secondly, the turbulence equation is solved using the latest pressure field, velocity field and related solution variables to update the turbulent kinetic energy and turbulent dissipation rate; finally, the energy equation is solved using the latest pressure field, velocity field, turbulent kinetic energy and turbulent dissipation rate and related solution variables to update the temperature field, thereby obtaining the transient physical quantity distribution of the current iteration step.

[0036] The present invention uses the interphase heat transfer rate and mass transfer rate as important source terms for the energy equation and the continuity equation to describe the nonlinear phase change heat transfer process. The interphase heat transfer rate is obtained by calculation of the heat transfer model, and the interphase mass transfer rate is obtained by calculation of the thermal phase change steam condensation rate model. The heat transfer model and the thermal phase change steam condensation rate model are defined in turn as follows:

[0037]

[0038] In the formula, Q gw is the heat transfer rate from gas phase to liquid phase, is the mass transfer rate from gas phase to liquid phase, h wi is the liquid side heat transfer coefficient, h gi is the air side heat transfer coefficient, A i is the interface area density, T sat 、T w 、T g are saturation temperature, liquid phase temperature and gas phase temperature respectively, H ws and H gs are the specific enthalpies of the liquid and vapor phases at saturation temperature, respectively. The saturation temperature is defined as a polynomial function of pressure.

[0039] In step 4, the complex nonlinear phase change heat transfer process is decomposed into two stages: heat transfer without phase change and heat transfer with phase change by using the staged source term reconstruction method. Different heat transfer coefficient definition methods are designed according to the numerical calculation requirements of each stage to realize the reconstruction of the mathematical model, and the source term of the energy equation is calculated based on the physical quantities obtained in step 3.

[0040] Among them, the liquid side heat transfer coefficient h wi It is given directly by the formula or calculated by the Nusselt correlation as follows:

[0041]

[0042] Where l is the characteristic length of the interface, Nu is the Nusselt number, and λ w is the thermal conductivity of the liquid, Nu is defined by a variety of heat transfer correlations, the characteristic length l is selected according to the applicable conditions of Nu, and the calculated liquid-side heat transfer coefficient is applied to the subsequent heat transfer calculations in all stages.

[0043] Air side heat transfer coefficient h gi The heat transfer rate between phases is defined in stages according to the actual heat transfer phenomena and the stability of numerical calculations.

[0044] Specifically, in this step, the user defined function (UDF) in ANSYS FLUENT software can be used to decompose the heat transfer process between superheated steam and subcooled water into two stages: heat transfer without phase change and heat transfer with phase change, so as to achieve a fine mathematical model reconstruction.

[0045] In the heat transfer stage without phase change, the air side heat transfer coefficient h gi Based on the law of heat conservation, the saturation temperature and the liquid side heat transfer coefficient h wi gives:

[0046]

[0047] The heat transfer model for the heat transfer stage without phase change is defined as follows:

[0048] Q gw =h wi A i (T sat -T w )

[0049] Q wg =h gi A i (T sat -T g )

[0050] Heat transfer coefficient h on the gas side with phase change heat transfer giAnd heat transfer models are divided into three categories according to the numerical calculation requirements.

[0051] The first type of calculation considers the condensation heat transfer when the interface is in a saturated state, the heat transfer model and the gas side heat transfer coefficient h gi The definition is as follows:

[0052]

[0053]

[0054] In the formula, α g is the volume fraction of the gas phase. The designed gas-side heat transfer coefficient aims to enhance heat and mass transfer while avoiding overcalculation of heat in cells with low gas content.

[0055] The second type of calculation considers the inevitable heat transfer of subcooled gas in numerical iteration, and the heat transfer model is defined as follows:

[0056]

[0057] In the formula, H wg is the enthalpy of the liquid at the gas temperature, H g is the gas enthalpy corresponding to the gas temperature; since only latent heat transfer is considered on the gas side, there is no need to define the gas side heat transfer coefficient in the second type of calculation.

[0058] The third type of calculation considers the gas temperature treatment in the area with extremely low gas content. Since the two-fluid method establishes temperature fields for the gas and liquid phases respectively, when the gas content of the flow field is very low, the gas temperature needs to be treated as the ambient liquid temperature to avoid unreasonable heat transfer calculations. The gas-side heat transfer coefficient h calculated in the third type is gi and liquid side heat transfer coefficient h wi The heat transfer model is defined as follows:

[0059] Q gw =h wi A i (T g -T w )

[0060] Q wg =h wi A i (T w -T g )

[0061] Step 5, add a temperature auxiliary model to the steam condensation rate source term of the continuity equation to strengthen the coupling between the energy equation and the continuity equation, and calculate the continuity equation source term based on the physical quantities such as velocity, pressure, and temperature obtained in step 3.

[0062] In this step, based on the thermal balance principle of the gas-liquid heat transfer model, a thermal phase change steam condensation rate model with gasification as the positive mass transfer is established, and a temperature auxiliary model is used to strengthen the coupling between the energy equation and the continuity equation. The steam condensation rate source term is determined by the thermal phase change steam condensation rate model and the temperature auxiliary model. The thermal phase change steam condensation rate model is defined as follows:

[0063]

[0064] Because the single thermal phase change steam condensation rate model has the problem of steam phase temperature lag in transient calculation, a temperature auxiliary model considering steam temperature is added to the steam condensation rate source term of the continuity equation as a phase distribution constraint. The temperature auxiliary model is defined as follows:

[0065]

[0066] Where Δt is the discrete time step, α g is the gas phase volume fraction, ρ g is the gas phase density, T sat and T g are saturation temperature and gas phase temperature respectively; the applicable time step range of the temperature-assisted model is 0.001 to 0.0001 s. The temperature-assisted model is designed to suppress the probability of occurrence of low-temperature gas, reduce unnecessary numerical calculations and parameter fluctuations, and is only used in the processing of the steam condensation rate source term of the continuity equation. The steam condensation rate and the temperature-assisted model are calculated based on the velocity, pressure, and temperature obtained by solving the control equation. The steam condensation rate source term of the continuity equation takes the minimum value of the temperature-assisted model and the thermal phase change steam condensation rate model to ensure that a large amount of supercooled gas does not appear in the condensation process with gasification as the forward mass transfer.

[0067] Step 6, explicitly relax the source term and update the source term of the control equation.

[0068] In this step, the relaxation factor that changes with the iteration step is used to explicitly relax the source terms of the control equation (i.e., gas-liquid heat transfer and steam condensation rate). The relaxation method is defined as follows:

[0069]

[0070] In the formula, is the updated value of the physical quantity source term, is the current value of the source term of the physical quantity, is the updated value calculated by the physical quantity model, and R is the relaxation factor; the physical quantities include the interphase heat transfer rate calculated by the heat transfer model, and the steam condensation rate determined by the thermal phase change steam condensation rate model and the temperature auxiliary model. The maximum number of iterations is set to 40, and the relaxation factor setting range for the first 20 steps of iterative solution is 0.3-0.6; the relaxation factor for 20-30 steps is 0.5 times the initial value; and the relaxation factor after 30 steps is 0.3 times the initial value.

[0071] Step 7: Update the number of iterations according to the change in time step.

[0072] In this step, the ADJUST function in the UDF is used to compile the iterative pedometer and update the number of iterations according to the change in the time step.

[0073] The solution process of the above model is as follows: first, refer to steps 1 to 7 to establish a complete model calculation case; for the first calculation, set the boundary and initial conditions according to step 2, and update the fluid properties with this physical quantity distribution; the subsequent iterative calculation updates the fluid properties according to the physical quantity distribution of the previous time step; use the ADJUST function defined in step 7 to update the number of iterations. Secondly, according to the dual-fluid control equation established in step 3, solve the momentum equation, continuity equation, turbulence equation, and energy equation in turn, and update the pressure, velocity, turbulent kinetic energy, turbulent dissipation rate, and temperature. Thirdly, in UDF, according to the methods of steps 4 and 5, use external function definitions and update the heat transfer coefficient, calculate the required enthalpy value, and thermal phase change steam condensation rate in the calculation; use the updated external function physical quantity to calculate the heat transfer model, and update the steam condensation rate in combination with the temperature auxiliary model; adjust and update the continuity equation steam condensation rate source term and the energy equation interphase heat transfer rate source term according to the relaxation method in step 6. Finally, the numerical model composed of steps 3 to 7 is iterated, and the flow field information is recorded after the calculation is stabilized and converged, and the pressure oscillation characteristics of the measuring points and the global physical quantity distribution of the flow field are obtained at different time steps, and finally the steam condensation behavior inside and outside the jet tube and the transient change process of temperature and pressure during the steam jet surge condensation process are clarified. The flow field physical quantity distribution results of this time step are saved, and the iterative calculation of the next time step is considered according to the calculation requirements.

[0074] Specifically, the transient changes in the distribution of physical quantities of steam jet condensation are recorded within 3 seconds of stable convergence of the calculation. Figure 5 This is a comparison of the pressure oscillation characteristics of 6 interface heat transfer coupling models under the same operating conditions. The interface heat transfer coupling model refers to the combination of the interface area density model and the liquid side heat transfer correlation; the tested interface area density models include the gradient model and the Ishii model; the gradient model is defined as follows:

[0075]

[0076] The Ishii model is defined as follows:

[0077]

[0078] Where d is the bubble diameter.

[0079] Liquid-side heat transfer correlation heat transfer models include Ranz-Marshall, Hughmark, Hughes-Duffey and Lee models; the Ranz-Marshall model is defined as follows:

[0080]

[0081] The Hughmark model is defined as follows:

[0082]

[0083] The Lee model is defined as follows:

[0084]

[0085] The liquid side heat transfer coefficient expression of the Hughes-Duffey model is defined as follows:

[0086]

[0087] In the formula, c p,w is the specific heat capacity of the liquid phase at constant pressure, λ w is the liquid phase thermal conductivity, v is the kinematic viscosity of the liquid, ε is the turbulent dissipation rate; Re is the relative Reynolds number, Pr w is the liquid phase Prandtl number, which are defined as follows:

[0088]

[0089]

[0090] The six coupling models are Lee-gradient model (abbreviated as Lee(G)), Ranz-Marshall-gradient model (abbreviated as RM(G)), Ranz-Marshall-Ishii model (abbreviated as RM(I)), Hughmark-gradient model (abbreviated as Hu(G)), Hughmark-Ishii model (abbreviated as Hu(I)), and Hughes-Duffey-gradient model (abbreviated as HD(G)). Figure 5 It is shown that the staged source term reconstruction method proposed in the present invention can ensure the stable calculation of strong nonlinear phase change heat transfer problems and support the application of various interface area density models and heat transfer models. Figure 6The prediction results of the Lee-gradient coupling model for steam surge condensation behavior under 6 operating conditions of steam mass flow rate and subcooled water temperature are shown. Typical steam morphology change stages are observed, namely steam injection, bubble expansion, interface oscillation, bubble necking, bubble detachment, bubble collapse, subcooled water backflow, compression and rupture of steam in the tube, etc.; the steam condensation behavior simulated in each operating condition is consistent with the experimental description and can effectively correspond to the condensation phase region; the influence of steam mass flow rate and temperature on surge frequency or bubble volume change frequency is consistent with the experimental conclusion, and the maximum relative error of surge frequency is less than 30%. Figure 5 and Figure 6 The calculation results show that the numerical simulation method established in the present invention has good engineering predictability and can effectively simulate the steam condensation behavior and pressure oscillation characteristics under various low steam mass flow rates and subcooled water temperatures.

[0091] It can be seen that the present invention proposes a method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction, which can support the stable calculation of various interface area density models and heat transfer model coupling schemes. Under a specific interface heat transfer model coupling scheme, the steam low-mass flow jet condensation information under various operating conditions can be accurately obtained. On this basis, a steam flow distribution strategy that matches the subcooled water temperature can be reasonably designed to enhance the equipment structural performance in the area affected by surge pressure oscillations and reduce the destructiveness of pressure oscillations on the steam jet condensation system. According to the present invention, the dynamic condensation behavior of steam and the impact position of pressure oscillations on the equipment can be accurately predicted, which has guiding significance for the efficient and safe design of the steam jet condensation system.

[0092] Based on the idea of ​​staged source term reconstruction, the present invention reconstructs the phase change heat transfer energy source term in the dual fluid control equation in a segmented manner, and designs a method for defining the interface heat transfer coefficient at different stages, thereby realizing the transient solution of the complex nonlinear phase change heat transfer process. The present invention comprehensively considers the non-steady-state heat and mass transfer process between superheated steam and subcooled water under various steam low mass flux operating conditions, and can accurately predict the surge condensation behavior and pressure oscillation characteristics of steam, which has guiding significance for the refined design of pressure relief condensation equipment such as jet tubes and bubblers.

Claims

1. A method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction, characterized in that: The steps include: Step 1: According to the real structure of the steam jet condensation system, a physical model of the surge condensation of superheated steam and subcooled water through the jet tube is constructed, and the fluid calculation domain is meshed to realize the discretization of the physical space; Step 2, using the mass flow rate and upstream temperature of the superheated steam as the inlet boundary conditions of the fluid calculation domain, and using the ambient pressure on the surface of the pool as the outlet boundary condition; Step 3: Use the k-ε model to describe the turbulence effect of the control equation, establish the mass, momentum and energy conservation equations for the gas phase and the liquid phase based on the Euler-Euler two-fluid model, and solve the transient physical quantity distribution of the current flow field according to the boundary conditions, initial conditions or the data obtained by the iterative calculation in the previous step; Step 4: Using the method of phased source term reconstruction, the nonlinear phase change heat transfer process is decomposed into two stages: heat transfer without phase change and heat transfer with phase change. Different heat transfer coefficient definition methods are designed according to the numerical calculation requirements of each stage to achieve mathematical model reconstruction, and the energy equation source term is calculated based on the physical quantities obtained in step 3. Step 5, adding a temperature auxiliary model to the steam condensation rate source term of the continuity equation, and calculating the continuity equation source term according to the physical quantity obtained in step 3; Step 6, explicitly relax the source term and update the source term of the control equation; Step 7, update the number of iteration steps according to the change of time step; Step 8, iteratively calculate the numerical model constructed by steps 3 to 7, record the flow field information after stable convergence, obtain the pressure oscillation characteristics of the measuring points at different time steps and the global physical quantity distribution of the flow field, and finally clarify the steam condensation behavior inside and outside the jet tube and the transient change process of temperature and pressure during the steam jet surge condensation process.

2. The method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction according to claim 1 is characterized in that: In the step 1, the grid independence of the average surge frequency solution and the average surge pressure peak solution is verified by comparing the transient pressure oscillation results calculated by different grids within a specific time range. If the average surge frequency and the average surge pressure peak are both grid-independent solutions, the calculation is carried out using a grid with a minimum number of grids that meets the requirement, otherwise the grid is further encrypted.

3. The method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction according to claim 1 is characterized in that: In the step 2, the actual water pool temperature during the actual operation of the steam jet condensation system is used as the liquid phase temperature for numerical calculation and applied to the initialization setting of the outlet boundary conditions and flow field information.

4. The method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction according to claim 1 is characterized in that: In step 3, the interphase heat transfer rate and mass transfer rate are used as source terms to describe the nonlinear phase change heat transfer process. The interphase heat transfer rate is obtained by calculation of the heat transfer model, and the interphase mass transfer rate is obtained by calculation of the thermal phase change steam condensation rate model. The heat transfer model and the thermal phase change steam condensation rate model are defined in sequence as follows: In the formula, Q wg is the heat transfer rate from liquid to gas, Q gw is the heat transfer rate from gas phase to liquid phase, is the mass transfer rate from liquid to gas, is the mass transfer rate from gas phase to liquid phase, h wi is the liquid side heat transfer coefficient, h gi is the air side heat transfer coefficient, A i is the interface area density, T sat , T w , T g are saturation temperature, liquid phase temperature and gas phase temperature respectively, H ws and H gs are the specific enthalpies of the liquid and vapor phases at saturation temperature, respectively. The saturation temperature is defined as a polynomial function of pressure.

5. The method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction according to claim 4 is characterized in that: The liquid side heat transfer coefficient h wi It is given directly by the formula or calculated by the Nusselt correlation as follows: Where l is the characteristic length of the interface, Nu is the Nusselt number, and λ w is the liquid thermal conductivity, Nu is defined by a variety of heat transfer correlations, the characteristic length l is selected according to the applicable conditions of Nu, and the calculated liquid side heat transfer coefficient is applied to the subsequent heat transfer calculations of all stages; Air side heat transfer coefficient h gi The heat transfer rate between phases is defined in stages according to the actual heat transfer phenomena and the stability of numerical calculations.

6. The method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction according to claim 1 is characterized in that: The step 4, the phased source term reconstruction method is completed with the help of UDF in ANSYS FLUENT software, and the heat transfer process between superheated steam and subcooled water is decomposed into two stages: heat transfer without phase change and heat transfer with phase change.

7. The method for simulating the heat and mass transfer process of steam jet surge condensation based on staged source term reconstruction according to claim 1 or 6, characterized in that: In step 4, in the heat transfer stage without phase change, the gas side heat transfer coefficient h gi Based on the law of heat conservation, the saturation temperature and the liquid side heat transfer coefficient h wi gives: The heat transfer model for the heat transfer stage without phase change is defined as follows: Q gw =h wi A i (T sat -T w ) Q wg =h gi A i (T sat -T g ) Gas side heat transfer coefficient h with phase change heat transfer gi and heat transfer models are divided into three categories according to the numerical calculation requirements; The first type of calculation considers the condensation heat transfer when the interface is in a saturated state, the heat transfer model and the gas side heat transfer coefficient h gi The definition is as follows: In the formula, α g is the gas phase volume fraction; The second type of calculation considers the inevitable heat transfer of subcooled gas in numerical iteration, and the heat transfer model is defined as follows: In the formula, H wg is the enthalpy of the liquid at the gas temperature, H g is the gas enthalpy corresponding to the gas temperature; The third type of calculation considers the gas temperature treatment in the area with extremely low gas content, and the gas side heat transfer coefficient h gi and liquid side heat transfer coefficient h wi The heat transfer model is defined as follows: Q gw =h wi A i (T g -T w ) Q wg =h wi A i (T w -T g ) 8. The method for simulating the steam jet surge condensation heat and mass transfer process based on staged source term reconstruction according to claim 1 is characterized in that: In step 5, the temperature auxiliary model is expressed as follows: Where Δt is the discrete time step, α g is the gas phase volume fraction, ρ g is the gas phase density, T sat and T g They are saturation temperature and gas phase temperature respectively; the applicable time step range of the temperature-assisted model is 0.001~0.0001s.

9. The method for simulating steam jet surge condensation heat and mass transfer process based on staged source term reconstruction according to claim 1 or 8, characterized in that: The steam condensation rate source term of the continuity equation takes the minimum value of the temperature auxiliary model and the thermal phase change steam condensation rate model to ensure that a large amount of supercooled gas does not appear in the condensation process with gasification as the forward mass transfer.

10. The method for simulating steam jet surge condensation heat and mass transfer process based on staged source term reconstruction according to claim 1, characterized in that: In step 6, a relaxation factor that varies with the number of iterations is used to explicitly relax the source term of the control equation. The relaxation method is defined as follows: In the formula, is the updated value of the physical quantity source term, is the current value of the source term of the physical quantity, is the updated value calculated by the physical quantity model, and R is the relaxation factor; the physical quantities include the interphase heat transfer rate calculated by the heat transfer model, and the steam condensation rate determined by the thermal phase change steam condensation rate model and the temperature auxiliary model; the maximum number of iterations is set to 40, and the relaxation factor setting range for the first 20 steps of iterative solution is 0.3-0.6; the relaxation factor for 20-30 steps is 0.5 times the initial value; and the relaxation factor after 30 steps is 0.3 times the initial value.

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