Numerical simulation method of iodine removal efficiency in venturi scrubber based on double-film theory coupled with chemical reaction kinetics

Abstract

The application discloses a numerical simulation method for iodine removal efficiency of a Venturi scrubber based on coupling of a double-film theory and chemical reaction kinetics, and steps of the method comprise: establishing a simplified geometric model of the scrubber, and locally encrypting a grid at a throat and a gas-liquid contact area; importing CFD software, setting a multiphase flow and a component transport model, and loading a user-defined function containing a double-film mass transfer rate equation and an iodine-sodium thiosulfate reaction kinetics equation; initializing a gas-phase iodine and a liquid-phase absorbent concentration field; coupling and solving momentum, continuity, component transport and energy equations, extracting a mass transfer rate and an iodine mass fraction distribution, and calculating the iodine removal efficiency from a difference between iodine mass flow rates at an inlet and an outlet. The application solves the defects of ignoring liquid film resistance and a chemical reaction enhancement factor in the existing CFD simulation, avoids errors caused by dependence on an empirical formula, can accurately predict gaseous iodine removal efficiency of the Venturi scrubber under different working conditions, and provides a theoretical basis for design and optimization of a containment filtration and exhaust system of a nuclear power plant.

Description

Technical Field

[0001] This invention belongs to the field of nuclear engineering safety and computational fluid dynamics application technology, specifically involving a numerical simulation method for iodine removal efficiency of a Venturi scrubber based on the coupling of chemical reaction kinetics with the two-film theory. Background Technology

[0002] Nuclear power plants provide clean and sustainable energy for power generation and heating. The safety of nuclear power plants and their ability to handle severe accidents have always been a primary concern. For example, in the Three Mile Island, Chernobyl, and Fukushima nuclear accidents, the containment structures designed to protect the environment from radioactive hazards were damaged. To address such severe accidents, containment filtration and emission systems were designed, which reduce containment pressure by releasing a portion of gases into the atmosphere. Radioactive iodine-131 is a major byproduct that needs to be removed in this system; its release into the atmosphere poses a serious threat to human health.

[0003] In containment ventilation systems (FCVS), Venturi scrubbers are widely used to remove harmful radioactive products from exhaust air, particularly iodine and its compounds (such as elemental iodine and organic iodine), to prevent their release into the external environment. Therefore, it is necessary to study the iodine retention phenomenon in Venturi scrubbers to effectively improve their removal efficiency. Venturi scrubbers employ a tapering, gradually expanding nozzle structure, facilitating the injection of the scrubbing solution into the throat. Harmful radioactive gases present in the air (mainly including elemental iodine and organic iodine forms such as iodomethane) are effectively absorbed by the scrubbing solution as they flow through the Venturi scrubber.

[0004] Currently, the evaluation of iodine removal performance of Venturi scrubbers mainly relies on bench experiments or simplified empirical formulas. Although computational fluid dynamics (CFD) techniques have been applied to two-phase flow simulations, existing technologies have the following limitations: Most current CFD simulations only consider gas film control or simple Henry's law equilibrium, neglecting liquid film mass transfer resistance, especially in chemical absorption processes where liquid film resistance significantly affects the overall mass transfer rate. Furthermore, it is difficult to obtain localized iodine concentration gradients and reaction rate distributions within the Venturi scrubber, limiting the optimization of the equipment's structure.

[0005] Therefore, developing a CFD numerical simulation method that can couple the two-membrane mass transfer theory with chemical reaction kinetics is of great practical significance for improving the accuracy of iodine removal efficiency prediction and optimizing the design of filter containment ventilation systems (FCVS). Summary of the Invention

[0006] The purpose of this invention is to provide a numerical simulation method for iodine removal efficiency in a Venturi scrubber based on the dual-film theory coupled with chemical reaction kinetics. This method solves the problem of neglecting liquid film resistance and chemical reaction enhancement effects in existing CFD simulations, and can more realistically reflect the gas-liquid mass transfer and reaction coupling process, thus obtaining more accurate iodine removal efficiency analysis results. This method is of great significance for the analysis of gaseous iodine removal characteristics in Venturi scrubbers. To achieve the above objectives, the present invention adopts the following technical solution: A numerical simulation method for iodine removal efficiency of a Venturi scrubber based on two-film theory coupled with chemical reaction kinetics includes the following steps: Step 1: Establish a simplified geometric model of the computational domain of the Venturi scrubber. The simplified geometric model includes a gas inlet, a liquid suction inlet, a throat mixing zone, and a diffusion separation zone. In this step, the computational domain boundary is determined based on the actual physical dimensions to ensure that the mesh can capture the characteristics of high-speed jets and droplet breakup at the throat.

[0007] Step 2: Mesh the simplified geometric model and refine the mesh locally in the throat and gas-liquid contact area to obtain the computational domain mesh model.

[0008] Step 3: Import the Venturi scrubber computational domain mesh model divided in Step 2 into the fluid dynamics software.

[0009] Step 4: Set up the multiphase flow model and component transport model in the fluid dynamics software, and load user-defined functions (UDFs). This UDF includes a mass transfer rate equation based on the two-film theory and a chemical reaction kinetic equation between iodine and sodium thiosulfate, wherein the governing equation is in the form of: (1) ——No. Phase volume fraction; ——No. Phase density, ; ——No. Phase velocity vector, ; — From the first to the first The mass transfer rate per unit volume of the phase, ; — From the first to the first The mass transfer rate per unit volume of the phase, ; ——No. Phase quality source term, .

[0010] Among them, from the first p to the first q Mass transfer of phase It indicates that, for The momentum equation for the phase is expressed by (2): (2) P -- pressure, ; ——No. q The stress tensor (shear stress and normal stress) within the phase. ; — Gravitational acceleration vector ; — Interface drag force, which is the viscous resistance or form resistance generated by relative motion (excluding momentum from mass transfer). ; — From the first p Phase transition to the first q Phase mass flow rate, ; — The velocity vector at the phase interface is usually taken as or , ; ——No. q Other external volumetric force sources of the phase (excluding gravity, lift, etc.). ; ——No. q The lift force experienced by the phase, ; ——No. q The wall lubrication force experienced by the phase, ; ——No. q The virtual mass force experienced by the phase, ; ——No. q The turbulent diffusion force it experiences .

[0011] Step 5: In the fluid dynamics software, the mass fraction of iodine vapor in the gas inlet region is set through region initialization assignment to generate an initial concentration field of iodine-containing air; simultaneously, the mass fraction of sodium thiosulfate solution in the liquid inlet region is set to generate an initial concentration field of the washing liquid containing the absorbent. This provides initial distribution conditions of the gas-liquid two-phase component concentrations for subsequent transient or steady-state solutions.

[0012] Step 6: Based on the established initial component concentration field, start the fluid dynamics software solver to perform coupled iterative solutions for the momentum equation, continuity equation, component transport equation and energy equation; extract the mass transfer rate and iodine mass fraction distribution of the gas-liquid two phases inside the Venturi scrubber based on the solution results, and calculate the overall iodine removal efficiency of the system based on the difference in iodine mass flow rate between the gas inlet and outlet sections.

[0013] Compared with the prior art, the present invention has the following advantages: 1. This invention establishes a mass transfer rate equation between gas and liquid phases based on the two-film theory, and simultaneously considers the mass transfer resistance on both the gas film side and the liquid film side. This overcomes the shortcomings of existing CFD simulations that simply regard the mass transfer process as gas film control while ignoring the liquid film resistance, making the prediction of the mass transfer rate more accurate.

[0014] 2. This invention couples the chemical reaction kinetic equation of iodine and sodium thiosulfate into the multiphase flow solver through a user-defined function (UDF), thereby achieving an accurate description of the enhancement effect of the chemical reaction on the mass transfer process and avoiding the errors caused by relying on empirical formulas to correct the mass transfer coefficient.

[0015] 3. By solving the momentum equation, continuity equation, component transport equation and energy equation in a coupled manner, this invention can comprehensively obtain the mass transfer rate and iodine mass fraction distribution of the gas-liquid two phases inside the Venturi scrubber, providing a visualization means for a deeper understanding of the iodine retention mechanism inside the Venturi scrubber.

[0016] 4. This invention can accurately predict the gaseous iodine removal efficiency of the Wenturi scrubber under different operating conditions (such as different gas phase flow rates, liquid-to-gas ratios, and scrubbing liquid concentrations), providing a reliable theoretical basis for the design optimization of the containment filtration and emission system of nuclear power plants. Attached Figure Description

[0017] Figure 1 Distribution curves of gas phase iodine mass fraction cloud map (top) and liquid phase iodine mass fraction cloud map (bottom) along the axial length of the Venturi tube under different gas phase volume flow rates.

[0018] Figure 2 The image shows the axial direction liquid phase iodine mass fraction cloud map (left) and gas phase iodine mass fraction cloud map (right) inside the Venturi scrubber.

[0019] Figure 3 This is a mass transfer rate distribution cloud map of iodine between the gas and liquid phases inside the Venturi scrubber and a detailed mass transfer cloud map of the throat. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments: This invention discloses a numerical simulation method for iodine removal efficiency of a Venturi scrubber based on the two-film theory coupled with chemical reaction kinetics, comprising the following steps: Step 1: Establish a simplified geometric model of the computational domain of the Venturi scrubber, including the fluid domain inside the Venturi throat and diffuser section. The specific steps are as follows: Step 1-1: Construct a simplified geometric model of the internal fluid domain of the Venturi scrubber, including the gas inlet, liquid intake inlet, throat mixing zone, and diffusion separation zone.

[0021] Steps 1-2: Determine the computational domain boundary based on the actual physical dimensions of the Venturi scrubber to ensure that the model can accurately capture the characteristics of the high-speed jet and droplet breakup at the throat.

[0022] Step 2: Mesh the simplified geometric model of the internal fluid domain of the Venturi scrubber obtained in Step 1. The specific steps are as follows: Using mesh generation software, generate a hexahedral structured mesh based on the geometric model established in Step 1-1, and refine the mesh locally in the throat and areas of intense gas-liquid contact to finally obtain the mesh model of the internal fluid domain of the Venturi scrubber.

[0023] Step 3: Import the computational domain mesh model of the internal fluid domain of the Venturi scrubber divided in Step 2 into the fluid dynamics software as the basis for subsequent numerical simulation calculations.

[0024] Step 4: Set up the multiphase flow model and component transport model in the fluid dynamics software, and load the user-defined function (UDF). This UDF contains the mass transfer rate equation based on the two-film theory and the chemical reaction kinetics equation between iodine and sodium thiosulfate. The governing equations are as follows: (1) Among them, from the first p to the first q Phase mass transfer In other words, for q The momentum equation for the phase is expressed by (2): (2) To describe the concentration changes of iodine in each phase, the solution is obtained in the component transport model. k Component in q mass fraction in the phase Its transport equation is: (3) — Components k In the q The diffusion coefficient in the phase, ; — Component source terms caused by interphase mass transfer, ; — Component source terms caused by chemical reactions, .

[0025] The mass transfer rate equation based on the two-film theory is as follows: (4) In the formula: — Iodine mass transfer flux ; — Overall mass transfer coefficient based on gas phase partial pressure, ; — The partial pressure of iodine in the bulk gas phase. ; — The Henry's constant for iodine in the washing solution. ; — The concentration of iodine in the bulk liquid phase. .

[0026] The chemical reaction kinetics equation for iodine and sodium thiosulfate (assuming a pseudo-first-order irreversible reaction) is as follows: (5) In the formula: — The rate of iodine consumption in the liquid phase, ; — Reaction rate constant ; — The concentration of iodine in the liquid phase, .

[0027] The above equation is coupled to the multiphase flow control equation in the following way: the mass transfer flux of iodine in equation (4) Multiply by the phase boundary area density and the molar mass of iodine Converted to volumetric mass transfer rate Substituting the source term into the interphase mass transfer term in equation (1) and the mass transfer source term in equation (3) The consumption rate of equation (5) is used as the source term. Substitute this into the liquid phase component transport equation (3). Through the coupling of the above UDF, the mass transfer and chemical reaction processes are updated synchronously in each iteration step, thereby accurately describing the migration of iodine from the gas phase to the liquid phase and the chemical reaction consumption process in the liquid phase.

[0028] Step 5: In the fluid dynamics software, establish the initial distribution conditions of the gas-liquid two-phase component concentrations in the solver through region initialization assignment. Set the mass fraction of iodine vapor in the gas inlet region to generate the initial concentration field of iodine-containing air; simultaneously set the mass fraction of sodium thiosulfate solution in the liquid inlet region to generate the initial concentration field of the washing liquid containing the absorbent.

[0029] Step 6: Based on the established initial component concentration field, start the fluid dynamics software solver to perform coupled iterative solutions to the momentum equation, continuity equation, component transport equation, and energy equation. Figure 1 The figures show the distribution curves of gaseous iodine mass fraction (top) and liquid iodine mass fraction (bottom) along the axial length of the Venturi scrubber under different gas phase volumetric flow rates. As can be seen from the figures, the driving force for mass transfer between the gas and liquid phases is greatest in the throat contraction section, resulting in a sharp decrease in the gas phase iodine mass fraction and a corresponding rapid increase in the liquid phase iodine mass fraction. Furthermore, the higher the inlet gas phase volumetric flow rate, the slower the concentration decay along the path, and the higher the residual concentration at the outlet. Based on the solution results, the mass fraction distribution cloud maps and mass transfer rate distribution cloud maps of the gas and liquid phases along the axial section inside the Venturi scrubber are extracted (e.g., [image of mass fraction distribution cloud maps]). Figure 2 , Figure 3 (as shown) Figure 2 The results show that the mass fraction of iodine in the gas phase gradually decreases from the inlet to the outlet, while the mass fraction in the liquid phase increases along the way, indicating that iodine continuously migrates from the gas phase to the liquid phase.

[0030] The overall iodine removal efficiency of the system is calculated based on the difference in iodine mass flow rate between the gas inlet and outlet sections, and the mass transfer rate distribution within the Venturi scrubber is obtained as follows: Figure 3 As shown, from Figure 3 It can be seen that the high values ​​are concentrated in the narrow throat region in the middle, indicating that the mass transfer is strongest in the constriction section of the Venturi tube and the two-phase mixing is most intense. The right figure is a magnified schematic diagram of the local details of the throat, which can intuitively show the mass transfer effect near the throat.

[0031] In this embodiment, the energy equation needs to be solved in a coupled manner for the following two reasons: firstly, the Henry's constant of iodine in the washing liquid. The temperature-dependent solubility of iodine decreases significantly with increasing temperature, and ignoring temperature changes would overestimate the mass transfer driving force. Secondly, the neutralization reaction of iodine with sodium thiosulfate is an exothermic reaction, and the chemical reaction heat... This will cause a local temperature rise, which in turn will change the saturation partial pressure at the gas-liquid interface and the chemical reaction rate constant. Therefore, in order to accurately predict the iodine removal efficiency in a Venturi scrubber, the energy equation must be solved simultaneously.

[0032] To describe the heat transfer and temperature change between the gas and liquid phases, an energy equation is added to the solver. q The energy equation for the phase is in the form of: (6) In the formula: ——No. q enthalpy of phase ; ——No. q Phase pressure, ; ——No. q Phase heat flux vector By Fourier's Law Given, among which For the first q thermal conductivity of the phase ; For the first q Phase temperature, .

[0033] No. q External heat source items (such as wall heating or cooling). ; From the p Phase passed to the first q The heat of the phase, ; The specific enthalpy at the phase interface is usually taken as . or , .

[0034] Among them, the heat of chemical reaction serves as the source term in the liquid phase energy equation. The expression for "join" is: (7) In the formula: — The rate at which iodine is consumed in chemical reactions. ; — The molar enthalpy change of the reaction between iodine and sodium thiosulfate is the heat of chemical reaction. (This reaction is exothermic.) Therefore ).

[0035] The energy equation is bidirectionally coupled with the aforementioned momentum, continuity, and component transport equations through the temperature dependence of physical property parameters and the heat source term of reaction: temperature changes affect the Henry's constant. and reaction rate constant It affects mass transfer and reaction rate, and changes in mass transfer and reaction rate are fed back to the temperature field through interphase heat transfer and reaction heat feedback.

[0036] The parts of this invention not described in detail are common knowledge to those skilled in the art.

Claims

1. A numerical simulation method for iodine removal efficiency of a Venturi scrubber based on dual-film theory coupled with chemical reaction kinetics, characterized in that, Includes the following steps: Step 1: Establish a simplified geometric model of the computational domain of the Venturi scrubber. The simplified geometric model includes a gas inlet, a liquid suction inlet, a throat mixing zone, and a diffusion separation zone. Step 2: Mesh the simplified geometric model and refine the mesh locally in the throat and gas-liquid contact area to obtain the computational domain mesh model; Step 3: Import the computational domain mesh model into the fluid dynamics software; Step 4: Set up a multiphase flow model and a component transport model in the fluid dynamics software, and load user-defined functions; the user-defined functions include mass transfer rate equations based on the two-film theory and chemical reaction kinetic equations between iodine and sodium thiosulfate. The mass transfer rate equation based on the two-film theory is as follows: In the formula, The mass transfer flux of iodine. The overall mass transfer coefficient is based on the partial pressure of the gas phase. This represents the partial pressure of iodine in the bulk gas phase. The Henry's constant for iodine in the washing solution. The concentration of iodine in the bulk liquid phase; the mass flux of iodine. Multiply by the phase boundary area density and the molar mass of iodine Converted into volumetric mass transfer source term Substitute these equations into the continuity equation and the component transport equations for the gas and liquid phases of the multiphase flow model. The chemical reaction kinetic equation for the reaction between iodine and sodium thiosulfate is as follows: In the formula, This represents the rate of iodine consumption in the liquid phase. Let be the reaction rate constant, representing the rate of iodine consumption in the liquid phase. Substitute the reaction source term into the liquid phase component transport equation; Step 5: Initialize the iodine vapor mass fraction in the gas inlet region and the sodium thiosulfate solution mass fraction in the liquid inlet region to establish the initial distribution conditions of the gas-liquid two-phase component concentrations; Step 6: Start the fluid dynamics software solver and perform coupled iterative solutions to the momentum equation, continuity equation, component transport equation and energy equation. Based on the solution results, extract the mass transfer rate and iodine mass fraction distribution of the gas and liquid phases inside the Venturi scrubber, and calculate the overall iodine removal efficiency based on the difference in iodine mass flow rate between the gas inlet and outlet sections.

2. The numerical simulation method for iodine removal efficiency of a Venturi scrubber based on dual-film theory coupled with chemical reaction kinetics as described in claim 1, characterized in that, The overall mass transfer coefficient based on gas phase partial pressure mentioned in step 4 The air-film mass transfer coefficient is determined as follows: and liquid film mass transfer coefficient Due to air film resistance With liquid film resistance The reciprocal of the sum is used to calculate the overall mass transfer coefficient based on the gas phase partial pressure, i.e. 。 3. The numerical simulation method for iodine removal efficiency of a Venturi scrubber based on dual-film theory coupled with chemical reaction kinetics as described in claim 1, characterized in that, The energy equation described in step 6 includes a heat source term arising from the exothermic reaction of iodine with sodium thiosulfate. ,in The molar enthalpy change of the reaction between iodine and sodium thiosulfate; temperature changes affect the Henry's constant. and reaction rate constant Feedback is used to calculate mass transfer rate and chemical reaction rate.

4. The numerical simulation method for iodine removal efficiency of a Venturi scrubber based on dual-film theory coupled with chemical reaction kinetics as described in claim 1, characterized in that, The simplified geometric model described in step 1 determines the computational domain boundary based on the actual physical dimensions of the Venturi scrubber to ensure that the model can capture the characteristics of high-speed jets and droplet breakup at the throat.

5. The numerical simulation method for iodine removal efficiency of a Venturi scrubber based on dual-film theory coupled with chemical reaction kinetics as described in claim 1, characterized in that, The mesh generation in step 2 uses a hexahedral structured mesh.

6. The numerical simulation method for iodine removal efficiency of a Venturi scrubber based on dual-film theory coupled with chemical reaction kinetics as described in claim 1, characterized in that, The method is used to predict and evaluate the gaseous iodine removal efficiency of the Chinese Churi scrubber in the containment filtration and emission system of a nuclear power plant.

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