CN-121997826-A - Calculation method of DNB-type critical heat flux density of high-flux arc stack plate-shaped fuel assembly

CN121997826ACN 121997826 ACN121997826 ACN 121997826ACN-121997826-A

Abstract

A calculating method of DNB type critical heat flux density of a high-flux arc stack plate-shaped fuel assembly comprises the steps of 1, carrying out geometric modeling and grid division on a coolant channel of the arc stack plate-shaped fuel assembly, 2, solving a control equation and a turbulence model of two fluid models to obtain phase field and flow field distribution, 3, solving a bubble number density group balance equation to calculate bubble diameter distribution, 4, solving bubble characteristic parameters by adopting a bubble stress balance dynamics model based on a near-wall flow field, 5, calculating heat flux components by adopting an improved wall heat flux distribution model, 6, establishing a heat conduction-convection-turbulence superposition interphase heat transfer model and calculating a phase change quality source item, 7, setting medium physical properties and boundary conditions, and coupling to solve gas-liquid volume fraction, speed, pressure and temperature, 8, carrying out step loading heat flux density calculation, and 9, judging critical heat flux density by boiling curve turning points, critical void share and outlet bubble component distribution. The method can effectively predict the critical heat flux density of the high-flux arc pile plate-shaped fuel assembly.

Inventors

GUO KAILUN
DI XIAO
YU XIN
YUAN YIDAN
GUO QIANG
TIAN WENXI
QIU SUIZHENG

Assignees

西安交通大学

Dates

Publication Date: 20260508
Application Date: 20260126

Claims (8)

1. A method for calculating DNB type critical heat flux density of a high-flux arc stack plate-shaped fuel assembly is characterized by comprising the following steps: step 1, establishing a high-flux arc pile plate-shaped fuel assembly coolant heat exchange channel geometric model and carrying out grid division to obtain a discretized calculation domain grid; step 2, establishing a gas-liquid two-phase flow control equation set based on an Euler-Euler two-fluid framework aiming at the established discretization calculation domain, and solving a coupling interphase action model and a Reynolds stress turbulence model to obtain phase field and flow field distribution under the current iteration step; step 3, calculating bubble diameter distribution and phase interface concentration by using a phase interface area density model based on a group balance equation based on the phase field and flow field distribution in the current iteration step; Step 4, based on near-wall flow field distribution, solving and obtaining a bubble detachment diameter, a buoyancy lifting diameter, detachment frequency and vaporization core density by adopting a bubble dynamics model based on force balance; Step 5, solving all heat flow components of wall boiling by adopting an improved wall heat flow distribution model based on the bubble characteristic parameters; step 6, calculating a boiling phase change quality source item by adopting a heat conduction-convection-turbulence linear superposition interphase heat transfer model based on the heat flow components and bubble diameter distribution of the wall boiling; Step 7, adding the boiling phase change mass source item, constructing a medium physical property model, setting boundary conditions, and solving two fluid equations to obtain the volume share, fluid velocity distribution, pressure distribution and temperature distribution of the gas-liquid two phases; step 8, supercooling boiling calculation is carried out by adopting a step-type loading heat flux method; and 9, determining the critical position and the critical heat flow density through the turning point of the boiling curve, the critical cavitation share and the volume fraction distribution of the large-diameter bubble group of the outlet section.
2. The method for calculating DNB type critical heat flux density of high-flux arc stack plate-shaped fuel assemblies according to claim 1 is characterized in that step 1 is specifically that the typical arc-shaped narrow slit runner structure size is determined for a reactor core 3-layer fuel zone total 5-type fuel assembly, geometric structure parameters comprise the curvature radius, the length of an active area and the center angle of a fan ring of the arc-shaped narrow slit runner, fluid development sections serving as an inlet runner and an outlet runner are respectively added at two ends of the narrow arc-shaped flow channel, the length of the fluid development sections is larger than 30 times of the hydraulic diameter, and the fluid development sections are divided by adopting a structured hexahedral grid, so that a discrete calculation domain for numerical calculation is obtained.
3. The method for calculating DNB type critical heat flux density of high-flux arc stack plate-shaped fuel assemblies according to claim 1, wherein in the step 2, for the established discretization calculation domain, a gas-liquid two-phase flow mass conservation, momentum conservation and energy conservation control equation set is established based on Euler-Euler two-fluid frames, an inter-phase action model is introduced to solve an inter-phase momentum transport equation, and a Reynolds stress turbulence model is adopted to simulate high-flux stack coolant flow characteristics of a high flow rate, a narrow flow channel and a secondary flow, so that phase field and flow field distribution under the current iteration step including local steam content, a speed field and liquid phase turbulence energy dissipation rate are obtained.
4. The method for calculating DNB type critical heat flow density of the high-flux arc stack plate-shaped fuel assembly according to claim 1, wherein in the step 3, a bubble group number density transport equation is discretely solved by adopting a uniform method through the local steam content, the speed field and the liquid phase turbulence energy dissipation rate obtained in the step 2, so that mass transfer of a multi-particle-size bubble group in a bubble flow system is realized, and the method comprises the steps of generating a mass source item by breaking large bubbles, generating a negative mass source item by breaking bubbles, converging the mass source item by small bubbles, merging the negative mass source item by combining the bubbles, and calculating the gas bubble diameter distribution and the phase interface concentration by combining the sauter average diameter.
5. The method for calculating DNB type critical heat flux density of high-flux arc stack plate-shaped fuel assemblies according to claim 1, wherein in step 4, according to the near-wall flow field distribution obtained in step 2, surface tension, bubble growth force, virtual mass force, steady-state drag force, shearing lift force, buoyancy, dynamic pressure and contact pressure suffered by bubbles on a vertical heating wall are comprehensively analyzed, a bubble dynamic model is established according to stress balance, bubble detachment is judged according to that the resultant force of the bubbles is larger than 0 in the direction parallel to the wall, bubble buoyancy is judged according to that the resultant force of the bubbles is larger than 0 in the direction perpendicular to the wall, and therefore bubble detachment diameter, buoyancy diameter, detachment frequency and vaporization core density are calculated.
6. The method for calculating DNB type critical heat flux density of high flux arc stack plate fuel assembly according to claim 1, wherein in step 5, the bubble characteristic parameters calculated in step 4 are substituted into an improved wall heat flux distribution model suitable for deviating from nucleate boiling state and critical heat flux density and a flow pattern section after evaporation to dryness to obtain four heat flux densities of liquid phase relative heat exchange heat flux, evaporation heat flux, quenching heat flux and gas phase relative heat exchange heat flux; The improved wall heat flow distribution model divides the wall heat flow density into four parts, namely a liquid phase relative flow heat exchange heat flow, an evaporation heat flow, a quenching heat flow and a gas phase relative flow heat exchange heat flow, and is calculated as follows: In the formula, q w is the total heat flow of the wall surface, and the unit is W M -2 ;q C -liquid phase convection heat flow with unit W M -2 ;q Q -quenching heat flow in W M -2 ;q E -the heat flow of evaporation, the unit is W M -2 ;q V -gas phase convection heat flow with unit of W M -2 ;f(α v ) -determining the transition function of the flow pattern. The calculation model of the heat flux density of each part is as follows: In the formula, h C is the liquid phase convection heat exchange coefficient with the unit of W m -2 K -1 ;T w , wall temperature in K, T l , liquid phase temperature in K, A b , bubble area fraction, lambda l , liquid phase heat conductivity in W m -1 K -1 ;η l -liquid phase thermal diffusivity, unit is m 2 S -1 , f, bubble detachment frequency, s -1 ;d d , bubble detachment diameter, m, N w , vaporization core density, site M -2 ;ρ g -gas phase density, unit is kg.m -3 ;h fv -vaporization phase transition latent heat, unit is J Kg -1 ;h V -gas phase convection heat transfer coefficient, unit is W m -2 K -1 ;T g -the gas phase temperature, in K.
7. The method for calculating DNB type critical heat flux density of high flux arc stack plate fuel assembly according to claim 1, wherein in step 6, based on each heat flux component of wall boiling and the bubble diameter distribution and phase interface concentration solved in step 3, energy exchange on a phase interface is obtained by establishing an inter-phase heat transfer calculation model of heat conduction-convection-turbulence linear superposition in a supercooling boiling bubble flow system, so as to calculate boiling phase change quality source items including a liquid phase evaporation item and a gas phase condensation item. Based on a spherical assumption single-bubble heat transfer mechanism, decomposing a gas-liquid phase inter-phase heat transfer mechanism into three independent mechanisms of heat conduction heat transfer, convection heat transfer and turbulent heat transfer, and establishing an inter-phase heat transfer calculation model in a linear superposition mode, wherein the total Knoop number is expressed as follows: In the formula, nu cond is a heat conduction Nu Cork number, nu conv is a convection NuCork number, nu turb is a turbulence NuCork number, ja sub is a liquid phase supercooling Jacobian number, re p is a bubble Reynolds number, pe is a Plantt number, alpha l is a partial liquid phase volume fraction, and tau turb is a turbulence time scale; Calculating the heat transfer quantity of the gas-liquid phase interface according to the total Knoop number : Wherein d b is the diameter of the bubble, and the unit is m; Assuming that the heat capacity of the phase interface is zero, taking condensation as the positive direction, calculating the condensation rate on the phase interface according to the heat transfer quantity of the gas-liquid phase interface, calculating the evaporation rate by the evaporation heat flow, and boiling the phase change quality source item Namely the difference between the two: wherein F/V is the ratio of the wall area to the volume of the near-wall grid, and the unit is m 2 M -3 , which function to convert the surface heat flux density into a volumetric heat source of the first layer of mesh.
8. The method for calculating DNB type critical heat flux density of high-flux arc stack plate-shaped fuel assembly according to claim 1, wherein step 9 comprises drawing a boiling curve of wall superheat along with heat flux density, determining boiling criticality by combining outlet section large-diameter bubble group volume fraction distribution when the boiling curve has a second turning point and heating wall near wall maximum void fraction exceeds critical void fraction, and determining DNB type critical heat flux density value and critical position.

Description

Calculation method of DNB-type critical heat flux density of high-flux arc stack plate-shaped fuel assembly Technical Field The invention relates to the technical field of nuclear reactor safety and analysis, in particular to a method for calculating DNB-type critical heat flux density of a high-flux arc stack plate-shaped fuel assembly. Background The high flux research pile is used as a neutron flux not lower than 1×10 14cm-2The non-power stack of s -1 has important application in the fields of scientific research, material performance test, medical treatment, industry and the like and radioisotope production. The ultra-high flux test reactor core fuel area based on the low self-shielding reactor core design concept consists of 6 novel arc plate-shaped fuel assemblies, and each circle of fuel arc plates form a closed fan ring on the section. The coolant flow channel in the arc-shaped fuel assembly is a narrow slit arc flow channel formed among a plurality of circles of fuel arc plates, the narrow slit height of the arc-shaped narrow slit flow channel is generally 2mm, and the arc-shaped narrow slit fuel assembly has larger specific surface area and stronger fluid disturbance characteristic, and is beneficial to enhancing the heat transfer effect. Due to the higher neutron flux density in the high flux reactor core, the volumetric power density of the fuel assembly is high and the coolant flow conditions are more complex. Critical heat Flux density (CRITICAL HEAT Flux, CHF) is one of the most important thermodynamic safety guidelines in nuclear reactor safety analysis, which characterizes the heat Flux density threshold at which heat transfer degradation occurs at the heated surface. For the novel arc-plate fuel assembly with higher coolant flow rate, when the wall heat flux density exceeds CHF, the heat exchange mechanism between the coolant and the heating surface is radically changed, the wall temperature is rapidly increased, and the phenomenon of deviation from nucleate boiling (Departure from Nucleate Boiling, DNB) under supercooled flow boiling can occur, so that serious accidents such as fuel element cladding failure and even core melting and the like can be caused. Therefore, accurate prediction of CHF is of critical importance for the safe design and operation of high-throughput stacks. A great deal of experiments and theoretical researches are carried out on the fuel element CHF at home and abroad, and the current prediction method of the CHF mainly comprises two types, namely, prediction is carried out by data driving means such as an empirical relation method, a Look-Up Table (LUT) or a neural network and the like based on the existing CHF experimental data and the statistical characteristics of influencing parameters, and the other type is based on a physical mechanism triggered by boiling critical, and analysis and prediction are carried out by adopting a Computational Fluid Dynamics (CFD) method based on a mechanism model. Chinese patent application number CN119227579 discloses a numerical simulation method for critical heat flow density of high burnup bubble plate type nuclear fuel. The method is to equivalent the air gap in the foaming cladding to solid thermal resistance, add source item to the whole solid domain of the fuel pellet as volume heat source, and realize the fuel pellet of heatFuel claddingFluid-to-solid coupled heat transfer calculation of coolant in the fluid domain. The two Euler-Euler fluid models are combined with the RPI wall surface boiling model to perform supercooling boiling calculation, but the bubble characteristic parameters in the RPI model are all in an empirical relation, such as the separation diameter is calculated by adopting a Tolubinsky-Kostanchuk empirical formula, and the bubble diameter is calculated by adopting a Kurul-Podowski empirical formula. Chinese patent application number CN118248361 discloses a numerical simulation calculation method for critical heat flow density of nuclear reactor core under ocean conditions. The method is based on the reactor core tilting And establishing a multiphase flow model and a wall boiling model under the motion condition of a space coordinate system according to the thermodynamic and hydraulic characteristics of three ocean conditions of tilting, swinging and ascending and diving, and coupling the two models to obtain the critical heat flow density of the rod bundle channel under the motion condition. Chinese patent application number CN120470980 discloses a supercooling boiling critical value simulation method, apparatus, medium and device. The method is used for coupling the wall surface heat flux density model and the non-inertial mixture model to obtain a two-phase boiling heat exchange and boiling critical calculation model. And solving the bubble parameters in the boiling submodel through the stress balance of the single bubble, and taking the wall surface dry area solved by assuming that the bubble obeys po