泡核沸腾两相CFD模拟的参数敏感性分析与模型验证

预测偏离泡核沸腾(DNB)型的临界热流密度(CHF)是压水堆热工水力分析的重要内容。基于计算流体力学(CFD)方法预测CHF需要准确预测空泡份额在截面上(尤其是壁面附近)的分布。本文使用商用CFD程序STAR-CCM+对泡核沸腾状态下DEBORA竖直上升流均匀加热圆管实验进行模拟。经过敏感性分析,找出对空泡份额、气体速度、液体温度和气泡直径四个物理量的径向分布以及轴向壁面温度分布有显著影响的模型参数。基于一组实验数据,通过调整关键模型参数重新标定了相间作用模型,并将标定过的计算模型应用到其他工况验证其适用性,得到了较好的结果。本研究为后续将两相CFD计算应用于DNB型CHF的预测打下了基础。     

摇摆条件下竖直圆管内干涸型临界沸腾的数值研究

针对摇摆条件下竖直圆管内干涸型临界热流密度（Dryout CHF）进行了三维数值计算,研究了摇摆条件下竖直圆管内相态分布特性、圆管内临界热流密度（CHF）的位置以及最高壁面温度,同时对管壁沿程换热系数特性进行了分析。结果表明:在摇摆条件下,圆管内相分布呈现周期性变化,CHF的位置也会发生周期性变化;同时发现摇摆运动会导致壁面最高温度更高,因此摇摆条件会使沸腾临界现象更严重。随着流型转变和沸腾传热机制的变化,管壁换热系数沿流动方向也会显著变化。本研究可以为摇摆条件下Dryout CHF的数值预测提供参考。      

竖直窄缝通道内轴向功率阶跃分布下临界热流密度理论分析

采用竖直窄缝通道内功率阶跃分布下的临界试验结果对非均匀加热临界预测模型进行验证。模型所预测临界触发位置、临界热流密度(CHF)及临界对应平均热流密度与试验结果的偏差均在合理范围内。采用该非均匀加热临界预测模型开展功率阶跃下CHF的数值计算,分析阶跃功率比、阶跃长度以及阶跃位置对沸腾临界的影响:随阶跃功率比的增大,CHF基本呈线性增大,但临界触发时的平均热流密度逐渐降低;阶跃长度的增大使得CHF逐渐减小,同时触发时的平均热流密度逐渐逼近均匀加热下的CHF值;随临界触发区域向流道上游迁移,临界触发位置将脱离功率阶跃区域的后端迁移至流道出口,此时的CHF值及临界触发时的平均热流密度值均趋近于均匀加热下的CHF值。     

基于汽泡壅塞流模型的竖直圆管临界热流密度理论研究

为研究单管壅塞流的临界热流密度（CHF）现象，建立了基于近壁处汽泡壅塞机理的CHF计算模型。模型通过求解相应的质量、动量和能量方程，再结合汽泡直径脱离模型、壁面临界空泡份额等模型，从而计算得到CHF。将模型计算结果同实验值比较，吻合良好，验证了模型的正确性。在此基础上，以建立的CHF模型为基础，研究了进口焓差、质量流速、管径和加热长度对CHF的影响，为预测壅塞流CHF提供依据。      

基于CFD方法的棒束通道内临界热流密度预测

为研究棒束通道内临界热流密度现象，采用基于对气、液两相分别建立基本守恒方程的欧拉两流体六方程模型和改进的壁面热流密度分配模型，利用CFD商用软件FLUENT 14.5对捷克大型水介质实验回路上开展的临界热流密度（CHF）实验进行数值模拟。通过计算获得CHF发生前、后计算域内重要热工水力参数的分布及CHF发生值，将CFD计算获得的CHF与实验测得值进行对比，结果表明，大多数工况的偏差在±30%以内，证明了欧拉两流体模型结合改进的壁面热流密度分配模型对CHF预测的准确性。本研究可为复杂结构的CHF预测提供依据。     

沸腾传热数值模拟方法及多管耦合应用特性研究

针对直流蒸汽发生器（OTSG）中全流型沸腾传热及一、二次侧耦合换热等复杂物理现象,计算流体动力学（CFD）数值分析普遍面临计算难度大、计算效率低及不确定性大等问题。基于欧拉两流体多相流模型与临界热流密度（CHF）壁面沸腾模型,建立了管内全流型流动沸腾传热数值分析模型,并验证了模型的有效性。基于所验证的模型,开展了数值模型在多管耦合传热下的应用特性研究,明确了该数值模拟方法在多管耦合下的可靠性,并对温度与相分布计算结果对相间作用力模型的敏感性进行了数值分析。研究结果表明：基于欧拉两流体多相流模型与CHF壁面沸腾模型,能够较准确地预测管内水介质由过冷到过热的全流型流动沸腾传热过程,计算的“干涸”点位置及壁面峰值温度与实验值符合较好,最大误差小于10%;基于欧拉两流体多相流模型与CHF壁面沸腾模型的数值方法对多管耦合工况有较好的适用性,计算的二次侧温度与实验结果吻合良好;两相间曳力对壁面温度及空泡份额的计算结果有较明显的影响,但非曳力对壁面温度的影响较小,因此对于大规模工程应用计算,可在分析中不考虑部分相间非曳力的影响。本文研究结果可为OSTG的三维精细化数值分析的模型选择提供有益参考。     

花瓣形燃料元件棒束通道内过冷流动沸腾特性数值研究

花瓣形燃料元件具有换热性能强和无需定位格架等优点，能进一步提高反应堆的功率密度和经济性。为此，本文利用欧拉两流体模型，同时结合RPI壁面沸腾模型，对2×2花瓣形燃料元件棒束通道内过冷流动沸腾特性开展数值研究。通过圆管过冷沸腾实验数据验证了模型的准确性。开展了流速和热流密度参数对花瓣形燃料元件棒束通道内流动、换热及空泡份额分布影响的数值研究。结果表明，通道内冷却剂的流动速度分布不均匀；横向流动沿主流方向存在波动；空泡份额在燃料元件的内凹弧与外凸弧处表现出较大差异；同时，由于流场和换热形式的不同，导致燃料元件的周向壁面温度呈现不均匀分布，横向流动的存在影响着壁面热流分配情况。     

轴向非均匀加热DNB型临界热流密度理论预测

基于微液层蒸干的临界触发机理,构建非均匀加热下的偏离核态沸腾(DNB)型临界预测模型。模型通过对临界触发点上游的非均匀热流分布进行沿程积分来确定临界位置附近的局部热工水力特性,通过临界点当地热流密度作用下汽块覆盖的微液层是否蒸干判断临界的发生,结合轴向功率分布曲线在流道沿程进行搜索来获取临界发生位置,最终实现了对轴向非均匀加热下DNB型临界热流密度的理论预测。分别采用入口峰、中间峰及出口峰功率曲线下的临界实验结果对该模型及功率因子修正法的预测能力进行对比验证。相对于功率因子修正法,所构建非均匀加热临界模型在3种不同功率曲线分布下均展示了较为准确的临界值和临界位置预测能力,其预测结果要优于功率因子修正法。     

竖直圆管内低压过冷沸腾相分布特性实验研究

实验采用双探头光学探针对内径24 mm竖直圆管内低压过冷沸腾局部空泡份额、界面面积浓度及汽泡尺寸等局部相界面参数径向分布特性进行了研究。实验结果表明：竖直圆管内过冷沸腾相分布形态呈现轴对称特性,随着热流密度的增大,相分布形态出现近壁峰值并逐渐向中间峰值分布形态的发展,较高热流密度工况下出现轴心峰值分布;随着质量流速的增加,局部空泡份额减小,并出现中间峰值向近壁峰值分布形态的转变;随着压力的增大,局部相界面参数减小。     

水平圆管临界热流密度实验研究

对水平圆管内低质量流速临界热流密度(CHF)进行了实验研究和分析。实验研究发现,水平流动圆管沸腾临界发生在圆管加热壁面顶部。通过对沸腾临界发生时圆管出口的质量含汽率和流型进行分析发现,本文研究的参数范围内沸腾临界时的出口含汽率高,流型为环状流,沸腾临界类型为干涸型(Dryout)。将经验公式预测值与实验结果进行比较发现,Bowring公式和Lookup table的预测值远大于CHF的实验值。导致此现象出现的主要原因为:Bowring公式和Lookup table是基于竖直流动CHF实验数据开发的模型,水平流动时在重力的作用下环状流液膜呈非均匀分布,顶部液膜干涸提前触发沸腾临界造成CHF值降低。     

Critical Heat Flux Prediction Method Based on Two-Phase Turbulence Model

In this paper, we present an analytical methodology to predict forced convective CHF (Critical Heat Flux) for DNB (Departure from Nucleate Boiling) type boiling transition that occurs inside of uniformly heated round tubes. Axial directional two-phase flow analysis was conducted based on one-dimensional two-fluid model and typical constitutive models. At the same time, the radial directional distribution of void fraction at any axial location was calculated based on the bubble diffusion model, which was coupled with two-phase turbulence model for boiling bubbly flow. The calculated void fraction showed the wall peak distribution, and was compared with experimental data, which was derived from subcool boiling experiments. IPNVG (Incipient Point of Net Vapor Generation), which means the starting point of two-phase flow analysis, was also investigated well, since it was revealed that IPNVG had a significant influence on CHF prediction. By using this methodology for calculating radial directional void fraction distribution, we carried out CHF prediction for water on the assumption that DNB would occur when the local void fraction near the heated wall exceeds a critical value. The predicted CHF agreed well with experimental data, and the accuracy was within about 20%.     

Development of a bundle correction method and its application to predicting CHF in rod bundles

A bundle correction method, based on the conservation laws of mass, energy, and momentum in an open subchannel, is proposed for the prediction of the critical heat flux (CHF) in rod bundles from round tube CHF correlations without detailed subchannel analysis. It takes into account the effects of the enthalpy and mass velocity distributions at subchannel level using the first derivatives of CHF with respect to the independent parameters. Three different CHF correlations for tubes (Groeneveld's CHF table, Katto correlation, and Biasi correlation) have been examined with uniformly heated bundle CHF data collected from various sources. A limited number of CHF data from a non-uniformly heated rod bundle are also evaluated with the aid of Tong's F-factor. The proposed method shows satisfactory CHF predictions for rod bundles both uniform and non-uniform power distributions.     

The Corrected Simulation Method of Critical Heat Flux Prediction for Water-Cooled Divertor Based on Euler Homogeneous Model

An accurate critical heat flux(CHF) prediction method is the key factor for realizing the steady-state operation of a water-cooled divertor that works under one-sided high heating flux conditions.An improved CHF prediction method based on Euler's homogeneous model for flow boiling combined with realizable k-ε model for single-phase flow is adopted in this paper in which time relaxation coefficients are corrected by the Hertz-Knudsen formula in order to improve the calculation accuracy of vapor-liquid conversion efficiency under high heating flux conditions.Moreover,local large differences of liquid physical properties due to the extreme nonuniform heating flux on cooling wall along the circumference direction are revised by formula IAPWSIF97.Therefore,this method can improve the calculation accuracy of heat and mass transfer between liquid phase and vapor phase in a CHF prediction simulation of water-cooled divertors under the one-sided high heating condition.An experimental example is simulated based on the improved and the uncorrected methods.The simulation results,such as temperature,void fraction and heat transfer coefficient,are analyzed to achieve the CHF prediction.The results show that the maximum error of CHF based on the improved method is 23.7%,while that of CHF based on uncorrected method is up to 188%,as compared with the experiment results of Ref.[12].Finally,this method is verified by comparison with the experimental data obtained by International Thermonuclear Experimental Reactor(ITER),with a maximum error of 6% only.This method provides an efficient tool for the CHF prediction of water-cooled divertors.     

An experimental study on the critical heat flux for low flow of water in a non-uniformly heated vertical rod bundle over a wide range of pressure conditions

An experimental study of the critical heat flux (CHF) has been performed for a water flow in a non-uniformly heated vertical 3 × 3 rod bundle under low flow and a wide range of pressure conditions. The experiment was especially focused on the parametric trends of the CHF and the applicability of the conventional CHF correlations to a return-to-power conditions of a main steam line break accident whose conditions might be a low mass flux, intermediate pressure, and a high inlet subcooling. The effects of the mass flux and pressure on the CHF are relatively large and complicated in the low pressure conditions. At a high mass flux or a low critical quality, the local heat flux at the CHF location sharply decreases with an increasing local critical quality. However, at a low mass flux or a high critical quality, the local heat flux at the CHF location shows a nearly constant value regardless of the increase of the critical quality. The CHF data at the very low mass flux conditions are correlated well by the churn-to-annular flow transition criterion or the flow reversal phenomena. Several conventional CHF correlations predict the present return-to-power CHF data with reasonable accuracies. However, the prediction capabilities become worse in a very low mass flux of below about 100 kg/(m² s).     

A critical heat flux model based on mass, energy, and momentum balance for upflow boiling at low qualities

A mechanistic modeling of critical heat flux (CHF) in upflow boiling at low qualities is performed. The developed model is based on a physical criterion of CHF occurrence and a mechanism limiting the thermal transport between a stagnant bubbly layer and bulk stream. The mechanism can be mathematically formulated by coupling the equation of limiting mixing mass flux, which is derived from momentum balance equations in two regions, with local mass and energy balance equations on the bubbly layer. The resulting form of the model is represented by a general and straightforward CHF formula involving two empirical constants related to the void fraction and the thickness of the bubbly layer. The predictions agree well with the extensive CHF data of water in uniformly heated tubes.     

Critical heat flux under zero flow conditions in vertical annulus with uniformly and non-uniformly heated sections

The experimental study of water CHF (critical heat flux) under zero flow conditions has been carried out in an annulus flow channel with uniformly and non-uniformly heated sections over a pressure range of 0.52–14.96 MPa. In the present boiling system, the CHFs occur in the upper region of the heated section, in contrast to the results in the experiments for boiling tubes conducted by several investigators. The general trend of the CHF with pressure is that the CHF increases up to a medium pressure of about 6–8 MPa and decreases as the pressure is further increased. A comparison of the present data with the existing flooding CHF correlations shows that the correlations depend greatly on the effect of the heat flux distribution. When the correction terms with the density ratio and the effect of the heat flux distribution proposed in the present work are used with the CHF correlation based on the Wallis flooding correlation, it predicts the measured flooding CHF within an RMS error of 9.0%.     

神经网络在CHF预测中的应用

利用人工神经网络理论对均匀加热垂直上升圆管内的临界热流密度（ＣＨＦ）进行预测和参数趋势分析。本研究采用局部条件假设,并选用Ｃｒｏｅｎｅｖｌｄ的ＣＨＦ查询表数据为本文神经网络训练的样本,采用训练成功的网络预测ＣＨＦ值可以得到比常规方法更好的效果,其均方差为１４．９％。     

含弯曲棒的全长燃料棒束沸腾传热CFD计算分析

采用计算流体动力学（CFD）分析方法模拟了含一根弯曲燃料棒（简称“弯曲棒”）的5×5全长燃料棒束内的沸腾传热现象。基于欧拉两流体模型和改进的壁面沸腾模型进行计算,并基于压水堆子通道和棒束实验( PSBT )基准题中的试验数据对计算方法进行了验证,计算所得截面平均空泡份额与试验数据吻合良好,说明了现有计算方法的可靠性。基于计算结果考察了弯曲棒对棒束通道内流场、温度场、空泡份额等关键参数的影响。研究结果表明,弯曲棒的存在对截面横向流动、流体温度、空泡份额等均未产生显著影响,但弯曲棒表面温度增加,气泡也易发生聚集,增加了发生临界热流密度（CHF）的风险。      

CHF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

In this study, the CHF enhancement using various mixing vanes is evaluated and the flow characteristics are investigated through the CHF experiments and CFD analysis.CHF tests were performed using 2 × 2 and 2 × 3 rod bundles and with R-134a as the working fluid. The test section geometry was identical to that of commercial PWR fuel assembly not including the heated length (1.125 m) and number of fuel rods. From the CHF tests, it was found that the CHF enhancement using mixing vanes under higher mass flux (1400 kg/m² s) and lower pressure (15 bar) conditions is larger than the CHF enhancements under other conditions. Among the mixing vanes used in this study, the swirl vane showed the best performance under relatively low pressure (15 bar) and mass flux (300-1000 kg/m² s) conditions and the hybrid vane performed best near the PWR operating conditions.The detailed flow characteristics were also investigated by CFD analysis using the same conditions as the CHF tests. To calculate the subcooled boiling flow, the wall partitioning model was applied to the wall boundary and various two-phase parameters were also considered. The reliability of the CFD analysis in the boiling analysis was confirmed by comparing the average void fractions of the analysis and the experiments: the results agreed well. From the CFD analysis, the void fraction flattening as a result of the lateral velocity induced by the mixing vane was observed. By the lateral motion of the liquid, the void fraction in the near wall was decreased and that of the core region was increased resulting in the void fraction flattening. The decrease of the void fraction in the near wall region promoted liquid supply to the wall and consequently the CHF increased. For the quantification of the void flatness, an index was developed and the applicability of the index in the CHF assessment was confirmed.     

CFD analysis on subcooled boiling phenomena in PWR coolant channel

Eulerian two-fluid model coupled with wall boiling model was employed to calculate the three dimensional flow field and heat transfer characteristics in a hot channel with vaned spacer grid in PWR. The heat transfer from pellet-gap-cladding to coolant was also taken into account by a system coupled code MpCCI. The wall boiling model utilized in this study was validated by Bartolomei experiment data, and a good agreement can be observed. By solving the governing equation in a two-way coupled method, the distribution of temperature in the pellet-gap-cladding region and the distribution of temperature, void fraction and velocity of two-phase flow in coolant channel can be obtained. The influences of spacer grid and mixing vane on the thermal-hydraulic characteristics were analyzed. The heat transfer capacity was strongly improved by the spacer grid and mixing vane, while the flow resistance was also enlarged. Localized volume fraction of vapor phase decreased due to mixing vane, which will decrease the possibility of the departure from nucleate boiling (DNB) and increase the critical heat flux (CHF). By analyzing the temperature and void fraction at cladding outer surface, the critical regions where hot spot may occur were determined.