矩形窄通道内DRYOUT模型研究

液膜干涸（DRYOUT）被广泛认为是诱发环状流区临界热流密度（CHF）的机理,已有DRYOUT模型对于矩形窄通道能否适用缺乏实验验证。本文通过比较几组不同的沉积率、夹带率关系式,得到了一优化的DRYOUT模型。计算结果表明：本文的模型较已有模型具有更高的精度,可用于矩形窄通道CHF的预测计算。     

基于液膜蒸干模型的低压低流量环状流CHF理论研究

为对低压低流量下的环状流临界热流密度（CHF）进行预测,建立了考虑液膜蒸发、液滴沉积和夹带的液膜蒸干模型,并用已有的实验数据对其进行验证。计算结果表明：在实验参数范围内,CHF计算值与实验值相对偏差在25%以内,两者符合较好。以建立的环状流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。该模型能够有效地计算低压低流量环状流CHF和分析CHF随不同参数的变化趋势。     

垂直向上圆管环状流临界热流密度研究

预测高含汽量下的临界热流密度对于直流蒸汽发生器和事故工况下反应堆堆芯的安全性具有工程价值.本文基于液滴夹带、沉积和液膜蒸发理论,对垂直向上的均匀加热圆管内环状流的液膜厚度和液膜质量流速沿轴向的变化进行了预测.结果表明,当液膜蒸干时,干涸发生,此时的热流密度即为临界热流密度.将理论计算的临界热流密度值与实测值相比较,实验数据偏高,偏差在30%以内.     

基于微液层理论的下封头外部流体CHF理论计算

在微液层(蒸干)模型的基础上,利用微液层中气-液相的守恒关系推导得到初始两相流边界层厚度,同时选取最大的夹带系数时的空泡份额,对下封头在饱和池式沸腾下的临界热流密度(CHF)进行理论预测.结果表明,两相边界层厚度开始发生变化时的角度会随半径的增加而增加；CHF的大小会随角度的增加而增加,最大值出现在90°处.通过CHF...     

高压工况下管内垂直向上流动沸腾CHF机理模型研究

针对高压工况下偏离泡核沸腾（DNB）型临界热流密度（CHF）的特点,重新构建了Weisman & Pei模型的本构关系式;针对高压工况下干涸（Dry-out）型CHF,比较分析了Kataoka、Celata以及Hewitt?3个沉积率和夹带率计算关系式的结果。基于以上两类改进的CHF模型,建立了一个适用于高压工况的、结合DNB型和Dry-out型沸腾临界机理的CHF模型。采用高压工况下管内垂直向上流动沸腾CHF实验数据对建立的CHF机理模型进行了验证,分析了热工参数和几何参数的趋势效应。      

液钠沸腾两相流流型与临界热流密度(CHF)机理实验研究

通过大量的液态金属钠临界热流密度 (CHF)的实验研究 ,结合液钠两相传热流动特性及液钠的物性特点 ,分析了起始沸腾流型 ,泡状流 ,块状流 ,环状流和双向环状流的热工水力特性 ;并从实验结果出发 ,深入分析了液钠发生临界热流密度时的气泡爆炸和液膜撕裂或局部蒸干的两种传热恶化机理     

自然循环临界热流密度实验现象及分析

对自然循环系统矩形通道内临界热流密度进行实验研究。研究发现:实验本体增加功率后,环状流液膜蒸干,壁温出现持续快速上升,实验本体出口发生沸腾临界。根据壁温的上升趋势和出口处流体的临界含汽率可以判断自然循环系统出现的临界热流密度(CHF)类型为干涸(Dryout)型。当自然循环系统沸腾临界出现时,自然循环流量出现明显的上升。根据理论分析可知:沸腾临界发生时导致自然循环流量上升的主要原因是环状流转变成弥散流,附在加热壁面的液膜消失,摩擦压降迅速减小。     

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。     

压水堆堆芯临界热流密度的预测方法综述

针对压水堆堆芯临界热流密度(CHF)预测这一重要科学问题,分析了棒束中存在的格架效应、冷壁效应和非均匀加热效应对CHF的影响,对比了基于不同假设的6类CHF机理模型,给出了棒束CHF关系式的开发途径,并得出了CHF机理模型和经验关系式在棒束CHF预测上的优缺点。建议进一步充实已有的CHF实验数据库,优化CHF关系式的开发方法,并积极开发棒束CHF机理模型。     

双面均匀加热矩形窄缝通道内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,构建了偏离泡核沸腾(DNB)型的临界预测模型。通过对临界点处的汽块进行受力平衡分析,基于优化后的侧面提升力系数CL,确定了汽块滑移速度、微液层液膜厚度等参数,实现对均匀加热下DNB型临界热流密度的理论预测。采用2种加热长度下矩形窄缝通道内的临界热流密度实验数据对该模型进行了验证。结果表明:模型所预测矩形窄缝通道内的临界热流密度值与实验结果的偏差均在±15%之内,该模型的预测精度高于Bowring公式和Bettis公式的预测精度。     

Prediction of the critical heat flux in annular regime in various vertical channels

The axial variation of film flow rate in annular regime was analyzed to predict the critical heat flux in water saturated flow boiling in various vertical channels. In the analyses, several quantities including the deposition and entrainment rates of droplets were evaluated with the correlations that were based on the separate experimental data of film flow rate and droplet flow rate in annular flow; model closure was achieved without introducing arbitrary constants. The predicted results were compared with extensive data of critical heat flux in flow boiling. Fairly good agreements were observed for the critical heat fluxes in round tubes with uniform and axially non-uniform heating and in non-circular shaped channels without sharp corners. It is hence expected that the important processes for the onset of critical heat flux condition in annular regime were satisfactorily described in the model. However, the critical heat fluxes were generally overestimated in thin rectangular channels with sharp corners. The non-uniform distribution of liquid film in channel cross-section would be the primary reason of the disagreement.     

An investigation of flow characteristics and critical heat flux in vertical upward round tube

Prediction of critical heat flux （CHF） in annular flow is important for the safety of once - through steam generator and the reactor core under accident conditions. The dryout in annular flow occurs at the point where the film is depleted due to entrainment, deposition, and evaporation. The film thickness, film mass flow rate along axial distribution, and CHF are calculated in vertical upward round tube on the basis of a separated flow modcl of annular flow. The theoretical CHF values are higher than those derived from experimental data, with error being within 30%.     

Film splitting model with entrainment

Critical heat flux (CHF) is one of the major parameters for calculating safety margin of nuclear power plant systems based on US-Nuclear Regulatory Commission's safety regulations (defined under 10CFR50-46). Exceeding the CHF point suddenly increases the fuel clad temperature so that it may result in damage to the nuclear fuel pins. Dryout is a special CHF physical phenomenon occurred in Boiling Water Reactors (BWRs). Firstly, the present work identifies several existing dryout correlations and notes the major assumptions for the derivation of them. It follows with the derivation of a film splitting model (originally proposed by Chun et al., in 2003) for annular geometry where the liquid film is broken into an inner and outer film. Film splitting can result in better prediction of the critical film thickness where dryout can occur due to local variations especially due to turbulence and other effects. Secondly, an alternate approach to solving the film splitting model of Chun et al. is employed to evaluate the model against the same experimental data. It was observed that liquid entrainment and deposition into and out of the film layer were the missing components in the model and the model predictions shown in Chun et al. (2003) so that the model predictions with alternative solution methods were compared against experimental data by excluding entrainment/de-entrainment phenomena. Thirdly, the film split model is updated by adding entrainment/de-entrainment models to make the film splitting model more realistic. Fourthly, new film splitting model is compared against both tubular and pipe data. It was observed that entrainment and deposition are significant contributors in the new mechanistic dryout model. The mass flux of the inner film is mostly overestimated while the mass flux of the outer film is overestimated for some of the experimental cases when compared to the original correlation.     

Phenomenological prediction of CHF under boiling water reactor (BWR) conditions

Critical Heat Flux (CHF) is an important parameter for the thermal design of any heat generating system, most importantly, nuclear reactors. Owing to the complex mechanisms of CHF there has been a large proliferation of the correlations, each having narrow range of validity, which shows that the empirical correlation is not an appropriate approach for the CHF prediction for a wide range of validity. This limitation has led to the development of the phenomenological approach of the CHF prediction. The film dryout mechanism is applicable to the high quality CHF corresponding to the annular flow pattern in which the progressive depletion of the liquid film leads to dryout. The basic concern in the prediction of dryout is the accuracy in the evaluation of the droplet deposition and entrainment. There are various models for the estimation of the entrainment and deposition of droplets. However, most of these models are based on the air-water data at the atmospheric conditions and hence their applicability to the BWR conditions needs to be confirmed. Some of the models are based on the steam-water data which needs to be validated for the dryout prediction under BWR conditions. In this paper, the film dryout modelling has been carried out for the prediction of CHF using appropriate models for entrainment fraction and deposition coefficient. The results have been compared with the CHF data generated to substantiate the appropriateness of the selected models under BWR conditions.     

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).     

Prediction of critical heat flux in vertical pipe flow

A previously developed semi-empirical model for adiabatic two-phase annular flow is extended to predict the critical heat flux (CHF) in a vertical pipe. The model exhibits a sharply declining curve of CHF versus steam quality (X) at low X, and is relatively independent of the heat flux distribution. In this region, vaporization of the liquid film controls. At high X, net deposition upon the liquid film becomes important and CHF versus X flattens considerably. In this zone, CHF is dependent upon the heat flux distribution. Model predictions are compared to test data and an empirical correlation. The agreement is generally good if one employs previously reported mass transfer coefficients.     

Development of the critical film thickness correlation for an advanced annular film mechanistic dryout model applicable to MARS code

It is generally assumed in the mechanistic film dryout model that the critical heat flux (CHF) arises when liquid film calculated from evaporation, droplet entrainment and deposition gets dryout. The dryout of film is usually assumed when film thickness becomes zero. However, it was indicated that the complete dryout assumption can estimate CHF well for uniform heating case but cannot simulate accurately for non-uniform heating case. The critical film thickness concept may be an appropriate approach physically because there is a possibility of instantaneous disappearance of liquid film when it gets very thin. Therefore, a critical dryout film thickness correlation was developed to properly model dryout phenomenon together with MARS code based on experimental data. The modified version of MARS implementing a newly developed critical dryout film thickness correlation was assessed using various dryout data including those of non-uniform heating case and flow reduction transient test. The prediction results showed improved agreement with the experimental data.     

Prediction of Liquid Film Dryout in Two-Phase Annular-Mist Flow in a Uniformly Heated Narrow Tube Development of Analytical Method under BWR Conditions

A method was developed based on the conservation lows to predict critical heat flux (CHF) causing liquid film dryout in two-phase annular-mist flow in a uniformly heated narrow tube under BWR conditions. The applicable range of the method is within the pressure of 3–9 MPa, mass flux of 500–2,000 kg/m²·s, heat flux of 0.33–2.0 MW/m² and boiling length-to-tube diameter ratio of 200–800.

The two-phase annular-mist flow was modeled with the three-fluid streams with liquid film, entrained droplets and gas flow. Governing equations of the method are mass continuity and energy conservation on the three-fluid streams. Constitutive equations on the mass transfer which consist of the entrainment fraction at equilibrium and the mass transfer coefficient were newly proposed in this study.

Confirmation of the present method were performed in comparison with the available film flow measurements and various CHF data from experiments in uniformly heated narrow tubes under high pressure steam- water conditions. In the heat flux range (q“<2MW/m²) practical for a BWR, agreement of the present method with CHF data was obtained as, (Averaged ratio)±(Standard deviation)=0.984±0.077, which was shown to be the same or better agreement than the widely-used CHF correlations.