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。     

液膜蒸干模型在液态金属CHF预测中的应用

为对圆管中环状流烧干型沸腾临界进行预测,建立了考虑液滴沉积夹带作用的液膜蒸干模型。沉积率、夹带率等相关关系式采用水等常规流体的已有关系式。在一定参数范围内,将模型预测结果与实验数据和经验关系式进行了比较。结果表明：基于常规流体的液膜蒸干模型大体可应用于液态金属,但在临界热流密度（CHF）较大时可能造成较大偏差;钠、钾两种液态金属在CHF较低时预测准确度区别不大,但在CHF较高时区别较为明显。为更加准确预测液态金属CHF,应开发专门的沉积率、夹带率等相关关系式。     

竖直环形狭缝通道内环状流的预测模型

对竖直环形狭缝通道内环状流流动沸腾传热理论模型进行了分析，以液膜质量、动量和能量守恒方程为基础，结合汽芯动量方程建立了竖直环形狭缝通道内环状流的数学物理模型。对该模型进行数值求解，得出了液膜厚度、液膜内的速度分布和温度分布、内—外管的换热系数以及通道内压降值，并与实验值进行了比较。     

环形窄缝通道内干涸型临界热流密度的理论研究

在双面加热的垂直环形窄缝通道内,对向上流动环状流的临界热流密度(CHF)进行理论研究,以质量、动量和能量守恒方程为基础建立数学物理模型。对该模型进行数值求解,得到了不同窄缝间隙通道内的CHF和临界含汽率的关系曲线,分析得出压力对CHF的影响,并将理论计算值与实验值进行比较。     

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。     

窄矩形通道自然循环流动停滞与临界热流密度研究

在华北电力大学自然循环实验室进行了自然循环条件下窄矩形通道内的临界热流密度(CHF)实验,对实验中出现的流动停滞及传热恶化现象进行了观察。提出自然循环饱和沸腾条件下,窄矩形通道内的流动停滞-传热恶化发生机理。即自然循环流量漂移发生后会产生流型变迁不稳定,继而造成流量的持续波动,并导致停滞现象,从而使出口附近的液膜层在一定的热流密度下被完全蒸发并引起CHF现象。而窄矩形通道内,由于受间隙尺寸的限制,蒸汽流对加热面上的液膜层产生挤压作用,加热面上液膜层厚度因此会变得较薄,在较小的加热量下便能发生传热恶化。基于机理分析,给出了相应的计算模型。引入了考虑窄通道间隙尺寸效应的无量纲约束数Nconf和反映自然循环流动特点的特征因子C,分别对模型进行了修正。根据实验结果,对计算模型进行了多元回归拟合,并对其准确性进行了验证。通过对实验结果与模型计算值的比较发现,随着通道入口流速和系统压力的增大,CHF均增大;而随着出口干度的增大,CHF会减小。     

基于两相CFD方法的竖直圆管环状流预测研究

基于欧拉-拉格朗日方法,结合壁面液膜模型,模拟圆管环状流中的液滴与连续气相的相互作用、液滴在壁面处的沉积与夹带、液膜的沸腾与蒸发等关键物理现象。通过与瑞典皇家理工学院的环状流实验结果比较,评价欧拉-拉格朗日方法对环状流的预测精度。比较结果表明,数值模拟得到的液膜质量流量与实验测量结果吻合良好,证明所采用欧拉-拉格朗日方法及本构模型能够较准确地模拟环状流动。      

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。     

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。     

RELAP5对自然循环下临界热流密度的数值分析

临界热流密度(CHF)是导致沸腾传热变化而使发热元件表面发生传热恶化的现象。RELAP5等系统程序的CHF模型对于传热系统的安全分析有重要影响。基于RELAP5程序对单棒及三棒束自然循环CHF实验进行建模,并在CHF实验数据基础上对RELAP5中CHF预测值进行对比分析。实验装置是带有一个向上流动通道的自然循环回路。其中单棒束加热测试段由一根轴向非均匀加热的电加热棒及圆管外壁组成的环管状流道,三棒束流道由三根相同的轴向非均匀加热棒与三叶型的外管组成。实验条件为低压、低流量的自然循环流动:入口压力110～270 kPa、入口过冷度为10～70 K、自然循环流量0～400 kg/(m2·s)。依次以质量流量、入口压力和过冷度为基准参数对比分析实验值和RELAP5预测值。结果表明在低压、低流量及自然循环条件下,RELAP5中的CHF预测值随着质量流量的增大而增大,与入口压力及过冷度之间的依赖关系不明显。通过对实验值与模型计算值的比较得到,单棒束RELAP5模型预测的CHF值偏高于实验值,而三棒束RELAP5模型的预测值较实验值偏低。     

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%.     

An utilization of liquid sublayer dryout mechanism in predicting critical heat flux under low pressure and low velocity conditions in round tubes

From a theoretical assessment of extensive critical heat flux (CHF) data under low pressure and low velocity (LPLV) conditions, it was found out that lots of CHF data would not be well predicted by a normal annular film dryout (AFD) mechanism, although their flow patterns were identified as annular–mist flow. To predict these CHF data, a liquid sublayer dryout (LSD) mechanism has been newly utilized in developing the mechanistic CHF model based on each identified CHF mechanism. This mechanism postulates that the CHF occurrence is caused by dryout of the thin liquid sublayer resulting from the annular film separation or breaking down due to nucleate boiling in annular film or hydrodynamic fluctuation. In principle, this mechanism well supports the experimental evidence of residual film flow rate at the CHF location, which can not be explained by the AFD mechanism. For a comparative assessment of each mechanism, the CHF model based on the LSD mechanism is developed together with that based on the AFD mechanism. The validation of these models is performed on the 1406 CHF data points ranging over P=0.1–2 MPa, G=4–499 kg m⁻² s⁻¹, L/D=4–402. This model validation shows that 1055 and 231 CHF data are predicted within ±30 error bound by the LSD mechanism and the AFD mechanism, respectively. However, some CHF data whose critical qualities are <0.4 or whose tube length-to-diameter ratios are <70 are considerably overestimated by the CHF model based on the LSD mechanism. These overestimations seem to be caused by an inadequate CHF mechanism classification and an insufficient consideration of the flow instability effect on CHF. Further studies for a new classification criterion screening the CHF data affected by flow instabilities as well as a new bubble detachment model for LPLV conditions, are needed to improve the model accuracy.     

曲率对竖直向上环形狭窄通道内环状流特性的影响

基于三流体分离流模型,以液膜质量、动量和能量守恒方程为基础,结合汽芯动量方程,对双面加热垂直向上流动环形狭窄通道内环状流特性进行数值模拟。将计算结果与实验结果相比较,两者符合较好。通过数值模拟,分析了曲率对环状流特性的影响,得到了曲率对液膜厚度、液膜内温度、液膜内速度、临界热流密度等的影响曲线。曲率越大,内液膜越薄,而外液膜越厚。内管干涸时,临界热流密度随曲率的减小而增大;外管干涸时,则反之。     

垂直向上流动通道内环状流干涸点的理论研究

本文基于分离流模型,建立了垂直向上流动环形通道内环状流的三流体模型,并对干涸点进行了数值模拟.比较计算结果和实验结果,发现两者符合较好.结果显示:当干涸点发生在内管并且外管热流密度不变时,临界含汽率随曲率和间隙的减小而增大,当干涸点发生在外管且内管热流密度不变时,情况相反;对于固定的间隙,当外管内径大于20 mm时,或间隙小于0.5 mm时,压力和质量流速对临界含汽率的影响非常微弱.     

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     

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.     

Applicability of Annular Flow Model to Counter-Current Flow in Debris Beds

Counter-current flow limitation (CCFL) is dominant phenomena for dryout in a debris bed which may be formed during a severe accident as observed in the Three-Mile Island unit-2. Actual CCFL situation in debris bed is very complex. It is difficult to treat the CCFL in the debris bed as it is. On the other hand, an annular flow model was developed to predict CCFL in a pipe by assuming a two-dimensional turbulent flow. If hypothetical flow channel were assumed for CCFL in the debris bed, CCFL in the debris bed could be treated with the same manner as for CCFL in a pipe. 'The purpose of this study is to investigate whether the annular flow model developed for CCFL in a pipe is applicable for CCFL in the debris bed or not. As the results, it is clarified that qualitative tendency of the CCFL in the debris bed consisting of larger particles than 3 mm is estimated by the annular flow model developed for CCFL in a pipe, although the difference between the calculation and the data is large in higher and lower gas velocity. It is also clarified that wall friction factor calculated with the present analysis is twice to forth larger than that in the single phase flow through porous media.     

Development of a Transient Boiling Transition Analysis Method Based on a Film Flow Model

A new single-channel, transient boiling transition (BT) prediction method based on a film flow model has been developed for a core thermal-hydraulic code. This method could predict onset and location of dryout and rewetting under transient conditions mechanically based on the dryout criterion and with consideration of the spacer effect. The developed method was applied to analysis of steady-state and transient BT experiments using BWR fuel bundle mockups for verification. Comparisons between calculated results and experimental data showed that the developed method tended to predict occurrence of rewetting earlier, however, onset time of BT and maximum rod surface temperature were well predicted within 0.6 s and 20°C, respectively. Moreover, it was confirmed that consideration of the spacer effect on liquid film flow rate on the rod surface was required to predict dryout phenomena accurately under transient conditions.     

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.