在双面加热的窄缝环形流道中流动沸腾换热研究

在窄缝流道内发生沸腾换热现象时,由于沸腾产生的汽泡受窄缝流道的限制,受压变形而消除了汽泡表面张力对传热的影响。因此对此现象进行基础性理论研究具有很重要的意义。本文在常压下用蒸馏水对窄缝间隙为 0.75mm的垂直环形流道,进行了流动沸腾传热实验研究。实验段的有效加热长度为 900mm,其加热方式为内外侧双面加热,实验的流量变化范围在 1.67× 10－ 5～ 5.83× 10－ 5m3/s。通过实验得到了在不同质量流速和热流密度下双面加热的窄缝流道中内外侧沸腾换热系数随干度变化的分布和特点。研究结果表明,由于在窄缝流道中存在着大量的运动聚合受压变形汽泡,因此使内外侧沸腾换热系数都很高 (可达 105W· m－ 2· K－ 1以上 )。     

竖直矩形窄缝通道内过冷沸腾传热特性的实验研究

以去离子水为工质,在进口压力为0.1～0.3 MPa、质量流速为200～1400 kg·m^-2·s^-1、热流密度为20～320k W·m^-2的参数范围内,对截面参数为50mm×2mm的竖直矩形窄缝通道展开了传热实验研究。实验获得通道内部工质由单相状态到过冷沸腾状态的传热过程曲线,将过冷沸腾段实验值与8个经验公式提供的预测值进行了对比与分析,采用相似原理以及回归分析法,建立了适用于竖直矩形窄缝通道的过冷沸腾准则关系式。研究结果表明,在竖直矩形窄缝通道内,热流密度对过冷沸腾传热具有主导作用;对于本实验的窄缝通道,Bertsch传热公式对于过冷沸腾段的预测效果相较于其他公式更好,本研究所建立的准则关系式与实验数据符合良好。因此,本研究建立的公式能够用于竖直矩形窄缝通道过冷沸腾传热系数的预测。     

单棒通道流动振荡诱发沸腾临界可视化实验研究

为深入分析沸腾两相流动振荡诱发沸腾临界的影响特性,本文以去离子水为工质,横截面19 mm×19 mm、中心为外径9.5 mm的单棒通道为研究对象,通过在不同热工参数下开展沸腾两相流动特性可视化实验研究,结合汽泡行为和汽-液界面特性,分析流动振荡诱发沸腾临界的影响特性。研究结果表明,低压力、低质量流速和低入口过冷度下,极易出现流动振荡,并导致沸腾临界提前发生,此时的临界热流密度与稳定工况下相比明显偏低;随着壁面热流密度不断增加,流道中两相流型先后出现泡状流、弹状流、合并弹状流、搅混流、剧烈搅混流、不稳定环状流;当流动出现剧烈振荡时,流道存在回流;发生沸腾临界时流道压降波动最大,对应的流型为不稳定环状流。因此,单棒通道内流动振荡可能会导致沸腾临界提前发生。      

Direct Measurement of Neutron Flux

An experimental study was carried out on steady-state boiling of sodium flowing in the annular channel formed around an electrically heated simulation of a fuel pin. In the present experiment, the inlet temperature and flow rate were held constant, and the heat flux was gradually increased up to the inception of boiling. Thereafter, the heat flux was further increased step by step until the surface temperature of the heater pin marked a sharp rise, indicating the occurrence of dry-out. Records were obtained of the changes brought by the increasing heat flux to boiling phenomena, with particular reference to the behavior of the two-phase flow pattern and to the characteristics of boiling noise, as well as of the frequency of bubble formation.

It was made clear that there exists a region in which steady-state boiling will be established, and under these conditions the two-phase flow pattern changes sequentially from bubbly flow to slug flow and then to annular flow. This behavior of sodium boiling in a narrow channel is quite similar to the case of water.

With rising heat flux, the level of noise intensity associated with boiling first increased sharply to attain a maximum point, then decreased somewhat and remained constant thereafter until dry-out.

The frequency of bubble formation depended on the size of the bubble. The product of bubble frequency and equivalent diameter was found to be constant.     

两流体模型在环形窄缝内的应用研究

采用EPRI最新开发的Chexal-Harrison相壁相间摩擦模型和简化的相壁相间传热模型,构造了适用于环形窄缝内沸腾传热和流动的两流体模型,并编制了热工水力计算程序——THYME程序.与实验数据比较,分析了环形窄缝套管在不同负荷下Relap5/Mod3.2程序和本文程序的计算结果.计算结果表明,Relap5/Mod3.2低估了环形蒸发管的蒸汽温度,本文计算结果与实验数据较为一致.     

Modeling of debris cooling with annular gap in the lower RPV and verification based on ALPHA experiments

For severe accident assessment in a light water reactor, heat transfer models in a narrow annular gap between the overheated core debris and the reactor pressure vessel (RPV) are important for evaluating RPV integrity and emergency procedures. Using existing data, the authors developed heat transfer models on the average critical heat flux (CHF) restricted by countercurrent flow limitation (CCFL) and local boiling heat fluxes, and showed that the average CHF depended on the steam–water flow pattern in the narrow gap and that the local heat fluxes were similar to the pool boiling curve. We evaluated the validity of heat transfer models by simple calculations for ALPHA experiments performed at Japan Atomic Energy Research Institute. Calculated results showed that heat fluxes on the crust surface were restricted mainly by thermal resistance of the crust after the crust formation, and emissivity on the crust surface did not have much effect on the heat fluxes. The calculated vessel temperature during the heat-up process and peak vessel temperature agreed well with the measurements, which confirmed the validity of the average CHF correlation. However, the vessel cooling rate was underestimated mainly due to underestimation of the gap size.     

中低压条件下矩形窄缝通道两相流动传热试验研究

在中低压条件下,对矩形窄缝通道两相流动传热进行试验研究,分析两相流动传热的变化规律,拟合出饱和沸腾传热系数计算关系式,并采用简化的一维分析方法对两相压降进行分析计算。试验结果表明:在相同热平衡含汽率(x)情况下,两相流动压降随系统压力(p)的降低而增大,随系统流量的增大而增大的变化规律;p越低,两相流动压降随x的增加而增大越剧烈;流量越大,两相流动压降随x的增加而增大越剧烈。通过数据回归方法得到汽相湿周长比例因子F并拟合了计算关系式,其计算值与试验值符合得较好。矩形窄缝通道内饱和沸腾平均传热系数受p、质量流量及热流密度的影响较大。     

矩形窄通道内流动沸腾阻力实验与计算方法研究

基于换热器小型化的研究背景,对水在矩形窄通道内流动沸腾阻力特性进行了实验研究与分析,并利用实验结果对常规通道和窄通道的两相摩擦压降计算的6种方法进行了评价。结果表明,应用于常规通道的关系式已不适于窄通道中流动沸腾压降的计算,而基于窄通道的Zhang-Mishima及Sun-Mishima关系式预测结果与实验值符合较好。实验结果和理论分析表明,利用分相流方法得到的分液相摩擦因子计算式中Chisholm系数C与Martinelli参数X存在指数关系,且随着质量流速的变化也有所不同,据此给出了新的分液相摩擦因子的计算方法,新方法具有更高的计算精度。     

Evaluation of Cooling Capability in a Heated Narrow Flow Passage

A numerical method for the thermal hydraulic phenomena in a narrow flow passage is developed to evaluate the gap cooling capability. Based on a drift flux model, the two-dimensional gas-liquid two-phase flow in the annular and hemispherical heated narrow flow passages is modeled. The drift velocity correlation is combined with the flooding correlation, which describes physical phenomena under cooling limits. Experiment on thermal hydraulic phenomena in the heated narrow flow passage is performed. Boiling two-phase flow behavior and dryout phenomena are observed. The critical heat flux data is obtained from measurement of the heating surface temperature. Counter-current two-phase flow, which is a key phenomenon in the gap cooling mechanism, is reproduced by the numerical analysis appropriately. The critical heat flux is predicted by assuming that deficiency of the liquid supply against the gas upward flow leads to occurrence of dryout. Validity of the newly developed numerical method is demonstrated through comparison of the predicted critical heat flux with the present and existing data in the gap width range from 0.5 to 5 mm and the pressure range from 1 to 50 bar.     

环形狭缝通道内环状流模型的数值分析

对环形狭缝通道内的环状流建立了分离流模型。应用质量、动量和能量守恒方程 ,加上相应的边界条件和使方程组封闭的经验关系式 ,对环形狭缝通道的内、外液膜厚度、液膜内的速度分布和温度分布 ,以及内、外管的换热系数进行了数值计算求解     

One-dimensional critical heat flux concerning surface orientation and gap size effects

Tests were conducted to examine the critical heat flux (CHF) on a one-dimensional downward heating rectangular channel having a narrow gap by changing the orientation of the copper test heater assembly in a pool of saturated water under atmospheric pressure. The test parameters include both the gap sizes of 1, 2, 5 and 10 mm, and the surface orientation angles from the downward-facing position (180°) to the vertical position (90°), respectively. Also, the CHF experiments were performed for pool boiling with varying heater surface orientations in the unconfined space at atmospheric pressure using the rectangular test section. It was observed that the CHF generally decreases as the surface inclination angle increases and as the gap size decreases. In consistency with several studies reported in the literature, it was found that there exists a transition angle at which the CHF changes with a rapid slope. An engineering correlation is developed for the CHF during natural convective boiling in the inclined, confined rectangular channels with the aid of dimensional analysis. This correlation agrees with the experimental data of this study within ±20%.     

窄缝通道内过冷沸腾区域汽泡脱离频率研究

采用高速摄像仪对矩形窄缝通道内垂直上升流过冷流动沸腾区域汽泡脱离频率进行可视化实验研究。结果表明,汽泡脱离频率随质量流速的增大而减小,随入口过冷度的增大而减小,随热流密度的增大而增大。将实验数据与文献中汽泡脱离频率计算模型进行比较,发现基于池式沸腾和饱和流动沸腾开发的计算模型不能准确预测过冷沸腾区域汽泡脱离频率。本文以无量纲参数的形式,分别用液相雷诺数、过冷雅各布数和核态沸腾热流密度表示质量流速、主流过冷度和热流密度对汽泡脱离频率的影响,获得矩形窄缝通道内过冷沸腾区域汽泡脱离频率预测关系式,关系式的平均预测误差为±17.1%。     

Experimental research on dryout point of flow boiling in narrow annular channels

1 Introduction There are basically two classes of critical heat flux (CHF) situations: departure from nucleate boiling (DNB) and dryout (DO) [1]. DO is also sometimes known as burnout or departure from forced convective boiling in vapor-continuous flow. From the point of view of engineering, the CHF caused by the DO mechanism is of particular importance since boiling annular flow is one of the most common flow patterns in gas–liquid two-phase flow and occurs in a wide range of vapor qua…     

基于CFD方法的矩形窄缝通道内过冷流动沸腾模型研究

板状燃料元件中的矩形窄缝通道具有宽高比大的几何特征,高度方向速度梯度大、分布陡峭,发生过冷沸腾时,近壁面汽泡运动行为将受其影响而改变,其中汽泡滑移现象对沸腾换热影响较大。本文针对矩形窄缝通道中的汽泡滑移行为,构建了包含滑移热流的壁面热流分配模型,并建立机理性的汽泡受力模型和滑移模型计算汽泡脱离直径、浮升直径和滑移距离等辅助参数,开发了一套适用于矩形窄缝通道内向上流动沸腾的壁面沸腾模型。选用Nuthel窄缝通道沸腾实验进行数值模拟验证,结果表明：本文模型可以较好地预测1～4 MPa中低压工况窄缝通道向上流动沸腾的壁面过热度,最大误差相比RPI模型由80%降低至17%;蒸发热流份额和近壁面空泡份额相比RPI模型更低。     

环形狭缝膜态沸腾弥散两相流动特性研究

在 3种不同间隙的同心竖直环形狭缝膜态沸腾试验段中 ,研究了以去离子水为工质的两相弥散流摩擦压降 ,比较了环形狭缝和圆形通道中两相流动摩擦压降的不同 ,还研究了环形间隙对摩擦压降的影响。结果表明 :不同间隙下的环形狭缝内膜态沸腾弥散流摩擦压降可用修正的Sadatomi方法计算 ,该方法可应用于工程上的环形狭缝膜态弥散流动     

基于流固共轭传热的两相流动数值模拟及燃料子通道CHF预测研究

为对过冷沸腾两相流动进行准确模拟,并探索临界热流密度（CHF）预测方法,本文基于共轭传热和两相CFD分析的方法,通过流固界面耦合,建立流固共轭传热两相流动耦合求解的数值模型。首先通过典型燃料棒栅元过冷沸腾两相流动的模拟,验证数值模型的正确性。随后对燃料子通道内两相流动进行模拟,并在两相流动模拟的基础上,通过准瞬态的方法,建立与CHF试验过程非常近似的CHF预测方法,将加热壁面的温度飞升作为CHF判定的标准,实现对燃料组件子通道CHF的数值预测。研究表明,本文建立的数值模拟方法,可为燃料组件或其他换热系统的CHF预测奠定基础,为燃料组件的设计提供新的辅助手段。     

环形流道两相流特性实验研究

在日本原子力研究所的一个两以试验台架上，进行了环形流道空气－水丙相流窟泡份额和逆向流试验，观察了其流型。通过试验和分析，提出了漂移流模型中分布系数Co和气相漂移速度Vgi的经验关系式；得到了逆向流Wallis表达式的拟合参数。对试验结果的分析表明，当流道间隙很小（Bond数小于1）时，流型和两相流动与间隙较大时有所不同。     

同心环形管内沸腾两相流动与传热实验研究

对窄缝为2．1mm的同心环形管,试验研究了外管加热条件下水的沸腾两相流动阻力与传热特性,得到了以下结果：窄缝环形管内两相流动的阻力较普通圆管内大,沸腾换热得到了较明显的强化,换热系数弓压力、热平衡干度、工质流量、加热负荷均有关系,且与缝隙宽度和加热方式有关;提出了环形管强化传热的微液膜蒸发机理与汽泡扰动机理的物理解释;得到了环形管内流动摩擦阻力系数与传热系数的实验关联式。     

小流速工况下窄间隙矩形通道临界热流密度研究

板型燃料堆芯常压池式反应堆在事故下的自然循环工况是由零流速逐步建立起来的．这一过程必须工作在临界热流密度(CHF)限值之下。对此进行了上升流小流速窄间隙矩形通道内的CHF试验研究,并与Sudo公式的预测值进行了对比。结果表明,低流速下的CHF随着质量流速的增加而增加．虽然大部分试验数据低于Sudo预测值,但仍然在Sudo公式-33％偏差范围内。     

竖直及倾斜环隙流道内自然对流沸腾临界热负荷

以水为工质 ,在常压下对垂直和倾斜环隙流道内的自然对流沸腾临界热负荷进行了实验研究和理论分析 ,得到了计及进出口局部阻力的计算公式 ,讨论了流道几何尺寸、几何形状、倾角、压力和进出口局部阻力等因素对临界热负荷的影响 ,最后提出了一个新的 ,可用于计算竖直环隙、圆管及长方形流道内自然对流沸腾临界热负荷的半经验公式 ,其计算精度和适用范围较现有的计算公式有显著提高 ,原则上不受H/De 值大小的限制。