垂直上升矩形流道内气液两相流流型图的数值模拟

两相流流型在分析换热、流动不稳定性以及临界热流密度方面具有基础性作用.本文基于VOF(Volume of Fluid)多相流模型,对垂直上升矩形流道内气液两相流动进行数值模拟,表观气速0.1～110 m/s,表观液速0.1～3.2 m/s.得到了流道内气液两相流的主要流型:泡状流、弹状流、搅混流和环状流,分析了流道内截面含气率分布与流型的对应关系,以及截面含气率与气液两相流容积含气率的关系;分析了各种流型下的压降分布特性,并绘制了基于气液表观动能通量的不同流量下气液两相流的流型图,直观的表示出各种流型的分布区域及各流型间的流型转换边界,与已发表文献的实验结果对比符合较好.     

竖直管道内间歇式两相流动沸腾特性分析

自然循环或重力注水过程的热功率、冷却剂流量等操作条件较小,易出现各种流动不稳定现象,影响核反应堆事故的发展进程,间歇式流动沸腾现象就属于其中的一种。以去离子水为工质,采用2×2加热棒束,对内径为32 mm竖直通道内的间歇式流动沸腾现象进行了实验研究,分析了不同热流密度下间歇式流动沸腾不稳定现象的变化规律,讨论了热流密度对间歇式沸腾周期的影响。结果表明,在一定的热流密度条件下,当加热通道内流体达到饱和并过热时,会发生周期性地剧烈喷涌及冷液回流现象,期间伴随泡状流、弹状流、搅混流及环状流等多种流动形态;间歇喷涌周期取决于沸腾停滞时间,随热流密度的不断增大,沸腾停滞时间缩短,间歇喷涌周期也缩短。当热流密度增大到一定程度时,间歇式流动沸腾现象消失,从而转变为另一种两相流动不稳定现象。     

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     

矩形窄缝通道内水沸腾流型理论研究

对矩形窄缝通道(1×20mm,2×20mm)中水沸腾的汽液两相流动流型转化准则进行了研究。根据实验结果,水平矩形窄缝加热通道中流型可以分为泡状流、间歇弹状流、搅拌流、环状流和汽液分离流。运用分相流模型对矩形窄缝通道中水沸腾的汽液两相流动流型进行了理论分析,得出了相应的流型转换准则,并与实验数据进行了初步比较,符合较好。     

摇摆状态下窄矩形通道内两相流流型特性研究

以空气和水为工质,在40mm×1.6mm的窄矩形通道内对竖直状态和摇摆状态下两相流流型进行了研究。流型由拍摄照片辨别,实验通道内观察到的流型有泡状流、弹状流、搅混流和环状流,绘制出窄矩形通道内的流型图,并与常规尺寸圆管内两相流型进行了对比。摇摆对窄矩形通道内流型的影响与常规尺寸圆管相似,但由于狭小空间的限制及表面张力的作用,摇摆对两相流动并无明显影响。     

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。     

棒束通道内气液两相流流型的实验研究

常温、常压条件下,在7×7矩形截面棒束通道内进行了垂直向上气液两相流动实验,气液两相折算速度的变化范围分别为0.04~14 m/s和0.238~1.860 m/s。实验中用高速摄像仪对流型进行记录,观察到了泡状流、泡状-搅混流、搅混流和搅混-环状流4种流型,发现搅混流是主要流型,并对Hewitt流型图的界限进行了修正。分析实验数据发现,摩擦压降在泡状流和搅混流区域的变化是相反的。根据实验数据,参考前人的研究得到棒束通道中泡状流向搅混流转变的边界。     

窄矩形通道内搅混流和环状流相界面参数的计算方法

空泡份额和界面浓度是两相流动中重要的相界面参数,准确获取窄矩形通道内搅混流和环状流工况下空泡份额和界面浓度是构建和完善两流体模型的关键。本文针对横截面为65 mm×2 mm的矩形通道开展了气液两相流动特性可视化实验研究,气相折算速度j_g＝1～9 m/s,液相折算速度j_f＝0.1～1.5 m/s,流型包含搅混流和环状流。提出了基于高速摄像法获取搅混流和环状流下空泡份额和界面浓度的分析计算方法,利用该方法所得空泡份额与窄矩形通道内经验关系式计算值的相对偏差约在10%以内。此计算方法可为研究复杂流型下窄矩形通道内的相界面参数提供理论依据。     

基于非监督机器学习方法的竖直环形流道流动沸腾流型研究

研究流动沸腾两相流动形态对封闭反应堆安全分析程序关键本构模型具有重要意义。本文基于非监督机器学习流型识别方法，提出将两相流物理知识融入数据驱动的机器学习模型，并构建输入特征的挑选原则：(1)机器学习在输入特征中捕捉到的应为流型相关信息；(2)机器学习的聚类准则应包络该流型下输入特征的所有可能性。依据挑选原则分析电导探针信号生成的汽泡分布特征，确定汽泡弦长累积分布函数数据可用于非监督机器学习流型判断。依据流型识别结果，获得了竖直环形流道内流动沸腾的二维局部流型特性，发现高位局部流型出现在流道中心位置并偏向内加热壁面；并判别了流道截面的全局流型，结果表明流动沸腾泡状流至弹状流的流型转变出现在空泡份额约为0.14位置。     

不同窄缝宽度下窄矩形通道中沸腾临界可视化流型及触发机理

窄矩形通道因具有结构紧凑等优点而被广泛应用于各个领域。为完善窄矩形通道中临界热流密度（Critical Heat Flux,CHF）的预测方法,提高反应堆安全性和经济性,本文进行了不同窄缝宽度下窄矩形通道内CHF可视化试验来探索CHF触发机理。实验同步采集不同窄缝宽度下可视化结果和热工水力数据,结果发现：当窄缝宽度分别为5 mm、3 mm、2 mm和1 mm时,在发生CHF时,通道内流型分别对应泡状流、弹状流、搅混流和环状流。在发生CHF前,在泡状流、弹状流和搅混流都存在温度波动。在环状流中CHF涉及到区域由初始的干斑逐渐扩展;在搅混流时CHF涉及到的区域较小;而弹状流涉及到的区域最广;在泡状流中加热壁面温度波动频率最高。当系统压力在1～4 MPa范围内、在窄缝宽度为1 mm时,系统压力与CHF之间存在非线性关系,而在其余通道中随着系统压力增加CHF增加。因此,窄缝宽度对窄矩形通道中CHF有非常重要的影响。本文分析结果可为CHF机理模型的建立提供思路。     

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.     

Natural Convection Sodium Boiling Experiments in 37-Pin Bundle Geometry

Decay heat removal capability under boiling condition was studied using an LMFBR fuel subassembly mockup loop. The sodium flow was driven by natural convection through the loop in which was installed a 37-pin bundle heated electrically over a length of 45 cm.

The heat flux furnished by the pins was increased stepwise, upon which the two-phase flow regime changed from bubble to slug flow and then to annular or annular mist flow. Dryout occurred even in slug flow regime, but only momentarily, and permanent dryout was not observed before establichment of annular flow. A suitable criterion for permanent dryout is considered to be 0.5 average exit sodium vapor quality. The results indicated that upon occurrence of sodium boiling, the coolability of fuel subassembly would be maintained by natural convection after reactor shutdown.     

非线性振动下水平通道内气液两相流动研究

在地震等行为产生的非线性振动下，两相流体会影响回路传热并对装置结构进行冲击，因此对气液界面行为的把握对核安全具有十分重要的意义。本文通过将振动装置与两相流实验回路相结合的方法，对非线性振动下水平通道内气液两相流问题进行了实验研究。基于FLUENT平台，结合动网格模型及UDF编程手段建立了数学模型，并对数学模型进行验证。研究结果表明：模拟结果与实验结果具有很好的一致性；振动工况下气液两相流动形式不同于稳态工况，会出现更复杂的气液界面，主要流型有泡状流、弹状流、搅拌流、波状流及环状流；瞬时摩擦压降的波动幅度随振动幅度和频率的增大而增大，且与振动幅度相比，振动频率对其影响更大。     

Numerical study on effect of gap width of narrow rectangular channel on critical heat flux enhancement

According to the flow passage characteristic of narrow rectangular channel and liquid film dry-out mechanics of annular flow critical heat flux (CHF), an annular flow CHF analytical model for narrow rectangular channel has been achieved. This model may be used to predict the CHF behavior of boiling two-phase flow in narrow rectangular channel with gap width of not being less than 0.0005 m (the equivalent diameter of this channel is 0.001 m). Through analyzing and calculating, when the inlet dimensionless gap width of narrow rectangular channel is within 30-85, the enhancement of CHF in channel is obvious. At the same time, according to the characteristic of two-phase flow, the new determinant laws of CHF in boiling two-phase flow system have been derived. Through analyzing and calculating, it is substantial that this determinant laws is appropriate. The best dimensionless gap width of heat flux enhancement has been achieved to be 45-75.     

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 )。     

Type of flow, pressure drop, and critical heat flux of a two-phase sodium flow

Little is known about the two-phase pressure loss, the flow pattern, and the critical heat flux conditions for boiling sodium under forced convection. The specific thermohydraulic properties of sodium prohibit extrapolation to sodium of experimental data obtained for other liquids. Therefore, some new test series were carried out in a sodium loop with an induction heated test section of 9 mm inner diameter and 200 mm heated length. The two-phase pressure loss and the film thickness were measured up to the critical cooling conditions. The experimental results are compared with values predicted by known models on annular flow and annular mist flow, respectively. Satisfactory predictions of the flow pattern and the critical heat flux conditions could only be obtained using the measured two-phase pressure losses.     

Heat Transfer to Steam-Water Annular Dispersed Flow in Vertical Annulus

The heat transfer coefficient and slow burnout heat flux were measured for a stream-water annular dispersed upward flow under pressures up to 3.5 ata in an electrically heated vertical annular channel.

An empirical equation was derived for the heat transfer coefficient as function of mass flow rate, steam quality and heat flux. The dominant mechanism of heat transfer to the annular dispersed two-phase flow is forced convection of liquid film on the heater surface even in the region of low steam quality (down to about 0.03). The observed slow burnout heat flux was near the point of intersection of the lines representing liquid film forced convective heat transfer and nucleate boiling heat transfer on the q vs. δT _sat diagram. A dryout mechanism is proposed in which increasingly violent evaporation comes to impede the rewetting of the dry patches generated on the heater surface, which thus spread to cover the whole surface. A maximum value is observed in the slow burnout heat flux plotted against exit steam quality. This can be explained as the effect of heat removal by droplet exchange between liquid film and steam flow.     

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。     

Two-phase Flow Patterns in a Four by Four Rod Bundle

Air-water two-phase flow patterns in a four by four square lattice rod bundle consisting of an acrylic channel box of 68mm in width and transparent rods of 12mm in diameter were observed by utilizing a high speed video camera, FEP (fluorinated ethylene propylene) tubes for rods, and a fiberscope inserted in a rod. The FEP possesses the same refractive index as water, and thereby, whole flow patterns in the bundle and local flow patterns in subchannels were successfully visualized with little optical distortion. The ranges of gas and liquid volume fluxes, {J_G} and {J_L}, in the present experiments were 0:1 ? {J_L} < 2:0 m/s and 0:06 < {J_G} < 8:85 m/s, which covered typical two-phase flow patterns appearing in a fuel bundle of a boiling water nuclear reactor. As a result, the following conclusions were obtained: (1) the region of slug flow in the {J_G}-{J_L} flow pattern diagram is so narrow that it can be regarded as a boundary between bubbly and churn flows, (2) the boundary between bubbly and churn flows is close to the boundary between bubbly and slug flows of the Mishima and Ishii's flow pattern transition model, and (3) the boundary between churn and annular flows is close to the Mishima and Ishii's model.