竖直圆管内泡状流界面参数分布特性

采用双头光纤探针对内径为50 mm竖直圆管内空气-水两相泡状流界面参数径向分布特性进行了实验研究。气液两相表观速度变化范围分别为0.004～0.05 m/s和0.071～0.283 m/s。结果表明,竖直管内向上泡状流局部界面面积浓度（IAC）、空泡份额及气泡频率径向分布相类似,即气相流速较低时管道中间很大范围内以上3个局部界面参数几乎恒定,近壁区迅速下降到较低值;随气相流速的增加,局部界面参数在管道中心出现峰值。本实验中气泡聚合与破碎现象较少发生,索特平均直径沿径向近似均匀分布,且随气液两相流速变化很小。通过气泡横向受力解释了局部界面参数分布的影响机理。     

竖直圆管内泡状流空泡份额径向分布实验研究

常温常压下,采用光学探针测量方法,对圆管（内径50 mm）内空气 水两相竖直向上泡状流空泡份额的径向分布特性进行了实验研究。结果表明,竖直圆管内泡状流空泡份额的径向分布随气液两相表观流速不同而变化。液相流速较高时空泡份额分布呈“壁峰型”,即中心区域变化平缓,近壁区出现峰值后迅速降低;液相静止时,随气相流速增加,空泡份额增加速度沿径向向外逐渐减小,气相流速较大时分布呈“核峰型”,即空泡份额随径向位置向外呈减小趋势;液相流速较低时分布呈现出过渡型。探针测量面积加权平均空泡份额与通过重位压降得到的空泡份额的相对偏差小于10%。     

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。     

倾斜管内上升泡状流界面参数分布特性实验研究

采用双头光纤探针对倾斜圆管内空气-水两相泡状流界面参数分布特性进行了实验研究,包括局部空泡份额、气泡通过频率、界面面积浓度及气泡当量直径径向分布特性。实验段内径为50 mm,液相表观速度为0.144 m/s,气相表观速度为0~0.054 m/s。结果表明倾斜管内向上泡状流气泡明显向上壁面聚集。局部界面浓度、空泡份额及气泡通过频率径向分布相似。倾斜条件下局部界面参数分布下壁面附近峰值相对于竖直状态被削弱甚至消失,上壁面附近峰值被加强,中间区域从下壁面往上逐渐增大,且随倾斜角度的增加变化更加剧烈。气泡等价直径随径向位置、气相速度及倾斜角度的不同无明显变化,气泡聚合和破碎现象较少发生。通过气泡受力分析解释了倾斜对泡状流局部界面参数分布的影响机理。     

振动幅度对两相流局部参数变化影响研究

为研究实验段振动对管内两相流局部参数变化的影响,利用电导探针技术对振动状态下局部两相流特性参数包括空泡份额、气泡直径和界面浓度进行了测量。实验首先在静态工况下进行,通过固定在实验段上方的偏心轮转动获得振动工况。实验段振动周期保持在0.5 s,偏心轮提供的振动幅度分别为4.8 mm、9.5mm和15.8 mm。实验结果表明,振动对环管内气-水两相流局部时均参数分布影响很小。但振动引起的附加惯性力作用使两相流局部参数径向分布在实验段振动周期中发生明显变化,而且局部参数的变化幅度随实验段振幅的增加而显著增大。在含气率较低的流动工况,当振幅增大到15.9 mm时振动工况下径向空泡份额峰值较静态工况下的空泡份额峰值的增量可以达到70%。但振动对局部流动参数的影响随气流量增大而降低。     

光纤探针方法测量界面面积浓度实验研究

采用光纤探针测量方法对垂直上升管中空气 水两相流动的界面面积浓度（IAC）分布规律进行研究。实验选用的管径为50 mm,气相、液相折算速度分别为0.01～1 m/s和0～1 m/s。通过高速摄影获取的影像数据对光纤探针法的测量精度进行评价和标定,获得IAC径向分布。利用IAC数据对几类IAC计算模型进行评价,基于截面含气率变密度模型给出平均IAC与中心截面含气率的关系式。通过与其他实验数据进行比较,表明新关系式具有较高的计算精度。     

竖直圆管内空泡份额径向分布特性形成机制

在常温常压下,采用光纤探针测量方法对垂直上升大圆管中空气-水两相流动的空泡份额径向分布特性及形成机制进行研究。实验选用圆管直径为100 mm,气相、液相折算速度的范围分别为0.004~0.053 m/s和0.071~0.213 m/s。结果表明空泡份额径向分布随着气-液流量的不同,呈现出"核峰"或"壁峰"型分布特点;通过分析气泡所受到的横向升力和壁面力,表明二者对气泡横向运动的综合作用是造成空泡份额径向分布呈现出"核峰"或"壁峰"型分布的主要原因。     

径向热流密度分布对并联多通道流动不稳定性的影响研究

以去离子水为工质,在系统压力为0.89~1.32 MPa、入口质量流速为500~750 kg/(m2?s)、入口温度58.5℃~132.3℃的参数范围内,研究了5根圆管[长1400 mm、外径Φ8 mm、壁厚2 mm（1400 mm×Φ8 mm× 2 mm）]加热通道内工质向上流动时,在不同径向热流密度分布条件下并联通道内发生流动不稳定时的特征,并对比了其流动不稳定边界。结果表明,在对热通道发生流动不稳定时的参数进行处理时,径向加热热流密度分布对并联多通道流动不稳定边界无明显影响;对于不同热流密度分布的并联多通道结构,其流动不稳定边界和完全对称加热的并联多通道的流动不稳定边界是一致的。      

倾斜、摇摆对矩形通道内泡状流局部界面参数分布的影响

借助高速摄像机,对倾斜和摇摆条件下矩形通道(43 mm×3.25 mm×2000 mm)内泡状流局部界面参数的横向分布特性进行实验研究,包括局部气泡比例、空泡份额及界面面积浓度(IAC)。实验结果表明,竖直、倾斜及摇摆条件下,局部气泡比例、空泡份额及IAC三者的横向分布形状相类似。竖直静止及摇摆至竖直位置时,通道内中间较大范围内各局部参数变化缓慢,在xi/(w/2)=±0.5(xi为以通道宽边中心轴线为原点到窄边内壁面的距离,w为宽边尺寸)附近出现峰值;随着倾斜和摇摆角度的增大,下壁面附近峰值被削弱,上壁面附近峰值被加强。实验摇摆参数范围内,摇摆时局部参数的横向分布与实验段倾斜至相同角度时的分布十分相近;主要原因为在横向上摇摆引起附加惯性浮力远小于气泡受到的浮力。     

竖直大圆管内两相流动界面输运过程研究

采用光纤探针测量方法研究了垂直上升管中空气-水两相流动的局部界面面积浓度（IAC）和空泡份额等分布规律。实验选用的圆管直径为100 mm,气相、液相表观速度的范围分别为0～0.1 m/s和0～1.0 m/s。结果发现,影响径向IAC分布的因素主要为气泡通过频率。基于Ishii-Kim界面输运模型,对轴向IAC进行了计算;通过分析4种气泡间相互作用对IAC的影响,发现工作压力是影响轴向IAC变化的主要因素,最后给出了引入工作压力影响的轴向IAC计算关联式。     

Measurement of Liquid Turbulent Structure in Bubbly Flow at Low Void Fraction Using Ultrasonic Doppler Method

Microscopic structure in bubbly flows has been a topic of interest in the study of fluid dynamics. In the present paper, the ultrasonic Doppler method was applied to the measurement of bubbly. The experiments were carried out for an air-water dispersed bubbly flow in a 20 mm × 100 mm vertical rectangular channel having a void fraction smaller than 3%. Two ultrasonic transducers were installed on the outer surface of the test section with a contact angle of 45^° off the vertical axis, one facing upward and the other facing downward. By applying statistical methods to the two directional velocity profiles, Reynolds stress profiles were calculated. Furthermore, to clarify the wake effect induced by the leading bubbles, the velocity profiles were divided into two types of data. The first one is for all of the liquid data and the other is the data which did not include the wake effect. For Re_m ≥1,593, it was observed that the bubbles suppressed the liquid turbulence. Furthermore, comparing with the Reynolds stress profiles in bubbly flow, it was found that Reynolds stress profiles varied with the amount of bubbles present in the flow and the effect of wake causes turbulence in the liquid.     

Measurement of interfacial area concentration in subcooled liquid-vapour flow

In two-fluid modelling, accurate prediction of the interfacial transport of mass, momentum and energy is required. Experiments were carried out to obtain a database for the development of interfacial transport models, or correlations, for subcooled water-steam flow in vertical conduits. The experimental data of interest included the interfacial area concentration, interfacial condensation heat transfer and bubble relative velocity. This paper focuses on the interfacial area concentration. The interfacial area concentration was obtained by measuring the distributions of bubble volume and surface area as well as the area-averaged void fraction at various axial locations in subcooled water-steam condensing vertical upward flow under low flow rate and low pressure conditions. The bubble size and surface area were determined using high-speed photography and digital image processing techniques. The area-averaged void fraction was measured by a single-beam gamma densitometer. The results were compared with existing correlations, which were developed on the basis of data obtained for air-water adiabatic flows. Poor agreement between the present data and the existing correlations was obtained. Accordingly, new correlations suitable for subcooled liquid-vapour bubbly flow are proposed.     

Distribution parameter and drift velocity for two-phase flow in a large diameter pipe

In view of practical importance of the drift flux model for two-phase flow analysis in general, and in the analysis of nuclear reactor transients and accidents in particular, the distribution parameter, and the drift velocity have been studied for two-phase flow in a vertical large diameter pipe. In this, study, local measurements were performed on flow parameters, such as void fraction, gas velocity and, liquid velocity in a vertical upward air–water two-phase flow in a pipe of 200 mm inner diameter and, 25 m in height by using the local sensor techniques such as hot-film probes, optical multi-sensor, probes and differential pressure gauges. Two-phase flow regimes in a vertical large diameter pipe, were classified into bubbly, churn and slug flows according to the visual observation. The values of the, distribution parameter and the mean drift velocity were determined directly by their definition using experiment data of the local flow parameters in a two-phase flow in a large diameter pipe. Various existing drift flux correlations were compared with the present experimental results and experimental data obtained by other researchers. A detailed discussion on the problems of these correlations was presented in this paper.     

振动工况下环管内气液两相流参数分布实验研究

为模拟地震对反应堆堆芯内两相流动的影响,将振动产生装置与实验段结合,并利用电导探针技术实验研究了振动工况下环管内气液两相流局部参数的分布特性。振动装置所采用的偏心轮的偏心距为15.875mm,并可通过调节电机转速获得不同的振动周期。环管实验段内径和外径分别为19.1mm和38.1mm,总长度为2.32m。实验以空气和水为工质,流动工况覆盖了泡状流、弹状流及搅混流等流型。实验结果表明,振动对管内气液两相流局部参数如含气率和界面浓度等分布具有重要影响。随振动周期的减小,管内两相流参数分布发生明显变化。另外振动对低含气率的泡状流影响较大,随含气率的增加,流型逐渐向搅混流过度,振动对两相流参数的影响逐渐减小。     

Effects of non-uniform inlet boundary conditions and lift force on prediction of phase distribution in upward bubbly flows with Fluent-IATE

This study investigates the profile effects of the boundary conditions in two-phase flows, such as the inlet void fraction, interfacial area concentration, and phase velocity, on the predictions of flow behaviors downstream. Simulations are performed for upward air-water bubbly flows in a 48.3-mm inner diameter pipe by employing Fluent's two-fluid model together with an interfacial area transport equation (IATE) model. The IATE was developed in the literature to model the interfacial area concentration by taking into account the bubble coalescence and disintegration, and phase change effects.In this study, two types of inlet boundary conditions are considered, one being a uniform-profile boundary condition in the radial direction with area-averaged experimentally measured values while the other being a non-uniform profile condition based on the actual measured profiles at the inlet. The numerical predictions of downstream profiles of the phase distributions indicate that the two types of boundary conditions yield similar results for the downstream flow behaviors for the bubbly flow conditions investigated. In addition, the results with and without the lift force demonstrated that the lift force is essential to obtain accurate lateral phase distribution.     

CFD application to prediction of void distribution in two-phase bubbly flows in rod bundles

This paper is concerned with the prediction of the void fraction distribution in two-phase bubbly flows in fuel rod bundles. Special attention has been devoted to the phenomena which govern the void fraction distribution in the lateral direction of a channel. A two-fluid model of two-phase flow has been formulated and implemented into a commercial computational fluid dynamics (CFD) code. The model has been used for the prediction of the void distribution in three different channels: a circular channel (inside diameter (ID), 34.5 mm) with a single heated rod of 13.9 mm outside diameter (OD), and circular channels (ID, 71 mm) with six heated rods (13.8 and 13.9 mm OD each). The predicted axial and lateral avoid fraction distributions in subcooled and bulk boiling regions have been area averaged in three lateral zones and compared with experimental data: in all cases, satisfactory agreement between the predictions and measurements has been obtained.