Investigation of liquid film behavior at the onset of flooding during adiabatic counter-current air–water two-phase flow in an inclined pipe

The liquid film characteristics at the onset of flooding in an inclined pipe (16 mm i.d. and 2.2 m in length) have been investigated experimentally. A constant electric current method and visual observation were utilized to elucidate the flow mechanisms at the onset of flooding. Two mechanisms are clarified to control the flooding in lower flooding and upper flooding, respectively. The lower flooding occurred at lower liquid flow rate and high pipe inclination angle. In this mechanism, the liquid film does not block the pipe cross-section. On the other hand, the upper flooding occurred at higher liquid flow rate and low pipe inclination angle. In this case, blocking of the pipe cross-section by large wave and entrainment plays an important role. The experimental data indicated that there was no reversal motion of liquid film at the onset of flooding during the operation of both lower flooding and upper flooding. The effects of pipe inclination angle on the onset of flooding are also discussed.     

Two-phase countercurrent flow in a model of a pressurized water reactor hot leg

The onset of flooding or countercurrent flow limitation (CCFL) determines the maximum rate at which one phase can flow countercurrently to another phase. In the present study, the experimental data of the CCFL for gas and liquid in a horizontal pipe with a bend are investigated. The different mechanisms that lead to flooding and that are dependent on the liquid flow rate are observed. For low and intermediate liquid flow rates, the onset of flooding appears simultaneously with the slugging of unstable waves that are formed at the crest of the hydraulic jump. At low liquid flow rates, slugging appears close to the bend; at higher liquid flow rates, it appears far away from the bend, in the horizontal section. For high liquid flow rates, no hydraulic jump is observed, and flooding occurs as a result of slug formation at the end of the horizontal pipe. The effects of the inclination angle of the bends, the liquid inlet conditions and the length of the horizontal pipes are of significance for the onset of flooding. A mathematical model of Ardron and Banerjee is modified to predict the onset of flooding. Flooding curves calculated by this model are compared with present experimental data and those of other researchers. The predictions of the onset of flooding as a function of the length-to-diameter ratio are in reasonable agreement with the experimental data.     

Modeling of bubble shape in horizontal and inclined tubes

An experimental and theoretical study on the bubble shape of intermittent flow in the horizontal and inclined pipes has been carried out. The experiment results show that the bubble shape depends on the Froude number, bubble length and pipe inclination. The bubble with staircase pattern tail is observed at low Froude numbers, which is corresponding to plug flow. A model for the prediction of the bubble shape in horizontal and inclined pipes is proposed. The model is able to predict the bubble shape, flow pattern transition between plug and slug flow regimes as well as nose-tail inversion phenomenon observed in the downwardly inclined pipe. Validation shows the model can well predict the bubble shapes in horizontal and inclined pipes. The model discloses that the transition between plug and slug flow regimes occurs within a region. The Froude number range for plug flow regime in the downwardly inclined pipe is much wider than that in the horizontal or upwardly inclined pipe. The assumption of fully developed liquid film under the long bubbles tends to under-estimate the liquid fraction in this part of the slug structure, especially, for the intermittent flow in the upwardly inclined pipe with high Froude numbers.     

Experimental study on the air/water counter-current flow limitation in a model of the hot leg of a pressurized water reactor

An experimental investigation on the air/water counter-current two-phase flow in a horizontal rectangular channel connected to an inclined riser has been conducted. This test-section representing a model of the hot leg of a pressurized water reactor is mounted between two separators in a pressurized experimental vessel. The cross-section and length of the horizontal part of the test-section are (0.25 m × 0.05 m) and 2.59 m, respectively, whereas the inclination angle of the riser is 50°. The flow was captured by a high-speed camera in the bended region of the hot leg, delivering a detailed view of the stratified interface as well as of dispersed structures like bubbles and droplets. Countercurrent flow limitation (CCFL), or the onset of flooding, was found by analyzing the water levels measured in the separators. The counter-current flow limitation is defined as the maximum air mass flow rate at which the discharged water mass flow rate is equal to the inlet water mass flow rate.From the high-speed observations it was found that the initiation of flooding coincides with the formation of slug flow. Furthermore, a hysteresis was noticed between flooding and deflooding. The CCFL data was compared with similar experiments and empirical correlations available in the literature. Therefore, the Wallis-parameter was calculated for the rectangular cross-sections by using the channel height as length, instead of the diameter. The agreement of the CCFL curve is good, but the zero liquid penetration was found at lower values of the Wallis parameter than in most of the previous work. This deviation can be attributed to the special rectangular geometry of the hot leg model of FZD, since the other investigations were done for pipes.     

Study of countercurrent flow limitation in a horizontal pipe connected to an inclined one

Experiments with air and water in small hot leg reproductions were carried out aiming to acquire a better understanding of the countercurrent flow limitation (CCFL) in this geometry. The effects of various geometrical parameters of the test section and of the inlet water flow rate on the onset of flooding, on the partial delivery of water and on the zero liquid penetration were investigated. It was observed that while the onset of flooding is affected by the inlet water flow rate, the zero liquid penetration is independent of this flow rate. The results with partial delivery showed that, for a fixed air velocity, an increase in the horizontal length, or in the inclined length, of the flow channel leads to an increase of the water carried over by the air. On the other hand, in pipes with larger diameters the drag of the water is smaller. The experimental results showed small differences in the results for tests with inclination of the riser lower than 90°. For an inclination equal to 90°, the water carried by the air tends to be lower than in the others angles for a fixed air velocity. The study led also to a new correlation for the flooding.     

Erratum to “Experimental study on the air/water counter-current flow limitation in a model of the hot leg of a pressurized water reactor”

This paper replaces the paper published in the journal by Deendarlianto et al. (2008). Because of an error in the implementation of the air flow meter some of the data given by Deendarlianto et al. (2008) are wrong. They are corrected within the present paper. The general results and conclusions remain unchanged.An experimental investigation on the air/water counter-current two-phase flow in a horizontal rectangular channel connected to an inclined riser has been conducted. This test-section representing a model of the hot leg of a pressurized water reactor is mounted between two separators in a pressurized experimental vessel. The cross-section and length of the horizontal part of the test-section are (0.25 m × 0.05 m) and 2.59 m, respectively, whereas the inclination angle of the riser is 50°. The flow was captured by a high speed camera in the bended region of the hot leg, delivering a detailed view of the stratified interface as well as of dispersed structures like bubbles and droplets. Counter-current flow limitation (CCFL), or the onset of flooding, was found by analyzing the water levels measured in the separators. The counter-current flow limitation is defined as the maximum air mass flow rate at which the discharged water mass flow rate is equal to the inlet water mass flow rate.From the high-speed observations it was found that the initiation of flooding coincides with the formation of slug flow. Furthermore, a slight hysteresis was noticed between flooding and deflooding. The CCFL data was compared with similar experiments and empirical correlations available in the literature. Therefore, the Wallis-parameter was calculated for the rectangular cross-sections by using the channel height as length, instead of the diameter. The agreement of the CCFL curve is good, but the zero liquid penetration was found at lower values of the Wallis parameter than in most of the previous work. This deviation can be attributed to the special rectangular geometry of the hot leg model of HZDR, since the other investigations were done for pipes.     

A simple model for vertical annular and horizontal stratified two-phase flows with liquid entrainment and phase transitions: one-dimensional steady state conditions

A simple one-dimensional three-fluid model is presented for the simulation and analyses of vertical annular and stratified horizontal or inclined two-phase flows. The model has been verified for various experimental data: developing annular flow, momentum transfer in an annular flow, plane flow with a hydraulic jump, flooding in a horizontal pipe, and stratified flow with direct steam condensation. Emphasis has been laid upon several mass, momentum and energy interfacial transfer processes. New correlations are proposed for the droplet entrainment intensity in annular flow and for steam direct contact condensation on the liquid film in a stratified flow. The liquid entrainment in the annular flow is correlated with the liquid film thickness. Direct contact condensation is correlated with the turbulent convective heat transfer in the liquid film. It has been shown that the present model is able to predict all dominant processes in both types of flow.     

垂直管内气液两相逆流极限管径效应实验

在压水堆安全性分析中,需准确预测气液逆流极限(CCFL)工况下两相流动关系。本文采用水下淹没排气的实验方法,对相同管长不同管径垂直管的CCFL特性进行可视化实验,并对垂直管CCFL关联式模型进行分析,主要结论有:①在CCFL工况下垂直管内流型为环状流动;表观气速较大时,大管径管内液膜呈搅拌状,小管径管内液膜呈波动状;随表观气速减小,均转为液面光滑的自由降膜流动;②Wallis数模型过度关联了管径变化对垂直管CCFL特性的影响;Kutateladze数和Froude-Ohnesorge数模型也不能良好关联垂直管CCFL特性的管径效应;③提出了新的CCFL无量纲参数和相应的实验关联式,由此可使垂直管CCFL特性的管径效应得以统一表征,还可以关联物性参数变化的影响。     

竖直管内淹没及流向反转的实验研究

本文对竖直管中气体由下向上流动,液体由上向下流动过程的淹没及流向反转问题进行了实验研究。给出了计算淹没及流向反转的关系式。通过实验发现,在一定的液体流量下,增加气体流量与减少气体流量时淹没点有所差别;管壁的干燥条件对流向反转有很大影响。     

竖直向下大、小支管气相夹带起始可视化研究

已有气相夹带起始模型均是基于竖直向下的小支管夹带所得到的,用于分析大支管气相夹带时并不适用。本文基于可视化实验,对竖直向下的大、小尺寸T型管的气相夹带起始点进行对比研究。选择与主管道直径比为0.625和0.1的大、小支管,并采用有机玻璃作为实验管道材料,空气和水为实验工质。其实验现象表明：大、小支管夹带起始均有漩涡,且漩涡现象大致相同,但在大支管条件下会出现气室,夹带气体进入支管后也会在气室下翻腾,且在相同液相折算速度下,大支管夹带起始液位会高于小支管。     

Off-take and Slug Transition at T-junction of Vertical-up Branch in the Horizontal Pipe

The off-take and the slug transition on air-water interface are experimentally investigated at the T-junction of the horizontal pipe with a vertical upward branch to simulate the loss-of-residual-heat-removal during a mid-loop operation in the Korea standard nuclear power plant. Scaling analysis is performed to scale down the experimental facility to the reference nuclear power plant. Two different diameters of branch pipes are used to verify the scaling laws and their scale effects. Air is used as working gaseous fluid and no water flow exists. Off-take behavior on horizontal stratified and slug flows is visually observed in the horizontal pipe. The experimental data are divided into three categories; onset of liquid entrainment at T-junction, onset of slug transition in the horizontal pipe, and discharge quality in the branch pipe. It is found out that the scale effect of the branch diameter on the onset of liquid entrainment is small and the existing correlations for it are applicable. Also, the onset of slug transition shows a discrepancy with Taitel-Dukler's correlation and has a strong influence on the discharge quality. New correlations for discharge quality are developed considering the critical dependency of the onset of slugging.     

Distribution of void fraction for gas-liquid slug flow in an inclined pipe

In order to investigate the effect of inclination angle on the spatial distribution of phases,experiments on gas-liquid two-phase slug flow in an inclined pipe were carried out by using the optical probe and an EKTAPRO 1000 high speed motion analyzer.It has been demonstrated that the inclination angle and the mixture velocity are important parameters to influence the distribution of void fraction for upward slug flow in the inclined pipe.At high mixture velocity,the gas phase profile is axial symmetry in the cross-section of the pipe.This is similar to that for vertical slug flow.In contrast.most of the gas phase is located near the upper pipe wall at low mixture velocity.By measuring the axial variation of void fraction along the liquid slug.it can be concluded that there is a high void fraction wake region with length of 3-4D in the front of liquid slug.In the fully developed zone of liquid slug.the peak value of the void fraction is near the upper wall.     

水平矩形管内气-液逆向流动特性实验研究

为探明管截面尺度及排气水深对水平矩形管内气-液逆流特性的影响,以空气和水为两相介质,对管长2 m、管截面尺寸106?mm×60 mm的矩形管分别在横置、竖置条件下和1 、3 m水深下进行了可视化流动实验。结果表明:管截面高度尺度对气-液逆流特性具有显著影响,在相同的管道流通面积下采用较高的流道高度会大幅强化水平管内液相倒流,从而有利于冷凝水由热段顺利回流至堆芯进行冷却;采用管截面高度作为气相及液相Wallis参数中的特征尺寸,可使不同管截面尺寸矩形管的无量纲逆流特性获得统一表征;在水深1~3 m范围,水深变化对水平矩形管内气-液逆流特性的影响并不明显。提出了预测水平矩形管内气-液逆向流动特性的无量纲实验关联式。      

Wall pressure measurements of flooding in vertical countercurrent annular air-water flow

An experimental study of flooding in countercurrent air-water annular flow in a large diameter vertical tube using wall pressure measurements is described in this paper. Axial pressure profiles along the length of the test section were measured up to and after flooding using fast response pressure transducers for three representative liquid flow rates representing a wide range of liquid Reynolds numbers (Re_L = 4Γ/μ; Γ is the liquid mass flow rate per unit perimeter; μ is the dynamic viscosity) from 3341 to 19,048. The results show that flooding in large diameter tubes cannot be initiated near the air outlet and is only initiated near the air inlet. Fourier analysis of the wall pressure measurements shows that up to the point of flooding, there is no dominant wave frequency but rather a band of frequencies encompassing both the low frequency and the broad band that are responsible for flooding. The data indicates that flooding in large diameter vertical tubes may be caused by the constructive superposition of a plurality of waves rather than the action of a single large-amplitude wave.     

基于CFD方法的高温热管特性研究

高温热管运行特性的分析与预测,对热管设计和应用具有重要意义。为分析高温热管内两相流动传热特性,首先建立钠热管的计算流体力学（CFD）分析模型,并对模型计算值与钠热管稳态实验数据进行对比校核,模拟结果与实验测点温度的绝对误差小于40℃,相对误差在5%以内;其次,利用本文模型与方法对不同传热功率和倾角下的热管内部流场特性进行分析研究。研究表明,均匀加热条件下,蒸气腔内的速度在蒸发段接近线性变化,而在冷凝段,气体流速减小致使压强回升,同时,蒸气的流动压降和速度随加热功率增加呈下降趋势;在热管水平和倾角运行工况,热管内两相流动压降中液相压降均占主导;而气液间剪切效应中,气体流动速度为主导效应。本文模型可为热管堆等高温热管应用领域提供热管设计与分析方法。      

AP1000稳压器波动管竖直管段空气-水CCFL实验研究

使用竖直管代替波动管模型开展稳压器波动管竖直管段内空气-水两相逆流限制(CCFL)特性可视化实验研究。实验现象表明:竖直管与上容器接口处的局部CCFL决定了进入竖直管内的液相流量;竖直管内的局部CCFL决定了从竖直管流出的液相流量;两处局部CCFL均随空气流量的增大而增强。在较低气量情况,进入竖直管内的液相能够完全或大部分流出,竖直管内的局部CCFL较弱,上容器和竖直管接口处的局部CCFL在整体CCFL中占主导地位,整体CCFL程度随着上容器液位升高而略有增强。在高气量情况,从上容器进入竖直管的液相大部分或者完全被限制而不能向下流出,竖直管内的局部CCFL强烈,在整体CCFL中占主导地位,整体CCFL特性不受上容器液位变化的影响。通过实验数据拟合得到了新的稳压器竖直管CCFL模型。稳压器波动管CCFL数据和稳压器竖直管CCFL数据基本重合,表明波动管CCFL主要由CCFL-U决定。     

螺旋管内单相及沸腾的强化换热与阻力特性实验

实验研究了三维内肋螺旋管内单相及沸腾的强化换热与阻力性能。单相对流换热实验采用光滑螺旋管和两种不同结构尺寸的三维内肋螺旋管。与光滑螺旋管相比,在测试的流动范围内．两种三维内肋管的平均换热系数增加了71％和103％．平均阻力增加了90％和140％;曲率δ=0．0605、测试段长0．58m的三维内微肋螺旋管内流动沸腾换热实验结果表明：在不同质量流速、热流密度工况下,三维内微肋螺旋管的平均换热系数比光滑螺旋管增加40％到120％．阻力增加18％到119％。     

起伏振动下倾斜管内单相流流动阻力特性分析

起伏振动状态下单相流流动阻力的正确计算对漂浮核电站的安全性有显著影响。实验研究了不同起伏振动工况和流动工况对倾斜圆管通道内单相水摩擦压降的影响,提出了方便计算的振动摩擦阻力系数。结果表明,振动摩擦压降大于稳定状态的,并呈周期性波动,波动周期与振动频率一致。振动摩擦阻力系数平均值随雷诺数和倾角的增大而减小,随管径和振动频率的增大先增大后减小,随振幅的增大而增大。通过实验数据拟合得到起伏振动下倾斜管内单相水振动摩擦阻力系数计算经验关系式,计算结果与实验结果吻合较好,为起伏振动单相水流动阻力的计算提供了新思路。     

反应堆失水事故条件下热管段内的两相流逆向流动研究

对反应堆失水事故条件下热管段内的两相流逆向流动过程进行了实验研究和理论分析。实验回路由竖直管、倾斜管和水平管三段组成 ,研究发现在这种回路中水平管的长度对逆向流动的溢流开始点影响较大。对逆向流动的溢流开始点进行了实验研究和理论分析 ,对现有的国外计算模型进行了分析和比较 ,推荐了热管段内溢流开始点的计算关系式。     

Admitting cold water into steam filled pipes without water hammer due to steam bubble collapse

Water hammer due to steam bubble collapse when cold water is admitted to vertical upward flowing, vertical downward flowing, and nominally horizontal pipes has been studied both experimentally and analytically. The work in horizontal pipes included a study of the effect of a slight downward inclination, a slight upward inclination, and the length of the pipe on the initiation of water hammer. Stability maps showing the combinations of filling velocities and liquid subcooling that cause water hammer and those which do not for each flow geometry were obtained from experiments. Analytical models were developed to predict those stability boundaries in the stability maps. All these models were tested with experimental data. Based on the verified models, a step-by-step approach for each flow geometry is presented for plant engineers and designers to follow in avoiding water hammer induced by steam bubble collapse.