Wave and drop periodicity in transient annular flow

Drop concentration frequency has been determined for the first time from time varying drop concentration measurement using light scattering technique. The study has been carried out using laser diffraction technique on a 19 mm internal diameter, 7 m length of vertical pipe using air and water as fluids. The gas superficial velocity was 13–43 m/s at liquid superficial velocities of 0.05 and 0.15 m/s. Additional tests were carried out with the gas velocity at 14 m/s for liquid superficial velocities of 0.03–0.18 m/s.Fluctuations of drop concentration and film hold-up with time have been analyzed. Both drop concentration and film hold-up shows evidence of periodicity with film thickness more periodic than drop concentration. Power spectrum density of auto-correlation function was generated from time series of the wave that produces the droplets and the droplet concentration to identify peak frequencies in both instances. Comparison of the peak structure frequencies shows that wave frequency is generally higher than frequency of the drops. This observation has been linked to drop coalescence rate and turbulent diffusion in the dispersed phase. Comparison of drop collision frequency with mechanistic model was carried out. Good agreement was achieved and a new, improved model proposed.The trend in drop frequency has been observed to be similar and directly proportional to the trend observed in drop size distribution (MMD). Traditional Strouhal number – Lockhart–Martinelli parameter provides a good correlation for the wave frequency. However, correlating drop frequency using this approach proves inadequate.     

Droplet deposition measurement with high-speed camera and novel high-speed liquid film sensor with high spatial resolution

A sensor based on the electrical conductance method is presented for the measurement of dynamic liquid films in two-phase flow. The so called liquid film sensor consists of a matrix with 64 × 16 measuring points, a spatial resolution of 3.12 mm and a time resolution of 10 kHz. Experiments in a horizontal co-current air-water film flow were conducted to test the capability of the sensor to detect droplet deposition from the gas core onto the liquid film. The experimental setup is equipped with the liquid film sensor and a high speed camera (HSC) recording the droplet deposition with a sampling rate of 10 kHz simultaneously. In some experiments the recognition of droplet deposition on the sensor is enhanced by marking the droplets with higher electrical conductivity. The comparison between the HSC and the sensor shows, that the sensor captures the droplet deposition above a certain droplet diameter. The impacts of droplet deposition can be filtered from the wavy structures respectively conductivity changes of the liquid film using a filter algorithm based on autoregression. The results will be used to locally measure droplet deposition e.g. in the proximity of spacers in a subchannel geometry.     

Development of Simplified Wave-type Vane in BWR Steam Dryer and Assessment of Vane Droplet Removal Characteristics

Air-water experiments were performed for the BWR steam dryer in order to elucidate droplet removal characteristics of the vane. Based on the results, a simplified vane was developed and its droplet removal characteristics were confirmed by air-water experiments using a whole dryer model.

Phase-Doppler anemometer was used to measure droplet diameter distributions. In the experiments of the current vane with four-wave stages and 120° bend angle, almost all of the droplets were found to be trapped in the first and second vane stages. For the air velocity of 3.1 m/s, 90% of the inlet droplets were trapped there and 4% were trapped in the third and fourth stages, resulting in 6% being carried over. Sauter mean diameters at the exit were 6 and 5μm while at the inlet they were 71 and 64μm for the respective air velocities of 1 and 3 m/s. Based on the Weber number evaluation, the possible mechanism for the fine droplet generation was considered to be the breakup of droplets due to impingement on the liquid film formed in the vane surface.

From the above results, the simplified vane with two-wave stages was developed. In order to remove fine droplets, the bend angle of the vane was reduced to 90°, in which a larger inertia force is exerted on finer droplets generated by breakup. Experiments for the whole dryer model showed that the developed vane could achieve the same droplet removal characteristics as the current vane without increasing the pressure drop of the dryer. A design having ten dryer bank rows and 1.2 m height, which is 0.8 m shorter than the current dryer, is possible for the dryer using the new simplified vane.     

倾斜条件对喷淋液滴运动特性的影响

本文以倾斜条件下喷淋液滴在空气环境下运动特性为工程背景,建立倾斜条件下单个液滴在常温、常压空气环境中动量方程,分析倾斜角度、喷射角度、液滴直径、初始速度对液滴运动轨迹的影响机理。计算结果表明：液滴初始喷射角度为45°、倾斜角度为30°时,距初始位置垂向距离为50 m的中心剖面的横向跨度约为5.5 m;倾斜角度越大、液滴直径越小、液滴初始速度越小,液滴覆盖范围越小;液滴初始喷射角度越大、液滴直径越小、液滴速度越小,倾斜角度对液滴喷射覆盖范围的影响越小;倾斜角度对倾斜同向和反向的液滴运动轨迹的影响相对大小,与液滴初始喷射角度、液滴直径和初始速度密切相关。研究还表明,液滴离开喷淋头后,按照正向和负向最大位移运动的两个液滴,液滴直径对液滴横向位移由初始状态到最大值的时间影响十分显著,而初始速度、喷射角度和倾斜角度对其影响有限。     

Vacuum sieve tray for tritium extraction from liquid Pb–17Li

Formation of droplet of liquid Li–17Pb released from a nozzle into vacuum was studied for the evaluation of the feasibility as a tritium extraction process. Size of droplets formed from the nozzles was estimated by theoretical and experimental methods. For the theoretical estimation, the non-dimensional comparison of the physical bulk property of liquid Pb–17Li with water (H₂O) at ambient temperature was applied. It was found to be reasonable to apply the Plateau-Rayleigh-Instability theory for the droplet size formula of the fluid Pb–17Li for the nozzle diameter 0.4 mm–1.0 mm, temperature 400 °C–500 °C, at initial velocity of 3 m/s. The experimental results of the droplet size showed good agreement with the theory. This device was used for the parametric study of extraction of deuterium during their free fall in vacuum. The scaling of the device suggests the engineering feasibility of the process.     

Experimental observation of the droplet size change across a wet grid spacer in a 6 × 6 rod bundle

During the reflood phase of a postulated loss of coolant accident in a nuclear reactor, entrainment of liquid droplets can occur at a quench front of reflooding water. It is widely recognized that the behavior of the entrained droplets crucially affects the reflood heat transfer phenomena by decreasing the superheated steam temperature and interacting with a rod bundle and spacer grids. For this reason, various experimental and numerical studies have been performed to examine droplet behavior such as the droplet size, velocity and droplet fraction inside a rod array. In this study, an experiment on the droplet behavior inside a heated rod bundle has been performed. The experiment was focused on the change of droplet size induced by a spacer grid in a rod bundle geometry, which results in the change of the interfacial heat transfer between droplets and superheated steam. A 6 × 6 rod bundle test facility in Korea Atomic Energy Research Institute was used for the experiment. Steam was supplied by an external boiler into the bottom of the test channel, and a droplet injection nozzle was equipped instead of simulating a quench front of reflooding water. The major measuring parameters of the experiment were the droplet size and velocity, which were measured by a high-speed camera and a digital image processing technique. A series of experiments were conducted with various flow conditions of a steam injection velocity, heater temperature, droplet size, and droplet flow rate. The experiments provided the data on the change of the Sauter mean diameter of droplets after collision with a wet grid spacer depending on flow conditions.     

Benchmark database on the evolution of two-phase flows in a vertical pipe

Based on many years of experience, a new extensive and high-quality database was obtained for steady-state upward air-water flows in a vertical pipe with an inner diameter of 195.3 mm using the wire-mesh sensor technology. During the experiments, the sensor was always mounted on the top of the test section while the distance between gas injection and measuring plane was varied up to 18 different L/D by using gas injection chambers at different vertical positions. The gas was injected via holes in the pipe wall. In this new test series the pressure was kept at 0.25 MPa (absolute) at the location of the active gas injection while the temperature was constant at 30 °C ± 1 K. The experiments were done for 48 combinations of air and water superficial velocities varying from 0.04 m/s to 1.6 m/s for water and 0.0025 m/s to 3.2 m/s for air. From the raw data time-averaged data as: radial gas volume fraction profiles, bubble size distributions, radial volume fraction profiles decomposed according to the bubble size and the radial profiles of the gas velocity were calculated. The consistency of this data was thoroughly checked. They are characterized by a high resolution in space, which makes them suitable for the development and validation of CFD-grade closure models, e.g. for bubble forces and coalescence and break-up. It is also an ideal base to validate CFD approaches for poly-dispersed flow. For this reason it is proposed to use the database as a benchmark for modelling poly-dispersed flows.     

Numerically investigating fire suppression mechanisms for the water mist with various droplet sizes through FDS code

With rapid progress in computer capability recently, it becomes feasible to investigate the sophisticated phenomena related to the fire, especially for the interaction of fire and water spray, by way of the computational fluid dynamics (CFD). In this paper, a fire simulation CFD code_FDS is used to numerically investigate the different droplet sizes on the fire suppression/extinguishment mechanisms. The CFD models adopted in the FDS are first assessed against the previous experimental work of Kim and Ryou. The droplet size interested is varied from 100 μm to 1000 μm that is located within the droplet size range for a water mist. Based on the sensitivity simulations with different droplet sizes, the dependency of fire extinguishing time on the discharged droplet size can be obtained. The fire extinguishing time decreases with the decreasing droplet size for a mist with relatively fine droplet size since both the evaporation cooling and the oxygen displacement are the dominant mechanisms of fire suppression. However, this trend is reverse for a mist with larger-size droplets for the sake that the direct cooling of flame is the major suppression mechanism. These conclusions are also confirmed by comparing the simulation distributions of gas temperature, oxygen concentration, and steam concentration after the mist actuation and just before the fire extinguishment.     

Experimental and numerical studies of liquid dispersal from a soft projectile impacting a wall

A medium-scale IMPACT test programme is currently being implemented at the Technical Research Centre of Finland (VTT). In these tests, deformable cylindrical steel or aluminium projectiles impact a solid concrete wall or a steel force plate. One part of the test is conducted with a missile filled with liquid water to study liquid dispersal phenomena (i.e., wet missile tests).The fluid-filled missile ranged in length from 0.5 to 1.5 m, the water mass inside the missile from 15 to 68 kg, and the impact velocity of missile from 70 to 177 m/s.This paper describes the methods used to measure the liquid dispersal processes, and presents the main results for preliminary simulations of liquid spread. Because the IMPACT tests have focused on structural aspects, it was necessary to develop cost-effective methods for measuring liquid phenomena. The tests measured some important parameters associated with liquid: the discharge speed and direction of the liquid core released from the ruptured missile, propagation speed of the spray front, liquid pooling on the floor, extent of liquid dispersal away from the target, and the drop size of the liquid spray.The experimental findings indicate that the liquid release starts along the surface almost perpendicularly to the incoming direction of the missile and forms a fairly “flat” and uniform splash pattern around the missile. Although the discharge speed of the liquid core may be initially much higher than the impact velocity of the missile, the propagation speed of the spray front decreases rapidly with increasing distance from the source. Results of the preliminary simulations show that the Fire Dynamic Simulator (FDS) program is a usable tool for simulating two-phase flows involving high-speed droplets, provided that the initial conditions (angle and speed of liquid release, droplet size, and initial air speed) can be specified appropriately. Given these requirements, FDS can reasonably well predict the formation of the water spray cloud and final distribution of water.     

竖直窄矩形通道内空气-水两相流中气弹运动速度研究

以空气和水为工质,应用高速摄像仪,对竖直窄矩形通道(3.25 mm×40 mm)内气液两相弹状流进行了可视化实验研究。气、液相表观速度分别为0.1~2.51 m/s和0.16~2.62 m/s,工作压力为常压。实验中发现窄矩形通道内弹状流与圆管中存在较大差别,气弹多发生变形,高液相流速时变形更为严重。窄边液膜含气量较高,在高液相流速时窄边液膜不下落,宽边液膜中含有由气弹头部进入和气弹尾部进入的气泡。气弹速度受气弹头部形状和宽度影响较大,受气弹长度影响较小。气弹速度可由Ishii & Jones-Zuber模型计算,但在低液相折算速度时偏差较大,其主要原因为漂移速度计算值较实验值偏小。     

One-dimensional three-field model of condensation in horizontal countercurrent flow with supercritical liquid velocity

A one-dimensional three-field model was developed to predict the flow of liquid and vapor that results from countercurrent flow of water injected into the hot leg of a PWR and the oncoming steam flowing from the upper plenum. The model solves the conservation equations for mass, momentum, and energy in a continuous-vapor field, a continuous-liquid field, and a dispersed-liquid (entrained-droplet) field. Single-effect experiments performed in the upper plenum test facility (UPTF) of the former SIEMENS KWU (now AREVA) at Mannheim, Germany, were used to validate the countercurrent flow limitation (CCFL) model in case of emergency core cooling water injection into the hot legs. Subcooled water and saturated steam flowed countercurrent in a horizontal pipe with an inside diameter of 0.75 m. The flow of injected water was varied from 150 kg/s to 400 kg/s, and the flow of steam varied from 13 kg/s to 178 kg/s. The subcooling of the liquid ranged from 0 K to 104 K. The velocity of the water at the injection point was supercritical (greater than the celerity of a gravity wave) for all the experiments. The three-field model was successfully used to predict the experimental data, and the results from the model provide insight into the mechanisms that influence the flows of liquid and vapor during countercurrent flow in a hot leg. When the injected water was saturated and the flow of steam was small, all or most of the injected water flowed to the upper plenum. Because the velocity of the liquid remained supercritical, entrainment of droplets was suppressed. When the injected water was saturated and the flow of steam was large, the interfacial shear stress on the continuous liquid caused the velocity in the liquid to become subcritical, resulting in a hydraulic jump. Entrainment ensued, and the flow of liquid to the end of the hot leg was greatly reduced.The influence of condensation on the transition from supercritical to subcritical flow as observed in the experimental data is also predicted with the three-field model. When the injected water was subcooled, condensation on the flow of continuous liquid caused a reduction in the flow of vapor and, consequently, a reduction in the interfacial shear stress. Therefore, the flow of liquid remained supercritical to the end of the hot leg at the upper plenum. The entire flow of injected water flowed to the end of the hot leg at higher flows of steam when the injected water was subcooled than when it was saturated. When the flow of vapor was large enough to cause a hydraulic jump in the subcooled liquid, the rate of entrained droplets was greatly increased. The interfacial surface area of the droplets was several orders of magnitude greater than for the continuous-liquid field, and condensation rate on the droplet field was also several orders of magnitude greater. When the flow of vapor from the upper plenum was at its greatest, most of the flow in the continuous liquid was entrained before reaching the upper plenum. The large flow of subcooled droplets caused three-quarters of the steam to condense.     

液滴撞击湿壁面实验研究

采用高速摄像仪以11 000 帧/s的拍摄速度对3种不同尺寸的液滴以不同速度分别撞击湿壁面（水平及不同倾斜角度）的过程进行了观测。研究了铺展、产生冠状不飞溅、飞溅3种实验现象。定性分析了初始液滴直径、液滴撞击速度、湿壁面倾斜角度对飞溅现象的作用。定量统计出液滴撞击湿壁面产生飞溅现象的各项临界参数,给出飞溅判据的经验关系式并进行误差分析。结果表明：随液滴撞击速度的增大,撞击湿壁面后会依次出现铺展、产生冠状不飞溅、产生二次液滴飞溅等现象。液滴直径的增加与湿壁面倾斜角度的降低均对飞溅的产生有促进作用。并通过拟合实验数据得到用于判别液滴撞击湿壁面是否发生产生二次液滴飞溅现象的无量纲参数K与无量纲液膜层厚度H^*关系K=3210-122 284.48H^*+2.234 26×10⁶H^*2（0.008 56*<0.033 18）。     

A numerical study on turbulence attenuation model for liquid droplet impingement erosion

The bent pipe wall thinning has been often found at the elbow of the drain line and the high-pressure secondary feed-water bent pipe in the nuclear reactors. The liquid droplet impingement (LDI) erosion could be regarded to be one of the major causes and is a significant issue of the thermal hydraulics and structural integrity in aging and life extension for nuclear power plants safety. In this paper two-phase numerical simulations are conducted for standard elbow geometry, typically the pipe diameter is 170 mm. The turbulence attenuation in vapor-droplets flow is analysed by a damping function on the energy spectrum basis of single phase flow. Considering the vapor turbulent kinetic energy attenuation due to the involved droplets, a computational fluid dynamic (CFD) tool has been adopted by using two-way vapor-droplet coupled system. This computational fluid model is built up by incompressible Reynolds Averaged Navier–Stoke equations using standard k–ε model and the SIMPLE algorithm, and the numerical droplet model adopts the Lagrangian approach, a general LDI erosion prediction procedure for bent pipe geometry has been performed to supplement the CFD code. The liquid droplets diameter, velocity, volume concentration are evaluated for the effects of carrier turbulence attenuation. The result shows that carrier turbulence kinetic energy attenuation is proved to be an important effect for LDI erosion rate when investigating the bent pipe wall thinning phenomena.     

竖直窄矩形通道内弹状流中液膜特性研究

气液两相弹状流广泛存在于工程领域,弹状流中液膜特性对弹状流模型的建立具有重要意义。为此利用高速摄像系统,对竖直窄矩形通道(3.25 mm×40 mm)内弹状流中液膜进行了可视化研究。实验中发现窄矩形通道中气弹左右两侧窄边液膜厚度不等且存在波动,但其对两侧液膜速度影响较小,两侧液膜速度相等。液膜脱离厚度主要受两相流速及气弹长度影响。液膜脱离速度随液相折算速度增加而增大;在低液相流速时,随气相折算速度增加而减小;当液相流速≥1.204 m/s时,液膜不下落,液膜脱离速度随气相速度变化较小,主要受液相流速影响。     

Flow structure of steam–water mixed spray

In this study, the flow structure of a steam–water mixed spray is studied both numerically and experimentally. The velocity and pressure profiles of single-phase flow are calculated using numerical methods. On the basis of the calculated flow fields, the droplet behavior is predicted by a one-way interaction model. This numerical analysis reveals that the droplets are accelerated even after they are sprayed from the nozzle. Experimentally, the mixed spray is observed using an ultra-high-speed video camera, and the velocity field is measured by using the oarticle image velocimetry (PIV) technique. Along with this PIV velocity field measurement, the velocities and diameters of droplets are measured by phase Doppler anemometry. Furthermore, the mixing process of steam and water and the atomization process of a liquid film are observed using a transparent nozzle. High-speed photography observations reveal that the flow inside the nozzle is annular flow and that most of the liquid film is atomized at the nozzle throat and nozzle outlet. Finally, the optimum mixing method for steam and water is determined.     

Simulation of drop deposition process in annular mist flow using three-dimensional particle method

The three-dimensional moving particle semi-implicit (MPS) method is employed to simulate the deposition process of single droplet on the liquid film. The model accounts for the presence of inertial, gravitation, viscous and surface tension and is validated by comparison with experimental results. The parameters of liquid droplets and film are calculated by a one-dimensional mixture model in which correlations and methods on void fraction, entrainment fraction and droplet velocity and size distribution are employed. The simulation results are analyzed to study the effect of splash on the deposition and re-entrainment processes in annular-mist flow. It is found that splash plays an important role in the deposition and re-entrainment processes in high quality conditions of BWR.     

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.     

丝网分离器水滴撞击水膜的机理研究

运用VOF多相流模型,对丝网分离器内水滴撞击网丝表面沉积水膜的过程进行数值模拟,研究和预测大水滴在较高汽速条件下撞壁产生二次携带的机理。深入研究水滴因素,如水滴大小、初始速度和水膜厚度对丝网的二次携带现象的影响,为丝网分离器设计提供参考。数值结果分析表明,水滴的大小和撞击速度是影响撞击飞溅发生的主要原因,飞溅生成的二次小水滴是造水装置丝网分离器二次携带的重要组成。丝网分离器内水滴撞击网丝水膜能否发生二次携带取决于阈值K_cr＝Oh•Re^a,当K大于K_cr时必然产生二次携带。     

Visualized measurement of extremely high-speed droplets in Venturi scrubber

Venturi scrubbers for filtered venting have been installed in nuclear power plants worldwide. Venturi scrubbers can eliminate fission products from a polluted gas by interaction through gas–liquid interfaces. Therefore, droplet diameter is important from the viewpoint of decontamination. When Venturi scrubbers are used in severe accidents, the gas flow velocity might be extremely high. In these studies, the authors did not measure droplet diameter under extremely high gas velocity conditions. In the scenarios, experimental data pertaining to droplet diameter, under the extremely high gas flow velocity, are required. Therefore, this objective is to evaluate the diameter of extremely high-speed droplets. A visualization experiment was conducted using a Venturi scrubber. The droplet diameter distribution and the Sauter mean diameter (SMD) were determined. By comparing between the experimental value of SMDs and the one evaluated using Nukiyama–Tanasawa equation, it was confirmed that the Nukiyama–Tanasawa equation can be used to evaluate SMD with good accuracy in the gas velocity range of 82–250 m/s, except the highest gas velocity conditions. Furthermore, the droplet generation mechanism in the Venturi scrubber was considered to clarify the main reason why the Nukiyama–Tanasawa equation can be used to evaluate SMD droplet diameter.     

Droplet de-entrainment by inertial impaction on vertical rods in an air-droplet mixture flow

We experimentally investigate the de-entrainment of droplets by inertial impaction on an array of vertical rods in an air-droplet mixture flow. The de-entrainment efficiencies are measured for a single rod, for a single row of rods, and for a multi-row of rods. We investigate the effects of the droplet mass flux (0.5–5.4 kg/m² s), the droplet Weber number (3000–8000), the air velocity (0–6 m/s), the rod geometry, and the surface roughness on the de-entrainment, and the rod diameter-to-pitch ratio effect on the de-entrainment. The results for a single rod show that the de-entrainment efficiency decreases slightly as the droplet mass flux increases; however, in our experimental ranges, there is negligible dependence on the droplet Weber number, the air velocity, and the surface roughness. The rod geometry affects the de-entrainment efficiency. The results for a single row of rods show that the existence of neighboring rods promotes de-entrainment due to droplet splashing, and we develop a correlation to show the effect of diameter-to-pitch ratio on the de-entrainment. Using information on the de-entrainment efficiencies of a single rod and those of a single row of rods, we propose a correlation that predicts the de-entrainment efficiency for a multi-row of rods with a staggered array. The RMS errors of the correlation from the de-entrainment efficiencies experimentally obtained are within 13.5%.