逆流喷雾式饱和器内两相速度场的实验研究

应用三维粒子动态分析仪(PDA)和二维粒子速度分析仪(PIV)对逆流喷雾式饱和器内湿化过程的气液两相流流场进行了实验测量。通过稳态和瞬态速度场的测量结果,获得了饱和器内各点上水滴的三维平均速度、平均粒径和平均体积流量,以及饱和器中心线上的测量区域内水滴的瞬态速度分布和流线,给出了逆流喷雾式饱和器内液相水滴的三维平均速度、平均粒径和平均体积流量的分布规律,揭示了其内部流动的高紊流特性。图4表1参7     

Numerical simulation of counter-flow spray saturator for humid air turbine cycle

The numerical simulations of simultaneous heat and mass transfer process in the counter-flow spray saturator and humid air turbine cycle are carried out in this work, according to the experimental conditions and actual size of a prototype saturator. This humidifying process involves two-phase flow of air and water droplets, also including interaction, breakup and collision of water droplets. Eulerian approach is used for gas phase flow, Lagrangian approach is used for liquid phase flow, and the two-way coupling is used between two phases. The simulations agree well with the experimental measurements. The simulations show the flow is with high turbulence intensity, the relative humidity and temperature of humid air increase along with the height of saturator, some water droplets carried by air escape from the saturator, and the humid air is mainly humidified at the lower part of saturator and is simultaneously humidified and heated at the upper part.     

模型饱和器冷态喷雾场的实验研究

针对HAT循环关键部件增湿饱和器内典型的传热，传质过程现象，为了最优化该过程和为今后数值分析提供有效的数据，设计了模拟饱和顺内传热，传质过程中的开放式冷态实验系统。使用相位多普勒分析仪DualPDA（Phase Doppler Analyzer）对冷态模型饱和器的喷雾场进行了详细的实验研究，测量了不同水压，不同气流速度下的喷雾场，得到喷雾场内液滴的三维平均速度分布，脉动速度分布，平均粒径大小分布等，分析了喷雾水压和鼓风气流速度对喷雾场的影响，分析结果表明，改良设计的离心喷嘴具用良好的喷雾性能和轴对称性，水压增大可以增大喷雾场中粒子速度，通量及降低平均直径，喷嘴喷雾长度随气流速度减小，气流速度可以改变喷雾场的分布，有利于液滴蒸发和液滴破裂。     

Dynamic behaviour of liquid water emerging from a GDL pore into a PEMFC gas flow channel

A numerical investigation of the dynamic behaviour of liquid water entering a polymer electrolyte membrane fuel cell (PEMFC) channel through a GDL pore is reported. Two-dimensional, transient simulations employing the volume of fluid (VOF) method are performed to explicitly track the liquid–gas interface, and to gain understanding into the dynamics of a water droplet subjected to air flow in the bulk of the gas channel. The modeled domain consists of a straight channel with air flowing from one side and water entering the domain from a pore at the bottom wall of the channel. The channel dimensions, flow conditions and surface properties are chosen to be representative of typical conditions in a PEMFC. A series of parametric studies, including the effects of channel size, pore size, and the coalescence of droplets are performed with a particular focus on the effect of geometrical structure. The simulation results and analysis of the time evolution of flow patterns show that the height of the channel as well as the width of the pore have significant impacts on the deformation and detachment of the water droplet. Simulations performed for droplets emerging from two pores with the same size into the channel show that coalescence of two water droplets can accelerate the deformation rate and motion of the droplets in the microchannel. Accounting for the initial connection of a droplet to a pore was found to yield critical air inlet velocities for droplet detachment that are significantly different from previous studies that considered an initially stagnant droplet sitting on the surface. The predicted critical air velocity is found to be sensitive to the geometry of the pore, with higher values obtained when the curvature associated with the GDL fibres is taken into account. The critical velocity is also found to decrease with increasing droplet size and decreasing GDL pore diameter.     

逆流喷雾式饱和器内湿化过程的实验研究

对逆流喷雾式饱和器内部空气的湿化过程进行实验研究,实验中不仅测量了饱和器进出口湿空气的相对湿度、温度和水的温度,而且也测量了饱和器内部几个高度截面上湿空气的相对湿度和气相、液相的温度。根据实验测量的湿空气的相对湿度和温度,计算出了饱和器内湿空气的含湿量和测量高度间湿空气的加湿量。由实验结果可见,随水气质量比的增大,饱和器出口湿空气的温度和温升也相应增大。湿空气的含湿量和水的蒸发量、出口温度随进口水温升高、水气质量比增大而增大。在所有实验工况下,饱和器出口湿空气接近或达到饱和。随空气速度增大水滴逃逸量增大。总体上饱和器内部下部主要是加湿进口空气,上部是加湿和加热空气。     

Numerical prediction of airfoil characteristics in a transonic droplet-laden air flow

A transonic airfoil moving in an air–water droplet two-phase flow is investigated numerically to study the effect of droplet size and volume fraction. The droplets we consider are in the size 1–100 μm and the volume fraction is in 0.01%–10%. A compressible two-fluid model is solved by the WAF-HLL scheme developed earlier by the authors which includes drag force, heat transfer, phase change, and droplet fragmentation of the droplets. The numerical results show that the droplet breakup layer can be extended to a later distance as large as about 60% of a chord length at the trailing edge. Also the droplets have made the shock wave dissolved in compression waves and the airfoil performance seriously deteriorated.     

Three-dimensional numerical simulations of water droplet dynamics in a PEMFC gas channel

The dynamic behavior of liquid water emerging from the gas diffusion layer (GDL) into the gas flow channel of a polymer electrolyte membrane fuel cell (PEMFC) is modeled by considering a 1000 μm long air flow microchannel with a 250 μm × 250 μm square cross section and having a pore on the GDL surface through which water emerges with prescribed flow rates. The transient three-dimensional two-phase flow is solved using Computational fluid dynamics in conjunction with a volume of fluid method. Simulations of the processes of water droplet emergence, growth, deformation and detachment are performed to explicitly track the evolution of the liquid–gas interface, and to characterize the dynamics of a water droplet subjected to air flow in the bulk of the gas channel in terms of departure diameter, flow resistance coefficient, water saturation, and water coverage ratio. Parametric simulations including the effects of air flow velocity, water injection velocity, and dimensions of the pore are performed with a particular focus on the effect of the hydrophobicity of the GDL surface while the static contact angles of the other channel walls are set to 45°. The wettability of the microchannel surface is shown to have a major impact on the dynamics of the water droplet, with a droplet splitting more readily and convecting rapidly on a hydrophobic surface, while for a hydrophilic surface there is a tendency for spreading and film flow formation. The hydrophilic side walls of the microchannel appear to provide some benefit by lifting the attached water from the GDL surface, thus freeing the GDL-flow channel interface for improved mass transfer of the reactant. Higher air inlet velocities are shown to reduce water coverage of the GDL surface. Lower water injection velocities as well as smaller pore sizes result in earlier departure of water droplets and lower water volume fraction in the microchannel.     

柱状旋流分离器内液滴运动轨迹的数值模拟

在不考虑相间相互作用的条件下,气相采用RNGk-ε湍流模型,液相采用随机轨道模型,对热气机排气系统中的气液分离器两相流动进行了数值模拟,揭示了气液分离器内的流动分布和液滴的运动轨迹及分离机理。计算结果表明：分离器内的速度场分布不均匀,并且靠近进出口的气流速度较大,柱体中心的气体切向速度很小;另外不同粒径的液滴运动轨迹差别很大,粒径大的液滴较容易分离;液滴入射的初始位置不同,其分离速度的差别较大,运动轨迹也明显不同。     

Monodisperse droplet heating and evaporation: Experimental study and modelling

Results of experimental studies and the modelling of heating and evaporation of monodisperse ethanol and acetone droplets in two regimes are presented. Firstly, pure heating and evaporation of droplets in a flow of air of prescribed temperature are considered. Secondly, droplet heating and evaporation in a flame produced by previously injected combusting droplets are studied. The phase Doppler anemometry technique is used for droplet velocity and size measurements. Two-colour laser induced fluorescence thermometry is used to estimate droplet temperatures. The experiments have been performed for various distances between droplets and various initial droplet radii and velocities. The experimental data have been compared with the results of modelling, based on given gas temperatures, measured by coherent anti-stokes Raman spectroscopy, and Nusselt and Sherwood numbers calculated using measured values of droplet relative velocities. When estimating the latter numbers the finite distance between droplets was taken into account. The model is based on the assumption that droplets are spherically symmetrical, but takes into account the radial distribution of temperature inside droplets. It is pointed out that for relatively small droplets (initial radii about 65 μm) the experimentally measured droplet temperatures are close to the predicted average droplet temperatures, while for larger droplets (initial radii about 120 μm) the experimentally measured droplet temperatures are close to the temperatures predicted at the centre of the droplets.     

Interface resolving two-phase flow simulations in gas channels relevant for polymer electrolyte fuel cells using the volume of fluid approach

With the increased concern about energy security, air pollution and global warming, the possibility of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy systems has achieved considerable momentum. A computational fluid dynamic model describing a straight channel, relevant for water removal inside a PEFC, is devised. A volume of fluid (VOF) approach is employed to investigate the interface resolved two-phase flow behavior inside the gas channel including the gas diffusion layer (GDL) surface. From this study, it is clear that the impact on the two-phase flow pattern for different hydrophobic/hydrophilic characteristics, i.e., contact angles, at the walls and at the GDL surface is significant, compared to a situation where the walls and the interface are neither hydrophobic nor hydrophilic (i.e., 90° contact angle at the walls and also at the GDL surface). A location of the GDL surface liquid inlet in the middle of the gas channel gives droplet formation, while a location at the side of the channel gives corner flow with a convex surface shape (having hydrophilic walls and a hydrophobic GDL interface). Droplet formation only observed when the GDL surface liquid inlet is located in the middle of the channel. The droplet detachment location (along the main flow direction) and the shape of the droplet until detachment are strongly dependent on the size of the liquid inlet at the GDL surface. A smaller liquid inlet at the GDL surface (keeping the mass flow rates constant) gives smaller droplets.     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical model of evaporative cooling processes in a new type of cooling tower

A numerical model for studying the evaporative cooling processes that take place in a new type of cooling tower has been developed. In contrast to conventional cooling towers, this new device called Hydrosolar Roof presents lower droplet fall and uses renewable energy instead of fans to generate the air mass flow within the tower. The numerical model developed to analyse its performance is based on computational flow dynamics for the two-phase flow of humid air and water droplets. The Eulerian approach is used for the gas flow phase and the Lagrangian approach for the water droplet flow phase, with two-way coupling between both phases. Experimental results from a full-scale prototype in real conditions have been used for validation. The main results of this study show the strong influence of the average water drop size on efficiency of the system and reveal the effect of other variables like wet bulb temperature, water mass flow to air mass flow ratio and temperature gap between water inlet temperature and wet bulb temperature. Nondimensional numerical correlation of efficiency as a function of these significant parameters has been calculated.     

Numerical simulation of condensate removal from gas channels of PEM fuel cells using corrugated walls

The removal of condensate water droplets from gas channels is necessary for proper operation of proton exchange membrane fuel cells. In the current work, it is shown that corrugated wall gas channels can help in the removal of condensate water droplets formed on the channel walls. Removal of sessile droplets from channels having semicircle, rectangular dent, and saw‐tooth corrugation and at different gas velocities is modeled numerically. It is shown that the time of condensate removal is much shorter in a corrugated channel as compared with that in an uncorrugated channel. Three different droplet removal regimes are identified: droplet, film, and misty flow regimes. The transition from one to another regime is mapped based on the inlet flow velocity and the type of the channel corrugate.     

Experimental investigation of pressurized packing saturator for humid air turbine cycle

Humid air turbine (HAT) cycle is an advanced power generation system, and its efficiency and output power are improved by humidifying the compressed air. This humidification process is completed in the saturator. Therefore, the humidifying performance of saturator has great influence on the performance of HAT cycle. In this work, a new type packing saturator was designed and a series of experiments were carried out to study its humidifying performance. In order to improve the uniformity of the saturator inlet, a twin-tangential annular flow gas distributor was designed. Then it was authorized by China invention patents (ZL201010200778.9). Now, the mal-distribution factor of inlet air is mainly between 0.15 and 0.35 in all experimental conditions. Some key parameters of air and water at the inlet and outlet of saturator were measured at different experimental conditions. These results show the outlet humid air temperature is an important parameter for determining the humidifying amount of the saturator. The humidifying performance of the saturator is mainly affected by the inlet water temperature and the liquid/gas (L/G) ratio. At the same operating pressure, the humidity ratio of outlet humid air increases with inlet water temperature and L/G ratio. At higher inlet water temperature, the L/G ratio has a greater effect on the humidity ratio of outlet humid air. The outlet water temperature is mainly affected by the inlet gas temperature. With the increasing of inlet air temperature, the outlet water temperature increases, and it is close to the wet-bulb temperature of inlet air.     

Analysis of evaporating mist flow for enhanced convective heat transfer

Enhancement of forced convective heat transport through the use of evaporating mist flow is investigated analytically and by numerical simulation. A two-phase mist, consisting of finely dispersed water droplets in an airstream, is introduced at the inlet of a longitudinally-finned heat sink. The latent heat absorbed by the evaporating droplets significantly reduces the sensible heating of the air inside the heat sink which translates into higher heat-dissipation capacities. The flow and heat transfer characteristics of mist flows are studied through a detailed numerical analysis of the mass, momentum and energy transport equations for the mist droplets and the airstream, which are treated as two separate phases. The coupling between the two phases is modeled through interaction terms in the transport equations. The effects of inlet mist droplet size and concentration on the thermal performance of the heat sink are analyzed parametrically. The results provide insight into the complex transport processes associated with mist flows. The simulations indicate that significantly higher heat transfer coefficients are obtained with mist flows as compared to air flows, highlighting the potential for the use of mist flows for enhanced thermal management applications.     

Phase splitting of a slug–annular flow at a horizontal micro-T-junction

The behavior of air–water two-phase flow at a horizontal micro-T-junction with main and side branch of 500 μm diameter was studied in this work. When the inlet flow pattern was a slug–annular flow, the impact of varying of upstream gas and liquid superficial velocity on phase distribution of two-phase flow in the T-junction was investigated in detail. It is found that the liquid taken off decreases as an increase in liquid superficial velocity, while it increases as an increase in gas superficial velocity. Liquid and gas superficial velocity have stronger impact on phase distribution when the inlet gas velocity is high. When the present data are compared to those from larger diameter T-junctions at similar inlet superficial velocities, it is found that decreasing the diameter of T-junctions increases the fraction of liquid taken off.     

An investigation of two-phase flow pressure drop in helical rectangular channel

Numerical simulations have been carried out to evaluate the two-phase frictional pressure drop for air-water two-phase flow in horizontal helical rectangular channels by varying configurations, inlet velocity and inlet sectional liquid holdup. The investigations performed using eight coils, five different inlet velocity and four different inlet sectional liquid holdups. The effects of curvature, torsion, fluid velocity and inlet sectional liquid holdup on two-phase frictional pressure drop have been illustrated. It is found that the two-phase frictional pressure drop relates strongly to the superficial velocities of air or water, and that the curvature and torsion have some effect on the pressure drop for higher Reynolds number flows in large-scale helical rectangular channel; the inlet sectional liquid holdup only increases the magnitude of pressure drop in helical channel and has no effect on the development of pressure drop. The correlation developed predicts the two-phase frictional pressure drop in helical rectangular channel with acceptable statistical accuracy.     

烟气除湿喷淋塔出口液滴逃逸率研究

为了降低火电厂烟气含湿量,以新型的上喷淋上进气喷淋塔为研究对象,采用FLUENT模型模拟喷淋塔内两相流体的运动,并与实验验证,采用单变量分析方法研究烟气流速、喷淋液滴直径、喷淋速度、喷淋流量四种因素对出口液滴逃逸率的影响。研究表明：操作条件对出口液滴逃逸率的影响程度液滴直径>进气速度>喷淋流量>喷淋速度;出口液滴逃逸率随着进气速度的增长而线性增长,喷淋流量和喷淋速度则反之;在进气速度3.5 m/s下,当液滴直径小于1.00 mm时,逃逸率随液滴直径减小而迅速上升;当直径大于1.00 mm时,液滴逃逸率几乎为0.0%,不受喷淋流量影响。     

Water management studies in PEM fuel cells,Part II: Ex situ investigation of flow maldistribution,pressure drop and two-phase flow pattern in gas channels

Two-phase flow of water and reactant gases in the gas distribution channels of proton exchange membrane fuel cells (PEMFCs) plays a critical role in proper water management. In this work, the two-phase flow in PEMFC cathode parallel channels is studied over a wide range of superficial air velocity (air stoichiometry) and superficial water velocity in a specially designed ex situ experimental setup, which enables the measurement of instantaneous flow rates in individual gas channels and simultaneous visualization of the water flow structure. It is found that the two-phase flow at low superficial air velocities (air stoichiometry below 5) is dominated by slugs or semi-slugs, leading to severe flow maldistribution and large fluctuations in the pressure drop. Slug residence time, measured from the video observation and the instantaneous flow rate data, is found to be a new parameter to describe the slug flow. At higher air velocities, a water film is formed on the channel walls if they are hydrophilic. The pressure drop for the film flow is characterized by smaller but frequent fluctuations, which are found to result from the water buildup at the channel-exit manifold interface. As the superficial air velocity increases further, mist flow is obtained where little water buildup is observed. The water buildup in the gas channels at the two-phase flow is well described by the two-phase friction multiplier, defined as the ratio of the two-phase pressure drop to the single gas phase pressure drop. It is found that the two-phase friction multiplier increases with increasing water flow rate. A flow pattern map is developed using superficial water and air velocities with clearly defined transition regions.     

水雾粒径及初速度对壁面材料火灾灭火效果的影响

从理论上分析了雾滴粒径与初速度对水平运动的影响,得到了不同粒径与初速度时的雾滴在水平方向上能达到的极限距离,分析了雾滴在水平穿越火焰区时蒸发量与火焰厚度以及雾滴速度之间的关系.从理论上分析了水雾水平作用于火焰与垂直作用于火焰的区别,提出了当水雾抑制竖直壁面火蔓延时,其灭火机理以表面相灭火为主,采用较小的雾滴粒径并不能达到更好的灭火效果.在标准燃烧室内开展了全尺寸实验,对理论分析结果进行了验证.