Investigation of gas flow characteristics in proton exchange membrane fuel cell

An investigation of electrochemical behavior of PEMFC (proton exchange membrane fuel cell) is performed by using a single-phase two-dimensional finite element analysis. Equations of current balance, mass balance, and momentum balance are implemented to simulate the behavior of PEMFC. The analysis results for the co-flow and counter-flow mode of gas flow direction are examined in detail in order to compare how the gas flow direction affects quantitatively. The characteristics of internal properties, such as gas velocity distribution, mass fraction of the reactants, fraction of water and current density distribution in PEMFC are illustrated in the electrode and GDL (gas diffusion layer). It is found that the dry reactant gases can be well internally humidified and maintain high performance in the case of the counter-flow mode without external humidification while it is not advantageous for highly humidified or saturated reactant gases. It is also found that the co-flow mode improves the current density distribution with humidified normal condition compared to the counter-flow mode.     

燃料电池新型流道对气体扩散层表面水去除的影响

质子交换膜燃料电池(Proton exchange membrane fuel cell,PEMFC)中气体扩散层(Gas diffusion layer,GDL)表面液态水的有效去除和输送对PEMFC的水管理非常重要。为了有效去除GDL表面液态水,提出一种新型流道结构,采用流体体积法对流道内液态水的传输过程进行三维数值研究,研究进气速度、表面润湿性和液滴尺寸对流道内液态水的传输过程和GDL表面液态水去除的影响。研究结果表明,新型流道结构可以有效去除GDL表面液态水。随着进气速度的增大,沿流动方向的空气剪切力增大,流道内水去除速率和压降增大,GDL表面水覆盖率降低。表面润湿性对液态水传输影响显著,GDL表面润湿性增强会减缓液滴运输,流道内阻力降低,压降减小,GDL表面水覆盖率增大。管表面润湿性增强,流道内压降和GDL表面水覆盖率降低。新型流道适用于流道内大液滴的去除。当θ_GDL=150°和θ_pipe=30°时,新型流道结构有较好的GDL表面除水性能。本研究工作为流道结构提供了一种新的选择,对GDL表面液态水的去除具有一定的指导意义。     

Prediction of flow crossover in the GDL of PEFC using serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for Polymer electrolyte fuel cells (PEFCs) since it ensures the removal of water produced in the cell with an acceptable parasitic load. The operating parameters such as temperature, pressure and flow distribution in the flow channel and gas diffusion layer (GDL) has a great influence on the performance of PEFCs. It is desired to have an optimum pressure drop because a certain pressure drop helps to remove excess liquid water from the fuel cell, too much of pressure drop would increase parasitic power needed for the pumping air through the fuel cell. In order to accurately estimate the pressure drop precise calculation of mass conservation is necessary. Flow crossover in the serpentine channel and GDL of PEFC has been investigated by using a transient, non-isothermal and three-dimensional numerical model. Considerable amount of cross flow through GDL is found and its influence on the pressure variation in the channel is identified. The results obtained by numerical simulation are also compared with the experimental as well as theoretical solution.     

Supercritical droplet dynamics and emission in low speed cross-flows

Droplet dynamics and emission of a supercritical droplet in crossing gas stream are numerically investigated. Effects of ambient pressure and velocity of nitrogen gas on the dynamics of the supercritical oxygen droplet are parametrically examined. Unsteady conservative axisymmetric Navier-Stokes equations. in curvilinear coordinates are preconditioned and solved by dual-time stepping method. A unified property evaluation scheme based on a fundamental equation of state and extended corresponding-state principle is established to deal with thermodynamic non-idealities and transport anomalies. At lower pressures and velocities of nitrogen cross flows, both the diffusion and the convection are important in determining the droplet dynamics. Relative flow motion causes a secondary breakup and cascading vortices, and the droplet lifetime is reduced with increasing in ambient pressure. At higher ambient pressures and velocities, however, the droplet dynamics become convection-controlled while the secondary breakup is hindered by reduced diffusivity of the oxygen. Gas-phase mixing depends on the convection and diffusion velocities in conjunction with corresponding droplet deformation and flow interaction. Supercritical droplet dynamics and emission is not similar with respect to the pressure and velocity of the ambient gas and thus provides no scale.     

Pressure response and droplet ejection of a piezoelectric inkjet printhead

The present study aims to investigate the pressure rise in the ink flow channel and the ink droplet formation process of a piezoelectric printhead after an electrical pulse is applied to the printhead. The ink flow channel is modeled as a straight circular pipe followed by a convergent nozzle. Both numerical analysis and experimental observations are performed in this study. In the numerical analysis, a characteristic method is used to solve the one-dimensional wave equation to obtain the transient pressure and velocity variations in the flow channel of the printhead. In this analysis, the channel is assumed to have a non-uniform cross section. In addition, a flow visualization system was set up to observe the ink droplet injection process. After the piezoelectric material is driven by the input electric pulse, the ink droplet images are immediately captured by a charge-couple device (CCD) camera converted to a digital image via a frame grabber, and stored in a computer. The results obtained from the experimental observations are also compared with the numerical prediction. The effects of electric pulse shape and voltage on the ink injection length and the ejected droplet weight are also presented.     

Combustion of a single droplet in the presence of an oscillating flow

The two-dimensional, unsteady, laminar conservation equations for mass, momentum, energy and species transport in the gas phase are solved numerically in spherical coordinates. This is to study the heat and the mass transfer, and the combustion around a single spherical droplet. The droplet mass and momentum equations are also solved simultaneously with the gas phase equations in order to investigate the effects of droplet entrainment in the oscillating flow with and without a steady velocity. The numerical solution for a single droplet combustion gives the droplet diameter variation as well as the gas phase velocity, temperature and species concentrations as a function of time. The effects of frequency, amplitude of oscillating flow, velocity ratio of oscillating flow amplitude to the steady velocity, ambient temperature and initial droplet diameter on the droplet combustion are also investigated. The droplet burning history is not governed by thed ²-law in the presence of oscillating flow, unlike to the case under quiescent ambient conditions.     

Three-dimensional computational fluid dynamic analysis of the conventional PEM fuel cell and investigation of prominent gas diffusion layers effect

A full three-dimensional, single phase computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) with both the gas distribution flow channels and the membrane electrode assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region are developed and numerically solved using a finite volume based computational fluid dynamics technique. In this research some parameters such as oxygen consumption, water production, velocity distribution, ohmic losses, liquid water activity and fuel cell performance for straight (base case) and prominent gas diffusion layers were investigated in more detail. The numerical simulations reveal that prominent gas diffusion layer improves the transport of the reactant gases through the porous layers; it is due to increase of the mentioned fuel cell efficiency, and prominent gas diffusion layers yield appreciably higher current density. Finally the numerical results of proposed CFD model (base case) are compared with the available experimental data that represent good agreement.     

聚合物气体辅助挤出中气体流动对熔体截面的影响

将聚合物熔体和低速热空气均视作不可压流体,针对一聚苯乙烯（PS）片材的全气体辅助挤出,建立了描述其气体-熔体两相分层流动的三维有限元模型,采用黏弹应力分离法（EVSS）和非协调流线迎风法（SU）等有限元方法,利用PolyFlow求解器对气体辅助流道中气体和熔体流动进行了计算,分析了熔体截面变化的规律及原因。研究结果表明：气体辅助流道内,气体对熔体有拖曳作用;沿挤出方向,熔体速度逐渐增大,而截面积逐渐减小,都在口模出口面上达到极值,同时截面形状有微小改变;口模出口面上熔体沿挤出方向的速度随入口气体体积流率的增大而近似线性增大,熔体截面积则近似线性减小。     

Flow near the meniscus of a pressure-driven water slug in microchannels

Micro-PIV system with a high speed CCD camera is used to measure the flow field near the advancing meniscus of a water slug in microchannels. Image shifting technique combined with meniscus detecting technique is proposed to measure the relative velocity of the liquid near the meniscus in a moving reference frame. The proposed method is applied to an advancing front of a slug in microchannels with rectangular cross section. In the case of hydrophilic channel, strong flow from the center to the side wall along the meniscus occurs, while in the case of the hydrophobic channel, the fluid flows in the opposite direction. Further, the velocity near the side wall is higher than the center region velocity, exhibiting the characteristics of a strong shear-driven flow. This phenomenon is explained to be due to the existence of small gaps between the slug and the channel wall at each capillary corner so that the gas flows through the gaps inducing high shear on the slug surface. Simulation of the shape of a static droplet inside a cubic cell obtained by using the Surface Evolver program is supportive of the existence of the gap at the rectangular capillary corners. The flow fields in the circular capillary, in which no such gap exists, are also measured. The results show that a similar flow pattern to that of the hydrophilic rectangular capillary (i.e., center-to-wall flow) is always exhibited regardless of the wettability of the channel wall, which is also indicative of the validity of the above-mentioned assertion.     

Proton exchange membrane fuel cells performance enhancement using bipolar channel indentation

The influence of bipolar channel indentation on the performance enhancement of proton exchange membrane (PEM) fuel cells is studied using three-dimensional computation of a whole single cell. It is assumed that the cell operates under a fixed dry condition of 80°C, 1 atm, and inlets’ relative humidity of 10%. Hence the sole influence of channel-bed shapes on the cell performance was considered. Three straight channels with different bed shapes were considered, namely, flat and indented (including semicircular and wavy) channel-beds. It is observed that the channel-bed shapes directly influence the fluid mechanics of the flow field within the channels, such as the magnitude and direction of the velocity vectors, pressure variations along the channels, and the consumption rates of oxygen and hydrogen within the catalyst layer. The results reveal that gas flow channel indentations in the anode and cathode sides enhance the net transport of reacting species through the porous layers toward the catalyst layer. The improvement of the cell due to channel indentation is observed to be in the range 18–22%.     

微小尺度下平板间气体流动机理及压力特性分析

基于平板间气膜内气体分子运动和碰撞的规律,提出气膜分层理论,将板间气膜内的气体划分为近壁层、稀薄层、连续流层。给出了划分稀薄层和连续流层的依据,建立分层物理模型并提出每层的控制方程,验证了分层理论的合理性。通过大规模原子/分子大型并行模拟器仿真板间气膜内气体流态并计算沿高度方向的压力,得出了如下结论：随着板间气体流速的增大,板间气膜有效压力减小,连续流层的厚度增大,稀薄层的厚度减小;当气体流速到达一定值时,气膜内压力不再分层,速度滑移现象可以忽略。     

Pressure data for various flow channels in proton exchange membrane (PEM) fuel cell

Micro flow channels in flow plates of fuel cells have become much narrower and longer to improve reactant flow distribution leading to increase of pumping power. Therefore it is very important to minimize the pressure drops in the flow channel because increased pumping power reduces overall efficiency. We investigated pressure drops in a micro flow channel at the anode and cathode compared to pressure losses for cold flow in straight, bended and serpentine channels. The results show that friction factors for cold flow channels could be used for parallel and bended flow channel designs for fuel cells. Pressure drop in the serpentine flow channel is the lowest among all flow channels due to bypass flow across the gas diffusion layer under reactive flow condition, although its pressure drop is highest for a cold flow condition. So the effect of bypass flow for serpentine flow channels should be considered when designing flow channels     

Parallel computation of two-phase flow in a microchannel using the lattice Boltzmann method

We present a numerical simulation of two-phase flow in a three-dimensional cross-junction microchannel by using the lattice Boltzmann method (LBM). At first, we validated our LBM code with the velocity profile in a 3-dimensional rectangular channel. Then, we developed a lattice Boltzmann code based on the free energy model to simulate the immiscible binary fluid flow. The parallelization of the developed code is implemented on a PC cluster using the MPI program. The numerical results of two-phase flow in the microchannel reveal droplet formation process, which compares well with corresponding experimental results. The size of droplet decreases with increase of the flow-rate ratio and the capillary number. The movement of a droplet through the microchannel induces three-dimensional circulating flow inside the droplet. This complex flow is thought to enhance the mixing and reaction of reagents.     

A study on the spray characteristics of a piezo pintle-type injector for DI gasoline engines

We studied the spray flow initiated from a piezo pintle-type injector for DI gasoline engines in an environment supplied by a constant volume vessel by means of laser diagnostics. To fully grasp the effects of the characteristic parameters, including designed spray angle, needle lift, injection pressure (P _inj ) and ambient pressure (P _b ), on the spray atomization and mixture preparation, particle image velocimetry (PIV) and phase Doppler anemometry (PDA) are used in the experiment, respectively. The gas perpendicularly enters into the outer periphery of the conical spray injected through the pintle-type injector activated by piezo, which creates two large-scale vortices: the vortex A and vortex B. The velocity standard deviation of the spray field is introduced to analyze the gas flow motion in the vicinity of nozzle. The droplet information of spray field is also recorded by PDA in variable boundary conditions. The time dividing method is used to study the droplet characteristics in four parts of spray. The injector with 98° designed spray angle has smaller droplet mean diameter (D10 and D32), due to a larger spray distribution. When the droplet velocity of the spray field is close to 0 m/s, the D10 and D32 hold at around 10 μm and 20 μm, respectively, in atmospheric pressure condition, which are about 20 μm and 40 μm, respectively, at ambient pressure of 1.1 MPa.     

Numerical simulation of droplet formation in a micro-channel using the lattice Boltzmann method

This study investigates droplet formation in a micro-channel using the lattice Boltzmann (LB) method. A cross-junction micro-channel and two immiscible, water and oil phase fluids, were used to form the micro-droplets. Droplets are formed by the hydrodynamic instability on the interface between two immiscible fluids when two immiscible fluids are imported simultaneously in a cross-junction micro-channel. The Shan & Chen model, which is a lattice Boltzmann model of two-phase flows, is used to treat the interaction between immiscible fluids. The detailed process of the droplet formation in the cross-junction micro-channel was illustrated. The results of the droplet formation by the LBM predicted well the experimental data by PIV (particle image velocimetry). The effect of the surface tension and the flow rate of water phase fluid on the droplet length and the interval between droplets was also investigated. As the surface tension increased, the droplet length and the interval between droplets were increased. On the other hand, when we increased the flow rate of the water phase fluid under the condition of the fixed oil-phase fluid flow rate, the droplet size was increased while the interval between droplets was decreased.     

水滴迷宫式调节阀空化特性计算与结构改进

针对超(超)临界机组中水滴迷宫式调节阀在高温高压工况下引起的严重气蚀问题,基于计算流体力学理论和空化机理,选用标准k-ε湍流模型、Mixture模型和Schnerr-Sauer空化模型,比较了改进前后调节阀在典型开度下的压力、速度、气相体积分数等结构性能.计算结果表明:原始碟片结构压降大,最大可至19.95 MPa,流...     

High data rate measurements of droplet dynamics in a vertical gas-liquid annular flow

Two-dimensional phase Doppler anemometry techniques have been used to achieve simultaneous measurement of the velocity and size of individual droplets entrained into the gas core during upward annular gas-liquid flow. Data rates greatly exceeding those of other experimental techniques have been achieved. Information is presented on the centreline variation of droplet velocity and droplet size, together with size and velocity correlations, for a range of flowrates and hydrodynamic development lengths from 135 to 228.     

影响微槽道流动扩散特性因素的研究

对压力驱动流下微槽道内流动的不同组分扩散进行了数值模拟。结果表明，雷诺数(RP)是影响微通道内物质扩散的主要因素，合适的RP对相应粒子直径物质的扩散很重要。此外，槽道的扩散尺寸(d)与通道深度(w)之比(d／w)对扩散也有一定的影响，当d／w较大时，沿硼的浓度分布明显为抛物线，此时上下壁面的影响是主要的；反之，当d／w较小时，沿d、w两个方向的速度分布都为抛物线，上下左右壁面影响都不可忽略。研究结果对微流控系统器件的设计与制造、T—sensor的数据采集与分析具有指导作用。     

Pressure drop and flow distribution characteristics of single and parallel serpentine flow fields for polymer electrolyte membrane fuel cells

This study numerically investigates pressure drop and flow distribution characteristics of serpentine flow fields (SFFs) that are designed for polymer electrolyte membrane fuel cells, which consider the Poiseuille flow with secondary pressure drop in the gas channel (GC) and the Darcy flow in the porous gas diffusion layer (GDL). The numerical results for a conventional SFF agreed well with those obtained via computational fluid dynamics simulations, thus proving the validity of the present flow network model. This model is employed to characterize various single and parallel SFFs, including multi-pass serpentine flow fields (MPSFFs). Findings reveal that under-rib convection (convective flow through GDL under an interconnector rib) is an important transport process for conventional SFFs, with its intensity being significantly enhanced as GDL permeability increases. The results also indicate that under-rib convection can be significantly improved by employing MPSFFs as the reactant flow field, because of the closely interlaced structure of GC regions that have different path-lengths from the inlet. However, reactant flow rate through GCs proportionally decreases as under-rib convection intensity increases, suggesting that proper optimization is required between the flow velocity in GCs and the under-rib convection intensity in GDLs.     

Multi – oil droplet recognition of oil-water two-phase flow based on integrated features

Multi-oil droplet target recognition is one of the applications of machine vision in the measurement of oil-water two-phase flow parameters, which could combine other algorithms to obtain the oil droplet velocity and the water holdup of oil water two-phase flow. Appropriate target representation features can improve the recognition effect of multiple oil droplets. However, due to shooting environment differences and quality differences of oil-water two-phase flow images, existing target representation features do not perform well in low-quality oil-water two-phase flow images. To improve the precision of multi-oil droplet target recognition in oil-water two-phase flow and reduce the miss rate, this paper constructs an integrated feature on the basis of aggregate channel features (ACF). The integrated feature named aggregate channel features with histogram of local gravitational feature（ACFHG） contains the color feature channels reflecting the overall color features of the oil droplet sample, the gradient amplitude channel reflecting the overall gradient of the oil droplet sample image, the gradient direction histogram feature channels reflecting the local gradient of the oil droplet sample image, and the local gravitational feature channels that ensure oil droplet target recognition in low quality photos and photos taken in complex shooting environments. Moreover, the rotation invariance is obtained by taking the oriented gradient histogram of the local gravitational feature to further improve the multi-oil droplet target recognition effect. Experiment results show that the average precision of multi-oil droplet target recognition using the integrated features is 83.38%, which is 9.93% higher than that with using ACF, and the miss rate is 9.13%, which is 57.18% lower than that with using ACF. Compared with other existing target detection methods, the method proposed in this paper still has an advantage in the rate of missed detection.