A numerical study on the performance of polymer electrolyte membrane fuel cells due to the variation in gas diffusion layer permeability

Convective flow in the under-rib regions of gas diffusion layers (GDLs) is a non-negligible transport process that can enhance the performance of polymer electrolyte membrane fuel cells (PEMFCs) by facilitating efficient utilization of catalyst layers (CLs) in those regions. The permeability of GDLs has been recognized as a dominant factor influencing the intensity of the under-rib convection in PEMFCs. In this study, the correlation between the permeability of GDLs and the performance of PEMFCs was numerically investigated through a detailed simulation of the transport and electrochemical processes in PEMFCs using a computational fluid dynamics (CFD) tool. Three serpentine flow fields with one, three, or five parallel paths were considered as reactant flow channels for an active cell area of 3 cm × 3 cm, while the permeability of GDLs was varied from 1×10⁻¹² m² to 1×10⁻¹⁰ m². The effects of the flow field design and the GDL permeability on the performance of PEMFCs were presented, along with their impacts on the local distribution of current density, water content, and reactant concentration.     

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.     

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     

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。     

Pressure drop of wet gas flow with ultra-low liquid loading through DP meters

The most common method to predict the gas and liquid flow rates in a wet gas flow simultaneously is to use dual pressure drops (dual-DPs) from two or even one single DP meter. In this paper, the metering mechanism of applying dual-DPs were overviewed. To fully understand the response of DP meters to wet gas flows, the pressure drops of wet gas flow with ultra-low liquid loading through three typical DP meters were experimentally investigated, including an orifice plate meter, a cone meter and a Venturi meter. The equivalent diameter ratio is 0.45. The experimental fluids are air and tap water. The pressure is in the range of 0.1–0.3 MPa and the Lockhart-Martinelli parameter (X_LM) is less than approximately 0.02. The results show that the upstream-throat pressure drop, the downstream-throat pressure drop and the permanent pressure loss of individual DP meters have unique response to liquid loading. The upstream-throat pressure drop of the orifice plate meter decreases at first and then increases as the liquid loading increases, while that of the cone meter and the Venturi meter increase monotonically. The non-monotonicity of the pressure drop for the orifice plate meter can be attributed to the flow modulation of trace liquid. The downstream-throat pressure drops of all the three test sections decrease at first and then increase. The reason is that the liquid presence in a gas flow increases the downstream friction and vortex dissipation. The permanent pressure loss of the orifice plate meter also shows non-monotonicity. To avoid non-monotonicity, the pressure loss ratio is introduced, which is defined as the ratio of the permanent pressure loss to the upstream-throat pressure drop. Results show that the pressure loss ratio of the Venturi meter has the highest sensitivity to the liquid loading.     

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.     

Numerical analysis of 3-D flow through LNG marine control valves for their advanced design

A numerical analysis of three dimensional incompressible turbulent flows through high pressure drop control valves was carried out by using a CFD-ACE code to develop anti-cavitation control valve used in LNG marine system. For this, numerical simulation was performed on several models of control valve that have different orifice diameters of anti-trim and the size of valve discharge. In this study, flow characteristics of control valves with complex flow fields including pressure drop, cavitation effect and variation of flow coefficient as well as correlation of discharge coefficient were investigated and analyzed. Comparing with conventional control valves, newly designed valves by using the CFD analysis showed an improved flow pattern with reduced cavitation and an anticipated performance characteristic. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Wet gas pressure drop across orifice plate in horizontal pipes in the region of flow pattern transition

As a basis for measuring the mass flow rate of wet gas using differential pressure meters, predicting the pressure drop of a wet gas flowing through orifice plates is important; however, this has not yet been solved satisfactorily, although many studies have reported on that subject. In this study, the pressure drop of wet gas across sharp-edged orifice plates was experimentally investigated in the region of flow pattern transition using air and water as the two phases, and the prediction performance of the available pressure drop models was compared based on the experimental data. The results show that the homogenous flow models overestimate the pressure drop, whereas those models based on the separated flow model often present underestimations. The models reported for wet gas are also incapable of predicting the pressure drop in this region with acceptable accuracy. Through an analysis of the prediction deviations, it is found that the Froude number of the liquid phase has a significant influence on the pressure drop of the wet gas, besides the Froude number of the gas phase. Then, three new correlations that are based on the homogeneous flow, Chisholm model, and Murdock model, respectively, were proposed based on the experimental result.     

A novel gas divider using nonlinear laminar flow

Gas dividers are important in emissions measurement since they continuously and accurately mix two gases to create a known gas concentration that is needed in the multi-point calibration of gas analyzers. A novel gas divider was designed using nonlinear laminar flow induced from the density change along the capillary channels due to the high-pressure drop (relative to the inlet gas pressure). The minor losses from entrance and exit effects can be ignored due to the high pressure loss from Hagen-Poiseuille's law relative to the minor losses. Small diameter wires inside of a tube were used to create capillary channels through which gas could flow. The gas divider, using nonlinear laminar flow, showed lower measurement uncertainty at high (90%) dilution levels than using linear laminar flow due to the higher-pressure drop at the same volumetric flow rates. Experiments showed the expected gas concentration from using the gas divider to be within 2% of the measured gas concentrations.     

Modeling of heat transfer and fluid flow during gas tungsten arc welding of commercial pure aluminum

In the present study, the temperature and the velocity fields during gas tungsten arc welding of commercial pure aluminum were simulated using the solution of the equations of conversation of mass, energy and momentum in three dimensions and under steady-state heat transfer and fluid flow conditions. Then, by means of the prediction of temperature and velocity distributions, the weld pool geometry, weld thermal cycles and various solidification parameters were calculated. To verify the modeling results, welding experiments were conducted on two samples with different thicknesses and the geometry of the weld pool was measured. It is found that there is a good agreement between the predicted and the measured results. In addition, dimensional analysis was employed to understand the importance of heat transfer by convection and the roles of various driving forces in the weld pool. It is observed that the molten metal convection strongly affects on the weld pool geometry. Also the predictions make it possible to estimate the morphology and the scale of the solidified structure through solidification parameter (G/R). The result show that as the net heat input increases, the importance of convection becomes higher and the value of G/R at the weld pool centerline increases.     

Experimental research of single-hole and multi-hole orifice gas flow meters

As energy efficiency is becoming more important today due to limited energy resources as well as their rising prices and environment issues, it is crucial to have reliable measurement data of different fluids in production processes. Because of its simplicity, affordability and reliability, orifice flow meters are again becoming subject of numerous researches. Conventional single-hole orifice (SHO) flow meter has many advantages but also some disadvantages like higher pressure drop, slower pressure recovery, lower discharge coefficient etc. Some of these disadvantages can be overcame by multi-hole orifice (MHO) flow meter while still maintaining advantages of conventional SHO meter. Both SHO and MHO flow meters with same β ratios were experimentally tested and compared. Results showed better (lower) singular pressure loss coefficient and lower pressure drop in favour of the MHO flow meter. Experimental data indicates that MHO flow meter is superior to the conventional orifice flow meter, but further research is necessary to make the MHO a drop-in replacement for a SHO flow meter.     

Thermal and flow analysis in a proton exchange membrane fuel cell

The effects of anode, cathode, and cooling channels for a Proton Exchange Membrane Fuel Cell (PEMFC) on flow fields have been investigated numerically. Continuous open-faced fluid flow channels formed in the surface of the bipolar plates traverse the central area of the plate surface in a plurality of passes such as a serpentine manner. The pressure distributions and velocity profiles of the hydrogen, air and water channels on bipolar plates of the PEMFC are analyzed using a two-dimensional simulation. The conservation equations of mass, momentum, and energy in the three-dimensional flow solver are modified to include electro-chemical characteristics of the fuel cell. In our three-dimensional numerical simulations, the operation of electro-chemical in Membrane Electrolyte Assembly (MEA) is assumed to be steady-state, involving multi-species. Supplied gases are consumed by chemical reaction. The distributions of oxygen and hydrogen concentration with constant humidity are calculated. The concentration of hydrogen is the highest at the center region of the active area, while the concentration of oxygen is the highest at the inlet region. The flow and thermal profiles are evaluated to determine the flow patterns of gas supplied and cooling plates for an optimal fuel cell stack design.     

动态压差控制阀流场及流量控制特性研究

针对变流量加热及冷却系统水力和热力失调的问题,设计一种动态压差控制阀.基于计算流体力学(CFD)方法,建立不同阀芯开度下动态压差控制阀三维流道模型.对比研究了不同阀芯开度下阀内流场分布以及流量变化,得出了动态压差控制阀在不同阀芯开度下阀内压降曲线的变化规律、阀芯节流口处速度曲线及湍动能曲线的分布规律,拟合了阀门出口流量...     

Effects of surface geometry and non-newtonian viscosity on the flow field in arterial stenoses

Hemodynamics including flow pattern, shear stress, and blood viscosity characteristics has been believed to affect the development and progression of arterial stenosis, but previous studies lack of realistic physiological considerations such as irregular surface geometry, non-Newtonian viscosity characteristics and flow pulsatility. The effects of surface irregularities and non-Newtonian viscosity on flow fields were explored in this study using the arterial stenosis models with 48% arterial occlusions under physiological flow condition. Computational flow dynamics based on the finite volume method was employed for Newtonian and non-Newtonian fluid. The wall shear stresses (WSS) in the irregular surface model were higher compared to those in the smooth surface models. Also, non-Newtonian viscosity characteristics increase the peak WSS significantly. The dimensionless pressure drop and the time averaged WSS in pulsatile flow were higher than those in steady flow. But pulsatility effects on pressure and WSS were less significant compared to non-Newtonian viscosity effects. Therefore, irregular surface geometry and non-Newtonian viscosity characteristics should be considered in predicting pressure drop and WSS in stenotic arteries.     

Air flow rate thermal control system at low pressure drop

Low pressure drop thermal Mass Flow Controllers are generally thought to fulfill needs concerning the realization of a dynamic reference gas mixture generator for accurate gas analysis. A small air flow rate at low pressure drop must be controlled in a stable and precise way in the generator. True operative pressure drop limits, set point reproducibility, calibration needs and flow rate stability during operations were investigated for a low pressure drop thermal Mass Flow Controller. The flow rate bias due to late calibration and flow rate short-term stability were measured and discussed. The Allan method was used to calculate stability during operation. Calibration uncertainty, bias for late calibration, stability and set point reproducibility were composed to calculate the total uncertainty of the flow rate as a function of the operation time. Results show that it is possible to operate below the target uncertainty stated for a dynamic generator of gas mixtures down to 100 Pa pressure drop. Stability gives the main contribution to total uncertainty at very short operation times, while calibration uncertainty gives the main contribution to total uncertainty at normal operation times. The calibration uncertainty at 0.1% is low enough to assure the target uncertainty for operation times over 10 s. Daily verification of calibration enhances the reliability of the measurement. An accurate voltmeter is necessary for the reproducibility of the set point.     

液固两相流压降规律及超声法压降测量

液固两相流广泛存在于能源动力、石油化工等工业过程,两相流压降作为重要的流动参数,有助于流动建模及流态分析。建立液固两相压降测量模型,提出了一种结合超声多普勒及超声透射衰减的液固两相超声压降测量方法。搭建液固两相流动实验平台,对两相压降规律进行研究。两相混合流速和固相体积分数升高时,液固两相压降均逐渐增加。在固相体积分数为0.28%～1.37%,两相混合流速为0.9～1.65 m/s时,根据液固两相压降测量模型及Churchill模型的超声法得到的两相压降与差压传感器测量的压降平均相对误差为4.93%和5.10%,验证了测量模型的准确性。针对非均匀分布的两相流态进行压降测量,进一步拓展了压降测量模型的应用范围。本研究工作为非侵入超声法测量液固两相压降提供了方法基础。     

Influence of the flow area around the ball valve on the flow characteristics of the injector control valve

The increase in common rail pressure can lead to increased cavitation inside the injector, resulting in degradation of injector performance and reduced life. The paper investigates the effect of the pressure block structure parameters (initial flow area around the ball valve) on the velocity field, pressure field, fuel gas phase volume fraction and drain rate of the control valve. The relationship between the initial flow area around the ball valve on the cavitation strength and unloading rate inside the valve was revealed. The results show that both the reduction of the flow area around the ball valve and the increase of the cavitation intensity inhibit the rate of oil discharge from the control valve. The reduction of the fuel flow area inhibits the expansion of the low-pressure region (0–1 MPa) within the flow layer, thus limiting the development of cavitation. The reduction of the cavitation area increases the fuel flow rate, however, the increase in flow rate increases the cavitation phenomenon, and these changes form a cycle (Reviewer 5. comment 2). The increase in cavitation inhibits the control valve pressure relief rate more significantly than the decrease in the initial flow area around the ball valve. Based on this, a stepped-pressure block model is proposed. The stepped pressure block model can effectively reduce the cavitation strength near the seal and enhance the oil discharge rate of the control valve. The study can provide a reference for the engineering optimization design of high-pressure common rail injector control valves.     

变循环发动机多涵道高隐身排气系统的气动研究

为揭示新型VCE排气系统多模式工作特征,构建了具有高隐身多涵道特征的变循环发动机二元S弯排气系统模型,并对其进行数值仿真。结果表明:单涵道模式下,二元S弯喷管流场呈现特有的二次流流场;多涵道模式结合了引射-混合器与喷管的工作特征。双涵道模式下,S弯喷管流场二次流强度增加,且总压、推力性能随着涵道与主流引射流量比增大而增大;三涵道模式下,随着第三外涵压力增大,喷管总压、推力性能有所降低,且保持在0.6%以内。     

Comprehensive experimental study of liquid-slug length and Taylor-bubble velocity in slug flow

Slug flow is an intermittent two-phase flow pattern that provokes undesirable pressure variations in pipes. Mathematical models are commonly used to study these variations; so that it is necessary to know the experimental liquid-slug length, Taylor-bubble length, and pressure drop to validate such mathematical models.In this work, we experimentally studied the water-air slug flow through an acrylic pipe loop 6 m long and 0.01905 m internal diameter. We assembled infrared sensors on the acrylic pipe to get voltage signals accordingly to the presence of liquid-slugs or Taylor-bubbles. We applied Fourier transform on the voltage signals to obtain dominant frequencies to determining the liquid-slug length.Moreover, we obtained the cross-correlation function to get the delay time between two groups of the voltage signals to determine the velocity of Taylor-bubbles. Additionally, we measured the liquid-slug length by video technique and pressure drop with a digital manometer. The liquid-slug lengths obtained by using dominant frequencies are in agreement with the ones measured by video technique.On the other hand, Taylor-bubbles could touch or not the wall pipe at different inclination pipe angles; this affects pressure drop. Then, we observed the inclination angle when the Taylor-bubble detaches from the wall of the pipe, under different flow conditions. We found that the Taylor-bubble detaching angle is 45°, and as the inclination angle is higher, the slug-liquid and Taylor-bubble lengths are smaller. The detaching angle can be used as a criterion to neglect the gas shear-stress into mathematical models to improve predictions of the hydrodynamic behavior of slug flow.     

Prediction of pressure drop in Venturi based on drift-flux model and boundary layer theory

Venturi, as the primary flow measurement sensor, is widely used in various industrial fields of oil and natural gas. Pressure drop of the Venturi is a crucial factor in the process design of exploitation and transportation of natural gas. Based on the drift-flux model and boundary layer theory, a pressure drop prediction model is established. Except for divergent section, a uniform void fraction model is established basing on drift-flux model. The thickness of boundary layer grows rapidly due to the existence of adverse pressure gradient in the divergent section, which results in an increase of the irrecoverable pressure drop. Considering the influence of slip between gas and liquid, weight coefficient is used to adjust the proportion of displacement thickness in the cross section of the Venturi. Compared to experiment, the theoretical model is applied to stratified wavy flow and annular mist flow. For different diameter, the relative deviations of experiment points are within ±15%.