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.     

Numerical simulation of two-phase flow in a multi-gas channel of a proton exchange membrane fuel cell

Understanding the two-phase distribution characteristics within the multi-gas channel of a fuel cell is important for improving fuel cell performance. In the paper, the volume of fluid model is used to predict the dynamic behaviour of water in the multi-gas channel, analyze the pressure drop, velocity distribution, and flow resistance coefficient between different channels, and investigate the influence of operating conditions, surface wettability and channel structure on the two-phase distribution characteristics in the channel. The results show that water undergoes the processes of growth, separation, single droplet transport, wall impact, droplet collision, liquid film formation, and liquid film transport in the multi-gas channel. Inlet velocity and surface wettability significantly affect the pressure drop, water saturation, and surface water coverage. As the inlet velocity and gas diffusion layer surface wettability increase, the flow resistance coefficient and unevenness of the distribution decrease, indicating that the in-channel flow distribution homogeneity is enhanced. The rectangular channel has better water removal and flow distribution uniformity than the tapered channel, and the unevenness of distribution decreases significantly with decreasing rectangular width, from 0.15715 to 0.00315. The research work is a guide to understanding water transport in multi-gas channels, accelerating water removal, and improving inter-channel flow distribution uniformity.     

Treatment of two-phase flow in cathode gas channel for an improved one-dimensional proton exchange membrane fuel cell model

It has been reported recently that water flooding in the cathode gas channel has significant effects on the characteristics of a proton exchange membrane fuel cell. A better understanding of this phenomenon with the aid of an accurate model is necessary for improving the water management and performance of fuel cell. However, this phenomenon is often not considered in the previous one-dimensional models where zero or a constant liquid water saturation level is assumed at the interface between gas diffusion layer and gas channel. In view of this, a one-dimensional fuel cell model that includes the effects of two-phase flow in the gas channel is proposed. The liquid water saturation along the cathode gas channel is estimated by adopting Darcy’s law to describe the convective flow of liquid water under various inlet conditions, i.e. air pressure, relative humidity and air stoichiometry. The averaged capillary pressure of gas channel calculated from the liquid water saturation is used as the boundary value at the interface to couple the cathode gas channel model to the membrane electrode assembly model. Through the coupling of the two modeling domains, the water distribution inside the membrane electrode assembly is associated with the inlet conditions. The simulation results, which are verified against experimental data and simulation results from a published computational fluid dynamics model, indicate that the effects of relative humidity and stoichiometry of inlet air are crucial to the overall fuel cell performance. The proposed model gives a more accurate treatment of the water transport in the cathode region, which enables an improved water management through an understanding of the effects of inlet conditions on the fuel cell performance.     

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.     

模拟贯穿裂纹内高速气液两相流动特性研究

压力管道和容器发生贯穿泄漏会引发严重的事故,合理估算贯穿泄漏量具有重要的工程意义.以矩形狭缝通道模拟贯穿裂纹,开展了高压氩气-水贯穿模拟裂纹的高速流动可视化试验研究,狭缝长度为20 mm,间隙宽度为80～180 μm.狭缝进口压力大于5 MPa,液体的表观速度为0.05～58.62 m/s,气体表观速度为1.71～34...     

Modeling two-phase flow in PEM fuel cell channels

This paper is concerned with the simultaneous flow of liquid water and gaseous reactants in mini-channels of a proton exchange membrane (PEM) fuel cell. Envisaging the mini-channels as structured and ordered porous media, we develop a continuum model of two-phase channel flow based on two-phase Darcy's law and the M² formalism, which allow estimate of the parameters key to fuel cell operation such as overall pressure drop and liquid saturation profiles along the axial flow direction. Analytical solutions of liquid water saturation and species concentrations along the channel are derived to explore the dependences of these physical variables vital to cell performance on operating parameters such as flow stoichiometric ratio and relative humility. The two-phase channel model is further implemented for three-dimensional numerical simulations of two-phase, multi-component transport in a single fuel-cell channel. Three issues critical to optimizing channel design and mitigating channel flooding in PEM fuel cells are fully discussed: liquid water buildup towards the fuel cell outlet, saturation spike in the vicinity of flow cross-sectional heterogeneity, and two-phase pressure drop. Both the two-phase model and analytical solutions presented in this paper may be applicable to more general two-phase flow phenomena through mini- and micro-channels.     

Experimental investigations on liquid water removal from the gas diffusion layer by reactant flow in a PEM fuel cell

The cross flow from channel to channel through gas diffusion layer (GDL) under the land could play an important role for water removal in proton exchange membrane (PEM) fuel cells. In this study, characteristics of liquid water removal from GDL have been investigated experimentally, through measuring unsteady pressure drop in a cell which has the GDL initially wet with liquid water. The thickness of GDL is carefully controlled by inserting various thicknesses of metal shims between the plates. It has been found that severe compression of GDL could result in excessive pressure drop from channel inlet to channel outlet. Removing liquid water from GDL by cross flow is difficult for GDL with high compression levels and for low inlet air flow rates. However, effective water removal can still be achieved at high compression levels of GDL if the inlet air flow rate is high. Based on different compressed GDL thicknesses, different GDL porosities and permeabilities were calculated and their effects on the characteristics of liquid water removal from GDL were evaluated. Visualization of liquid water transport has been conducted by using transparent flow channel, and liquid water removal from GDL under the land was observed for all the tested inlet air flow rates, which confirms that cross flow is practically effective to remove the liquid water accumulated in GDL under the land area.     

Modeling gas flow in PEMFC channels: Part I – Flow pattern transitions and pressure drop in a simulated ex situ channel with uniform water injection through the GDL

The two-phase flow in the gas channels of a proton exchange membrane fuel cell (PEMFC) is studied with an ex situ setup using a gas diffusion layer (GDL) as the sidewall of the channels. Air is supplied at the channel inlet manifold and water is supplied continuously and uniformly through the GDL along the length of the channel. This is different from the simultaneous air and water introduction at the inlet of the channel as studied by previous two-phase flow researchers. The GDL is compressed between the gas channels and the water chambers to simulate PEMFC conditions. The superficial velocity for air and water ranged from 0.25 to 34.5 m/s and 1.54 × 10⁻⁵ to 1.54 × 10⁻⁴ m/s, respectively. The ex situ setup was run in both vertical and horizontal orientations with two GDLs, – Baseline (Mitsubishi Rayon Co. MRC 105 with 5 wt.% PTFE and coated with an in-house MPL by General Motors) and SGL 25 BC – and three channel treatments – hydrophobic, hydrophilic, and untreated Lexan, with contact angles of 116°, 11° and 86°, respectively. No appreciable effect was noted because of the orientation, GDL type or channel coatings. The flow regime is observed at different locations along the channel and is expressed as a function of the superficial air and water velocities. Flow regime criteria are developed and validated against the range of ex situ data observations. A new variable water flow rate pressure drop model is developed in order to account for the variation of water entering the channel at multiple locations along the flow length. Pressure drop models are developed for specific flow regimes and validated against experimental data. The models are able to predict the experimental pressure drop data with a mean error of less than 14%.     

Numerical investigation of liquid water distribution in the cathode side of proton exchange membrane fuel cell and its effects on cell performance

A three-dimensional unsteady two-phase model for the cathode side of proton exchange membrane fuel cell (PEMFC) consisting of gas diffusion layer (GDL) with hybrid structural model is developed to investigate liquid water behaviors under different operating and geometrical conditions and to quantitatively evaluate effects of liquid water distribution on reactant transport and current density distribution. Simulation results reveal that liquid water transport processes and distributions are significantly affected by inlet air velocity, wall wettability and water inlet position, which in turn play a prominent role on local reactant transport and cause considerable disturbances of the current density. Liquid water film spreading on the gas channel (GC) top wall is identified as the most desirable flow pattern in the GC based on overall evaluations of current density magnitude, uniformity of current density distribution and pressure drop in the GC. Modification to GDL structure is proposed to promote the formation of the desirable flow pattern.     

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.     

Effect of wettability on water removal from the gas diffusion layer surface in a novel proton exchange membrane fuel cell flow channel

Effective water removal from the proton exchange membrane fuel cell (PEMFC) surface exposed to the flow channel is critical to the operation and water management in PEMFCs. In this study, the water removal process is investigated numerically for a novel flow channel formed by inserting a hydrophilic needle in the conventional PEMFC flow channel, and the effect of the surface wettability of the membrane electrode assembly (MEA) and the inserted needle on the water removal process is studied. The results show that the liquid water can be more effectively removed from the MEA surface for larger MEA surface contact angles and smaller needle surface contact angles. The pressure drop for the flow in the channel is also examined and it is seen to be indicative of the liquid water flow and transport in the flow channel, suggesting that pressure drop is a useful parameter for the investigation of water transport and dynamics in the flow channel.     

Investigation of two-phase transport phenomena in microchannels using a microfabricated experimental structure

Microchannels (0.05–1 mm) improve gas routing in proton exchange membrane fuel cells, but add to the complexities of water management. This work microfabricates experimental structures with distributed water injection as well as with heating and temperature sensing capabilities to study water formation and transport. The samples feature optical access to allow visualization and distributed thermometry for investigation of two-phase flow transport phenomena in the microchannels. The temperature evolution along the channel is observed that the temperature downstream of the distributed water injection decreases as the pressure drop increases. As the water injection rate is lower than 200 μl/min, there exists a turning point where temperature increases as the pressure drop increases further. These micromachined structures with integrated temperature sensors and heaters are key to the experimental investigation as well as visualization of two-phase flow and water transport phenomena in microchannels for fuel cell applications.     

Two-phase flow in GDL and reactant channels of a proton exchange membrane fuel cell

Understanding the effect of two-phase flow in the components of proton exchange membrane fuel cells (PEMFCs) is crucial to water management and subsequently to their performance. The local water saturation in the gas diffusion layer (GDL) and reactant channels influences the hydration of the membrane which has a direct effect on the PEMFC performance. Mass transport resistance includes contributions from both the GDL and reactant channels, as well as the interface between the aforementioned components. Droplet–channel wall interaction, water area coverage ratio on the GDL, oxygen transport resistance at the GDL–channel interface, and two-phase pressure drop in the channels are interlinked. This study explores each factor individually and presents a comprehensive perspective on our current understanding of the two-phase transport characteristics in the PEMFC reactant channels.     

Effects of needle orientation and gas velocity on water transport and removal in a modified PEMFC gas flow channel having a hydrophilic needle

Water management is critical to the performance and operation of the proton exchange membrane fuel cell (PEMFC). Effective water removal from the gas diffusion layer (GDL) surface exposed to the gas flow channel in PEMFC mitigates the water flooding of and improves the reactants transport into the GDL, hence benefiting the PEMFC performance. In this study, a 3D numerical investigation of water removal from the GDL surface in a modified PEMFC gas flow channel having a hydrophilic needle is carried out. The effects of the needle orientation (inclination angle) and gas velocity on the water transport and removal are investigated. The results show that the water is removed from the GDL surface in the channel for a large range of the needle inclination angle and gas velocity. The water is removed more effectively, and the pressure drop for the flow in the channel is smaller for a smaller needle inclination angle. It is also found that the modified channel is more effective and viable for water removal in fuel cells operated at smaller gas velocity.     

Experimental validation of two-phase pressure drop multiplier as a diagnostic tool for characterizing PEM fuel cell performance

Water management is a key area of interest in improving the performance of Proton Exchange Membrane fuel cells. Cell flooding and membrane dehydration are two extreme conditions arising from poor water management. Pressure drop has been recognized as a good diagnostic tool to determine the presence of liquid water in the reactant channels. Presence of liquid water in the channels increases the mass transport resistances and therefore reduces the cell performance, which is quantified by the cell voltage at a set current density. Since the two-phase pressure drop multiplier is uniquely related to the water content in the channel, it serves as a good diagnostic tool for directly predicting the cell performance. Experiments are carried out to establish the relationship between the pressure drop multiplier and cell voltage at different operating conditions. Cell temperature was varied from 30 °C to 80 °C and the inlet RH was varied from 0 to 95%. At the lower temperatures, a two-phase multiplier below 1.5 reduces flooding in the flow field. However, at the higher temperatures, a two-phase flow multiplier above 1.2 is preferred as it indicates the membrane remains hydrated for improved performance from the cell. The two-phase pressure drop multiplier has been successfully demonstrated as a diagnostic tool to predict cell flooding and membrane dehydration.     

Liquid water transport in parallel serpentine channels with manifolds on cathode side of a PEM fuel cell stack

Water management in a proton exchange membrane (PEM) fuel cell stack has been a challenging issue on the road to commercialization. This paper presents a numerical investigation of air–water flow in parallel serpentine channels on cathode side of a PEM fuel cell stack by use of the commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different air–water flow behaviours inside the serpentine flow channels with inlet and outlet manifolds were discussed. The results showed that there were significant variations of water distribution and pressure drop in different cells at different times. The “collecting-and-separating effect” due to the serpentine shape of the gas flow channels, the pressure drop change due to the water distribution inside the inlet and outlet manifolds were observed. Several gas flow problems of this type of parallel serpentine channels were identified and useful suggestions were given through investigating the flow patterns inside the channels and manifolds.     

Three dimensional numerical simulation of gas-liquid two-phase flow patterns in a polymer-electrolyte membrane fuel cells gas flow channel

Water management in polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on fuel cell performance and durability. To investigate the two-phase flow patterns in PEMFC gas flow channels, the volume of fluid (VOF) method was employed to simulate the air-water flow in a 3D cuboid channel with a 1.0 mm × 1.0 mm square cross section and a 100 mm in length. The microstructure of gas diffusion layers (GDLs) was simplified by a number of representative opening pores on the 2D GDL surface. Water was injected from those pores to simulate water generation by the electrochemical reaction at the cathode side. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. The water injection rate was also amplified 10 times, 100 times and 1000 times to study the flow pattern formation and transition in the channel. Simulation results show that, as the flow develops, the flow pattern evolves from corner droplet flow to top wall film flow, then annular flow, and finally slug flow. The total pressure drop increases exponentially with the increase in water volume fraction, which suggests that water accumulation should be avoided to reduce parasitic energy loss. The effect of material wettability was also studied by changing the contact angle of the GDL surface and channel walls, separately. It is shown that using a more hydrophobic GDL surface is helpful to expel water from the GDL surface, but increases the pressure drop. Using a more hydrophilic channel wall reduces the pressure drop, but increases the water residence time and water coverage of the GDL surface.     

An Experimental Study of Carbon Dioxide Condensation in Mini Channels

In this study, the local characteristics of pressure drop and heat transfer were investigated experimentally for carbon dioxide condensation in a multi-port extruded aluminum test section, which had 10 circular channels each with 1.31 mm inner diameter. The CO2 was cooled with cooling water flow inside the copper blocks that were attached at both sides of the test section. The temperatures at the outer surface of the test section were measured with 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured with 12 heat flux sensors to estimate the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO2 at the inlet and outlet of the test section were measured with 2 K-type thermocouples. The measurements were performed for the pressure ranged from 6.48 to 7.3 MPa, inlet temperature of CO2 from 21.63 to 31.33℃, heat flux from 1.10 to 8.12 kW/m2, mass velocity from 123.2 to 315.2 kg/m2s, and vapor quality from 0 to 1. The results indicate that pressure drop is very small along the test section, heat transfer coefficient in the two-phase region is higher than that in the single-phase, and mass velocity has important effect on condensation heat transfer characteristics. In addition, experimental data were compared with previous correlations and large discrepancies were observed.