Dominant frequency of pressure drop signal as a novel diagnostic tool for the water removal in proton exchange membrane fuel cell flow channel

In this work, a transparent assembly was self-designed and manufactured to perform ex situ experimental study on the liquid water removal characteristics in PEM fuel cell parallel flow channels. It was found that the dominant frequency of the pressure drop across the flow channels may be utilized as an effective diagnostic tool for water removal. Peaks higher than 1 Hz in dominant frequency profile indicated water droplet removals at the outlet, whereas relatively lower peaks (between 0.3 and 0.8 Hz) corresponded to water stream removals. The pressure drop signal, although correlated with the water removal at the outlet, was readily influenced by the two phase flow transport in channel, particularly at high air flow rates. The real-time visualization images were presented to show a typical water droplet removal process. The findings suggest that dominant frequency of pressure drop signal may substitute pressure drop as a more effective and reliable diagnostic tool for water removal in PEM fuel cell flow channels.     

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.     

填料式饱和器稳态和动态特性的试验研究

对填料式饱和器进行了稳态及动态特性试验研究,分析了进水流量、进气流量、进水温度对空气加热加湿过程的影响,主要考察了出水温度、出气温度、进出口空气含湿量差、填料段压降和持液量等性能参数的变化规律.结果表明：在相同进水流量下,提高水气比和进水温度,可显著提高加热加湿性能;当气相速度升高时,进出口空气含湿量差减小,填料段压降及持液量升高;在动态工况下,当进口参数发生扰动时,持液量及出水温度比压降需要更长的时间才能达到新的稳定点.     

The role of the gas diffusion layer on slug formation in gas flow channels of fuel cells

Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 μm liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells.     

Flow boiling in constructal tree-shaped minichannel network

Flow boiling in constructal tree-shaped minichannel network with an inlet diameter of 4 mm is numerically investigated using a one-dimensional model, taking into consideration the minor losses at junctions. The pumping power requirement, pressure drop, temperature uniformity and coefficient of performance of the constructal tree-shaped minichannel network are all evaluated and compared with those of the corresponding traditional serpentine channel, and the fluid stream undergoes a phase change from saturated liquid to saturated vapor. The effects of the length dimension and top view area (i.e. the path length) on saturated gas–liquid two-phase flow boiling heat transfer in tree-shaped minichannel networks are all analyzed and discussed. The results indicated that, the tree-shaped network configured with length dimension of two is able to maximum flow access; the path length plays a significant role in the determination of flow boiling in tree-shaped minichannel networks. In particular, compared to the traditional serpentine channel, flow boiling in constructal tree-shaped minichannnel network possesses less pressure drop, lower pumping power requirement, better temperature uniformity and higher coefficient of performance (COP).     

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 preferential accumulation in channels of PEM fuel cells with multiple serpentine flow fields

This work presents an experimental investigation on the preferential accumulation of liquid water in the channels of a multiple serpentine PEMFC with 50 cm² active area. Neutron imaging was used for visualizing the liquid water distribution during the cell operation for a wide range of operating conditions. Liquid water accumulation in the cathode channels was observed for most of the operating conditions, with a preferential accumulation in certain channels of the flow field. A statistical analysis was performed in order to determine the main characteristics of this accumulation (i.e. channel number and degree of accumulation). As cathode channels were positioned in vertical direction, it was found that gravity effects had an important influence in the accumulation, as well as the relative position of the channel with respect to the inlet and outlet locations. The gas flow direction had also a major impact on the water accumulation within the channels, with significantly more water accumulated in channels with upwards gas flow.     

Novel hybrid serpentine-interdigitated flow field with multi-inlets and outlets of gas flow channels for PEFC applications

In our previous study, a novel flow field design for a polymer electrolyte fuel cell (PEFC) called “Hybrid Serpentine-Interdigitated (HSI)” had been proposed. Although it was very promising in terms of performance and pressure drop, it still had a major drawback of the low oxygen concentration area. To improve its design and performance, three HSI configurations with different numbers of gas inlet and outlet, namely one inlet and one outlet HSI (1-IO HSI), one inlet and two outlets HSI (1I-2O HSI) and two inlets and two outlets HSI (2-IO HSI) were numerically investigated and compared with the conventional single channel serpentine (1S). The investigation on the cell performance and other transport behaviors has been carried out using CFD techniques via ANSYS FLUENT software. At a practical operating potential of about 0.6 V of 50 cm² PEFCs, the 2-IO HSI offered the best distributions of oxygen, current density and water due to the shorter channel length. More importantly, the 2-IO HSI could contribute to a reduction in cathode pressure drop by 90%, as compared with the 1S, resulting in the enhancement in the net power output by 6%, approximately.     

Pressure drop on water accumulation distribution for a micro PEM fuel cell with different flow field plates

Time-dependent measurements of pressure drop in different flow fields on the cathode of a PEM fuel cell with different operating conditions of mass flow rates and cell temperatures on water accumulation were conducted. The results show that, among four flow fields studied herein, the interdigitated flow channel has the biggest pressure drop as well as the largest water accumulation at an early phase (?30 min) compared to those of the other three channels. In addition, the more water produced, the bigger the pressure drop that occurs. Similarly, the effects of mass flow rates at a fixed cell temperature were also examined and discussed.     

Optimization of blocked flow field performance of proton exchange membrane fuel cell with auxiliary channels

The flow field optimization design is one of the important methods to improve the performance of proton exchange membrane fuel cell (PEMFC). In this study, a new structure with staggered blocks on the parallel flow channels of PEMFC and auxiliary flow channels under the ribs is proposed. Through numerical calculation method, the effect of blocks auxiliary flow field (BAFF) on pressure drop, reactant distribution and liquid water removal in the fuel cells are investigated. The results show that when the operating voltage is 0.5 V, the current density of BAFF is 21.74% higher than that of the straight parallel flow field (SPFF), and the power density reaches 0.65 W cm^?2. BAFF improves performance by equalizing the pressure drop across sub-channels, promoting the uniform distribution of reactant, and enhancing transport across the ribs. In addition, through parameter analysis, it is found that BAFF can discharge liquid water in time at the conditions of high humidification, high current density and low temperature, which ensures the output performance of the fuel cell and improves the durability of the fuel cell. This paper provides new ideas for the improvement of PEMFC flow field design, which is beneficial to the development of PEMFC with high current density.     

Neutron radiography and current distribution measurements for studying cathode flow field properties of direct methanol fuel cells

The influence of the cathode flow field properties on the water distribution and performance of direct methanol fuel cells (DMFCs) was studied. All measurements were performed with DMFC stack cells (A = 314.75 cm²). The local and temporal water distributions in the flow field channels during DMFC operation were visualized by means of through‐plane neutron radiography. Current and temperature distributions were measured simultaneously by the segmented cell technology. Additionally, the time‐dependent current distribution, cell performance and pressure drop were measured. Cathode flow field designs with channel and grid structures were compared. The cathode flow field channels were impregnated by either hydrophobizing or hydrophilizing agents or used as received. It turned out that hydrophobized and partially also untreated flow fields cause large water droplets in the cathode channels. The water droplets cause a blocking of the air flow and consequently a lower and more unstable (fluctuating) performance, less steady current and temperature distributions, and higher pressure drops between cathode inlet and outlet. Because of their two‐dimensional design, grid flow fields are less prone to water accumulations. The best results are achieved with a hydrophilized grid flow field that has a channel depth and width of 1.5 mm each (‘C‐GR15’). Copyright © 2013 John Wiley & Sons, Ltd.     

Fractal channel design in a micro methanol steam reformer

The aim of the present article is to study the fractal channel pattern design and the gradient catalyst layer in relation to their effects on the performance of a micro methanol steam reformer. A three-dimensional simulation model is established for the purpose of predicting the effects of bio-channel design on the performance of a micro-reformer. The CO concentration in the production gases, which is necessary to avoid the poisoned catalyst layers of low temperature fuel cells, is also investigated. In addition, the distributions of velocity and gas concentrations are predicted, and the methanol conversion ratios are also evaluated. Due to further decreases of the CO in product gases, a gradient catalyst layer arrangement is proposed to delay the timing of hydrogen generation and thus avoid the presence of hydrogen in the catalyst layer too long. This catalyst arrangement can effectively decrease the possibility of a reverse water gas shift reaction to reduce CO generation. Results showed that the fractal channel design increases the conversion ratio, decrease CO as well as decrease the pressure drop in the channels. Relative to a parallel channel design, the CO and methanol conversion ratio of this fractal channel design pattern with uniform catalyst layer can be decreased and increased by 17% and 8%, respectively, based on a 0.3 cc/min flow rate, respectively. Meanwhile, the pressure drops in the parallel channel design and in the fractal channel design were found to be 254 Pa and 51 Pa, respectively. From an energy consumption point of view, a low pressure drop also implies low input pumping power. Furthermore, compared to the fractal design with a uniform catalyst layer, the gradient catalyst layer was demonstrated to effectively increase the conversion ratio by 8.5% and decrease CO by 11% when the inlet liquid flow rate was fixed at 1.0 cc/min.     

Investigation on cold start of polymer electrolyte membrane fuel cells with different cathode serpentine flow fields

The flow velocity and pressure distribution of the three cathode flow fields are simulated in this study. Larger pressure drop and more rapid flow rate reduce residual water, resulting in minimal ice formation during the cold start process. The simulation results show that the single variable cross section serpentine flow field has the largest pressure drop and the most rapid flow rate.The evolution of the temperature and the segment current density characteristics of three different cathode flow fields during cold start process is studied by printed circuit board technology. The results show that the 2 to 1 serpentine flow field has the best cold start performance and the best current density uniformity when cold start at constant voltage mode above −5 °C. However, the single variable cross section serpentine flow field has the best performance when cold start temperature is below −5 °C. Based on these results, cold start at −30 °C can be realized in 97s by using hot antifreeze liquid.     

Dry gas operation of proton exchange membrane fuel cells with parallel channels: Non-porous versus porous plates

We present a study of proton exchange membrane (PEM) fuel cells with parallel channel flow fields for the cathode, dry inlet gases, and ambient pressure at the outlets. The study compares the performance of two designs: a standard, non-porous graphite cathode plate design and a porous hydrophilic carbon plate version. The experimental study of the non-porous plate is a control case and highlights the significant challenges of operation with dry gases and non-porous, parallel channel cathodes. These challenges include significant transients in power density and severe performance loss due to flooding and electrolyte dry-out. Our experimental study shows that the porous plate yields significant improvements in performance and robustness of operation. We hypothesize that the porous plate distributes water throughout the cell area by capillary action; including pumping water upstream to normally dry inlet regions. The porous plate reduces membrane resistance and air pressure drop. Further, IR-free polarization curves confirm operation free of flooding. With an air stoichiometric ratio of 1.3, we obtain a maximum power density of 0.40 W cm⁻², which is 3.5 times greater than that achieved with the non-porous plate at the same operating condition.     

Performance analysis of a proton exchange membrane fuel cell using tree-shaped designs for flow distribution

The performance of a proton exchange membrane (PEM) fuel cell is directly associated to the flow channels design embedded in the bipolar plates. The flow field within a fuel cell must provide efficient mass transport with a reduced pressure drop through the flow channels in order to obtain a uniform current distribution and a high power density. In this investigation, three-dimensional fuel cell models are analyzed using computational fluid dynamics (CFD). The proposed flow fields are radially designed tree-shaped geometries that connect the center flow inlet to the perimeter of the fuel cell plate. Three flow geometries having different levels of bifurcation were investigated as flow channels for PEM fuel cells. The performance of the fuel cells is reported in polarization and power curves, and compared with that of fuel cells using conventional flow patterns such as serpentine and parallel channels. Results from the flow analysis indicate that tree-shaped flow patterns can provide a relatively low pressure drop as well as a uniform flow distribution. It was found that as the number of bifurcation levels increases, a larger active area can be utilized in order to generate higher power and current densities from the fuel cell with a negligible increase in pumping power.     

Study of novel flow channels influence on the performance of direct methanol fuel cell

The existing flow channels like parallel and gird channels have been modified for better fuel distribution in order to boost the performance of direct methanol fuel cell. The main objective of the work is to achieve minimized pressure drop in the flow channel, uniform distribution of methanol, reduced water accumulation, and better oxygen supply. A 3D mathematical model with serpentine channel is simulated for the cell temperature of 80 °C, 0.5 M methanol concentration. The study resulted in 40 mW/cm² of power density and 190 mA/cm² of current density at the operating voltage of 0.25 V. Further, the numerical study is carried out for modified flow channels to discuss their merits and demerits on anode and cathode side. The anode serpentine channel is unmatched by the modified zigzag and pin channels by ensuring the better methanol distribution under the ribs and increased the fuel consumption. But the cathode serpentine channel is lacking in water management. The modified channels at anode offered reduced pressure drop, still uniform reactant distribution is found impossible. The modified channels at cathode outperform the serpentine channel by reducing the effect of water accumulation, and uniform oxygen supply. So the serpentine channel is retained for methanol supply, and modified channel is chosen for cathode reactant supply. In comparison to cell with only serpentine channel, the serpentine anode channel combined with cathode zigzag and pin channel enhanced power density by 17.8% and 10.2% respectively. The results revealed that the zigzag and pin channel are very effective in mitigating water accumulation and ensuring better oxygen supply at the cathode.     

Numerical studies on the geometrical characterization of serpentine flow-field for efficient PEMFC

Geometrical characterization of the serpentine flow-field is one of the key issues to be solved to enhance the performance of PEMFC in relation to pressure drop, discharge of condensed water, maximization of cell voltage, and uniformity of current density over the entire surface area. Three different channel heights and widths were compared with the base flow-field design of the serpentine channel whose width is 1 mm and 0.34 mm in height, each through a detailed numerical study of the distribution of temperature, pressure, water content, and local current density. As the channel height increases higher than the base design, the total pressure drop decreases and results in reduced load of BOP and accumulation of liquid water at the outlet of both anode and cathode. The accumulation of anode liquid water at the outlet caused by back diffusion is accelerated as the channel height increases. As the channel width expands wider than the base design, the pressure drop is lowered and the removal rate of liquid water becomes faster. The effect of the channel width increase on the water removal is greater than that of the channel height increase. Which can influence the dehydration and temperature of the MEA and thus cell performance and lifetime of PEMFC. The results obtained in this work are expected to be applied in developing an efficient serpentine flow-field channel with sub-channels and by-passes.     

非对称孔结构载体对DPF及柴油机性能的影响研究

运用GT-Power建立柴油机整机耦合柴油机颗粒捕集器(diesel particle filter,DPF)的模型,研究非对称孔道结构DPF的压降特性及其对柴油机整机性能的影响。研究结果表明:随着进/出口孔道比例增加,碳烟容量增大,DPF压降降低,但过度增大进/出口孔道比例会使出口孔径过小而造成压降升高;柴油机加装洁净DPF时,柴油机各项性能指标随进/出口比例增加而降低,但随着DPF碳烟和灰分沉积量的增加,柴油机各项性能指标随进/出口比例增加均有所改善。DPF进/出口孔道比例为1.3时,柴油机综合性能最佳。     

System instability of evaporative micro-channels

In parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow rate pumping, with a pressure regulating tank and a constant heating pre-heater, alternations between these two states of boiling and non-boiling could happen with a period of minutes. This cyclical system instability has been modeled, where the liquid phase flow occurs at conditions of high inlet subcooling and low surface heat flux that the boiling inception is hard to initiate. The system instability criteria are established in terms of a system binary states parameter, S, and a non-dimensional surface heat flux. This model has been validated experimentally.     

Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels

A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the microchannels. The fluid flow had no restriction in entering to and existing from the microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.