Two-phase hydrodynamics in a miniature direct methanol fuel cell

We present controlled experiments on a miniature direct methanol fuel cell (DMFC) to study the effects of methanol flow rate, current density, and void fraction on pressure drop across the DMFC anode. We also present an experimental technique to measure void fraction, liquid slug length, and velocity of the two-phase slug flow exiting the DMFC. For our channel geometry in which the diameter of the largest inscribed sphere (a) is 500 μm, pressure drop scales with the number of gas slugs in the channel, surface tension, and a. This scaling demonstrates the importance of capillary forces in determining the hydrodynamic characteristics of the DMFC anode. This work is aimed at aiding the design of fuel pumps and anode flow channels for miniature DMFC systems.     

The statistical relationship between flow channel geometry and pressure drop in a direct methanol fuel cell with parallel channels

In this study, the relationship between the pressure drop on the channels due to the methanol flow and the geometry of the flow channels on the anode side of a direct methanol fuel cell (DMFC) has been investigated. Parallel type channels are used as flow channels. The active area of the fuel cell is 5 × 5 cm². The system consists of channels that are optimally placed in the active area, with channel widths and distance of the channels kept constant. Combinations of 1, 1.5, 2, 2.5, 3 mm measurements were used for flow channel width and distance between channels. The ratio of the area created by the prepared geometries to the active area (percentage of contact area) is defined as a new parameter. The main motivation of this study is to be able to determine the effect of the geometric measurements of the designed flow channels on the intra-channel pressure drop by statistical method. There was a statistically significant difference between the flow channel widths and the distance between the channels and the pressure. Among the selected parameters, the effect of the channel width on the pressure drop was highest but it had a statistically moderate relationship. However, there was no significant relationship between the distance between channels and the pressure drop.     

Hydrogen pressure drop characteristics in a fuel cell stack

Research on hydrogen pressure characteristics was performed for a fuel cell stack to supply a rule of hydrogen pressure drop for flooding diagnostic systems. Some experiments on the hydrogen pressure drop in various operating pressure, temperature, flowrate and stack current conditions were carried out, and hydrodynamic calculation was managed to compare with the experiment results. Results show that the hydrogen pressure drop is strongly affected by liquid water content in the flow channel of fuel cells, and it is not in normal relation with flowrate when the stoichiometric ratio is inconstant. The total pressure drop can be calculated by a frictional pressure loss formula accurately, relating with mixture viscosity, stack temperature, operating pressure, stoichiometric ratio and stack current. The pressure drop characteristics will be useful for predicting liquid water flooding in fuel cell stacks before flow channels have been jammed as a diagnostic tool in electric control systems.     

Constructal flow distributor as a bipolar plate for proton exchange membrane fuel cells

A plate-type constructal flow distributor is implemented as a gas distributor for a proton exchange membrane fuel cell. A 3D complete model is simulated using CFD techniques. The fuel cell model includes the gas flow channels, the gas diffusion layers and the membrane-electrode assembly (MEA). The governing equations for the mass and momentum transfer are solved including the pertinent source terms due to the electrochemical reactions in the different zones of the fuel cell. Three constructal flow configurations were studied; each pattern is a fractal expansion of the original design, therefore, the only difference between them is the number of branches in the geometry. It was found that the number of branches is the key parameter in the performance of a fuel cell when using the constructal distributors as flow channels. The performance of the fuel cell is reported in I-V curves, power curves, and overpotential curves in order to determine which irreversibility is the main cause of energy losses. In terms of flow analysis, it was found that the constructal flow distributor presents a low pressure drop for a wide range of Reynolds number conditions at the inlet, as well as an excellent uniformity of flow distribution. Regardless of the outstanding hydrodynamic performance of the constructal distributors and the large current density values obtained, the implementation of these designs as flow patterns for PEMFCs need further optimization; first, the manufacturing of the plates have to be addressed in an efficient way; and secondly, the application in stacks will require an elaborate design to accomplish this task.     

Development of bipolar plates with different flow channel configurations for fuel cells

Bipolar plates include separate gas flow channels for anode and cathode electrodes of a fuel cell. These gases flow channels supply reactant gasses as well as remove products from the cathode side of the fuel cell. Fluid flow, heat and mass transport processes in these channels have significant effect on fuel cell performance, particularly to the mass transport losses. The design of the bipolar plates should minimize plate thickness for low volume and mass. Additionally, contact faces should provide a high degree of surface uniformity for low thermal and electrical contact resistances. Finally, the flow fields should provide for efficient heat and mass transport processes with reduced pressure drops. In this study, bipolar plates with different serpentine flow channel configurations are analyzed using computational fluid dynamics modeling. Flow characteristics including variation of pressure in the flow channel across the bipolar plate are presented. Pressure drop characteristics for different flow channel designs are compared. Results show that with increased number of parallel channels and smaller sizes, a more effective contact surface area along with decreased pressured drop can be achieved. Correlations of such entrance region coefficients will be useful for the PEM fuel cell simulation model to evaluate the affects of the bipolar plate design on mass transfer loss and hence on the total current and power density of the fuel cell.     

Optimization of anode performance in the ethanol electrochemical reforming for clean hydrogen production

Carbon-based fuel electrochemical reforming is considered as a promising hydrogen production method. Ethanol is one of the most appropriate carbon-based fuels. In this work, anode performance, especially the flow, ethanol electro-oxidization and energy consumption in the ethanol electrochemical reforming is numerically studied and experimental verified. Take the straight serpentine channel with square cross-section as a base structure in the electrochemical cell (EC), the effects of channel geometry and operating parameters are analyzed. Another five different configurations of flow channels, as well as another three different cross-sections are designed and explored. Results indicate that at the same cross-section area, the wider channel provides the higher effective area for proton transfer, and thereby improves the electrode reactions. The appropriate decrease of inlet velocity or increase of input voltage promotes the anode reaction and reduces the pressure drop in channel, while the operating temperature has the opposite effects on ethanol conversion and pressure drop. The arc channel is found optimal considering its highest ethanol conversion, although its pressure drop is a bit higher. The sector cross-section with uniform flow field distribution is found most favorable for the straight serpentine channel considering the ethanol electro-oxidization. These findings will favor the improvement of EC.     

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.     

Effect of flow field design on performances of high temperature PEM fuel cells: Experimental analysis

Among all types of fuel cells, attention is being drawn lately on high temperature Polybenzimidazole (PBI) PEM because their operative temperature range (120-180 °C) increases the tolerance to carbon monoxide. This feature allows working with low quality hydrogen produced by hydrocarbon reformation. Most of the literature on PBI PEM deals with membrane and MEA related issues, however, cell efficiency and specially, commercial feasibility are conditioned by other fuel cell components as bipolar plates. In the present study the focus is on the effect of the flow field geometry of high temperature PBI PEM composite bipolar plates on the overall performance of the cell. For this purpose, three different channel geometries are studied: two serpentine flow fields and parallel channels flow field. Results show that serpentine geometry yields higher performance though it introduces higher pressure drop along the cell as well.     

Numerical optimization of channel to land width ratio for PEM fuel cell

Flow field plays a vital role in proton exchange membrane (PEM) fuel cell where channel geometry being the primary factor. Most of the channel geometry analyses were limited to few number of case studies, whereas in this study total 73 case studies were analyzed for the optimization of channel and land width. A three dimensional isothermal single phase flow mathematical model is developed and further validated with experimental study to optimize the channel and land width through parametric sweep function for a staggering 73 number of case studies. The optimization analyses are carried out for a straight channel geometry considering a fixed operating voltage of 0.4 V and channel depth of 1.0 mm. Due to the large number of case studies, the analyzed performance parameters i.e. current density and pressure drop are easily understandable for the change in different channel and land width. The numerical results predicted that the pressure drop is more dependent on channel width compare to the land width and anode pressure drop is less significant than cathode pressure drop. However, both channel and land width have an equal importance on the cell current density. Considering channel pressure drop and current density, the optimization analyses showed that the channel to land width of 1.0 mm/1.0 mm would be best suitable for PEMFC channel geometry.     

Numerical study on thermal performance of non-uniform flow channel designs for cooling plates of PEM fuel cells

Due to the limited cooling capacity of air, large-scale proton exchange membrane (PEM) fuel cell stacks are generally cooled by liquid cooling where liquid water is circulated through the flow channels of cooling plates. Effective cooling is essential for the stability, durability, and performance of PEM fuel cells. In this study, cooling plates with conventional straight channel and novel non-uniform flow channel designs are investigated and analyzed by using a three-dimensional model. The simulated results are presented in terms of pressure drop, average temperature, maximum temperature, temperature difference between the maximum temperature and minimum temperature, and the temperature uniformity index. In addition, the effects of heat flux and inlet Reynolds number on the cooling performance are studied. It is concluded that the cooling performance is significantly improved as the novel flow channel designs are applied.     

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.     

Modelling and optimization of reactant gas transport in a PEM fuel cell with a transverse pin fin insert in channel flow

A proton exchange membrane (PEM) fuel cell has many distinctive features which make it an attractive alternative clean energy source. Some of those features are low start-up, high power density, high efficiency and remote applications. In the present study, a numerical investigation was conducted to analyse the flow field and reactant gas distribution in a PEM fuel cell channel with transversely inserted pin fins in the channel flow aimed at improving reactant gas distribution. A fin configuration of small hydraulic diameter was employed to minimise the additional pressure drop. The influence of the pin fin parameters, the flow Reynolds number, the gas diffusion layer (GDL) porosity on the reactant gas transport and the pressure drop across the channel length were explored. The parameters examined were optimized using a mathematical optimization code integrated with a commercial computational fluid dynamics code. The results obtained indicate that a pin fin insert in the channel flow considerably improves fuel cell performance and that optimal pin fin geometries exist for minimized pressure drop along the fuel channel for the fuel cell model considered. The results obtained provide a novel approach for improving the design of fuel cells for optimal performance.     

Thermal and Hydrodynamic Performances of Chaotic Mini-Channel: Application to the Fuel Cell Cooling

Currently, heat exchangers allowing the thermal management of low-temperature fuel cells (PEMFC) are integrated in the bipolar plates and are constituted of a network of straight channels. The flow regime is laminar and thus unfavorable to intense convective heat transfer. In order to increase the power density of the fuel cells, the use of chaotic geometries in the cooling system is envisaged to promote high convective heat transfer. In the present study, several chaotic three-dimensional mini-channels of rectangular cross-section (2 millimeters × 1 millimeter) are evaluated in terms of heat transfer efficiency, mixing properties, and pressure losses. Their performances are compared both to those of the straight channel geometry currently used in the cooling systems of the PEMFC and those of a square-wave mixer. Two Reynolds numbers are considered: 100 and 200. It is shown that a 3-D chaotic channel geometry significantly improves convective heat transfer over that of regular straight or square-wave mixer channels. Of all the geometries studied, one induces higher heat transfer intensification (mean Nusselt number equal to 20) with a strong pressure loss. With an alternative geometry, a better compromise between heat transfer and pressure loss is obtained. However, all of the chaotic geometries present similar mixing rate for the two Reynolds numbers studied.     

Heat transfer and pressure drop in corrugated channels

The convective heat transfer and pressure drop characteristics of flow in corrugated channels have been experimentally investigated. Experiments were performed on channels of uniform wall temperature and of fixed corrugation ratio over a range of Reynolds number, 3220 ≤ Re ≤ 9420. The effects of channel spacing and phase shift variations on heat transfer and pressure drop are discussed. Results of corrugated channels flow showed a significant heat transfer enhancement accompanied by increased pressure drop penalty. The average heat transfer coefficient and pressure drop enhanced by a factor of 2.6 up to 3.2 and 1.9 to 2.6 relative to those for parallel plate channel, respectively, depending upon the spacing and phase shift. The friction factor increased with increasing channel spacing and its phase shift. The effect of spacing variations on heat transfer and friction factor was more pronounced than that of phase shift variation, especially at high Reynolds number. Comparing results of the tested channels by considering the flow area goodness factor (j/f), it was better for corrugated channel with spacing ratio, ? ≤ 3.0 and of phase shift, Ø ≤ 90°. Comparisons of the present data with those available in literature are presented and discussed.     

Numerical modeling of anode-bleeding PEM fuel cells: Effects of operating conditions and flow field design

Operating the PEM fuel cell in the dead-ended anode mode reduces the overall cost and complexity of the system but causes a voltage loss and carbon corrosion in the cathode catalyst layer due to hydrogen starvation in the anode. Whereas allowing an ultra-low flowrate at the anode outlet offers a very high utilization of hydrogen and achieves a stable voltage transient. Here, a time-dependent pseudo-three-dimensional, two-phase, and non-isothermal model is developed to study the optimum bleeding rate, which maximizes the hydrogen utilization, achieves a stable cell voltage and avoids carbon corrosion, which is commonly observed when the bleed rate is set to zero, i.e. the dead-ended mode. The model is validated against the experimental data by comparing the polarization curves and cell voltage transients during the dead-ended anode operation of small experimental cells with serpentine and straight anode flow channels. Moreover, the effects of operating conditions on cell performance during the anode bleeding operation mode are investigated. Results demonstrate that the hydrogen utilization exceeds 99% in the anode-bleeding mode without hydrogen starvation, and the cell performance improves significantly for higher anode pressure, lower cell temperature, and lower relative humidity at the cathode inlet. Lastly, it is found that serpentine channels in the anode improve the uniformity of the distribution of hydrogen compared to straight and interdigitated channels in the anode-bleeding mode while the cathode flow field consists of serpentine channels.     

Comparison of water management between two bipolar plate flow-field geometries in proton exchange membrane fuel cells at low-density current range

This experimental research studies some aspects of water formation and management in polymer electrolyte membrane fuel cells (PEMFCs). To this end, two different single cells of 49 cm² active area have been tested, the first one with a serpentine-parallel geometry and the second with a cascade-type flow-field topology. In order to visualize the processes, flow-field channels have been machined on transparent plastic. Experiments have consisted in both image acquisition using a CCD camera, and simultaneous measurements of pressure drop in both hydrogen and oxygen gas flow paths. It has been observed that with the cascade-type flow-field geometry, water produced in the cathode does not flood the gas flow channels and, consequently, can be drained in an easy way. On the other hand, it has also been verified that saturated condition for the hydrogen gas flow at the anode side produces water condensation and channel flooding for the serpentine-parallel flow-field topology. Time fluctuations in the pressure drop of the gas flow have been detected and are associated to some transient process inherent to water formation and management.     

Numerical studies on ejector structure optimization and performance prediction based on a novel pressure drop model for proton exchange membrane fuel cell anode

The performance of hydrogen ejectors can be affected by the working conditions of the fuel cell system especially associating with the working pressure and pressure drop of the anode. However, the pressure drop characteristics model of the anode is correlated to the fuel cell parameters. In this work, a porous jump boundary is used as a pressure drop characteristics model of the anode which is weakly relevant to the parameters of fuel cells by employing the pressure drop characteristic curve of fuel cells. Based on the model, the influence of the condition parameters on the property of the ejector can be predicted. According to our results, the entrainment performance of the ejector can be influenced by anode inlet temperature, relative humidity, and differential pressure. Also, it is helpful for the design and prediction of the ejector in different fuel cell systems depend on the pressure drop.     

Characterization of sodium flow over hexagonal fuel subassemblies

Steady flow of liquid sodium over a bundle of heat generating hexagonal subassemblies has been investigated. The cross flow pressure drop and heat transfer are characterized using the general purpose CFD code STAR-CD. Analysis has been carried out for both laminar and turbulent regimes of interest to liquid metal fast reactors. Turbulence has been modeled using low Reynolds number (Re) k-ε model. The estimated pressure drop and heat transfer coefficients are compared against that of a straight parallel plate channel. It is seen that in the low Reynolds number range, the pressure drop for the hexagonal path is nearly equal to that of the parallel plate channel for the same length. However, in the high Reynolds number range, the pressure drop of the hexagonal path is much higher than that in the parallel plate channel, the ratio being 2 at Re = 2000 while it is 3.6 at Re = 20,000. Two competing factors, viz., (i) jet impingement/flow development effect and (ii) flow separation effect are found to influence the average Nusselt number (Nu). In the laminar regime, the latter effect dominates leading to a decrease of the Nusselt number with an increase in the Reynolds number. However, in the turbulent regime, the former effect dominates leading to an increase in the Nusselt number with Reynolds number. The Nusselt number in the hexagonal path is about twice that of the parallel plate channel due to under development of velocity/temperature profiles and the recirculation associated with the hexagonal path due to the changes in flow direction. Detailed correlations for both the pressure drop and the average Nusselt number have been proposed.     

The critical pressure drop for the purge process in the anode of a fuel cell

Purge operation is an effective way to remove the accumulated liquid water in the anode of proton exchange membrane fuel cells (PEMFCs). This paper studies the phenomenon of the two-phase flow as well as the pressure drop fluctuation inside the flow field of a single cell during the purge process. The flow patterns are identified as intermittent purge and annular purge, and the two purge processes are contrastively analyzed and discussed. The intermittent purge greatly affects the fuel cell performance and thus it is not suitable for the in situ application. The annular purge process requires a higher pressure drop, and the critical pressure drop is calculated from the annular purge model. Furthermore, this value is quantitatively analyzed and validated by experiments. The results show that the annular purge is appropriate for removing liquid water out of the anode in the fuel cell.     

Numerical study on characteristics of flow and heat transfer in a cooling passage with protrusion-in-dimple surface

The detailed flow structure and heat transfer characteristics in a newly designed heat transfer surface geometry were investigated. The surface geometry proposed is the combination of a conventional dimple cavity with a protrusion structure mounted within it. The underlying design concept of this surface geometry aims to enhance the flow mixing and the corresponding heat transfer in the flow-recirculating region that is generated by a conventional dimple cavity. Four different protrusion heights were considered as the main design parameter of the present study. The numerical simulations were carried out with a Reynolds number of 2800 and Prandtl number of 0.71 (air) corresponding to the mean velocity and channel height. The calculated pressure drop and heat-transfer capacity were assessed in terms of the Fanning friction factor and Colburn j factor. The overall performances, estimated in terms of area goodness factor for several protrusion-in-dimple cases, were higher than that found by a conventional dimple. Compared to the conventional dimple case, the pressure drop and heat-transfer capacity were slightly augmented in the case of a protrusion height of 0.05 since this leads to an improvement in the mixing of the turbulent flow in the dimple cavity.