Cuauhtemoc Rubio-Arana

Three Dimensional Analysis of a PEM Fuel Cell with the Shape of a Fermat Spiral for the Flow Channel Configuration

This work presents the analysis of a non-isothermal three-dimensional model in single phase of a PEM fuel cell with an innovative flow field path in the form of the Fermat spiral, i.e. two concentric spirals. The model is used to predict the current density contours and the water content in all of the zones of the fuel cell. The three-dimensional model includes: the gas flow channels with the shape of the new geometry proposed, the current collectors, gas diffusion layers, catalyst layers on both sides of the model, anode and cathode, and a proton exchange membrane in between. The model solves the energy equation, mass conservation, and species transport equations, including the source terms due the electrochemical effects occurring in the cell. The results show a higher average current density than the fuel cells with conventional flow paths, showing also that the current density attained is more uniform from the inlet to the outlet of the flow channels.Copyright

Improving the Performance of a PEMFC by Means of Achieving Uniform Flow Distribution

A performance analysis of a proton exchange membrane fuel cell is reported in this work. Two different flow patterns are modeled as gas distributors and current collectors of a PEM fuel cell. Both flow patterns have the same active area with similar channel distribution over the membrane electrode assembly. Three dimensional models are used in order to simulate the performance of the fuel cells. The Navier-Stokes equations as well as potential fields (potentiostatic and galvanostatic) are solved using computational fluid dynamics techniques. Two dimensionless parameters were computed to quantify and compare the uniformity of the flow over the reaction area. The present analysis shows that achieving a good flow distribution is a key parameter in the PEMFC performance. The reduction of the concentration losses is the main result when a parallel channel configuration operates with uniform reactants distribution. In this study is demonstrated that the conventional parallel channels flow pattern does not achieve similar flow conditions in each sub-stream and therefore, irregular energy generation is obtained.Copyright

Comparison Between Traditional Microchannels Heat Sinks and Microchannels Heat Sinks Based on Biomimical Tendencies

In the last few years high-tendency electronic devices have improved to a larger processing capability with smaller physical dimensions. This fact coupled to traditional cooling mechanisms, are not able to dissipate the high heat fluxes generated by these devices (around 200 W/cm2 .) Microchannel heat sinks are the new tendency in heat dissipation. Many of the studies done before had used single-phase water as cooling fluid in laminar flow. Operating within this regimen, and using water as the cooling fluid, the dissipated heat flux is not enough to keep optimal operational conditions in the electronic devices. Therefore, this work presents a thermal and hydraulic numerical analysis for a microchannel heat sink with circular cross section, fabricated in a silicon substrate. The channel cross section is variable, being a function of the heat sink longitudinal position, decreasing as the cooling fluid passes through the channel. The ratio between the inlet and outlet diameters is given as a function of the Biomimic tendency. These theories are based on the behavior that nature has for the mass transport in circular ducts. The cooling fluid used in this study is water in single-phase. These microchannels heat sink arrangements are based in the operational and geometrical parameters of previous works developed by several authors on microchannels heat sinks with constant and conventional cross sections.Copyright

Analysis of Pin-Fin Heat Sinks With an Unconventional Fin Profile

In this work the performance of pin-fin heat sinks having an unconventional fin profile is compared with the use of cylindrical fins. The fin profile is a sinusoidal function and a staggered array is considered. The overall thermal resistance and total pressure drop are reported for the pin-fin heat sinks. The effect of using a wave function for the fin is studied for different number of complete waves along the height of the fins and a geometric parameter defined as the ratio of the higher to the lower radius of the fins is proposed. The study is carried out for two different inlet velocities, and for two different fin densities, corresponding to 5×5 and 7×7 arrays. An entropy generation analysis for each pin fin heat sink configuration is carried out and reported. The results of the present analysis reveal that the proposed geometry has an improvement as compared to the conventional heat sinks profiles when there is a high number of waves per fin. The effect of the geometric parameters defined in this study for the thermal and hydraulic performance is identified and discussed as well.Copyright

The Effect of Gas Channels-Electrode Interface Area on SOFCs Performance

It is well known that the main overpotentials during the operation of a fuel cell are activation, ohmic and concentration overpotentials. In order to operate more efficiently these devices that convert the chemical energy of the fuel into electrical energy, it is necessary to reduce as much as possible the overpotentials aforementioned. Some of the components of a fuel cell are the so called current collectors. These components affect the fuel cell performance mainly by means of two overpotentials, the ohmic and concentration overpotentials. The second one, is however, affected indirectly by the current collector design, since it may only help to distribute more uniformly the gases over the electrodes. The activation overpotential is basically not affected because it is mainly related with the electrode properties such as the exchange current density. In this work, the effect of the current collectors design on the performance of planar Solid Oxide Fuel Cells (SOFCs) is assessed by means of fully three-dimensional numerical simulations by comparing the V-I and power density curves of a planar cell. The goal of this study is not to find the optimal design of the current collectors but a way in which the overpotentials relate with their design in order to propose some helpful recommendations during the design process of these fuel cell components. These recommendations may lead to design an improved or optimal flow distributor.Copyright

Forced Convection Analysis in a Vertical Square Cavity With a Vortex Promoter

This paper presents a 2-D numerical study of the effect on the heat transfer from a promoter of turbulence in a vertical square cavity. The transient, turbulent flow simulation is for a fluid with a Prandtl number of 0.7 and constant properties. The thermal boundary conditions considered are isothermal cavity walls. In the analysis various aspect ratios and blockage fractions (10%, 25% and 50%) were examined at a constant Reynolds number (Re = 5000). The governing equations are solved using the LES (Large Eddy Simulation) method. The results demonstrate that increasing the channel blockage makes it more difficult to generate turbulence but increases the channel residence time of the fluid.Copyright

Numerical Simulation in an Air Filled Cubical Enclosure With Protruding Heat Sinks

The present study evaluates the effects of a protruding heat sink located on one of the vertical walls of a cubic cavity, with a free convective laminar regimen. The best of three locations for the heat sink will be determined with the analysis of the velocity and temperature fields in the cavity. An optimal operation condition is required since the cavity will contain medical products with a specific temperature range. Three different locations for the heat sink, (in the +Y direction), are to be studied: 1/3 H, 1/2 H and 2/3 H, where H is the height of the cavity, for different values of Rayleigh Number 103 <Ra<106 . The heat exchange occurs between the heat sink and the isothermal walls. The results obtained show that it is recommended to place the heat sink at the position H/2, as the air had the best thermal and dynamic behavior.Copyright

Environmental Impact Index Using Exergy Destruction Within an Ecosystem: Methodology

Theoretical bases of the use of the irreversibilities concept in an ecosystem as an environment impact index are presented in this paper. Because an ecosystem is composed by different biotics and abiotics parts, each part has a specific function in the processes to transport energy inside the ecosystem, all these processes having a large dependence between them. When anthropogenic emissions is produced part of the useful energy of the process are used to assimilate or to absorb those emissions, and irreversibilities are produced in the processes of the ecosystem when emissions are made by the human beings. Thus, the work that an ecosystem can carry out varies as a function of the irreversibilities produced by anthropogenic sources; the permanency or loss of the ecosystem depends on how many irreversibilities it can support. The second law of thermodynamics through a systematic use of the exergy concept o lost work concept are the basis of this methodology. The ecosystem can be divided in subsystems, each subsystem interrelated with the other ones, and then an ideal work variation can be obtained from each variation in the ecosystem (being the subsystems the water, the soil, the atmosphere, the organisms and the society). Thus, a global index could be determined by adding the partial irreversibility of each subsystem, and could be used to determine the trend that an ecosystem will follow due to alterations of its pristine, original or environmental line base state. Applying the methodology presented here an index of environmental impact could be developed, not in a subjective or ambiguous way, but based on energy calculations. An index of environmental impact and its methodology, based on quantitative terms that enable to evaluate in a very clear way the harmful effects of industrial operations, such as the phenomenon of contamination of the environment, could be used as a tool in the analysis of sustainability, resource depletion or health effects.Copyright

CFD Analysis of a Constructal Flow Distributor as a Bipolar Plate for PEMFC’s

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. Similar configurations have been already presented in previous studies but using the flow distributor as a heat remover; however, the similarity between heat transfer and mass transfer phenomena has lead to investigate the performance of such distributors in fuel cells. 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. Some of the advantages of the constructal model, over traditional flow patterns, are the uniformity of current density distribution, the adequate consumption of species and the possibility to increase the bifurcation levels to cover a larger reaction area.Copyright

Natural Convection in an Air-Filled Cubical Enclosure With a Protruding Heat Sink: Application to a Thermoelectric Refrigerator

The present study evaluates the effects of a protruding heat sink located on one of the vertical walls of a cubic cavity, with a free convective laminar regimen. The best of three locations for the heat sink will be determined with the analysis of the velocity and temperature fields in the cavity. An optimal operation condition is required since the cavity will contain medical products with a specific temperature range. Three different locations for the heat sink, (in the +Y direction), are to be studied: 1/3 H, 1/2 H and 2/3 H, where H is the height of the cavity, for different values of Rayleigh Number 103 <Ra<106 . The heat exchange occurs between the heat sink and the isothermal walls. The results obtained show that it is recommended to place the heat sink at the position H/2. This configuration presented the best behavior of the fluid, dynamically and thermally. The results will be used for a further and more completed numerical analysis, and to determine the thermoelectric refrigerator parameters for a later experimental study with a prototype.Copyright