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Cited by 4 publications
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Institute of Gas Technolagy, Chicoso, Illinois 60616To study heat and mass transfer in fuel cells, we must have a good description of fluid motion in the geometrically complex gas compartments ( 2 ) . We have already shown that a good description of such a motion can be obtained by the use of Darcy's law (1,3) because the flow is slow. To describe two-dimensional motion we need two permeabilities. These can be either measured or obtained by the solution of the creeping flow equations. Unfortunately the solution of the three-dimensional Navier-Stokes equations, even for the typical geometric element involved in, for example, Allis-Chalmers plates, is excessively timeconsuming with present generation computers (3). Therefore we decided to take the empirical approach. We measured the two penneabilities. PRESSURE DROP M€ASUREMENTSThe apparatus and procedure for making pressure drop measurements have already been described (1) for the case of an actual oxygen plate. In this study the inlet and outlet ports of an Allis-Chalmers hydrogen plate were cut and distributor systems (called slotted ribs in Figure 1) introduced before and after the fuel cell plate (Section 11 in Figure 1). The purpose of the distributors was to obtain one-dimensional flow in the test section. The permeability was calculated using Equation Page 1010July, 1971Equation (1) by replacing the length B with the width of the plate A. By this method the values of the permeabilities were calculated to be k, = 2.2 x 2 1.7 x 10-9 sq. ft. and k, = 1.78 xHaving obtained the values of the permeabilities, it is now possible to verify the applicability of Laplace's equation to describe the flow inside the cavities. The Laplace's equation (1)
Institute of Gas Technolagy, Chicoso, Illinois 60616To study heat and mass transfer in fuel cells, we must have a good description of fluid motion in the geometrically complex gas compartments ( 2 ) . We have already shown that a good description of such a motion can be obtained by the use of Darcy's law (1,3) because the flow is slow. To describe two-dimensional motion we need two permeabilities. These can be either measured or obtained by the solution of the creeping flow equations. Unfortunately the solution of the three-dimensional Navier-Stokes equations, even for the typical geometric element involved in, for example, Allis-Chalmers plates, is excessively timeconsuming with present generation computers (3). Therefore we decided to take the empirical approach. We measured the two penneabilities. PRESSURE DROP M€ASUREMENTSThe apparatus and procedure for making pressure drop measurements have already been described (1) for the case of an actual oxygen plate. In this study the inlet and outlet ports of an Allis-Chalmers hydrogen plate were cut and distributor systems (called slotted ribs in Figure 1) introduced before and after the fuel cell plate (Section 11 in Figure 1). The purpose of the distributors was to obtain one-dimensional flow in the test section. The permeability was calculated using Equation Page 1010July, 1971Equation (1) by replacing the length B with the width of the plate A. By this method the values of the permeabilities were calculated to be k, = 2.2 x 2 1.7 x 10-9 sq. ft. and k, = 1.78 xHaving obtained the values of the permeabilities, it is now possible to verify the applicability of Laplace's equation to describe the flow inside the cavities. The Laplace's equation (1)
Fuel cell compartments are of two types: through-flow and dead-ended. Dead-ended anode and cathode compartments are used when pure reactants are available, such as hydrogen and oxygen in space vehicles. Gases enter the compartment only upon load demand due to a reduction in pressure in the cavity. But even ultrapure fuels and oxidants have some impurities which accumulate and cause a drop in voltage due to dilution of the reactants. In Part I of this study (1) we obtained concentration profiles as a function of time for the case when the flow in the fuel cell can be considered one-dimensional. We also suggested a method of improved operation of a fuel cell involving bleeding of inerts. This was based on the observation that the inerts accumulate near the dead end of the cell.The assumption of one-dimensionality of flow and diffusion in an actual fuel cell must, however, be questioned. This is due to the fact that the inlet ports are made quite small. The size of the opening, for example, in some of Allis-Chalmers cells, is about 1 .mil in diameter as compared with a cell width of 4 in. In the one-dimensional model, the inlet port corresponds to the full 4-in. width. In the present two-dimensional model, the ports can be point sources or have any desired width.In the one-dimensional model (1, 2 ) it was possible to obtain the solution for the buildup of inerts by considering only the reacting species equation and the continuity equation. The flow field was obtained by simple quadrature of the current density. For the two-dimensional problem, we have to consider the fluid mechanics in the fuel cell cavities. The flow would be simply creeping motion, called Hele-Shaw flow, were it possible to have empty cavities, since the Reynolds number is small ( 2 ) . However, structural rigidity, desirability to have current collectors contacting the electrodes, and the necessity to remove heat by conduction apparently led Allis-Chalmers to build the gas compartments with a complex internal structure. In an earlier study (3, 4 ) , it was shown that the flow field can be successfully described by the use of Darcy's law. Two empirical permeabilities were determined for a typical Allis-Chalmers flow configuration, one for the basic direction of flow and the other for the perpendicular direction. Using these values, theoretical pressure curves based on the solution of the resulting Laplace's equation for pressure were verified experimentally using an actual AllisChalmers fuel cell compartment (5). The agreement was such that we can safely use this model to describe our fluid mechanics in a dead-ended cell operated under load, with bleed or even during the purge operation, MATHEMATICAL MODELConsider a thin rectangular cavity into which a gas containing a small amount of impurity enters through a small opening (Figure 1). The gas reacts electrochemically on one side of the surfaces and disappears. The contaminant remains in the cavity and acts as a dilutant. The product water of the electrochemical reaction either leaves through the ...
Accumulation and continuous removal of impurities in fuel cells: I. One‐dimensional model
A mode of operation and o design technique hove been developed which permit the attainment of continuous purging of impurities directly from the gas compartments of a fuel cell-either anode, cathode, or both-with the realization of minimum reactont loss, most stable voltage and current output, and operating conditions with respect to reactant gas flow and electrolyte inventory. We have found o way to eliminate the complex periodic purge valves and attendant electronics by o fuel cell system which i s both simple in structure and operation and which has a high degree of reliability.The technique was suggested by the observation that in dead-ended fuel cell gas compartments, the inert impurities present i n reactant gases tend to accumulate at the dead-ended portion of the cell. Hence a small amount of bleed should be placed there. This observation has been made onolyticolly by solving numerically a system of two partial differential equations simultaneously.When hydrogen and oxygen or similarly pure reactants are used as fuel and oxidant and where the product of reaction i s a condensable gas such as water vapor, it i s common practice to make either the anode gas chamber or cathode gas chamber dead-ended. Such designs have been used recently, for example, on the oxygen or cathode chamber of the Apollo (1) fuel cell and for both the oxygen and hydrogen chamber of the matrix-type ( 2 ) fuel cell which may be used in the U S . Air Force's manned orbiting laboratory (MOL) fuel cell.Even though the hydrogen and oxygen used in these fuel cells are of high purity, there are, nevertheless, some impurities present in these reactants. These impurities are in the form of noncondensable gases, such as nitrogen, helium, or argon or similar such gases. During the course of operation of these dead-ended cells, the fuel and oxidant are continuously consumed and converted into water, which i s then removed from the cell in some fashion. In the case of the above-mentioned Apollo fuel cell, the product water i s removed by recirculation of the hydrogen gas through a water condensor. In the case of the MOL fuel cells, the water is removed through a water transport membrane located within the anode chamber of the fuel cell, When the cell i s dead-ended, the noncondensable impurity accumulates, that is, its concentration becomes higher in the gas compartments of the cell. The accumulation of these impurities causes a drop in operating voltage of the cell and a nonuniform current distribution within the cell. 80th features are most undesirable. A s a result, it i s common practice to purge the electrode chambers periodically to allow the impurity to escape from the cell. This periodic purging causes undesirable voltage transients, possible temporary interruption of power, and permanent loss of electrolyte from the cell, as well as other damaging transients, which, it i s believed, ultimately lead to total cell failure. Moreover, to accomplish the periodic purging, a complicated set of purge valves and accompanying electronics equipment...
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