Two‐dimensional inert accumulation in space hydrogen‐oxygen fuel cells

Abstract

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 ...

An Integral-Spectral Formulation for Convective-Diffusive Transport in a Packed-Bed With Adsorption at the Wall and Bulk Reaction

“…For the case of catalytic chemical reactors one can find only a few applications of integral methods (Solbrig and Gidaspow, 1967;Gidaspow, 1971;Lyczkowski et al;1971) in spite of the fact that integral equations are particularly suitable to obtaining the solution of differential reactor models. In general, reactor problems show that heat and 114 P. ARCE mass transfer processesare coupled to the chemical reactions and in addition, the reaction rate terms frequently display nonlinear forms.…”