Abstract
Gas flow in porous media can be seen in various engineering devices such as catalytic converters and fuel cells. It is important to understand transport phenomena in porous media for improvement of the performance of such devices. Porous media with pores as small as the mean free path of gas molecules are used in such devices as proton exchange membrane fuel cells. It is difficult to measure molecular transport through such small pores in the experimental approach. In addition, even when using theoretical or numerical approaches, gas flow through nanoscale pores must be treated by the Boltzmann equation rather than the Navier–Stokes equations because it cannot be considered as a continuum. Thus, conventional analyses based on the continuum hypothesis are inadequate and the transport phenomena in porous media with nanoscale pores are not yet clearly understood. In this study, we represented porous media by randomly arranged solid spherical particles and simulated pressure-driven gas flow through the porous media by using the direct simulation Monte Carlo (DSMC) method based on the Boltzmann equation. DSMC simulations were performed for different porosities and different sizes of solid particles of porous media. It was confirmed that Darcy’s law holds even in the case of porous media with micro-/nanoscale pores. Using the obtained results, we constructed expressions to estimate the pressure-driven gas transport in porous media with micro-/nanoscale pores and porosity ranging from 0.3 to 0.5. The flow velocities estimated by using the constructed expressions agreed well with those obtained in the DSMC simulations.
Highlights
Gas flow in porous media can be observed in various engineering devices such as catalytic converters and fuel cells
A porous medium was represented by arranging solid spherical particles randomly
Through a porous medium with micro-/nanoscale pores was reproduced by performing direct simulation Monte Carlo (DSMC) simulations
Summary
Gas flow in porous media can be observed in various engineering devices such as catalytic converters and fuel cells. Because of their large effective surface area, porous media are used to facilitate surface reactions such as dissociation of H2 in electrodes of proton exchange membrane fuel cells (PEMFCs). Gas flow through a porous medium is usually analyzed by using Darcy’s law (1856), which states that the discharge rate through a porous medium is proportional to the pressure gradient ∇p and the permeability K and is inversely proportional to the coefficient of viscosity μ of gas. Where p is the gas pressure and Us is the superficial velocity, which is defined as the volume flow rate through a unit cross-sectional area of the solid plus fluid of a porous medium (Bird et al 2007).
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