Convective Acceleration in Porous Media

Abstract

Convective acceleration occurs in porous media flows due to the spatial variations of the nonuniform flow channel geometry of natural pores. This article demonstrates that the influence of convective acceleration in a nonuniform a pore channel is analogous to that of a constricting pipe channel. Their fluid mechanical behaviour can be comparable, provided that their geometrical characteristics are described precisely in the same manner, and from the same point of reference with regards to the fluid velocity in the flow channels. The analogy of the dissipation mechanisms in nonlinear porous media flow to the "minor loss" approach in fluid mechanics of pipes is therefore appropriate. Conventional nonuniform pipe channel geometries obtain dissipation coefficients within the range 0 < CKL < 0.2. These pipe geometries are relevant reference points for natural porous media, and it is thus expected that most natural pore geometries will obtain values within this range. This assumption holds true for the nine different 3D porous media samples presented here. However, the results show that the rate of change in the pore geometry, and consequently the magnitude of induced convective acceleration, depends on: the area ratio a of the pore channel, the angle of approach θ and the rounding of the pore channel geometry. The rounding of the pore channel reduces the dissipation coefficient, as the rate of change becomes smoother along the channel length. The results also indicate that the pore tortuosity increase the magnitude of nonlinear dissipation, in good agreement with pipe flow behaviour. This knowledge can help improve our interpretation of experimental data and enhance the predictability of porous media equations that incorporate the appropriate dissipation coefficients CKL as a variable. Article Highlights The analogy of porous media flow to the "minor loss" approach in fluid mechanics of pipes is appropriate, and the angle of approach θ and the area ratio a of the pore channel govern the magnitude of induced convective acceleration in porous media The rounding of the pore channel geometry reduces the magnitude of induced convective acceleration The tortuosity of a pore influences the dissipation coefficient CKL and increase the magnitude of induced convective acceleration