Interaction of steady jets with an array of permeable screens

Abstract

Turbulent flows in porous media have important practical applications such as enhanced mixing of fuel and air, food drying, and cooling of electronics. However, experimental studies of turbulence in porous media are sparse due to the difficulties of measuring the complex flow environment. To this end, the interactions of steady jets with a porous medium formed from several parallel, transparent, permeable screens are studied using digital particle image velocimetry in a refractive indexed-matched environment. The permeable screens had porosities (open area ratios, ϕ) of 83.8, 69.0, 55.7, and 49.5 % and were held by a transparent frame that allowed the screen spacing to be changed. The steady jet results for Reynolds number (Re), which is defined based on the jet exit velocity and jet diameter, of 1000 showed laminar, predominantly steady flow that was segregated inside the porous medium, but for Re ≥ 2000, the flow was unsteady and turbulent with a mean velocity field that was relatively smooth inside the porous medium. As a result, more traditional jet features of self-similarity and increasing jet width were compared for the Re ≥ 2000 results. Decreasing the porosity was observed to increase the width of the jet significantly, especially for low porosity screens, and slowed the jet flow speed. Some of the typical features for axisymmetric jets were observed, even though the flow impinged on the permeable screens. In particular, self-similarity (or near self-similar behavior) was observed for the cross-sectional mean velocity profiles and for turbulence quantities for porosities larger than 55.7 % inside the porous medium. The effect of ϕ on turbulence quantities was significant for Re ≥ 2000. Although turbulence intensity increased on the downstream side of the first screen, the high dissipation forced drastic decrease of turbulence levels further downstream in the porous domain. Finally, the screens increased the removal of momentum from the jet as porosity decreased and screen spacing had a significant effect on the removal rate.