Delay of subsonic choking in slip regime by structured roughness in microchannel

Abstract

Roughness is an intrinsic property of a surface. Its presence is recognized at the micro-scale due to the high surface area to volume ratio. In the present experimental work, three-dimensional microchannels with structured roughness in the form of cuboidal protrusions called micro-ridges are fabricated. Ridge fraction (δ) is the ratio of the length of the ridge (s) to the distance between the ridges (L). δ is varied as 0.75, 0.50, 0.25, and 0 to check the occurrence of the choking phenomenon and its impact on the frictional resistance in gaseous slip flow. To this end, mass flow controllers, pressure sensors, and thermocouples are employed to explore the dependence of Poiseuille number (fRe) on Mach number (Ma) in the microchannel. It is demonstrated that the smooth microchannel (δ = 0) and the ridge with the shortest length (δ = 0.25) gets choked subsonically, but the longer ridges (δ = 0.50, 0.75) do not choke under the investigated conditions. Interestingly, fRe (δ = 0.50) > fRe (δ = 0.25) > fRe (δ = 0.75) ≈ fRe (δ = 0). Since choking limits the maximum amount of mass flow rate through a microchannel, its occurrence could be counter-productive or could even be beneficially employed, depending on the specific application.