Influence of three-dimensional transverse micro-ridges on the Poiseuille number in a gaseous slip flow

Micro-ridges are regular micro-engineering structures protruding outward from the microchannel wall surface. The role of these micro-ridges in changing the friction factor (f) of a microchannel with internal rarefied gas flows is not explored in the literature. In this work, we present three-dimensional numerical simulations in a microchannel containing ridges arranged transverse to the flow direction. The length of the ridges is varied with respect to the pitch of the ridges; this ratio being christened as ridge fraction (delta). The effect of delta, ridge height ratio (h/H), Reynolds number (Re), Knudsen number (Kn) and Tangential momentum accommodation coefficient (alpha(v)) on the flow friction in terms of Poiseuille number (fRe) is investigated. In the continuum and slip regimes, it is found that fRe decreases with an increase in delta and h and a significant reduction in flow friction was shown with respect to a straight microchannel. There exists a critical height after which decrease in fRe does not occur. Further, fRe was observed to be a strong function of Re at high delta, which is attributed to the intensity of acceleration-deceleration experienced by the gas during the flow. Also, fRe is established to be directly proportional to alpha(v). The various flow characteristics are scrutinised where vortices are found trapped inside the ridge affecting the dynamics of flow in the ridges, whose size increases on increasing the gas rarefaction.This comprehensive investigation of behaviour of slip flow in complex microchannels will be useful in designing micro-devices with reduced friction.