Exploring roughness effect on laminar internal flow-are we ready for change?

Laminar flow is often encountered in the channels of microdevices as a result of the small hydraulic diameters. The roughness introduced on the walls of these channels through various fabrication techniques, such as etching, micromachining, laser drilling, etc., results in a high value of relative roughness (defined as the wall surface roughness to channel hydraulic diameter ratio). Laminar flow in rough tubes, therefore, is an important topic in the study of transport processes in microdevices. This article begins with a review of the applications where roughness or wan surface features are present, such as mixing, enhanced mass transfer, and heat transfer. Next, the effect of roughness on fluid flow is reviewed in terms of eddy generation and transition to turbulence. The competing effects of destabilization of flow and relaminarization processes are considered. Drawing from the available literature, the criteria for transition to turbulence based on roughness Reynolds number are evaluated and some of the recent experimental data on repeated 2-D roughness structures are compared. The present article addresses these issues and provides a framework for quantifying the roughness effect at microscale.