Effect of Sawtooth Roughness on Pressure Drop and Turbulent Transition in Microchannels

Abstract

Roughness elements affect internal flows in different ways. One effect is the transition from laminar to turbulent flow at a lower Reynolds number than the predicted Re = 2300. Initial work at RIT in the subject area was performed by Schmitt and Kandlikar [1] and Kandlikar et al. [2], and this study is an extension of these efforts. The channel used in this study is rectangular, with varying separation between walls that have machined roughness elements. The roughness elements are saw-tooth in structure, with element heights of 107 and 117 μ m for two pitches of 405 μ m and 815 μ m, respectively. The resulting hydraulic diameters and Reynolds numbers based on the constricted flow area range from 424 μ m to 2016 μ m and 210 to 2400, respectively. Pressure measurements are taken at sixteen locations along the flow length of 88.9 mm to determine the local pressure gradients. The results for friction factors and transition to turbulent flow are obtained and compared with the data reported by Schmitt and Kandlikar [1]. The roughness elements cause an early transition to turbulent flow, and the friction factors in the laminar region are predicted accurately using the hydraulic diameter based on the constricted flow area.