Fabrication and experiment of micro-pin-finned microchannels to study surface roughness effects on convective heat transfer

Rectangular microchannels with three different micro-pin-finned spacings (2 μm, 4 μm and 8 μm) are fabricated using MEMS technique and the effect of the surface roughness on the convective heat transfer characteristics is examined under laminar flow conditions for 70 < Re < 250. The measured Poiseuille numbers (Po), defined as f Re as a measure of the friction, for the micro-pin-finned surfaces are lower than that for the smooth surface, which is attributed to the apparent slip associated with the flow separation and reattachment occurring in the regions between the pin-fins. Poiseuille number decreases with increasing pin-fin spacing because of its increased slip region. The measured Nusselt numbers, as a measure of the convective heat transfer enhancement, for the micro-pin-finned surfaces also decreases with increasing fin spacing, but Nu for 2 μm spacing is higher than that for the smooth surface and the Nusselt numbers for both 4- and 8 μm spacings are lower than the smooth surface case. Also, numerical predictions performed for the range of 2D pin fin spacings from 1 to 32 μm confirm that the competition between the heat transfer enhancement on the top surfaces of the fins and the heat transfer deterioration in the pin-fin spacing areas determines the overall heat transfer rate. The heat transfer on the top surfaces of the pin-fins tends to increase because of the locally developing flows both at the leading edge and the trailing edge of each fin. In contrast, the heat transfer in the pin-fin spacing areas decreases to the conduction level since the recirculation flows within the fin valleys are constricted by a suspended boundary.