Heat transfer in freestanding microchannels with in-line and staggered pin fin structures with clearance

The heat transfer in straight silicon microchannels with integrated in-line and staggered pin fin arrays is evaluated at clearance to diameter ratios of 0.50-0.77 in the laminar flow regime at Reynolds numbers ranging from 9.0 to 246. The channels have a small width, leading to a significant influence of the channel walls on fluid flow and heat transfer. Their influence is considered when measuring the temperature distribution along the channel length and the average heat transfer for a constant temperature boundary condition. For this purpose, platinum thermistors are integrated directly into the channel structures, which are released from the silicon substrate and made freestanding via deep reactive ion etching (DRIE) and selective dicing. The measurements show that a significant portion of the fluid flows below the pin fins in the clearance bypass region, leading to heat transfer results that show an improvement over an unfinned reference geometry only for small clearance to diameter ratios. Heat transfer correlations are developed with a new functional form which considers the strong influence of the clearance to diameter ratio on the overall heat transfer.