Heat transfer coefficient measurements in the thermal boundary layer of microchannel heat sinks

This study describes the use of optical pump-probe diagnostics to characterize the heat transfer coefficient (HTC) in a developing thermal boundary layer in a microchannel. We use a differential form of the anisotropic time-domain thermoreflectance (TDTR) technique to measure the HTC as a function of fluid flow rate (or Reynolds number, Re). The testing environment/geometry consists of single-phase, degassed water flowing in a rectangular microchannel (hydraulic diameter Dh ≅ 480 μm) with local spot heating by the pump TDTR laser beam. Relative to the HTC measured with non-flowing (static) fluids, we find a 30% increase in the HTC for single-phase water flowing at Re ~ 1800.