The demands for increasingly smaller, more capable, and higher power density technologies have heightened the need for new methods to manage and characterize extreme heat fluxes. This work presents the use of an anisotropic-version of the Time-domain Thermoreflectance (TDTR) technique to characterize the local heat transfer coefficient (HTC) of a water-cooled rectangular microchannel in a combined hot-spot heating and sub-cooled channel-flow configuration. Studies focused on room temperature, single-phase, degassed water flowing at an average velocity of ≈3.5 m/s in a ≈480 µm hydraulic diameter microchannel (e.g., Re ≈ 1850), where the TDTR pump heating laser induces a local heat flux of ≈900 W/cm2 in the center of the microchannel with a hot-spot area of ≈250 µm2. By using a differential TDTR measurement approach, we show that thermal effusivity distribution of the water coolant over the hot-spot is correlated to the single-phase convective heat transfer coefficient, where both the stagnant fluid (i.e., conduction and natural convection) and flowing fluid (i.e., forced convection) contributions are decoupled from each other. Our measurements of the local enhancement in the HTC over the hot-spot are in good agreement with established Nusselt number correlations. For example, our flow cooling results using a Ti metal wall support a maximum HTC enhancement via forced convection of ≈1060±190 kW/m2·K, where the Nusselt number correlations predict ≈900±150 kW/m2·K.
**TOPICS:**
Thermoreflectance, Water, Microchannels, Heat transfer coefficients, Heating, Forced convection, Natural convection, Pumps, Power density, Heat flux, Fluid dynamics, Flow (Dynamics), Heat, Temperature, Cooling, Fluids, Metals, Lasers, Heat conduction, Flux (Metallurgy), Coolants, Anisotropy, Channel flow, Convection