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 subcooled 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.

Two-phase cooling has become a promising method for improving the sustainability and efficiency of high energy-density and power-density devices. Fundamentally, however, two-phase thermal transport is not well understood for local, transient processes, especially at critical to near-critical heat fluxes at the macro, micro, and nano-scales. Here we report spatiotemporal characterization of the single-bubble ebullition cycle in a hot-spot heating configuration with heat fluxes approaching 3 kW cm −2 . In particular, we experimentally reconstruct the spatiotemporal heat transfer coefficient in terms of its proportionality at both the macro-scale (l >> 1 μm) and the micro-to-nanoscale (l < 1 μm). We show that the maximum rates of heat transfer occur during the microlayer evaporation stage of the ebullition cycle, corresponding to critical maxima in the heat transfer coefficient of~160 ± 40 kW m −2 K −1 and~5300 ± 300 kW m −2 K −1 at the macro-scale and micro-to-nanoscale, respectively.

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