Abstract
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.
Highlights
Two-phase cooling has become a promising method for improving the sustainability and efficiency of high energy-density and power-density devices
Four key, transient mechanisms are attributed to the heat transfer process during the vapor-bubble ebullition cycle[7,8,9,10]: microlayer evaporation, interline evaporation, transient conduction, and micro-convection—while other mechanisms such as condensation[11,12] and liquid imbibition[13,14,15,16] can contribute with varying significance at different heat flux conditions
We present the extended application of the time-domain thermoreflectance (TDTR) technique[33] to characterize both the local, transient wall temperature and the local, transient heat transfer coefficient (HTC) during subcooled flow boiling
Summary
Two-phase cooling has become a promising method for improving the sustainability and efficiency of high energy-density and power-density devices. All traditional methods suffer from insufficient resolution in at least one spatial, temporal, or temperature-diagnostic In this communication, we present the extended application of the time-domain thermoreflectance (TDTR) technique[33] to characterize both the local, transient wall temperature and the local, transient heat transfer coefficient (HTC) during subcooled flow boiling. We focus on the ebullition cycle of a single vapor bubble over a micron-sized hot-spot in a microchannel using two different cooling configurations: (1) subcooled flow boiling and (2) subcooled pool/confined boiling In both of these subcooled boiling configurations, localized wall-to-fluid heat fluxes beyond 2 kW cm−2 are achieved with in-situ HTC characterization during the consecutive bubble ebullition cycles. In light of the extreme local heat fluxes imposed in this work, our reproduction of commonly measured macroscale HTCs offers promise for the adept use of conventional HTC correlations in extreme thermal gradient processes
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