Collaborative Research: Computational and Experimental Investigation of High-Flux Heating of Supercritical Fluids in Microscale Geometries

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At a sufficiently high temperature and pressure, fluids become supercritical. When a supercritical fluid is heated, it does not undergo a change in phase from a liquid to vapor like in familiar boiling processes. Rather the fluid undergoes a continuous transition from "liquid like" to "gas like" properties without the formation of bubbles. During this "pseudo-critical" transition, the fluid can absorb a great quantity of heat per degree of temperature increase, which potentially enables new high-intensity heat transfer technologies without the instabilities, critical heat flux, and maldistribution issues that cause challenges in two-phase boiling solutions. By harnessing the unique properties of supercritical fluids, it may be possible to further develop higher power, compact computer chips and more efficient high temperature solar thermal systems. To explore these possibilities, the research team will study a new thermal management framework employing supercritical carbon dioxide (sCO2) in the pseudo-critical region in microscale flow geometries. Under these conditions sCO2 has extremely high volumetric heat capacity and thermal conductivity, enabling management of high heat fluxes (~500 W cm-2). New engineering resources and models formulated in this study will enable rapid adoption of supercritical cooling in real-world technologies. Additionally, improved understanding of microscale supercritical fluid transport can yield advances in diverse technologies including enhanced oil extraction, near-zero global warming potential (GWP) supercritical refrigeration and power cycles, and solvent/impregnation processes. The research team will conduct an integrated experimental and computational investigation to elucidate the complex governing phenomena during heating of sCO2 and general supercritical fluids at pseudocritical conditions in microscale geometries. Local fast-response temperature measurements and infrared thermal imaging will resolve the existence of, and quantify the effects of key supercritical phenomena at this scale, including pseudo-boiling, intrinsic flow pulsations, heat-transfer enhancement and/or deterioration due to sharp property variations, pin-fin wake interactions, and conjugate heat transfer effects. Experiments will be conducted with supercritical CO2 at high pressure (75 P 200 bar) and heat fluxes, and microfabrication techniques will be leveraged to explore advanced geometries of interest for electronics cooling and solar thermal applications. Extensive detached eddy simulation (DES) computations will be performed to provide detailed local flow data to complement experiments, and enable generalization beyond specific working fluids and operating conditions. DES represents a new approach to study supercritical convective transport that captures key unsteady turbulent phenomena in the channel bulk, unlike Reynolds averaged formulations employed in previous studies, but without the extreme computational costs of LES/DNS that also fully resolve near-wall regions. Once experimentally validated, simulations will be employed to extend results to supercritical flows with diverse configurations and geometries. In these studies individual fluid property trends will be modulated independently to isolate and quantify critical effects that have previously only been characterized through lumped parameter descriptions. This approach will enable generalization of results to the broader family of supercritical flows, rather than just specific fluids employed in experiments. Integration of detailed experimental and computational data will yield consolidated analytical transport models, facilitating rapid translation of fundamental research to practical thermal management technologies.