At high temperatures and pressures, fluids can exist above a critical point where they can transition between “liquid-like” and “gas-like” properties without undergoing a phase change (such as the formation of bubbles or droplets). Recently, supercritical fluids have gained significant interest as working fluids in highly efficient engines, advanced aerospace applications, and electronics cooling. Supercritical fluids near the critical point have unique thermophysical properties that often provide enhanced heat transfer, but these properties can also lead to damaging flow instabilities and degraded heat transfer during certain operating conditions. The cause and control of these damaging near-critical degradations and oscillations are not well understood. This project will use a novel experimental approach to understand the physical phenomena that govern the heat transfer properties of near-critical fluids in order to predict and control their potentially damaging effects. In addition to the research objectives, this work will be linked with the development of a unique educational program dedicated to developing globally competent engineers prepared to bring supercritical technology to the market.

Specifically, this research will use fiber optic sensors to measure transient convective heat transfer of supercritical fluids, and use the data to develop transient models of conjugate heat transfer in tailored, non-homogenous, heat exchanger substrates. This proposed work will provide fundamental insights on the relationship between buoyancy, flow acceleration, and varying thermophysical properties on the unsteady convective heat transfer coefficient of supercritical fluids, and new information on the evolution of high- and low-frequency flow oscillations during transients of supercritical fluids. Established theory and models developed for steady state conditions will be evaluated using experiments more representative of actual supercritical system operation. Data will be used to guide the modeling of conjugate effects in non-homogeneous substrates, laying the groundwork for components fabricated through new manufacturing techniques. Finally, the demonstration of the fiber optic instruments for distributed high accuracy heat transfer experiments is of value to the broader thermal transport community.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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Oregon State University
United States
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