Deciphering the Physics of Critical Heat Flux (CHF)

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Boiling is a common mechanism of heat transfer that has numerous applications ranging from cooling and refrigeration systems used in most buildings to large boilers used in energy and process industries. The performance of the boiling heat transfer process is limited by a phenomenon commonly known as the critical heat flux. Critical heat flux is the highest heat flux a heater can exchange with a boiling fluid before the formation of a vapor layer of low thermal conductivity that isolates the surface from the liquid. Despite nearly a century of research on critical heat flux, its underlying physics is still not fully understood because the relevant phenomena are transient, geometrically complex and limited by multiple, coupled physical mechanisms. This research project aims to address fundamental physical questions about the nature of critical heat flux. Outcomes of this research could potentially benefit many applications in which thermal management is a limiting factor, such as X-band radars, laser diodes, semiconductor-based power transformers, data centers and more reliable, compact nuclear reactors. The results of this study also will enrich thermofluid science courses through inclusion of new knowledge on physics of boiling heat transfer. This research project aims to describe the physical mechanisms of critical heat flux in boiling and to provide design rules to maximize critical heat flux for a wide range of fluids. The first aim of this proposal is an investigation of the coupling between hydrodynamic enhancement and wickability. Using the new configuration of a phobic membrane above the surface to optimize multiphase flows, the respective contributions of hydrodynamics, wicking and liquid pressure on critical heat flux will be identified and individually optimized. This research project also aims to build on the hypothesis that extended area ratios enhance critical heat flux when specific geometries with sub-millimeter fins are used. An extensive array of experiments involving low and high-surface tension fluids, high-speed visualization and state-of-the-art heat flux mapping will provide the data to validate, quantify and generalize the above contributions. The effort will culminate in a science base for the limits of critical heat flux and with design tools to maximize critical heat flux for a wide range of heater materials and fluids. The fundamental knowledge and theories generated under this work serve as major validation thresholds in boiling science that can facilitate development of next generation two-phase systems with a drastically improved performance.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.