CAREER: Enhanced Two-phase Thermal Management Using Self-sustained Flow Oscillations at the Microscale

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0748249NarayananThe broad goal of this CAREER project is to establish an integrated research and educational framework in the field of thermal management. Dissipation of heat loads at levels of ~ 102 to 103 W/cm2 is of great interest in the cooling of high-power electronics. Current predominant cooling methods include spray and liquid jet impingement evaporation, and flow boiling in microchannel heat sinks. Although these methods have demonstrated capability of removing high heat fluxes, significant challenges still exist in improving cooling efficiencies. The primary intent of the proposed research is to demonstrate, by the use of inherent flow oscillations, enhancement in heat transfer rate per unit coolant mass flux beyond that currently achieved using evaporative liquid jet and spray impingement, with no additional pumping power penalty. An organ pipe resonance mechanism will be used to create a self-sustained, self-excited oscillatory jet (SOJ). The hypotheses to be considered include the following: (1) Flow oscillations enhance heat transfer rates through both periodic renewal of the hydrodynamic and thermal boundary layers and the increased convective heat transport by bubble oscillations at the surface, (2) Critical heat flux (CHF) can be increased (beyond that attained by free-surface liquid jets and sprays) due to the effective rewetting of the surface by transverse flow oscillations, and (3) Use of flow oscillations mitigates surface deposition and aggregation of nanoparticles when using nanofluids.To test the above hypotheses, a predominantly experimental approach is proposed to document the heat transfer rate and CHF onset. Key momentum and thermal transport mechanisms will be identified by quantitative imaging of fluid temperature using laser induced fluorescence, jet flow field using particle image velocimetry, bubble dynamics using high-speed imaging, and surface temperature using IR thermography. The effect of flow oscillations on surface microstructures and nanofluids will also be studied.Several aspects of the research will be integrated into the University Honors College (UHC) curriculum through (a) an undergraduate Heat Transfer course, (b) a proposed Honors colloquium, and (c) UHC theses. At the graduate level, research outcomes will be disseminated through two existing classes as well as through a new special topics class based on the research area. Each summer, motivated high-school students will participate in research activities through the Apprenticeship in Science and Engineering program (www.saturdayacademy.org). The intellectual merit pertains to the following novel aspects: (a) study of the jet flow oscillation phenomenon under phase change conditions, (b) coupling of jet flow oscillations with existing enhancement mechanisms such as microstructured surfaces and nanofluids, and (c) performing detailed imaging to delineate the physical mechanisms of convective heat transport. Broader Impacts include education of three PhD, one MS, and several undergraduate and high-school students in the field of thermal management. Enhancement of heat transfer rates and developing methods to delay the onset of CHF in two-phase thermal management are of critical importance to the performance of high-power electronics and avionics, as well as for computer chip cooling. Utilization of passive enhancement methods to achieve enhancement in thermal and fluid transport fosters reductions in energy use by effective use of available resources.