Narayanan The 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.
The broad goal of this project was to establish an integrated research and educational framework in the field of thermal management. Dissipation of heat loads at levels of 100 to 1000 watts per square centimeter is required in the cooling of modern and future 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 this research project was to demonstrate, by the use of inherent flow oscillations, enhancement in heat transfer rate per unit coolant mass flux can be achieved, with no additional pumping power penalty. An organ pipe resonance mechanism was used to create a self-sustained, self-excited oscillatory jet. A predominantly experimental approach was undertaken to document boiling curves through global measurements of temperature and heat flux. In addition, local and/or transient measurements of bubble dynamics using high-speed imaging, and surface temperature using IR thermography were performed. The results of the project are summarized in 7 archival journal articles and 14 conference papers. At the undergraduate level, several aspects of the research were integrated into two University Honors College courses. At the graduate level, research outcomes were disseminated through two courses. Two motivated high-school students were recruited to participate in 8-week summer projects through Apprenticeship in Science and Engineering program. The intellectual merit of the project pertains to the following novel aspects: (a) study of the jet flow oscillation phenomenon under phase change conditions, and (b) detailed imaging to delineate the physical mechanisms of convective heat transport during phase change heat transfer. We have demonstrated that it is possible to generate self-sustained flow oscillations in liquids undergoing phase change, and that flow oscillations can enhance the heat transfer coefficient and critical heat flux. Through detailed infrared thermal imaging, we have demonstrated that for flow boiling, it is essential to perform local or regional characterization of heat transfer rate. Through high-speed imaging, we have identified the complexities associated with bubble generation and entrainment during convective flow boiling. Broader Impacts of the project include education of four 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. The use of passive methods to achieve enhancements, such as the one demonstrated through this project, is critical to enhanced energy efficiency. A generalized correlation for jet impingement boiling, developed during this project, will enable implementation of new thermal management solutions.
