A miniature active cooling system capable of providing multiwatt refrigeration for thermally protecting downhole instruments used in hot geothermal wells is researched, developed and designed. The engineering design process is used to develop design criteria and design constraints and to select potential refrigeration processes. A literature search identifies nine physical and chemical processes and twenty six systems appropriate for a feasibility study. Each refrigeration process is investigated to search for suitable high temperature components and a refrigerant, thereby eliminating only three processes and nine systems. Consistent analysis of an ideal thermodynamic cycle for each of the six remaining refrigeration processes further reduces the choices to three processes and seven systems.; Final selection requires consideration of refrigerator and geothermal logging system thermal, mechanical and electrical interactions to define failure modes, insure compatibility with existing hardware, and allow adaptability to changes in design criteria. Results of calculations from postulated design criteria changes provide insight into design simplicity and data for further selection. A decision model is constructed and used to organize design constraints and criteria and sort out those useful for making a final selection. Final selection is based on maximizing system simplicity and providing graceful degradation in case of refrigerator failure. The consistent design, systematic analysis and unbiased selection process represent a body of research results that is new to this technology and provides a potential for advance not realized to date.; The selected thermoacoustic process and the designed refrigeration system are analyzed in detail, with numerical models constructed for each subsystem, and component and for the component interactions. The thermoacoustic wave equation is extended to allow sloped rather than parallel acoustic plates. This extension increases acoustic power and reduces dissipative losses. The resonator losses model is extended for rectangular geometry and sloped stacks to aid in reducing viscous losses. A modular layered resonator design separates high temperature and high pressure vessel operating problems and provides adaptability to changes in operating conditions. A miniature electric resistance heater and two sets of stacked micro heat pipes at each acoustic stack leading or trailing edge provide localized high flux heat transfer. The micro heat pipe design represents an extension in micro heat pipe construction technology to avoid typical wicking limits.; The miniature thermoacoustic refrigerator design provides long term active cooling for downhole instruments and represents a promising and substantial advancement in downhole thermal protection technology.
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