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Investigation of the Use of an Inorganic Aqueous Solution (IAS) in Phase Change Heat Transfer Devices


Research with the goal of understanding the chemistry of an Inorganic Aqueous Solution (IAS) used as a working fluid in copper and aluminum phase change heat transfer devices, finding the advantages and limits of IAS in various heat transfer applications, and providing a general guideline of how to correctly use IAS for performance enhancement and surface passivation purposes are described.

First, a comparison between IAS and water is done both physically and chemically of being used as a working fluid in phase change heat transfer devices. Above all, the chemical constituents in an IAS fluid are investigated, and a method is given to produce it. Afterwards, the physical properties of IAS are measured and compared to those of water. Moreover, a chemical analysis is performed, and the chemical reactions involved between IAS and the surfaces of copper or aluminum made devices where it is to be used are listed and categorized by their contributions to the heat transfer performance or the surface passivation. In addition, OLI software, commercial software to simulate electrolytes, is used to estimate the amount of the solid products, generated by redox reactions or temperature increases, at different temperatures and ratios of being concentrated. At the end, a coating study is performed to understand how each chemical coats the surface and how it contributes to the surface wettability. The positive and negative effects of each coating are discussed and demonstrated by data from SEM observations and contact angle tests.

Second, using IAS in copper made phase change heat transfer devices is discussed, and the main focus is how IAS improves the heat transfer performance by a smaller thermal resistance and a larger critical heat flux. At the beginning, a 1-D diffusion model is built and used to estimate the concentration profiles of chemicals along the liquid flow path and the location where each coating begins to be deposited. Next, a capillary rise test is performed to show how each chemical contributes to the improvement of the surface wettability. The heat transfer performance of water and IAS are compared for a thermo-syphon test at different inclination angles, a grooved flat heat pipe test, and a sintered heat pipe test. The test results validate the conclusions of the diffusion model and the capillary rise test. Based on the results of the comparison tests, the advantages and limits of using IAS, for performance enhancement purposes in various heat transfer applications, are discussed.

Last, an investigation of the aluminum passivation, with IAS being used as the working fluid in aluminum made phase change heat transfer devices, is done, and the key factors, that lead to failures, are discussed. Initially, an electrochemical theory of aluminum passivation is introduced, and the existence of an electrochemical cycle is demonstrated by an aluminum thermo-syphon test. Afterwards, the importance of a continuous liquid back flow to aluminum passivation is pointed out, and a vertical thermo-syphon test with natural convection cooling is used to demonstrate that a discontinuous liquid back flow is the main reason of the failures. At the end, using IAS in steel or stainless steel phase change heat transfer devices is discussed.

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