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A Multi-scale Study of Inorganic Aqueous Solution (IAS) for Advanced Heat Pipe Applications


This dissertation is a comprehensive study of a novel category of working fluids, referred to as an Inorganic Aqueous Solution (IAS), for use in phase-change heat transfer devices (e.g. capillary heat pipes). The motivation for exploring IAS comes from investigation of an inorganic aqueous solution that was central to the Qu-tube cooling device. Although the tubes performed well, the explanation of their heat transfer mechanism was suspect. This study in part examines the heat transfer mechanisms occurring in an IAS-heat pipe device and leads to the introduction of a novel wicking mechanism termed Thermo-Hydro-Chemo (THC) wicking.

A broad exploratory study was performed to understand what the Qu-fluid (and IAS in the broader sense) is and the role it plays in a phase-change based cooling device. Prior to this dissertation there was no systematic verification of the performance of the Qu-fluid. The most significant outcomes of this exploration are the development and successive improvements of IAS and the introduction to THC mechanism.

A custom flat heat pipe (FHP) facility was designed and built to enable systematic comparison of the working fluids with different wick geometries by enabling one to make precise measurements and to visualize the thermophysical processes. Using the FHP facility, the thermal performance of IAS fluids was investigated and compared to the performance of water and to understand how IAS working fluids behave during heat pipe operations. This part of the work is mainly concerned with quantifying the methods and capacity that designer fluids have to enhance heat transfer. Different hypotheses the observed performance were explored using the FHP device and it was found that the impact of IAS on heat transfer performance and the involved mechanisms were more complex than previously thought.

The parameters that could be the reason for the enhanced performance of IAS were determined and investigated. A biporous wick experiment served as a model to determine the corresponding causal variables. To this end, each of the parameters that could contribute were investigated, i.e. the thermophysical properties of the Qu-fluid were measured and compared to those of water.

In addition, the chemistry of the Qu-fluid was unraveled, leading to development of an IAS. After in-house formulation of the IAS fluid, the effort to explain the prevalent thermal mechanisms of IAS continued. In particular, a multi-scale study was performed to better understand the IAS thermal performance and micro-scale studies were performed to understand the results of the device-level measurements. These studies are supported by empirical evidence obtained using optical microscopy, SEM/EDS analysis, and theoretical studies using a commercial computing engine developed by OLI Systems, Inc.

Finally, an exploratory optimization study of IAS was carried out using rectangular capillary grooved wick geometry. The existence of an optimum molality, and an optimum chemical composition that could maximize the heat removal capability of the copper grooved-capillary heat pipe was established. This study was limited to certain parental chemical constituents, based on which a "family of IAS" was designed and presumed to be the best generation of IAS, which thus far holds valid.

The findings presented in this thesis along with the work of other UCLA researchers resulted in a patent for the IAS fluid, which has enabled the use of water in aluminum heat pipes for the first time, a particularly important feature for the aerospace industry. The IAS fluid not only increases the cooling capacity for a broad range of closed evaporative based heat transfer devices, but also opens several research avenues for further exploration of its fundamentals and general applications. In particular, the new THC pumping mechanism requires an interdisciplinary approach to be fully characterized even more improvements.

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