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Theoretical and Experimental Investigation of the Effects and Limits of using Inorganic Aqueous Solutions to Resist NCG Generation in Aluminum Thermosiphons


Water is one of the most capable and widely used working fluids in heat pipes and thermosiphons. Aluminum is a high conductivity, lightweight, and high strength choice for heat pipe casing material. However, aluminum and water are a heat pipe combination that is not considered viable due to the rapid generation of hydrogen, a noncondensable gas (NCG), resulting from chemical reactions between water and aluminum. A great deal of past research has been done evaluating the compatibility of many pure fluids with different metal heat pipe casings, yielding a large volume of lifetime tests demonstrating success or failure of various combinations. On the other hand, very little research was found analyzing what, if any, progress can be made to take low compatibility combinations and make them more compatible through the use of chemical inhibitors. This is particularly true for the combination of water and aluminum. Recently, inorganic chemicals in an aqueous solution, with the proper concentrations and pH range, have been shown to suppress the unwanted reactions and subsequent hydrogen formation rates in a manner that could prove useful enough to allow aluminum heat pipe casings to operate with aqueous based solutions as a working fluid.

The goal of this thesis is to produce, understand, and experimentally show the robustness of using inorganic inhibitors in aqueous heat pipe fluids for aluminum devices. Thermodynamic predictions were made to estimate conditions in which a stable oxide layer of the base metal, as well as the oxidizing inhibitors, will form within the pipe, thus giving a better chance at preventing NCG generation. E-pH, or Pourbaix, diagrams will be generated as a function of temperature to allow for prediction at any given heat pipe operating temperature range. Chemical reactions and processes responsible for NCG suppression will be explained. Inhibitor species’ concentrations will be investigated as an independent variable to determine recommended inhibitor amounts. Aluminum thermosiphons and heat pipes will be experimentally tested to verify efficacy of various inhibitor solutions in different thermosiphon geometries tested under different test conditions. Simple corrosion tests will also be carried out within a vacuumed chamber to determine relative amounts of hydrogen generated when an aluminum sample is in direct, static contact with a test fluid. The results of this thesis will show that while there is good evidence that inorganic inhibitors can significantly reduce the production of NCG in an aluminum-water heat pipe, the complex nature of two phase fluid circulation in such devices still presents serious challenges to the reliability and adoption of inhibitor fluids for critical thermal management needs. Reasons behind this phenomenon will be presented along with pathways for future research. These future work paths are believed to provide great opportunities for overcoming the challenges found in this work as well as potentially expanding the application of inhibiting heat pipe fluids to other active metals such as steel devices. The creation of a dilute aqueous solution which maintains the high latent heat of water but is also compatible with aluminum heat pipes would allow for significantly higher heat transport per device unit mass than currently used aluminum and ammonia heat pipes and provide another option for intermediate temperature heat pipes at low cost.

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