Advanced Working Fluids for Flat Heat Pipes
Heat pipes and other phase change devices have vast application to thermal management systems due to their efficient heat transfer capabilities. Current trends for improving the thermal performance of heat pipes have been focused on optimization of different wick geometries, to give the most efficient liquid transport while lowering the overall thermal resistance of the device. Advances in manufacturing have opened research to high capillarity micro- and nano-structured wicks. The choice of working fluid has been limited in the past. However, recently discovered advanced working fluids can offer several advantages. A novel working fluid for phase change heat transfer devices was investigated at UCLA. The Inorganic Aqueous Solution (IAS) has shown significant thermal performance enhancement due to surface augmentation but the deposition mechanisms of the IAS surface coating haven’t been fully observed or understood. The goal of this work was to observe the deposition and wetting dynamics of the fluid during operation in a heat pipe and use information from those experiments to develop a predictive thermal and hydrodynamic model of flow in a heat pipe. In this work, a background in phase change heat transfer and advanced working fluids is first presented. Three tasks, coating characterization, heat pipe experiment, and heat pipe modeling are outlined. Deposition mechanisms of a new working fluid were examined through accurate flat heat pipe experiments. Surprising observations of the deposition patterns in a grooved wick yielded valuable insight into the fundamental heat transfer concepts in which the fluid enhances heat pipe thermal performance. The role of interesting dynamics of the fluid, including the role of advective flow in a heat pipe, or the creeping nature of salt deposits, in enhancing heat transfer were identified. These observations were then utilized to develop a heat pipe model to study the effects of the deposition on heat pipe thermal resistance and dry-out limit theoretically. Strategic application of experimental closure to the model lead to even more interesting findings. Important information regarding the dry-out dynamics of flows with and without coatings was learned that will help develop the fluids.