Enhancing Heat Pipe Efficacy: Thermosyphon Analysis, New Generation Refrigerant Transition, and Design Optimization
Creators: Kumar, Vivek
Publisher: name: University of Alberta Library
Year: 2023

<p>The efficacy of heat pipes is often constrained by their ability to effectively return condensed liquid from the condenser to the evaporator section through capillary pumping. However, when the heat pipe wick is absent or flooded due to overfilling, capillary pumping becomes irrelevant, resulting in a thermosyphon system. In a close-to-horizontal orientation, the driving force for liquid return in a thermosyphon is derived from the difference in liquid pool depth between the evaporator and condenser. Increasing the depth of condensed liquid augments the driving force for flow. However, an excessively deep liquid pool in the condenser can limit radial heat transfer and hinder heat rejection. Therefore, it is crucial to strike a balance that favors intermediate-depth liquid pools. Increasing the fill ratio beyond the optimized value leads to escalated manufacturing costs and adverse effects on performance. This study employs a theoretical approach based on the lubrication approximation to the Navier-Stokes equations to determine the fill ratio that maximizes thermosyphon performance. Additionally, we explore the variations of this ratio in relation to factors such as the axial temperature difference along the thermosyphon. Our analysis overcomes a common simplification observed in conventional thermosyphon descriptions by considering the incremental flow resistance resulting from axial variations in the liquid film thickness. Neglecting this aspect can lead to inaccuracies in estimating the axial heat flux. The modeling of heat pipe and thermosyphon results aids in the selection between a thermosyphon and a heat pipe, while also utilizing the hydrostatic-driven flow limit in the thermosyphon to replace the capillary limit in the heat pipe ``fundamental diagram.&#39;&#39; \\</p>

<p>Furthermore, we compare the efficacy of R513a, an HFC/HFO refrigerant blend with lower global warming potential, to the commonly used R134a refrigerant in the context of heat pipe applications. Considering the adverse environmental impact of refrigerants on global warming, it is imperative to identify and implement eco-friendly alternatives with reduced global warming potential. Tests were conducted on both smooth and grooved heat pipes under uniform environmental conditions to evaluate the principal variations in performance and operation between the two refrigerants. Our study suggests that, in most cases, R513a can serve as a one-to-one substitute for R134a, demonstrating superior performance and enhanced heat transfer capacity. In certain situations, an integrated approach that involves adjusting only the fill mass is necessary to achieve comparable outcomes. This study illuminates a critical transition strategy towards alternative refrigerants, highlighting the potential for eco-friendly substitutes that rival or outperform conventional refrigerants.\\</p>

<p>Additionally, we present a Matlab-based, GUI-driven algorithm developed for heat pipe design and optimization. The algorithm predicts the thermodynamic performance of a heat pipe by considering various limiting conditions imposed by viscosity, capillary action, entrainment, boiling, and compressibility. This standalone tool assists in selecting the appropriate working fluid for a heat pipe and constructs a fundamental diagram over a wide temperature range. Furthermore, it facilitates the identification of optimal design parameters in a given setting. We utilized this tool to achieve an optimized design of axial heat transfer with enhanced performance, leveraging experimental data from the previous section specifically for a helical grooved heat pipe.\\</p>

<p>To sum up, our study contributes to the enhancement of heat pipe efficacy through an analysis of thermosyphon behavior, a comparison between eco-friendly refrigerants, and the development of a Matlab-based algorithm for heat pipe design and optimization. These findings offer valuable insights into achieving optimal heat transfer and provide a foundation for selecting suitable heat pipe configurations and working fluids for various applications.</p>