Thermal Performance Characterization of a Miniature Loop Heat Pipe in Vacuum: Fixed and Variable Conductance Modes of Operation

This study presents an experimental investigation of a miniature loop heat pipe operating inside a thermal vacuum chamber, providing novel insights into its behavior under vacuum conditions with a focus on fixed versus variable conductance modes of operation. Results demonstrate that, unlike in ambient conditions where variable conductance mode is typically observed, the miniature loop heat pipe operates exclusively in the fixed conductance mode under vacuum. This is due to the absence of convective heat exchange between the liquid line and the surroundings, which is critical for autonomous temperature regulation. The experiments systematically evaluate the effects of sink temperature (−10 °C to 35 °C) and heat load variations (up to 120 W) on thermal performance, revealing that thermal resistance decreases with lower sink temperatures, ranging from 0.10 to 0.20 K/W. Additionally, the evaporator heat transfer coefficient stabilizes at higher heat loads, peaking at 4 kW/(m2·K) for a 0 °C sink. These findings advance the understanding of miniature loop heat pipe behavior in vacuum environments, supporting the development of reliable thermal control systems for space applications. Numerical parametric analyses conducted with the model indicate that enlarging the vapor line diameter and increasing wick porosity enhance thermal performance by decreasing pressure drops.