Marangoni-driven oscillations in a binary mixture wickless heat pipe: image-based analysis in microgravity

Abstract

Heat pipes are used in many fields for thermal management but are specifically important at the microscale. As systems become smaller and their heat loads grow it becomes increasingly important to understand how performance can be improved. One such method is the use of mixture systems to increase performance, but the increase in performance and changes in reliability are not well characterized. A wickless heat pipe using a 50:50 mixture of pentane and 2-methylpentane was used on the international space station (ISS) to further characterize the liquid-vapor interface when using a perfectly miscible mixture species. During experimentation it was found that above a temperature difference from heater to condenser of 50 °C, oscillation in pressure, temperature, and film thickness could be seen laterally throughout the fluid. It was theorized that when using the 50:50 mixture there becomes a large enough gradient in composition across the heat pipe that Marangoni flow can occur causing oscillations. This work aimed to analyze recordings of fringe oscillations by calculating fringe velocity as the system oscillates. It was found that both advancing and receding velocity increase with temperature difference but both have observable plateaus at 65 °C and 60 °C, respectively. Receding velocity remains plateaued across the operating range while advancing velocity decreases at 80 °C. When comparing between system temperature difference and temperature differences found locally based on thermocouple measurements, there is an additional clustering of experimental points within three regimes. An additional trend between fringe velocity and condenser temperature was found that as condenser temperature decreases, fringe velocity increases. Further experiments with constant temperature difference and varied condenser temperature are necessary to confirm this finding.