Self-sustaining oscillations in a heat pipe using a binary liquid mixture

Heat pipes are used in satellites, space vehicles and laptop computers for thermal management. Few have been fully commercialized using liquid mixtures because their reliability and performance are difficult to predict. A transparent, wickless, heat pipe containing a 50:50 mixture of pentane and 2-methylpentane was run on the international space station (ISS) to investigate how the liquid–vapor interface behaves when a perfectly miscible fluid mixture is used. As the heater temperature exceeded a threshold of 45–50 °C and the temperature difference between the heater and the condenser exceeded 50 °C, spontaneous, single-frequency oscillations in temperature, pressure, and film thickness appeared along the axis connecting the heater and the condenser. The oscillations remained steady for at least 7–8 h. Oscillations were not observed in ground-based experiments, nor when a 94:6 liquid mixture was previously operated on the ISS. Oscillations arise due to evaporative distillation at the heater end and an entropy-driven recombination of the liquids at the condenser end that also leads to a solutal Marangoni flow. The oscillation period was slow, ∼16–25 s, exhibited a distinct turn-on temperature at the heater end, and the oscillation frequency could be adjusted by changing the heater temperature or the temperature difference between the heater and condenser. The mixture outperformed a pure fluid, leading to a 47% overall improvement in heat rejection and double the heat removal rate at the heater wall.