(397g) A Wickless Heat Pipe Using a Binary Working Fluid: Results from Microgravity

A transparent, wickless heat pipe containing a 50:50 mixture of pentane and isohexane was operated on the International Space Station in September 2023. The purposes of the experiment were twofold; to expand on the original mixture experiments of 2012 that showed a 94:6 mixture of pentane and isohexane acted as a self rewetting working fluid to enhance heat pipe performance and to see how a 50:50 mixture would behave in a similar system. Terrestrial surface tension measurements indicated that this 50:50 composition exhibited the largest deviation from ideality. Placing the mixture in a heat pipe configuration would amplify any deviations that could be measured on Earth. The heat pipe was square in cross section, 3 mm on a side, and the internal vapor bubble occupied 43 mm of the 45 mm total length of the device. A pressure transducer and set of 28 thermocouples allowed us to monitor the temperature profile along the hot/cold axis of the system and the internal pressure of the system. Optical interferometry was used to visualize the liquid film on the walls of the device. Due to the high concentration of components, we had expected to see a decrease in heat transfer performance from the 94:6 system due to vapor diffusion in the evaporator and the condenser sections. We also expected to see more complicated liquid behavior and faster and more chaotic meniscus oscillations in a direction transverse to the hot-cold axis of the system.

What we actually saw in these experiments were slow oscillations along the hot-cold axis of the system similar to those that occur in a pulsating heat pipe. However, in this linear system, we had only a single, large bubble attached to the heater, the liquid fill ratio was about an order of magnitude smaller than used in pulsating heat pipes, and the oscillation frequency was coherent. The oscillations occurred at much lower temperatures than the anomalies we saw in our experiments with a pure working fluid and thermal Marangoni effects were negligible at these heater temperatures. The oscillations started abruptly over a temperature of about 0.25 Ëš C and ended just as abruptly as the temperature passed a threshold point. The closest analogy is that to a cavity oscillator so we dubbed the response Flow Amplification by Sustained Thermal Reinforcement (FASTR). Finally, thermal performance was much better than using a pure fluid or the 94:6 mixture due to the bulk movement of the liquid.