ISS: Constrained Vapor Bubbles of Ideal Mixtures

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CBET - 1637816PI: Plawsky, Joel L.Heat pipes are heat transfer devices that are used regularly for cooling a variety of electronic equipment, including laptop computers. Heat pipes use a fluid to transfer heat, but an essential feature of the device is that the fluid undergoes change of phase between liquid and vapor. The detailed motion of the liquid and vapor, and the motion and dynamics of the interface between the two phases, can strongly affect the performance of heat pipes and similar systems. Recent results suggest that the use of a fluid mixture, instead of a pure fluid, can help overcome limitations on performance and unanticipated failures of multiphase heat transfer systems. This project will use the constrained vapor bubble system on board the International Space Station (ISS) to examine various interfacial phenomena for mixtures of organic fluids. Parallel experiments conducted on Earth will help reveal the influence of gravity on interfacial dynamics and heat transfer performance characteristics. Results from the project will provide information that practitioners can use to improve phase-change heat and mass transfer operations, which are becoming increasingly important in fields such as energy conversion, distillation, microelectronics cooling, coating processes, and space exploration. The project will also provide opportunities for students to participate in research through Rensselaer's Undergraduate Research Program. This project will investigate interfacial phenomena and thermal-fluid performance for pentane/isohexane fluid mixtures in a wickless heat pipe. Parallel experiments on Earth and on the ISS will address fundamental issues, including the effects of Marangoni forces on fluid motion and device performance. Condensation and the presence of Leidenfrost droplets at the hot ends of the device are of particular interest. The project will examine mixtures near 50:50 composition for which ground-based experiments have shown deviations from ideal mixture behavior. These deviations will be magnified in microgravity and will alter interfacial characteristics and heat transfer performance of the wickless heat pipe. The experiments will map the vapor-liquid interface using multi-wavelength interferometry. This technique can isolate curvature gradients that drive fluid flow and can determine whether a phase change is occurring locally within the liquid menisci present throughout the device. Particular focus will be given to regions near the heated end of the device where the competition between Marangoni and capillary forces can lead to a new limitation on performance and where disjoining pressure/intermolecular force effects appear to promote unexpected condensation. The interface mapping will be combined with temperature profile measurements to understand, and ultimately predict, the thermal performance of these devices. Using stopped motion and video applications of the interferometry technique, the stability of the liquid menisci and how stability depends upon heat input and liquid composition will be explored. Thermal-fluid models will be developed to help explain the observations both at the meniscus and the device levels. Results of the project will lead to the design of better thermal management devices that rely on phase change heat transfer and will help outline the limits of evaporation and condensation processes, the stability of liquid menisci, and the applicability of the ideal fluid mixture approximation, one of the cornerstones of phase equilibrium thermodynamics.