The Constrained Vapor Bubble (CVB) is a prototype for a wickless heat pipe and was developed into an experiment that was run in the microgravity environment of the International Space Station during 2010. Since the CVB is transparent, we can visualize the flow processes within the device in a way not possible before. Results from the experiment indicate that the CVB operates at higher pressures and temperatures in microgravity, a consequence of radiation being the only mechanism for removing heat from the device. The temperature profile data along the heat pipe and corresponding heat transfer calculations indicate that CVB performance is enhanced in the microgravity environment due to increased capillary flow even though heat transfer to the external environment is diminished by the absence of natural convection. Image data of the liquid profile in the grooves of the heat pipe indicate that the curvature gradient is considerably different from that on Earth and supports the conclusion that capillary flow and internal heat transfer is increased. Operations with the 20 mm version of the device allowed us to view explosive nucleation within the CVB upon device start-up. In this scenario, bubble nucleation occurred spontaneously and periodically at the hot end of the device. The nucleation process sent a shock wave through the pipe that collapsed the original bubble as a new vapor space was generated. The newly formed bubble returned to its original size, shape and location as heat loss from the CVB reestablished the original, pseudo-steady-state temperature and pressure profiles.

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