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Abstract This short article describes the major findings from the CVB experiment performed in the LMM on the International Space Station from 2010–2012. CVB was the first experiment to run in the new facility and focused on understanding the heat transfer and fluid mechanics occurring inside a wickless miniature heat pipe. The LMM was used to map the location of the vapor-liquid interface inside the device and to measure the film thickness profile on the walls of the device. Several interesting and unexpected phenomena were observed in microgravity including flooding of the heater end with liquid as the heat input increased, explosive nucleation of vapor bubbles at the heater end in the shortest version of the heat pipe tested, condensation on highly superheated surfaces, and the spontaneous formation of rip currents as the device tried to enhance the contact line area available for evaporation of the liquid. 

Abstract Bubble nucleation was investigated in a 20-mm-long, wickless heat pipe on the International Space Station. Over 20 h of running experiments using pentane as the working fluid, more than 100 nucleation events were observed. Bubble nucleation at the heater end temporarily boosted peak pressures and vapor temperatures in the device. At the moment of nucleation, the heater wall temperature significantly decreased due to increased evaporation and the original vapor bubble collapsed due to increased pressure. A thermal model was developed and using the measured temperatures and pressures, heat transfer coefficients near the heater end of the system were extracted. Peak heat transfer coefficients during the nucleation event were over a factor of three higher than at steady-state. The heat transfer coefficient data were all collapsed in the form of a single, linear correlation relating the Nusselt number to the Ohnesorge number. 

Although engineers can control the internal geometry of materials down to the micro-scale, it is unclear what configuration is ideal for a given transport process. We explore the use of mazes as abstract representations of two-phase systems. Mazes can be easily generated using many different algorithms and then represented as graphs for analysis. The three, dimensionless graph parameters of effective tortuous resistance, average tortuosity, and minimum-cut-size were derived and then correlated to the maze’s effective transport property (e.g., permeability), average residence time, and robustness, respectively. It was shown that by tuning the settings of the maze algorithm, one can obtain desired maze performance. Finally, a composite maze was constructed and shown to mimic the geometry and permeability of a real commercial membrane. In principle, a surrogate maze geometry can be optimized/tuned for a given transport process and then used to guide the rational design of the engineered system it represents.