Design and Development of a Next Generation High Capacity, Light Weight, 20 K Pulse Tube Cryocooler for Active Thermal Control on Future Space Exploration Missions

This project is a collaborative effort between groups at University of California Davis and Auburn University. The central hypothesis of the proposed work is that asymmetry in surface microstructures can cause self-generated directional motion of the condensate. If such directional motion of the condensate can be achieved, a condenser with such microstructures can be combined with a similarly designed evaporator, developed under a previous NASA grant, to create a pumpless thermal management loop. The hypothesis will be tested on three specific asymmetric surface morphologies and compared against symmetric surface microstructures. The first two surface morphologies will consist of hydrophilic and hydrophobic micro-structured asymmetric ratchets respectively. The third morphology will consist of a combination of hydrophilic and hydrophobic faces on each ratchet. In accordance with the hypothesis, the overall objective is to characterize the effects of surface microstructures on droplet dynamics and/or film dynamics, and on heat transfer rate, by variation of the microstructure size or surface conditions. Both highly wetting fluids that are typically used in electronics thermal management as well as water are to be studied such that specific surfaces can be engineered for specific fluids. The approach includes modeling coupled with experiments. The experiments will be aimed at providing input data for the model as well as for model validation. Two sets of experiments are proposed- one aimed at revealing the droplet dynamics and another at obtaining condensation heat transfer rates. The proposed concept and work plan are strongly aligned with NASA’s roadmap on heat rejection through phase-change. The NASA Technology Area 14 roadmap and an NRC Committee on Microgravity Research identify a need for fundamental studies in condensation heat transfer in partial and microgravity environments towards design of thermal control systems and further human exploration of space. Design of gravitationally independent phase-change components has been identified by NASA as a critical area of research in order to further its mission. Ongoing and past work by the group has resulted in demonstration of thermally actuated pumping during boiling on mm-sized ratcheted surfaces. If the present project were to be successful, there is potential to create a passive phase-change thermal management system. 
The most significant achievements were (a) to develop and solve a model of film drainage on symmetric and asymmetric ratchets, and (b) to collect data of water droplet formation, mobility and heat transfer coefficients on hydrophobic ratchets. One extended abstract on the experiments performed on brass ratchets with a highly wetting fluid, PF5060 was accepted for presentation at an upcoming conference in March 2016. A draft paper on water droplet mobility on hydrophobic ratchets has been submitted for review to a second conference.