Passive pumping, evaporation based system for multiscale thermal management




Crawford, Robert Vincent

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Drawing from the lessons of plant transpiration, this dissertation explores a biomimiced system for fluid transport and thermal regulation. This system utilizes evaporation and benefits from the associated passive pumping with an application of a rooftop solar radiation barrier in mind. By directing the incoming energy towards the phase change of water, lower surface temperatures can be maintained thus reducing heat transfer into the structure by conduction. In order to design and construct such a bio-inspired system, several parameters, i.e., the evaporation surface, the delivery path and the working fluid, must be understood as to how they affect and limit operations. Performance factors such as evaporation rate and suction pressure were monitored for the various design constraints of feeding tube length and diameter, membrane area, and working fluid. Additionally, as a heat flux was imposed on the membrane from above and below, the substrate temperature became important. Over the range of parameters tested, hydrodynamic resistances of the delivery path were shown to affect pumping height but not the evaporation rate. Instead, the evaporation rate was controlled by the substrate temperature. Furthermore, the normalized evaporation rate was found to be inversely related to the evaporation surface area. Under contaminated working fluid conditions, particles deposited in the membrane caused decreases in evaporation rates. When applied to a simulated roof situation, the evaporation system was successful at maintaining considerably lower surface temperatures than other conventional and unconventional roof albedos, which, in turn, would reduce heat flux into the interior by conduction. Lastly, in estimating the water consumption, on a typical August day in Austin, TX, the system could use up to 2 gallons/m² while providing enhanced cooling. When the system's resources were compared to being purposed in other ways, they were arguably better utilized in providing evaporative cooling.



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