AFRL-OSR-VA-TR-2014-0183 (YIP 11) Advanced Nanostructures for Two-Phase Fluid and Thermal Transport

This report summarizes our three-year effort on advanced micro and nanostructures for fundamental studies of fluid manipulation and enhanced two-phase heat transfer. First, we studied the role of micro/nanostructures on pool boiling heat transfer. We fabricated well-defined microstructured surfaces in silicon and performed systematic pool boiling experiments in which we demonstrated that increasing surface roughness increased critical heat flux. We developed a force-balance based model, and elucidated the important role of the surface roughness in increasing the contact line length, which as a result augments the capillary force pinning the contact line of the bubble and so delaying vapor film formation. Next, we translated this understanding to investigate the effect of microstructures on two-phase heat transfer in microchannels. We fabricated and characterized microchannels with micropillars arrays on the heated channel wall. Small fluctuations in the measured heater surface temperature (± 3-8 °C) indicated increased flow stability, and the heat transfer coefficient for the structured surface microchannel was 37% higher compared to the flat surface microchannel. More importantly, the mechanism for the increased flow and thermal stability is that the structures, with an enhanced capillary wicking capability, help maintain a liquid film on the heated surface. Finally, we successfully developed a flexible uniform responsive microstructure (μFUR) array for dynamic manipulation capability that can be used to promote bubble departure real time in boiling systems. We demonstrated uniform, continuous, extreme tilt angles with precise control and an instantaneous response. Furthermore, we showed that μFUR is capable of real-time manipulation of fluid spreading directionality, fluid drag, and can tune optical transmittance over a large range simply by adjusting the applied magnetic field. The collective fundamental insights gained from our work promises the development of advanced thermal management approaches, among others, for various defense systems. Motivation and Program Goals: In this research program, we studied the role of advanced nanostructures to manipulate coupled fluidic and heat transport processes for high performance thermal management devices. Thermal management is a critical bottleneck for the advancement of a variety of important defense, space, and commercial applications. Pumped phase-change based microfluidic systems promise compact solutions with high heat removal capability. However, challenges in implementation lead to poor heat transfer performance. One of the primary limitations is the inability to remove