Study of Micro/Nano Scale EHD-Driven Flow Distribution Control and Heat Transfer Enhancement for Thermal Control Systems

In aerospace applications, optimizing size, weight and power (SWaP) is necessary to keep up with the increasing performance demands of space-bound hardware while minimizing the cost of bringing payloads into orbit or beyond. The more complex, condensed and miniaturized electronics have to be to meet these increasing demands, the more challenging it is to cool them using traditional means. Electrohydrodynamic (EHD) conduction pumping is an innovative refrigerant pumping technology that uses strong electric fields applied on dielectric liquids in order to generate flow motion. EHD conduction pumps have no moving parts, and their designs are flexible and can be now be scaled down to the micro-scale using innovative manufacturing techniques. These pumps can pump thin liquid films and have been shown to enhance heat transfer in two-phase flows. They are reliable over great periods of time without requiring maintenance, consume very little power, and have been tested in microgravity on both variable gravity flights and the ISS with an overall TRL of 7. The purpose of this NSTRF project was to obtain a fundamental understanding of how EHD conduction pumps behaves when scaled down, and how their performance changes when incorporated into a multi-channel flow distribution system for smart, autonomous, small-scale aerospace thermal systems. These thermal systems could then be used in future generations of CubeSats, deep space missions, manned missions and many more. During the course of this project numerical simulations of the charge distributions, pressure generation, and overall flow field inside micro-scale EHD conduction pumps with multiple electrode pairs was developed. The model was used to observe the effects of externally applied flows and pressure drops, as well as the behavior of the pumps in parallel branches with joint inlets and outlets, as would be the case in distributive systems. These simulations showed which orientations enhance EHD conduction pumping pressure generation, explained the non-linear increase in pressure generation with each added electrode pair observed in experiments, and showed how well these pumps can be used to overcome a maldistribution of flow. These simulations were compared with data from a two-branch EHD conduction driven flow distribution control experiment for both single and two-phase flows, with evaporation occurring downstream of the pumping section. In addition, PIV measurements of EHD conduction driven thin film flows were also taken using different pumping orientations and compared with the relevant simulations. Additional fundamental work included an investigation of the mathematical model of the two regimes in which EHD conduction pumps operate in, based on the different behaviors of charge and current with different sets of characteristic parameters, as pertaining to different size scales. The results show that macro-scale EHD conduction pumps generally operate in one of these regimes, while micro-scale ones operate in the other. Since pressure generation is a different function in each of these regimes due to the different charge behaviors, this investigation provided a fundamental explanation for seemingly contradictory reports of the change in EHD conduction pressure generation performance as a function of working fluid temperature. In general, the performance of micro-scale pumps will be enhanced in low temperature environments, while macro-scale pumps will benefit from a high temperature environment. Finally, an innovative evaporator-embedded micro-scale EHD conduction pump was designed, manufactured at the NASA Goddard Space Flight Center’s Advanced Manufacturing Branch, and tested in single and two-phase flows. The results show how the EHD conduction pump performs in different vapor qualities, and how critical heat flux can be enhanced in this small scale. The conclusions of this project will allow for more informed designs of micro-scale EHD conduction pumps for future NASA projects involving this technology, such as the Electrically Driven Thin Film Flow Boiling Experiment set to launch to the ISS in 2021.