Exploration of Unsteady Spray Cooling for High Power Electronics at Microgravity Using NASA Glenn’s Drop Tower

At present, there is little understanding of the application of spray cooling to electronics in the microgravity environment. Typically in closed cycle terrestrial spray cooling systems, since not all of the liquid impinging on a hot substrate is evaporated, some residual liquid is separated from its vapor component by gravity and returned to the pump. This technique of phase separation is not available to spacecraft designers. Methods to predict spray cooling performance for ground systems do exist, but they are absent for the space environment. Particularly for NASA spacecraft, there is a need to design spacecraft that use high power laser systems and other systems that use evaporative spray cooling in microgravity. Such knowledge is very important for the performance and life of the device. Reliable analytical methods of predicting thermal response of a spray cooled substrate when considering a transient heat load, such as that found during start up and shut down of a space-based laser or other high heat flux electronics, do not exist. Our goal was to use NASA Glenn’s 2.2 second drop tower to investigate unsteady heat transfer at low Bond numbers and residual fluid behavior in spray cooling. The work contrasts other experiments aboard the NASA Glenn KC-135 low gravity aircraft [1]. Our future plans are to continue the experimental work and include the use of the NASA Glenn 5 second drop tower. This paper will report on some preliminary results of an interesting experimental study performed at NASA Glenn in the summer of 2004. The high speed camera and specially-designed “S.L.O.B.” drop rig provided video and data to assess the fluid management problems that arise in a microgravity spray environment, for both heated and unheated cases. The data show unexpected residual fluid management issues, such as the development of multiple spherical liquid globs, with apparent ordered and repeatable geometry, at the point of impact. The results of these experiments provide direction for further investigation in the future.