Enhancement of Heat Transfer with Pool and Spray Impingement Boiling on Microporous and Nanowire Surface Coatings

The National Renewable Energy Laboratory (NREL) is leading a national effort to develop next-generation cooling technologies for hybrid vehicle electronics, as part of the Advanced Power Electronics and Electrical Machines program area in the U.S. Department of Energy's (DOE's) Vehicle Technologies Program. The overarching goal is to reduce the size, weight, and cost of power electronic modules that convert direct current from the batteries to alternating current for the motor, and vice versa. Aggressive thermal management techniques help in achieving the goals of increased power density and reduced weight and volume, while keeping the chip temperatures within acceptable limits. The viability of aggressive cooling schemes such as spray and jet impingement in conjunction with enhanced surfaces is being explored as part of the program. In this work, we present results from a series of experiments with pool and spray boiling on enhanced surfaces, such as a microporous layer of copper and copper nanowires, using HFE-7100 as the working fluid. Spray impingement on the microporous coated surface showed an enhancement of 100%-300% in the heat transfer coefficient at a given wall superheat with respect to spray impingement on a plain surface under similar operating conditions. The critical heat flux also increased by 7%-20%, depending on the flow rates. Heat transfer coefficients obtained on the nanowire-grown surface are considerably better than those obtained on the plain surface, although the enhancement is lower than those obtained on the microporous surface. The critical heat flux is also considerably lower for the nanowire surface than for the plain surface.