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

Abstract

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.

Flow boiling heat transfer of HFE-7000 in nanowire-coated microchannels

“…This non-monotonic behavior of wetting and wicking in turn leads to significant decrease of CHF. This trend was also observed by previous studies using nanowire-coated boiling surfaces on highly-wetting dielectric fluids [20,21]. It may be attributed to coupled interactions between capillary flow and liquid entrainment, which lead to a flow regime transition from the annular flow into a wispy-annular flow or a churn flow.…”

Bubble dynamics and nucleate pool boiling heat transfer on microporous copper surfaces

“…Kim et al [10] studied the mechanism of pool boiling with FC-72 on cylindrical platinum wire with a diameter of 390 lm using high-speed photography. The 17-lm-thick coating was made from dielectric DOM (8)(9)(10)(11)(12) lm synthetic diamond particles/OMEGABOND 101 epoxy binder/methyl-ethyl-ketone carrier). They found that the diameter at departure of bubbles on the microporous surface was lower than the plain surface.…”

“…At high heat fluxes, convection heat transfer became larger than latent heat, possibly due to the decrease in the superheated liquid layer at the surface, which inhibits latent heat transfer. Thiagarajan et al [11,12] studied pool boiling and spray impingement boiling on a vertically oriented 10-mm  10-mm copper surface coated with a 100-lm-thick microporous layer (similar to the one used in the current work). They found that the pool boiling heat transfer coefficient was enhanced by 500% on the microporous surface compared to the plain surface [10].…”

“…They found that the pool boiling heat transfer coefficient was enhanced by 500% on the microporous surface compared to the plain surface [10]. With spray impingement, the heat transfer coefficient was enhanced by 300-600% and the CHF was enhanced by up to 80% over the plain surface [11].…”

Microdroplet evaporation on superheated surfaces

“…This is especially true at micro-and nano-scale dimensions, where interactions at liquid interfaces can have a dramatic influence on the macroscopic properties of the system, dictating physical properties like the fluid viscosity, thermal conductivity, and surface tension. Over the past several years, several groups have shown that micro-and nanostructures can enhance the overall heat transfer coefficient in pool boiling [3][4][5], flow boiling [6,7], spray cooling [8][9][10], heat pipes, and post-CHF film boiling [11]. Nanowire-like structures have also been predicted to enhance the long-range van der Waals interactions at solid interfaces [12,13], which, in turn, dictate the formation of specific liquid-wetting [14] and gas-spreading [15] states at a liquid-solid interface.…”