Staying Cool: Research Shows How Spray Cooling Works

FAYETTEVILLE, Ark. — Heat is a critical obstacle to developing smaller electronic devices. Engineering researchers at the University of Arkansas have developed computer models that explain the complex process of spray cooling, an increasingly popular method used to remove heat from microsystems in computers and other electronic devices. The research could lead to further development of even smaller microscopic electronic systems, circuits and chips.

Panneer Selvam, professor of civil engineering and director of the university’s Computational Mechanics Lab, created computational models of the impact of spray cooling on heated surfaces and found that the complex interaction of conduction and convection are the major phenomena of spray cooling.

Conduction is the transfer of heat through matter by communication of energy from particle to particle, and convection is the circulatory motion that occurs in a fluid due to variation of the fluid’s density and the action of gravity. Selvam also discovered that density of individual droplets affects the droplet’s ability to cool a heated surface.

“Spray cooling has been used in the computer and electronics industries for several years,” Selvam said. “But an overall theoretical understanding of the process is limited because of the complex interaction or tension between liquid, vapor, gravity and droplet impact. We have contributed to a better understanding of this process.”

Supported by the Office of Naval Research, the U.S. Air Force and the National Aeronautics and Space Administration, the research will help engineers design high-powered electronic devices in radar used by the military. NASA could use Selvam’s research to design efficient, system-level hardware for lasers and communication equipment in space shuttles and vehicles that perform research missions to Mars and Venus.

During operation, machines produce heat that must be dissipated into the external environment to prevent malfunction or destruction. A general rule is that densely packaged machinery -- chips and integrated circuits in computers and hundreds of other electronics devices with little or no open space in the surrounding environment -- will produce greater heat than machinery in a larger space. As electronic devices decrease in size, engineers have struggled to develop cooling systems that are effective yet small enough to fit inside the devices. 

Heat produced by electronic circuits is usually measured in density, or watts per unit volume or area. A personal computer, for example, produces less than 1 watt per square centimeter. Traditionally, heat sinks, and fans have been used to transfer heat away from electronic chips and circuits in computers and other electronic devices. But a personal computer is a relatively spacious machine, generally not tightly packaged.

For microsystems that generate hundreds or thousands of watts per square centimeter, engineers have increasingly relied on spray cooling, a high-flux, heat-removal technique that involves applying water to individual chips. Liquid droplets exploit the processes of conduction, convection and evaporation to improve cooling. Heat from a chip produces a thin, liquid film and vapor bubbles on the surface of the chip. Spray droplets, which collectively make a fine mist, interact with the liquid film and vapor bubbles on a heated surface and cause high-heat transfer away from the microsystem.

 Although spray cooling has been used for several years, scientists and engineers have only a superficial understanding of the interaction between a liquid droplet and the liquid film and vapor on a hot surface, Selvam said. Several years ago, he decided to pursue a better theoretical understanding of this dynamic by analyzing the composition and behavior of a single droplet.

Researchers in Selvam’s lab created a computer model of a droplet’s flow and interaction with the liquid film or vapor bubble. The model allowed them to study fluid properties, such as density, viscosity and thermal conductivity. They worked with complex numerical equations to understand the droplet’s effect on gravity, surface tension and phase change. 

Selvam discovered that a droplet’s density affects its ability to cool the surface. At low-density ratios, the droplet failed to burst vapor bubbles within the liquid film. At high-density ratios, droplets burst vapor bubbles upon impact and thus allowed the process of conduction and convection to begin. In other words, energy created by the interaction of the droplet with the thin liquid layer and vapor bubble caused heat to convey or transfer away from the surface.

Findings thus far have come from two-dimensional models. Selvam said future research will focus on three-dimensional models of surface tension between liquid, vapor, gravity and viscosity.