For high heat flux applications, air cooling has to be replaced by liquid cooling, which has higher heat flux removal capability than air cooling for the following three major reasons.
1) Liquids have higher thermal conductivity than air. Therefore, for the same value of Nusselt number, the liquid flow has higher heat transfer coefficient.
2) Liquids have higher volumetric heat capacities than air. Therefore, for the same volume flow rate, the liquid flow has lower advection heat transfer resistance .
3) If boiling takes place, the two-phase flow has even higher heat transfer coefficient.
Spray Cooling:
One cooling solution for high-heat-flux thermal control is spray cooling, in which coolant is forced through spray nozzle to break up into a field of small droplets. The droplets impinge on the hot surface to remove heat through liquid heating and liquid-vapor phase change. Spray cooling features high heat flux removal capability, uniform cooling surface temperature, and low fluid inventory.
Heat transfer of a stream of droplets impacting flowing liquid film:
Jet Impingement Cooling:
