Heat transfer characteristics of high flow rate electrospray and droplet cooling

Abstract

Electrospray cooling is experimentally examined for high volumetric flow rates (80 mL/h) stabilized by a novel hemispherical nozzle. Stability of the Taylor cone-jet mode of this nozzle is increased by interaction of the discharging liquid with the outer wall of a hemispherical cap, installed at the tip of a simple nozzle. The liquid can then be discharged at high flow rates at the same mode. The liquid is ethanol and the test surface temperature as well as the cooling heat flux are measured. The results of the surface temperature, the heat flux and the heat transfer coefficient are compared between the three cases. The first and main case is the electrospray using the novel hemispherical nozzle. The second case is the electrospray using a corresponding simple conventional nozzle. The third case is again the simple nozzle but without any applied voltage which results in a train of droplet impact cooling. All cases are studied and compared at the same flow rates, and the distance from the nozzle to the surface. Depending on the flow rate and the applied voltage, the critical heat flux for the case of electrospray cooling with hemispherical nozzle varies between 5.66 and 15.13 W/cm2. At the same flow rate and distance to the test surface, the cooling is improved compared to the electrospray cooling with a simple nozzle (maximum heat flux between 1.75 and 7.84 W/cm2) and drop impact cooling (maximum heat flux between 3.80 and 11.54 W/cm2). Finally, a parametric study indicates an improvement in maximum heat flux (and maximum heat transfer coefficient) by increasing the liquid flow rate and applied voltage, and by reducing the distance from the nozzle to the test surface.