The influence of a hemispherical porous extender on electrospray stability and cooling performance

Electrospray cooling emerges as a promising two-phase heat removal method. The performance of high flow rate electrospray cooling generated by an impermeable hemispherical extender has already been explored. Here, we examine a new extender with surface porosity and experimentally investigate the performance of electrospray cooling of the two hemispherical extenders—one impermeable and one with the surface porosity, injecting a mixture of water and ethanol. The focus is on the critical parameters influencing heat transfer characteristics, as well as exploring the stability region of the cone-jet mode. Key findings reveal that the porous extender significantly expands the operational flow rate from 100 to 250 ml/h by enhancing surface tension and assisting Maxwell stress balance. The study systematically examines the effects of three primary parameters on heat transfer performance: the flow rate, the applied voltage, and the nozzle to surface distance. The research identifies three distinct cooling mechanisms: thermal conduction (40–60 °C), nucleate boiling [60 °C to critical heat flux (CHF)], and a transitional regime characterized by intermittent droplet-surface interactions. The porous extender increases the flow rate from 100 to 250 ml/h, and about 95% rise in CHF, while inducing a 50% reduction in the cooling time. Variations in the Nusselt number are studied under different operating conditions to observe general trends in heat transfer. The enhancement ratio, defined as the convective heat transfer coefficient to neutral (zero-voltage) conditions, is also used to evaluate the effect of electric field on the performance.