Experimental investigation on heat transfer performance during electrospray cooling with ethanol–R141b mixture

Electrospray (ES) cooling is promising to achieve energy-efficient liquid atomization and high-heat-flux thermal dissipation at relatively low operating temperatures (<75 °C) when using an ethanol–R141b mixture as a coolant, making it suitable for micro-sized electronics. Nevertheless, there has been little research on ES cooling using the ethanol–R141b mixture. This paper presents an experimental investigation on the cooling performance triggered by ES impingement. Results demonstrate that adding ethanol to R141b improves the electrohydrodynamic (EHD) atomization, namely reducing Sauter mean diameter and increasing spray angle. Meanwhile, it also causes the deposition of large-size isolated liquid films on the surface. As the mass concentration of ethanol increases from 10% to 60%, the cooling performance in terms of the temperature drop (T_s,d) and time-averaged heat transfer coefficient (h_m), first increases and then decreases or almost remains unchanged. This is a complicated consequence of microscopic droplet behaviors, film deposition, and thermal properties, which are dependent on the mass concentration of ethanol. For volumetric flow rates of 100 and 150 mL/h, the ES sprays with mass fractions of 50% and 20% produce preferable cooling capabilities with respective h_m equivalent to 1.6 and 1.9 kW/(m²·K) under the current tests. A Nusselt number correlation with the electric Weber, hydrodynamic Weber, and Prandtl numbers is derived. This correlation covers more than 97.7% of the experimental data within a ± 15% deviation. The derived correlation implies the increase in the flow rate has a superior contribution to the heat transfer in comparison with the applied voltage.