Second-law-based experimental investigation of flow regimes in an ethanol electrospray cooling system

Electrospray cooling has emerged as a high-potential thermal management solution for electronic devices with high heat fluxes. While most studies focus on first-law efficiency, the thermodynamic irreversibility and exergy destruction mechanisms of these systems have been generally overlooked. This study investigated the cooling performance and second-law efficiency of an ethanol-based electrospray system under various flow rates (0.1, 0.15, and 0.2 mL min−1) and applied voltages (0, 4, 8, 12, and 16 kV). A 22 G nozzle was used to cool a 4 × 4 cm2 surface. The thermodynamic performance was evaluated by considering electrical work as a high-grade energy source through exergy analysis and entropy production rate calculations. Morphological observations identified four distinct regimes: micro-dripping, cone-jet, stable multi-jet, and unstable multi-jet. The stable multi-jet mode was found to be the optimal operating region, providing a uniform evaporation film and a significant surface temperature drop (up to 50 °C at 0.2 mL min−1). However, despite an increase in cooling efficiency when the voltage was raised beyond the steady-state conditions, a significant decrease in exergy efficiency was observed due to the increased electrical energy loss and entropy production. The results demonstrate that the operating regime associated with maximum cooling performance does not necessarily correspond to the regime of maximum thermodynamic efficiency. Stable multi-jet operation yielded superior cooling performance, whereas the highest exergy efficiency was obtained under micro-dripping conditions.