Spray cooling on uniform and gradient radial groove surfaces: Bubble dynamics and phase-change heat transfer

Spray cooling is an efficient thermal management approach for high heat flux applications, and its performance is related to surface structure. However, the influence of gradient grooves remains insufficiently understood. In this study, spray cooling experiments were conducted on four radial groove configurations with maximum depths of 0.2–0.8 mm. Under fixed spray conditions (flow rate 6.0 L h −1 , inlet pressure 0.4 MPa, nozzle height 6 mm), the effects of groove geometry on heat transfer performance were investigated. The results showed that heat transfer enhancement varied with groove depth and heat transfer regime. In the single-phase regime, retained nonboiling gas bubbles hindered local liquid film renewal. Width-gradient and depth–width dual-gradient grooves improved liquid spreading and bubble removal, while the latter also facilitated drainage at groove depths of 0.6–0.8 mm. These effects were associated with favorable heat transfer performance in the single-phase regime, particularly for width-gradient grooves at shallow depths of 0.2–0.4 mm. Under high heat flux conditions, deep uniform grooves tended to show higher heat transfer performance, likely because their larger effective heat transfer area in the spray-core region favored phase-change heat transfer. Depth-gradient grooves showed limited enhancement, which was likely associated with the reduced effective heat transfer area caused by shallow central grooves. Among all tested surfaces, U4 achieved the highest heat flux of 433.02 W cm −2 at 105.43 °C, representing a 54.7% enhancement compared with the flat surface. These findings may provide insights into the design of enhanced surfaces for spray cooling.