Numerical study on the effect of gravity angle on airborne spray cooling with different coolants

In spray cooling systems designed for airborne equipment or confined packaging conditions, the angle between the spray direction and gravity, known as the gravity angle, significantly affects cooling performance. This study employed the Euler–Lagrange method to build a numerical model for spray cooling with four commonly used working fluids: R134a, HFE-7100, ethyl alcohol, and water. The study investigated the flow and heat transfer characteristics of spray cooling at different gravity angles. The results show that changes in the gravity angle alter the forces acting on the droplets during movement, promoting the aggregation of small droplets into larger ones, which leads to differences in droplet size distribution. This also causes varying degrees of viscous dissipation, resulting in a decrease in average droplet velocity. A nonzero gravity angle introduces a gravity component along the surface direction, causing asymmetry in the velocity and thickness of the liquid film. The region opposite this component exhibits a low-speed, thick liquid film, while the other side shows a high-speed, thin liquid film. These changes in liquid film flow due to the gravity angle affect heat transfer performance. The effect of the gravity angle varies for different working fluids. When the gravity angle increases from 0° to 180°, for low-viscosity and low-surface-tension fluids, such as R134a and HFE-7100, the maximum surface temperature difference exhibits an M-shaped variation, and the mean temperature remains constant. For high-viscosity and high-surface-tension fluids, such as ethyl alcohol and water, both the maximum temperature difference and mean temperature show a U-shaped variation, with optimal heat transfer performance at a gravity angle of 90°. These differences are attributed to the physical properties of the working fluids, which result in variations in liquid film distribution and energy conversion during the spray-cooling process.