A Design-Oriented Experimental Method for Evaporative Cooling under Automotive Boundary Conditions

Minimizing cooling air mass in passenger cars substantially improves aerodynamic efficiency. In low-temperature circuits, the small temperature difference between ambient air and component limits increases cooling air demand. Peak cooling is often achieved with a closed refrigeration cycle, at the expense of power, complexity, and cost. Open-loop evaporative water injection can boost cooling capacity during short-term peaks, but automotive-relevant prediction and design methods are missing. Here, water is misted upstream of a low-temperature radiator using various hollow-cone spray nozzles. An holistic, experimentally validated test-bench methodology quantifies three superimposed mechanisms: air precooling by upstream evaporation, evaporation on the radiator surface, and sensible heat uptake by residual liquid water. Nozzles are first characterized in free-stream using a laser diffractometer to measure Mean Sauter Droplet (SMD) diameter and water mass flow rate. The influence on heat rejection performance is then measured at component level using a realistic low-temperature automotive cooling circuit operated as a calorimeter. Evaporative operation increases radiator heat transfer performance by up to 25-55%, depending on droplet size and injected water mass flow and environmental conditions. A trade-off is observed between enhanced heat transfer and reduced effective air mass flow due to partial blockage of the heat exchanger by liquid water. Finer atomization achieves comparable enhancement at significantly lower water consumption, making droplet size a key design parameter within practical water limits. The results confirm open-loop evaporative cooling as a viable peak-load augmentation concept that can enable downsizing of conventional cooling hardware and reduced cooling air demand, supporting improved aerodynamic efficiency and lower energy use. The methodology provides a physically consistent basis for future model development, demand-oriented operating strategies, and validation under driving-speed airflow with optimized low-consumption nozzle concepts.