# Method for Measuring Cooling Efficiency of Water Droplets Imping onto Hot Metal Discs
## 6 Discussion  
A method for measuring the cooling efficiency of water droplets impinging onto hot metal surfaces was developed. The test apparatus was described, measurements were performed on stainless steel and aluminum discs, and the recordings were processed to reveal the cooling efficiency in the temperature range between 85-400 \({}^{\circ}\)C. It was also investigated whether the method can reveal any differences in the cooling efficiency as a function of the surface roughness. Therefore, the 50-mm diameter and 10-mm thick metal discs tested were prepared with roughnesses of Ra 0.4 and Ra 3 for both materials. The test setup was validated by droplets (with a 2.4-mm diameter) generated by the acceleration of gravity from a hypodermic injection needle impacting onto the disc center at a speed of 2.2 m/s and a rate of 0.02 g/s, i.e., about three droplets per second. The horizontally aligned disc was suspended by four thermocouples penetrating the disc 15-mm radially, along the horizontal symmetry plan. This solution minimized contact between the disc and other objects, and allowed for recording the disc temperature during the cooling phase with and without cooling the disc by water droplets. Several researchers placed a thermocouple junction either flush with or 0.5 mm below the droplets' impact point [11; 14]. This allowed them to record the surface temperature at the impact point. The thermocouple then induces a disturbance in the metal exactly at the point where the droplets hit the surface. The present method aimed at recording the temperature within the metal disc, with four symmetrically located thermocouples giving minimal disturbances to the droplet impact point. This gave a better representation of the water-cooling process of metallic objects, as such, during the cooling process. The tradeoff by losing information about the impact point surface temperature was thus outweighed by the possibility for better determining the water droplet cooling efficiency. For the aluminum discs, the water droplet cooling efficiency associated with the critical heat flux, i.e., at about 125 \({}^{\circ}\)C, was in the range of 80% to 85%. The lowest water droplet cooling efficiency, i.e., in the range of 6% to 10%, was observed at a disc temperature of about 230 \({}^{\circ}\)C. Since this temperature was associated with a minimum in the cooling efficiency, it may be defined as the Leidenfrost temperature. The cooling efficiency then slightly increased with increasing temperature up to 400 \({}^{\circ}\)C due to the increasing temperature difference between the disc surface and the evaporating levitated droplet. For stainless steel discs, at the recorded temperatures associated with the critical heat flux, i.e., about 150 \({}^{\circ}\)C to 180 \({}^{\circ}\)C, the droplet cooling efficiency was found to be in the range of 70% to 80%. The reason for this slightly lower maximum cooling efficiency compared to aluminum is likely due to the differences in thermal properties, i.e., a thermal conductivity of 150 W/mK for aluminum versus 15 W/mK for stainless steel. For aluminum, the surface temperatures recovered more between each droplet impact compared to stainless steel, i.e., the impingement point was more thermally disturbed by the time the next droplet hit the stainless steel surface. Small variations in droplet impact point and droplet movements just after impact likely explained the higher standard deviations in the recorded efficiency for the stainless steel discs. Large standard deviations in measurements from the temperature regions near the temperature associated with the critical heat flux are also reported in other studies [16], although those researchers did not focus on droplet cooling efficiency. Clear differences in cooling efficiency were demonstrated between aluminum and stainless steel (same roughness for both metal disc surfaces). It was more challenging to reveal the differences in cooling efficiency within the same material, i.e., for the two different roughnesses studied (Ra 0.4 and Ra 3).