# Method for Measuring Cooling Efficiency of Water Droplets Imping onto Hot Metal Discs
## 6 Discussion
The Student's \(t\)-test (alpha < 0.05) was applied to indicate whether there were statistically significant differences between the water droplet cooling efficiency obtained for Ra 0.4 and Ra 3. The observed water droplet cooling efficiency differences for Ra 0.4 and Ra 3 were for the aluminum discs significant (alpha > 0.05) only for temperatures between 220-310 \({}^{\circ}\)C, where the higher surface roughness (Ra 3) gave a slightly higher cooling efficiency. This may be explained by the better possibilities for the water vapor generated to escape through the crevices below the droplet, i.e., resulting in less distance between the droplet and the aluminum surface peaks. Due to the larger standard deviation in cooling efficiency observed for the stainless steel discs, any observed differences between the surface roughnesses of Ra 0.4 and Ra 3 were not statistically significant, i.e., Student's \(t\)-test alpha > 0.05. Indeed, for temperatures above 265 \({}^{\circ}\)C, the cooling efficiency was practically equal for stainless steel surface roughnesses of Ra 0.4 and Ra 3. It is more difficult to explain this observation. However, many parameters do control the heat and mass transfer in the film boiling regime [7]. For stainless steel, the influence of crevices and peaks may be less prominent due to the heat transfer capacity being 10 times lower than that of similar peaks on the aluminum surface. The results indicate that the application of fire water early in a fire scenario is very important, i.e., while the water cooling will likely be most efficient and thereby may prevent further temperature increase past the temperature associated with a boiling crisis. Upon reaching the film boiling regime, the cooling efficiency of the stainless steel discs was shown to become as low as about 12% to 14% due to the stable vapor film below the droplet. Even with application of the recommended 10 L/min-m\({}^{2}\), i.e., 0.167 kg/s\(\cdot\)m\({}^{2}\) recommended by NORSOK [19], the spray would only be able to withdraw 50-60 kW/m\({}^{2}\) from the fire exposed surface. This is significantly lower than the heat flux that objects may be exposed to in industrial fires, i.e., 250 kW/m\({}^{2}\) to 350 kW/m\({}^{2}\). This is a paradox for fire safety engineering. The literature study did not reveal research papers investigating the absolute cooling efficiency of water droplets applied to hot metal surfaces of e.g., 10-mm wall thickness. Therefore, the present work may be the first to attempt quantifying this for objects of representative wall thicknesses. However, it should be noted that a variety of metal qualities are used in process plants, and that the present study is limited to horizontally aligned stainless steel discs of 10-mm thickness. In the process industry, equipment and pipe work may also be thermally insulated and shielded by stainless steel cladding. Such cladding, with thicknesses of 0.5 mm or 0.7 mm, is unfortunately too thin to be analyzed by the presented setup. Omitting the fine mesh screen and moving the core setup to the classroom for demonstrating water droplet behavior and cooling efficiency worked well for demonstrating the mentioned physical phenomena to BSc and MSc students in fire safety engineering. The students expressed an increased understanding of water droplets' cooling efficiency as a function of substrate temperature. The potential of very low water cooling efficiency in fire scenarios was noted. Therefore, it can be concluded that this simple setup serves both scientific and educational purposes. [PERSON] conceived the concept. [PERSON] and [PERSON] designed the experimental setup. [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON]-[PERSON] contributed materials and equipment and [PERSON] contributed as an advisor throughout; [PERSON], [PERSON] and [PERSON] wrote the paper. [PERSON] was supported by the Norwegian Research Council, Grant No. 257901 and Gassco Inc., Norway, Grant No. PO 4500024195.