Effects of surface conditions on spray cooling characteristics

The influence of surface conditions such as scale thickness and surface roughness on water spray cooling and air jet cooling characteristics was investigated experimentally. SUS304 stainless steel with the thickness of 20 mm was used as the cooled sample. An artificial scale layer was formed on the sample surface by thermal-spraying using Al₂O₃ powder. The thickness of the Al₂O₃ layer was varied from 50 μm to 210 μm. A sample without an artificial scale layer was also studied; in this case, the surface was roughened by shot blasting up to 20 μmRa. As a result, the artificial scale layer showed a thermal resistance function in both water spray cooling and air jet cooling. In water spray cooling, the characteristics of which depend on surface temperature, the cooling rate during film boiling and the apparent quenching temperature at the interface increased with Al₂O₃ scale thickness. Surface roughness enhanced the cooling rate during film boiling and resulted in a higher quenching temperature in spray cooling. In air jet cooling, heat flux increases with surface roughness, but this tendency can be seen only with larger flow rates. Surface roughness has a much stronger influence on heat flux in water spray cooling, even though the average heat flux is not as large. In this research, the heat flux during impingement of water droplets was estimated to be much higher than that in air jet cooling. This is thought to explain the difference in the influence of surface roughness on cooling characteristics with the two cooling methods.