| Surface properties play a significant role in droplet-wall-interaction phenomena. To enable detailed experiments considering the effect of surface roughness, two different methods for the manufacturing of wall elements with a graded surface roughness depth and width were developed. A calibrated etching method and a calibrated sandblasting method are employed. Two complete sets of wall elements with roughness depths in the range of 0 μm to 100 μm and different roughness widths in the range of 83.8 μm to 203 μm were manufactured. As primary droplets, a droplet chain with droplet diameters in the range from 40 μm to 200 μm was used. Visualization and particle tracking velocimetry (PTV) are used as diagnostic techniques. Experiments showed that both surface parameters had an influence on secondary droplet diameter, secondary droplet velocity, spray cone angle and morphology of the wall film. Furthermore, the heat transfer from the surface to the wall film is affected by roughness, too. The observed results can be explained by a thin vapour layer underneath the wall film damping primary droplet impact and affecting the heat transfer from the wall to the film. |
