Investigation of deposition and self-cleaning mechanism during particulate fouling on dimpled surfaces

Structuring of surfaces increases the efficiency of heat exchangers, but influences the deposition of particles suspended in the fluid. It is assumed, that spherical dimples suppress the deposition of particles or even induce a permanent cleaning of the occupied surface. In this study the self-cleaning mechanism of dimpled surfaces in heat exchangers is investigated to describe the influencing factors and mechanisms of particulate fouling. To get a detailed insight into fundamental deposition mechanisms, experiments on different dimpled surfaces have been carried out. According to conditions of industriell problems, e.g. cooling water fouling using river or sea water, concentrations of particles (spherical glas, d(p,50) = 3 mu m) of c = 2 g/l to c = 10 g/l were used. Influences of enforced turbulence due to structuring of surface with dimples or increasing fluid velocity were investigated and visualialized with mu PIV technique. Furthermore, different test durations and number as well as the geometry of the dimples were considered and evaluated with specially developed analytical methods. In extension of preceding studies on the effect of a single dimple the influencing parameters were quantified and the effect of multiple dimples in a row were investigated experimentally. A repeatable fouling pattern was observed for the different structured surfaces. The quantitative results show that the surface coverage is generally decreased downstream of the dimples. Therefore, the results confirm earlier findings suggesting an advantage of dimpled surfaces against other surface structures with respect to thermo-hydraulic efficiency as well as reduced fouling propensit. Thus, this study has shown the occurrence of self-cleaning mechanisms of the surface downstream of the dimple and provides the possibility to estimate values for the reduction of particulate depositions on dimpled surfaces. All results presented were obtained by analyzing the surface around the dimple.