Interactions between Drops and Hot Surfaces

Kurzfassung

Heat transfer and phase change phenomena occurring during drop-surface interactions are of a great importance in a number of technical applications. Surfaces whose temperatures are above and below the saturation and solidification point of the liquid need to be distinguished. In internal combustion engines, in spray cooling processes and in fire extinguishing dropsimpinge on surfaces that are far above the saturation temperature. The impact of drops on surfaces whose temperature is below the solidification point of the liquid is of contrary concern in icing of airfoils and in coating processes by thermal sprays, respectively. In the former case the formation of an ice layer shall be avoided while in the latter case a deposition and solidification of molten metal drops is desired. In all these cases many different flow phenomena such as spreading and splashing that are well-known from impacts without heat exchange, can be found. The characteristics of these flows are enhanced by the addition of new phenomena such as solidification, nucleate boiling or the reflection of drops by a vapor cushion. Radial temperature gradients can cause the onset of surface tension driven flows. In this chapter we will concentrate on the interaction of drops with surfaces whose temperature is so high that evaporation plays a dominant role. Solidification processes occurring during the interaction of drops with cold surfaces is the topic of the preceding chapter ``Heat Transfer and Solidification During the Impact of a Droplet on a Surface'' by Poulikakos et al.. Processes occurring during spray impingement on hot surfaces are determined by a large number of parameters relating to the characteristics of the spray such as size and velocity distribution of drops, to the material properties of the liquid and of the ambient gas, and to the properties of the hot wall. This renders a general understanding of impinging sprays extremely complex. In order to obtain insights into some of the more important mechanisms governing the interaction of sprays with hot surfaces it is very useful to study phenomena occurring during the interaction between hot walls and single drops. This is the subject of the present chapter. We will divide the discussion into two major parts. In a first part we will concentrate on the quasisteady evaporation of drops that are already close to a hot surface. In a second part the dynamic behavior of drops impinging onto hot surfaces is considered. Particular stress is laid on modeling the so-called dynamic Leidenfrost phenomenon, i.e., the reflection of drops off a hot wall due to vapor formation.