Evaporation and Explosion of Liquid Drops on a Heated Surface

The literature pertinent to various aspects of drop evaporation on a heated surface is reviewed. Both the laser shadowgraphic and direct photographic methods are employed to study thermal stability and flow structures in evaporating drops in all heating regimes. It is revealed that four flow regions exist in stable and unstable type drops at low liquid-film type vaporization regime. As the surface temperature is raised, the flow regions reduce to two. In the nucleate-boiling type vaporization regime, the interracial flow structure changes due to a reduction in the Marangoni number as well as the dielectric constant of the liquid. An evidence of bubble growth in the drops is disclosed. The micro explosion of drops is found to occur in the transitionboiling type heating range. No drop explosion takes place in the spheriodal vaporization regime except when the drop rolls on to a microscratch on the heating surface. It is concluded that the mechanisms for triggering drop explosion include the spontaneous nucleation and growth phenomena and the destabilization of fihn boiling. heated surface in the form of small drops or large portions of liquid. Figure 1 shows a typical curve illustrating the dependence of the total vaporization time of a liquid drop on the temperature of the heating surface T,, (Tamura and Tanasawa, 1959). Its corresponding pool boiling curve is included in Fig. 1. When T,, is below the boiling temperature of the liquid Tb, liquid drops spread into a thin layer and evaporate slowly. This range a b is called tile liquidfilm type vaporization regime in which the heat is transferred from the surPace to the liquid by convection. At tile point b when T., is equal to 7"/,, nucleate boiling is observed in the spreading liquid with a vapor bubble arising in the central part of the drop. With a further increase in T,,, the number of nucleation sites increases and