The effect of plate thickness, surface tension and fluid flow on detachment of drops from a plate

The pinch-off of drops from the downstream end of plates is an important re-entrainment mechanism of condensate in compact condensers. The present experimental study complements a previous one [A.S. Lexmond, C.W.M. van der Geld, The effect of fluid flow on detachment of drops from the downstream end of a flat plate, Exp. Therm. Fluid Sci. 29 (3) (2005) 363-370], in which ethanol and water drops were created for various mass densities and gas velocities at the downstream end of a plate that was fully wetted. In the present study, the effects of plate thickness and surface tension are studied with ethane drops detaching in ethane vapor flow at near-critical conditions. If the thickness of the plate, D, is introduced as a governing length scale, the Bond number ΔρgD2/σ is found to indicate whether or not the plate thickness affects drop detachment. A theoretical prediction for the drop size after pinch-off in otherwise quiescent vapor is compared with experiments, which results in a correlation valid for high Bond numbers that are based on the drop diameter. The predictive method presented in the previous study took account of the effect of flow on detaching drop size. It has now been extended and generalized. The combined effect of plate thickness and high vapor velocity is found to lead to a bifurcation in the main drop size after detachment. This phenomenon is due to the facts that the position of the drop foot underneath the plate is unstable, and that detachment near the rim of a plate is essentially different from that from the center of the plate.