Condensation on rotating axisymmetric bodies

Abstract

An analysis is made for tho laminar film condensation on a rotating axisymmetrical general curved body of which the cylinder, the disc and the cones are special examples. The body rotates about a vertical axis and the film is assumed to drain under the influence of gravitational and centrifugal accelerations. The problem is formulated as a solution of the Navier-Stokes and energy equations. A differential equation which governs the growth of the condensate film is obtained and solved to yield one equation which is applicable to all rotating axisymmetrical bodies. The equation involves the geometry of the body and requires a single integration for the complete solution. Theoretical results of condensate layer thickness and heat transfer coefficients are given for a curved body whose generator forms a circular are of 90 for a speed range of 0.1 to 10(^6) rev/min. It is shown that when the curvature is neglected, the analysis over estimate the local film thickness by 71 per cent. Experimental results for the curved body confirm the theoretical findings. However, due to the formation of roll-waves on the condensate layer and hence a departure from the laminar model, the experimental results for the heat transfer coefficient were 1.055 to 1.440 times greater than the theoretical predictions. No detachment of drops from the film of condensate was observed at the experimental speed range of 0.0 to 1000 rev/min. and the condensate drained as a film along the generator. The criterion for the detachment of drops given by Howe is re-appraised and a new criterion which states that "the non-dimensional film thickness η, should be equal to or greater than 1.8 for the onset of the detachment of dropq" is proposed.