An approximate analysis for general film condensation transients

This work presents a simple, powerful technique for analyzing a broad class of film condensation transients. The analysis shows that general film condensation transients are governed by the propagation of a kinematic wave along the film. Scaling arguments establish conditions for the use of quasi-steady profiles in the integral conservation equations. An elementary method permits simple solutions of the governing hyperbolic equation for time-step changes with arbitrary initial conditions. The application of this method yields closed-form solutions for step changes of body force, vapor shear, and wall temperature for a laminar film and for step changes of body force and wall temperature for a film within a porous medium. These approximate results agree excellently with numerical solutions of the complete boundary-layer equations. This technique has applications to a wide class of film condensation transients and to film boiling and convective vaporization transients