Stability and heat transfer characteristics of unsteady condensing and evaporating films

The stability and heat transfer characteristics of an unsteady condensing and evaporating n-pentane film on the underside of a cooled, flat, horizontal plate was studied experimentally. Unsteady conditions were produced by varying the system pressure in a cyclic fashion. The film was imaged using a double-pass shadowgraph system, and an embedded heat flux sensor measured the spatially averaged heat flux. Surface conditions were obtained using an inverse method. Images and data point collection were synchronized to permit direct correlation between thermal data and film behavior. The heat flux was affected by the Rayleigh–Taylor instability after an initial rise due to condensate formation. Hysteresis was observed in the heat flux over each pressure variation cycle, where the heat flux during condensation varied differently with the degree of subcooling than during evaporation. An additional study examined the film stability of non-condensing, growing films with mass addition but without thermal effects. Experiments showed that the film thickness at the point of first droplet break-off increased with increased pumping rate.