Unsteady film condensation and pumped films on the underside of a flat plate

This work studies the fluid and heat transfer characteristics of a condensing film on the underside of a cold, flat horizontal plate exposed to a vapor with a cyclically varying saturation pressure and temperature. The motivation is to understand the startup and shutdown processes relevant to engineering devices that use condensation, such as two phase thermal control systems used in space applications. The working fluid is n-pentane. An embedded sensor is used to measure the time-varying, spatially averaged, rate of heat transfer due to film condensation. The condensing surface conditions are estimated using a 1-D numerical model. The model solves the unsteady heat conduction equation using an inverse method. The condensing film is imaged using a double-pass shadowgraph system. The data acquisition system is coordinated such that each thermal data point has an accompanying image of the condensing film to allow correlation between the film behavior and heat transfer characteristics. An additional study simulates condensation by mass addition using silicone oil through a porous surface. In this case film growth occurs, but in absence of any thermal and phase change effects. The pendant drop size, the formation wavelengths, the interval between drops and the rate of formation are studied visually. It was found that 1) Heat flux increases most dramatically upon the initial formation of a thin film of condensate. The spatially-averaged heat flux does not appear to be significantly impacted by the appearance of the Rayleigh- Taylor instability or drop formation during a pressure ramp-up; 2) In a cyclic pressure environment, the condensing surface temperature variation increases, for a given range of saturation temperature, with a decrease in block average temperature; 3) Inverse methods can be successfully implemented to determine cyclic surface conditions with data from an embedded probe; 4) A certain amount of hysteresis is exhibited in the case of cyclic pressure variation, in that the heat flux does not increase and decrease linearly with the measured degree of subcooling for the case of condensing and evaporating films. This hysteresis suggests other factors such as film thickness variation due to drop formation, thermal mass and thermal resistance of the system influence the overall heat flux. 5) Pumped-film experiments show that the film thickness at the point of first droplet break-off increases with increased pumping rate. No change in the instability wavelength with pumping rates was observed