Electrowetting-based enhancement of droplet growth dynamics and heat transfer during humid air condensation

Abstract

Condensation heat transfer can be significantly enhanced by condensing vapor as droplets (instead of a film), which rapidly roll-off. This work studies the use of electrowetting to enhance coalescence, growth and roll-off dynamics of condensed droplets, thereby enhancing the condensation rate and associated heat transfer. This enhancement depends on the nature of fluid motion (translation of droplets, oscillations of the three phase line), which in turn depends on the magnitude and frequency of the applied electrical actuation waveform. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation under the influence of an electric field. It is seen that droplet growth is enhanced as the voltage and frequency of AC electric fields is increased, with AC electric fields seen to be more effective than DC electric fields. Roll-off dynamics also depends on the frequency of the AC field. Overall, electric fields alter the droplet size distribution and move the condensate to more favorable states for removal from the surface. The condensation rate depends on the roll-off diameter of the droplet, frequency of roll-off events, and on the interactions of the rolled-off droplets with the remainder of the droplets. An analytical heat transfer model is utilized to relate the measured condensation rate with condensation heat transfer. It is noted that this study deals with condensation of humid air, and not pure steam. Overall, this study reports more than 30% enhancement in condensation rate resulting from the applied electric field, which highlights the attractiveness of electrowetting for condensation heat transfer enhancement.