Electrowetting Enhancement of Critical Heat Flux

Critical heat flux (CHF) represents the upper limit of nucleate boiling heat transfer, beyond which boiling transitions to the inefficient film boiling regime. Potentially catastrophic burnout conditions may ensue and endanger the safe and reliable operation of the boiling device. Thus, it is highly desirable to augment CHF in order to boost the thermal performance of various energy-intensive applications that rely on boiling to transport a large amount of thermal energy.
In this work, CHF enhancement was explored both theoretically and experimentally by capitalizing on the ability of electrowetting (EW) to modulate the liquid-vapor interfacial stabilities and the liquid-vapor-solid three-phase contact line motion. To do so, a Leidenfrost drop (i.e., a liquid drop hovering over a highly superheated solid surface) was first employed as a model system, due to its simplicity and close connection to film boiling and CHF, to investigate the effect of the electric field on the dynamics of the vapor film that separates the drop from the hot surface. It was found that the electrostatic attraction force alone cannot destabilize the vapor film, instead, it is the accelerated vapor flow that changes the critical wavelength of the Kelvin-Helmholtz instability, thus causing the film to collapse. The results show that, without the need for any complicated surface micro/nanostructures, the Leidenfrost point (LFP) temperature of water can be increased from 200°C to 380°C with a moderate voltage of 56 V a frequency of 50 Hz.
Subsequently, to better understand the impact of EW on the liquid-vapor interfacial behaviors, the dynamics of EW-induced motion of both liquid droplets and vapor bubbles was studied. Computational fluid dynamics models were developed by using the Volume of Fluid (VOF)-Continuous Surface Force (CSF) method to scrutinize the response of a droplet when subject to EW actuating signals. In particular, a dynamic contact angle model based on the molecular kinetic theory was implemented as the boundary condition at the moving contact line, which considers the effects of both the contact line friction and the pinning force. The droplet shape evolution and the interfacial resonance oscillation were investigated in detail. On the bubble aspect, the nucleation, growth and departure of vapor bubbles on a hydrophilic surface, a hydrophobic surface both with and without the influence of EW, were compared, which revealed the significant effect of the EW force on the contact line and, therefore, on the bubble dynamics. Lastly, to demonstrate the EW enhancement of CHF, a synchronized high-speed optical imaging and infrared (IR) thermographic technique was used to characterize boiling heat transfer at the CHF conditions. Simultaneous measurements of the bubble dynamics and the wall temperature and heat flux distributions on the boiling surface were acquired. The results showed that CHF can be enhanced by 133% by the use of EW. Additionally, by considering the force balance at the contact line of a nucleate bubble, a theoretical model was developed to delineate the threshold conditions for CHF to occur, which show very good agreement with the experimental measurements.