Electrostatic control and enhancement of film boiling heat transfer
Boiling heat transfer is severely degraded at high surface temperatures due to the formation of a vapor layer at the surface, commonly known as the Leidenfrost effect. Heat transfer is limited to a critical heat flux (CHF); higher heat fluxes lead to surface dryout and temperature excursions. An externally applied electric field in the vapor layer can significantly enhance boiling heat transfer for electrically conducting or polar liquids. In such liquids, the electric field is concentrated in the vapor layer, and promotes liquid-surface contact, which can significantly enhance boiling heat transfer. This dissertation is a fundamental study of the influence of concentrated interfacial electric fields on film boiling heat transfer for liquids with finite electrical conductivity (like water and organic solvents). This dissertation describes experimental, analytical and numerical studies on various aspects of the physics underlying electrostatic suppression of film boiling. This dissertation also quantifies the heat transfer benefits associated with electrostatic suppression of film boiling. This dissertation is divided into five main studies, which analyze different aspects of electrostatic suppression of the Leidenfrost state. The first part of this dissertation (Chapter 2) describes droplet-based experimental investigations on electrostatic suppression of the Leidenfrost state. It is demonstrated that the Leidenfrost state can be suppressed and surface dryout can be prevented using externally applied electric fields (AC or DC). Elimination of the Leidenfrost state increases heat dissipation capacity by more than one order of magnitude. In preliminary experiments, heat removal capacities exceeding 500 W/cm² are measured for water, which is five times the CHF of water on common engineering surfaces. A multiphysics analytical model is developed to predict the vapor layer thickness in the Leidenfrost state. The second part of this dissertation (Chapter 3) analyzes the fundamental mechanisms underlying electrostatic suppression of Leidenfrost state. It is shown that the interplay of destabilizing and stabilizing forces determines the minimum (threshold) voltage required to suppress the Leidenfrost state. Detailed linear instability analysis is conducted to investigate the growth of electrostatically-induced perturbations on the liquid-vapor interface in the Leidenfrost state, and predict the threshold voltage required for suppression. The third part of this dissertation (Chapter 4) focuses on suppression of the Leidenfrost state on soft, deformable surfaces, like liquids. It is seen that the nature of electrostatic suppression on a deformable liquid substrate is drastically different from that on a solid substrate. This is due to the existence of an electric field inside the substrate and the deformability of the substrate. A multiphysics analytical model is developed to predict the vapor layer thickness on deformable liquids. The fourth part of this dissertation (Chapter 5) includes experimental studies on suppression of film boiling during high temperature quenching of metals. It is shown that an electric field can fundamentally change the boiling patterns, wherein the stable vapor layer (film boiling) is replaced by intermittent wetting of the surface. This fundamental switch in the heat transfer mode significantly accelerates cooling during quenching. An order of magnitude increase in the cooling rate is observed, with the heat transfer seen approaching saturation at higher voltages. An analytical model is developed to extract voltage dependent heat transfer rates from the measured cooling curve. The fifth part of this dissertation (Chapter 6) develops the concept of using acoustic signature tracking to study electrostatic suppression of film boiling. It is shown that acoustic signature tracking can be the basis for objective measurements of the threshold voltage and frequency required for suppression. Acoustic signature tracking can also detect various boiling patterns associated with electrostatically-assisted quenching. With appropriate calibration, this technique can be used to estimate surface temperatures, heat flux and onset of dryout associated with electrically enhanced boiling. In summary, this dissertation has led to seminal contributions in the field of boiling heat transfer, and essentially opened up a new area of study in the field. This work has shown that electric fields can make the CHF limit irrelevant, and reshape the boiling curve. The present work lays the foundations for electrically tunable boiling heat transfer with conducting liquids. The impact of the proposed work is evident in the area of quenching, where electrically tunable cooling offers a new tool to control the microstructure and mechanical properties of metals.