Boiling heat transfer influences the performance of many industrial processes like steam generation, desalination and quenching. Enhanced heat transfer improves the energy efficiency of thermal processes and reduces the operating temperatures. At high temperatures, boiling heat transfer is drastically reduced due to the formation of an insulating vapor layer between the surface and the liquid. This well-known Leidenfrost effect (film boiling) is responsible for adversely affecting the performance of industrial equipment which involves boiling. An electrical voltage applied across this vapor layer can promote liquid-surface wetting, thereby eliminating dryout and enhancing heat transfer. Similarly, an electric field can control bubbles, which also influences boiling heat transfer. The proposed research is a fundamental study on the influence of electric fields in multiple regimes of boiling heat transfer. Electrical suppression of the Leidenfrost state will be analyzed and the resulting heat transfer enhancement will be quantified. Electric-field-based control of physical phenomena associated with bubbles will be analyzed to quantify electric field-enhanced heat transfer in the nucleate boiling regime (which involves bubbles). The proposed research includes experimentation, analytical modeling and numerical simulations. Overall, the proposed research will develop a new area of study in the field of boiling heat transfer. This will also set the stage for the development of novel heat transfer technologies which impact the energy and materials processing sectors. The impact of this work is particularly evident in the area of quenching (ultrafast cooling), where electrically tunable cooling offers a new tool to control the microstructure and mechanical properties of metals.
The proposed research studies the influence of interfacial electric fields on film and nucleate boiling heat transfer, for liquids with very low (but finite) electrical conductivity (like organic solvents and deionized water). In such liquids, the applied voltage is expressed across the electrically insulating vapor gap or bubble. This localized interfacial electric field can influence boiling-related phenomena more strongly than the volumetric electric field in electrically insulating liquids. The proposed research will directly measure the heat transfer coefficients and analyze the fundamental mechanisms underlying heat transfer enhancement. This work involves the development of a high heat flux, high temperature pool boiling test facility to measure the critical heat flux (CHF), which is the maximum heat flux that avoids dryout. The physics underlying Leidenfrost state suppression will be captured by single droplet experiments and analysis. The influence of electric fields on microfluidic phenomena like bubble growth, oscillations, and detachment will be studied by single bubble high speed visualizations and analytical modeling. Experiments will be conducted with deionized water and isopropanol/methanol as working fluids. Specific outcomes of this work include measurements of electrically enhanced heat transfer coefficients and CHF?s, and an in depth understanding of the micro/mesoscale thermal-fluid-electrical phenomena influencing boiling heat transfer. Overall, the proposed research will lead to seminal contributions in the field of boiling heat transfer. This research can potentially reshape the boiling curve by making the CHF limit irrelevant and increasing heat transfer coefficients. This work also lays the foundations for electrically tunable boiling heat transfer.
