Heat Transfer and Bubble Detachment in Subcooled Pool Boiling From a Microheater Array Under the Effect of a Nonuniform Electric Field

Abstract

The effects of a nonuniform electric field on vapor bubble detachment and heat transfer rate were studied in pool boiling at different subcooled conditions for various wall temperatures. Dielectric fluid (FC-72) was used as the working fluid at 1 atm at earth gravity with two extreme gas concentration levels. An array of 3×3 independently controlled microheaters each 0.7×0.7 mm2 in size were maintained at constant temperature using electronic feedback loops, enabling the heat transfer from each heater to be determined. An electric field was applied between the horizontal upward facing microheater array, which was grounded, and a spherical, off-axis top electrode. Boiling heat transfer results with and without the electric field are presented in this study. Without the electric field, a single large “primary” bubble was observed to form due to the coalescence of the individual “satellite” bubbles which nucleated directly from each single heater array. Before its detachment, a dry spot formed underneath this primary bubble resulted in a reduction in heat transfer. With the electric field applied, three or more small “secondary” bubbles that nucleated and grew more rapidly and detached more frequently were observed. Due to the nonuniformity of the electric field, bubbles moved away from the top electrode (into the weaker region of the electric field) during their development. Higher overall heat transfer rates were measured from the heater array. In addition, the bubble behavior showed agreement with our previous investigation of injecting air bubbles into a stagnant, isothermal liquid through orifices.