Analysis and modeling of nucleate boiling in highly wetting liquids.

摘要

Boiling has proven to be an efficient heat transfer mode for removing very high heat fluxes and accommodating large increases in local heat flux with only modest increases in surface temperature. In recent years, the incessant drive towards higher heat flux and power density in nearly all categories of electronic equipment has turned attention to the use of dielectric liquids, such as the Fluoro-Fluorocarbons (Fluorinerts) and the Chloro-Fluorocarbons (Freons) for the thermal control of electronic components. Since these liquids, referred to as highly-wetting liquids, possess some unusual characteristic properties as compared with conventional liquids, it is expected that the boiling process and dominant heat transfer mechanisms of highly-wetting liquids may be different from those of conventional liquids.; Nucleate boiling incipience is one of the most important parameters in boiling heat transfer. In the present study, the effect of contact angle and contact angle hysteresis on boiling incipience is investigated; a set of models for predicting the critical bubble radius and the incipience superheat, as well as the superheat excursion, are presented. Analyses of air/gas trapping in microcavities and of the bubble growth process have been used to relate boiling incipience superheat to the maximum value of the inverse bubble radius. For highly-wetting fluids the critical bubble radius has been shown, in all cases, to lie below the radius of the cavity at the surface.; Since both Fluorinerts and Freons possess unusually high gas solubility, the effect of dissolved gas on boiling incipience has been investigated. The calculated results show that for a closed system, both the wall temperature and saturation temperature decrease with an increase in the dissolved gas content in a coolant.; A theoretical analysis and photographic study of bubble flow regimes in nucleate boiling were performed. The results suggest that thermal transport models such as Rohesnow's, based on isolated bubble behavior, may be applicable to nearly the entire range of nucleate boiling of electronic cooling fluids.; The orientation effect on nucleate boiling has been addressed for inclined surfaces facing downward. Mathematical models for latent heat transfer in the bubbles and sensible heat transfer associated with removal of the superheat layer by these bubbles, are developed under low-heat-flux conditions to explain why the boiling curves shift with the surface orientation angle.