Enhancements of Nucleate Boiling and Critical Heat Flux Under Microgravity Conditions

Abstract

Two means are presented for enhancing nucleate boiling and critical heat flux under microgravity conditions: using microconfigured metal-graphite composites as the boiling surface and using dilute aqueous solutions of longchain alcohols as the working fluid. In the former, thermocapillary force induced by the temperature difference between the graphite-fiber tips and the metal matrix plays an important role in bubble detachment. Thus, boiling heat transfer performance does not deteriorate in a reduced-gravity environment. In the latter case, the surface-tension-temperature gradient of the long-chain alcohol solutions turns positive as the temperature exceeds a certain value. Consequently, the Marangoni effect does not impede, but rather aids in bubble departure from the heating surface. This feature is most favorable in microgravity conditions. As a result, the bubble size of departure is substantially reduced at higher frequencies. Based on the existing experimental data, and a two-tier theoretical model, correlation formulas are derived for nucleate boiling on the copper-graphite and aluminum-graphite composite surfaces, in both the isolated and coalesced bubble regimes. In addition, performance equations for nucleate boiling and critical heat flux in dilute aqueous solutions of long-chain alcohols are obtained.