Enhancements of nucleate boiling and critical heat flux under microgravityconditions

Two means are presented for enhancing nucleate boiling and critical heat fl ux under microgravity conditions: using microconfigured metal-graphite comp osites as the boiling surface and using dilute aqueous solutions of long-ch ain alcohols as the working fluid. In the former, thermocapillary force ind uced by the temperature difference between the graphite-fiber tips and the metal matrix plays an important role in bubble detachment. Thus, boiling he at transfer performance does not deteriorate in a reduced-gravity environme nt. In the latter case, the surface-tension-temperature gradient of the lon g-chain alcohol solutions turns positive as the temperature exceeds a certa in value. Consequently, the Marangoni effect does not impede, but rather ai ds in bubble departure from the heating surface. This feature is most favor able in microgravity conditions. As a result, the bubble size of departure is substantially reduced at higher frequencies, Based on the existing exper imental data, and a two tier theoretical model, correlation formulas are de rived for nucleate boiling on the copper-graphite and aluminum-graphite com posite surfaces, in both the isolated and coalesced bubble regimes, In addi tion, performance equations for nucleate boiling and critical heat flux in dilute aqueous solutions of long chain alcohols are obtained.