Investigation of Decisive Mixture Effects in Nucleate Boiling of Binary Mixtures Using a Theoretical Model

Abstract

Abstract In the present paper an attempt is made to clarify the influence of mixture effects upon heat transfer in nucleate boiling of binary mixtures. The studies are based on a theoretical model that is briefly summarized. Evaluating heat and mass transfer around a single vapor bubble emphasizes a strong influence of the so-called micro region where the liquid-vapor phase interface approaches the wall. Due to the preferential evaporation of one component of the mixture, strong concentration gradients occur in the micro region. These microscale composition effects cause diffusive mass transfer, Marangoni convection, and a variation of the liquid-vapor phase equilibrium as well as a variation of the thermophysical properties. From a macroscopic point of view the bubble site density and the departure diameter vary with the composition of the liquid. By means of parameter studies decisive mixture effects are identified and their relevance in the nucleate boiling process is stated. The heat transfer coefficient crucially depends on the bubble site density and departure diameter. For increasing bubble site density, the influence of microscopic concentration gradients increases. But only the variation of liquid-vapor phase equilibrium becomes important, while diffusive mass transfer and Marangoni convection can be neglected.