Flow pattern maps and transition criteria for flow boiling of binary mixtures in a diverging microchannel

This paper presents a visualization study of flow boiling of binary mixtures (methanol-water and ethanol-water mixtures) in a diverging microchannel. The flow pattern and transition criteria are studied in terms of effects of mass flux, heat flux, and molar fraction of the more volatile component (i.e., methanol or ethanol). Four boiling regimes are identified: bubbly-elongated slug flow, annular flow, liquid film breakup, and dryout. Further, generalized flow pattern maps are constructed using coordinates of nondimensional parameter space (boiling number, Weber number, and Marangoni number), wherein relatively distinct boundaries between the flow patterns are identified. Criteria for transitions between flow patterns are proposed in the form of nondimensional groups and are successfully used to predict the experimental results. More than 92% of the data are correctly located within transition boundaries. The criterion for the onset of nucleate boiling - the boundary between single-phase flow and bubbly-elongated slug flow - is also determined for both methanol-water and ethanol-water mixtures on the basis of the same set of nondimensional parameters.