Measurements and flow pattern visualizations of two-phase flow boiling in single channel microevaporators

Two-phase flow boiling is being used in different applications because of its high heat flux capacity compared to single phase flow. However, the fundamentals of boiling fluid flow and heat transfer in microscale geometries are not yet fully understood. The aim of this work is to contribute to a better understanding of the underlying physical phenomena in flow boiling of water in small channels. For this purpose experiments were conducted to investigate flow patterns, boiling curves, and heat transfer coefficients in single channel, microevaporators with a channel depth, width and length of 198 × 241 × 21900 μm and 378 × 471 × 21900 μm. High speed visualization (up to 30,000 fps) were performed simultaneously with heat transfer and pressure drop measurements to support the quantitative experimental data for better understanding of two-phase flow characteristics in microchannel evaporators. The influence of the heat flux and mass flux in the flow patterns, boiling curve and heat transfer coefficient were studied. Six different flow patterns were observed and classified using the most commonly accepted terms as bubbly, slug, churn, annular, wavy-annular, and inverted annular flow. The flow patterns were closely coupled with mass flux, heat flux, and channel size. Bubbly flow was mainly developed at lower heat fluxes and progressed to slug, churn, and annular flow as the heat fluxes increased. High speed visualization provided a means to characterize the formation of intermittent flows and the subsequent re-wetting of the channels that lead to pressure drop oscillations, the evolution of flow patterns and provided a physical explanation for the occurrence of reversed flow.