Local flow patterns distribution during flow boiling in a micro channel array.

The main aim of this study is to experimentally investigate two-phase flow patterns during flow boiling of R245fa in a micro channel array. The array consists of 10 rectangular micro channels connected in parallel, each having a hydraulic diameter of 1 mm. Base area of the channels is 14.5 x 180 mm. Micro channels without and with 0.5 mm wide inlet restrictions are examined to determine their influence on the flow. Four two-phase flow patterns are observed using a high-speed camera: bubbly, slug, churn-annular and annular flows. Two former are grouped together as intermittent flow, and two latter as annular flow. In the paper a special focus is put on the determination of the transition vapor quality corresponding to the change from intermittent to annular flow as well as how it is influenced by heat flux, mass flux and saturation temperature. Tested heat flux ranges from 30 to 50 kW/m 2 , mass flux from 400 to 1000 kg/m 2 s, examined saturation temperatures are 49, 63 and 82 ° C and inlet subcooling varies between 13 and 19.4 K. It was found that transition vapor quality increases at lower mass flux, higher heat flux or saturation temperature. Coupling data on two-phase flow patterns with the data from our previous study on heat transfer (Halon et al., 2022) shows that flow patterns do not determine the dominant heat transfer mechanism. Intermittent flow occurs both in forced convection and nucleate boiling dominated zones, and annular flow, typically associated with the domination of forced convection, accompanies nucleate boiling dominated heat exchange. Micro channels with restrictions exhibit similar behavior, but transition vapor qualities shift to higher values. Comparison with four recognized models for transition vapor quality shows that it is best predicted by the model of Costa-Patry and Thome (2013) with M A P E of 47.9% calculated for both studied geometries. • Two-phase flow patterns during flow boiling in micro channels are investigated. • Two micro channel geometries are tested: without and with inlet restrictions. • The focus is on transition from intermittent to annular flows. • Transition shows dependence on heat flux, mass flux and saturation temperature. • The link between flow patterns and the dominant heat transfer mechanism is analyzed. [ABSTRACT FROM AUTHOR]