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This work performs an experimental analysis of the onset of flow reversal in upflow condensation of refrigerant 134a in an inclinable 5-mm diameter tube. As the upward unidirectional annular flow regime is established, solitary waves are generated on the liquid substrate and are initially carried upwards by the vapor. With an increase in the condensate flow rate, the liquid film grows thicker and more disturbed, leading to a wider range of film flow structures as well as oscillatory changes in the liquid velocity. These changes mark the onset of flow reversal in the liquid film and the subsequent transition to churn flow. Image processing techniques have been applied to high-speed video sequences of the two-phase flow to identify the onset of film flow reversal and properties of the interfacial waves. A film structure char- acterized by the existence of the so-called primary and secondary waves was observed to exist near the region of transition to bidirectional flow. Spatio-temporal maps of the liquid film thickness have been generated aiming to quantify the influence of the operating conditions on the wave behavior. Adaptations of existing correlations for the critical vapor velocity associated with the point of flow rever- sal and wave frequency (Strouhal number) have been proposed.
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