Corrugation wall effect on friction factor and air-water flow threshold velocities in vertical annular channels

This paper presents an experimental investigation of the wall corrugation effect on three threshold velocities for air–water flow. Those threshold velocities refer to different boundaries of the flooding phenomenon as follows: onset flooding (OF), end flooding (EF), and onset deflooding (OD). Four vertical annular channels were tested; three were corrugated with different corrugation pitches and heights, and one was smooth. The corrugation height varies from 0.5 mm to 2.4 mm, and the pitch ranges from 5 mm to 10 mm. The average hydraulic diameter of the annular channels remains constant at 13.9 mm, and the Reynolds number, based on the hydraulic diameter, air properties, and velocity, varies in the range of 5,000 to 30,000. Based on empirical analysis, it is found that an increase in the corrugation height corresponds to an increase in the OF velocity. Furthermore, it is shown that corrugations had no evident impact on the EF and OD threshold velocities compared to those observed in smooth channels. Variations in corrugation dimensions exhibited no significant influence on these velocities. Another investigated parameter is the effect of corrugation placement within the combined channel, where both smooth and corrugated sections coexist. It is observed that a consistent trend and equal values were apparent for each threshold velocity across all three cases examined.