Air-Water Flow in a Vertical Pipe with Sudden Changes of the Superficial Water Velocity

A constant air flow rate was injected into an upwards water flow, while the superficial water velocity was suddenly changed by activating a butterfly valve, the flap of which was perforated. In this way, both a jump-like decrease and increase of the liquid flow rate were studied. Measurements were taken by two wire-mesh sensors mounted at a short distance behind each other. They were placed 3030 mm downstream of the gas injection. Time sequences of the average gas fraction, radial gas fraction and gas velocity profiles as well as bubble size distributions were derived from the wire-mesh sensor data. The velocities were obtained by cross-correlating the signals of the two successive sensors.
Closely after a sudden increase of the liquid superficial velocity, the content of the pipe segment between gas injection and sensor contains still bubbles that were generated before the perturbation. In the first approximation, the cross-section averaged gas fraction should therefore remain the same as before for the duration of the passage of this portion of two-phase mixture. However, the wall peak in the radial gas fraction profile became more pronounced and shifted towards the wall. This was predicted with a CDF code (CFX-4) as well. However, the experimental observations are indicating a significant decrease of the cross-section averaged gas fraction, which can only be explained by an increase of the drift velocity. The latter was supported by gas velocity profiles obtained by cross-correlation of the signals of the two available sensors. We conclude that there is an effect of growing bubble rise velocity with increasing liquid superficial velocity. The nature of this effect is still unclear. In addition, the described effect was observed with inversed tendencies in case of a sudden reduction of the liquid flow rate.