COUNTERCURRENT BUBBLE AND SLUG FLOWS IN A VERTICAL SYSTEM

A model for estimating void fraction in countercurrent bubbly and slug flows in vertical systems, based on the drift-flux approach, is prese nted. In countercurrent flow, as in cocurrent flow, the in situ gas ve locity is aided in the upward direction by the buoyancy of the gas pha se as well as its tendency to flow through the channel center. These t wo factors allow us to express the in situ gas velocity and the void f raction in terms of the bubble rise velocity, the mixture velocity, an d the flow parameter, C0. Single bubble rise velocity data gathered fo r various falling liquid velocities agreed well with the Harmathy corr elation. The parameter C0 in countercurrent bubbly flow was found to b e 2.0, a value much higher than the generally accepted one of 1.2 for cocurrent flow. Significant liquid recirculation at the pipe wall duri ng countercurrent bubbly flow, causing changes in the velocity and bub ble concentration profiles similar to those observed in bubbly flow th rough stagnant liquid columns in large diameter pipes, is postulated t o be the cause for this high value of C0. The Taylor bubble rise veloc ity data gathered at various falling liquid velocities agreed well wit h the Nicklin equation. Visual observation indicated little distortion in the shape of Taylor bubbles due to liquid flow in the opposite dir ection. Void fraction during slug flow is computed using two different approaches. Good agreement is found between theory and experiment.