Liquid entrainment, droplet concentration and pressure gradient at the onset of annular flow in a vertical pipe

There is a dearth of data on flow parameters in the transition region between churn and annular flow. To address this deficiency, adiabatic air–water experiments were carried out in a vertical test section (31.8 mm internal diameter, 10.8 m long) in which an isokinetic probe was employed to measure the local mass fluxes of gas and of entrained liquid droplets in the core region; pressure gradient was also measured. The tests covered pressures ranging from 1.7 to 5 bara and liquid superficial velocities ranging from 0.012 to 0.33 m s−1. Average liquid entrained fraction and pressure gradient exhibited minima at gas flow rates that, according to a widely applied flow reversal criterion, correspond to the point of transition to upwards co-current annular flow. Also, the profiles of local droplet concentration characterise churn flow as a region in which the radial gradients of droplet concentration tend to disappear with increasing gas flow rate. As annular flow takes place, the local concentration is virtually constant with respect to radial position and gas flow rate. An empirical correlation is finally proposed for the prediction of liquid entrained fraction at the onset of annular flow.