The phenomenon of negative frictional pressure drop in vertical two-phase flow

Highlights: • Negative frictional pressure drop was experimentally observed in vertical two-phase flow. • Energy loss has another part in addition to the one associated with frictional pressure drop. • Energy loss can be still positive in the case of negative frictional pressure drop. • The potential for negative frictional pressure drop is demonstrated in vertical two-phase flow. -- Abstract: The phenomenon of negative frictional pressure drop could occur in vertical two-phase flow in some conditions. At gas–liquid ratios ranging from 100 to 10,000, the two-phase flow in a liquid-loading gas well was experimentally simulated in a vertical pipe. The superficial gas velocities range from 0.1 m/s to 20 m/s and the superficial liquid velocities range from 0.00001 m/s to 0.2 m/s corresponding to the gas–liquid ratios. The negative frictional pressure drop was observed in this experiment. The uncertainty analysis shows that the negative frictional pressure drop could not be undoubtedly attributed to measurement error. Further investigation indicates that the energy loss is associated with a buoyancy-like term in addition to the frictional pressure drop in vertical two-phase flow. By taking this buoyancy-like term into account, the energy losses obtained from experimental data are all positive in the cases of negative frictional pressure drops. The results demonstrate the potential for the negative frictional pressure drop in which case the energy loss is positive and hence the rule of energy conversion is not violated in vertical two-phase flow