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A slug pattern for gas-liquid flow in horizontal pipelines is observed when slugs of liquid block the whole pipeline and move as a coherent mass downstream at a velocity approximately equal to the gas velocity. At low gas velocities they are easily observed in a transparent pipe. However, at high gas velocities it is difficult to differentiate slugs from large amplitude waves which momentarily close off the pipe cross section. This paper shows how the pressure measurements at two locations, separated from each other in the flow direction, can be used to detect slugs.

New results are presented on interfacial patterns observed for air and water flowing in horizontal 2.54 and 9.53 cm pipelines close to atmospheric conditions. This work differs from previous studies in that measurements of pressure fluctuations at two locations separated in the streamwise direction are used to detect slugs. The liquid flow needed to initiate slugs at low gas velocities is strongly affected by pipe diameter and appears to depend on a linear instability. At high gas velocities the transition is approximately independent of pipe diameter and is explained by a nonlinear mechanism associated with the coalescence of roll waves. The initiation of slugs in the annular flow regime is determined to occur at much lower liquid flows than had been reported by previous investigators. The transition from stratified to annular flow is different in smaller-diameter pipes than in larger pipes because wave wetting plays a more important role.

This paper explores the application of linear stability theory to explain the onset of slugging. It is shown that the inviscid Kelvin-Helmholtz theory correctly predicts stability of a stratified flow only for very large liquid viscosities. In general, however, inviscid theory is in error because it ignores the destabilizing effect of liquid inertia. Good agreement is noted between the linear stability analysis and observations of the initiation of slugs in 2.54- and 9.53-cm horizontal pipes at superficial gas velocities less than 3.3 m/s.