Bullet shaped bubbles

Street JR, Tek MR (1965) Dynamics of bullet shaped bubbles encountered in vertical gas liquid slug flow. AIChE J ll(4) 644-650... 

At increased gas throughput, bubbles interact and coalesce to give rise to large bullet shaped plugs occupying most of the pipe cross-section, except for a thin liquid film at the wall of the pipe which is thicker towards the bottom of the pipe. 

In contrast, additional complications arise when dealing with bubbles and drops as both their shape and terminal velocity are influenced by the bounding walls. The literature is less extensive on this aspect of bubble motion even in Newtonian liquids. Irrespective of the type of liquid, in narrow tubes, bubbles tend to assume an elongated bullet shape with a hemispherical fore end and nearly flat at the rear end. The limited experimental results of Coutanceau and Hajjam (1982) confirm this type of behavior in tubes such that rf/D> 0.5. 

Slug flow The bubble dimensions are close to tube pipe diameter and the bubbles have a characteristic shape similar to a bullet with a hemispherical nose with a blunt tail end. They are commonly referred to as Taylor bubbles after the instability of that name. Taylor bubbles are separated from one another by slugs of liquid, which may include small bubbles. 

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