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New data on flow pattern transition for gas liquid flow in small diameter tubes (4 to 12 mm) is presented. The experimental results are compared with previously published models for horizontal and vertical flows considered to be valid for medium and large diameter pipes. The effect of surface tension which might be expected to be important in small diameter pipe flow has been found to affect only the stratified‐slug transition in horizontal flow. A modification to the model to include this effect is proposed.

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This paper is dedicated to Professor Gad Hetsroni on the occasion of his 65th birthday. Gad, through his leadership and devotion to science in general and multiphase¯ows in particular, has been the beacon of this ®eld. As the Founder and the Editor of the International Journal of Multiphase Flow, Gad has marched this ®eld forward at a rapid pace and positioned it at the focus of science and engineering. We would like to express our deep gratitude to Gad for all he has done for the ®eld of multiphase¯ow and we wish him many more productive and enjoyable years. AbstractThe motion of a single elongated (Taylor) bubble propagating in a transparent vertical pipe is studied experimentally in stagnant liquid, as well as in upward and downward liquid¯ow. Digital image processing of a sequence of video images serves as the main experimental method for the study of the Taylor bubble motion. In addition, the distribution of the velocities in front of the bubble and in the liquid ®lm is measured using Particle Image Velocimetry. The relation between the Taylor bubble motion and the velocity ®eld in front of it is discussed. #