The present results show experiments in which drag-reducing polymers (DRP) were added to air-water flow in horizontal pipes. It has been noted that the pressure drop reduction occurs almost in all flow pattern configurations, but the more interesting aspect is the findings that the addition of drag reducing polymer alters the flow pattern transition. The studies were carried out for air-water flowing in a horizontal transparent pipe with diameters of 25.4 and 95.3 mm. The various flow pattern configurations were observed at different gas and liquid velocities. However, by adding polymers a higher superficial liquid velocity, USL, needed to convert a stratified flow to pseudo-slug or pseudo-slug to slug flow pattern and also a higher superficial gas velocity, USG, is needed to convert stratified to annular. Most importantly it has been noticed that the maximum drag reductions occur when the annular flow pattern changes to stratified or a slug to pseudo-slug flow pattern. The finding that the slug flow regime change to wavy stratified pattern or the annular regime to stratified pattern at large drag reduction could be very important in some applications.

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