Abstract: In the present work, an experimental research was carried out on the air-water two phase flow patterns and their transition characteristics in the shell-side of a segmentally baffled horizontal shell -tube heat exchanger model of horizontally-cut baffles. In the inlet crossflow zone, the two -phase flow was in vertical downflow across an in-line (square) horizontal tube bundle with a pitch to diameter ratio. p/D of 1. 28. The flow pattern sketch was obtained by visual observation. The flow pattern transition characteristics and their criteria were presented and dimensionless correlations for determining flow patterns were established.