AbstractAdiabatic cocurrent flow of air and water through a narrow passage between two flat plates 240 mm long and 99mm wide with gap‐widths of 0.778 mm and 1.465 mm was investigated for six different orientations: Vertically upward and downward, 45°inclined upward and downward, and horizontal flows between horizontal plates and between vertical plates. Except for horizontal flow between vertical plates, the effects of gap width and flow channel orientation on flow pattern, void fraction and friction pressure drop were found to be small in narrow channels. The void fraction and two‐phase friction multiplier data could also be reasonably correlated in terms of the Martinelli parameter. For horizontal flow between vertical plates, both the void fraction and friction multiplier data showed strong mass velocity effects. Several friction pressure drop correlations were tested for applicability to the narrow channels including a separated flow model proposed in this