Experimental investigation on flow regimes and transitions of steam-water two-phase flow in narrow rectangular horizontal channels

Experimental investigations on atmospheric steam-water two-phase flow regimes and transitions in two transparent narrow rectangular channels with cross-section areas of 1 mm × 20 mm and 2 mm × 20 mm were carried out under conditions of 0.007 ≤ J_f ≤ 0.27 m s⁻¹, 0.05 ≤ J_g ≤ 65.2 m s⁻¹, 0 < q ≤ 18 kW m⁻². Four flow patterns were observed in narrow rectangular horizontal channels under heated condition, i.e., bubbly flow, slug flow, churn flow and annular flow. New dryout phenomenon of annular flow was observed under high wall heat flux conditions. The liquid phase occupies half of the middle of narrow channel above the bottom rectangular plate. Below the top plate, liquid phase is concentrated to a slender streamline in the middle of upper half narrow channel by high speed steam flow. For the flow regime transitions, the critical values of void fraction for transitions from bubbly flow to slug flow, slug flow to churn flow, and churn flow to annular flow in narrow rectangular horizontal channels under heated condition are 0.7, 0.8 and 0.9, respectively. The heat flux decays flow regime transitions because the generated bubbles from heated wall surfaces promote the separation of bubbles in the upstream and downstream. Flow regime transitions under larger surface tension condition need greater vapor phase velocities to disturb the mainflow. In a general, new flow regime transition criteria were developed and can present excellent predictions for the present data of flow regimes and transitions in narrow rectangular horizontal channels under heated condition.