Experimental study on gas–liquid two-phase flow regimes in rectangular channels with mini gaps

Abstract Experimental investigations were conducted for adiabatic air/water two-phase flow in vertical rectangular channels. Three test sections with the narrow gaps of 0.3, 0.6 and 1.0 mm respectively were studied. The flow regimes were observed by using a high-speed video-camera and were identified by examining the video images. The flow regimes in channels with the gaps of 1.0 and 0.6 mm are similar to those found by previous studies, and can be classified into bubbly, slug, churn and annular flow. With decreasing the channel gaps, the transition lines for bubbly-slug, slug-churn and churn-annular flow shift to left in the flow regime maps with the superficial gas velocity as the horizontal coordinate and the superficial liquid velocity as the vertical coordinate. However, the flow regimes in the channel with the gap of 0.3 mm are dramatically different from the previous studies. Bubbly flow was never observed even at very low gas flow velocity. Cap-bubbly flow can exist in the channels stably. Due to the increased influences of the surface tension and friction shear stress in the mini channels, the liquid droplets adhere on the wall surface and are pushed by the flowing gas. The flow regimes can be classified into cap-bubbly, slug-droplet, churn and annular-droplet flow.