BUBBLE-SIZE AND ENTRANCE LENGTH EFFECTS ON VOID DEVELOPMENT IN A VERTICAL CHANNEL

The effects of bubble size and entrance length on the void distribution in a vertical upward cocurrent air-water two-phase flow were studied systematically based on the measurements of a dual-sensor resistivity probe. The experiments were carried out under various fixed gas and liquid fluxes, with only the bubble size being changed at the flow entrance. Profiles of void fraction, bubble frequency, bubble velocity and bubble size were measured along the test section diameter at four axial positions with an entrance length (L)-to-internal diameter (D) ratio of 30, 60, 90 and 120, respectively, to study the void development phenomena. The test conditions cover both the wall and core peaking void distributions of two-phase bubbly flow. It is found that the bubble lateral migration and flow regime transition are very sensitive to the variation in bubble size and the bubble coalescence effect during the development of bubbly flow. Hence, existing models, which are to be valid over a wide range of conditions, should include the effect of bubble size.