Effects of liquid viscosity on flows inside and outside a bubble diffuser pipe

•Effects of viscosity on distribution of gas flow rate and slugging are investigated.•Increase in viscosity mitigates slugging and makes gas flow rate uniform.•A slugging criterion for inviscid liquids is also applicable to viscous liquids.

Effects of liquid viscosity on flows inside and outside a bubble diffuser pipe were investigated in this study. Glycerol–water solutions and air were used for the liquid and gas phases, respectively. A circular pipe having five aeration holes on the topside and larger opening on the bottom side was placed in a rectangular water tank. The pipe diameter was 20 mm and the hole diameter was 5 mm. The liquid viscosity μL ranged from 1 to 100 mPa s and the total gas flow rate QIN ranged from 3 × 10−4 to 7 × 10−4 m3/s. The flows inside and outside the pipe were observed using a high-speed video camera and the gas flow rates from each aeration hole were measured by capturing bubbles generated from each hole. The liquid entered into the pipe through the aeration hole and the opening, resulting in wavy flow pattern with intermittent slugging. The conclusions obtained are as follows: (1) the increases in QIN and μL decrease the frequency of slugging disturbing aeration, so that the gas flow rate from each hole becomes uniform at high QIN and μL, (2) the main triggers of slugging are interfacial waves at low μL and interface deformation caused by liquid drops falling from the aeration holes into the pipe at high μL, (3) a slugging criterion for inviscid fluids proposed by Mishima and Ishii (1980) is applicable not only to low μL liquids but also to high μL liquids because the wave growth rate is much larger than the viscous damping rate even at high viscosities tested in the present experiments, and (4) the Davidson–Schüler correlation (Davidson and Schüler, 1960) gives reasonable estimations of the bubble diameter not only for low viscosity liquids but also for high viscosity liquids, provided that the influence of slugging is small.