An Experimental Study of Fluid Dynamics and Mixing in Bubbly Gas-Liquid Flow Systems

Abstract: In the present project, a new experimental method enabling Planar Laser-Induced Fluorescence (PLIF) measurements in bubbly gas-liquid flows is presented. Due to reflection and refraction of the laser light, the bubbles will cause an uneven and unknown distribution of light intensity in the measurement plane. Introducing a tracer as a “reference” tracer with a known concentration, this can be compensated. Based on this method, the mixing of a “determining” tracer in the liquid phase with two different experimental set-ups, i.e. grid-generated turbulent flow and a double Rushton turbine stirred tank, was studied. The flow structure of the liquid phase and some related properties were also studied using the PIV measurement technique. It was also the aim of this work to experimentally characterize mixing as influenced by interaction phenomena between the liquid and gas phases. The purpose of the studies using the first set-up, grid-generated turbulent flow, was to obtain experimental data on the concentration and turbulent diffusion of the determining tracer plume from an injection nozzle in the self-preserving region. In this region, concentration profiles showed relatively good self-similar behaviour in both single-phase and bubbly two-phase flow. In single-phase flow the concentration profiles have rather similar Gaussian shapes, whereas in bubbly two-phase flow the profiles are not Gaussian. At the centre of the plume, the flow was found to have a periodic coherent structure, probably of vortex shedding character. Observations showed that the period of oscillation is higher in the case of two-phase flow than in single-phase flow. To determine the diffusivity, a method based on the relation between the diffusivity and the variation of the variance of the mean concentration profiles was used. The results show that the bubbles do not affect the diffusivity of the superimposed distribution of the plume in the central region of the test section, while it is increased in the outer region. In the stirred tank reactor, the mixing characteristics were measured at 3 different rotational impeller speeds, 225, 300 and 400 rpm. Knowledge of the flows induced by the impellers is very useful for the optimization of mixing processes with respect to the power consumption, quality of mixing and mixing time. Therefore, the discharge flow rates, and the dimensionless pumping numbers for both single- and bubbly two-phase flow systems were calculated. The mixing time was obtained from the response to a pulse injection of a Rhodamine-590 tracer into the stirred tank.