Experimental investigation of the flow in a bubble column

Computational Fluid Dynamics (CFD) models for two-phase flows always require closure relations representing the nature of the forces acting at the interface between the participating phases. This is vital for the modelling of momentum transfer term. In case of a gas-liquid flow, it is expressed in terms of forces acting on bubbles. The present study uses instantaneous high-speed video observations of the bubble motion to validate models for the interfacial forces.
The turbulent diffusion coefficient of the gaseous phase is obtained by a statistical evaluation of the movement of bubbles in a swarm, which is affected by the turbulence of the liquid phase. After the bubble trajectories were identified, the lateral displacement of bubbles was statistically analysed by constructing probability density functions. The dispersion coefficients of the standard distributions were found to grow proportionally to the square-root of the time. This supports the assumption of a diffusion model for the lateral displacement. Deviations from the linear dependency were identified to be the result of deterministic oscillatory motions of the rising bubbles.
Another objective is objective is to demonstrate the capabilities of the high-speed stereo-imaging technique to study the single (isolated) bubble dynamics in a vertical bubble column. The reconstruction algorithm of the 3D bubble trajectories from the stereo images of the bubble projection is presented. The bubble size, shape, orientation as well as bubble aspect ratio are investigated.
The first results of experiments on bubble motion in a vary narrow flat bubble column (width-100 mm, depth-8 mm, height – 600 mm) obtained for the validation of numerical predictions achieved using DNS-based analyses (Wörner et al. FZK) are presented. Results of high-speed video measurements of bubble parameters (bubble shape, size, rising velocity etc.) as well as PIV measurements of velocity of the liquid phase are discussed.