A Particle Image Velocimetry Study of Bubbly Gas-Liquid Flow

Detta är en avhandling från Xiaoyun Tu, Division of Food Engineering, Department of Food Technology, Engineering and Nutrition

Sammanfattning: The aim of this work is to build a framework that can be used for characterizing the bubbly gas liquid turbulent flow in the stirred bioreactor tank. To this end we have developed and modified techniques with which to measure the velocity of both dispersed gas and carrier liquid phases by the optical measurement technique, a 2-CCD camera Particle Image Velocimetry (PIV) system; and to characterize the turbulence by the wavelets tool, Proper Orthogonal Decomposition (POD). Further modification and development of the method have been done in order to measure the gas bubble velocity by applying a numerical filter on the bubble image and by applying the normalized cross-correlation algorithm for velocity calculation were made. The bubble size and its reflection behaviour make it unqualified to be a seeding particle used in the PIV technique. In order to obtain the accurate velocity result at the higher resolution, and hence the slip velocity between the two phases, the post-process of the bubble image is required and the proper algorithm for the velocity calculation needs to be examined. Turbulence introduced through the gas bubbles presence in the grid-generated liquid turbulent flow is investigated in term of excess turbulent kinetic energy of carrier liquid flow. The momentum equilibrium between the two phases is evaluated by the use of the Stokes number and the energy spectra of the turbulent flow. The research work starts from the well-controlled, grid-generated turbulence for investigating the methodology of both experimental and data interpretation stage, and then switching to the real stirred bioreactor tank. We have for the first time introduced POD methodology to analyse the turbulence of both phases for characterizing the mutual effect in between, and attempted to connect the mode index in the POD energy spectra with the real physical turbulent spatial length scale. The experiment shows that the modified energy spectra agree with the Stokes number. The flow regime address in this work is restricted to disperse bubbly gas liquid two-phase flow. The flow regimes addressed in this article are restricted to dispersed bubbly gas liquid two-phase flow in a non-Newtonian surrounding fluid with significant influence of gravity or convection on the magnitude of relative particle velocity, but negligible influence of heat and mass transfer effects.

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