In recent optical flow experiments on a transparent volute-type radial centrifugal pump, an accumulation of air bubbles to adherent gas pockets within the impeller blade channels was observed. A transition of unsteady bubbly flow towards an attached gas pocket at the blade suction side was found for increasing air loading of the liquid water phase. This steadily attached pocket shows a distinctive unsteady wake. A reproduction of the transition from bubbly to pocket flow in a three-dimensional flow simulation demands the treatment of dispersed bubbly flow, on the one hand, and of coherent air regions, on the other hand. Therefore, a hybrid flow solver is adopted based on an Euler-Euler two-fluid method for dispersed flows and features Volume-of-Fluid properties when air accumulations form. A scale-adaptive turbulence model is utilized to account for highly unsteady flow regions. For the time being, a monodisperse bubble size distribution is assumed for the dispersed part of the flow. For an operation range close to the design point and rising air loading, the flow transition from bubbly to pocket flow is well captured by the hybrid simulation method. Even an alternating pocket flow in between bubbly and pocket flow regime is predicted. The simulation method is still limited by an appropriate choice of a monodisperse bubble diameter. Therefore, the disperse model part of the hybrid flow solver will be coupled with population balance and bubble interaction models in future studies.
In this work, steady-state droplet size distributions in a DN300 stirred batch vessel with a Rushton turbine impeller are investigated using an insertion probe based on the telecentric transmitted light principle. High-resolution droplet size distributions are extracted from the images using a convolutional neural network for image-analysis in order to investigate the influence of impeller speed and phase fraction (up to 50 vol.-%). In addition, Sauter mean diameters were calculated and correlated with two semi-empirical approaches, while the standard approach only accomplished 5.7% accuracy, and the correlation of Laso et al. provided a relative mean error of 4.0%. In addition, the correlated exponent in the Weber number was fitted to the experimental data of this work yielding a slightly different value than the theoretical (−0.6), which allows a better representation of the low coalescence tendency of the system, which is usually neglected in standard procedures.
The fluid dynamic (flow rates) and hydrodynamic behavior (local droplet size distributions and local holdup) of a continuous DN300 pump-mixer were investigated using water as the continuous phase and paraffin oil as the dispersed phase. The influence of the impeller speed (N = 375 to 425 rpm), the feed phase ratio (φF = 10 to 30 vol.-%), and the flow rate (V˙tot ≈ 0.5 to 2.3 L/min) were investigated by measuring the pumping height, local holdup of the disperse phase, and the droplet size distribution (DSD). The latter one was measured at three different vessel positions using an image-based telecentric shadowgraphic technique. The droplet diameters were extracted from the acquired images using a neural network. The Sauter mean diameters were calculated from the DSD and correlated with an extended model based on Doulah (1975), considering the impeller speed, the feed phase ratio, and additionally the flow rate. The new correlation can describe an extensive database containing 155 experiments of the fluid and hydrodynamic within a 15% error range.
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