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B ubble size distribution is among the important factors controlling interfacial mass transfer rate in gas liquid stirred tank reactors. This distribution is determined by a balance of coalescence and breakage rates. Despite a large number of articles reporting bubble coalescence, most of the previous investigations, such as those by Ueyema et al. (1993), Kim and Lee (1987), and Oolman and Blanch (1986), were done for a pair of bubbles under non-turbulent flow conditions.Bubble coalescence in a stirred tank reactor (STR) has been shown photographically for the first time by Rennie and Valentin (1968). Tse et al. (1998) have tried to visualise bubble coalescence in a coalescence cell, a STR and a bubble column. Unfortunately, the use of low record rate (50 frames per second, fps) in visualising bubble coalescence in the STR made it impossible to observe the coalescence occurring in time scale shorter than the record rate.Several researchers (Takahashi and Nienow, 1993;Bakker, 1992;Barigou and Greaves, 1992) show that Sauter mean diameter, d 32 , becomes larger as the radial distance from the impeller increases. They suggest that coalescence of bubbles takes place as the bubbles move away from the high shear rate region near the impeller blades toward the tank wall. Takahashi and Nienow (1993) have tried to quantify the coalescence rate solely based on measurement of bubble size during passage of bubbles from the impeller to the tank wall. In their approach, they did not differentiate the break-up from coalescence rate. Therefore their result cannot be used in developing population balance(s) coupled with equations describing hydrodynamics of gas-liquid flows.Near the wall, d 32 generally increases as the axial distance from the impeller plane toward the liquid surface increases indicating the coalescence takes place near the tank wall (Takahashi and Nienow, 1993;Bakker, 1992;Barigou and Greaves, 1992). The increase is more pronounced in the leeward or downstream side of the baffle compared to the windward or upstream side, especially in a short distance above and below the impeller plane (Takahashi and Nienow, 1993;Barigou and Greaves, 1992). Recently, Sudiyo et al. (2002) have observed consecutive frames using a fast CCD camera (up to 2000 fps), to quantify the coalescence rate and have shown that bubble coalescence occurs much more frequent on the leeward side than that in the windward side. This phenomenon is not clearly understood due to the lack of measurement data of liquid velocities and turbulent properties. Bubble coalescence has been studied in a 2.6 L stirred tank. Instantaneous velocity fields were measured using PIV and corresponding turbulent kinetic energy, dissipation rate, various length and timescales were estimated. All of these data, combined with data of local gas hold-up, bubble size and coalescence rate obtained with shadowgraph were used to assess bubble coalescence at different positions. Results show that bubble coalescence takes place mostly near the tank wall, especially on the leeward...

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The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In the present study, the flow around the 25° Ahmed body has been investigated using Large Eddy Simulations (LES) and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used Sub-Grid Scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Excellent agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the Wall-Adopting Local Eddy-Viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modelling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.

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