Modelling of breakup and coalescence in vertical bubbly two-phase flows

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Numerical simulations of gas-liquid two-phase flow with high superficial velocity in a vertical pipe were conducted with the use of the commercial software package STAR-CD 3.27. Detailed description of the Sγ model is presented. The applicability and performance of the Sγ model in Eulerian modelling of gas-liquid bubbly flow were studied. The sensitivity of the Sγ model to the distribution moment γ, and the drainage mode were also investigated. The numerical results were compared with the available experimental data of Hibiki et al., (2001). Good agreement was achieved for the phase axial velocity and radial void fraction for all tested cases. It is found in this work that the second-moment Sγ model in STAR-CD is capable of reasonably predicting bubble size and its distribution even in high void fraction. Except in the near wall region, simulated bubble size and therefore the interfacial area density does not fit well with the experiment measurements. It is observed that the predicted bubble size and interfacial area density obtained from both S0 model and S2 model are more or less the same, which indicates that the numerical results are independent of the distribution moment γ. It is further found that, the drainage mode greatly affects the bubble size: the increase of the mobility of the bubble surface enhances the coalescence in the current model and leads to an over-prediction of the bubble size in the pipe core. The bubble size increases with the increase of the gas phase superficial velocity while the interfacial area density varies less as the interfacial area density is a combined function of the bubble size and local gas hold-up.

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Simon Lo
Dongsheng Zhang