网格和数值格式对TBPBM-CFD模型模拟鼓泡塔的影响

基于气泡动力学属性的现有认识,把气泡分成大、小气泡,首次建立了完整的双气泡相-群平衡模型(TBPBM)以预测气泡尺寸.通过编写用户自定义程序实现了TBPBM模型、Luo破碎模型以及Prince 聚并模型,并耦合TBPBM与CFD双流体模型对直径440 mm鼓泡塔进行数值模拟,详细考察了网格与数值格式对TBPBM-CFD模型模拟结果的影响.结果表明,网格与数值格式对各物理变量的模拟结果影响非常大,特别是网格和体积分数方程对流项离散格式的影响最为显著.随着计算精度的提高,湍流耗散率和整体气含率分布梯度增大,气泡平均直径减少,大气泡所占气相比率降低,液相速度及气含率径向分布与实验值更趋吻合.

Influence of Grid and Numerical Schemes on Bubble Column Simulation with TBPBM-CFD Model

A two-bubble-groups population balance model (TBPBM) was proposed to predict the bubble size in this paper which treats gas phase (bubbles) to consist of small and large bubble groups.The population balance model was applied and solved respectively for each bubble group to predict the bubble sauter diameters.It was noted that the proposed TBPBM model effectively reflected the features of small and large bubbles in bubble column due to the coalescence and breakup of daughter bubbles.The TBPBM model, Luo's bubble breakup model and Prince's bubble coalescence model were respectively implemented into Fluent 6.3 using User Define Function (UDF).The use of twofluid model coupled with the TBPBM was tested to simulate hydrodynamics in a bubble column with a diameter of 440 mm and the effect of grid and numerical schemes on simulation results with TBPBMCFD model was carefully assessed.The simulation demonstrated that the influence of grid and numerical schemes was significant, especially for grid and discretisation of the convection term in volume fraction transport equation.With refinement of grid and increasing accuracy of numerical scheme, the turbulence dissipation rate and the gradient of the overall gas holdup increased but the averaged sauter diameter and volume fraction of large bubbles decreased.The simulation also indicated that the prediction accuracy for evaluation of axial liquid velocity and gas holdup distribution was improved when refining the grid.Adoption of a coarse grid gave rise to under-prediction of the axial liquid velocity.