低雷诺数下气固两相中浓稀相界面对微尺度曳力的影响及建模

Influence of interface between dense and dilute phases on microscopic drag force in gas-solid suspensions at low Reynolds numbers and its modeling

采用具有二阶精度的浸没边界-格子玻尔兹曼法对低雷诺数下流体流经不同颗粒聚团结构的过程进行了解析到颗粒表面的直接数值模拟(PR-DNS)。结果显示在颗粒聚团表面的浓稀相界面处,传统的微观均匀BVK曳力模型[AIChE Journal,2007,53(2):489-501]的预测结果与PR-DNS结果有明显的差别;同时文献中所构建的考虑界面影响的微观曳力模型(Int.J.Multiphase Flow,2020,128:103266)也无法准确预测稀相固含率不为0的情况。因此,本工作提出了一种将界面附近网格分解求取曳力的方法。通过与不同颗粒聚团结构的PR-DNS结果及其他曳力模型预测结果对比发现,新模型不但在稀相固含率趋近于0时与文献中模型具有相近的预测能力,且在稀相固含率不为0时,具有明显优于文献模型的相关系数及拟合优度。

Gas-solid two-phase flow is widely encountered in energy and chemical industries and the interphase drag force is believed to be the dominant factor affecting the flow. Although the homogenous microscopic drag models, e.g. the BVK drag law(AIChE Journal, 2007, 53(2): 489-501), could accurately predict the drag force for homogenous regions, i. e. the dilute or dense phases, they failed to predict the drag at the interface of dense and dilute phases where the surface of particle clusters was located.To study the influence of the interface on the drag force, the particle-resolved direct numerical simulation(PR-DNS) was performed on the process of fluid flowing through different particle clusters at low Reynolds numbers. The results showed that at the interface of dense and dilute phases, the predictions of the BVK drag model were significantly different from the PR-DNS results. Meanwhile Chen et al.’s model(Int. J. Multiphase Flow, 2020, 128: 103266), which considered the influence of the interface, cannot accurately predict the drag force where the solid holdup of the dilute phase was not zero. Therefore, this work proposed a method of decomposing the mesh near the interface to predict the drag force. To validate the proposed model, PR-DNSs of flow past various particle clusters, were performed and the predictions of several drag force models were calculated. It was found that the new model not only had a similar predictability with Chen et al. ’s model when the dilute phase solid holdup approaches 0, but also had better Pearson correlation coefficients and fitness than the Chen et al. ’s model when the solid holdup of the dilute phase was not zero. In conclusion, the proposed drag model could accurately account for the influence of the interface on drag force with different dilute and dense solid volume fractions, and it recovered the BVK law when the local gradient of volume fraction approached zero.