基于含晕气泡曳力模型的鼓泡床流化行为分析

Analysis of flow behaviors of bubbling fluidized bed via clouded bubble-based drag model

摘要:

气固鼓泡流态化系统具有非平衡、非线性的多尺度特征。为研究含晕鼓泡床内复杂流动行为,利用含晕气泡多尺度曳力模型对三维鼓泡床反应器开展数值模拟,获得不同位置颗粒速度与体积分数的变化规律。对颗粒体积分数时间波动序列进行小波变换,分析轴向和径向位置颗粒脉动和气泡波动的变化。结果表明:基于含晕气泡的多尺度曳力模型能够较好地预测床内流动状态,床内整体呈现颗粒在中心上升而沿壁面下落的环-核流动形式;与颗粒速度波动相比,颗粒体积分数的波动频率更高且更加难以分辨;小波分析获得的颗粒相和气泡相的波动强度沿床高增加,但随着靠近壁面逐渐减弱;A类颗粒鼓泡床内气泡更强烈的破碎与聚并行为,使径向气泡分布较为均匀,能量分率径向差异较小;入口流速的增大增强了颗粒径向分布的非均匀性,且增大了颗粒相和气泡相波动信号的能量分率。

Abstract:

Gas-solid bubbling fluidization system exhibits non-equilibrium, non-linear, and multiscale characteristics. In order to study the complex flow behavior inside the fluidized bed with clouded bubbles, this paper conducts numerical simulations on a three-dimensional bubbling fluidized bed using the clouded bubble-based multi-scale drag model. The variations of particle velocity and concentration at different positions are obtained. The time fluctuation sequences of particle concentration are processed by wavelet transformations. The particle fluctuation and bubble oscillation at the axial and radial positions are compared. The results indicate that the clouded bubble-based multiscale drag model can provide good predictions of flow behaviors in the bed. A core-annulus flow pattern where the particles rise in the center and fall along the walls is shown in the bed. Compared to the fluctuation of particle velocity, the fluctuation of particle volume fraction appears a higher frequency, which becomes more difficult to discern. The wavelet analysis reveals that the fluctuation intensities of both particle phase and bubble phase increase with bed height but gradually weaken towards the wall. The stronger fragmentation and aggregation behaviors of bubbles in a bubbling fluidized bed with Geldart A particles results in more uniform distribution of bubbles in the radial direction and smaller radial difference of energy fraction. Furthermore, increasing the inlet gas velocity enhances the non-uniformity of particle radial distribution and the energy fraction of particle and bubble fluctuation signals.