CFD simulation and PIV validation of the gas/liquid behavior in an UASB reactor

ABSTRACT As the world population increases, the need to develop more efficient wastewater treatment systems requires the use of new technologies. Software aided project and optimization of bioreactors and bioprocesses have become a matter of interest in recent years, especially due to the advance in the state-of-the-art of computational resources. This work aimed to perform gas/liquid numerical simulations using the Fluent 16.2 software and to validate this model through Particle Image Velocimetry (PIV) and shadow imaging techniques. Eulerian-Eulerian, laminar, tridimensional and transient simulations were carried out. The results for the mass imbalance for the gas and liquid phases, gas volumetric fraction, gas velocity, bubble size, liquid magnitude and upflow velocity and the velocity profiles for the liquid phase were successfully validated against experimental data. Concerning the dispersed phase, it was found a difference of 4.37% for the gas volumetric fraction between experiments and simulations. Simulated results showed a difference for the bubble mean velocity of 1.73% when compared with shadow imaging results. No coalescence was observed along the experiments, and the flow regime was characterized as dispersed bubble flow. Regarding the liquid phase, it was found a difference of 3.2% for the mean velocity, between simulated and PIV results. Simulated and experimental velocity profiles showed a better agreement at the center of the reactor. Some differences were observed in those profiles, due to geometry simplifications assumed in order to get a better mesh. Considering the good agreement between simulation and experiments, the model was considered validated.

摘要 伴随全球人口增长，开发更高能效污水处理系统的需求愈发迫切，亟需引入新型技术。近年来，借助软件辅助开展生物反应器与生物过程的设计优化已成为研究热点，这一趋势的背后离不开计算资源领域的前沿突破。本研究采用Fluent 16.2软件开展气/液两相数值模拟，并通过粒子图像测速法（Particle Image Velocimetry, PIV）与阴影成像技术对所建模型进行验证。本次模拟采用欧拉-欧拉（Eulerian-Eulerian）两相模型、层流假设、三维瞬态求解框架。针对气液相质量失衡量、气体体积分数、气体流速、气泡尺寸、液相流速幅值与上升流速，以及液相速度分布等结果，均以实验数据为参照完成了有效验证。就分散相而言，实验与模拟所得气体体积分数的相对偏差为4.37%。与阴影成像结果相比，模拟得到的气泡平均流速相对偏差为1.73%。实验过程中未观察到气泡聚并现象，流型被界定为分散气泡流。针对液相而言，模拟与PIV实验所得平均流速的相对偏差为3.2%。模拟与实验得到的速度分布在反应器中心区域吻合度更高；由于为优化网格而进行的几何简化，部分区域的速度分布存在一定差异。鉴于模拟结果与实验数据具备良好的吻合度，本模型被认定为验证合格。