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>Experimental investigation and two-fluid model large eddy simulations of the hydrodynamics of re-circulating turbulent flow in rectangular bubble columns.
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Experimental investigation and two-fluid model large eddy simulations of the hydrodynamics of re-circulating turbulent flow in rectangular bubble columns.
Experimental and numerical studies have been carried out to study the hydrodynamics of rectangular bubble columns. Wire-mesh tomography was used for the measurement of void fraction distribution and bubble size distributions in plumes with different aspect ratios. Digital image processing algorithms have been developed to track the bubbles in a Lagrangian sense and estimate bubble sizes and bubble velocities. The image processing techniques were successfully applied to estimate flow parameters in the non-oscillating region of the long bubble plume.; Large Eddy Simulations (LES) of short and long bubble plumes have been performed using a commercial CFD code FLUENT. The Smagorinsky sub-grid scale stress model was programmed into FLUENT using user defined functions. Large eddy simulations were performed on a 5 mm grid (filter width of 5 mm) and with a 0.5 millisecond time step. LES was found to provide a better spatial resolution of the flow field. The large eddy simulation results were found to be in excellent qualitative and quantitative agreement with the experimental data. Previous LES studies in bubble columns were limited to superficial gas velocities of less than 2 mm/s. The current study extends this range to 10 mm/s. Previous LES studies used lift coefficient values of 0.5 which is twice the experimentally measured values lying in the range 0.25--0.28. This study used an optimal value of 0.25 for the lift coefficient and a parametric study on the effect of the lift coefficient for a flow with a superficial gas velocity of 10 mm/s has been reported. The feasibility of two fluid model LES is established through the work performed as part of this dissertation. Transient computations were also performed with the mixing length and the k-epsilon turbulence models. The cross-section of the bubble column has a large aspect ratio (width to depth ratio of 5) and the length scale (column depth of 2 cm) governing turbulence and therefore the large scale structure was resolved equally well by the three turbulence models.
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机译：已经进行了实验和数值研究来研究矩形气泡塔的流体动力学。丝网层析成像用于测量具有不同长宽比的羽流中的空隙率分布和气泡尺寸分布。已经开发了数字图像处理算法以在拉格朗日意义上跟踪气泡并估计气泡大小和气泡速度。图像处理技术已成功应用于估计长气泡羽流的非振荡区域中的流动参数。使用商用CFD代码FLUENT对长短气泡羽流进行了大涡模拟（LES）。使用用户定义的函数将Smagorinsky子网格比例应力模型编程为FLUENT。在5毫米网格（过滤器宽度为5毫米）上以0.5毫秒的时间步长进行了大型涡流模拟。发现LES可以提供更好的流场空间分辨率。发现大的涡流模拟结果与实验数据在定性和定量方面具有极好的一致性。以前在鼓泡塔中进行的LES研究仅限于表观气体速度小于2 mm / s。当前的研究将该范围扩展到10 mm / s。 LES以前的研究使用的升力系数值为0.5，这是实验测量值在0.25--0.28范围内的两倍。这项研究对升力系数使用了0.25的最佳值，并且已经报道了对表观气体速度为10 mm / s的流的升力系数影响的参数研究。通过本文的工作，确定了两种流体模型LES的可行性。还使用混合长度和k-ε湍流模型进行了瞬态计算。气泡塔的横截面具有较大的长径比（宽高比为5）和控制湍流的长度标度（列深度为2 cm），因此，通过三种湍流模型可以很好地解析大型结构。
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