(676f) Development of a CFD Methodology to Simulate Vortex around Partially Immersed Rotating Shaft: Application to Evaluate Designs of Vortex Breaker

Partially immersed rotating shafts are used in many industrial applications in chemical and nuclear industries. Such rotating shafts may generate deep vortex which may eventually lead to gas entrainment. Gas entrainment needs to be avoided in many applications. Various types of vortex breakers are used in such applications to reduce or eliminate vortex around rotating shafts. In this work, we have developed a new computational methodology to simulate vortex around rotating shafts. The methodology is applied for a specific case and was used to evaluate different designs of vortex breaker. Partially immersed shaft generates inertia dominant flow in the vessel and due to high rotational pump speeds (~300-700 rpm), the flow is turbulent in nature. Centrifugal force due to shaft rotation causes steep radial pressure gradients in the fluid which lead to formation of a vortex around rotating shaft. The shaft rotation may also lead to Taylor vortices near the wall of the shaft. At vortex tip, meridian flow is in vertical downward direction. Pressure gradients are steeper in this region than rest of the vessel. The probability of gas entrainment enhances due to formation of low pressure region. Entrained gas in the form of bubbles gets carried into the vessel. Possibility of gas entrainment can be reduced by damping the swirling flow in the vessel. The design of such dampeners or vortex breakers is not straightforward and may produce adverse effect due to sharp swirl flow gradients. Computational Fluid Dynamics (CFD) can provide detailed flow information which would be useful in several aspects such as, characterization of vortex shape, evaluation of gas entrainment criterion and designing anti-vortex breakers. Conventionally Volume of Fluid (VOF) approach is used for simulating free surface flows like vortex (Ranade, 2002). However, VOF approach requires very fine mesh resolution and small time steps demanding significant resources in terms of CPU time and memory. In this work we present an alternative method to simulate vortex shape based on single phase simulation. The proposed method uses pressure field predicted with the single phase flow model to estimate shape of free surface (which is considered flat initially). The estimated free shape is then used to carry out next iteration of single phase flow simulation. The method converges to the final free surface shape within 5 iterations. The performance of this method is evaluated by comparing the predicted results with those obtained by the VOF approach for simple systems. The developed methodology is then applied to a case of a vortex around a large partially immersed shaft of pump system typically used in nuclear industry. The developed CFD methodology is used to investigate the influence of various parameters on interface depth and shape of vortex in pump vessel. Without considering any internals, maximum vortex depth showed an increasing trend with the shaft rotational speed, the rate of which gradually tapers off (see for example, Figure 1). Efficiency of various internals to reduce the swirling flow and therefore vortex depth are investigated computationally. Sample of results is shown in Figure 2. The results are also used to assess the possibility of gas entrainment and investigate various ways to mitigate such entrainment possibility. Ranade, V.V. (2002), Computational Flow Modeling for Chemical Reactor Engineering, Academic Press, London.