Sliding mesh approach

FIGURE 10.3 Approaches to modeling flow in stirred reactors, (a) Black box approach, (b) sliding mesh approach, (c) multiple reference frame or inner-outer approach, (d) snapshot approach. 

Ranade, V.V, Tayaliya, Y. and Choudhury, D. (1997), Modeling of flow in stirred vessels comparison of snapshot, multiple reference frame and sliding mesh approaches. Presented at 16th NAME Meeting, Williamsberg, June 22-21. 

The third standard for modeling rotating impellers is the sliding mesh approach (Rai 1985). Here, a grid is attached to the impeller that does not extend much beyond the outer radius of the impeller. This is the most computationally intensive of the three standard techniques, but also the most accurate. In principle, it fully captures the effect of the agitator on the flow. 

At present, the simulation of flow in stirred tanks requires particular attention to accurate treatment of the impeller. The rotating impeller is difficult to simulate directly in the context of a stationary CFD domain. Even with the introduction of sliding mesh techniques which allow the impeller to rotate in a fixed tank, and thus reproduce the trailing vortices behind the impeller blades [13,23]), only 5 to 10 rotations of the impeller have been reported [13]). Laroche reported that 16 sliding mesh steps, for 90° of tank simulation, took over 10 hours on a Cray [23]. Since the (time varying) bulk flows of interest take of the order of 50 rotations to become established, and the process results of interest may span 10,000 rotations (60 rpm for 3 hours on an industrial scale), this approach is still impractical for the typical user. 

To model the geometry of the impeller exactly, a 3D simulation must be performed. A number of solution approaches are available to incorporate the motion of the impeller, and the computational grid used must be able to adapt to the solver method employed. The models in popular use today are reviewed in the following sections. Particular attention is paid to the sliding mesh model, the most rigorous of them all. The solver methods described are aU designed to capture the motion of a rotating impeller in a stationary tank, but they vary in accuracy. Three of the models are steady-state and one is time-dependent. 

An alternative way to bypass calculation of the startup period is to solve for a steady-state solution first using the MRF model. The MRF model (Section 5-5.2) provides a solution for the moving impeller at a fixed orientation relative to the baffles. Tools are available in commercial codes to use the solution data from the MRF simulation and apply it to the sliding mesh simulation as an initial condition. A moderately coarse time step can be nsed initially (say, corresponding to a 10° rotation, as in the example above) and rednced at a qnicker rate than would otherwise be advisable. This approach can also be nsed if inflow and outflow boundaries are present or if a multiphase calculation is to be performed. In the case of multiphase flows, however, care must be taken to wait until the periodic steady-state condition has been reached before introducing the secondary phase. 

Yeoh, S.L., Papadakis, G., and Yianneskis, M. (2004) Numerical simulation of turbulent flow characteristics in a stirred vessel using the les and rans approaches with the sliding/deforming mesh methodology. Chem. Eng. Res. Des., 82 (A7), 834-848. 

Murthy et al [63] were the first who used the sliding grid method for the simulation of unsteady flows in mixing vessels. The model formulation used with moving meshes is of the arbitrary-Lagrangian-Eulerian (ALE) t rpe [70]. In this particular approach the flow domain is divided into two cylindrical, non-overlapping sub-domains, each gridded as a separate block. The outer... 

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