Revision as of 12:25, 12 February 2013

Particle-laden swirling flow

Application Challenge AC3-12 © copyright ERCOFTAC 2024

Key Fluid Physics

The introduced swirling flows are highly turbulent and as known, the turbulence structure is strongly anisotropic. Moreover, the flow is characterized by a central recirculation region and a flow separation in the pipe expansion. Mostly such kind of flows is not stationary, but exhibits some fluctuations of the vortex core (precessing). This effect also influences the particle behaviour which is manifested in the formation of particle ropes. These are caused by slight fluctuations of the particle-laden primary jet induced by the vortex precession. Eventually these ropes move spirally along the test section wall downward. As a consequence of the locally high particle concentration two-way coupling effects and also inter-particle collisions might become of importance.

Application Uncertainties

The flow geometry is relatively simple and can be accurately specified and discretised. The inlet conditions were measured 3 mm downstream the exit of the inlet tubes so that the variation of the flow during the first 3 mm (i.e. from the exact geometrical exit) can be neglected. In previous calculations, as shown above, the particle size across the central tube inlet was specified according to that provided in Fig. 2 (i.e. no variation). The first measured profile reveals that a spatial variation of the particle size distribution at the exit can be neglected. Possibly however, the mean velocity and the rms values for the different particle size classes might be slightly different. It should be also kept in mind that the measurements were only done for one profile across the test section. Hence any asymmetries of the flow could bias the results.

Computational Domain and Boundary Conditions

Previous calculations, as shown above, have been done based on the two-dimensional axisymmetric conservation equations. As a matter of fact however the flow should be considered as fully three-dimensional and possibly the computations should be done using an unsteady approach in order to capture the slight precessing of the swirling vortex. This will also affect the particle behaviour and it is possible to numerically predict particle rope formation and dispersion (Lipowsky and Sommerfeld 2007, Sommerfeld et al. 2010).

Discretisation and Grid Resolution

For full three-dimensional calculations of the considered swirling flow at least 300,000 control volumes should be used when applying RANS methods. In the case of LES, the grid resolution should be much higher. Apte et al. (2003) have for example used 1.6 million hexahedral volumes.

For steady-state calculations several hundred thousands of particles should be sufficient. For unsteady simulations the number of considered particles needs to be higher in order to ensure good statistical averaging.