Choice of Urea-Spray Models in CFD Simulations of Urea-SCR Systems

Andreas  Lundstrom, Chalmers University of Technology

The work presented here has investigated droplet-flow interactions in a urea-spray, originating from the exhaust pipe wall, using commercial CFD software (Fluent). This has been modelled both using a Eulerian-Lagrangian discrete phase model to capture the behaviour of the entire spray, and using a Volume of Fluid (VOF) multiphase model to simulate individual droplet behaviour.

It has been found that the choice of models influences the results from the simulations. The choices considered have included the different forces in the Eulerian-Lagrangian force balance, models for droplet distortion and drag coefficient, and the models involved in predicting turbulent dispersion of droplets.

In an assessment of the various forces acting on the droplets in the exhaust gas stream, it was found that the most important forces to include in a Eulerian-Lagrangian model are the forces due to gravity and drag. Other forces are estimated to be of minor importance in the urea-SCR system. Proof for this is obtained both from comparison with correlations in the literature as well as VOF simulations of single droplets.

The distortion of the droplets will affect their drag coefficients, which can be accounted for through the use of a dynamic particle drag law, where the Taylor Analogy Breakup (TAB) model supplies the current level of droplet distortion. Comparison with VOF simulation results suggests that this approach provides a satisfactory description of the droplet distortion amplitude and time constant for oscillation. Also the predicted drag coefficients and drag forces agree well for the two methods.

Turbulent effects on droplet trajectories are large enough to require modelling. Inclusion of a stochastic model such as the Discrete Random Walk (DRW) model will help describe these turbulent effects, but also makes the simulation results sensitive to the quality of the turbulence model’s prediction of the turbulent fluctuating velocities. Comparison with a Reynolds stress model suggests the DRW model gives acceptable results if used with the RNG k-e model for turbulent gas flow in a straight pipe, but that it should be used with caution for other flow situations.

The droplets will experience a perceptible change of material properties as the water evaporates and the urea starts to decompose. The influence of the quality of droplet material data on the predictions by the various models is therefore discussed.

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