Kiln aerodynamics : influence from turbulence models and boundary conditions

LKAB (Luossavaara-Kiirunavaara AB) is an international high-tech minerals group that produces iron ore products for the steel industry. LKAB's main product is iron ore pellets. In the pelletising process nitrogen oxides, among other emissions, are produced. Stricter environmental laws results in higher demands on the companies and their environmental impact. LKAB is therefore running several development projects with the objective to reduce NOx emissions from their pellet production plants. In this project the focus has been on the grate-kiln process and especially on the rotary kiln. A down-scaled model of the real kiln has been created and with help of Computational Fluid Dynamics (CFD) the flow field has been investigated. The objective was to get a deeper understanding of the flow field in order to decide if changes of the geometry up-stream the burner can change the flow field, improve the combustion and reduce the environmental harmful NOx emissions. The commercial code ANSYS CFX was used and different boundary conditions and turbulence models and their impact on the flow field were studied. The turbulence models used were the SST-model, which is a two-equation model, and the BSL-model, which is a Reynolds stress model. The most prominent result was the differences between the two turbulence models. The turbulence model's ability to reveal secondary flow affected the flow field pattern and the result with plug profiles as inlet boundary conditions deviated from the one where velocity profiles were used. One conclusion was that a more advanced turbulence model can be used when simulating a down-scaled model like this one. If the secondary flows are of interest, two-equation models should not be used. The two-equation model showed trouble with convergence in certain cases, especially when velocity profiles were used as inlet boundary conditions. The more advanced Reynolds stress model created stable simulations with good convergence in every case at no noticeable increased run time.Validerat; 20101217 (root)