Modelling the aerodynamics of iron ore pelletizing kilns

Abstract: In an iron ore pelletizing plant, crude ore is upgraded to pellets to be used as feedstock in steel-making plants. As part of a grate-kiln pelletizing plant, the rotary kiln is an indurating furnace in which the pellets are sintered. The rotary kiln involves complex flow of large amounts of gas and the process is strongly coupled to the fluid dynamics, which is not well understood. The present work focuses on increasing the understanding of the aerodynamics of the rotary kiln.Though the kiln geometry is relatively simple a rather complex flow arises, which is known to occur for turbulent flows in similar geometries. In order to isolate the underlying flow mechanisms, simplified models of the kiln are studied both numerically using Computational Fluid Dynamics (CFD) and experimentally using Particle Image Velocimetry (PIV). The understanding of the flow phenomena that arises for the simplified models is essential for maintaining a solid comprehension of the fluid dynamics when increasing the complexity of the models. Computations are validated against available experimental data to evaluate the capability of the numerical procedure in capturing the underlying physics of the flow. In this way, the reliability of the predictions is improved when increasing the complexity of the model.In Paper A the unsteady non-reacting flow is computed and a preliminary coal combustion model is proposed, which is in need of further development to yield reliable predictions of the reacting flow. Paper B is an experimental investigation of a down-scaled model of the kiln and also an extension to previous experimental work by introducing an inclination of the upper inlet duct to the kiln and carrying out a more thorough analysis of the fluid dynamics. In Paper C, the periodic flow observed in Paper A is investigated further using a more sophisticated turbulence closure and carefully validating the predictions against available experimental data.For the simplified models under investigation, it is concluded that the flow is dominated by the periodic shedding and downstream convection of von-Karman-like vortices originating in the free shear layers enclosing the recirculation zone formed in the inlet end of the kiln. Both numerical and experimental investigations show a strong dependence of momentum flux ratio between the two inlet ducts on the flow field. The large-scale periodic fluctuations, which are resolved in an unsteady computation but completely neglected in a steady computation, are seen to contribute significantly to the turbulent transport in the recirculation zone. This indicates the need for unsteady computations to accurately predict the transport processes. The recirculation zone is important for flame stabilization as it feeds back hot gas to the near-burner region. Hence, a challenging requirement of the numerical model is to accurately capture the physics of the recirculating flow. The use of a second-moment turbulence closure is shown to significantly improve the predictions over the use of an eddyviscosity turbulence model and give promising results for further work on more complex models of the kiln.