Analysis of the cold flow field in a rotary kiln

Abstract: The pelletizing process where the crude ore from the mine is upgraded to pellets is a process which includes several stages involving complex fluid dynamics. In this thesis, focus is on the grate-kiln pelletizing process and especially on the rotary kiln, with the objective to get a deeper understanding of the aerodynamics and its influence on the combustion process. The aim is to discover flow features taking place in the kiln, and the kiln hood, by using Computational Fluid Dynamics (CFD) on simplified models of the real kiln, and to validate the set-ups of the numerical model with physical experiments using Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). By starting as simple as possible, studying only the cold flow field without combustion and validating the simulations, a foundation for future geometrical optimizations can be achieved. Later on more realistic geometries may be studied with the validated simulations as a base. In Paper A the initial down-scaled, simplified model of the real kiln is studied, and both numerical and experimental analyses of the flow field are performed. Paper B focuses on the turbulent secondary flow that arises in ducts with non-circular cross-section. One of the inlet ducts to the kiln of interest here is close to semi-circular in cross section, hence the focus of this work. Numerical and experimental results are reported. Paper C is a development of the model, where instead of parallel inlet ducts as in Paper A, the top one has an inclination angle to the kiln axis. A thorough experimental analysis of the flow field is performed in this case. Conclusions are that steady state simulations can be used to get an overview over the main features of the flow field. Precautions should though be taken when analyzing the recirculation zone which is important for the flame stabilization. A stable flame is safe and crucial for efficient combustion. Steady state simulations do not capture the transient, oscillating behavior of the flow seen in the physical experiment. These oscillations will under certain conditions considerably affect the size of the recirculation zone. Another parameter affecting the size of the recirculation zone is the inclination of the upper inlet duct, where a decrease in recirculation length is seen although the actual inclination of the incoming jet is only about 3-4º. The choice of turbulence model affects the prediction of turbulent secondary flow. If this flow feature needs to be revealed, a more advanced turbulence model should be used.