Reduced model for flow simulation in the Burner region of lime shaft kilns

Autor :Zhiguo Xu
Herkunft :OvGU Magdeburg, Fakultät für Verfahrens- und Systemtechnik
Datum :19.10.2010
Dokumente :
Dataobject from HALCoRe_document_00008887
Typ :Dissertation
Format :Text
Kurzfassung :The knowledge of temperature distribution and radial gas mixing plays a very important role in design of lime shaft kilns, yet is poorly understood. This is due to the geometric complexity and the movement of the lumpy processed material which make it impossible to measure the main parameters, such as temperature and gas components, as well as flow velocity. As one of the critical “enabling technologies”, computational fluid dynamics (CFD) is used to study the different aspects of the internal phenomena of the kilns by means of flow visualization. Geometrical models are initially developed based on a normal lime shaft kiln. To model such a complicated system, simplifications are inevitable. For instance, the arbitrarily shaped and stacked processed material granules are simplified to be uniform spheres and packed in regular structure. Regarding the main body, the feeding and the discharging ports including the cooling air distributor are excluded in this stage. As a rule of thumb, investigation is firstly carried out in two dimensions thanks to the great advantage in the computational time. Considering the serious deficits caused by the geometric limitations, however, the main purpose of the 2-D modeling is to obtain some general information about the jet flow in packed bed and thereby help develop the 3-D geometry. The results show that the flow structure in the jet expanding zone is independent on the height of the bed. This means that modeling of the reaction zone of shaft kiln is assumed to be sufficient to optimize the firing system. It has also been proved that the jet flow pattern is slightly influenced by the heat transfer between the solid and gas phases. In other words, heat transfer between phases can be ignored as long as the flow structures are the interest of study. Such simplifications, in a certain degree, make the 3-D modeling of an industrial shaft kiln economically and technically possible based on the present computer power. The 3-D geometric model in this work is a 30°segment with a bed height of about 0.8 m. Simulations based on this geometry are performed to investigate the dominant factors influencing the radial temperature distribution. The in-kiln flow structures are distinctly visualized by images of temperature contours and pathlines etc. It appears that the increase in the lance depth may prevent the refractory wall being overheated but have slight effect on the ratio of the cold and hot area, which is in agreement with 2-D simulations. Mixing between the combustion gas and the cooling air can be improved by reducing the burner diameter or by preheating the combustion air, as both of which accelerate the jet velocity and provide energy aspirating the cooling air. In the simulation of furnace with diffusion burner,longer flame and deeper jet penetration illustrate the preference of the shaft kilns in the lime industry. In addition, simulations are also performed with defining the bed as porous media. As expected, the radial penetration of the combustion gas is underestimated relatively. This is mostly due to the over-prediction of the momentum loss caused by the inertial terms in the model. Considering its great advantages of simple geometry and short computational time, however, further study in porous medium modeling is expected. As this study demonstrates, simulations upon the scalable segment geometric model are capable of providing the main flow features in the reaction zone of lime shaft kilns, which are nearly impossible to study in experimental setups. With further development and evaluation, CFD approach is expected to be one of the effective methods that could substantially reduce physical testing in the design of lime shaft kilns.
Schlagwörter :CFD; Shaft kiln; Packed bed; Combustion; Flow simulation
Rechte :Dieser Text ist urheberrechtlich geschützt
Größe :XXII, 152 S.
Erstellt am :02.11.2010 - 11:08:01
Letzte Änderung :02.11.2010 - 11:10:01
MyCoRe ID :HALCoRe_document_00008887
Statische URL :http://edoc.bibliothek.uni-halle.de/servlets/DocumentServlet?id=8887