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CFD Simulation of Jet in Asymmetric Co-flows in a Down-scaled Rotary Kiln Model
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Rotary kilns are industrial furnaces that have been widely used in limestone calcination, cement industry and hazardous waste incineration for centuries. In this work a rotary kiln used for the iron ore pellet sintering process in the grate-kiln pelletizing system has been studied. In order to increase the energy efficiency, a large amount of air is supplied to the kiln through air channels connected to the cooler. This air is necessary for the coal combustion process and the heat transport to the kiln bed. However, the geometry of the kiln hood connecting the air channels and the cooler is complicated. As a consequence, the jet flame is unstable. In order to improve the performance of the jet flame it is therefore necessary to study the kiln aerodynamics to reveal the flow field. Even though it is a complicated problem containing fluid dynamics, combustion and heat and mass transfer, it can be simplified into a down-scaled cold model to make it feasible to understand the flow field both experimentally and numerically. In this work, the whole kiln is generalized as a high Reynolds number turbulent round jet interacting with asymmetric co-flows. With the aid from previous PIV measurement data of a down-scaled water model of the kiln, Computational Fluid Dynamics (CFD) simulations using the commercial code ANSYS CFX 16.0 have been pursued for two main purposes: 1) To find a turbulence model that is computationally inexpensive and able to capture the main features of the mean flow field; 2) With the turbulence model chosen in 1), to study the geometrical effect on the development of the primary jet. In Paper A, three turbulence models were employed, the standard k-epsilon model, a modified k-epsilon model with slightly higher turbulence production and the SSG Reynolds stress model. Wall functions were applied since resolving the viscous wall region was not a concern in this work. It is found that the standard k-epsilon model fit the experimental data best compared to the other two models and that all three turbulence models predict an asymmetric development of the primary jet, especially far downstream, In Paper B, again using the down-scaled kiln model, isothermal cases with four different nozzle diameters were simulated with the standard k-epsilon model. The aim is to investigate the effect of initial Reynolds number on the jet development in asymmetric co-flows from the air channels. It is found that, with increasing Reynolds number, the jet becomes shorter and the mixing between the primary jet and surrounding flow is better. A low-velocity region or external recirculation zone (ERZ) form near the kiln upper wall and shrink with increased nozzle diameter or decreased initial Reynolds number. The ERZ may stabilize the flame since it is a low-velocity region and consequently attract the jet to reside predominantly in it or in the shear layer. As a conclusion, by enlarging the jet exit diameter, the jet can be prolonged, while to a certain extent, the benefit from the ERZ would need to be sacrificed. Since the asymmetric development and the ERZ in the kiln were not studied during the previous PIV measurement campaign, more experimental studies are planned to provide more experimental evidences of the details of the flow and to lay grounds for validation of the CFD simulation results.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2017.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-61175ISBN: 978-91-7583-789-5 (print)ISBN: 978-91-7583-790-1 (electronic)OAI: oai:DiVA.org:ltu-61175DiVA: diva2:1058311
Opponent
Supervisors
Funder
VINNOVA, 197426
Available from: 2016-12-21 Created: 2016-12-20 Last updated: 2017-10-19Bibliographically approved
List of papers
1. CFD Simulation of Jet Mixing with Asymmetric Co-flows in a Down-scaled Rotary Kiln Model
Open this publication in new window or tab >>CFD Simulation of Jet Mixing with Asymmetric Co-flows in a Down-scaled Rotary Kiln Model
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2016 (English)In: Proceedings of the ASME 2016 International Mechanical Engineering Congress and Exposition, ASME Press, 2016, Vol. 7, IMECE2016-65637Conference paper, Published paper (Refereed)
Abstract [en]

Rotary kilns used in the iron pellet production in the grate-kiln pelletizing process normally have two asymmetric secondary air channels. The primary jet is ejected from a burner located in the middle of a back plate. As a consequence of the high flow rates and irregular-shaped secondary air channels, the aerodynamics in the kiln is strongly connected to the combustion and sintering performance. In this work a Computational Fluid Dynamics study is performed on a downscaled, simplified kiln model established in earlier numerical and experimental work. Comparisons are made with the experiment and among three turbulence models, the standard k-ε model, a k-ε model modified for turbulent axisymmetric round jets and Speziale-Sarkar-Garski Reynolds Stress Model (SSG-RSM hereafter). Recirculation regions with negative axial velocity are found at the upper side of the kiln and behind the back plate. Results from the standard k-ε model have the best fit to the experimental data regarding the centerline decay and the jet spreading of the velocity. The spreading rate of the scalar concentration calculated from the results with the modified k-ε model and the SSG-RSM fit better with the experiment, but they both underestimate the centerline decay and the spreading of the velocity. The modified k-ε model yields a more physical and realistic flow field compared to the standard k-ε model, and the results are close to those obtained with the SSG-RSM. Unlike the isotropic development of the jet predicted with the standard k-ε model, the modified k-ε model and the SSG-RSM show different development of the jet in the horizontal and vertical directions.

Place, publisher, year, edition, pages
ASME Press, 2016
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-60693 (URN)10.1115/IMECE2016-65637 (DOI)000400877800062 ()2-s2.0-85021639527 (Scopus ID)978-0-7918-5061-9 (ISBN)
Conference
ASME 2016 International Mechanical Engineering Congress and Exposition (IMECE 2016), Phoenix, Arizona, USA, November 11–17, 2016
Projects
Faste Laboratory
Funder
VINNOVA, 197426
Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2017-10-19Bibliographically approved

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