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Research Papers: Flows in Complex Systems

The Flow Field in a Virtual Model of a Rotary Kiln as a Function of Inlet Geometry and Momentum Flux Ratio

[+] Author and Article Information
I. A. Sofia Larsson

Division of Fluid and Experimental Mechanics,
Luleå University of Technology,
Luleå SE-97187, Sweden
e-mail: sofia.larsson@ltu.se

T. Staffan Lundström

Division of Fluid and Experimental Mechanics,
Luleå University of Technology,
Luleå SE-97187, Sweden
e-mail: staffan.lundstrom@ltu.se

B. Daniel Marjavaara

Luossavaara-Kiirunavaara AB (Publ.),
Kiruna SE-981 86, Sweden
e-mail: daniel.marjavaara@lkab.com

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 6, 2014; final manuscript received May 3, 2015; published online June 15, 2015. Assoc. Editor: Zhongquan Charlie Zheng.

J. Fluids Eng 137(10), 101102 (Oct 01, 2015) (11 pages) Paper No: FE-14-1292; doi: 10.1115/1.4030536 History: Received June 06, 2014; Revised May 03, 2015; Online June 15, 2015

The rotary kiln is the middle part of a grate-kiln iron ore pelletizing process and consists of a large, cylindrical rotating oven with a burner in one end. The flame is the heart of the process, delivering the necessary heat. The combustion process is largely controlled by the turbulent diffusion mixing between the primary fuel jet and the combustion air, called the secondary air, which is mostly induced through the kiln hood. The relatively high momentum of the secondary air implies that the resulting flow field has a significant impact on the combustion process, justifying a systematic study of the factors influencing the dynamics of the secondary air flow field, by neglecting the primary fuel jet and the combustion. The objective of this work is thus to investigate how the geometry and the momentum flux ratio of the inlets affect the flow field in the kiln. Down-scaled models of the kiln are investigated numerically. It is found that the resulting flow field is highly affected by both the geometry and momentum flux ratio of the inlet flows, including effects from pressure driven secondary flow occurring in the semicircular inlet ducts. The dynamics of the flow is further investigated using proper orthogonal decomposition (POD) resulting in a deeper understanding of the forming, interaction and convection of the vortical structures.

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Figures

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Fig. 1

Schematic picture of the kiln

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Fig. 2

Geometries with coordinate system

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Fig. 3

Numerical grid seen from two perspectives

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Fig. 4

The sensitivity of the calculated flows to grid refinement, showing velocity profiles of streamwise velocity along the kiln centerline

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Fig. 5

Contours of instantaneous absolute velocity (U/Ub) in the vertical center (VC)

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Fig. 6

Time-averaged streamwise velocity contours and sectional streamlines in the VC

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Fig. 7

Contours of the instantaneous vorticity in the z-direction (ωzD/Ub) in the VC

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Fig. 8

Time-averaged streamwise velocity (Ux/Ub) contours with normalized vectors showing the in-plane motion in the transverse plane at the kiln inlet (x/D = 0)

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Fig. 9

Development of the secondary flow in the transverse plane in the upper inlet duct at positions x/D = -0.05,-0.7,-1.3 from bottom to top. The arrows are colored by time-averaged absolute velocity projected on the xy-plane. Sectional streamlines highlight the flow pattern.

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Fig. 10

Instantaneous absolute velocity (U/Ub) contours in the VC

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Fig. 11

Time-averaged streamwise velocity contours and sectional streamlines in the VC

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Fig. 12

Time-averaged streamwise velocity along the centerline

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Fig. 13

Development of the time-averaged streamwise velocity (Ux/Ub) in the end of the kiln. The profiles are extracted at x/D=4,6,8,9; all positions have been marked with a vertical straight line.

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Fig. 14

Time-averaged streamwise velocity contours in the horizontal center (HC); sectional streamlines show the flow pattern

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Fig. 15

Time-averaged absolute velocity (U/Ub) contours and vectors showing the flow distribution in the transverse plane at positions x/D=0.5,2,4,6,8

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Fig. 16

Vorticity (ωzD/Ub) contours in the VC, instantaneous to the left and time-average to the right

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Fig. 17

Isosurfaces of Q-criterion = 1450 (Rtot = 5.44,ud, Rtot = 1) or 1970 (Rtot = 5.44,ld). The structures are colored by absolute velocity.

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Fig. 18

Energy content of the first ten modes in both the vertical and HC

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Fig. 19

Scatterplot of the POD coefficients of modes 1–4 in both the vertical and HC

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Fig. 20

Mean flow and modes 1–4, VC to the left and HC to the right. The color contours in the POD modes show the out-of-plane component.

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Fig. 21

Qualitative visualization of the dominant structures with Q-criterion = 522 in the first four modes in the vertical (left) and first six modes in the horizontal (right) center plane. The contour plot shows absolute velocity.

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Fig. 22

The first snapshot and the reconstruction of the first snapshot using either the first four (VC) or first six (HC) POD modes. The contours show absolute velocity. VC to the left and HC to the right.

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