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Control of Heat Flux Profiles from Rotary Kiln Burners by Modification of Mixing.

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Dev. Chem. Eng. Mineral Process., 7(3/4),pp.333-344, 1999.
Control of Heat Flux Profdes from Rotary
Kiln Burners by Modification of Mixing
JJ. Parham*#,GJ. Nathan*, J.P. Smart+, B.G. Jenkins’
and R.E. Luxton*
*Depament of Mechanical Engineering, University of Adelaide,
Adelaide, South Australia, 5005.
+Fueland Combustion Technology (International)Ltd.
The effect of flame shaping using a Precessing Jet (PJ) burner on heat jlux profiles
was measured at the semi-industrial scale (2MW) in the cement kiln zone simulator at
the International Flame Research Foundation. Natural gas was used as the fuel and
measurements were taken at air preheat temperatures of 400”C, 640°C and 840°C.
Results showed that the radiation from the shaped PJ flames was greater than that
produced by momentum controlled flames from a multi-channel burner. Increasing
the proportion of shaping jet to Precessing Jet also “stretched” the shape of the heat
flux profile such that the location of the peak heat jlux was shifted down the kiln and
reduced in magnitude slightly. The changes in the heatflux profiles are related to the
changes in the mixing field of the non-reacting jet jlows recorded using a semiquantitativeflow visualisation technique.
Introduction
The Precessing Jet (PJ) nozzle has been developed in the Department of Mechanical
Engineering, University of Adelaide, where ongoing research into its properties
continues. The fluidic PJ nozzle has been shown to modify the structure of the
turbulence mixing in a way which enhances combustion in a number of high
’Author for correspondence
333
J.J. Parham et al.
temperature combustion applications, particularly that of cement manufacture [l, 2,
31. The precessing jet technology is the fundamental component of the commercial
“Gyro-Them’’ burner.
The Gyro-Them burner combines a precessing jet flow, which alone produces a
rapidly spreading jet and has a high heat release close to the nozzle, with a high
momentum jet to provide control of the flame shape and heat flux [4]. However,
while the efficiency of this technique has been demonstrated in practice by Hill et al
[5], the nature of the interaction between the two flows has been unknown and the
relative heat flux has yet to be investigated under controlled conditions. The present
research investigates the character of the interaction between a non-reacting
precessing jet flow and an axial jet flow using imaging techniques and quantifies
spreading rates. The heat flux is quantified in a semi-industrial scale combustor and
related to the mixing characteristicsof the jets.
Combustion Experimental Details
Semi-industrial scale combustion tests were conducted in a 2MW cement kiln zone
simulator at the International Flame Research Foundation, Netherlands. The facility is
a refractory-lined cylindrical combustor of 756mm internal diameter with air pre-heat
and probe access. This design simulates a typical cement kiln from the firing end to
about the end of the “burning zone” (a term referring to the bed material, not the
flame) and has a length to internal diameter ratio of 15. The shell is constructed of
fifteen water-cooled segments, each 707mm in length. The secondary air was heated
in two stages using an indirect fired air preheater feeding air into a natural gas directfired pre-combustor. The secondary air passes through a “U” bend between the precombustor and the kiln. Two high pressure-drop “honey-combs”,made from castable
refractory are placed at the inlet to the kiln to correct the flow asymmetries introduced
by the bends. More information on the cement kiln simulator and experimental
apparatus is given by van de Kamp [6,7]and Parham [8].
A schematic of the burner nozzle used for the test program is shown in Figure 1.
The chamber dimensions of the PJ nozzle and centrebody conform to the optimal
ratios determined by Hill [9].The PJ throat was sized to provide 100% of the thermal
334
Control of heat flux profiles from rotary kiln burners
input (160kgihr) of natural gas at 200kPa The PJ nozzle chamber internal diameter is
56mm.
Figure 1. A simplified diagram of the burner showing the precessing jet noale and
the centrebodyjet usedf o r f i m e shaping,and theflow patterns.
The design of the conventional momentum controlled burner is an IFRF MultiChannel Burner (MCB), with capacity for solid fuel firing. The MCB consists of two
channels for fuel supply and three channels for primary air. The inner primary air
channel has 45"swirl vanes and a cross-sectional area of 147mm2.The burner was set
up to produce two different flames, based on the extent of the recirculation within the
kiln, as determined by the Craya-Curtet number [lo]. An under recirculatory flame
( C W . 4 ) was achieved with a primary airflow of 2.8% (20kg/hr through each
channel), while an optimal recirculatory flame (CG2.2) corresponded to a primary
airflow rate of 13% (107kg/hr hr through each channel).
The heat flux profile of each flame was measured using an ellipsoidal radiometer.
The radiometer measures total hemi-spherical radiation and thus includes the
radiation from the wall and from the flame.
Flow Visualisation Experimental Details
Laser-Induced Fluorescence (LIF') was used to conduct the flow visualisation
experiments in non-reacting conditions. The set-up for the tests is represented in
Figure 2. Jet fluid is supplied through the nozzle, which is mounted on a frame
335
J.J. Parharn et al.
separate from the tank, which is of dimensions 75Ox75Ox153Omm. The PJ nozzle
chamber diameter is 44mm so the confinement is very low. Fluid is removed from the
tank by an ovefflow drain that is mounted on the top of the tank.
Figure 2. Schematic diagram of the h e r and imaging set-up used for non-reacting
Laser-Induced Fluorescence (LJF) experiments
The dyes marking the jet flows were mixed in two separate 200L capacity tanks
adjacent to the main tank.Fischer and Porter flowmeters were used to control the flow
to the precessing jet and centre-body jet. A Spectra-Physics 265 Exciter Argon-Ion
laser was used to create a thin light sheet by first passing the beam through a spherical
lens of lm focal length and then passing it through a cylindrical lens to diverge it. The
light sheet was aligned so that it passed through the axis of the nozzle. All lines of the
Argon-ion laser were used and the laser output power was estimated to be 2.3W.
Two different fluorescent dyes were used to allow the flowfields of the precessing
and shaping jets to be distinguished. Rhodamine B was used to mark the precessing
336
Control of heat fluxprojiles from rotary kiln burners
jet, while Fluorescein was used to mark the shaping jet. Dye concentrations of
O.Wmg/L for Fluorescein and 0.045mgL for Rhodamine B were used in accordance
with the recommendations of Arcoumanis [ 111. The dimensions of the precessing jet
nozzle conform to the optimal geometry as detexmined by Hill [9].
To minimise spurious illumination data acquisition was performed with the room
darkened. Since the tank has no co-flow, the run time is limited to about two minutes,
prior to which about one minute is required to eliminate bubbles and ensure steady
state conditions. Background noise becomes significant with longer run times. A
shutter speed of 1/1OOO of a second was required to adequately freeze the motion of
the exiting fluid. A video camera was used to provide time-resolved images and a
photographic camera with ISO-800film was used to provide high-resolution images
of the jets. A filter to minimise s c a t t e d light was found to be unnecessary provided
sufficienttime was allowed for bubbles to clear.
Combustion Results
.
150 --
- .
I
Figure 3. Heatjlux profilesfroma precessing jet burner with different proportions of
centrebodyjetflow. Conditions: 2MW input, 840°C air preheat.
33 7
J.J. Parham et al.
Figure 3 demonstrates the trend of translating the heat profile downstream with
increased proportion of centrebody jet flow for a fixed total input. The distance from
the tip of the burner to the position of the peak of the heat-flux profile increases by
about one kiln diameter as the ratio of CBJ:PJ gas is increased from 0% to 31%. The
peak heat flux for 100%PJ and 85%PJ is approximately 3% greater than for 69%PJ
and 75%PJ.
-85%
PJ
*MCB65%0.4
-0-MCB 65W.2
I
0
2
8
Axhl Distance (dD)frum burner tip
4
6
10
12
Figure 4. Heat flux profilesfrom a precessing jet burner with differentproportions of
centrebody jet flow, compared to a low recirculation jlame (MCB 654b0.4) and an
optimal recirculation flame (MCB 6552.2) from a Multi-Channel Burner (MCB).
Conditions: 2MW input, 640°C air preheat.
Figure 4 shows that both precessing jet flames produce more total heat flux than
the either of the MCB flames. The general shape of the heat flux profile of the optimal
Craya-Curtet burner is similar to those produced by the burner with various ratios of
PJ to shaping jet flows. However, the total heat flux from the 75%PJ and 85%PJ
flames is 13% greater than that of the MCB (CC=2.2) flame, and the peak heat flux is
approximately 8% greater. The shape of the low Craya-Curtet number flame is much
longer with a peak some two kiln diameters further down the kiln. The total heat flux
338
Control of heat flux profilesfrom rotary kiln burners
from the C M . 4 MCB flame is also much lower than that from the other flames. This
is consistent with Moles [12], which finds that low Craya-Curtet flames provide poor
mixing and long heat flux profiles. The effect of flame shaping can also be seen in
that the peak heat flux for the 75%PJ is further downstream than that of the 8595PJ
flame.
Flow Visualisation Results
Figure 5. Flow visualisation of a shaped precessing jet flow using LIF. The ratio of
precessing jet flow to centrebodyj e t f i w is 85-15 Precessing jet fluid appears dark
grey, while the centrebodyjetfluid is whiter due to the different coloured dyes.
The flow visualisation experiments demonstrate that two broad classes of flow regime
can be generated, depending on the proportion of centrebody jet flow to the total
flowate. With small ratios of centrebody jet flow (0-25% of the total) the mixing
field is dominated by the large-scale flow structures generated by jet precession that
have been described by Newbold et a1 [13] and Nathan and Luxton [14] The rapid
initial spread and large coherent motion across this region are evident in Figure 5.
339
J.J. Parham et al.
Ambient fluid can also be observed to be drawn onto the axis of the flow. The two
colour images, [8], demonstrate that the centrebody jet merges with the PJ fluid so
that the combinedjet precesses about the jet axis.
Figure 6. Flow visualisation of a shaped precessing jetflow using U F . The ratio of
precessing jet flow to centrebodyjetflow is 70:30, Precessing jet fluid appears dark
grey, while the centrebodyjetfluid is whiter due to the different coloured dyes.
At higher proportions of centrebody jet flow (greater than 30% of total), the
centrebodyjet doknates the mixing field, as shown in Figure 6. In this condition, the
size of the large-scale flow structures is diminished, and the spread of the jet tends
toward that of a non-precessing jet flow. The velocity of the jet at a corresponding
location is deduced to be higher than for a PJ dominated flow, although this was not
measured. The @ansition from PJ to CBJ dominated regimes corresponds to a
momentum ratio of centrebody jet to precessing jet (calculated at the upstream
orifice) of 0.25-0.3.
Quantitative measurement of spreading rates can be obtained from the video
images by calculation based on locally normaiised data. If the divergence of the laser
340
Control of heat flux profiles from rotary kiln burners
beam through the measured area is small (here the half-spread angle is less than lo")
then the local half-width of the jet can be calculated relative to a local centreline value
without requiring any additional corrections (such as for light sheet variations or
absolute reference of "pure" jet fluid). Only the left side of each image is presented in
Figure 7 since the laser sheet was incident from the left, resulting in stronger signal
strengths there.
Figure 7. The average hay-width of shaped precessing jet jbws, determined from
local normalisation under the following conditions: (a) 100% Precessing Jet, spread
angle=4O0; (b) 85%PJ, 15% Centrebody Jet, spread angle=345. (c) 70%PJ, 30%
CBJ, spread angle=25O; (d) 60%PJ, 40% CBJ, spread angled4" (high background
noise levels are present in this image).
The reduction in spreading angle with increased centrebody jet ratio can be clearly
seen. The half-angle reduced by about 10" for each increase in CBJ ratio. The
instantaneous images demonstrate that coherent large scale motions occur across the
entire width of the jet, so variation in the ratio of the two flows provides means of
controlling the scale of the largest turbulent mixing scales. A decrease in spreading
angle can be expected to correlate with increased jet velocity and results in the flame
being pushed further downstream from the nozzle. This is consistent with the
observed change in heat-flux profile measured in the combustion experiments.
341
J.J. Parham et al.
Discussion
In natural gas flames, the rate of combustion process is mixing limited. Hence gaining
control of the mixing leads to control of the combustion characteristics. Gutmark et al
[15] has related the control of mixing in cold flows to the control of combustion
characteristics. Gutmark observed that similarity in flow dynamics between flames
and cold flows suggested that the combustion process can be controlled by using
similar methods to those used in cold flows. Nathan et al [161 similarly concluded, on
the basis of experiments in cold-flow air experiments and in flames, that differences
in non-reacting mixing characteristicswithin the region corresponding to the onset of
flame stabilisation will correspond to differences in the mixing within the flame. The
present cold flow data show that the combustion of a precessing jet and an axial jet
provides a means of controlling the turbulence structure and hence the mixing.
A precessingjet has been shown to-enhancethe formation of large-scale structures
at the expense of fine-scale turbulent energy within the region where combustion
occurs. This has been related to a reduction in the characteristic local strain rate
within a flame [13,17]. Kent and Bastin [18] showed that increased soot formation in
a simple jet flame correlates with reduced exit strain rate. Hence the reduction in
strain rate cause by precession is postulated to be significant in achieving the increase
in radiant fraction from precessing jet flames [19]. Increased emissivity from
precessingjet flames compared to conventional type flames has been demonstrated in
open flames, [ 171, and observed in the results of industrial installations. However, the
results presented in this paper represent the first time increased radiant heat transfer
has been quantified in a confined environment.
Although the size of the largest flow structures generated by the PJ is observed to
vary with centrebodyjet proportion, there does not seem to be a significant change in
total radiant energy with increasing CBJ proportion. Only the shape of the heat flux
profile changes. In addition the NOx emissions were measured to increase with CBJ
proportion [8].This finding is the subject of ongoing investigation but is consistent
with previous findings by Manias and Nathan [20]. Further research to quantify the
change in mixing and the turbulence spectrum with centrebody jet proportion in a
confined flow will be conducted to resolve these issues.
342
Control of heat flux profiles from rotary kiln burners
Conclusions
The trends observed in the flow visualisation experiments show that:
Two types of flow regime exist: a PJ dominated flow and a CBJ dominated flow.
The flow field changes from precessing jet dominated to centrebody jet
dominated if the momentum ratio of CBJ to PJ is above 0.25-0.3.
Control of the half-width spread angle of the non-reacting jet from 14" to 40",
with a corresponding change in the largest scale of turbulent motions, can be
achieved by variation of the proportion of centrebodyjet flow from 40% to 0%.
The general trends in the heat-flux data are consistent with the trends observed in the
flow visualisation and industrial installations:
The ratio of centre-body jet (CBJ) gas to precessing jet (PJ) gas provides good
control of the heat flux profile. Increased CBJtotal gas flow over the range 0-31%
pushes the peak heat flux location approximately one kiln diameter downstream
from the burner tip. The profile is also broadened so that the peak heat flux is
reduced by approximately 3%. However, the total heat release is does not change
significantly;
The general shape of the heat release profile from an optimised swirYmomentum
burner with a Craya-Curtet number (CC) of 2.2 is comparable with those of the PJ
burner flames, although its integrated heat flux is 8% lower than the PJ flames.
The heat flux profile from the low recirculation flame ( C W . 4 ) is much longer
and peaks two kiln diameters further downstream than the other flames. The total
heat flux is also 24% lower than the total heat flux from the PJ flames.
Hence control of the mixing, and therefore of the heat flux profile can be achieved by
variation in the ratio of CBJ to PJ flows. An increase in the CBJ:total flow ratio
reduces the spreading angle of the jet and lengthens the heat flux profile. It is
postulated that the characteristic strain rate in the initial part of the flame also varies
with the ratio of CBJ:total flow. However, how this propagates through the confined
flame is not yet known.
343
J.J. Parham et al.
Acknowledgment
The authors wish to thank the staff of the IFRF,and in particular Jochen Haas, for
their assistance in conducting the combustion experiments and analysing the data.The
help of Mr. G.J.R. Newbold and Mr. D.S.Nobes in setting up the flow visualisation
experiments was also invaluable. The support of Fuel and Combustion Technology
Ltd (FCT) and the Australian Research Council through the Collaborative Grant
Schemes, is also gratefully acknowledged.
References
1. Manias. C.G. and Nathan, GJ. 1993. The Mes s ing Jet Gas Burner - A Low NOx Burner Providing
Recess Efficiency and Product Quality Improvements. World Cement, March. 4 -1 1.
2. Manias, C.G. and Nathan, G.J. 1994. Low NOx Clinker Production. World Cement, 25(5), 54-56.
3. Nathan, G.J. and Manias, C.G. 1995.The Role of Rocess and Flame Intaaction in Reducing NOx
Emissions Combustion and Emissions Control, The Institute of Energy London. December,309-318.
4. Rapson, D.. Stokes, B. and Hill,S. 1995. Kiln Flame Shape Opthisation Using a Gyro-Thcrm Gas
Burner. World Cement, 26(7).2-5.
5. Hill,S.J., RapsoO.S. and Nathan,G.J. 1995. Control of Flame Shape and Heat Flux in a Rotary Kiln.
The Australian Symposium on Combustion,Gawler, S.A., November.
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and Fuel Propertieson the charactcn'stics of Cement Kiln Flames. IFRF Doc.No. F97/y/3.
7. Van De Kamp, W. L 1996. Evaluation of Ccment Kiln Flames from Coal, Delayed Coke, Sewage
Sludge and Plastic Waste. IFRF Doc.No. D91/y/12.
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F d Rotary Kilns -herim Report (tobe submitted). Dept Mech. Eng. University of Adelaide.
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submitted). Dept. Mech. Eng. Univesity of Adelaide.
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11. Arcoumanis, C., McGuirk, J.J., and Palma, J.M.LM. 1990. On the Use of Fluomcent Dyes for
Concentration Measurements In Water Flows. &pt. Fluids. 10.177-180.
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Precessing Jet Flame. Corn Sci. Tech., 126(1-6). 71-95.
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Visuulisation,No. 14.The Visualisation Society of Japan.
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King, K.D. 1997. Exploring the Relationship Between Mixing. Radiation and NOx Emissions from
Natural Gas Flames.3rd Intenrationu1 Conference on Combustion and Emissions Control.
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R.V. 19%. The Influence of Fuel Jet Precession on the Global
Properties and Emissions of Unconfined Turbulent Flames.Comb. Sci. Tech.. 112.21 1-230.
18. Kent, J.H. and Bastin, S.J. 1984. Parametric Effects on Sooting in Turbulent Acetylene Diffusion
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Emission Characteristics of Unconfined Turbulent Jet Diffusion Flames. Intemional Wonkrhop on
Thermal Energy Engineering and the Environment, Adelaide, Australia, 9-10February.
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344
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