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.

+Fuel and 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 heat flux profiles are related to the

changes in the mixing field of the non-reacting jet jlows recorded using a semi-

quantitative flow 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

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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 characteristics of 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 direct-

fired pre-combustor. The secondary air passes through a “U” bend between the pre-

combustor 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

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Control of heat flux profiles from rotary kiln burners

input (160 kgihr) 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 centrebody jet used forf ime shaping, and theflow patterns.

The design of the conventional momentum controlled burner is an IFRF Multi-

Channel 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

(CW.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

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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 flux projiles from rotary kiln burners

150

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 [ 1 11. 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-800 film was used to provide high-resolution images

of the jets. A filter to minimise scat ted light was found to be unnecessary provided

sufficient time was allowed for bubbles to clear.

-- I

Combustion Results

. - ..

Figure 3. Heatjlux profiles from a precessing jet burner with different proportions of centrebody jetflow. 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 4 6 8 10 12 Axhl Distance (dD) frum burner tip

Figure 4. Heat flux profiles from a precessing jet burner with different proportions 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: 2M W 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 profiles from 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 centrebody j e t f i w is 85-15 Precessing jet fluid appears dark grey, while the centrebody jetfluid 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.

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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 combined jet precesses about the jet axis.

Figure 6. Flow visualisation of a shaped precessing jetflow using U F . The ratio of precessing jet flow to centrebody jetflow is 70:30, Precessing jet fluid appears dark grey, while the centrebody jetfluid is whiter due to the different coloured dyes.

At higher proportions of centrebody jet flow (greater than 30% of total), the

centrebody jet 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

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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.

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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 characteristics within 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 precessing jet has been shown to-enhance the 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

precessing jet 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 centrebody jet 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.

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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 centrebody jet 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 (CW.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.

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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.

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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.

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