Flame movement mechanisms and characteristics of gas fired cyclone combustors

Eighteenth Symposium (International) on Combustion

The Combustion Institute, 1981

F L A M E M O V E M E N T M E C H A N I S M S A N D C H A R A C T E R I S T I C S O F GAS FIRED CYCLONE COMBUSTORS S. E. NAJIM, A. C. STYLES AND N. SYRED

Department of Mechanical Engineering and Energy Studies, University College, Newport Road, Cardiff CF2 ITA, Wales.

Cyclone combustors are finding application in several fields, ranging from b u r n i n g waste vegetable refuse, low calorific value gases, M H D applications and as highly flexible modulatable combustors. The combustor, described in this paper, is based on design experience with coal fired cyclone burners and is run on a premixed supply of natural gas and air at varying mixture ratios. Three principal modes of combustion and associated flame movement mechanisms are described. Mode I occurs at rich mixture ratios, combustion taking place partially inside the combustor, but for the most part downstream of the exit. Mode II occurs at near stoichiometric mixture ratios, the flame being located primarily in the inlet/outlet mainifold of the combustor but extending inside the cyclone chamber. Mode III occurs at weak mixture ratios, the flame is of annular form and located at 0.75 of the cyclone chamber diameter. The flame movement mechanism between Mode I and II is caused by differences in heat release rates due to the degree of flammability of the air/fuel mixture. The distinct flame movement mechanisms between Mode II and III is caused by (a) a reduction in entrainment from the descending wall flow to the exhaust flow, (b) a reduction in the annular width of the wall flow, (c) a change in angular and axial momentum flux distributions inside the combustor. The effect of combustion is seen to significantly reduce the local dynamic swirl n u m b e r throughout the combustor, due to flow acceleration, and is predicted by a simple expression using spatially averaged mean densities.

Introduction In response to the d e m a n d for efficient utilization of existing and alternate fuel sources, recent work on the development of cyclone combustors has proved successful in the incineration of solid refuse (1, 2), e.g. cotton husks, car tyres and for low calorific value gas produced by various chemical processes (3, 4, 5). High temperature coal fired versions (6) are being applied to M H D applications, the cyclone combustor being of similar configuration to that reported in this paper. It is also evident that such combustors have potential for burning premium fuels in multi-stage low pollution combustion systems or multi-mode (modulatable) combustors (4, 7, 8). In the latter several distinct modes of combustion are apparent, each corresponding to a given range of mixture ratio. It is possible to tailor flame characteristics to a particular operating condition

by altering the mixture ratio or fuel entry position (3), thus providing greater flexibility and optimum performance over a wide range. In the cyclone combustor described in this paper, three principal modes of combustion exist due to the presence of regions within the flow structure where the aerodynamics and air/fuel ratio are conducive to flame stabilisation. The cyclone combustor is based on the cyclone dust separator design and previous coal fired cyclone combustor experience. Three main modes of combustion are obtained when fired with a premixed supply of natural gas and air. Two regions of the combustor have aerodynamic properties suitable for flame stabilisation:-(a) Within the cyclone, a long thin annular recirculation zone exists located close to the chamber walls. Long residence times, due to the position of fuel entry, and extremely high centrifugal forces

1949

1950

FURNACES

(= 200 - 300 g) are produced which substantially increase the flame speed of the air/fuel mixture (10, 11). Increases in blow-off limits and combustion efficiencies are achieved with low pollutant emissions. This region, where flame stabilisation of weak air/fuel mixtures or low calorific value gases is possible, is also useful as the first stage of a low pollution combustor. (b) At the exit, flame stabilisation of rich or near stoichiometric mixture ratios is achieved with a flow structure similar to that which occurs in swirl burners of geometric swirl number, S, of 1.0. Alternatively the region can be employed as the second stage of a multi-staged combustor, as mentioned above, in which additional fuel is injected to provide complete combustion. Moreover the wall flow region which is subject to extremely high centrifugal force fields enables flame stabilisation to occur at a radius within the combustor which is dependent on the interaction between flame speed, mixture ratio and heat removal rates from the walls. Thus insulation of the combustor walls, by refractory, moves the flame radially outwards for certain modes of combustion (5). The three modes of combustion have been characterised with respect to temperature, gas composition and velocity in order to determine flame characteristics and flame movement mechanisms which occur at each mode change. The mode changes are primarily produced by variation in the internal flow structure or changes in the local swirl number, as a consequence of varying heat release rates. The experimental work has proved valuable in optimising combustors to applications where rapid changes in fuel quality and calorific value (CV) occur (i.e. change in CV of a fuel gas from 11.5 to 1.5MJ/m 3 in a 15 minute period). The authors have also found the work useful in designing liquid and solid fuel fired cyclone combustors.

Experimental Apparatusand Techniques The cyclone combustor was constructed from stainless steel specifically for velocity measurements and was machined to incorporate two symmetrically positioned quartz windows. The combustor was designed in two sections--the inlet/outlet manifold with exhaust sleeve and the cyclone chamber. Natural gas and air were supplied premixed to the manifold through eight circular tangential inlets. The prevention of gases immediately leaving the combustor was achieved by an extension sleeve to the exit inside the cyclone chamber. The flow was therefore directed against the chamber walls and towards the base of the combustor. Relevant dimensions of the combustor are as follows:--

Diameter of cyclone chamber Diameter of exit Diameter of tangential inlets Length of cyclone chamber Length of exhaust sleeve Area of single inlet

DO D, D, Lo L, A,

152.5 78 18.7 309 125 283.5

mm mm mm mm mm mm ~

The geometric swirl number, S, based on input and exit parameters (12) is ~r De Do S- - - 4.21 4 A, Sampling ports were provided along the length of the cyclone chamber for temperature and concentration measurements. The quartz windows were of adequate length to allow corresponding velocity measurements. Results were obtained axially between X / D r = - 4 to + 2 in 0.25 X / D e intervals and radially in 0.14 r / R e intervals. Mean temperatures were measured using thermocouples constructed from 7.5 Ixm diameter P t / P t - 13% Rh wire enclosed in a suitable twin bore refractory tubing. Catalytic degradation of the thermcouple wire was reduced by a coating of hexamethyldisiloxane. Gas samples were collected at various spatial positions in the flow using a peristaltic pump and uncooled quartz microprobe (1 mm bore/30 ~ taper) designed to give a sonic throat velocity, thus quenching any further reaction of chemical species. Analysis of the stable species, e.g., CO 2, H 2, 02, CH 4, CO and NO x levels was made using gas phase chromatography and a chemiluminescent gas analyzer. The mean and fluctuating components of velocity were measured with a single component laser Doppler anemometer operating in the forward scatter fringe mode. The arrangement utilized a 2W Argon ion laser operating at a wavelength of 488 nm and output power of 120 mW. A bleached, 36000 line radial diffraction grating was employed as a beam splitter from which two equally intense first order diffraction beams were brought parallel and focussed to form a measuring control volume of approximately 0.1 mm diameter by 1.2 mm length. Generally associated with torroidal flows are regions of high turbulence intensity and reverse flow which are undetected if frequency shifting of the Doppler frequency is not employed. The sense of the flow was therefore determined by rotating the diffraction grating which enabled a maximum frequency shift of 5.4 MHz to be used in regions of negative velocity occurrences. Light scattered by micron sized MgO particles passing through the control volume was collected by a combined optics and photomultiplier unit. The

1951

GAS FIRED CYCLONE COMBUSTORS resultant frequency modulated signal was processed by a digital pulse counter, interfaced to a PDP 11/03 microcomputer programmed to function as a multichannel analyzer. Data acquisition was handled by a machine code program and linked to an 8K version of BASIC language for on-line analysis (8).

/ 8 Circulor Tangential Inlets

/

Results and Discussion The principal combustion modes, discussed in this paper, are referred to as Modes I-III and represent rich (I), near stoichiometric (II) and weak (III) air/fuel mixtures. The flame characteristics and flame movement mechanisms which occur between modes are described. The extent of each mode, within the range of fuel loading considered, is identified in Fig. 1 and is seen to be primarily a function of mixture ratio. However at high fuel loadings, the useful range of the combustor, especially for Mode II and III is considerably reduced. Mode IV, as shown in Fig. 1, is of no practical significance as it produces inefficient combustion and is not discussed here. The present measurements describe one mixture ratio within each mode regime as shown below.

Mode

Mixture Ratio 4)

Air Flow Rate kg/hr

I II Ill

0.66 1.1 1.68

35 60 89

1 ~

3.1 kg/hr natural gas

Spatial distributions of mean temperature for the three modes of combustion, schematically represented in Fig. 2, are shown in Fig. 3. At rich mixture ratios, referred to as Mode I in Fig. 2(a), combustion occurs in an annular region located at r / R e = 1.0, the size of which is described by the 1200~ temperature contour in Fig. 3(a). Downstream of the exit, further combustion is supported by oxygen entrained from the surrounding ambient air. As shown in Fig. 4(a), combustion efficiencies, calculated as the ratio of the molar fraction of CO x to the sum of the molar fractions of CO and CO 2, are low in the central region of the flame, Fig. 2, where there is an oxygen deficiency and high levels of intermediate inflammables, e.g. H 2 and CO; however 100% efficiencies-are reached at the flame boundary. Low levels of NO, typically 17 ppm, are produced which illustrate the potential of Mode I as a possible first stage in a multi-stage low pollution combustor. At near stoichiometric mixture ratios (i.e. Mode II) combustion as shown in Fig. 2(b), is initiated in the exhaust sleeve region immediately downstream of the tangential inlets and continues in the exit region. Maximum recorded temperatures, in Fig.

N~

o

0

f / I

f

~\

\

Mixture Ratio

FIG. 1. Modes of operation for fully premixed cyclone combustor 3(b) inside the cyclone chamber, are 1500~ which are reduced by dilution effects downstream of the exit. Much of the cyclone chamber is not used in the combustion process, due to the initial stoichiomerry of the mixture. Figure 4(b) indicates combustion efficiencies of 100% with higher NO emission levels, of the order of 50 ppm. For Mode III, there is a distinct change in flame front, being annular in form, Fig. 2(c), and located at r/R~ = 1.5, with slight afterburning in the exhaust due to some flow being immediately drawn into the exhaust sleeve by the central vortex. Maximum flame temperatures, in Fig. 3(c) are low, of the order of 1180~ due to the high air/fuel mixture ratio. However 100% combustion efficiencies are achieved with negligible production of NO pollutant emission, Fig. 4(c). Having described the combustion characteristics of the three modes in terms of mixture ratio, it is pertinent to consider the factors which cause the flame movement mechanism. Aerodynamic evaluation of both combustion and isothermal states has proved useful. Spatial distributions of mean tangential velocity, t~, for the modes of combustion are presented in Fig. 5 with corresponding mean axial velocity, ti, and mean stream function shown in Fig. 6. Mean streamlines were calculated using the mean axial velocities and computed density data derived from the measured species concentration and mean temperatures. The contours of Fig. 5 appear similar in profile

FURNACES

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Mode I

(a) Rich Mixture ~<

Mode TIT

Mode II

(c) Weok Mixture

(b) Neor Stoichiometric

1.0

1.0<~<1-/.

<

1 . 4 < ~ <1,8

•

Location of Combustion Recircutation Zone

....

F1c. 2. S c h e m a t i c d i a g r a m of the three m o d e s of c o m b u s t i o n

,--r

,--'/6

z

(a) MODE I

I

!%--!

o

,

(b) MODE ]I

2

!

'--~,~

~

~--~

2

b

I

(s MODE TIT

F1c. 3. Spatial d i s t r i b u t i o n o f m e a n t e m p e r a t u r e for M o d e s I - I I I

GAS FIRED CYCLONE COMBUSTORS ;4/De = 15

o~.col H ~cop. 2O

1953

Xl0e:15

XYDe=+5

o~.co~

C~

~'~o%(pp,'n)

_~0

(

"/

~

NOCO~

CO2%,CH~% WOippmi

H~%,CO~

~p

.75 5o

O ............. ~ 25 I, O'5 rt~

r~ e

(b) MODEII

(a) MODEI

(c) MODE m

Fic. 4. Radial profiles of gas composition downstream of the exit at X/D~ = 1.5 for Modes I-III

for Mode I and II, the levels of mean tangential velocity, in each case, reflecting the inlet flow rate conditions. Changes in the boundary and flame stabilisation between the two modes is considered not due to aerodynamic forces but differences in the heat release rates associated with the degree of flammability for the two mixture ratios. This is further substantiated later by Fig. 6(a) and (b). The angular momentum flux in Mode II is concentrated in the upper section of the cyclone chamber and is consistent with the flame position, Fig. 3(b). By comparison the contours for Mode III inside the cyclone chamber are more uniform and flame stabilisation is therefore likely to occur along the corn-

~--%!

r/.

\i/~i",~}

2 r-

plete length of the chamber, Fig. 3(c). In the exit, despite the lower inlet flow rates of Mode II, maximum tangential velocities for Mode II are equivalent to those measured for Mode III due to the acceleration of gases in the less dense temperature region in the exit. Stream function data, for the combustive case, is shown in the form of streamlines in Fig. 6, as calculated from the integration of the measured mean axial velocity component, ft. R

= 21r

I

p fir

dr

o

~ !r/R--

/

7'

tAli l tI~51~ /

x~,

I'): ,\ll'I! ;

i

(a}MODEI

JD,

(b) MODETr

(c)MOOEliT

FIG. 5. Spatial distribution of mean tangential velocity for Modes I-III

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FURNACES =

--4==

L

t

(a) MODEI

I

6

I

(b) MODEII

(c) MODEllI

FIG. 6. Spatial distribution of stream function and mean axial velocity profiles for Modes I - I I I

where ~ is the mean density derived from temperature and species concentration measurements and ~o is the value of the stream function at the exit. Velocity measurements were not attempted inside the exhaust sleeve; however the end of the recirculation zone was determined using tracer techniques. Owing to the small size of the recirculation zone in this region, the streamlines of the exit were extrapolated to those of the internal flow. Mode I and Mode II, as described above, are seen to be aerodynamically similar despite the change in inlet flow rate. Due to the axial momentum flux of the central vortex, considerable quantities of combustion gases are entrained from the wall flow and do not travel the length of the combustor. For Mode II, approximately 40% of the gases are drawn into the exhaust flow by X / D e = - 2 . 2 and within 0.4 D,, upstream of the exhaust sleeve. Comparison of the streamlines for Mode II and III indicates significant differences within the cyclone chamber. For Mode III, approximately 20% of the gases are entrained by X / D e = -2.2, the remaining 80% penetrating the lower section of the combustor. This is also consistent with the more uniform distribution of angular momentum flux, as shown in Fig. 5(c). The location of the flame front is coincident with the inflexion point of the streamlines. For Mode II, the flame front is located at r / R e = 1.0 and moves radially outwards to r / R e = 1.5 as the air/fuel mixtures become weaker. The flame movement mechanism, which distinctly separates Mode II from Mode III is therefore caused by:--

(b) TangentiolTurbule~e Irftereity 6(

e RichBurning m NearStoichBurning ~ , ' ~

t~

2(

0's

rme

1'0

l!s

20

(o) Axv31TurbulenceInte~y

11 ,l j! I

I

05

10

I

15

2.0

dRe

FIG. 7. Radial profiles of angular and axial local turbulence intensities inside the eombustor at X / D e = - 2 . 0 for Modes I - I I I

GAS FIRED CYCLONE COMBUSTORS (1) a reduction in entrainment from wall flow to exhaust. (2) a reduction in the annular width of the descending wall flow. (3) a change in angular and axial momentum flux distributions inside the combustor. Fig. 7 represents the local turbulence intensity, It, Ix (r.m.s/mean velocity) for tangential and axial components at an axial distance of X / D e = -2.0. Turbulence intensities shown in Fig. 7(a) are of the order of 20% over three-quarters of the chamber diameter except close to the central axis where higher levels e.g. 60%, are encountered due to fluctuation in the position of the vortex core. For rich and near stoichiometric combustion, there is a tendency for I~ to increase towards the wall where, in the upper section of the combustor, high angular momentum fluxes are obtained. Axial turbulence intensities, I~, at X / D e = -2.0, and near the axis of the combustor, i.e. r / R e = 0.5, indicate significant velocity fluctuations, typically 40%, for all modes of combustion, due to the presence of the central vortex core. The flame front is located at the position of the zero mean velocity indicated by the discontinuity in Fig. 7(b) at the 1000~ isotherm of Fig. (3). Here

flame characteristics for Mode I and II are seen to be similar. Fig. 7(b) also depicts the movement of the flame front towards the wall for weaker air/fuel mixtures. This corroborates earlier conclusions. Fig. 6(c), of a reduction in size of the wall flow, for Mode III, and a distinct change in flame characteristics between Mode II and Mode III. The flame movement mechanism is further illustrated in Fig. 8, by the axial variation in local dynamic swirl number, S*, for both combustion and isothermal conditions. S), is defined by the expression:-GO S*=

GaR, where G o and G a are the axial fluxes of angular and axial momentum respectively and Re is a local radius to which G o and G a are integrated. The graph was obtained from a knowledge of the angular and axial momentum fluxes derived by integration of the measured mean velocity profiles. The largest variation in swirl number is in the cyclone chamber with the S ' v a l u e s obtained with Mode III combustion, due to higher ratios of angular/axial momentum fluxes. This is not primarily a result of higher

\\

si"

_••a

1955

ox~

9

Isothermal

I

o

Rich burning

1-[

[3

Near sto~h burning

TIT ~

Weak burning

O

Main body

=

-[-

L

I

I

-4

-3

-2

-I-

Exhaust sleeve I

-1

~ ~ --

X/De

0

I~

-lil~

I

1

FIG. 8. Local dynamic swirl number along the length of the combustor for isothermal and burning conditions

19~

FURNACES

tangential inlet velocities but of lower axial velocities due to a reduction in the mean temperature. Calculated swirl numbers for Mode I and II indicate a similarity in flow which is shared by all modes downstream of the exit. Slight differences in the exhaust region between the three modes is due to the presence of small recirculation zones. The presence of a reverse flow zone is important from the aspect of flame stabilisation. However, similarity of S~,values, in the exit region, indicates that the parameter is not sensitive enough to determine the difference in sizes of the reverse flow zones for the modes of combustion, Fig. 6(a) and (c). For the isothermal case where the flow rate was comparable to that of Mode III measurements reported elsewhere (8) show a large exit reverse zone and generally lower axial velocities throughout the combustor. Fig. 8 also infers this as shown by the large changes in swirl number between the isothermal and combustion states. The effect of combustion on the flow aerodynamics causes a reduction in the local swirl number, S~by an average factor of 3, primarily due to the increase in axial momentum. This result substantiates previous work (9, 12) and suggests a simple empirical expression (8) for the calculation of the swirl number with combustion:-Poutlet S(~om|,uslion) ~- S(is,,th . . . . . I) X -P inlet

Applying the correction to the isothermal data, given in Fig. 8, provided a theoretical line corresponding to Mode III combustion.

Conclusions Three possible modes of combustion have been found to exist in the cyclone combustor, described in this paper, by alteration of the mixture ratio. These modes correspond to rich (I), near stoichiometric (II) and weak (III) air/gas mixtures and their combustion aerodynamics and heat release patterns have been described in detail. From the measurements undertaken in this study, the following conclusions have been d r a w n : As there is a wide range of burner loading for which the modes of combustion exist, principles of similarity can be applied, thus enabling the effect of aerodynamic forces and heat release rates to be separated despite the differences in inlet air flow rate. The flame movement mechanism, between Mode I and Mode II, is caused by increases in heat release rates due to the degree of flammability of the air/gas mixtures. The distinct flame movement mechanism, be-

tween Mode II and Mode III, is caused by a reduction in the heat release/unit volume of inlet air and aerodynamic forces:-(1) a reduction in entrainment from the descending wall flow to the exhaust flow. (2) a reduction in the annular width of the wall flOW.

(3) a change in angular and axial momentum flux distributions inside the combustor. The effect of combustion is seen to significantly reduce the local dynamic swirl number throughout the combustor, due to flow acceleration. It is possible to predict the reduction using a simple empirical expression using spatially averaged mean densities. However, this swirl number is not sensitive enough to detect differences in the size of exit reverse flow zone for the three modes of combustion. Acknowledgements

The authors gratefully acknowledge the generous financial support of the Science Research Council (U.K.). S. E. Najim was supported by a grant from the Iraqi Government.

REFERENCES 1. AGREST, J. 1. Inst. Fuel, Vol. 38, pp. 344-348, August 1966. 2. SCUMIDT,K. R. V. D. I.--Bericht, Vol. 146, pp. 543-561, 1970. 3. SYRED,N., DAHMEN,K. R., STYLES,A. C. ANDNAIIM, S. E. I. Inst. Fuel, Vol. 1, No. 405, pp. 195-207, December 1977. 4. SYaEO, N. ANt)DAHMEN,K. R. "The Combustion of Low Calorific Waste Gases," Proc. 2nd European Symposium on Combustion, Orleans, France, pp. 414-419, 1975. 5. NAJIM,S. E., STYLES,A. C. ANDSYRED,N. "'Combustion and Turbulence Characteristics of Cyclone Combustors for Burning Low Calorific Value Fuels," A.I.A.A. Paper No. 80-0075 presented at A.I.A.A. 18th Aerospace Sciences Meeting, Jan. 1980, Pasadena, California. 6. KATSNELSON, B. D. ANDBOGOANOV,B. A. Thermal Engineering, Vol. 17, No. 4, pp. 82-86, 1970. 7. STYLES,A. C., SYRED,N. ANDNAIIM, S. E. "Turbulent Flow Structures and Recirculation Patterns associated with Cyclone Combustors and their Effect on Flame Stabilisation," Proc. Shear Flows Symposium held at Pennyslyvania State Univ., p. 725, April 1977. 8. NAJIM,S. E. "An Aerodynamic Study of Cyclone Combustion with Gaseous Fuels," Ph.D. Thesis, University of Wales, August 1979. 9. BELTA(,UI,S. A. ANDMACCALLUM,N. R. L. 1. Inst.

GAS F I R E D C Y C L O N E C O M B U S T O R S

of Fuel, Vol. 49, No. 401, pp. 183-200, 1976. 10. LEWIS, D. Proc. 13th Syrup. on Combustion (Int.), The Combustion Institute, Pittsburgh, Pa, p. 625, 1971. 11. LEWIS,D. Proc. 14th Syrup. on C o m b u s t i o n (Int.)

1957

The C o m b u s t i o n Institute, Pittsburgh, Pa, p. 413, 1973. 12. SYRED, N., B~ER, J. M. " C o m b u s t i o n in Swirling Flows: A Review," C o m b u s t i o n and Flame, Vol. 23, p. 143, 1974.

COMMENTS I. T. Reardon, University of Waterloo, Canada. H o w did you obtain the flow patterns s h o w n in one of the earlier slides (blue) which showed eddies with opposing rotation? T h i s flow pattern also disagrees with your later stream function distributions.

Author's Reply. The flow patterns commented on by Dr. Reardon were obtained from a water model study. All results presented in this paper were obtained with combustion. C o m b u s t i o n causes very substantial changes in the aerodynamics of cyclone combustors, especially in the exhaust region, where the reverse flow/recirculation zone is drastically reduced in size. Further details may be obtained from the following reference.

REFERENCE

combustors. Care m u s t be taken in p l e n u m chamber design and gas distribution systems to minimise Helmholtz and standing wave oscillations. This combustor has been tested in a partially diffused form, w h e n the natural g a s / a i r mixture was only partially premixed. More details are given in reference 2 below. In brief, with near stoichiometric combustion the flame is moved into the exhaust region, whilst the position of the flame with weak mixture ratios (corresponding to Mode III in this paper) is scarcely changed, except for increased luminosity.

REFERENCE 2. STYLES, A. C., SYRED, N., NAJ1M, 8. A. ]. Inst. Energy, Vol. 52, no. 413, p. 159-168, December 1979.

1. NAIIM, S. A. Ph.D., Thesis, University of Wales, 1980.

S. Galant, Societe Bertin, France. Have you encountered acoustic instability modes in your study? have you tried to make the combustor in a diffusional model? Author's Reply. We have encountered acoustic instability modes in this and other studies of cyclone

M. Murphy, Battelle Columbus Lab., USA. Were the p p m values given in the report of emissions data as measured or were they corrected to some percent oxygen? Author "sReply. E m i s s i o n s data were as measured. Correction to a specified percentage oxygen is possible using data from reference 1.