The effect of turbulence on premixed flame noise

THE E F F E C T O F T U R B U L E N C E ON PREMIXED FLAME NOISE J. K. KILHAM AND N. KIRMANI Department of Fuel & Combustion Science, The University, Leeds, England

The effects of turbulence intensity and integral length scale on combustion noise have been investigated in cylindrical burners with fully developed pipe flow and in a nozzle burner with grid induced turbulence. Mean velocity and turbulence intensity measurements in flames over a wide range of Reynolds numbers have been made by laser anemometry and the results for equivalent air jets have been verified by hot-wire anemometry. Semi-empirical relationships for combustion noise have been tested over a wide range of flow conditions and equivalence ratios and it has been confirmed that these expressions give accurate predictions for fuel lean and stoichiometric flames on cylindrical burners. Grids producing higher turbulence levels in flames give rise to higher values of turbulent burning velocity and combustion noise, indicating that the upstream turbulence is also responsible for the increase in amplitude of combustion noise. Integral length scale measurements by auto-correlation and two point velocity correlation techniques in the axial and radial directions in air jets indicate that the magnitude of the macro-scale is independent of the grid geometry. Assuming that the integral scale in equivalent air jets is identical to that in flames, it appears to have no effect on combustion noise.

1. Introduction All turbulent combustion processes are accompanied by the emission of noise and a number of experimental investigations have attempted to determine the dependence of this noise on such parameters as flow velocity, burner diameter and fuel type but discrepancies exist in the resulting correlations. The first reported investigation of noise from premixed turbulent flames on industrial type burners was made in the A.G.A. laboratories. ~'2 The results indicated that upstream turbulence was an important factor in flame noise, Smith & Kilham, 3 as a result of a systematic investigation of laboratory scale turbulent flames, concluded that the noise generation process could be represented by a distribution of monopole sources caused by a fluctuating heat release. This work was later extended by Strahle et al 8 to include larger burners and a wider flow velocity range in order to produce scaling laws. Custard et al 4 & Putnam ~ have also demonstrated the importance of turbulence in their work with industrial burners. Several theories of combustion noise have appeared in the literature, starting with that of Bragg 7 which was based on the wrinkled laminar flame concept of turbulent flames. In a series of papers 327

Strahle s.9.~oadopted a more rigorous approach, using a Lighthill type wave operator to obtain semi empirical expressions for acoustic power. Chiu and Summerfield x~ used the convected wave equation to allow for source convection within the flame zone but the resulting expressions contain source terms which are not easy to determine experimentally. The scaling laws derived by Strahle appear to be limited in their applicability, particularly in respect of changes in the combustible gas and the fuel-air equivalence ratio. Some of the uncertainties can be ascribed to a lack of knowledge of the turbulence characteristics of the flames which have, in general, been assumed to be the same as those of the cold approach flow. The present work involved the direct measurement of turbulence properties by laser anemometry and an attempt to apply these values to the relationship between turbulence and noise.

2. Experimental Apparatus 2.1 Burner Configuration Two types of burner were used to investigate premixed flames of methane, propane, ethylene and

COLLOQUIUM ON TURBULENT-COMBUSTION INTERACTIONS

328

propylene with air at various equivalence ratios. In order to ensure fully developed turbulent pipe flow at the burner port, seamless copper cylindrical burners of 12.7, 10.0 and 7.5 mm internal diameter and length to diameter ratio of greater than 50 were used. A 1 mm annulus was formed round the main burner port by surrounding it with a large diameter tube and a hydrogen retention flame was burned on this annulus to anchor the main turbulent flame at high flow rates. It was found that better stabilisation resulted if the surrounding tube terminated about 2 mm below the main burner port. Variation of turbulence parameters independently of the burner diameter was achieved by means of the nozzle burner shown in Fig. 1. The nozzle, of area contraction ratio 25:1, produced a jet with a uniform velocity profile across the exit and a very low level of turbulence. The reactants were admitted at the burner base and mixing occurred upstream of fine mesh wire screens which smoothed out turbulent fluctuations, The burner exit was surrounded by a 0.75 mm annulus to accommodate the retention flame. The turbulence characteristics could be altered by using turbulence generators in the form of grids of different solidity ratios upstream of the burner port. The turbulence promoters employed took the form of perforated metal plates, the dimensions of which are given in Table I. Preliminary experiments showed that optimum results were obtained by using grid 1 at location B in the burner and grids 2 & 3 at location C. 2.2 Laser Anemometer The laser anemometer operated in the real fringe mode with forward scattered light, using an argon ion laser with 700 mW power output at 488 nm wavelength. The integrated optical unit produced a beam intersection half angle of 4~ The light collecting system ensured that the measuring control volume was 2.5 mm in length and 0.144 mm in diameter, and that approximately 40 fringes were focused at the receiving aperture of an EMI 9558B photomultiplier. The signal processing equipment was based on a DISA 55L20 frequency tracker which gave rise

to some limitation in the maximum turbulence intensity that could be measured. A few results at higher turbulence levels were obtained by incorporating a DISA L02 frequency shift device in the optical system. Seeding of the flames with particles was accomplished by the fluidisation technique, which involved passing a proportion of the combustion air supply (about 0.33 ls -~) through a fluidised bed of magnesium oxide particles which had been reduced to a mean diameter of 1/xm by fluid energy milling. The aerosol formed was directed tangentially into a settling chamber to remove any large agglomerates and to smooth out fluctuations in particle concentration before being united with the main air supply. 2.3 Hot-Wire Anemometer A conventional DISA 55D hot-wire anemometer system, together with platinum-tungsten probes 5 /~m in diameter and 1.25 mm long, was used for turbulence measurements in air jets at flows equivalent to those of the flames. Turbulence frequency spectra were obtained by feeding the anemometer output into either a Briiel and Kjaer 2107 constant percentage bandwidth frequency analyser or a 1615 third octave filter set. After bandwidth corrections the power spectral density and hence the integral length scale of turbulence was calculated by the method of Laurence. je Power spectral densities were also obtained by feeding the linearised signal from the anemometer into a Hewlett Packard Correlator, type 3721A, to give the autocorrelation function and performing a Fourier transform automatically by means of a type 3720A Spectrum Display Unit. In order to determine integral length scales in the radial direction, a two point velocity correlation technique was used involving two hot-wire probes and a D1SA 55A06 Random signal indicator and correlator. The separation of the probes was increased in 1 mm steps and the corresponding correlation coefficients were plotted against the probe separation. The area under the curve was divided by the maximum value of the correlation function to yield integral length scales. This technique was

TABLE I Dimensions of turbulence promoters (ram)

T.G. no.

Thickness

Hole diameter

Distance between centres

1.5 1.0 0.75

3.2 6.4 6.4

5.3 8.6 9.8

Solidity 0.72 0.50 0.73

79

~~

0.75

nnulus 9

..~,~

A Turbulence grid locations

smoothing ~screens I

0 0

~l

l

l

l

l

lf

l

l

l

I

//J

I/.//,///~ e

dimensions

~plosion window

mm Fl~. 1. Nozzle burner.

330

COLLOQUIUM ON TURBULENT-COMBUSTION INTERACTIONS -: * o ~ , o , ,

not used in the axial direction because of the possibility of flow disturbance by the upstream probe.

xX

~

x

~. x a

10

2.4 Noise Measurements Sound pressure level measurements were made in a ventilated anechoic chamber by means of a calibrated Briiel and Kjaer 4145 condenser microphone in conjunction with a 2619 preamplifier and a 2107 frequency analyser set to linear response. Readings were taken at a distance of 1 m from the burner port at intervals of 15~ in azimuth and after converting to sound pressure they were integrated to yield the acoustic power output,

l

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~

xo

s

Ko ~o

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x

4 2 0

l D: D : 'l O m m

XlD ~

Re:B,000 U :11.2 m/$

I o HOT

Turbulent burning velocities were determined by the area method proposed by Bollinger and Williams.IS Direct photographs of the flames were taken, using an exposure time of 1/60 s and the estimated surface of maximum luminosity was traced from the enlarged negative. The turbulent burning velocity was calculated by dividing the volumetric flow rate of the gaseous mixture by the area of maximum luminosity.

Measurements of the mean velocity and turbulence intensity on the axis of air jets were made at Reynolds numbers of 8000 and 16160 for burner diameters of 7.5 and 10,0 ram. Figure 2 shows a comparison between the measurements by hot-wire and laser anemometry for an air jet at Re = 8000 on the 10 mm diameter burner. The results indicate reasonably good agreement between the two techniques. A constant value of mean velocity appears to extend up to X/D = 4 which confirms the existence of a potential core. The turbulence intensity shows a gradual rise to X/D - 3, followed by a steeper increase where the interaction between the expanding iet and the surrounding air is prominent. Values of length scale on the jet centre line were calculated from the autocorrelation function of the hot-wire anemometer measurements. An example of the results is given in Fig. 3 which indicates that the integral length scale increases almost linearly with axial distance and the values appear to be almost independent of flow velocity. No directly comparable results could be found in the literature but it appears to be generally agreed that integral length scales increase linearly up to X / D = 8, beyond which the eddies commence to decay. Measurements of mean velocity and turbulence

I

DOPPLERJ

10 8

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ox

o x

. x

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3. Results and D i s c u s s i o n

3.1 Cylindrical Burners

WI~E

I x LASER

2.5 Turbulent Burning Velocity

X/D

FiG. 2, Comparison of axial variation of mean velocity and turbulence intensity on air jet centre line measured by laser and hot wire anemometers. intensity in fuel lean and stoichiometric flames of various gases, for the same burner, are shown in Figs. 4 and 5. In the case of ethylene flames, there is a slight peak in mean velocity at the combustion zone. Beyond the cone tip the flow is decelerated 0"4

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Fic, 3. Variation of integral length scales on air jet centre line.

TURBULENCE ON PREMIXED FLAME NOISE

331

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Fxc. 4. Axial variation of mean velocity and turbulence intensity in methane and propane flames.

F~c. 5. Axial variation of mean velocity and turbulence intensiW in ethylene and propylene flames.

due to radial expansion and further downstream there is a slight increase. The turbulence intensity appears to fluctuate somewhat randomly between 4 and 7% and beyond the combustion zone it increases steeply. The velocity profile of methane and propane flames is devoid of the peak at the combustion zone, which seems to be a unique feature of ethylene-air flames. Turbulent burning velocities were calculated by the area method outlined in Section 2.5 and the results agreed with the values predicted by the empirical expression of Bollinger and Williams m reasonably well, especially in the case of fuel lean

Acoustic powers for fuel lean, stoichiometric and fuel rich flames in the range Re = 20,000-30,000 have been evaluated from the sound pressure level measurements and some of the results are shown in Fig. 6. Ethylene-air flames have the highest acoustic power output as a result of their higher burning velocity. The acoustic powers of fuel rich flames were slightly higher in the case of methane, propane and propylene flames but in a few cases the acoustic power of ethylene flames was found to be lower for fuel rich than for stoichiometric flames. This tendency has also been reported by Strahle et al6 who found ethylene flames less noisy between ~b = 1.0 and 2.0 but were unable to put forward a satisfactory explanation.

flames.

C O L L O Q U I U M ON T U R B U L E N T - C O M B U S T I O N I N T E R A C T I O N S

332

applicable over the range of variables

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The dependence of acoustic power on reactant velocity, burner diameter and laminar b u r n i n g velocity has received a considerable amount of attention but the exact values of the exponents are still a matter of some controversy. In a series of publications Strahle, 6'8'1~ has proposed scaling laws of the form P = K U a D b S~,F"

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where U is the reactant velocity, D the burner diameter, SL the laminar b u r n i n g velocity and F the fuel mass fraction. The values of K, a, b, c and d were derived by means of regression analysis of the experimental observations yielding a final correlation P = 0.825 X 10-~ U 26r D T M SL" ~ F - o ~ watts

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TURBULENCE ON PREMIXED FLAME NOISE The fuel mass fraction in the reactants has been expressed in the form:

F =

~b(F/1 - F) .....

(3)

1 + ~b (F/1 - F) .....

The values of (F/1 - F) stoic for methane, propane, ethylene and propylene-air flames were taken to be respectively 0.0574, 0.064, 0.0677 and 0.0625.

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0.5

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The acoustic powers of the stoichiometric and lean flames in the present investigation at Re = 20,000, 25,000 and 30,000 have been compared with those calculated from equation 3 and some of the results are presented graphically in Figs. 7 and 8. In all cases the agreement is within 2 dB and is especially close for fuel lean propylene and ethylene flames.

Measurements on air jets, using the nozzle burner with turbulence promoting grids showed the existence of a potential core, where the mean velocity was almost constant, up to X/D = 4. An increase in turbulence intensity, however, occurred at X/D = 2 which is earlier than with fully developed turbulent pipe flow. Measurements in the radial direction showed an almost fiat velocity profile up to axial distances of X/D = 0.53, beyond which the profile became parabolic. Integral length scales in the axial direction were determined by the method outlined in Section 2.3, using the different turbulence generating grids. A typical example of the results is shown in Fig. 9

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X/D

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3

4

;

Fro. 9. Axial variation of integral length scale in air jets with turbulence promoters.

COLLOQUIUM ON TURBULENT-COMBUSTION INTERACTIONS

334

which indicates that, after an almost linear increase, the length scale attains a maximum value in the range X/D = 3-4. This differs from the results obtained with the cylindrical burners, where, after a more gradual increase, the maximum occurred at X/D = 8. Variation of Reynolds number does not appear to influence the length scale and the effects of grid geometry cannot be distinctly identified. Values of the length scale in the radial direction were about 1/3 of those in the axial direction presumably because of eddy stretching by the nozzle in the latter case. The eddies emerging from the perforations in the turbulence promoters pass through the contraction region of the nozzle and, as a result, some influence of nozzle geometry on length scale is to be expected. The assumption that integral length scale is a function of the perforation diameter is not justified in this situation. The influence of promoters on turbulence inten-

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o

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,

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sity in stoichiometric and lean flames is illustrated in Figs. 10 and 11. The level induced by TG 3 appears to be slightly higher in all cases but the peak in turbulence intensity of stoichiometric ethylene flames which is usually apparent at low Reynolds numbers is not evident at Re = 20,000. The results of turbulent burning velocity measurements at Re = 20,000 are shown in Table II. All flames with TG 3 have higher burning velocities than those with TG 2 which again are higher than those with TG 1. Comparing the turbulence intensities under identical flow conditions, although flames with TG 3 show slightly higher values, no significant distinction can be made between the intensity values induced by TG 1 and TG 2. It is tempting to assume that the increase in turbulent burning velocity is the result of integral scale but the experimental measurements have not been able to distinguish

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T U R B U L E N C E ON PREMIXED FLAME NOISE

335

TABLE II The effect of turbulence promoters on burning velocity (Re = 20,000)

~=

Equivalence ratio f u e l / a i r ratio

Turbulent burning velocity (ms -~)

stoic, fuel/air ratio Methane

Propane

Ethylene

Propylene

1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8

scale differences generated by the 3 grids. A feature of methane-air is that the lean flames have a higher burning velocity than rich flames. Similar results have also been reported by Wohl and Shore ~5 and are considered to be the effect of the high diffusivity of methane compared with that of oxygen, which leads to the formation of wrinkles under the influence of approach flow turbulence in lean flames. The acoustic powers of flames at Re = 20,000 and 30,000 using the 3 turbulence promoters are given in Table III. Examination of the results shows that in all cases the lean flames produced less noise than the stoichiometric and rieh flames. This is to be expected in the case of propane, ethylene and propylene because of the lower turbulent burning velocity of the lean flames. Table II, however, indicates that lean methane flames have a higher turbulent burning velocity than the rich flames and it is difficult to understand why they should be less noisy. The noise output of flames using TG 1 to promote turbulence was always slightly higher than that from flames on the cylindrical burners at equivalent Reynolds numbers. Flames using TG 3 produced more noise than those with TG 2 and TG 1, in accordance with both higher turbulent burning velocities (Table II) and higher turbulence intensities (Fig. 10 and 11). The higher acoustic power of flames with TG 2 compared with those using TG 1 is again in agreement with their higher turbulent burning velocities but it was difficult to distinguish any appreciable differences in the turbulence intensity mea-

TG 1

TG 2

TG 3

0.57 0.79 0.75 0.80 0.85 0.74 1.50 1.53 1.32 0.72 0.89 0.70

0.60 0.81 0.76 0.85 0.94 0.76 1.60 1.74 1.51 0.87 0.94 0.84

0.63 0.86 0.84 0.92 0.95 0.86 1.65 1.79 1.63 0.88 0.95 0.86

TABLE III The effect of turbulence promoters on acoustic power output

Methane

Re

~b

Acoustic power (watts • 10 -4) TG 1 T G 2 T G 3

20,000

1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8 1.2 1.0 0.8

2.11 3.37 2.82 4.93 1.56 1.94 3.17 8.24 11.44 18.81 5.95 7.62 14.60 4.52 6.29 9.31 4.11 5.85 5.66 3.80 5.34 6.20 1.92 2.85 3.26 7.93 12.37 13.31 9.05 14.72 16.47 5.69 7.69 9.68 13.39 17.04 26.14 12.64 16.32 25.50 6.32 9.02 14.82 19.36 26.86 43.23 21.40 28.09 49.07 12.99 19.42 31.14 4.32 5.87 7.62 3.83 6.27 8.31 1.97 3.27 4.59 7.10 11.70 14.08 8.60 13.53 20.00 5.49 6.89 11.67

30,000

Propane

20,000

30,000

Ethylene

20,000

30,000

Propylene

20,000

30,000

1.71

2.42

336

C O L L O Q U I U M ON TURBULENT-COMBUSTION INTERACTIONS

surements. Kilham & Smedley ~6 ascribed the effect of grids to differences in length scale and, in the absence of any direct measurements assumed that the length scale was proportional to the perforation size in the turbulence promoters. They obtained an expression for the acoustic power involving the length scale to the power 0.68. The experimental measurements of integral length scales on equivalent air jets in the present investigation (e.g. Fig. 9) does not confirm this assumption and until direct measurements are made on flames, e.g. by the two point velocity correlation technique using laser anemometry, the dependence of combustion noise on integral length scale cannot be confirmed. Giammar & Putnam ~7 have also investigated the effect of grids on the noise output of a premixed burner and concluded that it does not appear to be affected by changes in length scale.

4, Conclusions Semi-empirical scaling laws for the combustion noise generated by open turbulent flames on cylindrical burners have been tested over a range of combustible gases, flow conditions and equivalence ratios and it has been shown that these expressions are capable of predicting values within about 2 dB for fuel lean and stoichiometric flames. Turbulence characteristics have been varied independently of the burner diameter by the use of turbulence generators in the form of grids of different solidity ratios and their effect on flame noise output has been investigated. In general grids producing higher turbulence intensities gave rise to higher turbulent burning velocities and increased combustion noise. Integral length scale measurements in equivalent air jets appeared to yield results independent of the grid geometry and, assuming that the length scale is identical to that in flames, no effect on combustion noise was found. Further work involving direct measurement of integral length scales in flames is required before this conclusion can be confirmed.

Acknowledgments The authors thank the British Gas Corporation

for financing this project and for permission to publish the results. REFERENCES 1. AMERICANGAS ASSOCIATION."Noise produced by industrial gas burners as affected by the mixture burned and the character of the installation preceding the burner body," AGA Testing Laboratory Report No. 692 (1932). 2. AMERICANGAS ASSOCIATION."Noise produced by industrial gas burners as affected by the features of burner & tunnel design," AGA Testing Laboratory Report No. 724 (1933). 3. SMIX•, T. J. B. ANOKILHAM,J. K.: J. Acoust. Soc. Amer. 35, 715 (1963). 4. CUSTARD,G., FmCKErLN. Ano HUGRES,C.: Second European Combustion Symposium, p. 420, The Combustion Institute (1975). 5. PUTNAM,A. A.: J. Inst. Fuel 49, 135 (1976). 6. SHIVASHANKARA,B. N., STRAHLE, W. C. AND HANDLE~, J. C.: AIAA Paper 73-1025 (1973). 7. BRAGG,S. L.: J. Inst. Fuel 36, 12 (1963). 8. STRAHLE,W. C.: J. Fluid Mech. 49, 399 (1971). 9. STRAHLE,W. C.: J. Sound Vib. 23, 133 (1972). 10. STRAFILE, W. C. AND SHIVASHANKARA,B. N.: Fifteenth Symposium (International) on Combustion, p. 1379, The Combustion Institute, 1975. 11. Cmu, H. H. ANO SUMMERFIELD, M,: Astronaut. Acta 1, 967 (1974). 12. LAURENCE,J. C.: Intensity, scale and spectra of turbulence in mixing region of free subsonic jet. NACA Report 1296 (1956). 13. BOLUNCER,L. M. ANDWILLIAMS,D. T.: Effect of Reynolds number in turbulent flow range on flame speeds of Bunsen burner flames--NACA Report 932 (1949). 14. STRAHLE,W. C.: The convergence of theory and experiment in direct combustion generated noise. AIAA Paper 75-522 (1975). 15. WOHL, K. AND SrtORE, L.: Ind. Eng. Chem. 47, 828 (1955). 16. K1LHAM,J. K. ANDSMEDLEY,C.: Second European Combustion Symposium, p. 444. The Combustion Institute (1975). 17. GIAMMAR,R. D. AND PUTNAM, A. A.: "Noise generation by turbulent flames." American Gas Association Catalog No. M00080 (1971).