On combustion generated noise from turbulent diffusion gaseous flames

ON COMBUSTION GENERATED NOISE FROM T U R B U L E N T DIFFUSION GASEOUS FLAMES

A. K. GUPTA* and J. M. BEER*

Department o[' Chemical Engineering and Fuel Technology, Sheffield University, SheJfield (Great Britain)

SUMMARY

A study is made into the mechanism oJnoise emission processJrom an "inverse' open turbulent dijJi~sion gaseous double-concentric jet flame. Correlations are presented between the mean temperatures, mean and fluctuating velocity, and the fluctuating pressure on one hand and the overall noise emission on the other Jrom the natural gas-air flames and the He-air jets. Time mean and temporal component o f velocity were measured using a laser doppler anemometry system. The LDA was operated in the fringe mode with a radial rotating diff?action grating. Signal processing was carried out. by a counter linked to an on-line mini computer. A comparison is presented between the experimental results and the theoretical predictions based on a rational correlation and shows a good agreementJor the peak J?equeney of the radiated sound power.

INTRODUCTION

There is great interest in reducing noise emission from high-intensity industrial burners. This problem of noise emission is becoming acute especially with the current trend towards compact high-intensity burners. The noise levels in such 'compact' firing systems (domestic or industrial) reach unacceptable levels and steps to minimise the sound pressure levels (SPL) have to be taken in order to ensure acceptable working conditions. Design modifications of a combustor, therefore, have to be based on a better understanding of the noise-generating mechanism. Several theoretical and experimental investigations have been undertaken to attempt to identify the mechanism of noise production together with its variation * Present address: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 35 Applied Acoustics (11) (1978) © Applied Science Publishers Ltd, England, 1978 Printed in Great Britain

36

A. K. GUPTA, J. M. BEI~R

with relevant geometric and flow parameters. It is now generally agreed that the combustion noise is a monopole source of noise and in a combustion region it is equivalent to a statistical distribution of monopole sources in addition to the aerodynamic noise. Theoretical predictions do not always agree with the experimental results due to the lack of understanding of the mechanism of turbulent combustion and also due to the lack of data regarding turbulence within the flames. A number of publications have dealt with the noise-emission characteristics of turbulent flames both pre-mixed and of the diffusion type.~ 5 More recently, an attempt was made to predict the sound power output and the spectral content from burner geometry and input parameters based on known turbulent flame speed correlations and the theory of isotropic turbulence decay along the burning jet. 0"7 Another study 3'4 demonstrated that effective noise-reducing design modifications can be made from the information on the correlation of the spatial distribution of fluctuating and mean temperatures, fluctuatingpressure and sound pressure levels in turbulent diffusion flames. The incomplete information on turbulence parameters in this latter investigation did not allow the correlations produced for pre-mixed flames 6 to be tested and extended for diffusion flames. In this paper therefore, results of a detailed experimental investigation on an qnverse' turbulent methane-air diffusion flame are reported. Experiments were carried out also with the same air flow through the central nozzle but with helium gas replacing the fuel in the annulus surrounding the air jet. In this way the fluids density ratio could be compared for the effects of combustion upon the noise emission. A comparison is also presented between the experimental results and the theoretical predictions for the peak frequency of the radiated sound power. EXPERIMENTAL APPARATUS

The double-concentric burner

Conventional double-concentric burners having fuel in the centre and the air surrounding the central nozzle have very low stability limits with respect to air/fuel ratio and flow throughput. The stability limits can be, however, much improved by interchanging the respective positions of fuel and air entry into the burner. The experimental flame was produced from the above double-concentric burner (inner diameters of the inside and outside tubes being 8 mm and 27 mm, respectively) in which natural gas was introduced through an annulus surrounding the central air jet. The flame from this 'inverse' burner had wide stability limits. The flame length varied between 8-20 outer-burner diameters, depending upon the burner loading and the mixture ratio. In this type of 'inverse' double-concentric jet flame, the fuel gas is entrained upstream of the region where turbulent jet similarity applies and the length of the flame is therefore not entirely independent of the flow rate through the nozzle.

COMBUSTION NOISE FROM TURBULENT DIFFUSION GASEOUS FLAMES

37

Mean temperature measurements

Mean temperature measurements were made using a 90-#m diameter Pt versus Pt-13 per cent Rh thermocouple. The thermocouple was butt welded to form a very small fine bead. The thermocouple was coated with silica to eliminate catalytic reaction that takes place on the surface of the thermocouple junction at high temperatures within the flame. The response time of this thermocouple was large (about 0.1 sec) so that only low fluctuations could be followed (up to 1.6 Hz). Concentration measurements

Gas samples were collected from within the flame at various spatial positions with an uncooled quartz microprobe (2-mm diameter, 30 ° taper). An inlet mass flow rate of 80 cc/min assured a ten-fold pressure drop and thus critical flow conditions in the orifice. The drop in pressure and temperature accompanying the expansion of the gas quenched the gas composition. A Pye Unicam Series 104 isothermal gas chromatograph was used to measure the concentrations of the stable species, e.g. CO2, H2, 02, CH4 and CO. The relatively-small size of this uncooled quartz probe has advantages over the conventional water-cooled probe with respect to the disturbance to the flow. A previous study s demonstrated that a good agreement between the two types of conventional type water cooled and the uncooled quartz probes (within 1 per cent) can be obtained by using a properly-designed quartz probe. Fluctuating pressure measurements

The noise-producing regions in the flame can be located by a mapping of the fluctuating pressures within the turbulent flame brush. A water-cooled horn-coupled condenser microphone, with a frequency response linear to within 4 dB from 30 Hz to 8 kHz, was used as the measuring instrument. This microprobe has a pressureequalising hole in the membrane of the microphone so that the microphone only measures p', the pressure fluctuations. In the absence of a flame, when the flow direction is perpendicular to the microphone diaphragm, the pressure due to the fluctuating component, as registered by the condenser microphone (assuming v' and w' are small and not noted by the probe and the static pressure fluctuations are small compared to the influence of u') is given by 3 p' = k(½pu '2 + pgJu')

However the microphone probe as well as the other probes deriving velocity fluctuations from pressure readings suffer from the independent fluctuation of density. With fluctuating density the turbulence intensity, T(U), can be derived from 9

38

A. K. GUPTA, J. M. BEER

The correlation factor, a, of u' and p' is

P'~m~

U'rm~

a'--

assuming that the phases of u' coincide with the phases o f p ' , the value of u' can be calculated from the measured p' according to

l(P'2~ 2=

(l+a+

In hot-air jets and diffusion flames the values of a are found to lie between 0.6 to 1.0 (ref. 9). The above method has been successfully used to measure the local distribution of turbulence intensity in jet flames. 9'1° In the present study, however, measurements of the distribution of pressure fluctuations were made not for the purposes of irvestigating turbulence intensities but to provide information on the spatial distribution of noise sources in flames. The damping effect of the microphone probe as a function of the probe tube dimensions is discussed in reference 11. The resulting pressure fluctuations from within the jets (with or without combustion) were recorded in dB using the B and K frequency analyser set for linear frequency response in the 2-40,000 Hz range (i.e. used as a linear amplifier).

Combustion noise measurements Combustion noise measurements were made in the near field of the source in a semi-free field. The total acoustic energy emitted by the burner in terms of S PL was measured by means of a 25-4-mm diameter condenser microphone. Amplitudefrequency spectra of the noise were recorded using a frequency analyser, type B and K 2107, band pass filter set, type B and K 1615, and a level recorde L type B and K 2305. The frequency analyser 2107 was adjusted for sensitivity and calibrated with a pistonphone, type B and K 4240. The rms sound pressure level produced by the pistonphone is around 124 dB (reference 2 x 10- 5 N / m 2) ___0.2 dB at 250 Hz. This arrangement enabled the spectral distribution to be obtained in the 25-20,000 Hz audio frequency range in steps of~ octave bands together with the noise amplitude in the A, B, C and linear filter network ranges.

Mean and fluctuating velocity measurements Time mean and temporal component of velocity (axial and radial) were measured using a laser doppler anemometry system shown in Fig. 1. It consists o f a 4-W argonion laser (Spectra Physics model 164-03) which was operated at 5145A having a maximum power of 1400 mW. The laser beam after passing through the bleached radial diffraction grating is diffracted into various orders. The grating used here had 21,600 bleached line pairs drawn radially on a 160-mm diameter plate glass disc and

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A. K. GUPTA, J. M. BEI~R

40

was chosen since the large number of line pairs offered a high shift at low rotational speeds and the large diameter offered low deformation of the diffracted beams. The respective mean efficiencies of the diffracted light intensity to the total light intensity were 57-5 per cent on the first order and 20 per cent for the second order. Only 8 per cent of the incident light was transmitted into the zero order. The diffraction grating acts as a beam splitter and a frequency shifter, the frequency shift (fs) obtained from this at 5360 rev/min of the grating being 3-86 MHz. The plus one and minus one order beams after passing through the focusing lens intersect to form the measuring control volume (MCV). By simply exchanging the focusing lens (or the beam separation with the optical unit) and hence the intersection angle • of the two beams at the MCV, the fringe spacing a = 20/[2 sin (~/2)] and the signal frequency f = (U/a) can be adapted to the velocity range of interest. The angle of intersection of the two beams at the MCV here in this study was 9.02 ° and the fringe spacing, a = 3.2595/tin. This gave a signal frequency of 0.'3068 MHz/m/sec, but, however, with frequency shifting (.1~), f is given by: f=.f~ +

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or

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The optical receiver system consisted of a piano-convex lenswith F = 300 mm; the scattered light collected through this lens using an iris diaphragm was focused onto the aperture in front of the photomultiplier (PM). The magnification of the MCV at the aperture of the PM was 1.69. The PM (E.M.I. 9668B) is equipped with a modified $20 cathode that provides a 19 per cent quantum efficiency in the green region of the spectrum. The signal from the PM is fed to the laser anemometer interface and then to the P D P 8/E on-line mini computer for signal processing. The data is stored on a paper or magnetic tape for later analysis of the statistical quantities of the flow, e.g. mean and fluctuating velocity, turbulence intensity, skewness factor, flatness factor and the probability density distribution of velocity. 12 The calculation of Reynolds stresses in the main flow direction was performed on the basis of two consecutive measuring runs on the rms values of velocity with the beam plane rotated through 4-45 ° from the axial direction (i.e. U velocity). The uv was therefore calculated by means of the formula

where a~ = rms value of U f o r + 4 5 ° and a 2 = rms value of U f o r - 4 5 ° CORRELATIONSFOR FREQUENCYCONTENTOF THE RADIATEDSOUND Strahle and Shivashankara ° presented an empirical correlation of the frequency of maximum sound power from the burner geometry and input parameters based on

COMBUSTION NOISE FROM TURBULENT DIFFUSION GASEOUS FLAMES

41

known turbulent flame speed correlations and the theory of isotropic turbulence decay along the burning jet. The sound power and the frequency of m a x i m u m sound power were given by: P = 3.7 x l O - 6 U 2 6 7 S l a 3 F - ° ' 4 D 2"78

(1)

f~ = 2"3U°'lSS°L'SSF-l':lD-°'13

(2)

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where U is the axial velocity (ft/sec), S c is the laminar flame speed (ft/sec), D is the burner diameter (ft), F is the fuel mass fraction, f~ is the frequency of m a x i m u m radiated acoustic power (Hz) and P is the acoustic power (watts). Taking the natural gas-air mixture at ~ = 1.0 2, U = 18m/sec

and

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For this case the following properties can be calculated: F = 0.054,

Re = 3.18 x 104

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Using formulae for isotropic turbulence 13 for the nearly- isotropic core o f the pipe flow, it is found that at the pipe exit plane: u' --- 2.3 m/sec (about the same as found in the experimental results and 1e ~ 0 . 3 D = 0.008 m (where 1e = integral scale of turbulence). Using above data, the frequency of macroscopic mixing is given by: U'

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These frequencies are found to be of the same order of magnitude to those found on the double-concentric 'inverse' jet flames as shown in the Results and Discussions. The chemical frequency is given by: Jchem ~

~x

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where e, the thermal diffusivity, is far too high to be considered representative of a process important in combustion noise. In turbulent diffusion combustion processes, therefore, the inhomogeneities caused at the fuel-air interface by the turbulent mixing process (at the energy containing eddies size scale) can also be considered as a possible source of combustion noise.

A. K. GUPTA, J..M. BEI~R

42

RESULTS AND DISCUSSIONS

The stability limits of this 'inverse' double-concentric burner, depicted in Fig. 2, were found to be better and higher than the conventional type due to the entrainment of the fuel gas with air on both sides. The Reynolds' Number at an air-flow throughput of 95 litre/min was 1-7 x 104 based on the burner diameter. At higher flow throughput, the flame was only stable for (1) > 1 and shows an imbalance between the flame velocity and the gas velocity. In this type of double-concentric burner, therefore, the stabilisation is aerodynamic and depends on the existence of a region where the gas flow balances the flame velocity. Mean axial and rms turbulence axial velocity at various axial distances downstream of the burner exit with varying air flow throughput and equivalence ratio is shown in Figs. 3(a) and 3(b). As was to be expected, maximum mean velocity occurs on the central axis of the burner at the exit (maximum velocity = 26 m/sec, Fig. 3b) and the velocity profiles are Gaussian in nature. Proceeding downstream away from the burner exit, a decrease in maximum velocity occurs and is due to the jet spread. The maxima of the rms axial velocity occur at the maximum gradient of the mean velocity. In general, axial velocity fluctuations lie between 1.5 and 3.5 m/sec (Fig. 3a) and 2 and 4-5 m/sec (Fig. 3b). The increase in velocity fluctuations with increase in mean velocity is as expected. The

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radial distribution of mean and rms turbulence radial velocity at various axial distances downstream of the burner exit is shown in Fig. 4, and shows that the shape and amplitude of the velocity profile remain essentially unchanged with increasing distance downstream away from the burner exit. The radial distribution of axial velocity corresponding to Fig. 4 is shown in Fig. 3(b). The mean radial velocity was found to vary between 0.8 and 1.5 m/sec. The rms turbulence radial velocity was found to lie between 1.0 and 2.5 m/sec, the maximum being close to the burner lip. The minimum radial velocity occurs at the maximum mean axial velocity and the maxima of mean radial velocity coincide with the spatial location maximum of mean axial velocity gradient (Figs. 3 and 4). The size and shape of the flame obtained from this 'inverse' double-concentric burner are shown in Fig. 5 and correspond to the flow configuration as in Figs. 3(a) and 3(b). The location of the reaction zone can be clearly seen and lies in between the fuel- and air-rich boundary. Near stoichiometric conditions are therefore achieved at this reaction zone. The flame length varied between 8-20 outer-burner diameters, depending upon the burner loading and the equivalence ratio. In this type of double-concentric jet flame, the fuel gas is entrained upstream of the region where turbulent jet similarity applies and the length of the flame is therefore not entirely independent of the flow rate through the nozzle. A comparison of the coherent jet structure under both burning and non-burning conditions was investigated by keeping a similar density ratio between the inner and the outer jet. Such conditions were obtained by replacing the combusting natural gas with helium in the outer annuli. As was to be expected, the velocity profiles show good agreement (Figs. 3 and 6). Local turbulent intensity Urms/Uhas a maximum at the outer edge of the jet where mean velocity is at a minimum and is due to mean velocity beingvery near to zero. The rms axial velocity is found to lie between 1 and 5 m/sec, similar to that found in the burning case, except that this replaces the natural gas. This detailed study of the coherent jet structure under the non-burning and burning conditions has therefore demonstrated that, for modelling purposes, one can take data under isothermal conditions as long as the density difference between the two jets is taken into consideration. A comparison of the results under non-burning and burning conditions showed that both the mean and rms velocity (axial and radial) increases with combustion. The radial distribution of the skewness factor for various equivalence ratios and Reynolds' Number (burning case) and also for the He-air jet (isothermal case) is shown in Fig. 7 and shows a non-Gaussian nature of the probability density distribution of velocity. In all cases, the probability density distribution is positively skewed, once again showing a similarity between the burning and the non-burning cases. The probability density distribution shown in Fig. 7 can give quantitative insight into the large-scale turbulent motion associated with flow re-circulation, particularly near to the boundary of the jet, demonstrating the unsteadiness of the flow field. This large-scale mixing structure, particularly near the boundaries of the

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jet, results in significant deviations from Gaussian turbulence, confirmed also by high values of directional intermittency. The high values of directional intermittency have also been reported in swirling flows near to the boundary of the internal recirculation z o n e . 14,15 .. The radial distribution of Reynolds' stress, obtained on the basis of two consecutive measuring runs with the beam plane rotated through___ 450 f r o m the axial direction, is shown in Fig. 8. Shear stress is maximum at the burner exit at r/R --- 0.35 and decays with increasing distance downstream from the burner exit for the case of He-air jet. A similar phenomenon is observed for the gas diffusion flame except for X/D = 7.85, wherein a negative shear stress is observed. This negative shear stress indicates a stretch in the structure of eddies as they travel across the flame front. This negative shear stress is in agreement with the findings of Durst. 16 Isotherms of mean temperatures within the flame are shown in Fig. 9. Maximum flame temperatures were found at 7-8 diameters downstream of the burner exit. The coherent structure of the jet with negative shear stress at X/D = 7.85 is also in agreement with the isotherms of mean temperatures. The maximum visible flame length at this Reynolds' N u m b e r and equivalence ratio was about 14 burner diameters. The radial distribution of species concentration obtained by collecting gas

50

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He - AIR JET AIR = 4B [ / MIN He = 1 351. / MIN

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samples from within the flames at X/D = 2-3, 4.55 and 7.15 is shown in Fig. 10. The quality of combustion efficiency, r/c, was determined from r/f = (Xcoz)/Xco2 + )(co ) where [Xco, and )(co are the mole fractions of C O 2 and CO, respectively. Relatively large concentrations of CO and H 2 (e.g. at X/D = 7.15) infer poor mixing within these double-concentric jet type of burners. Maximum C O 2 concentrations were found to be in the same spatial position as the mean maximum temperatures. The radial distribution of pressure fluctuations within the He air jet and the natural-gas diffusion flame is depicted in Figs. 1 l(a) and (b). Maximum pressure fluctuations were found in the vicinity of the maximum velocity fluctuations, notably near to the reaction zone. Compared with the usual double-concentric jet flame arrangement, i.e. fuel in the central jet and air in the annular jet, in this "inverse' system the maxima of the mean temperature traverses are further from the jet axis (Fig. 9) and generally outside of the regions where the fluctuating properties have their maximum values. This is significant from the point of view of the noiseemission mechanism; a low-density interface can act as a partial reflector to acoustic waves emanating from the inner regions of the flames. 3 ' ~ Amplitude-frequency spectra and the total noise-emission levels in terms of sound pressure levels determined by means of a Bruel and Kjaer instrument, described

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previously, are depicted in Fig. 12. The frequency spectra of noise and the overall noise levels in Fig. 12 show the effect of variation of Reynolds' Number and the mixture ratio in the flame. The maximum noise levels were found to be within the 250--350 Hz frequency range. There was a general increase in overall SPL with increase in Reynolds' Number and mixture ratio. The increase in SPL with increase in equivalence ratio (even greater than unity) from this type of double-concentric burner is not surprising because, although the overall input equivalence ratio to the burner is fuel-rich, the local equivalence ratio at the interface of the reactants must be near stoichiometric, thus emitting increased noise levels. Comparing Fig. 12 with Figs. 3 and 4, it is apparent that the noise output from double-concentric jet flames

52

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increases with increase in turbulence levels. In the present study, however, it has not been possible to find out the contribution of the turbulent length scale upon the noise output. However, a reasonably good agreement is found between the predicted frequency content of the maximum sound power and the experimental results. The efficiency of noise production was found to be of the order of 10-a, typical for small-range size of the burners.

CONCLUSIONS

The basic mechanism of the noise generation process from an 'inverse' open turbulent diffusion double-concentric jet flame has been investigated. Noisegeneration process can be explained by downstream velocity fluctuations caused by a fluctuating heat-release rate which is in turn caused by large-scale fluctuations in the turbulence. Good correlation is obtained between the non-burning and burning

A. K. GUPTA, J. M. BEI~R

54 PRESSURE

b

FLUCTUATIONS DIFFUSION

FLAME

AIR = 4 8

[ / MIN

FUEL = 5

[ / MIN

.x/o iDi1116

'10 dB

t FUEL

Fig. ll(b).

AIR

FUEL

Radial distribution o f pressure fluctuations in the diffusion flame at respectively. Air flow rate, 48 l/min; ~ = 1.02.

9ol

SPECTRUM. FUEL(t/MIN).AIR(I/MIN). A 5 48 B 10 68 C 15 95 D 20 105

~ 1.02 1.43 1.54 186

X/D = 0-11 and 1.26,

dB (LIN) 84 B? 93 95

DIFFUSION FLAME

z 80 ,.L i.u cE

,=70

O

w

D

60

50 10 ~

103

10 z

FREQUENCY,

Hz

Fig. 12. N o i s e e m i s s i o n levels.

10 ~

COMBUSTION NOISE FROM TURBULENT DIFFUSION GASEOUS FLAMES

55

conditions provided the density difference between the two jets is taken into consideration as shown in this paper. With the natural gas-air and the He-air jets, good agreement is found for the peak frequency of the radiated sound power based on the theory of isotropic turbulence and the present experimental results.

ACKNOWLEDGEMENT

The authors acknowledge the financial support of the Science Research Council (UK).

REFERENCES 1. S. L. BRAGG, Combustion noise, J. Inst. Fuel, 36 (1963) pp. 12 16. 2. R. D. GIAMMARand A. A. PUTNAM, Noise Generation by Turbulent Flames. A.G.A. Catalog No. 00080 (1971). 3. A. K. GUPTA, N. SYREDand J. M. BEER, Fluctuating Temperature and Pressure Effects on the Noise output of Swirl Burners, 15th Symposium (Intl.) on Combustion, The Combustion Institute (1974) pp. 1367 77. 4. A. K. GUPTA,N. SvgEDand J. M. BEER,Noise emission from swirl combustors, Applied Acoustics, 9 (April, 1976) pp. 151-63. 5. T.J.B. SMITHand J. K. KILHAM,Noise generation by open turbulent flames, J. Acous. Soc. America, 35(5) (1963) pp. 715-23. 6. W. C. STRAHLEand B. N. SHIVASHANKARA,,4 Rational Correlation of Combustion Noise Results fi'om Open Turbulent Pre-mixed Flames, 15th Symposium (Intl.) on Combustion, The Combustion Institute (1974) pp. 1379 85. 7. M. MUTHUKRISHMAN, W. C. STRAHLE and J. C. HANDLEY, The Effect of Flameholders on Combustion Generated Noise, A.I.A.A. 14th Aerospace Sciences Meeting, Washington, D.C. (January, 1976) Paper No. 76-39. 8. A. K. GUPTA, J. SWITHENBANK and G. ROCK, Combustor Modelling: Comparison of Some Theoretical and Experimental Results, 2nd European Symposium on Combustion, The Combustion Institute, France (1975) pp. 763 8. 9. R. G~3NTHER, MethodsJbr Turbulence Measurements in Flames. A.I.A.A. 14th Aerospace Sciences Meeting, Washington, D.C. (January, 1976) Paper No. 76-36. 10. B. LENZE, Turbulenzverhalten und Ungemischthecht Von Strahlen und Strahlflammen. Doktorlngenieurs dissertation, Karlsruhe, 1971. 11. 1. EaRAHIMI, Combustion and Flame (1967) p. 255. 12. A. K. GUPTA and J. M. BEER, Departmental Report No. CEFT/102, Department of Chemical Engineering and Fuel Tech., Shet~eld University (October, 1975). 13. J. O. HINZE, Turbulence, McGraw-Hill Book Co. (1959) p. 184. 14. A. K. GuPra, J. SWIT.ENaANK and J. M. BEER, 16th Symposium (intl.) on Combustion, MIT, Cambridge, Massachusetts (August, 1976) pp. 79-.91. 15. F. K. OWEN, Laser Velocimeter Measurements t~! a Con~ned Turbulent Di[Jusion Flame Burner, A.I.A.A. 14th Aerospace Sciences Meeting, Washington, D.C., Paper No, 76-33 (January, 1976). 16. F. DURST, private communications (1976).