Sound generation by turbulent pre-mixed flames

SOUND GENERATION BY TURBULENT PRE-M|XED FLAMES

J. A. CHILLERY Acoustics Group, Universityof Salford, Salford. Lancs (Great Britain) (Received: 12 December, 1974)

SUMMA R Y

Previous work l° on noise generation by small open burner stab!iised turbulent pre-mixed flames, performed elsewhere, showed that such aflame may be represented acoustically as an assembly Of monopole sound sources distributed throughout the reaction zone of (he flame. Over a limited frequency range a.strong correlation was observed between the far field sound pressure and the first time derivative of the heat release rate in the flame: The letter quantity was measured by monitoring the emission intensity of free radicals present in the reaction zone of the flames. The present work was [nte~l~d to confirm these observation~ for a single ethylene/air flame and to extend she investigation to a double flame system. Good qualitative and quantitative correlation was observed over a iimited frequeacy range for both single and double/lames. The frequencies beyond which the correlation deteriorated were identified accd,for the double flame, were found t~ depend on the spatial separation of the flames.

INTRODUCTION

Efforts to increase the intensity of combustion in modern appliances have produced combustors which are highly efficient but, unfortunately, also have high acoustic outputs. Until recently, little attention has been paid to the noise output from such devices because the fraction of energy radiated as sound is so small that the overall efficiency is left relatively unimpaired. However, increasing pressure of public opinion for the suppression of noise has prompted recent research into the mechanisms by which combustors generate noise. The fact that flames could generate noise was first reported nearly two centuries ago by Higgins I and the action of the 'singing flame' was first explained by Lord 281

Applied Acoustics (8) (1975)--© Applied Science Publishers Ltd, England, 1975

Printed in Great Britain

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Rayleigh 2 over 80 years later. The topic then remained of little but academic interest until World War II when the advent of the 'buzz bomb' and the post war interest in rockets prompted intensive research into oscillatory combustion. In noise generating systems of this type resonances between fluctuations of the flame and natural modes of vibration of the containing vessel give rise to sounds of definite discrete frequencies. A considerable amount of research has been carried out aimed at eliminating this kind of phenomenon, initially because of its sometimes destructive properties, but more lately because the high sound intensities generated constitute health hazards and are frequently the subject of complaints of annoyance.

COMBUSTION NOISE

There also exists another class of flame-generated noise typified by the ordinary Bunsen burner. This, non-resonating combustion noise, has received scant attention in the literature until quite recently. It is similar in many respects to jet noise. It does not require a solid boundary with which to resonate and has no precise freq.'hey peaks in a wide band spectrum that extends over ten or more octaves. The first significant publication dealing with this topic was that by Bragg 3 who, using the 'wrinkled flame' concept of turbulent combustion, developed an ingenious method of calculating the sound power output from a combustion zone. Bragg's treatment was general enough to appertain to both pre-mixed and diffusion flames but, owing to a lack of concomitant data, he was not able to verify his conclusions. This work was followed closely by that of Kilham and Smith* who produced the first reliable data on broad spectrum noise generated by flames. Investigating tunnel-burner noise, KJlham and Smith began by examining, under approximately free field conditions, the sound fields generated by turbulent pre-mixed flames stabilised on open straight pipes. They described the intensity, frequency spectra and directionality of the sound fields and the variation of these parameters with both flow velocity and burning velocity of the fuel/air mixture as well as with burner geometry. An expression relating the acoustic power radiated to the various flow parameters was presentect and shown to be characteristic of a monopole source. This observation and considerations of the mechanism of congestion in a turbulent region led Kilham and Smith to postulate that flames burning under such conditions could be represented a s a random distribution o f bu~ning elements of the combustion m/,~ture, each generating an increased volume of heated gases. According to this model, any. turbulent flame may he repnesented acoustically b y a collection of simple monopole sources of varying s ~ h s and frequencies distributed throughout the turbot/on zone. The publication of these results by Kilham and Smith inspired a new body of

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research into flame-generated noise. In particular, a classic series of experiments was carried out at Shell Research Centre, Thornton, by various co-workers. The first report of this work by. Thomas and Williams 5 investigated noise generation by a spherically expanding flame and conclusively demonstrated that such a flame acted as a monopole sound source. A Schlieren technique was employed to monitor movements of an expanding flame front originating from a spark ignition source centrally mounted within a spherical volume of combustible mixture. An expression was derived relating sound pressure to these measurements and the fuel parameters. Close agreement was obtained between predicted and measured values of the instantaneous sound pressure in the far sound field of such a system. A second paper, by Hurle e t a / . ° extended the relationships established by Thomas and Williams to the more complex system of a turbulent pre-mixed flame. Thomas and Williams had pointed out that the pressure in the radiated sound wave arising from changes in the evolution of volume of gases in the flame was proportional to the rate of change of the rate of chemical reaction within the combustion zone. It was recognised by Hurle et al. that the rate of generation of free radicals within the reaction zone was also fundamentally related to the rate of combustion. An optical technique, relying on observations of changes in intensity of emission of free radicals whose existence was known to be almost exclusively confined to the inner reaction zone of a pre-mixed flame, was accordingly developed using a photomultiplier as a detector. Suitable modification of the original expression relating pressure and volume generation was performed and, using the resulting equations, very good agreement, both qualitative and quantitative, was obtained between measured values of far field sound pressure level and those predicted from measurements of the time differential of the radical emission intensity.

INTERACTION OF FLAMES WITH APPLIED FIELDS

In recent years the interaction of electromagnetic fields with flame gases has achieved considerable prominence as a topic of research. This rapid growth is due largely to the various practical consequences and potential applications, the most notable being the possibility of direct generation of electricity (Lawton and Weinberg~). Several r ~ t publications have described methods of inducing premixed flames to generate, sound waves. Here it has been found possible, after the manner of Burchard 8 and Babcock et al., 9 to cause a pre-mixed flame to emit pure sinusoids under the influence of an oscillating electric field. It has also been recorded by various workers thatlflames respond to acoustic waves, either present in the unburnt gas flow or directed on to the reaction zone externally. It was proposed here that a combination of these two field effects with the optical technique developed by Hurle et al. would provide useful information about the behaviour of flames and, under certain conditions, it might prove

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possible to attenuate the acoustic output from a turbulent flame by phase inverting the output from a photomultiplier focused on that flame and an adjacent laminar flame andimposing the resuitingsignal on electrodes inserted in the laminar flame. The s o l d field resulting from such a system could b e expected to he strongly dipole over a restrictedbandwidthand, as such, less effteieat as a radiator of sound. The first stage in the investigation has been to corroborate the findings of Hurle et al. for single turbulent flames and to extend the technique to studies of two juxtaposed flames. In this paper, measurements of sound pressure level, frequency spectra and field contours of the noise generated by the double flame configuration are reported and comparisons drawn between these and corresponding measurements for single flames. The variations produced in such measurements by changes in certain basic flame~parameters have also been recorded. EXPERIMENTAL DETAILS

Experimentally~ the research was strongly influenced, in the initial stages reported here, by the work of Hurle et aL A schematic diagram of the arrangement is shown in Fig. 1. Pre-mixed ethylene/air flames were stabilised on long, straight seamless tubes. Hydrogen retention flames were supplied to the main flames when at high flow velocities by surrounding the main burner with a coaxial tube, there being a t mm gap between the two tubes. The tube projected horizontally from one wall of a chamber built from 12 panels, each 8 ft by 4 ft by2¼ in, formed into a cube of internal dimensions 8 ft by 8 ft by 8 ft. The inside of the chamber was lined with foam wedges 14 in long. It was demonstrated that this chamber provided a reasonably sound-proof, anechoic environment down to as low a frequency as 200 Hz. The burner tubes were 6 ft long and mixing of the constituents of the fuel took place some considerable distance upstream. It was shown that even at the highest flow rates used, the noise existing in the unlit gas flow was barely distinguishable from ambient even at high frequencies. The fuel/air ratio and the volume flow of the mixture was controlled and monitored using valves and meters. Both ethylene and air were supplied from cylinders. Acoustic information was extracted using a standard Bruel & Kjaer type 4133 ½ in condenser microphone attached to a gantry system controlled by strings and pulleys from outside the chamber. Assuming the sound fields generated to be axially symmetric, it was only essential to make measurements in that half of a plane containing the burner axis and bounded on one side by this axis. This allowed the microphone to be attached to the gantry at a fixed height approximately halfway between the floor and ceiling of the chamber. The plane traced out by movements of the microphone was then defined as a reference•plane, and both the burner axis and the principal axis of the optical system were adjusted to be in that plane.

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When destined for presentation as oscilloscope traces the signal was taken to a B & K type 2607 microphone amplifier with auxiliary variable bandwidth Dawe type 1462A filters. This was then taken straight to one vertical amplifier of a Tektronix type 565 dual-beam oscilloscope. In addition to acting as a normal microphone amplifier, the type 2607 also gives a dc output proportional to the sound pressure level corresponding to the input. This facility was used when plotting contours of the sound field surrounding the flame. Using a data logging system, the de signal could be transferred to punch tape suitably coded for the University KDF9 computer. By tracking the microphone the broad band sound pressure level was measured at the intersection points of a 5 cm grid network covering an area 1.25 m by 0.6 m. These readings were transferred automatically on to the punch tape and, after suitable manipulation, were submitted as data for the graph plotting programme available in the computer library. Typical results are shown in Figs. 2, 3, 4 and 5. Frequency analyses were carried out using a B & K type 1612 ~ octave filter coupled to the microphone amplifier and driven by a B & K type 2305 level recorder. Typical results are shown in Figs. 6 and 7. If necessary, narrow band analyses were performed using a B & K type 2607 narrow band analyser driven by the level recorder. The optical instrumentation was mounted on a 2 m optical bench which projected slightly into the chamber through one of a pair of access holes cut in opposing walls. An EMI type 9634QR photomultiplier viewed an image of the flame, focused on to its cathode by a 6 in condenser lens, through a 25 A bandwidth interference filter centred on 5165 A. Dynode voltages were supplied from a Hewlett-Packard type 6516A voltage supply via a suitable chain of zener diodes and resistors. The output was taken to a Tektronix type 3A8 vertical amplifier plugged into the oscilloscope. The type 3A8 contains two operational amplifiers. The first of these, connected as a high impedance input non-inverting amplifier, acted as a buffer stage between the photomuitiplier output and a second Dawe 1462A variable bandwidth filter. An auxiliary buffer was inserted between the output of the 1462A and the second operational amplifier which was connected as a differentiator. In this way the photomultiplier output was easily measured, differentiated, and either presented on the screen o f the oscilloscope or at the output terminals of the second operational amplifier for further analysis.

RESULTS AND DISCUSSION

Frequency spectra The sound fields generated by the double flame configuration were found not to differ in any remarkable way from what would be expected from two separate, but closely adjacent, flames. Typical frequency spectra are shown in Figs. 6 and 7.

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The spectra were of the form to be expected from combustion noise, broad band extending over several octaves. The peak frequency is somewhat indeterminate as is normal but it may be seen to lie in the octave between 500 and 1000 Hz. This is lower than the peak frequency generated by the corresponding single flame as shown in Fig. 6. This, the single flame peak frequency, is not as high as predicted by the empirical relationship presented by Briffa e t al. 1 o For ethylene/air flames, Briffa e t a l . found the wavelength of the peak frequency to be approximately fifty Sound pressure level (dB)(orbitrory)

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times the burner diameter. For ¼ in diameter burners this gives a frequency slightly in excess of 1000 Hz; Kilham and Smith suggested multiplying by a factor between 70 and 100, dependent on the experimental conditions, which would fit more closely with the data recorded here. Figure 6 shows frequency spectra recorded at a distance of 25-5 cm from the centre of the burner mouth for the single flame and at an 'equal distance from a

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corresponding point midway between the two axes for the double flame. It may be seen that the single flame contoJns more high frequency components than the double flame. The overall sound pressure levels recorded in these two situations gave the SPL for the double flame, approximately 3 d~ higher than that for the single flame; a result to be expected from simple acotuttic theory. The contour plots shown in Figs. 4 and 5 indicate that whilst the overall SPL contours for the double flame remain monopolar, the contours at the higher frequencies, in this case an octave at 8 kHz, exhibit a certain amount Sound pressure level (dB) (orbitrory) 45 20

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of directionality. These characteristics, less high frequency content and high frequency directionality, may l ~ r ~ be explained by considering the burner separation, in this case 2 cm from axis to axis. A simple calculation shows that at approximately 4000 Hz, the quarter-wave length of emitted sound wii| be of the order of 2 cm. If the double flame ~nfiguration were considered to be a single monopole source, then this frequency would be critical. Above ~ H z it would

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be expected that the sound intensity would decrease and deviations from monopolarity would be observed. Thus, the lower high frequency content in the double flame noise, together with the observed monopolarity at intermediate frequencies and the lack of this at high frequencies, would seem to indicate that two separate but identical noisy pre-mixed flames may be considered acoustically as a single monopole source over a frequency bandwidth specified by the size and separation of the flames. Figure 7 shows the variation in noise output from the double flame with fuel flow rate. These results are similar to those obtained from experiments on single flames. The SPL increases with flow rate in the same way but with no clearly discernible increase in peak frequency. Field contours

Figures 2, 3, 4 and 5 are half-field contour plots generated by the method of measurement described above. When calibrating the anechoic chamber and in the early stages of experimentation it was found that the fields being plotted were symmetric about the burner axis and thus only half-field plots were necessary. This saved considerable labour. As stated above, measurements were made at the intersections of a 5 cm grid network covering an area 1.25 m by 0.6 m, entailing in excess of 300 separate measurements of SPL. This array of data was then presented to the computer which was instructed to draw contours through specified levels. These levels were selected to give a sensitivity compatible with the limits of measurement accuracy and without confusing the end product with unncessary contours. The results are quite satisfactory, giving a good visual representation of the sound field existing within the anechoic chamber. Figures 2 and 3 show contour plots drawn from data gained when measuring single flame noise, firstly over a broad band and, secondly, over an octave centred at 8 kHz. Comparing these with Figs. 4 and 5, which are the corresponding measurements for a double flame, the effect on the high frequency condition may clearly be seen. The results displayed here were gained using a burner axis separation of 2 cm. For larger separations the critical frequency, defined by the quarterwave length limit, decreased. However, for the frequency bandwidth of interest here, 25-2500 Hz, the sound field generated by the double flame configuration was found to be monopole. Optical measurements

Figures 8 and 9 show the results of measurements of changes in emission intensity of the C--C radical within the flame reaction zone. Hurle et al. showed that, over a limited frequency bandwidth, the acoustic signal generated by a turbulent pre-mixed flame was directly proportional to the time differential of the variations in emission intensity of such radicals. Mathematically this was expressed as

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1). k. d l / d t (1) where: ro = density of medium of propagation; d = distance between source and acoustic measurement point; E = volume expansion ratio and k = constant determined experimentaily~ The constant k was determined by observing dc changes in emission intensity with increasing flow rate. AS shown by Hurle e t al. : p(t) = (ro/4 . pi" d) . (E -

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therefore, k could be determined by finding the gradient of a straight line plot of volume flow rate of unburnt fuelagainst corresponding values of emission intensity. Although not shown here, the results of similar experiments carried out here on both single and:double flames were much the same as those obtained by Hurle e t ai. Up to the maximum Reynolds Number available, 3600, the relationship was a straight line with the same gradient whether in the laminar, transition or turbulent states.

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Fig. 9(a). Sinile ethylene/air stabilised on ~ i n diametff burner. Meuurmmmt bandwidth: 23-3150 Hz. Top trace: optical signal. Bottom trace: acoustic signal.

Fig. 9(b). Two ¼ in_diameter burners with axis ~paration of 2 cm. Bandwidth: 25-3150 Hz. Yop trace: optical signal. Bottom ttage: acoustic signal.

Fig. 9(c). Two ¼ in diameter burners with separation of 7 cm. Bandwidth: 25-3150 Hz. Top trace: optical signal. Bottom trace: acoustic signal.

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is shown with the corresponding acoustic analysis in Fig. 8. As may be seen, the two signals are very similar; only at high frequencies, where the differentiated electronic noise is predominant, do they differ. The similarity between the two signals is also clearly shown in Fig. 9(b) which shows the separation ar be clearly seen a~ Lcing proof

The techniques d e v e ~ ~ Helle era/i :have ~ e s ~ : t o investigate sound fields generated by both single and double flames. For the single flame the results of Hurle et al. have beencorroborated and ~ frequency range over which correlation between the acoustic signal and the differentiated emission intensity exists has been shown to ve length fre, The tec nt pre-mixed he single flan :e, but over a ~-d by both s3 ue which has be investigatq

I. B. HIOO,NS, Nicholsona Journal, I(130)(1802)~

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2. LORDRAYLEIGH,Theory of sound, Vol. II (1894) Dover, NY1945, p. 224. 3. S. L. BR^cg3, Combustion noise, J. Inst. Fuel, 26 (1963) p. 12. 4. J. K. FdLHAMand J. B. SMITH,Noise generation by open turbulent flames, JASA, 35 (1963) p. 715. 5. A. THOMASand G. T. WILL~LS, Flame noise: sound emission from spark ignited bubbles of combustible gas, Proc. Roy. Soc., A294 (1961) p. 449. 6. I. R. HUXLE,R. B. l~lcE and T. M. SUODEN,Optical studies of the structure of and generation of noise in turbulent flames, Proc. Roy. Sot., A303 (1968) pp. 409-27. 7. J. LAW'tONand F. J. WEINSERO,Electrical aspects of combustion, Clarendon Press, Oxford, 1967. 8. J. K. BURCHARD,Preliminary investigation of the electrothermal loudspeaker, Combustion and Flame, 13 (1969) p. 82. 9. W. R. BAIDCOCK,K. L. BAKERand A. G. CATrANEO,Musical flames, Nature, 216 (1967) p. 676. 10. F. E. J. BRIFFA,C. J. CLARKand G. T. WILL,MS,Combustion noise: 'Noise and the industrial use of fuel,' Syrup. of the Inst. of Fuel, Univ. of Southampton, 1972.