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Experimental investigation of noise generated by large turbulent diffusion flames
EXPERIMENTAL INVESTIGATION OF NOISE G E N E R A T E D BY LARGE T U R B U L E N T D I F F U S I O N FLAMES C. BERTRAND* AND S. MICHELFELDER International Flame Research Foundation, IJmuiden, The Netherlands
The sound power of combustion noise emitted by a natural gas fired experimental burner (1-3 MW) was measured under free-field conditions and compared with the noise emitted inside a refractory furnace. Measurements under free-field conditions show that the noise power output is proportional to'the average gas velocity to the 2.8 power and to the burner throat diameter to the 5.6 power. The peak frequency of the combustion noise ranges between 125 Hz and 300 Hz depending on the swirl number but not on the firing rate. Measurements inside the furnace show the predominance of resonance phenomena at low frequencies. The actual combustion noise could be evaluated by the sound pressure level measured in the 250 Hz or 500 Hz octave band. The increase of combustion noise with firing rate is greater than under free-field conditions, which can be attributed to an increase of the flame turbulence rather than to an increase of the gas temperature. Preheating the combustion air greatly increases the sound power output as well as the peak frequency. A study of noise emitted by three industrial burners inside the furnace confirmed these results, notwithstanding the very different flame shapes obtained. Oil flames were found to be less noisy than natural gas flames when the combustion air was not preheated, whereas this difference disappeared on preheating the combustion air or using a pregasification burner.
Introduction Experimental studies on noise generation processes in flames have, to date, been performed mainly on premixed flames and small size burners. 1-3. For such systems theoretical laws have been derived to account for the f u n d a m e n t a l parameters governing the comb u s t i o n noise. Using the wrinkled flame front model due to Karlovitz,4 Bragg 5 first described the t u r b u l e n t flame zone as a collection of small volumes of combustible mixture through which a laminar flame propagates. He derived a theoretical relationship in which the sound power level emitted is proportional to the laminar flame velocity and to the third power of the gas velocity. His expression gave a reasonable order of m a g n i t u d e for the value of the sound power level, b u t his analysis failed
*Presently at C.N.R.S., Centre de Recherches sur la Chimie de la Combustion et des Hautes Temp6ratures, Orl6ans (France).
b y predicting a high sensitivity of the peak frequency of combustion noise to the gas velocity. Strahle 6 also assumed this monopole source character (demonstrated by Hurle et ah v) of the combustion noise emitted by t u r b u l e n t premixed flames, and gave a more rigol'ous derivation based on the inhomogeneous wave equation of Lighthill's theory. 8 With a few assumptions, Bragg's equation could be derived from this theory, but the author 6 preferred a distributed reaction model of the flame a n d derived a theoretical relationship which can be summarized as: p oc Ue u ? D 3 Thus, a striking difference between combustion noise and aerodynamically generated noise is that the former theoretically varies with the third (Bragg) or second (Strahle) power of the jet velocity, whereas the latter varies with the eighth power, s This has been confirmed experimentally, although in some cases the combustion noise has been found
1757
1758
TURBULENT FLAMES AND COMBUSTION
to vary with the fourth power of the jet velocity. 2 More recently, Strahle and Shivashankara 9 have established experimental scaling laws based on extensive measurements on p r e m i x e d turbulent flames in a simple configuration (Bunsen-type burner), and have given theoretical support to these empirical relationships. The n u m b e r of studies on turbulent diffusion flames is more limited. Experimentally, Giammar and Putnam 1o have given extensive data for a particular burner configuration (octopus burner) a n d observed the noise o u t p u t to vary with the square of the firing rate. T h e influence of b u r n e r design on noise e m i s s i o n has been studied b y Smith and Lowes. u Theoretically, Strahle 12 has given an "order of magnitude" relationship which takes into account the t u r b u l e n c e parameters of the flow (turbulence intensity, integral scale of t u r b u lence). More experimental noise data are r e q u i r e d to determine the correlations between the fundamental flame parameters and noise emission from large turbulent d i f f u s i o n flames. An investigation has therefore b e e n undertaken to s u p p l y such data from measurements on industrial size burners. In addition, an attempt has b e e n made to compare the noise emitted by a b u r n e r under free-field conditions and the noise emitted by the same burner fired
in a furnace where resonance p h e n o m e n a occur, which is a p r o b l e m often encountered in industrial practice. All the measurements were performed at thermal inputs ranging from 1 to 4 MW.
Experimental Apparatus Sound pressure levels were measured in two different environments: free-field conditions and furnace conditions. Under free-field conditions the burner was fired upward in the open air from a position five meters above ground level. T h e s o u n d pressure levels were measured by m o v i n g a General Radio condenser microphone (1 inch) over an imaginary sphere of three-meter radius, centered on the b u r n e r mouth. This radius (seventeen burner throat diameters) was sufficiently small for the measurements to be easily accomplished, and sufficiently large for the attenuation law of sound pressure level to be approximately followed (pressure proportional to the reciprocal distance). A schematic diagram of the experimental burner used is given in Fig. 1. The throat diameter could be changed from 176 m m to 116 ram. The swirl n u m b e r of the c o m b u s t i o n air was defined as 13 :
bloc~ks
swirl generator
J
/ w \ x x \ x \
i fuel ---g
.~ Ill
- -
) /
/
/
/
wQler oir
0 I
50 cm I
FIG. 1. Experimental burner and flow pattern of the flames (measured inside the furnace); full lines: S = 0, dotted lines: S = 1.7.
TURBULENT DIFFUSION FLAME NOISE axial flux of angular m o m e n t u m S=
f D/2 G,= G,~ =
_ 2G,
axial flux of linear m o m e n t u m x radius of burner throat
with
~'0
~ 9,"
2~rpWUrZdr
D12 2wp U2 rdr 0
a n d could be varied c o n t i n u o u s l y from S = 0 to S = 1.7. The gas was injected radially through a 16-hole injector placed at the b u r n e r
1759
DG~
throat. This geometry was chosen because it produces very stable short flames for any value of S. This type of flame limits the influence of the s u r r o u n d i n g atmosphere on the combustion parameters and avoids u n w a n t e d noise due to oscillations of the flow pattern between two different configurations. 14 It was also expected that such a diffusion flame would behave more like a premixed flame, since mixing between gas and combustion air occurred very early a n d was quite intense. The same b u r n e r was fired in the experi-
'ii iI !iii,i
air
fuel
1// ///J
I
'ec~ r
-
. . . .
~
i
~
(c0a-Hz0)
L: FIG. 2. Industrial burners: (a) burner A, (b) burner B, (e) burner C
1760
TURBULENT FLAMES AND COMBUSTION
mental furnace of the I F R F . This is a horizontal refractory-lined tunnel furnace of approximately square cross-section, with internal dimensions 2 x 2 x 6.25 m. It was e q u i p p e d with a water-cooled steel hearth. The refractory wall temperature ranged from 800~ to 1100~ d e p e n d i n g on the firing rate. The sound pressure levels inside the furnace were recorded b y means of a 1 / 2 - i n c h condenser m i c r o p h o n e protected b y a watercooled brass cap. The m i c r o p h o n e h e a d " v i e w e d " the furnace through a c y l i n d r i c a l hole, 0.5 mm d i a m e t e r and 1 cm long. This probe could be m o v e d to any location inside the furnace a n d was suitable for m e a s u r i n g noise frequency spectra b e l o w 1000 Hz, w h i c h in all cases was a b o v e the peak f r e q u e n c y of the combustion roar (<500 Hz). Fig. 1 shows the forward and reverse flow patterns measured in the furnace for S = 0 a n d S = 1.7. The good stabilization of the flame is due to the internal recirculation zone w h i c h exists even for S = 0. Finally, three commercial burners were fired in the same furnace: - - B u r n e r A (Fig. 2a). A muffle burner w i t h axial fuel injection (gas or oil). This b u r n e r was used with c o m b u s t i o n air preheated to 450~
- - B u r n e r B (Fig. 2b). A swirl burner with axial fuel injection (gas or oil). - - B u r n e r C (Fig. 2c). A self-recirculating gasification b u r n e r (gas or oil), based on the staged c o m b u s t i o n principle. The sound pressure levels were measured with the opening of the m i c r o p h o n e probe p e r p e n dicular to the axis of the burner and for each position the overall ( d B F ) , A - w e i g h t e d (dBA), and octave-band s o u n d pressure levels were recorded. A d d i t i o n a l l y , 1 / 1 0 octave b a n d frequency analyses were performed to characterize the resonance p h e n o m e n a occurring in the furnace and the frequency spectra of the noise emitted b y the burner under free-field conditions.
Experimental Results
Free-field Conditions The intensity distribution of the noise emitted was investigated by performing octave band analyses from 24 positions on a circle surrounding the b u r n e r and containing the burner axis. F i g u r e 3 shows the behavior of the different octave b a n d sound pressure levels
i~
clB
FIC. 3. Directivity diagram of the sound pressure level. Swirl number S = 1.7; Throat diameter = 176 mm; Load = 2 MW
TURBULENT DIFFUSION FLAME NOISE
1761
flames. 1~ The influence of the swirl number on the combustion noise can be seen more clearly by c o m p a r i n g the 1 / 1 0 octave band analyses of the noise emitted by a n o n s w i r l i n g flame and by a h i g h l y swirling flame (Fig. 5). Increasing the swirl n u m b e r produces two different effects:
with respect to m e a s u r i n g position. This diagram was obtained for a h i g h l y turbulent flame (S = 1.7) at a load of 2 MW. No p r o n o u n c e d bias in the spatial d i s t r i b u t i o n is observed at frequencies below 500 Hz, except for a shift of 2 to 3 dB in the direction of the gas flow, w h i c h can be explained b y convection and refraction effects, 3,12 A slight directional bias, with two maxima'ton either side of the burner axis, appears for the 500 Hz octave band, and this tendency increases with the frequency so that the 4000 Hz octave b a n d exhibits highly nonuniform distribution with two m i n i m a on the burner axis and two maxima situated approximately in the p l a n e of the burner mouth. A m o n o p o l e source theory of the combustion noise can thus r e a s o n a b l y account for our experimental results, since in the region of the peak frequency (250 Hz octave band) the sound pressure levels u n d e r free-field conditions exhibit no d e p e n d e n c e on spatial direction. At higher frequencies the multiple sources become more important and they pred o m i n a t e at frequencies above 2000 Hz, b u t their contribution to the overall sound p o w e r remains weak. The variation of the s o u n d power level with the load, i.e. with the m e a n velocity of the gases, was then investigated for different values of the swirl number. The sound p o w e r yields are plotted on Fig. 4 against the swirl n u m b e r with the load as a parameter. The curves exhibit inflection points at swirl numbers between 0.5 a n d 1, and the sound p o w e r yield is found to vary with load to the 1.8 power (average). This gives a variation of the sound power with average gas velocity to the 2.8 power w h i c h is very close to the value of 2.67 given by Strahle and Shivashankara in their recent work on t u r b u l e n t premixed
- - a n increase in the m a x i m u m value of the noise emitted; - - a shift of the peak frequency to the high frequencies so that, in the range 10-150 Hz, the sound power emitted b y the n o n s w i r l i n g flame is higher than that emitted by the h i g h l y swirling flame. T h e same figure also gives the frequency spectrum p r o d u c e d by the air flow (curve c) and a comparison with curve (a) shows that the noise of the isothermal air jet is much lower than the c o m b u s t i o n noise (20 dB). However, the frequency distribution is very similar, with the exception of a large peak at 160 Hz. This peak is characteristic of highly swirling jets, and can be attributed to the precessing vortex core (P.V.C.) described by Be6r et a l ) 3 The P.V.C. is either amplified or d a m p e d by combustion, according to the value of the f u e l / a i r ratio. It is a m p l i f i e d only in very lean mixtures. In our conditions (10% excess air), the peak is completely damped, w h i c h is in agreement with previous experimental observations. It must be noted that the frequency spectra in Fig. 5 were recorded u s i n g constant percentage b a n d w i d t h filters. A constant correction of - 3 riB/octave has to be a p p l i e d to obtain the equivalent of an analysis performed with constant b a n d w i d t h filters. T h e two maxima of the frequency spectra of combustion noise would then a p p e a r not at 220 Hz
1,0 ,,?.1o 6 0,75-
jJ
0.5
0,25- -7--
0
/o....-- ---o
o
--~-~"----'-'0
jf
J~ --~c
I MW
~
o,5
j
e"
1,o
e
1,5
S
FIc. 4. Sound power yield versus swirl number in free-field conditions.
2.0
1762
TURBULENT FLAMES AND COMBUSTION 120 dB 110
100'
/-
/ 50
100
200
300
500
frequency (Hz)
1000
FIG. 5. Narrow band frequency spectra of cold jet and flame noise under free-field conditions: (a) combustion noise, 2 MW, S = 1.7; (b) combustion noise, 2 MW, S = 0; (d) cold jet noise, S = 1.7
and 400 Hz, but at 125 Hz and 300 Hz. Strahle and Shivashankara 9 have given two experimental equations to describe the s o u n d power and the peak frequency of the combustion noise emitted by premixed g a s / a i r flames:
turbulence intensity a n d the integral scale of turbulence:
P=K
Po - P l t 2 D2 U2 ~ o F ] i t2 "r u?
P = 3.7 10 -6 x U 267 x uf lsa x F 0.4 x D "2"r8 (1)
(3)
fc = 2.3 x U 0"I8 X U? '88 X F -xm X D -~
(2)
Introducing typical values corresponding to our 2 MW flame in these equations (U = 100 ft/s, uf = 1.2 f t / s 16, F = 0.07 and D = 0.59 ft) gives a predicted peak frequency of i 6 5 Hz, and a predicted sound power yield of 4 x 10 7. This peak frequency is in agreement with our experimental values, and the s o u n d power yield is lower by a factor of eight than the sound power yield measured for the n o n swirling flame. This comparison is satisfactory in view of the very different conditions, but our results show the need for scaling laws to d e p e n d on turbulence parameters, since we have seen that both peak frequency and sound power depend on the swirl n u m b e r , and c o n s e q u e n t l y on the t u r b u l e n c e intensity. Strahle 12 has derived an equation for jet diffusion flames, taking into account the
If the integral scale of turbulence is proportional to the b u r n e r throat diameter, P b e c o m e s proportional to the product D 4 U 2. This equation suggests that c h a n g i n g the burner diameter at a given load a n d at a given swirl n u m b e r should not cause any change in the s o u n d power emitted. To verify whether such a result could be applied in our case, we varied the burner throat diameter from 116 mm to 176 mm for a given load (1.5 MW) and fixed swirl (S = 1.0). The results are summarized in Fig. 6 and Table I. The sound pressure level remains constant, and is consistent with P cc D 2x U x where the value of x has b e e n determined as 2.8. It must be kept in mind, of course, that the high dependence of P on D also accounts for the dependence of i t on the burner diameter. The shift to higher frequencies for smaller diameters shown on Fig. 6 can also be explained if we assume that the integral scale of t u r b u -
TURBULENT DIFFUSION FLAME NOISE
1763
110
%
dB
p
100
/,w,
90.
80.
70
60.
50. octave - band
centre frequencyIHal ,.
31,5
63
125
,
250
500
,
1000
2000
4000
8000
Fie. 6. Octave band frequency spectra of combustion noise under free-field conditions: (a) 1.5 MW, burner throat diameter D = 176 mm, S = 1.0; (b) 1.5 MW, burner throat diameter D = 116 mm, S = 1.0
lence is proportional to the burner diameter a n d that the peak f r e q u e n c y is given by l2: L = u'/1,. No attempt has been m a d e to verify the theoretical d e p e n d e n c e of the sound p o w e r on laminar b u r n i n g velocity under free-field conditions. The only way to vary this parameter i n d e p e n d e n t l y is to b u r n pure gases whose laminar b u r n i n g velocities with air are significantly different from that of methane (e.g. ethylene, acetylene or hydrogen). This presented too m a n y difficulties for our particular investigation. It is also difficult to comment on the importance of the t u r b u l e n c e intensity in the scaling laws. In some recent work, Baker et al. '5 have shown that the turbulence intensity contours measured in swirling (S = 0.52) a n d n o n s w i r l i n g flames were quite different, a n d that a knowledge of the complete flow pattern was necessary to evaluate a turbulence intensity in the c o m b u s t i o n zone.
Furnace Conditions T h e problem of investigating combustion noise inside furnaces is often complex since the furnace is not an acoustically simple envir o n m e n t and it influences the sound p o w e r directly by a m p l i f y i n g or d a m p i n g some freq u e n c y ranges of the noise emitted by the burner. Another major difference, as compared to outdoor conditions, is that the flame is s u r r o u n d e d by a hot atmosphere, the tempera-
ture of which is d e t e r m i n e d b y flow pattern, c o m b u s t i o n characteristics a n d furnace geometry, One aim of this study was to investigate experimentally how the scaling laws could be a p p l i e d in this particular environment. For this purpose, the sound pressure levels were measured outside and inside the furnace, using the same burner as under free-field conditions. F i g u r e 7 presents the results of frequency analyses performed inside the furnace, at 350 cm (Fig. 7a) and 500 cm (Fig. 7b) from the b u r n e r along the burner axis, and outside the furnace (Fig. 7c). Two high resonance peaks at about 60 Hz and 120 H z represent the fundamental and the first harmonic of the longitudinal mode of resonance (X = 12.5 m equal to double the furnace length, with c = 705 m / s ) . The second a n d the third harmonics, as well as the pressure node of the 60 Hz
TABLE I Variation of sound pressure level with burner throat diameter D (mm)
Sound pressure level (dB)
116 132 152 176
107 108 108 107
1764
TURBULENT FLAMES AND COMBUSTION
120 dB 110
100
er
go
50
A J \A \ 100
(a}
\.
!
200
300
00
500 frequency (Hz)
120 dB 110
100
"
A
90
,% A
80
Flc. 7. Narrow band frequency spectra of noise emitted by the experimental burner fired in the furnace. (a) 350 cm far from burner mouth inside the furnace, (b) 500 cm far from burner mouth inside the furnace, (c) outside the furnace
resonance, appear clearly on Fig. 7a. T h e results of the measurements of octave b a n d s (centered around 63 Hz and 125 Hz) at different distances along the burner axis show the sound pressure level characteristics inside the furnace in more detail (Fig. 8). These resonance effects became less important above 200 Hz, so that octave b a n d measurements at 250 Hz a n d / o r 500 Hz could be considered as representative of the "actual" combustion noise emitted by the burner. It must be pointed out that at a distance of more than 150 cm from the b u r n e r mouth, the fluctuating gas velocities in the furnace are low and cannot cause any interference i n the s o u n d pressure measurements. With the m i c r o p h o n e probe positioned on the burner axis, 500 cm from the burner, the influence of load, swirl, and preheat was investigated. I n order to test the influence of the preheat temperature on the noise power output, the
130 dB 125-
~ 120
63 Hz
// .Y
\
115'
\/
110
0
1
distonce from burner mouth(m
3
Flc. 8. Axial profiles of sound pressure level inside the furnace; full line: 125 Hz octave band, dotted line: 63 Hz octave band
TURBULENT DIFFUSION FLAME NOISE
1765
level with the swirl n u m b e r is similar to that recorded under free-field conditions, whereas the overall sound pressure level stays constant a n d even shows a t e n d e n c y to decrease at very h i g h swirl numbers. This t e n d e n c y is consistent with observations u n d e r free-field conditions which show that increasing the swirl n u m b e r can decrease the s o u n d power at frequencies b e l o w 100 Hz. Plotting the dB values d i v i d e d b y 10 (which vary linearly with the logarithm of sound p o w e r emitted) against the logarithm of the load (proportional to the mean gas velocity) gives a slope of 3.6 for the 250 H z octave b a n d s o u n d pressure level a n d 2.1 for the overall sound pressure level. T h e con> b u s t i o n noise thus appears to increase faster inside the furnace than u n d e r free-field conditions. This observation cannot be explained b y the increase of the flame temperature (approximately 100~ w h i c h can be measured w h e n the load is c h a n g e d from 1 to 3 MW. T r e a t i n g t h i s effect as the result of a theoretical preheat to about 150~ of the combustion air a n d using Eq. (3) w o u l d indeed reduce the 2.8 p o w e r derived under free-field conditions. This effect must then more p r o b a b l y be attributed to a change in the turbulence characteristics of the flow due to a large increase of gas recirculation inside the furnace. Another parameter that it was interesting to vary was the fuel, a n d this was done by c o m p a r i n g natural gas a n d heavy fuel oil flames. Under the same conditions of swirl a n d preheat, the peak of c o m b u s t i o n roar was f o u n d to occur at the same frequency. The
c o m b u s t i o n air of natural gas flames was heated up to 250~ at constant swirl numbers. T w o effects similar to the influence of swirl u n d e r free-field conditions were observed: - - A shift of the peak f r e q u e n c y to the high frequencies, so that a clear maximum of c o m b u s t i o n noise is o b t a i n e d in the 500 Hz octave band; - - A n increase of the m a x i m u m value of the combustion noise equal to 11 dB. Preheating the c o m b u s t i o n air must produce at least three effects: - - A n increase of the average gas velocity; - - A n increase of the laminar flame velocity; 16 - - A change in the b o u n d a r y values of the density, sound speed a n d thermal diffusivity parameters in Eq. (3). Calculations using Eq. (3) show that preheating the air should decrease the overall noise p o w e r output if the t u r b u l e n t characteristics of the flame remain u n c h a n g e d . The results seem to indicate that the t u r b u l e n c e characteristics have changed drastically, a fact which can be related to the shift of the peak frequency to higher values. However, it must be noted that, for a similar effect on the frequency spectrum, increasing the swirl from S = 0 to S = 1.7 increased the overall s o u n d power only b y 4.5 dB under free-field conditions. The load of the gas flames was then varied between 1 a n d 3 M W for different swirl numbers, and the results are given on Fig. 9. The variation of the 250 Hz octave b a n d sound pressure
130
3MW
dB ~
~
"
2M
x
120 0
110X
100
1MW~ ~-~ o---.-o/
90-
0
0,5
1,0
1,5
s
Fro. 9. Sound pressure level inside the furnace versus swirl number; full lines: 250 Hz' octave band, dotted lines: overall level
1766
TURBULENT FLAMES AND COMBUSTION
FIc. 10. Photographs of typical flames of the industrial burners: (a) burner A; fuel oil flame--2.25 MW, 450~ preheat, (b) burner B; fuel oil flame--4 MW, 20~ preheat, (c) burner C; fuel oil flame--2.32 MW, 20~ preheat
only difference was a lower (3 dB) level of noise at the same load in oil flames, w h i c h is probably due to the gasification process acting as a l i m i t i n g factor on the flame b u r n i n g velocity. Indeed the levels of noise emitted
by oil and gas flames are almost the same when the c o m b u s t i o n air is preheated. 11 Finally, the three industrial burners were fired in the furnace (Fig. 10 and 11). I n spite of very different flame shapes, the results are
TURBULENT DIFFUSION FLAME NOISE
The noise emitted b y oil flames without preheat of the c o m b u s t i o n air in b u r n e r C is not significantly different from the noise emitted b y gas flames u n d e r the same conditions. This can be attributed to the pregasification process occurring inside the burner quarl w h i c h tends to suppress the difference between the c o m b u s t i o n characteristics of gas and oil.
130
(a)
dB 120
110'
100
/ 31,5
/
63
f r e q ~ (Hz) 250 5()0 1000
125
130
(b)
dB 120
110
v x
100
\
frequent,(Hz)'~ 31,5
63
125
250
500
1000
130
(c)
dB 120
.o
..
_
1767
- - T h e 250 Hz or 500 Hz (maximum of combustion noise) octave b a n d sound power levels are proportional to the load raised to the 3.7, 4.0 a n d 4.2 powers for burner A b u r n i n g gas at loads between 1.25 and 2.25 M W with air p r e h e a t e d to 450~ for burner A b u r n i n g oil at loads between 1.12 and 2.25 M W with air preheated to 450~ and for burner B b u r n i n g oil at loads between 1 and 4 M W respectively. These results should be c o m p a r e d with the 3.6 power derived for gas flames with the experimental burner. - - V a r y i n g the excess air on burner C from 5% to 30% does not give any significant difference in noise emission, which suggests that the increase (---3 dB ) resulting from the increase of the average gas velocity is compensated b y a decrease of other parameters involved in the scaling law.
_%
Conclusions
~~,\\
100
frequency (Hz) 31,5
3
125
250
5013
1000
FIG. 11. Octave band analyses of the noise emitted by the industrial burners; full lines: gas flames, dotted lines: fuel oil flames. (a) burner A--2.25 MW, 450~ preheat, (b) burner B--3 MW, 20~ (c) burner C--2.32 MW, 20~
very similar to the ones obtained using the experimental burner: - - T h e m a x i m u m of the c o m b u s t i o n roar occurs a r o u n d 250 H z with no preheat (Fig. l l b a n d l l c ) and a r o u n d 500 Hz with preheat (450~ Fig. l l a ) ; - - S w i r l i n g flames exhibit a sharp m a x i m u m of the combustion roar (Fig. l l b ) ; - - T h e combustion noise emitted b y oil flames is significantly lower than that emitted b y gas flames when the c o m b u s t i o n air is not preheated (Fig. l l b ) , whereas no significant difference is noticed at high preheat levels (Fig. l l a ) .
The following conclusions may be drawn from this study: - - I t is valid to use a m o n o p o l e source theory to describe the emission of c o m b u s t i o n noise from large turbulent diffusion flames. (Fig. 3). - - T h e results indicate a variation of the, sound power with the b u r n e r throat diameter to the 5.6 power and the average gas flow velocity to the 2.8 power. - - E x i s t i n g scaling laws do not account for the effect of preheat on c o m b u s t i o n noise, unless the turbulent characteristics of the flame are strongly d e p e n d e n t on this parameter. - - T h e peak frequency of the combustion noise appears to d e p e n d m a i n l y on the turbulent mixing characteristics of the flame.
Nomenclature c D F
sound speed burner diameter fuel-air mass ratio
1768 fc lt P r S U uf u' W "q p
T U R B U L E N T FLAMES AND COMBUSTION peak f r e q u e n c y of c o m b u s t i o n n o i s e integral scale of t u r b u l e n c e sound power radius swirl n u m b e r axial v e l o c i t y laminar flame velocity f l u c t u a t i n g v e l o c i t y (rms) tangential v e l o c i t y thermal diffusivity sound power yield wavelength density turbulence intensity
Subscripts 0 1
v a l u e in c o l d gases v a l u e in b u r n t gases
Acknowledgments The investigation reported in this paper was carried out with financial aid of the Commission of the European Communities (Contracts Nr. 6210-096-006 and 6254-40-6-250). The authors gratefully acknowledge the contribution of their colleagues at the I F B F Research Station in the execution of the experimental work, as well as the cooperation of the burner manufacturers Bloom, Nippon Furnace Kogyo and Hoogovens IJmuiden BV. REFERENCES 1. SMITH, T. J. B., AND KILHAM J. K.: J. Aeoust. Soc. Am, 35, 715 (1963).
2. KOTAKE,S., AND HATrA, K.: Bulletin of J.S.M.E., 8, no. 30, 211 (1965). 3. BRIFFA, F. E. J., CLARK, C. J., AND WILLIAMS, G. T.: J. Inst. Fuel, 207 (May 1973). 4. KaRLOVITZ,B.: J. Chem. Phys., 19, 541 (1951). 5. BRACe, S. L.: J. Inst. Fuel, 12 (January 1963). 6. ST~HLE, W. C.: J. Fluid. Mech., 49, 399 (1971). 7. HUm,E, I. R., PRICE, R. B., SUGBEN T. M., AND THOMAS,A.: Proc. Roy. Soc. (London)A303, 409 (1968). 8. L1GHTHILL,M. J.: Proe. Roy. Soc. (London A211, 564 (1952). 9. ST~rfLE, W. C., ANn SmVASHANra~, B. N.: Fifteenth Symposium (International) on Combustion, p. 1379, The Combustion Institute, Pittsburgh (1974). 10. GIAMMAR,R. D., AND PUTNAM, A. A.: J. Eng. for Power, Trans. ASME ser. A., 92, 159 (1970). 11. LowEs, T. M., HEAP, M. P., AND SMITH, B. R.: La Rivista dei Combustibili 29, No. 5-6, 197 (May-June 1975). 12. STRAHLE, W. C.; Fourteenth Symposium (International) on Combustion, p. 527, The Combustion Institute, Pittsburgh (1973). 13. SYRED, N., GUPTA, A. K., AND BEI~R, J. M.: Fifteenth Symposium (International) on Combustion, p. 587, The Combustion Institute, Pittsburgh (1974). 14. FRICKER, N.: 2nd Members Conference IFRF, IJmuiden (May 1971). 15. BAKER,R. J., HUTCHINSON,P., KHALIL,E. E., AND WHITELAW, J. H.: Fifteenth Symposium (International) on Combustion, p. 553, The Combustion Institute, Pittsburgh (1974). 16. VAN TIGGELEN, A., ET AL.: Oxydations et Combustion, ed. Technip (1968).
COMMENTS Abbott A. Putnam, Battelle Columbus Laboratories, USA. There is a number of interesting features brought out in this study. The first is the verification of the monopole hypothesis for combustion roar, up to 500 Hz. The noise then shifts toward a dipole source type, which might be related to the vortex shedding phenomenon. But could one also explain this result on the basis of back-shielding by the burner body and front-shielding by the flame itself? Be6r has hypothesized a flame shielding effect to explain results on a vortex burner, and Putnam has noted a cancellation by the flame surface at high frequencies, which spreads to lower frequencies as the flame size increases. A second point made was in respect to the differences between the combustion roar of a flame
in an acoustically infinite situation, and in a furnace. It appears that one might associate an amplification speetrmn with a furnace, at a given firing rate, and apply it to any burner for which the aeoustieally open noise spectrum is known, to judge comparative noise pollution effects. One notes that decreasing either the burner noise input or the amplification factor would help. But, from the combustion standpoint, what types of burners might be fired in an acoustically open situation without modifying their burner performance? The problem was also brought up of the relative performance of multiple burners of a small size as compared to one burner of large size. In studying several burner sizes in a series, one cannot "a priori" assume that the burners are properly sealed aeousti-
T U R B U L E N T D I F F U S I O N FLAME NOISE tally. For instance, h o w does one scale the swirl factor, which is dimensional. Some burners are actually scaled on a constant intensity of combustion basis, which changes the prediction rules. But assuming none of these problems are involved, could it be that the multiple flame fronts that are associated with multiple burners could result in multiple noise cancellations and quieter combustion on this basis, rather than on the basis of the relative burner size?
Authors" Reply. It is indeed difficult to determine how much the burner performance is influenced by changing the environment, from which the flame jet entrains, as drastically as one does when the burner is fired in free field conditions instead of into a furnace. Of course it is clear that the burner performance is least affected when the flame length is small. A tunnel b u r n e r delivering completely combusted products will hardly change its performa n t e with the environment. The burner studied produced a short flame with a length of only 1.5 burner diameters, so that the effect of the environinent on its performance was only small. The method suggested for the assessment of noise emission from furnaces would, however, require a very expensive anechoic chamber in order not to confine the method to extremely short flames. Although we believe that the burner scaling criteria used for the case where we compared the noise emission from one 3 M W burner with three 1 M W burners represent a proper scaling also from an acoustic point of view (based on same velocity and swirl number whieh results in an identical total tangential and axial m o m e n t u m flux for the single and the multiple burners), the noise reduction with three 1 MW burners could h e explained also b y the suggested flame shielding effect a n d / o r by a shift of the noise spectrum to higher frequencies. The frequency shift results in less amplification of the low resonance frequencies of the furnace and thus contributes to a reduction in noise emission.
George E. Abouseif, M.I.T., USA. Your observations show that, indeed, the low frequency standing
1769
waves are being fed with energy from the combusting mixture. Did you do any theoretical analysis to understand this coupling mechanism w h i c h should help understanding such problems a n d consequently would help attenuating or maybe avoiding these waves? Our study, here at M.I.T., indicates very clearly that the lower the frequency, the more the waves are amplified. Besides at low frequency and also at high temperatures, the rate of dissipation is m u c h smaller than otherwise. This shows that these low frequency waves are more likely to survive as long as there is unsteady reaction.
Authors'Reply. We have not yet put enough effort into the theoretical analysis of the data to enable the formulation of coupling mechanisms. However, this is obviously the only way which eventually will allow predictions of furnace noise emission to be made and hence would be an invaluable tool for the minimisation of noise pollution. Indeed, we also find that the amplification factor seems to b e larger in the lower frequency ranges, and also that the noise emission seems to increase with furnace temperature.
B. M. Belgaumkar, IAEC (MOB.) P'v~F., LTD., lndia. According to the latest issue of the B&K journal, the damage criteria need looking into. The 90 db criteria can be exceeded in frequencies other than 4 KHz. Therefore one need not unnecessarily exaggerate the noise pollution problem.
Authors" Bepl~t. We are aware of the data on damage criteria for the h u m a n ear published in the latest B&K journal and see that from this~point of view the noise pollution problem should not be overemphasized, however, noise unfortunately does affect other parts of the h u m a n body too and therefore should be kept as low as possible to prevent any unnecessary damage.
The sound power of combustion noise emitted by a natural gas fired experimental burner (1-3 MW) was measured under free-field conditions and compared with the noise emitted inside a refractory furnace. Measurements under free-field conditions show that the noise power output is proportional to'the average gas velocity to the 2.8 power and to the burner throat diameter to the 5.6 power. The peak frequency of the combustion noise ranges between 125 Hz and 300 Hz depending on the swirl number but not on the firing rate. Measurements inside the furnace show the predominance of resonance phenomena at low frequencies. The actual combustion noise could be evaluated by the sound pressure level measured in the 250 Hz or 500 Hz octave band. The increase of combustion noise with firing rate is greater than under free-field conditions, which can be attributed to an increase of the flame turbulence rather than to an increase of the gas temperature. Preheating the combustion air greatly increases the sound power output as well as the peak frequency. A study of noise emitted by three industrial burners inside the furnace confirmed these results, notwithstanding the very different flame shapes obtained. Oil flames were found to be less noisy than natural gas flames when the combustion air was not preheated, whereas this difference disappeared on preheating the combustion air or using a pregasification burner.
Introduction Experimental studies on noise generation processes in flames have, to date, been performed mainly on premixed flames and small size burners. 1-3. For such systems theoretical laws have been derived to account for the f u n d a m e n t a l parameters governing the comb u s t i o n noise. Using the wrinkled flame front model due to Karlovitz,4 Bragg 5 first described the t u r b u l e n t flame zone as a collection of small volumes of combustible mixture through which a laminar flame propagates. He derived a theoretical relationship in which the sound power level emitted is proportional to the laminar flame velocity and to the third power of the gas velocity. His expression gave a reasonable order of m a g n i t u d e for the value of the sound power level, b u t his analysis failed
*Presently at C.N.R.S., Centre de Recherches sur la Chimie de la Combustion et des Hautes Temp6ratures, Orl6ans (France).
b y predicting a high sensitivity of the peak frequency of combustion noise to the gas velocity. Strahle 6 also assumed this monopole source character (demonstrated by Hurle et ah v) of the combustion noise emitted by t u r b u l e n t premixed flames, and gave a more rigol'ous derivation based on the inhomogeneous wave equation of Lighthill's theory. 8 With a few assumptions, Bragg's equation could be derived from this theory, but the author 6 preferred a distributed reaction model of the flame a n d derived a theoretical relationship which can be summarized as: p oc Ue u ? D 3 Thus, a striking difference between combustion noise and aerodynamically generated noise is that the former theoretically varies with the third (Bragg) or second (Strahle) power of the jet velocity, whereas the latter varies with the eighth power, s This has been confirmed experimentally, although in some cases the combustion noise has been found
1757
1758
TURBULENT FLAMES AND COMBUSTION
to vary with the fourth power of the jet velocity. 2 More recently, Strahle and Shivashankara 9 have established experimental scaling laws based on extensive measurements on p r e m i x e d turbulent flames in a simple configuration (Bunsen-type burner), and have given theoretical support to these empirical relationships. The n u m b e r of studies on turbulent diffusion flames is more limited. Experimentally, Giammar and Putnam 1o have given extensive data for a particular burner configuration (octopus burner) a n d observed the noise o u t p u t to vary with the square of the firing rate. T h e influence of b u r n e r design on noise e m i s s i o n has been studied b y Smith and Lowes. u Theoretically, Strahle 12 has given an "order of magnitude" relationship which takes into account the t u r b u l e n c e parameters of the flow (turbulence intensity, integral scale of t u r b u lence). More experimental noise data are r e q u i r e d to determine the correlations between the fundamental flame parameters and noise emission from large turbulent d i f f u s i o n flames. An investigation has therefore b e e n undertaken to s u p p l y such data from measurements on industrial size burners. In addition, an attempt has b e e n made to compare the noise emitted by a b u r n e r under free-field conditions and the noise emitted by the same burner fired
in a furnace where resonance p h e n o m e n a occur, which is a p r o b l e m often encountered in industrial practice. All the measurements were performed at thermal inputs ranging from 1 to 4 MW.
Experimental Apparatus Sound pressure levels were measured in two different environments: free-field conditions and furnace conditions. Under free-field conditions the burner was fired upward in the open air from a position five meters above ground level. T h e s o u n d pressure levels were measured by m o v i n g a General Radio condenser microphone (1 inch) over an imaginary sphere of three-meter radius, centered on the b u r n e r mouth. This radius (seventeen burner throat diameters) was sufficiently small for the measurements to be easily accomplished, and sufficiently large for the attenuation law of sound pressure level to be approximately followed (pressure proportional to the reciprocal distance). A schematic diagram of the experimental burner used is given in Fig. 1. The throat diameter could be changed from 176 m m to 116 ram. The swirl n u m b e r of the c o m b u s t i o n air was defined as 13 :
bloc~ks
swirl generator
J
/ w \ x x \ x \
i fuel ---g
.~ Ill
- -
) /
/
/
/
wQler oir
0 I
50 cm I
FIG. 1. Experimental burner and flow pattern of the flames (measured inside the furnace); full lines: S = 0, dotted lines: S = 1.7.
TURBULENT DIFFUSION FLAME NOISE axial flux of angular m o m e n t u m S=
f D/2 G,= G,~ =
_ 2G,
axial flux of linear m o m e n t u m x radius of burner throat
with
~'0
~ 9,"
2~rpWUrZdr
D12 2wp U2 rdr 0
a n d could be varied c o n t i n u o u s l y from S = 0 to S = 1.7. The gas was injected radially through a 16-hole injector placed at the b u r n e r
1759
DG~
throat. This geometry was chosen because it produces very stable short flames for any value of S. This type of flame limits the influence of the s u r r o u n d i n g atmosphere on the combustion parameters and avoids u n w a n t e d noise due to oscillations of the flow pattern between two different configurations. 14 It was also expected that such a diffusion flame would behave more like a premixed flame, since mixing between gas and combustion air occurred very early a n d was quite intense. The same b u r n e r was fired in the experi-
'ii iI !iii,i
air
fuel
1// ///J
I
'ec~ r
-
. . . .
~
i
~
(c0a-Hz0)
L: FIG. 2. Industrial burners: (a) burner A, (b) burner B, (e) burner C
1760
TURBULENT FLAMES AND COMBUSTION
mental furnace of the I F R F . This is a horizontal refractory-lined tunnel furnace of approximately square cross-section, with internal dimensions 2 x 2 x 6.25 m. It was e q u i p p e d with a water-cooled steel hearth. The refractory wall temperature ranged from 800~ to 1100~ d e p e n d i n g on the firing rate. The sound pressure levels inside the furnace were recorded b y means of a 1 / 2 - i n c h condenser m i c r o p h o n e protected b y a watercooled brass cap. The m i c r o p h o n e h e a d " v i e w e d " the furnace through a c y l i n d r i c a l hole, 0.5 mm d i a m e t e r and 1 cm long. This probe could be m o v e d to any location inside the furnace a n d was suitable for m e a s u r i n g noise frequency spectra b e l o w 1000 Hz, w h i c h in all cases was a b o v e the peak f r e q u e n c y of the combustion roar (<500 Hz). Fig. 1 shows the forward and reverse flow patterns measured in the furnace for S = 0 a n d S = 1.7. The good stabilization of the flame is due to the internal recirculation zone w h i c h exists even for S = 0. Finally, three commercial burners were fired in the same furnace: - - B u r n e r A (Fig. 2a). A muffle burner w i t h axial fuel injection (gas or oil). This b u r n e r was used with c o m b u s t i o n air preheated to 450~
- - B u r n e r B (Fig. 2b). A swirl burner with axial fuel injection (gas or oil). - - B u r n e r C (Fig. 2c). A self-recirculating gasification b u r n e r (gas or oil), based on the staged c o m b u s t i o n principle. The sound pressure levels were measured with the opening of the m i c r o p h o n e probe p e r p e n dicular to the axis of the burner and for each position the overall ( d B F ) , A - w e i g h t e d (dBA), and octave-band s o u n d pressure levels were recorded. A d d i t i o n a l l y , 1 / 1 0 octave b a n d frequency analyses were performed to characterize the resonance p h e n o m e n a occurring in the furnace and the frequency spectra of the noise emitted b y the burner under free-field conditions.
Experimental Results
Free-field Conditions The intensity distribution of the noise emitted was investigated by performing octave band analyses from 24 positions on a circle surrounding the b u r n e r and containing the burner axis. F i g u r e 3 shows the behavior of the different octave b a n d sound pressure levels
i~
clB
FIC. 3. Directivity diagram of the sound pressure level. Swirl number S = 1.7; Throat diameter = 176 mm; Load = 2 MW
TURBULENT DIFFUSION FLAME NOISE
1761
flames. 1~ The influence of the swirl number on the combustion noise can be seen more clearly by c o m p a r i n g the 1 / 1 0 octave band analyses of the noise emitted by a n o n s w i r l i n g flame and by a h i g h l y swirling flame (Fig. 5). Increasing the swirl n u m b e r produces two different effects:
with respect to m e a s u r i n g position. This diagram was obtained for a h i g h l y turbulent flame (S = 1.7) at a load of 2 MW. No p r o n o u n c e d bias in the spatial d i s t r i b u t i o n is observed at frequencies below 500 Hz, except for a shift of 2 to 3 dB in the direction of the gas flow, w h i c h can be explained b y convection and refraction effects, 3,12 A slight directional bias, with two maxima'ton either side of the burner axis, appears for the 500 Hz octave band, and this tendency increases with the frequency so that the 4000 Hz octave b a n d exhibits highly nonuniform distribution with two m i n i m a on the burner axis and two maxima situated approximately in the p l a n e of the burner mouth. A m o n o p o l e source theory of the combustion noise can thus r e a s o n a b l y account for our experimental results, since in the region of the peak frequency (250 Hz octave band) the sound pressure levels u n d e r free-field conditions exhibit no d e p e n d e n c e on spatial direction. At higher frequencies the multiple sources become more important and they pred o m i n a t e at frequencies above 2000 Hz, b u t their contribution to the overall sound p o w e r remains weak. The variation of the s o u n d power level with the load, i.e. with the m e a n velocity of the gases, was then investigated for different values of the swirl number. The sound p o w e r yields are plotted on Fig. 4 against the swirl n u m b e r with the load as a parameter. The curves exhibit inflection points at swirl numbers between 0.5 a n d 1, and the sound p o w e r yield is found to vary with load to the 1.8 power (average). This gives a variation of the sound power with average gas velocity to the 2.8 power w h i c h is very close to the value of 2.67 given by Strahle and Shivashankara in their recent work on t u r b u l e n t premixed
- - a n increase in the m a x i m u m value of the noise emitted; - - a shift of the peak frequency to the high frequencies so that, in the range 10-150 Hz, the sound power emitted b y the n o n s w i r l i n g flame is higher than that emitted by the h i g h l y swirling flame. T h e same figure also gives the frequency spectrum p r o d u c e d by the air flow (curve c) and a comparison with curve (a) shows that the noise of the isothermal air jet is much lower than the c o m b u s t i o n noise (20 dB). However, the frequency distribution is very similar, with the exception of a large peak at 160 Hz. This peak is characteristic of highly swirling jets, and can be attributed to the precessing vortex core (P.V.C.) described by Be6r et a l ) 3 The P.V.C. is either amplified or d a m p e d by combustion, according to the value of the f u e l / a i r ratio. It is a m p l i f i e d only in very lean mixtures. In our conditions (10% excess air), the peak is completely damped, w h i c h is in agreement with previous experimental observations. It must be noted that the frequency spectra in Fig. 5 were recorded u s i n g constant percentage b a n d w i d t h filters. A constant correction of - 3 riB/octave has to be a p p l i e d to obtain the equivalent of an analysis performed with constant b a n d w i d t h filters. T h e two maxima of the frequency spectra of combustion noise would then a p p e a r not at 220 Hz
1,0 ,,?.1o 6 0,75-
jJ
0.5
0,25- -7--
0
/o....-- ---o
o
--~-~"----'-'0
jf
J~ --~c
I MW
~
o,5
j
e"
1,o
e
1,5
S
FIc. 4. Sound power yield versus swirl number in free-field conditions.
2.0
1762
TURBULENT FLAMES AND COMBUSTION 120 dB 110
100'
/-
/ 50
100
200
300
500
frequency (Hz)
1000
FIG. 5. Narrow band frequency spectra of cold jet and flame noise under free-field conditions: (a) combustion noise, 2 MW, S = 1.7; (b) combustion noise, 2 MW, S = 0; (d) cold jet noise, S = 1.7
and 400 Hz, but at 125 Hz and 300 Hz. Strahle and Shivashankara 9 have given two experimental equations to describe the s o u n d power and the peak frequency of the combustion noise emitted by premixed g a s / a i r flames:
turbulence intensity a n d the integral scale of turbulence:
P=K
Po - P l t 2 D2 U2 ~ o F ] i t2 "r u?
P = 3.7 10 -6 x U 267 x uf lsa x F 0.4 x D "2"r8 (1)
(3)
fc = 2.3 x U 0"I8 X U? '88 X F -xm X D -~
(2)
Introducing typical values corresponding to our 2 MW flame in these equations (U = 100 ft/s, uf = 1.2 f t / s 16, F = 0.07 and D = 0.59 ft) gives a predicted peak frequency of i 6 5 Hz, and a predicted sound power yield of 4 x 10 7. This peak frequency is in agreement with our experimental values, and the s o u n d power yield is lower by a factor of eight than the sound power yield measured for the n o n swirling flame. This comparison is satisfactory in view of the very different conditions, but our results show the need for scaling laws to d e p e n d on turbulence parameters, since we have seen that both peak frequency and sound power depend on the swirl n u m b e r , and c o n s e q u e n t l y on the t u r b u l e n c e intensity. Strahle 12 has derived an equation for jet diffusion flames, taking into account the
If the integral scale of turbulence is proportional to the b u r n e r throat diameter, P b e c o m e s proportional to the product D 4 U 2. This equation suggests that c h a n g i n g the burner diameter at a given load a n d at a given swirl n u m b e r should not cause any change in the s o u n d power emitted. To verify whether such a result could be applied in our case, we varied the burner throat diameter from 116 mm to 176 mm for a given load (1.5 MW) and fixed swirl (S = 1.0). The results are summarized in Fig. 6 and Table I. The sound pressure level remains constant, and is consistent with P cc D 2x U x where the value of x has b e e n determined as 2.8. It must be kept in mind, of course, that the high dependence of P on D also accounts for the dependence of i t on the burner diameter. The shift to higher frequencies for smaller diameters shown on Fig. 6 can also be explained if we assume that the integral scale of t u r b u -
TURBULENT DIFFUSION FLAME NOISE
1763
110
%
dB
p
100
/,w,
90.
80.
70
60.
50. octave - band
centre frequencyIHal ,.
31,5
63
125
,
250
500
,
1000
2000
4000
8000
Fie. 6. Octave band frequency spectra of combustion noise under free-field conditions: (a) 1.5 MW, burner throat diameter D = 176 mm, S = 1.0; (b) 1.5 MW, burner throat diameter D = 116 mm, S = 1.0
lence is proportional to the burner diameter a n d that the peak f r e q u e n c y is given by l2: L = u'/1,. No attempt has been m a d e to verify the theoretical d e p e n d e n c e of the sound p o w e r on laminar b u r n i n g velocity under free-field conditions. The only way to vary this parameter i n d e p e n d e n t l y is to b u r n pure gases whose laminar b u r n i n g velocities with air are significantly different from that of methane (e.g. ethylene, acetylene or hydrogen). This presented too m a n y difficulties for our particular investigation. It is also difficult to comment on the importance of the t u r b u l e n c e intensity in the scaling laws. In some recent work, Baker et al. '5 have shown that the turbulence intensity contours measured in swirling (S = 0.52) a n d n o n s w i r l i n g flames were quite different, a n d that a knowledge of the complete flow pattern was necessary to evaluate a turbulence intensity in the c o m b u s t i o n zone.
Furnace Conditions T h e problem of investigating combustion noise inside furnaces is often complex since the furnace is not an acoustically simple envir o n m e n t and it influences the sound p o w e r directly by a m p l i f y i n g or d a m p i n g some freq u e n c y ranges of the noise emitted by the burner. Another major difference, as compared to outdoor conditions, is that the flame is s u r r o u n d e d by a hot atmosphere, the tempera-
ture of which is d e t e r m i n e d b y flow pattern, c o m b u s t i o n characteristics a n d furnace geometry, One aim of this study was to investigate experimentally how the scaling laws could be a p p l i e d in this particular environment. For this purpose, the sound pressure levels were measured outside and inside the furnace, using the same burner as under free-field conditions. F i g u r e 7 presents the results of frequency analyses performed inside the furnace, at 350 cm (Fig. 7a) and 500 cm (Fig. 7b) from the b u r n e r along the burner axis, and outside the furnace (Fig. 7c). Two high resonance peaks at about 60 Hz and 120 H z represent the fundamental and the first harmonic of the longitudinal mode of resonance (X = 12.5 m equal to double the furnace length, with c = 705 m / s ) . The second a n d the third harmonics, as well as the pressure node of the 60 Hz
TABLE I Variation of sound pressure level with burner throat diameter D (mm)
Sound pressure level (dB)
116 132 152 176
107 108 108 107
1764
TURBULENT FLAMES AND COMBUSTION
120 dB 110
100
er
go
50
A J \A \ 100
(a}
\.
!
200
300
00
500 frequency (Hz)
120 dB 110
100
"
A
90
,% A
80
Flc. 7. Narrow band frequency spectra of noise emitted by the experimental burner fired in the furnace. (a) 350 cm far from burner mouth inside the furnace, (b) 500 cm far from burner mouth inside the furnace, (c) outside the furnace
resonance, appear clearly on Fig. 7a. T h e results of the measurements of octave b a n d s (centered around 63 Hz and 125 Hz) at different distances along the burner axis show the sound pressure level characteristics inside the furnace in more detail (Fig. 8). These resonance effects became less important above 200 Hz, so that octave b a n d measurements at 250 Hz a n d / o r 500 Hz could be considered as representative of the "actual" combustion noise emitted by the burner. It must be pointed out that at a distance of more than 150 cm from the b u r n e r mouth, the fluctuating gas velocities in the furnace are low and cannot cause any interference i n the s o u n d pressure measurements. With the m i c r o p h o n e probe positioned on the burner axis, 500 cm from the burner, the influence of load, swirl, and preheat was investigated. I n order to test the influence of the preheat temperature on the noise power output, the
130 dB 125-
~ 120
63 Hz
// .Y
\
115'
\/
110
0
1
distonce from burner mouth(m
3
Flc. 8. Axial profiles of sound pressure level inside the furnace; full line: 125 Hz octave band, dotted line: 63 Hz octave band
TURBULENT DIFFUSION FLAME NOISE
1765
level with the swirl n u m b e r is similar to that recorded under free-field conditions, whereas the overall sound pressure level stays constant a n d even shows a t e n d e n c y to decrease at very h i g h swirl numbers. This t e n d e n c y is consistent with observations u n d e r free-field conditions which show that increasing the swirl n u m b e r can decrease the s o u n d power at frequencies b e l o w 100 Hz. Plotting the dB values d i v i d e d b y 10 (which vary linearly with the logarithm of sound p o w e r emitted) against the logarithm of the load (proportional to the mean gas velocity) gives a slope of 3.6 for the 250 H z octave b a n d s o u n d pressure level a n d 2.1 for the overall sound pressure level. T h e con> b u s t i o n noise thus appears to increase faster inside the furnace than u n d e r free-field conditions. This observation cannot be explained b y the increase of the flame temperature (approximately 100~ w h i c h can be measured w h e n the load is c h a n g e d from 1 to 3 MW. T r e a t i n g t h i s effect as the result of a theoretical preheat to about 150~ of the combustion air a n d using Eq. (3) w o u l d indeed reduce the 2.8 p o w e r derived under free-field conditions. This effect must then more p r o b a b l y be attributed to a change in the turbulence characteristics of the flow due to a large increase of gas recirculation inside the furnace. Another parameter that it was interesting to vary was the fuel, a n d this was done by c o m p a r i n g natural gas a n d heavy fuel oil flames. Under the same conditions of swirl a n d preheat, the peak of c o m b u s t i o n roar was f o u n d to occur at the same frequency. The
c o m b u s t i o n air of natural gas flames was heated up to 250~ at constant swirl numbers. T w o effects similar to the influence of swirl u n d e r free-field conditions were observed: - - A shift of the peak f r e q u e n c y to the high frequencies, so that a clear maximum of c o m b u s t i o n noise is o b t a i n e d in the 500 Hz octave band; - - A n increase of the m a x i m u m value of the combustion noise equal to 11 dB. Preheating the c o m b u s t i o n air must produce at least three effects: - - A n increase of the average gas velocity; - - A n increase of the laminar flame velocity; 16 - - A change in the b o u n d a r y values of the density, sound speed a n d thermal diffusivity parameters in Eq. (3). Calculations using Eq. (3) show that preheating the air should decrease the overall noise p o w e r output if the t u r b u l e n t characteristics of the flame remain u n c h a n g e d . The results seem to indicate that the t u r b u l e n c e characteristics have changed drastically, a fact which can be related to the shift of the peak frequency to higher values. However, it must be noted that, for a similar effect on the frequency spectrum, increasing the swirl from S = 0 to S = 1.7 increased the overall s o u n d power only b y 4.5 dB under free-field conditions. The load of the gas flames was then varied between 1 a n d 3 M W for different swirl numbers, and the results are given on Fig. 9. The variation of the 250 Hz octave b a n d sound pressure
130
3MW
dB ~
~
"
2M
x
120 0
110X
100
1MW~ ~-~ o---.-o/
90-
0
0,5
1,0
1,5
s
Fro. 9. Sound pressure level inside the furnace versus swirl number; full lines: 250 Hz' octave band, dotted lines: overall level
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TURBULENT FLAMES AND COMBUSTION
FIc. 10. Photographs of typical flames of the industrial burners: (a) burner A; fuel oil flame--2.25 MW, 450~ preheat, (b) burner B; fuel oil flame--4 MW, 20~ preheat, (c) burner C; fuel oil flame--2.32 MW, 20~ preheat
only difference was a lower (3 dB) level of noise at the same load in oil flames, w h i c h is probably due to the gasification process acting as a l i m i t i n g factor on the flame b u r n i n g velocity. Indeed the levels of noise emitted
by oil and gas flames are almost the same when the c o m b u s t i o n air is preheated. 11 Finally, the three industrial burners were fired in the furnace (Fig. 10 and 11). I n spite of very different flame shapes, the results are
TURBULENT DIFFUSION FLAME NOISE
The noise emitted b y oil flames without preheat of the c o m b u s t i o n air in b u r n e r C is not significantly different from the noise emitted b y gas flames u n d e r the same conditions. This can be attributed to the pregasification process occurring inside the burner quarl w h i c h tends to suppress the difference between the c o m b u s t i o n characteristics of gas and oil.
130
(a)
dB 120
110'
100
/ 31,5
/
63
f r e q ~ (Hz) 250 5()0 1000
125
130
(b)
dB 120
110
v x
100
\
frequent,(Hz)'~ 31,5
63
125
250
500
1000
130
(c)
dB 120
.o
..
_
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- - T h e 250 Hz or 500 Hz (maximum of combustion noise) octave b a n d sound power levels are proportional to the load raised to the 3.7, 4.0 a n d 4.2 powers for burner A b u r n i n g gas at loads between 1.25 and 2.25 M W with air p r e h e a t e d to 450~ for burner A b u r n i n g oil at loads between 1.12 and 2.25 M W with air preheated to 450~ and for burner B b u r n i n g oil at loads between 1 and 4 M W respectively. These results should be c o m p a r e d with the 3.6 power derived for gas flames with the experimental burner. - - V a r y i n g the excess air on burner C from 5% to 30% does not give any significant difference in noise emission, which suggests that the increase (---3 dB ) resulting from the increase of the average gas velocity is compensated b y a decrease of other parameters involved in the scaling law.
_%
Conclusions
~~,\\
100
frequency (Hz) 31,5
3
125
250
5013
1000
FIG. 11. Octave band analyses of the noise emitted by the industrial burners; full lines: gas flames, dotted lines: fuel oil flames. (a) burner A--2.25 MW, 450~ preheat, (b) burner B--3 MW, 20~ (c) burner C--2.32 MW, 20~
very similar to the ones obtained using the experimental burner: - - T h e m a x i m u m of the c o m b u s t i o n roar occurs a r o u n d 250 H z with no preheat (Fig. l l b a n d l l c ) and a r o u n d 500 Hz with preheat (450~ Fig. l l a ) ; - - S w i r l i n g flames exhibit a sharp m a x i m u m of the combustion roar (Fig. l l b ) ; - - T h e combustion noise emitted b y oil flames is significantly lower than that emitted b y gas flames when the c o m b u s t i o n air is not preheated (Fig. l l b ) , whereas no significant difference is noticed at high preheat levels (Fig. l l a ) .
The following conclusions may be drawn from this study: - - I t is valid to use a m o n o p o l e source theory to describe the emission of c o m b u s t i o n noise from large turbulent diffusion flames. (Fig. 3). - - T h e results indicate a variation of the, sound power with the b u r n e r throat diameter to the 5.6 power and the average gas flow velocity to the 2.8 power. - - E x i s t i n g scaling laws do not account for the effect of preheat on c o m b u s t i o n noise, unless the turbulent characteristics of the flame are strongly d e p e n d e n t on this parameter. - - T h e peak frequency of the combustion noise appears to d e p e n d m a i n l y on the turbulent mixing characteristics of the flame.
Nomenclature c D F
sound speed burner diameter fuel-air mass ratio
1768 fc lt P r S U uf u' W "q p
T U R B U L E N T FLAMES AND COMBUSTION peak f r e q u e n c y of c o m b u s t i o n n o i s e integral scale of t u r b u l e n c e sound power radius swirl n u m b e r axial v e l o c i t y laminar flame velocity f l u c t u a t i n g v e l o c i t y (rms) tangential v e l o c i t y thermal diffusivity sound power yield wavelength density turbulence intensity
Subscripts 0 1
v a l u e in c o l d gases v a l u e in b u r n t gases
Acknowledgments The investigation reported in this paper was carried out with financial aid of the Commission of the European Communities (Contracts Nr. 6210-096-006 and 6254-40-6-250). The authors gratefully acknowledge the contribution of their colleagues at the I F B F Research Station in the execution of the experimental work, as well as the cooperation of the burner manufacturers Bloom, Nippon Furnace Kogyo and Hoogovens IJmuiden BV. REFERENCES 1. SMITH, T. J. B., AND KILHAM J. K.: J. Aeoust. Soc. Am, 35, 715 (1963).
2. KOTAKE,S., AND HATrA, K.: Bulletin of J.S.M.E., 8, no. 30, 211 (1965). 3. BRIFFA, F. E. J., CLARK, C. J., AND WILLIAMS, G. T.: J. Inst. Fuel, 207 (May 1973). 4. KaRLOVITZ,B.: J. Chem. Phys., 19, 541 (1951). 5. BRACe, S. L.: J. Inst. Fuel, 12 (January 1963). 6. ST~HLE, W. C.: J. Fluid. Mech., 49, 399 (1971). 7. HUm,E, I. R., PRICE, R. B., SUGBEN T. M., AND THOMAS,A.: Proc. Roy. Soc. (London)A303, 409 (1968). 8. L1GHTHILL,M. J.: Proe. Roy. Soc. (London A211, 564 (1952). 9. ST~rfLE, W. C., ANn SmVASHANra~, B. N.: Fifteenth Symposium (International) on Combustion, p. 1379, The Combustion Institute, Pittsburgh (1974). 10. GIAMMAR,R. D., AND PUTNAM, A. A.: J. Eng. for Power, Trans. ASME ser. A., 92, 159 (1970). 11. LowEs, T. M., HEAP, M. P., AND SMITH, B. R.: La Rivista dei Combustibili 29, No. 5-6, 197 (May-June 1975). 12. STRAHLE, W. C.; Fourteenth Symposium (International) on Combustion, p. 527, The Combustion Institute, Pittsburgh (1973). 13. SYRED, N., GUPTA, A. K., AND BEI~R, J. M.: Fifteenth Symposium (International) on Combustion, p. 587, The Combustion Institute, Pittsburgh (1974). 14. FRICKER, N.: 2nd Members Conference IFRF, IJmuiden (May 1971). 15. BAKER,R. J., HUTCHINSON,P., KHALIL,E. E., AND WHITELAW, J. H.: Fifteenth Symposium (International) on Combustion, p. 553, The Combustion Institute, Pittsburgh (1974). 16. VAN TIGGELEN, A., ET AL.: Oxydations et Combustion, ed. Technip (1968).
COMMENTS Abbott A. Putnam, Battelle Columbus Laboratories, USA. There is a number of interesting features brought out in this study. The first is the verification of the monopole hypothesis for combustion roar, up to 500 Hz. The noise then shifts toward a dipole source type, which might be related to the vortex shedding phenomenon. But could one also explain this result on the basis of back-shielding by the burner body and front-shielding by the flame itself? Be6r has hypothesized a flame shielding effect to explain results on a vortex burner, and Putnam has noted a cancellation by the flame surface at high frequencies, which spreads to lower frequencies as the flame size increases. A second point made was in respect to the differences between the combustion roar of a flame
in an acoustically infinite situation, and in a furnace. It appears that one might associate an amplification speetrmn with a furnace, at a given firing rate, and apply it to any burner for which the aeoustieally open noise spectrum is known, to judge comparative noise pollution effects. One notes that decreasing either the burner noise input or the amplification factor would help. But, from the combustion standpoint, what types of burners might be fired in an acoustically open situation without modifying their burner performance? The problem was also brought up of the relative performance of multiple burners of a small size as compared to one burner of large size. In studying several burner sizes in a series, one cannot "a priori" assume that the burners are properly sealed aeousti-
T U R B U L E N T D I F F U S I O N FLAME NOISE tally. For instance, h o w does one scale the swirl factor, which is dimensional. Some burners are actually scaled on a constant intensity of combustion basis, which changes the prediction rules. But assuming none of these problems are involved, could it be that the multiple flame fronts that are associated with multiple burners could result in multiple noise cancellations and quieter combustion on this basis, rather than on the basis of the relative burner size?
Authors" Reply. It is indeed difficult to determine how much the burner performance is influenced by changing the environment, from which the flame jet entrains, as drastically as one does when the burner is fired in free field conditions instead of into a furnace. Of course it is clear that the burner performance is least affected when the flame length is small. A tunnel b u r n e r delivering completely combusted products will hardly change its performa n t e with the environment. The burner studied produced a short flame with a length of only 1.5 burner diameters, so that the effect of the environinent on its performance was only small. The method suggested for the assessment of noise emission from furnaces would, however, require a very expensive anechoic chamber in order not to confine the method to extremely short flames. Although we believe that the burner scaling criteria used for the case where we compared the noise emission from one 3 M W burner with three 1 M W burners represent a proper scaling also from an acoustic point of view (based on same velocity and swirl number whieh results in an identical total tangential and axial m o m e n t u m flux for the single and the multiple burners), the noise reduction with three 1 MW burners could h e explained also b y the suggested flame shielding effect a n d / o r by a shift of the noise spectrum to higher frequencies. The frequency shift results in less amplification of the low resonance frequencies of the furnace and thus contributes to a reduction in noise emission.
George E. Abouseif, M.I.T., USA. Your observations show that, indeed, the low frequency standing
1769
waves are being fed with energy from the combusting mixture. Did you do any theoretical analysis to understand this coupling mechanism w h i c h should help understanding such problems a n d consequently would help attenuating or maybe avoiding these waves? Our study, here at M.I.T., indicates very clearly that the lower the frequency, the more the waves are amplified. Besides at low frequency and also at high temperatures, the rate of dissipation is m u c h smaller than otherwise. This shows that these low frequency waves are more likely to survive as long as there is unsteady reaction.
Authors'Reply. We have not yet put enough effort into the theoretical analysis of the data to enable the formulation of coupling mechanisms. However, this is obviously the only way which eventually will allow predictions of furnace noise emission to be made and hence would be an invaluable tool for the minimisation of noise pollution. Indeed, we also find that the amplification factor seems to b e larger in the lower frequency ranges, and also that the noise emission seems to increase with furnace temperature.
B. M. Belgaumkar, IAEC (MOB.) P'v~F., LTD., lndia. According to the latest issue of the B&K journal, the damage criteria need looking into. The 90 db criteria can be exceeded in frequencies other than 4 KHz. Therefore one need not unnecessarily exaggerate the noise pollution problem.
Authors" Bepl~t. We are aware of the data on damage criteria for the h u m a n ear published in the latest B&K journal and see that from this~point of view the noise pollution problem should not be overemphasized, however, noise unfortunately does affect other parts of the h u m a n body too and therefore should be kept as low as possible to prevent any unnecessary damage.