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Combustion inside refractory tubes
592
CELLULAR FLAMES AND OSCILLATORY COMBUSTION 76
C O M B U S T I O N INSIDE REFRACTORY TUBES By A. H. HOWLAND AND W. A. SIMMONDS 1.
INTRODUCTION
The rate of heat transfer which can be obtained from a flame depends largely on the temperature of the products of combustion. In many cases these products are diluted with excess secondary air (1) and consequently cooled. The hottest products of combustion are obtained when the combustion occurs with the complete exclusion of secondary air. The simplest way of carrying this out is to burn a correctly proportioned fuel gas-air mixture inside a refractory tube and use the stream of hot products emerging. Burners employing this principle have been used for many years in both Great Britain and America (2) and, in addition, it has been found that the flames obtained with these burners remain stable inside the refractory tube for very high gas velocities so that high rates of heat release are obtained per unit volume. As the reason for this stability was not known the following investigation into this type of combustion was undertaken by the Gas Research Board. 2.
THE
LIMITS
OF
INSIDE
STABILITY REFRACTORY
FOR
The effect of varying the tube length on the limits of stability of the combustion has also been investigated. I t has been shown that as the tube length is increased the upper limit at first increases
o
*o
c_~_t._ _c~_~ _ o ~ . . . . . . . . . . . . . . . . . . . . . . . . .
l
] ~ s
6o
,~ "60 ~o ,=,L . . . . . . . /w,, Fie. 1. Variation of air/methane ratio with total flow for upper and lower limits. Tube diam. 3.2 cms. Inlet diam. 1.0 ems. Tube length 55.0 cms.
COMBUSTION
TUBES
400 litm/R/~. o
The limits of stability for the combustion of various fuel gas-air mixtures has been determined in refractory tubes of various lengths and diameters. The experimental technique used in this work was simply to burn the fuel gas inside the tube until the wails became red hot and then with a constant flow of fuel the air supply was increased until the flame blew out of the tube. The fuel and air flows at this point gave the upper limit for the combustion. The lower limit was similarly obtained by decreasing the air supply until the flame left the tube, usually to burn at its mouth. Typical results of these experiments are illustrated in figure 1 which is a plot of air/fuel gas ratio against total flow showing the upper and lower limits for methane-air mixtures burning in a tube of diameter 3.2 cms. I t will be noted that the lower limit does not depend greatly oil the total flow. The upper limit, however, tends to decrease as the total flow is increased. Some evidence has been obtained to indicate that at very high total flows the upper and lower limits join up.
~t
v.
1~a~Ntk LIEt~'Th C M
FIG. 2. Variation of air/methane ratio for upper and lower limits with tube length at constant total flow. Tube diam. 3.2 cms. Inlet diam. 1.0 cms. and then becomes appreciably constant. The lower limit is constant at an air-fuel gas ratio of about 6 over the range of tube lengths considered. This is illustrated in figure 2. 3.
FACTORS INSIDE
AFFECTING
COMBUSTION"
A REFRACTORY
TUBE
At the outset of this work, several factors were considered which could affect the combustion reactions occurring in the refractory tube and lead
COMBUSTION INSIDE REFRACTORY TUBES to an increased rate of combustion. For example, the gas stream inside the refractory tube is highly turbulent and it is possible that this would result in very efficient ,nixing of the gases in the reaction zone with a resultant increase in the reaction rate and flame speed. It is welt known that turbulence can increase combustion rates (3) and this factor must be of some considerable importance. It is unlikely, however, that this could be sufficient to account for the whole of the increase in the rate of heat release per unit volume since similar turbulent conditions occur in other burners in which this rate, although high, is not of the order found here. The combustion inside the refractory tube occurs in a hot confined space and it is possible that the consequent heating of the reaction zone will have the effect of increasing the reaction rate. I t is unlikely that this will be of the order necessary to give the increase in the rate of heat release per unit volume that is actually obtained. In the operation of these burners it is necessary to run the burner at a low rate of gas input for a short time until the refractory walls are hot. The input can then be greatly increased and a stable flame obtained inside the tube. This suggests that the hot walls of the tube may he affecting the combustion processes. It is well known that in the slow combustion of gases the nature of the walls of the containing vessel can have a considerable effect on the combustion rate and it has also been shown by Pannetier and Laffitte (4) that the wall conditions may affect the speed of flame propagation. Leason (5), however, has studied the temperature distribution in tunnel burners of various shapes and has found no evidence for any catalysis of the combustion by the refractory walls. I t is also possible that the combustion process may be influenced by radiation from the walls of the refractory tube. Norrish (6) has examined the effect of high intensity light beams on the slow c o m b u s t i o n of ethylene and has shown that light of wave length shorter than 3,600/~ increased the combustion rate. By working close to the ignition limits of ethylene it was found possible to convert the slow combustion into actual ignition by irradiation. The effect of the light is believed to be due to activation of formaldehyde molecules which are formed as intermediaries in the slow combustion and which possess strong absorption bands between 3,600-2,600 ~. H.2CO +
hv =
H~.CO
H2CO + O~ = H~CO~. + O
593
There is no evidence, however, that light of very high intensity will affect the normal combustion of gases and in any case the intensity of radiatinn of wave lengths shorter than 3,600 A from the refractory walls will be small. Furthernmre, experimental investigations carried out by the Gas Research Board into the effect of intense radiation on combustion in a bunsen flame of town gas showed that such effects were very small. No information is available on the effect of radiation on intense flames, but it is unlikely that it will be great enough to produce the known increase in the rate of heat release per unit volume. None of the factors considered appear to result in effects large enough to explain the observed stability of flames inside refractory tubes. 4. INVESTIGATION OF THE NOISE PRODUCED BY COMBUSTION IN*SIDE A RI~'RACTORY T U B E
A characteristic property of combustion in a refractory tube is the generation of a great deal of noise and it was decided at an early stage in the work to investigate this. The burner used in the initial experiments consisted of a long insulated silica tube, 2 cms inside diameter, in which was inserted an iron draw tube fitted with a silica nozzle 0.6 cm inside diameter. The length of the burner could be altered by adjusting the position of the iron tube. The wave form of the noise emitted was registered on one of the beams of a double beam cathode ray oscillograph using a microphone silualed approximately 1 cm away from the burner mouth, behind an asbestos screen to protect it from the hot combustion products. The intensity of the noise was measured approximately from the amplitude of the wave, and its frequency by comparison with a 1,000 cycle signal introduced on to the other beam of the oscillograph. In the latter case the two wave forms were photographed simultaneously and the noise frequency measured from the photograph. In order to minimise errors these determinations were generally (lone in duplicate. With a constant rate of flow thr(mgh the burner and a constant air/gas ratio it was found that the noise intensity varied with the refractory tube length passing through a position of maximum intensity as shown for a typical set of measurements using town gas as fuel in figure 3. The relation between the signal amplitude on the oscillograph and the noise intensity is non-linear and thus any ratio of these amplitudes does not give an accurate ratio of the noise intensities. The general
594
CELLUI.AR FLAMES AND OSCILLATORY COMBUSTION
form of the variation of noise intensity with tube length will be correct. In experiments carried out over a number of rates of flow in the range 220490 litres/minute for town gas/air mixtures burning in tubes of various diameters with a fixed inlet diameter of 0.6 cm it has been found that the length of tube corresponding to the maximum noise intensity was always between 20 and 25 cms. I t is assumed that under these conditions the burner is acting as an organ pipe sealed at one end and that these positions of maximum noise intensity correspond to resonance.
but
V
=
XnT
so
V
=
4L,n,
Using values for the velocity of sound obtained in this manner it is possible to calculate the average temperature (TI) of the gas stream inside the tube by using the relation
V= ,t//~ v p~ where p~ is the pressure of the gas inside the burner, P~ is its density, and 3' is the ratio of the specific --
A m FLOW S't/* n t r e . a / * ~ . , G~s FLOW 1tO m , ~ / ~ , . . a*m a,~s
HYDROGEN//AIn f~[XTURE - AIR FLO~V 34~t I l t r ~ / m i h , HYDGIOGEN FLOW 11911tres/min.
s,as,
t u e ~ o~/*~ ].icm, iNLET DRAM. 0 " 6 c ~ .
ArI~/HYOROGEN 2 . 8 7 TUBE OlAM, 2 * O c m . rNLET D I A M . 0 . 6 r
0
~so
i.ooo z
.
o ~
'
~,
8
o
2
6 ~us~
e
2o z'2 2 ueNGT~
26 ze
~o 3~t )
36 )a
O
Cu.
2
.
.
4
.
.
6
8
*O
,
i
12
14:6 TUBE
*
i . . . . IB ~O 22 2 4 LENGTH C M .
26
' 28
' 30
i 32
L 34
, 35
FIG. 3. Variation of noise intensity with tube length for air/town-gas mixture.
Fro. 4. Variation of noise intensity with tube length for air/hydrogen mixture.
If this assumption is correct resonance should occur at a series of burner lengths (L) corresponding to
heats of the gas at constant pressure and constant volume. The pressure inside the tube will fluctuate over a large range rising instantaneously to a few atmospheres when an explosion occurs and falling rapidly afterwards. I t is clear, however, that its mean value is not much greater than 1 alto since the gas is driven through the burner by an excess pressure at the inlet of 1 or 2 lbs/sq in. The value of a; is obtained from p0, the density at room temperature To, since the change in volume on combustion of a town gas/air mixture of the type being burnt is small when both the air/gas mixtures and the products of combustion are measured at room temperature, so that
L, -- (2r + 1) X/4
(1)
where X is the wave length of the vibrations of frequency n generated in the tube and r may be 0, 1, 2, 3, etc. When burning town gas it was only possible to obtain the first resonant position at L -~ X/4. In experiments using hydrogen/air mixtures two positions of maximum noise intensity were obtained at tube lengths of 8 cms and 24 cms corresponding to L -~ X/4 and L - 3X/4 respectively at the measured sound frequency of 2,600 cycles/second as shown in figure 4. I t is possible to calculate the mean temperature of the products of combustion in the tube from the resonant lengths. If the refractory tube length LT and the noise frequency n, are measured at the position of maximum noise intensity and V is the velocity of sound inside the tube and X its wave length, then as L, corresponds to resonance Lr = ),/4
'V i.e.
po To
T1 -- 16L2rn2"Topo "Ypl
(2)
I t is clear from this formula that this method for determining Tl is liable to serious inaccuracies since T1 is proportional to L~~ and n3. Inspection of the results shows that errors of up to 5 per cent
COMBUSTION INSIDE REFRACTORY TUBES may occur in the determination of each of these quantities and consequently there may be errors of up to 10 per cent in the values of both L 3 and n3. Therefore, the error in T~ may be as large as 20 per cent. In addition, equation (1) is approximate since there is an end effect, which is a function of the diameter d of the tube, to be added to (2r + 1) X/4. For sound waves at room temperature, this correction has the magnitude of 0.29d. The magnitude of this correction is unknown for conditions of operation of this burner. In view of the inaccuracies known to be present in the determination of T1 no correction has been applied.
5O II
d: i
t I z_
§ ?s
TUNNEL
LENGTM
CM.
Fro. 5. Variation of noise intensity with tube length for air/methane mixture. Tube diam. 3.2 cms. Inlet diam. 1.0 cm. Total flow 210 litres/min. Air/methane ratio 9.,5. Dotted curve = unlagged tube. Full curve = }agged tube. Values of T~ obtained in this way range between 1,400 and 2,300~ which is of the order expected for this type of flame and in agreement with sodium line reversal measurements of temperature on the emergent flame at the tube exit. The temperature of this flame will not be greater than that of the flame in the tube, and was measured as 1800~ This approximate agreement is some confirmation that resonance is occurring in the refractory tube. I t may be noted that under certain conditions very much more complicated noise intensity/tube length curves may be obtained than indicated in figures 3 and 4. The relation between noise intensity and tube length for a methane/air mixture burning in a tunnel of inside diameter 3.2 eros and inlet diameter 1.0 cm is indicated in figure 5. Noise intensity maxima will be observed at 25, 40, 50, 63 cms. The dotted curve indicates results obtained using an unlagged silica tube while the full curve was obtained with about an inch of
595
asbestos string lagging around the tube. I t will be seen that the presence of the lagging appears to affect the value of the noise intensity especially at the maximum value at 41) cms. Now, the existence of a resonant length of burner can only occur if there is some periodic phenomenon taking place inside the burner and giving rise to the vibrations to which the burner is resonating. The highly turbulent flow conditions in the gas stream entering the refractory tube will, of course, give rise to a range of frequencies, but since a different tube length will resonate to each of these frequencies, the flow conditions cannot be the source of the vibrations leading to the existence of a single resonant length. Bearing in mind that a very high rate of heat release is obtained in this type of combustion, it is clear that the flame in the burner must have a very high burning velocity. A mechanism of combustion that explains the required high flame speed and at the same time provides the source of the vibrations occurring will be for successive explosions to take place inside the refractory tube. The hypothesis was, therefore, adopted that the combustion inside the refractory tube was taking place in a series of explosions; the mechanism being that the premixed supply entered the tube and after a certain time was ignited when the whole of the mixture inside the tube then exploded. The ignition would take place by heating of the unignited mixture by heat transfer from the walls and by turbulent mixing with the products of combustion of the previous explosion and also by the introduction into the fresh mixture of active species formed in this explosion. The products of combustion would be swept from the tube by lhe fresh supply, which would in turn ignite and the cycle would be repeated, thus giving the periodic phenomenon required to explain the variation of noise intensity with tube length. The explosion is not propagated upstream through the mixture because of the thermal losses by cooling through the narrow metal supply tube which is at room temperature. .5. STROBOSCOPIC EXAMINATION OF TIIE EMERGENT PLAME PROM THE BURN'ER \u a burner of this type is operating the visible flame is almost completely inside the lube, there being only a small "fl'ather" of flame at the tube exit. If the hypothesis stated above is correct, this "feather" of flame should increase in size each time an explosion occurs, in other words, it should
596
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
be flickering with the frequency of the explosions. This frequency can be readily measured by the method already given, so that if the flame emerging from the refractory tube is examined stroboscopically, it should be possible to "stop" the flame at a certain frequency of view of the stroboscope and
and noise traces (figure 6) showed their frequencies to be the same. The experiments were repeated over a range of flows with similar results as shown in table 1. The appearance of the flow when arrested is indicated in figure 7 which consists of four frames from a cine film record taken through the stroboscope wheel at a speed of 16 frames/second on Ilford 16 mm. H.P. 3 film. The use of a cine camera is necessary here as the phenomenon is rather illusive and requires careful control on the speed of the stroboscope. In the photographs the emergent flame due to one explosion can be clearly seen and in addition there is a very faint flame from the previous ex-
FIG. 6. Oscillogram of noise (upper) and stroboscope frequencies (approx. 1,000 cycles/see). Burner length 22 cms. Inlet diam. 0.6 cm. Tube diam. 2.0 cms. TABLE 1
Comparison of noise frequency with stroboscope frequency when emergent flame appears to stop (over a range of flows and air~town gas ratios). Tube diameter 2.0 cm. Inlet diameter 0.6 cm. Tube length em
23.6
23.1
15.5
Noise frequency cycles/see
960 903 780 836 956 935 935 941 845 900
Stroboscope frequency cycles/sec
1,130 895 820 930 984 850 905 896 847 900
1,040 970
1,030 920
1,180
1,215
this frequency should be the same as that of the explosion. In the following experiments the burner was arranged to be at resonance at the fows considered and the walls of the tunnel coated with salt in order to make the flame as luminous as possible. By adjusting the speed of the stroboscope it was found possible to arrest the movement of the flame and photographs taken of the stroboscope
FI6. 7. Frames from cine record taken through stroboscope of emergent flame from tube with stroboscope frequency approx, equal to noise frequency (16 mm H.P. 3 film 16 frames/see). plosion which has by then travelled downstream a few inches. This evidence supports the hypothesis adopted to explain the mechanism of combustion. 6.
VARIATION OF NOISE F R E Q U E N C Y W I T H R E F R A C TORY T U B E
L E N G T H AND D I A M E T E R AND W1TH
THE
OF
RATES
FLOW
OF
AIR
AND
GAS
According to the above mechanism, for the same rates of flow of air and gas and the same burner diameter, the frequency of the explosions and, consequently, of the noise emitted, should be independent of the burner length provided that length is sufficient for ignition to occur inside the refractory tube. For lengths shorter than this, the frequency will vary. The variation of the noise frequency with tube length has been measured experimentally, for various rates of flow of air and town gas (shown in figure 8) for a tube diameter of 2.0 cms and an inlet nozzle diameter of 0.6 cm.
COMBUSTION INSIDE REFRACTORY TUBES The form of this variation agrees with that predicted from the theory given above and allows the length of the tube (L0) at which the frequency will vary from the constant value for long tubes to be assessed.
O ~o . A t R
$76 Ittrcs/~,n G&5FLOW~,o I,tett/~,m AIm/GAS
o
~ aS
11cm
ueE OlA~ INLET el~
o
2
4
&
8
~0
o 6r
o
li
1A.
16
TUBE
lS
i'O 2i'
~
2~
26
2~ 3C 32 3 ~t 36
~8
C~4
FIG. 8. Variation of noise frequency with length for tmvn gas/air mixture.
597
length is different fo~" various diameters, l:,~r lengths just shorter than the critical length the frequency increased for the two smaller diameter tubes while the mixture failed to ignite wilh lhe largest diameter tube. The results of these experiments are given in table 2, in which the combined rate of film of air and town gas, the air/gas ratio, the noise frequency for tube lengths greater than L0, the temperature Tz determined from the velocity- {}f sound, aim the experimentally determined value of L. arc given. Examination of these results shows thal wilhin the limits of experimental errors the noise frequency for refractory tube lengths greater than L0 does not vary greatly with the combined rate of flow, the air/gas ratio and the refractory tube diameter.
TABLE 2
Variation oJ critical length with flow and tube diameter
L Tube diameter (d) cm
2.0
3.1
4.9
Total flow (Q)
Air/Gas ratio
Mean temperMean noise [ ature of frequency combustion above critical I products in length(n)
tube (TO
lilres/min
~K
Calculated critical length (Lc)
Experimental crlt ical length
d
,t
cm
229 317 364 426 488
4.7 4.3 4.1 4.1 4.2
1,040 1,100 975 950 750
1,850 1,850 1,425 1,730 2,300
8 11 10 15 29
3.5 12 18 21 32
4 5.5 5 7.5 14.5
254 328 499 686
4.4 4.2 5.1 5.3
1,050 1,000 1,020 1,020
1,658 1,500 1,770 1,900
3.3 4 7 10
4.2 10 12 14
l.l 2.3 3.2
1.4 3.2 3.9 4.5
272 341 415 48O
2.7 4.0 3.0 4.9
1,050 1,025 1,050 1,025
1,975 2,225 2,460 2,330
1.7 2.5 3.3 3.6
5.1 6.0 7.2 8.9
0.4 0.5 0.7 0.74
1.0 1.2 1.5 1,8
Two similar series of experiments were carried out with refractory tubes of diameter 3.1 cms and 4.85 cms, several different rates of flow of air and town gas being used with each diameter. In all cases it was found, as predicted, t e a t the noise frequency was independent of the length of the refractory tube for lengths above a certain critical value, L0, and this length was determined in each case. I t is of interest that the variation of explosion frequency for tube lengths less than the critical
1.3
1.~ (J
9 10.5
On the basis of the theory suggested f.r the mechanism of the combustion it is possible lo calculate L0 from the explosion frequencies. I.et Q be the volume of mixture flowing per second, d the refractory tube diameter, and n the m)isc frequency, then the volume of mixture flowing into the tube between successive explosions is ( ) : , . As previously explained this may be taken as lhe volume of the i)roducts of combustion at room temperature so that the volume of these al the mean temperature T~ (as obtained from determina-
598
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
tion of the velocity of sound in the mixture) is given by:
QxT2 n
7'0
The length of tube occupied by this volume is
ON 7'1
~
To X
The magnitude of this effect is determined by the intercept on the Le/d axis as about 0.5. This means the point of ignition is inside the tube at a distance from the end equal to the radius. The agreement between the calculated and experimental values for the critical length, when allowance has been made for the end effect, and
4
tS t4 t] t~
~d---~
and this equals L0, so
Lo-
tt
(3)
QTz4
sO 9
n To ~ d ~
The values of L0 calculated in this way are also given in table 2. The agreement between the experimental and calculated values is as close as SO
O--T~BE DI~'( ZOcm = " 9 1-1cm
all= a
TUBE DIAM. 2"0 cm. i N L E T DIAM O 6CR ,
4C
2
)
4
S e, ?
8
~4o
H t2 ~3 14 t~ 16 t? 11
MEASURED FRO~4 IrR~QUENCY CURVES.
Fro. 10. Variation of L,/d with L,/d ~
U p
,&"CALCULATED
O //
20
/, Io
Jz""
1500 1000
I
IO0
200
TOT*L
3OO
400
500
6O0
(O,S+A,,) FLO~u~s/M,,
J
Z
FIG. 9. Variation of critical length with total flow.
9 ",,
000
"%
Air/town gas mixtures. ]~ soc
could be expected in view of the difficulty in determining Tz for the smallest tube diameter (see figure 9) but becomes worse for the larger diameters. A discrepancy of this nature indicates the existence of an end effect proportional to the diameter. This would arise if the point of ignition of the explosion for a tube of the critical length is not situated at the end of the tube but at some small constant fraction of the diameter from the end. The actual position is determined by the flow conditions in the tube. The existence of such an end effect was determined by plotting the ratio of the experimental value of the critical length to the diameter of the tube (L,/d) against the same ratio for the calculated value of this length (L~/d) for all the results. A straight line was obtained which did not pass through the origin (see figure 10) thus showing that there is an end effect proportional to the diameter.
o;
TUNN*"
UE~T.
CM.
FIG. 11. Variation of noise frequency with tube length for methane/air mixture. Tube diam. 3.2 cms. Inlet diam. 1.0 cm. Total flow 210 litres/min. Air/ methane ratio 9.5. the agreement of the form of the variation of frequency with tube length with the predicted form, both confirm the suggested theory for the mechanism of the combustion. Further work is in progress at the moment to examine the variation of noise frequency with tube length for gases other than town gas and confirm the above findings. It has been shown that the form of the variation of noise frequency
COMBUSTION I N S I D E REFRACTORY" TUBES with tube length is similar in the case of m e t h a n e / air mixtures to t h a t obtained with town g a s / a i r mixtures. T h e results of an experiment carried out in a tube of diameter 3.2 cms is shown in figure 11. I t will be seen t h a t the noise frequency becomes c o n s t a n t a t a b o u t 400 cycles/second for tube lengths greater t h a n 40 cms. In addition, a kink will be observed in the curve a t a tube length of 25 cms corresponding to a frequency of 700 cycles/ second. E x a m i n a t i o n of the corresponding noise i n t e n s i t y - t u b e length curve (fig. 5) shows t h a t resonance is occurring at this point. 7.
599
source of light in order to obtain the very short exposures necessary to arrest the turbulence in the stream. T h e effects of the explosions on the stream of combustion products can be clearly seer, from these pictures and provide a striking confirmation of the rflechanism of combustion occurring inside the tube.
EXAMINATION OF T H E FLOW P A T T E R N I N THE E M E R G E N T STREAM OF COMBUSTION PRODUCTS
F u r t h e r verification of the suggested mechanism of combustion m a y be obtained b y considering the flow p a t t e r n of the stream of combustion products
FIG. 13. Spark sehlieren photograph of combustion products leaving tube. Tube length 23.5 cms. Inlet diam. 0.6 cm. Tube diam. 2.0 cms.
0
0.16
0.32" I0" ~,:~8
FIG. 12. Spark schlieren photograph of unignited hot gas stream leaving tube. Tube length 23.5 cms. Inlet diam. 0.6. cm. Tube diam. 2.0 cms. emerging from the refractory tube. The effect of the successive explosions on this stream will be to superimpose pressure pulses or regions of higher pressure. W h e n a region of the stream in which one of the explosions has occurred emerges from the tube, the lateral spread will be greater t h a n t h a t of a n ordinary fluid jet u n d e r these conditions. T h e flow structure of the stream of emergent gases was, therefore, investigated using a schlieren technique. Figures 12 a n d 13 shows up very well the difference in flow p a t t e r n between the e m e r g e n t gas stream from the refractory tube when combustion is occurring inside the tube and when it is not. These photographs were t a k e n with a spark
L___~ ............. l...................... 0.45 0.64 0.80 qO'"~,e<-on~ FIO. 14, High speed schlieren photographs of emergent stream of combustion products from tube showing passage of pocket of gas (6,300 frames/sec). An examination of the stream has also been carried out using a high-speed schlieren camera. T h e camera was specially constructed for this work a n d is of the rotating mirror type: a complete description of the camera and ancillary spark light source has been published elsewhere (7). Figure 14 shows a series of high-sl)eed schlieren photographs taken with the camera at a rate of 6,3oo frames/second of the emergent stream of
600
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
products leaving the tmmel. Superimposed on the general turbulent background it is possible to see the passage of a pocket of gas leaving the tube. A graph has been drawn (fig. 15) which shows the variation of position of the front of this pocket from the end of the burner with time. The two curves show the progress of the first pocket of gas and the start of the second about a millisecond
$
7.
o!s TIME
,!o
;'5
X |O -$ SEC~
FIG. 15. Variation of position of front of gas pocket rom burner mouth with time.
form of combustion considered here will differ to some extent from that occurring in bunsen type flames. These differences may affect the ultraviolet emission spectrum of the combustion. It was therefore decided to carry out a spectrographic study of the flame both inside and at the mouth of the tube. Some of the results of this work have already been indicated in an earlier publication (8) for the combustion of town gas/air mixtures. The flame burning inside a refractory tube is generally of low luminosity. At the inlet nozzle to the tube, however, there is usually a diffuse blue "cone" which tends to become less luminous as the walls of the tube become hotter. Comparison of the ultra-violet spectrum of this region with that of the inner cone of a normal bunsen burner flame under the same air/gas ratio shows certain differences in the band systems present. The Swan system due to C~, for instance, is of much lower intensity than in the bunsen inner cone. Figure 16 shows the ultra-violet emission spectrum of the flame taken at about 4.5 cms from the inlet nozzle to the tube using a burner fitted with quartz windows. The fuel used here was methane at an air/gas ratio of 10. The spectrum of the inner cone of a bunsen burner burning methane is included as comparison. The OH band systems at 3064 and 2811 ~, and the C H bands at 4313 .~ and 3900 A which occur strongly in the bunsen burner inner
FI6. 16. Comparison of ultra-violet spectrum of combustion at 4.5 cms from inlet to refractory tube (upper spectrum) with bunsen inner cone (lower spectrum), both spectra at constant air/methane ratios. Tube -ength 25.4 cms. Tube diameter 3.2 cms. Total flow 220 lltres/minute. Air/methane ratio 10. later. The emergent velocity of the stream of gases Ieaving the burner in this case is about 190 metres/second. It is intended to continue this work with further studies of the gas stream leaving the refractory tube using the high-speed camera. The effects of different fuels and flow rates on the velocity distribution in the stream leaving the tube are to be investigated, as information of this nature is of considerable interest in the study of heat transfer from the flame gases. 8. SPECTROGRAPHIC STUDY OF THE FLAME I t is likely that the course of the combustion reactions and the mechanism of ignition in the
cone may be identified in the spectrum of the tunnel burner flame. The ethylene flame bands which occur in the region 3100-3900 &, and are believed to be due to HCO, also appear to be present in fair strength. The reason for the diminution in intensity of the Swan band system in combustion inside a refractory tube is not at the moment known, but it is clear that the differences in emission spectra between the inner cone of a bunsen burner and this type of combustion must be due to differences in the combustion mechanism. Further work is proceeding at the moment to obtain additional information which may provide an explanation of this. It is intended to study the spectrum of the flame
COMBUSTION INSIDE REFRACTORY TUBES inside the refractory tube using a stroboscope in order to examine the various stages of the explosions which are occurring. When the combustion occurs under resonance conditions a great deal of noise is generated and it is possible that this may affect the band systems by causing pressure broadening. This is also to be investigated. 9. CONCLUSIONS
The experiments described prove that when a premixed supply of air and gas is burnt inside a refractory tube the combustion is of a novel and interesting type since it occurs in a series of explosions. The mechanism of the combustion is that fresh mixture flows into the refractory tube driving out the products of combustion of the previous explosion. The mixture becomes ignited by heating due to radiation and convection from the hot walls and turbulent mixing with the products of combustion and also by mixing of active species from these products. The whole of the mixture then explodes, the explosion being prevented from travelling upstream in the fresh mixture by cooling in the cold, narrow inlet tube. The products of the explosion are replaced by fresh mixture and the cycle repeated continuously. The scope for the further industrial development of this type of combustion to give more rapid rates of heating appears to be limited only by (a) the supply of pressure available for the premixed fuel and (b) the length of tube required to give satisfactory operation. It is possible that this length could be decreased by further increase of the mixing of the fresh fuel mixture with the previous products of combustion by the presence of baffles in the tube, although this must be accompanied by an increased pressure drop through the burner. The noise produced by the burner is inherent in its mechanism of operation and cannot be eliminated. It can be substantially reduced by selecting a tube length which does not resonate to the frequency produced by the burner. Further reduction could be obtained by the use of acoustic screening and absorption by silencers as applied in internal combustion engines where the practical conditions of use allow this. Further work is proceeding at the Gas Research Board to study the mechanism by which ignition of the explosions occurs involving spectrographic and high-speed camera studies of the flame both inside and at the mouth of the refractory tube. The study of the stability limits when various gases are burnt inside the refractory tube should also
601
give additional information as to the means uf ignition. As iudicated in the paper the stream of combustion products has a form considerabbdifferent from that of a normal fluid jet and this must affect the heat transfer froni the stream. As a start to the study of heat transfer under such conditions an examination of the velocity distribution in the emergent stream is being carried out using the high-speed camera. REFERENCES 1. SIM~ONOS,W. A., AND WILSON, .~[- J. G.: GRB 61 (1951). 2. PALSER, J. (Birmingham Gas Dept.): Ind. Gas Times, 9, 65 (194S). 3. WILLIAMS, D. T , AND BOLLINGER, L. ~I.: Third Symposium on Combustion, Flame and Explosion Phenomena, p. 176. Baltlmorc, The Willlams & Wilkins Co. (1949). 4. PANNF.TIER, G., ANn) LAFFITTE, P.: Compt. rend., 224, 1429 (1947). 5. LEASON, D. B.: Thesis in Gas Engineering, l.eeds University, March (1947). 6. NORRISH,R. G. W., AND PATNAIK,D. : Nature, 163, 883 (1949). 7. HOWLAND, A. H., AND WILSON,M. J. G.: Fuel, in press. 8. HOWLAND, A. H., AND SIMMONDS, W. A.: J. Inst. Fuel, 24, 252 (1951). DISCUSSION BY H. ~{. NICttOLSON*
It is felt worthwhile to bring to the notice of this Symposium some recent findings in the investigation of oscillations in an actual jet engine, which exemplify both the flame sustained resonant organ pipe oscillations and the pressure waves generated at the flanae front which have been described in the four preceding papers. This investigation was carried out at the National Gas Turbine Establishment in England and was concerned with a stud)' of pressure pulsations in the Derwent aircraft jet engine. Oscillations were studied which involved the whole gas colunm within the engine and also the gas column in the afterburner alone. Simultaneous pressure recordings at four different stations along the duct showed that during rapid accelerations the whole gas column, from diffuser intake to exit nozzle, fell into organ pipe oscillation. Early in the acceleration when the mean pressure in the engine was only slightly greater than atmospheric the amplitude of oscillation recorded was as much as =1=2.7 p.s.i. This severe oscillation was accompanied 1)3"a shudder~ ing and vibration of the whole engine in its mountiug. The frequency of the gas column oscillation was of the order SO cycles/sec, and was typified by a very clean sinusoidal form which was "in phase" at all the measur* Applied Physics Laboratory, Johns Hopkins lTni versity.
602
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
ing stations. This oscillation increased in frequency and decreased in relative amplitude as the pressure level built up and the temperature on the gas column increased. It was noted too thxt a discontinuous frequency change occurred when the nozzle guide vanes choked. It is interesting that one of the conclusions arrived at was that such oscillations can best be discouraged by locating the combustion chamber at a point of minimum velocity oscillation, the reasoning being that in a gas turbine engine employing a high pressure constant flow fuel injection system the heat release is much less sensitive to pressure oscillation than to velocity oscillation, which markedly affects efficiency. It seemed therefore that locating of the combusion chamber midway between the velocity and pressure nodes would produce a condition most susceptible to oscillation. Pressure recordings in the afterburner showed a different type of pulsation which was the result of the forward propagation of pressure waves generated in the flame zone. These waves had a steep leading edge and their progress from point to point was recorded at the different probes. The maximum magnitude of the pressure rise in such wave fronts was of the order 15 p.s.i. when the mean afterburner pressure was approximately 2 atmospheres. It was speculated that such pressure fronts passing over the constant flow fuel injectors would induce variations in air fuel ratio because of the variation of instantaneous mass flow during the passage of the wave front. These variations in air fuel ratio would then be carried down stream with the mean flow velocity and in turn give rise to further pressure waves on reaching the combustion zone. This picture of the mechansim of generation of pulsations leads to valuable design rules concerning the spatial distribution of the injector and flame stabilizing components. This investigation is described in more detail in N. T. G. E. Report No. R85 by H. M. Nicholson and A. Radcliffe. In subsequent investigations at N. T. G. E. the theoretical aspects of this type of oscillation have been further investigated by D. G. Stewart who has used the method of characteristic mapping to determine the pressure wave system generated when an enriched zone
of mixture passes into the combustion zone. His preliminary findings indicate that in a typical ram jet flow regime, if the temperature rise during combustion be increased from 500~ to 1600~ due to the passage of a zone where fuel air ratio increases linearly for 3.4 msec., the forward going compression wave reduces the Mach number transiently from an initial value of 0.2 to zero. This investigation is about to be published as an N. G. T. E. report. DISCUSSION BY E. A. DEZuBAu The authors, in the derivation of equation (2) given in the form of Tt
16L~ n~ 7'0 p0
ypi
imply that the initial state of the mixture designated by To and o0 together with the tube pressure pl must be known to evaluate the flame temperature T1. Since the acoustic velocity is
dp
V~ = ~
at constant e n t r o p y or
V = "~v/yRV, where R is the gas constant in appropriate units. Hence by substituting the resonance condition V = 4L~ m yields
yR which is the same as the form deduced by the authors if P0 = pt. It must be emphasized that the acoustic velocity as used in this paper can only he dependent on the conditions of the burning gas and cannot be dependent on the inlet conditions. * Westinghouse Research Laboratories, East Pittsburgh, Pa.
CELLULAR FLAMES AND OSCILLATORY COMBUSTION 76
C O M B U S T I O N INSIDE REFRACTORY TUBES By A. H. HOWLAND AND W. A. SIMMONDS 1.
INTRODUCTION
The rate of heat transfer which can be obtained from a flame depends largely on the temperature of the products of combustion. In many cases these products are diluted with excess secondary air (1) and consequently cooled. The hottest products of combustion are obtained when the combustion occurs with the complete exclusion of secondary air. The simplest way of carrying this out is to burn a correctly proportioned fuel gas-air mixture inside a refractory tube and use the stream of hot products emerging. Burners employing this principle have been used for many years in both Great Britain and America (2) and, in addition, it has been found that the flames obtained with these burners remain stable inside the refractory tube for very high gas velocities so that high rates of heat release are obtained per unit volume. As the reason for this stability was not known the following investigation into this type of combustion was undertaken by the Gas Research Board. 2.
THE
LIMITS
OF
INSIDE
STABILITY REFRACTORY
FOR
The effect of varying the tube length on the limits of stability of the combustion has also been investigated. I t has been shown that as the tube length is increased the upper limit at first increases
o
*o
c_~_t._ _c~_~ _ o ~ . . . . . . . . . . . . . . . . . . . . . . . . .
l
] ~ s
6o
,~ "60 ~o ,=,L . . . . . . . /w,, Fie. 1. Variation of air/methane ratio with total flow for upper and lower limits. Tube diam. 3.2 cms. Inlet diam. 1.0 ems. Tube length 55.0 cms.
COMBUSTION
TUBES
400 litm/R/~. o
The limits of stability for the combustion of various fuel gas-air mixtures has been determined in refractory tubes of various lengths and diameters. The experimental technique used in this work was simply to burn the fuel gas inside the tube until the wails became red hot and then with a constant flow of fuel the air supply was increased until the flame blew out of the tube. The fuel and air flows at this point gave the upper limit for the combustion. The lower limit was similarly obtained by decreasing the air supply until the flame left the tube, usually to burn at its mouth. Typical results of these experiments are illustrated in figure 1 which is a plot of air/fuel gas ratio against total flow showing the upper and lower limits for methane-air mixtures burning in a tube of diameter 3.2 cms. I t will be noted that the lower limit does not depend greatly oil the total flow. The upper limit, however, tends to decrease as the total flow is increased. Some evidence has been obtained to indicate that at very high total flows the upper and lower limits join up.
~t
v.
1~a~Ntk LIEt~'Th C M
FIG. 2. Variation of air/methane ratio for upper and lower limits with tube length at constant total flow. Tube diam. 3.2 cms. Inlet diam. 1.0 cms. and then becomes appreciably constant. The lower limit is constant at an air-fuel gas ratio of about 6 over the range of tube lengths considered. This is illustrated in figure 2. 3.
FACTORS INSIDE
AFFECTING
COMBUSTION"
A REFRACTORY
TUBE
At the outset of this work, several factors were considered which could affect the combustion reactions occurring in the refractory tube and lead
COMBUSTION INSIDE REFRACTORY TUBES to an increased rate of combustion. For example, the gas stream inside the refractory tube is highly turbulent and it is possible that this would result in very efficient ,nixing of the gases in the reaction zone with a resultant increase in the reaction rate and flame speed. It is welt known that turbulence can increase combustion rates (3) and this factor must be of some considerable importance. It is unlikely, however, that this could be sufficient to account for the whole of the increase in the rate of heat release per unit volume since similar turbulent conditions occur in other burners in which this rate, although high, is not of the order found here. The combustion inside the refractory tube occurs in a hot confined space and it is possible that the consequent heating of the reaction zone will have the effect of increasing the reaction rate. I t is unlikely that this will be of the order necessary to give the increase in the rate of heat release per unit volume that is actually obtained. In the operation of these burners it is necessary to run the burner at a low rate of gas input for a short time until the refractory walls are hot. The input can then be greatly increased and a stable flame obtained inside the tube. This suggests that the hot walls of the tube may he affecting the combustion processes. It is well known that in the slow combustion of gases the nature of the walls of the containing vessel can have a considerable effect on the combustion rate and it has also been shown by Pannetier and Laffitte (4) that the wall conditions may affect the speed of flame propagation. Leason (5), however, has studied the temperature distribution in tunnel burners of various shapes and has found no evidence for any catalysis of the combustion by the refractory walls. I t is also possible that the combustion process may be influenced by radiation from the walls of the refractory tube. Norrish (6) has examined the effect of high intensity light beams on the slow c o m b u s t i o n of ethylene and has shown that light of wave length shorter than 3,600/~ increased the combustion rate. By working close to the ignition limits of ethylene it was found possible to convert the slow combustion into actual ignition by irradiation. The effect of the light is believed to be due to activation of formaldehyde molecules which are formed as intermediaries in the slow combustion and which possess strong absorption bands between 3,600-2,600 ~. H.2CO +
hv =
H~.CO
H2CO + O~ = H~CO~. + O
593
There is no evidence, however, that light of very high intensity will affect the normal combustion of gases and in any case the intensity of radiatinn of wave lengths shorter than 3,600 A from the refractory walls will be small. Furthernmre, experimental investigations carried out by the Gas Research Board into the effect of intense radiation on combustion in a bunsen flame of town gas showed that such effects were very small. No information is available on the effect of radiation on intense flames, but it is unlikely that it will be great enough to produce the known increase in the rate of heat release per unit volume. None of the factors considered appear to result in effects large enough to explain the observed stability of flames inside refractory tubes. 4. INVESTIGATION OF THE NOISE PRODUCED BY COMBUSTION IN*SIDE A RI~'RACTORY T U B E
A characteristic property of combustion in a refractory tube is the generation of a great deal of noise and it was decided at an early stage in the work to investigate this. The burner used in the initial experiments consisted of a long insulated silica tube, 2 cms inside diameter, in which was inserted an iron draw tube fitted with a silica nozzle 0.6 cm inside diameter. The length of the burner could be altered by adjusting the position of the iron tube. The wave form of the noise emitted was registered on one of the beams of a double beam cathode ray oscillograph using a microphone silualed approximately 1 cm away from the burner mouth, behind an asbestos screen to protect it from the hot combustion products. The intensity of the noise was measured approximately from the amplitude of the wave, and its frequency by comparison with a 1,000 cycle signal introduced on to the other beam of the oscillograph. In the latter case the two wave forms were photographed simultaneously and the noise frequency measured from the photograph. In order to minimise errors these determinations were generally (lone in duplicate. With a constant rate of flow thr(mgh the burner and a constant air/gas ratio it was found that the noise intensity varied with the refractory tube length passing through a position of maximum intensity as shown for a typical set of measurements using town gas as fuel in figure 3. The relation between the signal amplitude on the oscillograph and the noise intensity is non-linear and thus any ratio of these amplitudes does not give an accurate ratio of the noise intensities. The general
594
CELLUI.AR FLAMES AND OSCILLATORY COMBUSTION
form of the variation of noise intensity with tube length will be correct. In experiments carried out over a number of rates of flow in the range 220490 litres/minute for town gas/air mixtures burning in tubes of various diameters with a fixed inlet diameter of 0.6 cm it has been found that the length of tube corresponding to the maximum noise intensity was always between 20 and 25 cms. I t is assumed that under these conditions the burner is acting as an organ pipe sealed at one end and that these positions of maximum noise intensity correspond to resonance.
but
V
=
XnT
so
V
=
4L,n,
Using values for the velocity of sound obtained in this manner it is possible to calculate the average temperature (TI) of the gas stream inside the tube by using the relation
V= ,t//~ v p~ where p~ is the pressure of the gas inside the burner, P~ is its density, and 3' is the ratio of the specific --
A m FLOW S't/* n t r e . a / * ~ . , G~s FLOW 1tO m , ~ / ~ , . . a*m a,~s
HYDROGEN//AIn f~[XTURE - AIR FLO~V 34~t I l t r ~ / m i h , HYDGIOGEN FLOW 11911tres/min.
s,as,
t u e ~ o~/*~ ].icm, iNLET DRAM. 0 " 6 c ~ .
ArI~/HYOROGEN 2 . 8 7 TUBE OlAM, 2 * O c m . rNLET D I A M . 0 . 6 r
0
~so
i.ooo z
.
o ~
'
~,
8
o
2
6 ~us~
e
2o z'2 2 ueNGT~
26 ze
~o 3~t )
36 )a
O
Cu.
2
.
.
4
.
.
6
8
*O
,
i
12
14:6 TUBE
*
i . . . . IB ~O 22 2 4 LENGTH C M .
26
' 28
' 30
i 32
L 34
, 35
FIG. 3. Variation of noise intensity with tube length for air/town-gas mixture.
Fro. 4. Variation of noise intensity with tube length for air/hydrogen mixture.
If this assumption is correct resonance should occur at a series of burner lengths (L) corresponding to
heats of the gas at constant pressure and constant volume. The pressure inside the tube will fluctuate over a large range rising instantaneously to a few atmospheres when an explosion occurs and falling rapidly afterwards. I t is clear, however, that its mean value is not much greater than 1 alto since the gas is driven through the burner by an excess pressure at the inlet of 1 or 2 lbs/sq in. The value of a; is obtained from p0, the density at room temperature To, since the change in volume on combustion of a town gas/air mixture of the type being burnt is small when both the air/gas mixtures and the products of combustion are measured at room temperature, so that
L, -- (2r + 1) X/4
(1)
where X is the wave length of the vibrations of frequency n generated in the tube and r may be 0, 1, 2, 3, etc. When burning town gas it was only possible to obtain the first resonant position at L -~ X/4. In experiments using hydrogen/air mixtures two positions of maximum noise intensity were obtained at tube lengths of 8 cms and 24 cms corresponding to L -~ X/4 and L - 3X/4 respectively at the measured sound frequency of 2,600 cycles/second as shown in figure 4. I t is possible to calculate the mean temperature of the products of combustion in the tube from the resonant lengths. If the refractory tube length LT and the noise frequency n, are measured at the position of maximum noise intensity and V is the velocity of sound inside the tube and X its wave length, then as L, corresponds to resonance Lr = ),/4
'V i.e.
po To
T1 -- 16L2rn2"Topo "Ypl
(2)
I t is clear from this formula that this method for determining Tl is liable to serious inaccuracies since T1 is proportional to L~~ and n3. Inspection of the results shows that errors of up to 5 per cent
COMBUSTION INSIDE REFRACTORY TUBES may occur in the determination of each of these quantities and consequently there may be errors of up to 10 per cent in the values of both L 3 and n3. Therefore, the error in T~ may be as large as 20 per cent. In addition, equation (1) is approximate since there is an end effect, which is a function of the diameter d of the tube, to be added to (2r + 1) X/4. For sound waves at room temperature, this correction has the magnitude of 0.29d. The magnitude of this correction is unknown for conditions of operation of this burner. In view of the inaccuracies known to be present in the determination of T1 no correction has been applied.
5O II
d: i
t I z_
§ ?s
TUNNEL
LENGTM
CM.
Fro. 5. Variation of noise intensity with tube length for air/methane mixture. Tube diam. 3.2 cms. Inlet diam. 1.0 cm. Total flow 210 litres/min. Air/methane ratio 9.,5. Dotted curve = unlagged tube. Full curve = }agged tube. Values of T~ obtained in this way range between 1,400 and 2,300~ which is of the order expected for this type of flame and in agreement with sodium line reversal measurements of temperature on the emergent flame at the tube exit. The temperature of this flame will not be greater than that of the flame in the tube, and was measured as 1800~ This approximate agreement is some confirmation that resonance is occurring in the refractory tube. I t may be noted that under certain conditions very much more complicated noise intensity/tube length curves may be obtained than indicated in figures 3 and 4. The relation between noise intensity and tube length for a methane/air mixture burning in a tunnel of inside diameter 3.2 eros and inlet diameter 1.0 cm is indicated in figure 5. Noise intensity maxima will be observed at 25, 40, 50, 63 cms. The dotted curve indicates results obtained using an unlagged silica tube while the full curve was obtained with about an inch of
595
asbestos string lagging around the tube. I t will be seen that the presence of the lagging appears to affect the value of the noise intensity especially at the maximum value at 41) cms. Now, the existence of a resonant length of burner can only occur if there is some periodic phenomenon taking place inside the burner and giving rise to the vibrations to which the burner is resonating. The highly turbulent flow conditions in the gas stream entering the refractory tube will, of course, give rise to a range of frequencies, but since a different tube length will resonate to each of these frequencies, the flow conditions cannot be the source of the vibrations leading to the existence of a single resonant length. Bearing in mind that a very high rate of heat release is obtained in this type of combustion, it is clear that the flame in the burner must have a very high burning velocity. A mechanism of combustion that explains the required high flame speed and at the same time provides the source of the vibrations occurring will be for successive explosions to take place inside the refractory tube. The hypothesis was, therefore, adopted that the combustion inside the refractory tube was taking place in a series of explosions; the mechanism being that the premixed supply entered the tube and after a certain time was ignited when the whole of the mixture inside the tube then exploded. The ignition would take place by heating of the unignited mixture by heat transfer from the walls and by turbulent mixing with the products of combustion of the previous explosion and also by the introduction into the fresh mixture of active species formed in this explosion. The products of combustion would be swept from the tube by lhe fresh supply, which would in turn ignite and the cycle would be repeated, thus giving the periodic phenomenon required to explain the variation of noise intensity with tube length. The explosion is not propagated upstream through the mixture because of the thermal losses by cooling through the narrow metal supply tube which is at room temperature. .5. STROBOSCOPIC EXAMINATION OF TIIE EMERGENT PLAME PROM THE BURN'ER \u a burner of this type is operating the visible flame is almost completely inside the lube, there being only a small "fl'ather" of flame at the tube exit. If the hypothesis stated above is correct, this "feather" of flame should increase in size each time an explosion occurs, in other words, it should
596
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
be flickering with the frequency of the explosions. This frequency can be readily measured by the method already given, so that if the flame emerging from the refractory tube is examined stroboscopically, it should be possible to "stop" the flame at a certain frequency of view of the stroboscope and
and noise traces (figure 6) showed their frequencies to be the same. The experiments were repeated over a range of flows with similar results as shown in table 1. The appearance of the flow when arrested is indicated in figure 7 which consists of four frames from a cine film record taken through the stroboscope wheel at a speed of 16 frames/second on Ilford 16 mm. H.P. 3 film. The use of a cine camera is necessary here as the phenomenon is rather illusive and requires careful control on the speed of the stroboscope. In the photographs the emergent flame due to one explosion can be clearly seen and in addition there is a very faint flame from the previous ex-
FIG. 6. Oscillogram of noise (upper) and stroboscope frequencies (approx. 1,000 cycles/see). Burner length 22 cms. Inlet diam. 0.6 cm. Tube diam. 2.0 cms. TABLE 1
Comparison of noise frequency with stroboscope frequency when emergent flame appears to stop (over a range of flows and air~town gas ratios). Tube diameter 2.0 cm. Inlet diameter 0.6 cm. Tube length em
23.6
23.1
15.5
Noise frequency cycles/see
960 903 780 836 956 935 935 941 845 900
Stroboscope frequency cycles/sec
1,130 895 820 930 984 850 905 896 847 900
1,040 970
1,030 920
1,180
1,215
this frequency should be the same as that of the explosion. In the following experiments the burner was arranged to be at resonance at the fows considered and the walls of the tunnel coated with salt in order to make the flame as luminous as possible. By adjusting the speed of the stroboscope it was found possible to arrest the movement of the flame and photographs taken of the stroboscope
FI6. 7. Frames from cine record taken through stroboscope of emergent flame from tube with stroboscope frequency approx, equal to noise frequency (16 mm H.P. 3 film 16 frames/see). plosion which has by then travelled downstream a few inches. This evidence supports the hypothesis adopted to explain the mechanism of combustion. 6.
VARIATION OF NOISE F R E Q U E N C Y W I T H R E F R A C TORY T U B E
L E N G T H AND D I A M E T E R AND W1TH
THE
OF
RATES
FLOW
OF
AIR
AND
GAS
According to the above mechanism, for the same rates of flow of air and gas and the same burner diameter, the frequency of the explosions and, consequently, of the noise emitted, should be independent of the burner length provided that length is sufficient for ignition to occur inside the refractory tube. For lengths shorter than this, the frequency will vary. The variation of the noise frequency with tube length has been measured experimentally, for various rates of flow of air and town gas (shown in figure 8) for a tube diameter of 2.0 cms and an inlet nozzle diameter of 0.6 cm.
COMBUSTION INSIDE REFRACTORY TUBES The form of this variation agrees with that predicted from the theory given above and allows the length of the tube (L0) at which the frequency will vary from the constant value for long tubes to be assessed.
O ~o . A t R
$76 Ittrcs/~,n G&5FLOW~,o I,tett/~,m AIm/GAS
o
~ aS
11cm
ueE OlA~ INLET el~
o
2
4
&
8
~0
o 6r
o
li
1A.
16
TUBE
lS
i'O 2i'
~
2~
26
2~ 3C 32 3 ~t 36
~8
C~4
FIG. 8. Variation of noise frequency with length for tmvn gas/air mixture.
597
length is different fo~" various diameters, l:,~r lengths just shorter than the critical length the frequency increased for the two smaller diameter tubes while the mixture failed to ignite wilh lhe largest diameter tube. The results of these experiments are given in table 2, in which the combined rate of film of air and town gas, the air/gas ratio, the noise frequency for tube lengths greater than L0, the temperature Tz determined from the velocity- {}f sound, aim the experimentally determined value of L. arc given. Examination of these results shows thal wilhin the limits of experimental errors the noise frequency for refractory tube lengths greater than L0 does not vary greatly with the combined rate of flow, the air/gas ratio and the refractory tube diameter.
TABLE 2
Variation oJ critical length with flow and tube diameter
L Tube diameter (d) cm
2.0
3.1
4.9
Total flow (Q)
Air/Gas ratio
Mean temperMean noise [ ature of frequency combustion above critical I products in length(n)
tube (TO
lilres/min
~K
Calculated critical length (Lc)
Experimental crlt ical length
d
,t
cm
229 317 364 426 488
4.7 4.3 4.1 4.1 4.2
1,040 1,100 975 950 750
1,850 1,850 1,425 1,730 2,300
8 11 10 15 29
3.5 12 18 21 32
4 5.5 5 7.5 14.5
254 328 499 686
4.4 4.2 5.1 5.3
1,050 1,000 1,020 1,020
1,658 1,500 1,770 1,900
3.3 4 7 10
4.2 10 12 14
l.l 2.3 3.2
1.4 3.2 3.9 4.5
272 341 415 48O
2.7 4.0 3.0 4.9
1,050 1,025 1,050 1,025
1,975 2,225 2,460 2,330
1.7 2.5 3.3 3.6
5.1 6.0 7.2 8.9
0.4 0.5 0.7 0.74
1.0 1.2 1.5 1,8
Two similar series of experiments were carried out with refractory tubes of diameter 3.1 cms and 4.85 cms, several different rates of flow of air and town gas being used with each diameter. In all cases it was found, as predicted, t e a t the noise frequency was independent of the length of the refractory tube for lengths above a certain critical value, L0, and this length was determined in each case. I t is of interest that the variation of explosion frequency for tube lengths less than the critical
1.3
1.~ (J
9 10.5
On the basis of the theory suggested f.r the mechanism of the combustion it is possible lo calculate L0 from the explosion frequencies. I.et Q be the volume of mixture flowing per second, d the refractory tube diameter, and n the m)isc frequency, then the volume of mixture flowing into the tube between successive explosions is ( ) : , . As previously explained this may be taken as lhe volume of the i)roducts of combustion at room temperature so that the volume of these al the mean temperature T~ (as obtained from determina-
598
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
tion of the velocity of sound in the mixture) is given by:
QxT2 n
7'0
The length of tube occupied by this volume is
ON 7'1
~
To X
The magnitude of this effect is determined by the intercept on the Le/d axis as about 0.5. This means the point of ignition is inside the tube at a distance from the end equal to the radius. The agreement between the calculated and experimental values for the critical length, when allowance has been made for the end effect, and
4
tS t4 t] t~
~d---~
and this equals L0, so
Lo-
tt
(3)
QTz4
sO 9
n To ~ d ~
The values of L0 calculated in this way are also given in table 2. The agreement between the experimental and calculated values is as close as SO
O--T~BE DI~'( ZOcm = " 9 1-1cm
all= a
TUBE DIAM. 2"0 cm. i N L E T DIAM O 6CR ,
4C
2
)
4
S e, ?
8
~4o
H t2 ~3 14 t~ 16 t? 11
MEASURED FRO~4 IrR~QUENCY CURVES.
Fro. 10. Variation of L,/d with L,/d ~
U p
,&"CALCULATED
O //
20
/, Io
Jz""
1500 1000
I
IO0
200
TOT*L
3OO
400
500
6O0
(O,S+A,,) FLO~u~s/M,,
J
Z
FIG. 9. Variation of critical length with total flow.
9 ",,
000
"%
Air/town gas mixtures. ]~ soc
could be expected in view of the difficulty in determining Tz for the smallest tube diameter (see figure 9) but becomes worse for the larger diameters. A discrepancy of this nature indicates the existence of an end effect proportional to the diameter. This would arise if the point of ignition of the explosion for a tube of the critical length is not situated at the end of the tube but at some small constant fraction of the diameter from the end. The actual position is determined by the flow conditions in the tube. The existence of such an end effect was determined by plotting the ratio of the experimental value of the critical length to the diameter of the tube (L,/d) against the same ratio for the calculated value of this length (L~/d) for all the results. A straight line was obtained which did not pass through the origin (see figure 10) thus showing that there is an end effect proportional to the diameter.
o;
TUNN*"
UE~T.
CM.
FIG. 11. Variation of noise frequency with tube length for methane/air mixture. Tube diam. 3.2 cms. Inlet diam. 1.0 cm. Total flow 210 litres/min. Air/ methane ratio 9.5. the agreement of the form of the variation of frequency with tube length with the predicted form, both confirm the suggested theory for the mechanism of the combustion. Further work is in progress at the moment to examine the variation of noise frequency with tube length for gases other than town gas and confirm the above findings. It has been shown that the form of the variation of noise frequency
COMBUSTION I N S I D E REFRACTORY" TUBES with tube length is similar in the case of m e t h a n e / air mixtures to t h a t obtained with town g a s / a i r mixtures. T h e results of an experiment carried out in a tube of diameter 3.2 cms is shown in figure 11. I t will be seen t h a t the noise frequency becomes c o n s t a n t a t a b o u t 400 cycles/second for tube lengths greater t h a n 40 cms. In addition, a kink will be observed in the curve a t a tube length of 25 cms corresponding to a frequency of 700 cycles/ second. E x a m i n a t i o n of the corresponding noise i n t e n s i t y - t u b e length curve (fig. 5) shows t h a t resonance is occurring at this point. 7.
599
source of light in order to obtain the very short exposures necessary to arrest the turbulence in the stream. T h e effects of the explosions on the stream of combustion products can be clearly seer, from these pictures and provide a striking confirmation of the rflechanism of combustion occurring inside the tube.
EXAMINATION OF T H E FLOW P A T T E R N I N THE E M E R G E N T STREAM OF COMBUSTION PRODUCTS
F u r t h e r verification of the suggested mechanism of combustion m a y be obtained b y considering the flow p a t t e r n of the stream of combustion products
FIG. 13. Spark sehlieren photograph of combustion products leaving tube. Tube length 23.5 cms. Inlet diam. 0.6 cm. Tube diam. 2.0 cms.
0
0.16
0.32" I0" ~,:~8
FIG. 12. Spark schlieren photograph of unignited hot gas stream leaving tube. Tube length 23.5 cms. Inlet diam. 0.6. cm. Tube diam. 2.0 cms. emerging from the refractory tube. The effect of the successive explosions on this stream will be to superimpose pressure pulses or regions of higher pressure. W h e n a region of the stream in which one of the explosions has occurred emerges from the tube, the lateral spread will be greater t h a n t h a t of a n ordinary fluid jet u n d e r these conditions. T h e flow structure of the stream of emergent gases was, therefore, investigated using a schlieren technique. Figures 12 a n d 13 shows up very well the difference in flow p a t t e r n between the e m e r g e n t gas stream from the refractory tube when combustion is occurring inside the tube and when it is not. These photographs were t a k e n with a spark
L___~ ............. l...................... 0.45 0.64 0.80 qO'"~,e<-on~ FIO. 14, High speed schlieren photographs of emergent stream of combustion products from tube showing passage of pocket of gas (6,300 frames/sec). An examination of the stream has also been carried out using a high-speed schlieren camera. T h e camera was specially constructed for this work a n d is of the rotating mirror type: a complete description of the camera and ancillary spark light source has been published elsewhere (7). Figure 14 shows a series of high-sl)eed schlieren photographs taken with the camera at a rate of 6,3oo frames/second of the emergent stream of
600
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
products leaving the tmmel. Superimposed on the general turbulent background it is possible to see the passage of a pocket of gas leaving the tube. A graph has been drawn (fig. 15) which shows the variation of position of the front of this pocket from the end of the burner with time. The two curves show the progress of the first pocket of gas and the start of the second about a millisecond
$
7.
o!s TIME
,!o
;'5
X |O -$ SEC~
FIG. 15. Variation of position of front of gas pocket rom burner mouth with time.
form of combustion considered here will differ to some extent from that occurring in bunsen type flames. These differences may affect the ultraviolet emission spectrum of the combustion. It was therefore decided to carry out a spectrographic study of the flame both inside and at the mouth of the tube. Some of the results of this work have already been indicated in an earlier publication (8) for the combustion of town gas/air mixtures. The flame burning inside a refractory tube is generally of low luminosity. At the inlet nozzle to the tube, however, there is usually a diffuse blue "cone" which tends to become less luminous as the walls of the tube become hotter. Comparison of the ultra-violet spectrum of this region with that of the inner cone of a normal bunsen burner flame under the same air/gas ratio shows certain differences in the band systems present. The Swan system due to C~, for instance, is of much lower intensity than in the bunsen inner cone. Figure 16 shows the ultra-violet emission spectrum of the flame taken at about 4.5 cms from the inlet nozzle to the tube using a burner fitted with quartz windows. The fuel used here was methane at an air/gas ratio of 10. The spectrum of the inner cone of a bunsen burner burning methane is included as comparison. The OH band systems at 3064 and 2811 ~, and the C H bands at 4313 .~ and 3900 A which occur strongly in the bunsen burner inner
FI6. 16. Comparison of ultra-violet spectrum of combustion at 4.5 cms from inlet to refractory tube (upper spectrum) with bunsen inner cone (lower spectrum), both spectra at constant air/methane ratios. Tube -ength 25.4 cms. Tube diameter 3.2 cms. Total flow 220 lltres/minute. Air/methane ratio 10. later. The emergent velocity of the stream of gases Ieaving the burner in this case is about 190 metres/second. It is intended to continue this work with further studies of the gas stream leaving the refractory tube using the high-speed camera. The effects of different fuels and flow rates on the velocity distribution in the stream leaving the tube are to be investigated, as information of this nature is of considerable interest in the study of heat transfer from the flame gases. 8. SPECTROGRAPHIC STUDY OF THE FLAME I t is likely that the course of the combustion reactions and the mechanism of ignition in the
cone may be identified in the spectrum of the tunnel burner flame. The ethylene flame bands which occur in the region 3100-3900 &, and are believed to be due to HCO, also appear to be present in fair strength. The reason for the diminution in intensity of the Swan band system in combustion inside a refractory tube is not at the moment known, but it is clear that the differences in emission spectra between the inner cone of a bunsen burner and this type of combustion must be due to differences in the combustion mechanism. Further work is proceeding at the moment to obtain additional information which may provide an explanation of this. It is intended to study the spectrum of the flame
COMBUSTION INSIDE REFRACTORY TUBES inside the refractory tube using a stroboscope in order to examine the various stages of the explosions which are occurring. When the combustion occurs under resonance conditions a great deal of noise is generated and it is possible that this may affect the band systems by causing pressure broadening. This is also to be investigated. 9. CONCLUSIONS
The experiments described prove that when a premixed supply of air and gas is burnt inside a refractory tube the combustion is of a novel and interesting type since it occurs in a series of explosions. The mechanism of the combustion is that fresh mixture flows into the refractory tube driving out the products of combustion of the previous explosion. The mixture becomes ignited by heating due to radiation and convection from the hot walls and turbulent mixing with the products of combustion and also by mixing of active species from these products. The whole of the mixture then explodes, the explosion being prevented from travelling upstream in the fresh mixture by cooling in the cold, narrow inlet tube. The products of the explosion are replaced by fresh mixture and the cycle repeated continuously. The scope for the further industrial development of this type of combustion to give more rapid rates of heating appears to be limited only by (a) the supply of pressure available for the premixed fuel and (b) the length of tube required to give satisfactory operation. It is possible that this length could be decreased by further increase of the mixing of the fresh fuel mixture with the previous products of combustion by the presence of baffles in the tube, although this must be accompanied by an increased pressure drop through the burner. The noise produced by the burner is inherent in its mechanism of operation and cannot be eliminated. It can be substantially reduced by selecting a tube length which does not resonate to the frequency produced by the burner. Further reduction could be obtained by the use of acoustic screening and absorption by silencers as applied in internal combustion engines where the practical conditions of use allow this. Further work is proceeding at the Gas Research Board to study the mechanism by which ignition of the explosions occurs involving spectrographic and high-speed camera studies of the flame both inside and at the mouth of the refractory tube. The study of the stability limits when various gases are burnt inside the refractory tube should also
601
give additional information as to the means uf ignition. As iudicated in the paper the stream of combustion products has a form considerabbdifferent from that of a normal fluid jet and this must affect the heat transfer froni the stream. As a start to the study of heat transfer under such conditions an examination of the velocity distribution in the emergent stream is being carried out using the high-speed camera. REFERENCES 1. SIM~ONOS,W. A., AND WILSON, .~[- J. G.: GRB 61 (1951). 2. PALSER, J. (Birmingham Gas Dept.): Ind. Gas Times, 9, 65 (194S). 3. WILLIAMS, D. T , AND BOLLINGER, L. ~I.: Third Symposium on Combustion, Flame and Explosion Phenomena, p. 176. Baltlmorc, The Willlams & Wilkins Co. (1949). 4. PANNF.TIER, G., ANn) LAFFITTE, P.: Compt. rend., 224, 1429 (1947). 5. LEASON, D. B.: Thesis in Gas Engineering, l.eeds University, March (1947). 6. NORRISH,R. G. W., AND PATNAIK,D. : Nature, 163, 883 (1949). 7. HOWLAND, A. H., AND WILSON,M. J. G.: Fuel, in press. 8. HOWLAND, A. H., AND SIMMONDS, W. A.: J. Inst. Fuel, 24, 252 (1951). DISCUSSION BY H. ~{. NICttOLSON*
It is felt worthwhile to bring to the notice of this Symposium some recent findings in the investigation of oscillations in an actual jet engine, which exemplify both the flame sustained resonant organ pipe oscillations and the pressure waves generated at the flanae front which have been described in the four preceding papers. This investigation was carried out at the National Gas Turbine Establishment in England and was concerned with a stud)' of pressure pulsations in the Derwent aircraft jet engine. Oscillations were studied which involved the whole gas colunm within the engine and also the gas column in the afterburner alone. Simultaneous pressure recordings at four different stations along the duct showed that during rapid accelerations the whole gas column, from diffuser intake to exit nozzle, fell into organ pipe oscillation. Early in the acceleration when the mean pressure in the engine was only slightly greater than atmospheric the amplitude of oscillation recorded was as much as =1=2.7 p.s.i. This severe oscillation was accompanied 1)3"a shudder~ ing and vibration of the whole engine in its mountiug. The frequency of the gas column oscillation was of the order SO cycles/sec, and was typified by a very clean sinusoidal form which was "in phase" at all the measur* Applied Physics Laboratory, Johns Hopkins lTni versity.
602
CELLULAR FLAMES AND OSCILLATORY COMBUSTION
ing stations. This oscillation increased in frequency and decreased in relative amplitude as the pressure level built up and the temperature on the gas column increased. It was noted too thxt a discontinuous frequency change occurred when the nozzle guide vanes choked. It is interesting that one of the conclusions arrived at was that such oscillations can best be discouraged by locating the combustion chamber at a point of minimum velocity oscillation, the reasoning being that in a gas turbine engine employing a high pressure constant flow fuel injection system the heat release is much less sensitive to pressure oscillation than to velocity oscillation, which markedly affects efficiency. It seemed therefore that locating of the combusion chamber midway between the velocity and pressure nodes would produce a condition most susceptible to oscillation. Pressure recordings in the afterburner showed a different type of pulsation which was the result of the forward propagation of pressure waves generated in the flame zone. These waves had a steep leading edge and their progress from point to point was recorded at the different probes. The maximum magnitude of the pressure rise in such wave fronts was of the order 15 p.s.i. when the mean afterburner pressure was approximately 2 atmospheres. It was speculated that such pressure fronts passing over the constant flow fuel injectors would induce variations in air fuel ratio because of the variation of instantaneous mass flow during the passage of the wave front. These variations in air fuel ratio would then be carried down stream with the mean flow velocity and in turn give rise to further pressure waves on reaching the combustion zone. This picture of the mechansim of generation of pulsations leads to valuable design rules concerning the spatial distribution of the injector and flame stabilizing components. This investigation is described in more detail in N. T. G. E. Report No. R85 by H. M. Nicholson and A. Radcliffe. In subsequent investigations at N. T. G. E. the theoretical aspects of this type of oscillation have been further investigated by D. G. Stewart who has used the method of characteristic mapping to determine the pressure wave system generated when an enriched zone
of mixture passes into the combustion zone. His preliminary findings indicate that in a typical ram jet flow regime, if the temperature rise during combustion be increased from 500~ to 1600~ due to the passage of a zone where fuel air ratio increases linearly for 3.4 msec., the forward going compression wave reduces the Mach number transiently from an initial value of 0.2 to zero. This investigation is about to be published as an N. G. T. E. report. DISCUSSION BY E. A. DEZuBAu The authors, in the derivation of equation (2) given in the form of Tt
16L~ n~ 7'0 p0
ypi
imply that the initial state of the mixture designated by To and o0 together with the tube pressure pl must be known to evaluate the flame temperature T1. Since the acoustic velocity is
dp
V~ = ~
at constant e n t r o p y or
V = "~v/yRV, where R is the gas constant in appropriate units. Hence by substituting the resonance condition V = 4L~ m yields
yR which is the same as the form deduced by the authors if P0 = pt. It must be emphasized that the acoustic velocity as used in this paper can only he dependent on the conditions of the burning gas and cannot be dependent on the inlet conditions. * Westinghouse Research Laboratories, East Pittsburgh, Pa.