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Burning velocity of methane-air flames
BURNING
VELOCITY
OF
METHANE-AIR
FLAMES
I. F E L L S A N D A. G~ R U T H E R F O R D Department of Chemical Engineering, University of Newcastle upon Tyne. Burning velocity measurements using stationary flames have been refined to give results with a mean error as low as _+1 per cent. The method employsnozzle burners and a single pass schlieren system. Precautions have been taken to minimize errors at each stage of the experiments and calculations. The influenceof burnerdesign and the use of approximations in burning velocity measurements have also been investigated. The maximum burning velocity of methane in air at 25°C and 760 mm of mercury is concluded to be 39.6 + 0.4 cm/sec at a methane concentration of 10.16 + 0~)6per cent by volume. The effects of the following additives on methane-air burning velocities are reported: nitrogen, carbon dioxide, water vapour, methanol, acetaldehyde, formaldehyde, nitromethane, nitrogen dioxide and hydrogen peroxide. The retarding effects on burning velocity of the first three, additives were in the order nitrogen < water vapour < carbon dioxide. These effects could be explained on the grounds of dilution and heat abstraction. No additive produced any flame-promoting effect and the burning velocity results obtained could be explained by the thermal properties of the additives or by their actions as fuels or oxidants. Nitromethane was found to be capable of acting simultaneously as both a fuel and an oxidant.
lutroduetion
Gas temperatures were carefully controlled to 25°+_ 0~5°C and the burning velocities reported are for an ambient pressure of 760 m m of mercury. The optical arrangement adopted to locate the flame front was a monochromatic single pass schlieren system using high quality lenses with a triangular cut-off. A very narrow schlieren image of the flame front was obtained, free from spherical aberration.
THE combustion characteristics of methane are of particular significance in Britain today since the advent of North Sea natural gas. The burning velocity is an important parameter and although a considerable amount of work has been published on the topic, there is some disagreement on the exact value of maximum burning velocity and on the composition at which this occurs. The object of the research reported here was to refine the measurement of burning velocity of stationary flames, investigate burner effects on the measurements and to compare the merits of various approximations commonly used in calculation. The possibility of increasing the burning velocity of methane by the use of additives has also ~ been studied along with effects of some common gaseous impurities. Further details of theory, background literature and experimental techniques have been reported by Rutherford ~.
(i) Burner design Type A : This bell-shaped burner was based on the design of Scholte and Vaags 2 but a watercooling jacket was added. F o r each burner, extension tings of various thicknesses were made so that the burner neck could be lengthened. Type B: This design, illustrated in Figure 1, is a sharp-edged orifice. Various types and sizes of orifice were interchangeable by making the top plate removable.
Experimental The methane was 99-5 per cent pure and contained less than 0-004 per cent water vapour.
(ii) Performance tests on burners The success of a nozzle burner in producing perfectly conical flames depends upon the 130
April 1969
BURNING
VELOCITY
OF METHANE--AIR
131
FLAMES
1-0-
,1
L Water jacket
0.8
2~2in. Gauze screens J
21/~in.
I
0uO-
(a)
Bert-shaped nozzle
6
0
~ O-4
!
21'zin
0
1
t~____Water jacket
(a)
0-2 =
Gauze screens
|
~.
2 ~ in.
(b)
6 4 2 0 2 4 6 Distance from nozzle centre, mm
FIGURE 1. Sharp-edged burner designs
production of a gas jet with a uniform velocity and temperature profile. Velocity profiles were measured on 0.5 in. diameter burners using a very small total head impact tube and the results are shown in Figure 2. The velocities are expressed as fractions of the maximum velocity head in each case. The sharp-edged orifice produced a slightly narrower jet and steeper velocity gradient than the bellshaped nozzle but this information is limited in its application as it does not indicate the direction of flow at the stream boundaries, i.e. only the vertical components are measured. Temperature profiles were obtained with a small thermocouple and each reading was obtained both for a cold nozzle, i.e. before ignition, and after extinguishing a flame which had been burning for at least 30 minutes. The results are shown in Figure 3. Two observations are important with regard to the bell-shaped nozzle: (i) Although the water cooling is nearer to the burner rim than in the sharp-edged nozzle, there is a definite temperature rise in the jet boundaries. (ii) The temperature rise is outside the limits of the burner rim.
1"0
0-8
Sharp-edged orifice
,*2
u0"6 O > O >
0"4 0
(b)
0"2
0 ~ ~
!
I
!
I
1
l
4 2 0 2 4 6 Distance from orifice centre, mm
6
FIGURE 2. Velocity profiles (a) Bell-shaped nozzle: (b) Sharp-edged orifice
(iii) The production of conicalflames On nozzle burners the most common type of flame obtained with a slow burning mixture has convex sides as shown in Figure 4(a). Scholte and V a a g s 2 solved this problem by adding rings
132
Vol. 13
1. FELLS AND A. G. RUTHERFORD
to extend the burner neck and so produce more drag in the boundary layers This procedure was found undesirable in the research reported here as results sb,owed that burning velocity could be considerably affected by the presenceof rings. It was found that convex distortions could usually be removed by increasing the flowrate or using a smaller diameter nozzle, i.e. by increasing the linear gas velocity.
(a)
-"55 ~o'-' 25C
(a) 25.s
2
5
_
.
0
(b)
~
2/-,.5
.
24-0
25.0[ 24"5[
8-5 e5 4 - 5 2 - 5 0 2 - 5 4 " 5 6"58-5 Distance from nozzle centre, m m
~
- 24.ok.~" 3
. .
23.5 T--
(c) .
.
.
.
.
.
.
.
-
I..
E
2
5
.
o
~
(d)
24"5~ .
.
98?654321
.
.
.
.
.
.
.
.
0123456?89
Distance from o r i f i c e c e n t r e , m m
(b)
FIGURE3. Temperature profiles: (a), (b) Bell-shaped nozzle; (c), (d) Sharp-edged orifice
FIGURE 4. (a) Common type of non-conical flame" (b) Conical flame
The effect of nozzle size on burning velocity was investigated by comparing values obtained using identical linear gas velocities and gas compositions on the bell-shaped and sharpedged nozzles with diameters varying from 0.4 to 0-6 in. No dependence on nozzle diameter was detected. There was, however, a difference between burning velocities on the bell-shaped burners and those on the sharp-edged orifices. This is discussed later. The effects of gas velocity over the range tested, which was 2-1 to 3.6 ft/s (64 to 1l0 cm/s), are pcesentdd in Figure 5.
For the bell-shaped burner there appeared to be a slight decrease in burning velocity with increasing gas speed although this variation was of the same order as the experimental error. With the sharp-edged orifice, however, no such variation could be detected. A variation of burning velocity similar to that shown in Figure 5(a) has been reported by other authors (Halpern 3, Caidwell et al. 5) and a possible reason is heat transfer from the nozzle to the unbumt gases. At high flowrates this would be less serious and it would seem that the lower values of burning velocity would be the more
April 1969
the volume flow rate, V, of a combustible mixture is known, and the area of the flame front, A, can be measured, the burning velocity, S,, is given by
(a)
38 37
36
S,, --V/A
[I]
If the flame front is a perfect right cone, then from the apex half-angle, ~, and the linear gas velocity, U
(b) 38
133
BURNING VELOCITY OF METHANE--AIR FLAMES
-
S= = U sin ~
:t
(c)
60
70
80
9
100
110
Uu, cm/s Methane percentages (a) 9-28, (b)10-20, (c) 11-26
38I
(d)
37 36 35
E 40 u 39 38
I
(e)
36I
35 34 33
(f) 0
v
i
i
L
60
'70
80
i
i
i
90 100 110 Uu, crn/s M e t h a n e percentages : (d) 9.28, (e) 10-20, (f) 11.26
FIGURE 5. Effect of gas velocity on burning velocity. (a), (b), (c) Bell-shaped nozzle; (d), (e), (f) Sharp-edged orifice
accurate. This hypothesis is supported by the effects of burner design which are discussed later. (iv) Measurement from schlieren photographs Burning velocity is defined as the speed of propagation of a flame, normal to the flame front, relative to the unburnt mixture. Hence if
[2]
This obviates the measurement of the flame area but the work of Halpern 3 and the research reported here have shown that this method can be inaccurate by as much as + 30 per cent. Halpern 3, who used a bell-type burner, suggested that the reason for this was that the inner flame cone rarely had a diameter equal to that of the burner port. This is confirmed by the present work. Flame cone diameters were found to vary with gas velocity and with fuel/air ratio and the flames on the bell-shaped burners all had diameters larger than the burner port. This was probably due to the well known Coanda effect 4 in which a fluid emerging from a nozzle tends to cling to the walls and spill out over the edges. Using sharp-edged nozzles it was found that cone diameters were always less than the orifice diameter and showed a much smaller dependence on gas velocity. This demonstrated the lack of flow divergence using this design of burner. When rings were added to a sharpedged nozzle a marked increase in flame base diameter occurred. As cone base diameter, and hence cone apex angle, is a function of both gas velocity and burner design, the cone angle technique cannot be used as an accurate method of burning velocity measurements. Area techniques--When a perfectly straightsided cone was produced there was no problem in measuring schlieren cone surface areas. There was very little scatter of results if the least exposed photographic negatives were chosen, i.e. those with very narrow reaction zones. All deviations from corneal shape were of a convex nature as shown by Figures 4(a) and 6. The most accurate method of area determination in such cases is to divide the cone into a large
I. FELLS AND A. G. RUTHERFORD
134
The approximations tested were as follows" (1) Using D and !, the flame was treated as a perfect cone. These were used to calculate sin 0c which was substituted into equation 3. (Low burning velocities could be expected since the calculated surface area is slightly too large.) (2) Using sin a from above, but 1), instead of O, equation 3 was again used, i.e. using the actual cone angle and base diameter, the flame was treated as a perfect cone. (This represents the cone shown by the broken lines of Figure 6. hence high burning velocities can be expected.) (3) Using a planimeter, or by counting squares when the cone was drawn on graph paper, the true cross-sectional area, A~, was calculated. This was substituted into equation 4 and the value of A thus obtained was inserted in equation 1. (This gives A~ correctly but on using equation 4 a perfect cone is assumed and l would be too large. L3w burning velocities could thus be expected.) (4) Having calculated h as in method 1, it was used with D, and the appropriate perfect cone equations, i.e. a new value of i or sin • had to be calculated and equations 3 or 4 employed. (This assumes that the flame front is a perfect cone with base diameter Dc and height h.)
l ~f
i-' !
i r----
~
! !
Vol. 13
!
I I
FIGURE 6. Parameters of a non-conical flame
number of segments. Most authors have, however, approximated in some way and it was therefore decided to examine possible approximations to determine the best. Nine such flames were chosen covering a range of mixture compositions and flowrates on both bell-shaped and sharp-edged nozzles. The approximations tested are described with reference to Figure 6.
TABLE 1. Results of approximation methods when measuring burning velocity
Burner tvpe Methane and diameter Flowrate (°'°) (in.) (cm 3/s) 9-73 9.97 10-44 10.2.0 11-26 11.26 11.44 11.63 12-05
B 0.5* B 0-5 B 0.5 A 0-5 B 0-5 A 0.5 B 0.4 A 0.5 B 0,5
127.1 100-3 139-4 139.4 129.3 129-3 87.4 163.6 102.6
1.
Burning velocity (cm/s) Method 2, 3,
35-53 33-32 36-18 34.44 32-24 30.43 30.81 31-82 25.72
40.79 44-32 40-75 45.33 38.94 41-1! 36.88 34.66 30.29
3604 34438 36-30 35-16 32-55 31-11 30.86 32-36 26-16
4.
Burning velocity (t'm/s) by segmentation
38.40 39-82 38.82 40.46 35-65 36-21 33.94 33-31 28.17
38.72 39.50 38.95 40436 35.04 35.54 33-94 32-81 29.44
* A denotes bell-shaped; B is sharp-edged orifice type.
Three equations can be used: S. = V / A
[1]
S, = 4 V sin ot/nD~
[3]
A = rcAll/h
[4]
where A t is the cross-sectional cone area and A is the total surface area.
The results are presented in Table 1 and it can be seen that method 4 gave the best approximation. This method also gave the best reproducibility of results. If perfect cone equations are to be used for flames which deviate even slightly from conical shape, they should be based on a cone area defined by the vertical height and the base
April 1969
nURmNGVELOCITYov MmrHANE--Am ELAM~
diameter of the actual flame. This involves measuring D, and h of Figure 6 and ignoring the actual values of D, I and ~. For results given in this report, segmentation was used for flames which showed any deviation from conical shape. (v) The effect of lmrner design on burniru3velocity A series of methane-air compositions and flowrates were chosen which gave good conical flames on both types of burner. Burning velocities were measured with rings added and without. The results are presented in Figure 7.
42 40 38 36
E tO to
34o = S h a r p - e d g e d orifice ' I ~
32
• = S h a r p - e d g e d orifice and rings I
A = Bell-shaped
30
nozzte
t
~ ,
• ' = Betl.-shaped nozzle and r=ngs
28 26
I
9
I
10
I
I
11 % Methane
I
I
12
FIGURE 7. Burning velocity plotted against methane-air composition for the various types of burners
If rim quenching were invoked to explain the curves of Figure 7, the highest curve would be nearest to the true burning velocity. The other curves would be lower supposedly due to heat abstraction by the nozzles. From their design and the efli~ctiveness of their water cooling without rings, the bell-shaped burners would be expected to give lower results than the sharpedged orifices. The latter, with their very thin rims, would be expected to give the highest results of all when in fact they yield the lowest. The theory advanced here is that the lowest curve is nearest to the true burning velocity
135
and the other values are increased due to heat transfer from the nozzle to the unburnt gases. This is supported by the results of temperature and velocity profiles, the effect of gas velocity illustrated in Figure 5(a) and by the observed heating of the burner rims: the extension rings, when in use, got hot (over 200°C) and presumably more of this heat reached the unburnt gases from the rings on the sharp-edged orifices due to the less efficient water cooling. It has been mentioned that cone base diameters with sharp-edged orifices were less than the burner port diameters, i.e. the flames stabilized within the profile of the burner port whereas on the bell-shaped burners the flames spread out over the rim. This factor, together with the very small contact area between gas and burner rim, must account for the lack of rim heating with the sharp-edged orifices. For the final burning velocities obtained on these orifices (without rings) it is believed that ~,eat loss can be considered to be negligible. (vi) Accuracy From the estimated accuracy of measurement and the observed variances in reproducibility, the total experimental errors over all burning velocities measured w e r e ' m e a n error" 1.9 per cent; maximum error 4.3 per cent. The maximum error was only approached by the very worst flames, i.e. those at low flowrates, with fairly 'thick' reaction zones and deviating somewhat from conical shape. If these flames are ignored it is believed that the mean error for conical flames is about + 1 per cent. The Influence of Additives on Burning Velocity Additives chosen were: nitrogen, carbon dioxide, water vapour, methanol, nitromethane, formaldehyde, acetaldehyde, nitrogen dioxide and hydrogen peroxide. Hydrogen, oxygen and carbon monoxide were excluded as their effects have been extensively studied, e.g. by Jahn 6, Scholte and Vaags 2. The reasons for this choice were: nitrogen is present in all natural gases; carbon dioxide is produced in large quantities during gas reforming and the effect of water vapour is important in storage and distribution problems. Carbon dioxide and water vapour are of course
L FELLSAND A. G. R ~ O R D Vol. 13 136 It can be seen from Figure 8 that the presence also products of the combustion reaction. All of nitrogen in North Sea natural gas (around the other additives are either oxidants or fuels three per cent by volume) is unlikely to affect structurally related to methane, and are known flame stability. Carbon dioxide, and to a lesser to dissociate into radicals and atoms which have extent water vapour, is likely to be a more been observed in methane-air flames (Fristrom, serious problem if present in excessive amounts Grunfelder and FavinT). (say five per cent or more). Fortunately, such Two flames were chosen for these experiamounts are not encountered in natural gases ments: Flame A: 9-86 per cent methane, 150 and the partial removal of carbon dioxide from cmJ/s, i.e. on the fuel-lean side of maximum reformed gases is a comparatively easy operaburningvelocity. tiorL Flame B: 10.59 per cent methane, 140 cm3/s The slopes for carbon dioxide and nitrogen i.e. on the fuel-rich side of maximum burning are in reasonable agreement with the more velocity. restricted data of Scholte and Vaags 2 who added Both flames gave straight-sided cones on the five per cent carbon dioxide and five and ten 0.5 in. sharp-edged orifice burner. The above per cent nitrogen to a maximum burning compositions had similar burning velocities and velociL methane-air flame. Their slope values were suitable for showing up any changes in were m(N2) = 0.186 and re(CO2) = 0.412. burning velocity due merely to fuel or oxidant enrichment. (ii) Methanol, acetaldehyde, formaldehyde, nitrogen dioxide and hydrogen peroxide (i) Nitrogen, carbon dioxide and water vapour With all these additives the effects of additions The results are shown collectively in Figure 8 upon burning velocity could be explained on where average lines have been drawn for the the grounds of the additives' action as a fuel or effects on flames A and B of each additive. oxidant. In no case was there evidence for catalytic promotion or retardation of reaction. Frame A+N2e + I'L,zO • + CO2 = Small additions of a combustible additive in39 F t a m e B + N 2 o + H2Oo +CO2~ creased the burning velocity of flame A (the fuel-lean flame) through a maximum burning 3e velocity which was close to that for pure methane-air flames. Similar additions caused a 37 corresponding decrease in the burning velocity when the additive was an oxidant. Quantitative 36 differences in the changes of burning velocity could be explained by invoking thermal or E 35 diffusion concepts or on the extra oxygen required or supplied by the additive. Figure 9(a) m 3/. has been included here showing the typical effects of combustible additives (methanol and 33 acetaldehyde) for comparison with the nitro32 methane additions discussed below. i/1
g
31 .L .. 0
l 0-05
i 0-10
i 015
Additive A d d i t i v e + CHz, S[opes:m{N~ =0.1L, ---"-re|H20)=0-31 m(C02}=0.44
cm.s'l(% additive) "1
FIGURE 8. Effects on methane-air burning velocities of nitrogen, carbon dioxide and water vapour additions
(iii) N itromethane The results of nitromethane addition are shown in Figure 9(b). As this compound is also a fuel (heat of combustion 170 kcal) the enrichment effects of small additions are as expected, i.e. with 0.5 per cent additions burning velocity beg:ns to increase with flame A and decrease with flame B. Using acetaldehyde and methanol
April 1969
B U R N I N G VELOCITY OF METHANE--AIR FLAMES
40
38 36
3/,
~ 32 3(1 28 26 24 0
1.0 2-0 A dd i t ive ~% total flow
3-0
* Flame A • Flame B
40-
~ 3e
3/`
70
,
,
I
*
I
*
10 2-0 CHINO z, % t o t a l f l o w
I
30
FIGURE 9. (a) Effe,.'ts of methanol and acetaldehyde and (b) effects of nitromethane, ,m methane-air burning velocities
[Figure 9(a)] large: additions resulted in a fairly rapid decrease in burning velocity as expected from the enrichment concept. With nitromethane, however, it can be seen that this fall-off in burning velocity was very slow indeed. Even with additions of three per cent of the total flow (i.e. about 30 per cent of the methane flow) the burning velocities of flames A and B were maintained above 38 crn/s and 35 em/s respectively. As fairly high burning velocities have been maintained in fuel-rich mixtures, the effect is similar to adding both a fuel and an oxidant to the methane. Cottrell, Graham and Reid 8 studied the pyrolysis of nitromethane i n a static system and concluded that the most probable first step was C - - N bond rupture. This could occur in the
137
pre-reaction period of a premixed flame, supplying the reaction zone with combustible methyl radicals and nitrogen dioxide as an oxidant. (The flames reported here showed a definite pale green/yellow colouration with nitromethane additions. When nitrogen dioxide was added a similar radiation was observed.) Hillenbrand and Kilpatrick 9 have studied nitromethane pyrolysis in a flow system at higher temperatures and found considerable amounts of formaldehyde in the early stages of the reaction. It would therefore seem that nitromethane may be capable of maintaining the chain reactions of methane combustion by production of active species. This would explain the high burning velocities of Figure 9(b) in spite of fuel enrichment. It could also explain why Powling 12 observed widening of flammability limits of n-butane in air when nitromethane was added. A fuller investigation would seem necessary but if the above reasoning proves correct nitromethane may have practical value in hydrocarbon fuel mixtures where the supply of oxidant is limited.
Conclusians The use of sharp-edged orifice burners and total area measurement of the schlieren cones proved successful in burning velocity determinations. Accurate measurement of gas flowrates is essential and all physical variables must be strictly controlled. It is also necessary to produce a primary reaction zone as near as possible to perfectly conical, in particular, the sides should be straight. The exposure time of photographs should be as short as possible and special care is needed in their interpretation. The optical system must be free from all spherical aberrations. Heat transfer to the burner rim can result in increased burning velocities, by preheating the boundaries of the unburnt gases. This effect can be avoided, or at least minimized, by using sharp-edged orifices. The maximum burning velocity of methane in air at 25 C and 760 mm of mercury was found to be 39.6 + 0.4 cm/s at a methane concentration of 10.16 + 0.06 per cent by volume. The addition of 'inert' additives showed that nitrogen had only a small dilution effect and its
VoL 13 I. FELLSAND A. G. RUTHERFORD 138 We are indebted to the Gas Council for propresence in natural gas is unlikely to cause invision of a Scholarship to A. G. Rutherford which stability problems. Carbon dioxide proved more made this work possible. retarding and water had an effect roughly intermediate between those of nitrogen and (Received March 1968; revised June 1968) carbon dioxide. However, the vapour pressure of water at normal temperatures is too low to cause any serious flame stability problems in References natural gas, even if the latter were saturated. I RUTHERFORD, A. G. Ph.D. Thesis. University of NewThe effects of all additives could be explained castle upon Tyne (1966) by their actions as fuels, oxidants or diluents. In 2 SCHOLTE,T. G. and VAAGS,P. B. Combustion & Flame, 3, no case was there evidence of true catalysis of 495 (1959) the flame reaction. Similar results were obtained 3 I-I~PL~N, C. J. Res. Nat. Bur. Stand. 60, 535 (1958) 4 REINr~, M. Physics Today, 9, 16 (1956) by Yumlu 13 who worked with propane-air s CALDW~,.~, F. R., BaO~DA, H. P. and DOVER, J. J. flames and added nitrogen, oxygen, nitrous Industr. Engng Chem. (lndustr.), 43, 2731 (1951) oxide, hydrogen, hydrogen sulphide, acetylene, 6 JAHN, G. Der Ziindvorgang in Gasgemischen. Oldenburg: ber,zene and methane. Yumlu found that for all Berlin (1934) additives except hydrogen sulphide, which was FRISTROM, R. M., GRUNFELDER, C. and FAr,N, S. J. phys. Chem. 65, 587 (1961) inhibitory, burning velocities were influenced s Corrar.LL, T. L., GRAHAM,T. E. and RE~D, T. J. Trans. only by changes in the stoichiometry and final Faraday Soc. 47, 584 (1951) flame temperature of the mixtures. 9 HILLENBRAND, L. J. and KILPATRICK, M. L. J. chem. Nitromethane produced the most interesting Phys. 21, 525 (1963) to ROSSER, JR, W. A. and WIsE, H. Stanford Res. Inst. results and was found to be capable of sustaining Project No. SU- 1715 (I 957) fairly high burning velocities at large additions. it DE J~ffX;RE, S. and VAN Tm~3ELEN, A. Combustion & This was attributed to a dual fuel-oxidant Flame, 3, 187 (1959) • action and/or the supply of active centres by the t z PowusG, J. Ph.D. Thesis?University of London (1946) nitromethane. t3 YUMLU, V. A. Combustion & Flame, 12, 14 (1968)
VELOCITY
OF
METHANE-AIR
FLAMES
I. F E L L S A N D A. G~ R U T H E R F O R D Department of Chemical Engineering, University of Newcastle upon Tyne. Burning velocity measurements using stationary flames have been refined to give results with a mean error as low as _+1 per cent. The method employsnozzle burners and a single pass schlieren system. Precautions have been taken to minimize errors at each stage of the experiments and calculations. The influenceof burnerdesign and the use of approximations in burning velocity measurements have also been investigated. The maximum burning velocity of methane in air at 25°C and 760 mm of mercury is concluded to be 39.6 + 0.4 cm/sec at a methane concentration of 10.16 + 0~)6per cent by volume. The effects of the following additives on methane-air burning velocities are reported: nitrogen, carbon dioxide, water vapour, methanol, acetaldehyde, formaldehyde, nitromethane, nitrogen dioxide and hydrogen peroxide. The retarding effects on burning velocity of the first three, additives were in the order nitrogen < water vapour < carbon dioxide. These effects could be explained on the grounds of dilution and heat abstraction. No additive produced any flame-promoting effect and the burning velocity results obtained could be explained by the thermal properties of the additives or by their actions as fuels or oxidants. Nitromethane was found to be capable of acting simultaneously as both a fuel and an oxidant.
lutroduetion
Gas temperatures were carefully controlled to 25°+_ 0~5°C and the burning velocities reported are for an ambient pressure of 760 m m of mercury. The optical arrangement adopted to locate the flame front was a monochromatic single pass schlieren system using high quality lenses with a triangular cut-off. A very narrow schlieren image of the flame front was obtained, free from spherical aberration.
THE combustion characteristics of methane are of particular significance in Britain today since the advent of North Sea natural gas. The burning velocity is an important parameter and although a considerable amount of work has been published on the topic, there is some disagreement on the exact value of maximum burning velocity and on the composition at which this occurs. The object of the research reported here was to refine the measurement of burning velocity of stationary flames, investigate burner effects on the measurements and to compare the merits of various approximations commonly used in calculation. The possibility of increasing the burning velocity of methane by the use of additives has also ~ been studied along with effects of some common gaseous impurities. Further details of theory, background literature and experimental techniques have been reported by Rutherford ~.
(i) Burner design Type A : This bell-shaped burner was based on the design of Scholte and Vaags 2 but a watercooling jacket was added. F o r each burner, extension tings of various thicknesses were made so that the burner neck could be lengthened. Type B: This design, illustrated in Figure 1, is a sharp-edged orifice. Various types and sizes of orifice were interchangeable by making the top plate removable.
Experimental The methane was 99-5 per cent pure and contained less than 0-004 per cent water vapour.
(ii) Performance tests on burners The success of a nozzle burner in producing perfectly conical flames depends upon the 130
April 1969
BURNING
VELOCITY
OF METHANE--AIR
131
FLAMES
1-0-
,1
L Water jacket
0.8
2~2in. Gauze screens J
21/~in.
I
0uO-
(a)
Bert-shaped nozzle
6
0
~ O-4
!
21'zin
0
1
t~____Water jacket
(a)
0-2 =
Gauze screens
|
~.
2 ~ in.
(b)
6 4 2 0 2 4 6 Distance from nozzle centre, mm
FIGURE 1. Sharp-edged burner designs
production of a gas jet with a uniform velocity and temperature profile. Velocity profiles were measured on 0.5 in. diameter burners using a very small total head impact tube and the results are shown in Figure 2. The velocities are expressed as fractions of the maximum velocity head in each case. The sharp-edged orifice produced a slightly narrower jet and steeper velocity gradient than the bellshaped nozzle but this information is limited in its application as it does not indicate the direction of flow at the stream boundaries, i.e. only the vertical components are measured. Temperature profiles were obtained with a small thermocouple and each reading was obtained both for a cold nozzle, i.e. before ignition, and after extinguishing a flame which had been burning for at least 30 minutes. The results are shown in Figure 3. Two observations are important with regard to the bell-shaped nozzle: (i) Although the water cooling is nearer to the burner rim than in the sharp-edged nozzle, there is a definite temperature rise in the jet boundaries. (ii) The temperature rise is outside the limits of the burner rim.
1"0
0-8
Sharp-edged orifice
,*2
u0"6 O > O >
0"4 0
(b)
0"2
0 ~ ~
!
I
!
I
1
l
4 2 0 2 4 6 Distance from orifice centre, mm
6
FIGURE 2. Velocity profiles (a) Bell-shaped nozzle: (b) Sharp-edged orifice
(iii) The production of conicalflames On nozzle burners the most common type of flame obtained with a slow burning mixture has convex sides as shown in Figure 4(a). Scholte and V a a g s 2 solved this problem by adding rings
132
Vol. 13
1. FELLS AND A. G. RUTHERFORD
to extend the burner neck and so produce more drag in the boundary layers This procedure was found undesirable in the research reported here as results sb,owed that burning velocity could be considerably affected by the presenceof rings. It was found that convex distortions could usually be removed by increasing the flowrate or using a smaller diameter nozzle, i.e. by increasing the linear gas velocity.
(a)
-"55 ~o'-' 25C
(a) 25.s
2
5
_
.
0
(b)
~
2/-,.5
.
24-0
25.0[ 24"5[
8-5 e5 4 - 5 2 - 5 0 2 - 5 4 " 5 6"58-5 Distance from nozzle centre, m m
~
- 24.ok.~" 3
. .
23.5 T--
(c) .
.
.
.
.
.
.
.
-
I..
E
2
5
.
o
~
(d)
24"5~ .
.
98?654321
.
.
.
.
.
.
.
.
0123456?89
Distance from o r i f i c e c e n t r e , m m
(b)
FIGURE3. Temperature profiles: (a), (b) Bell-shaped nozzle; (c), (d) Sharp-edged orifice
FIGURE 4. (a) Common type of non-conical flame" (b) Conical flame
The effect of nozzle size on burning velocity was investigated by comparing values obtained using identical linear gas velocities and gas compositions on the bell-shaped and sharpedged nozzles with diameters varying from 0.4 to 0-6 in. No dependence on nozzle diameter was detected. There was, however, a difference between burning velocities on the bell-shaped burners and those on the sharp-edged orifices. This is discussed later. The effects of gas velocity over the range tested, which was 2-1 to 3.6 ft/s (64 to 1l0 cm/s), are pcesentdd in Figure 5.
For the bell-shaped burner there appeared to be a slight decrease in burning velocity with increasing gas speed although this variation was of the same order as the experimental error. With the sharp-edged orifice, however, no such variation could be detected. A variation of burning velocity similar to that shown in Figure 5(a) has been reported by other authors (Halpern 3, Caidwell et al. 5) and a possible reason is heat transfer from the nozzle to the unbumt gases. At high flowrates this would be less serious and it would seem that the lower values of burning velocity would be the more
April 1969
the volume flow rate, V, of a combustible mixture is known, and the area of the flame front, A, can be measured, the burning velocity, S,, is given by
(a)
38 37
36
S,, --V/A
[I]
If the flame front is a perfect right cone, then from the apex half-angle, ~, and the linear gas velocity, U
(b) 38
133
BURNING VELOCITY OF METHANE--AIR FLAMES
-
S= = U sin ~
:t
(c)
60
70
80
9
100
110
Uu, cm/s Methane percentages (a) 9-28, (b)10-20, (c) 11-26
38I
(d)
37 36 35
E 40 u 39 38
I
(e)
36I
35 34 33
(f) 0
v
i
i
L
60
'70
80
i
i
i
90 100 110 Uu, crn/s M e t h a n e percentages : (d) 9.28, (e) 10-20, (f) 11.26
FIGURE 5. Effect of gas velocity on burning velocity. (a), (b), (c) Bell-shaped nozzle; (d), (e), (f) Sharp-edged orifice
accurate. This hypothesis is supported by the effects of burner design which are discussed later. (iv) Measurement from schlieren photographs Burning velocity is defined as the speed of propagation of a flame, normal to the flame front, relative to the unburnt mixture. Hence if
[2]
This obviates the measurement of the flame area but the work of Halpern 3 and the research reported here have shown that this method can be inaccurate by as much as + 30 per cent. Halpern 3, who used a bell-type burner, suggested that the reason for this was that the inner flame cone rarely had a diameter equal to that of the burner port. This is confirmed by the present work. Flame cone diameters were found to vary with gas velocity and with fuel/air ratio and the flames on the bell-shaped burners all had diameters larger than the burner port. This was probably due to the well known Coanda effect 4 in which a fluid emerging from a nozzle tends to cling to the walls and spill out over the edges. Using sharp-edged nozzles it was found that cone diameters were always less than the orifice diameter and showed a much smaller dependence on gas velocity. This demonstrated the lack of flow divergence using this design of burner. When rings were added to a sharpedged nozzle a marked increase in flame base diameter occurred. As cone base diameter, and hence cone apex angle, is a function of both gas velocity and burner design, the cone angle technique cannot be used as an accurate method of burning velocity measurements. Area techniques--When a perfectly straightsided cone was produced there was no problem in measuring schlieren cone surface areas. There was very little scatter of results if the least exposed photographic negatives were chosen, i.e. those with very narrow reaction zones. All deviations from corneal shape were of a convex nature as shown by Figures 4(a) and 6. The most accurate method of area determination in such cases is to divide the cone into a large
I. FELLS AND A. G. RUTHERFORD
134
The approximations tested were as follows" (1) Using D and !, the flame was treated as a perfect cone. These were used to calculate sin 0c which was substituted into equation 3. (Low burning velocities could be expected since the calculated surface area is slightly too large.) (2) Using sin a from above, but 1), instead of O, equation 3 was again used, i.e. using the actual cone angle and base diameter, the flame was treated as a perfect cone. (This represents the cone shown by the broken lines of Figure 6. hence high burning velocities can be expected.) (3) Using a planimeter, or by counting squares when the cone was drawn on graph paper, the true cross-sectional area, A~, was calculated. This was substituted into equation 4 and the value of A thus obtained was inserted in equation 1. (This gives A~ correctly but on using equation 4 a perfect cone is assumed and l would be too large. L3w burning velocities could thus be expected.) (4) Having calculated h as in method 1, it was used with D, and the appropriate perfect cone equations, i.e. a new value of i or sin • had to be calculated and equations 3 or 4 employed. (This assumes that the flame front is a perfect cone with base diameter Dc and height h.)
l ~f
i-' !
i r----
~
! !
Vol. 13
!
I I
FIGURE 6. Parameters of a non-conical flame
number of segments. Most authors have, however, approximated in some way and it was therefore decided to examine possible approximations to determine the best. Nine such flames were chosen covering a range of mixture compositions and flowrates on both bell-shaped and sharp-edged nozzles. The approximations tested are described with reference to Figure 6.
TABLE 1. Results of approximation methods when measuring burning velocity
Burner tvpe Methane and diameter Flowrate (°'°) (in.) (cm 3/s) 9-73 9.97 10-44 10.2.0 11-26 11.26 11.44 11.63 12-05
B 0.5* B 0-5 B 0.5 A 0-5 B 0-5 A 0.5 B 0.4 A 0.5 B 0,5
127.1 100-3 139-4 139.4 129.3 129-3 87.4 163.6 102.6
1.
Burning velocity (cm/s) Method 2, 3,
35-53 33-32 36-18 34.44 32-24 30.43 30.81 31-82 25.72
40.79 44-32 40-75 45.33 38.94 41-1! 36.88 34.66 30.29
3604 34438 36-30 35-16 32-55 31-11 30.86 32-36 26-16
4.
Burning velocity (t'm/s) by segmentation
38.40 39-82 38.82 40.46 35-65 36-21 33.94 33-31 28.17
38.72 39.50 38.95 40436 35.04 35.54 33-94 32-81 29.44
* A denotes bell-shaped; B is sharp-edged orifice type.
Three equations can be used: S. = V / A
[1]
S, = 4 V sin ot/nD~
[3]
A = rcAll/h
[4]
where A t is the cross-sectional cone area and A is the total surface area.
The results are presented in Table 1 and it can be seen that method 4 gave the best approximation. This method also gave the best reproducibility of results. If perfect cone equations are to be used for flames which deviate even slightly from conical shape, they should be based on a cone area defined by the vertical height and the base
April 1969
nURmNGVELOCITYov MmrHANE--Am ELAM~
diameter of the actual flame. This involves measuring D, and h of Figure 6 and ignoring the actual values of D, I and ~. For results given in this report, segmentation was used for flames which showed any deviation from conical shape. (v) The effect of lmrner design on burniru3velocity A series of methane-air compositions and flowrates were chosen which gave good conical flames on both types of burner. Burning velocities were measured with rings added and without. The results are presented in Figure 7.
42 40 38 36
E tO to
34o = S h a r p - e d g e d orifice ' I ~
32
• = S h a r p - e d g e d orifice and rings I
A = Bell-shaped
30
nozzte
t
~ ,
• ' = Betl.-shaped nozzle and r=ngs
28 26
I
9
I
10
I
I
11 % Methane
I
I
12
FIGURE 7. Burning velocity plotted against methane-air composition for the various types of burners
If rim quenching were invoked to explain the curves of Figure 7, the highest curve would be nearest to the true burning velocity. The other curves would be lower supposedly due to heat abstraction by the nozzles. From their design and the efli~ctiveness of their water cooling without rings, the bell-shaped burners would be expected to give lower results than the sharpedged orifices. The latter, with their very thin rims, would be expected to give the highest results of all when in fact they yield the lowest. The theory advanced here is that the lowest curve is nearest to the true burning velocity
135
and the other values are increased due to heat transfer from the nozzle to the unburnt gases. This is supported by the results of temperature and velocity profiles, the effect of gas velocity illustrated in Figure 5(a) and by the observed heating of the burner rims: the extension rings, when in use, got hot (over 200°C) and presumably more of this heat reached the unburnt gases from the rings on the sharp-edged orifices due to the less efficient water cooling. It has been mentioned that cone base diameters with sharp-edged orifices were less than the burner port diameters, i.e. the flames stabilized within the profile of the burner port whereas on the bell-shaped burners the flames spread out over the rim. This factor, together with the very small contact area between gas and burner rim, must account for the lack of rim heating with the sharp-edged orifices. For the final burning velocities obtained on these orifices (without rings) it is believed that ~,eat loss can be considered to be negligible. (vi) Accuracy From the estimated accuracy of measurement and the observed variances in reproducibility, the total experimental errors over all burning velocities measured w e r e ' m e a n error" 1.9 per cent; maximum error 4.3 per cent. The maximum error was only approached by the very worst flames, i.e. those at low flowrates, with fairly 'thick' reaction zones and deviating somewhat from conical shape. If these flames are ignored it is believed that the mean error for conical flames is about + 1 per cent. The Influence of Additives on Burning Velocity Additives chosen were: nitrogen, carbon dioxide, water vapour, methanol, nitromethane, formaldehyde, acetaldehyde, nitrogen dioxide and hydrogen peroxide. Hydrogen, oxygen and carbon monoxide were excluded as their effects have been extensively studied, e.g. by Jahn 6, Scholte and Vaags 2. The reasons for this choice were: nitrogen is present in all natural gases; carbon dioxide is produced in large quantities during gas reforming and the effect of water vapour is important in storage and distribution problems. Carbon dioxide and water vapour are of course
L FELLSAND A. G. R ~ O R D Vol. 13 136 It can be seen from Figure 8 that the presence also products of the combustion reaction. All of nitrogen in North Sea natural gas (around the other additives are either oxidants or fuels three per cent by volume) is unlikely to affect structurally related to methane, and are known flame stability. Carbon dioxide, and to a lesser to dissociate into radicals and atoms which have extent water vapour, is likely to be a more been observed in methane-air flames (Fristrom, serious problem if present in excessive amounts Grunfelder and FavinT). (say five per cent or more). Fortunately, such Two flames were chosen for these experiamounts are not encountered in natural gases ments: Flame A: 9-86 per cent methane, 150 and the partial removal of carbon dioxide from cmJ/s, i.e. on the fuel-lean side of maximum reformed gases is a comparatively easy operaburningvelocity. tiorL Flame B: 10.59 per cent methane, 140 cm3/s The slopes for carbon dioxide and nitrogen i.e. on the fuel-rich side of maximum burning are in reasonable agreement with the more velocity. restricted data of Scholte and Vaags 2 who added Both flames gave straight-sided cones on the five per cent carbon dioxide and five and ten 0.5 in. sharp-edged orifice burner. The above per cent nitrogen to a maximum burning compositions had similar burning velocities and velociL methane-air flame. Their slope values were suitable for showing up any changes in were m(N2) = 0.186 and re(CO2) = 0.412. burning velocity due merely to fuel or oxidant enrichment. (ii) Methanol, acetaldehyde, formaldehyde, nitrogen dioxide and hydrogen peroxide (i) Nitrogen, carbon dioxide and water vapour With all these additives the effects of additions The results are shown collectively in Figure 8 upon burning velocity could be explained on where average lines have been drawn for the the grounds of the additives' action as a fuel or effects on flames A and B of each additive. oxidant. In no case was there evidence for catalytic promotion or retardation of reaction. Frame A+N2e + I'L,zO • + CO2 = Small additions of a combustible additive in39 F t a m e B + N 2 o + H2Oo +CO2~ creased the burning velocity of flame A (the fuel-lean flame) through a maximum burning 3e velocity which was close to that for pure methane-air flames. Similar additions caused a 37 corresponding decrease in the burning velocity when the additive was an oxidant. Quantitative 36 differences in the changes of burning velocity could be explained by invoking thermal or E 35 diffusion concepts or on the extra oxygen required or supplied by the additive. Figure 9(a) m 3/. has been included here showing the typical effects of combustible additives (methanol and 33 acetaldehyde) for comparison with the nitro32 methane additions discussed below. i/1
g
31 .L .. 0
l 0-05
i 0-10
i 015
Additive A d d i t i v e + CHz, S[opes:m{N~ =0.1L, ---"-re|H20)=0-31 m(C02}=0.44
cm.s'l(% additive) "1
FIGURE 8. Effects on methane-air burning velocities of nitrogen, carbon dioxide and water vapour additions
(iii) N itromethane The results of nitromethane addition are shown in Figure 9(b). As this compound is also a fuel (heat of combustion 170 kcal) the enrichment effects of small additions are as expected, i.e. with 0.5 per cent additions burning velocity beg:ns to increase with flame A and decrease with flame B. Using acetaldehyde and methanol
April 1969
B U R N I N G VELOCITY OF METHANE--AIR FLAMES
40
38 36
3/,
~ 32 3(1 28 26 24 0
1.0 2-0 A dd i t ive ~% total flow
3-0
* Flame A • Flame B
40-
~ 3e
3/`
70
,
,
I
*
I
*
10 2-0 CHINO z, % t o t a l f l o w
I
30
FIGURE 9. (a) Effe,.'ts of methanol and acetaldehyde and (b) effects of nitromethane, ,m methane-air burning velocities
[Figure 9(a)] large: additions resulted in a fairly rapid decrease in burning velocity as expected from the enrichment concept. With nitromethane, however, it can be seen that this fall-off in burning velocity was very slow indeed. Even with additions of three per cent of the total flow (i.e. about 30 per cent of the methane flow) the burning velocities of flames A and B were maintained above 38 crn/s and 35 em/s respectively. As fairly high burning velocities have been maintained in fuel-rich mixtures, the effect is similar to adding both a fuel and an oxidant to the methane. Cottrell, Graham and Reid 8 studied the pyrolysis of nitromethane i n a static system and concluded that the most probable first step was C - - N bond rupture. This could occur in the
137
pre-reaction period of a premixed flame, supplying the reaction zone with combustible methyl radicals and nitrogen dioxide as an oxidant. (The flames reported here showed a definite pale green/yellow colouration with nitromethane additions. When nitrogen dioxide was added a similar radiation was observed.) Hillenbrand and Kilpatrick 9 have studied nitromethane pyrolysis in a flow system at higher temperatures and found considerable amounts of formaldehyde in the early stages of the reaction. It would therefore seem that nitromethane may be capable of maintaining the chain reactions of methane combustion by production of active species. This would explain the high burning velocities of Figure 9(b) in spite of fuel enrichment. It could also explain why Powling 12 observed widening of flammability limits of n-butane in air when nitromethane was added. A fuller investigation would seem necessary but if the above reasoning proves correct nitromethane may have practical value in hydrocarbon fuel mixtures where the supply of oxidant is limited.
Conclusians The use of sharp-edged orifice burners and total area measurement of the schlieren cones proved successful in burning velocity determinations. Accurate measurement of gas flowrates is essential and all physical variables must be strictly controlled. It is also necessary to produce a primary reaction zone as near as possible to perfectly conical, in particular, the sides should be straight. The exposure time of photographs should be as short as possible and special care is needed in their interpretation. The optical system must be free from all spherical aberrations. Heat transfer to the burner rim can result in increased burning velocities, by preheating the boundaries of the unburnt gases. This effect can be avoided, or at least minimized, by using sharp-edged orifices. The maximum burning velocity of methane in air at 25 C and 760 mm of mercury was found to be 39.6 + 0.4 cm/s at a methane concentration of 10.16 + 0.06 per cent by volume. The addition of 'inert' additives showed that nitrogen had only a small dilution effect and its
VoL 13 I. FELLSAND A. G. RUTHERFORD 138 We are indebted to the Gas Council for propresence in natural gas is unlikely to cause invision of a Scholarship to A. G. Rutherford which stability problems. Carbon dioxide proved more made this work possible. retarding and water had an effect roughly intermediate between those of nitrogen and (Received March 1968; revised June 1968) carbon dioxide. However, the vapour pressure of water at normal temperatures is too low to cause any serious flame stability problems in References natural gas, even if the latter were saturated. I RUTHERFORD, A. G. Ph.D. Thesis. University of NewThe effects of all additives could be explained castle upon Tyne (1966) by their actions as fuels, oxidants or diluents. In 2 SCHOLTE,T. G. and VAAGS,P. B. Combustion & Flame, 3, no case was there evidence of true catalysis of 495 (1959) the flame reaction. Similar results were obtained 3 I-I~PL~N, C. J. Res. Nat. Bur. Stand. 60, 535 (1958) 4 REINr~, M. Physics Today, 9, 16 (1956) by Yumlu 13 who worked with propane-air s CALDW~,.~, F. R., BaO~DA, H. P. and DOVER, J. J. flames and added nitrogen, oxygen, nitrous Industr. Engng Chem. (lndustr.), 43, 2731 (1951) oxide, hydrogen, hydrogen sulphide, acetylene, 6 JAHN, G. Der Ziindvorgang in Gasgemischen. Oldenburg: ber,zene and methane. Yumlu found that for all Berlin (1934) additives except hydrogen sulphide, which was FRISTROM, R. M., GRUNFELDER, C. and FAr,N, S. J. phys. Chem. 65, 587 (1961) inhibitory, burning velocities were influenced s Corrar.LL, T. L., GRAHAM,T. E. and RE~D, T. J. Trans. only by changes in the stoichiometry and final Faraday Soc. 47, 584 (1951) flame temperature of the mixtures. 9 HILLENBRAND, L. J. and KILPATRICK, M. L. J. chem. Nitromethane produced the most interesting Phys. 21, 525 (1963) to ROSSER, JR, W. A. and WIsE, H. Stanford Res. Inst. results and was found to be capable of sustaining Project No. SU- 1715 (I 957) fairly high burning velocities at large additions. it DE J~ffX;RE, S. and VAN Tm~3ELEN, A. Combustion & This was attributed to a dual fuel-oxidant Flame, 3, 187 (1959) • action and/or the supply of active centres by the t z PowusG, J. Ph.D. Thesis?University of London (1946) nitromethane. t3 YUMLU, V. A. Combustion & Flame, 12, 14 (1968)