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The quenching of flames of various fuels in narrow apertures
The Quenching of Flames of Various Fuels in Narrow Apertures K. N. PALMER and P. S. TONKIN Fire Research
Station,
Boreham
Wood, Herts.
(Received August 1962) The quenching of flames propagating in a range of combustible vapours mixed with air has using perforated metal sheeting and blocks with narrow apertures. The been investigated, perforated metal was mounted inside an explosion tube along which the flame propagated. Measurement of flame velocities &owed that the velocities of vapourdir pames just able to propagate through the perforated metal were similar to those of propane-air fames. Experiments with ethylene games showed that. when the diameter of the apertures in the perforated metal was nearly as large as the quenching diameter, the flames were able to propagate through the apertures at considerably reduced velocities. This is discussed on a of cool flames by perforated metal sheeting was also theoretical basis. The quenching investigated.
on the quenching of cool flames, propagating rich fuel-air mixtures.
Introduction PERFORATED metal sheeting and blocks with narrow apertures can be used as flame arresters in pipe systems carrying flammable gas mixtures, to prevent the continued propagation Previous of flame through the gas mixture. experiments with perforated metal1 and wire propane-air gauzes, mainly with stoichiometric flames, showed that the flame propagated through apertures in metal or gauze if the flame velocity exceeded a critical value, which usually depended on the aperture diameter and depth. The flame velocity varied with the composition of the gas mixture and the layout of the pipe system, and to quench flames propagating with high velocities, the apertures in the perforated metal needed to be considerably narrower than the standard quenching diameter for the gas mixture. In the present experiments the critical velocities of stoichiometric fuel-air flames just able to propagate through perforated metals of various aperture diameters and depths were measured to determine whether these critical flame velocities varied with the fuel. As results were already available for propane flames, these were taken as a convenient reference for comparison with the results for the fuels examined. Finally, additional experiments were carried out
in
Experimental Afparatus and materials The sheeting and blocks were of brass and were perforated with circular apertures; the dimensions and spacing of the apertures in the sheeting are given in Table 1 and were accurate to within +_1 per cent. The centres of the apertures were spaced in a rectangular pattern, with an additional aperture in the centre of the rectangle; the spacings given in Table 1 are the lengths of the sides of the rectangles. The perforated blocks were 1 cm thick, drilled with apertures 0.175 cm in diameter in the same pattern as the sheeting, and could be combined in packs with the apertures accurately aligned. Table I.
Characteristics
of perforated
Thickness of sheeting,
Spacings between centres of apertures
Diameter of aperture, d cm
Y cm
0*560
0.175 0.100 0.075 0.055 121
,
O-124 0.073 0.072 0.052 0.046
cm 0.845, 0.285, 0.175, 0.140, 0.110,
1.400 0.465 0.290 0.250 0.180,
sheeting Area of aperture per unit area of sheeting P
0.41 0.37 0.31 0.26 0.24
K. N. Palmer and P. S. Tonkin
122
Table
2.
Maximum
velocities of flames quenched and minimum velocities for various fuels and sizes of perforated sheeting
T Stoichio-
metric mixture (per cent fuel in air)
Fuel, and grade of purity
Hexane (standard) Acetone (analytical) Acetaldehyde (standard) Ethyl acetate (analytical) .___ -. Benzene (analytical) Diethyl ether (analytical)
Vol.
T
Diameter 0.055 Max.
in perforated
850
1 200
1 000
000
550
0.175
cm
Min. velocity
Max. velocity quenched cm/s
l-
-I-
passed
sheeting
0,100 cm
Min. velocity passed cm/s
velocity quenched cm/s
(
._
600
cm/s
cm/s
120
140
1 000
650
800
650
1 000
of apertures
cm
of flames
7
-.-
1 100
.I
I1 050
The explosion tube was horizontal for all experiments with normal flames and for some of those with cool flames, but for the remainder of the cool flame experiments it was vertical. The tube was of Perspex, 6.4 cm in internal diameter and of wall thickness 0.6 cm, and was open at one end and closed at the other. The perforated metal was fixed across the tube, part way along its length, and the distance from the igniter (the run-up) was varied from 11.4 to 313 cm. The gas mixture was ignited at the open end of the tube either by a spark gap or, for cool flames, by a small flame. The six liquid fuels used are listed in Table 2, together with their specified purity. An additional fuel, ethylene, was 98.2 per cent pure. With the liquids the fuel-air mixtures were prepared by vaporizing the fuel, in a metered airstream in a vaporizing vessel. The vessel was a glass tube containing steel balls, to give a large heat exchange surface; and was heated externally by a thermostatically controlled heating tape. The fuel was delivered into the vaporizer by a pump, which could be set by a micrometer control to deliver at the required The fuel-air mixture was cooled in a rate. water-bath before passing into the explosion tube.
Procedure
Circular discs of the perforated metal were made to the same diameter as that of the outside of the explosion tube, and were sealed between two plane ends of the tube with transparent adhesive tape. The tube was filled with fuel-air mixture, the supply of which was then cut off, and the mixture ignited. From a photograph of each explosion in the stoichiometric mixtures, the velocity of approach of the flame to the perforated metal was measured and observation was made of whether the flame propagated through the metal. Cool flames were too dim to be photographed in this manner, but as they propagated slowly their velocities were measured by visual observation. Results
With stoichiometric fuel-air mixtures the flame velocity varied with the run-up length between the igniter and the perforated metal; when the flame velocity of liquid fuels exceeded a critical value, the flame propagated through the perforated metal. The maximum velocities at which flames of liquid fuels were quenched, and the minimum velocities at which they propagated through perforated metal sheeting, are given in Table 2; in some cases the maximum velocity
June
1963
The quenching
of flames of various
for quenching was appreciably lower than the minimum velocity for propagation, but intermediate flame velocities were not obtained in the limited series of experiments carried out. The results for ethylene flames with perforated metal sheeting were more erratic and are given in Figure 1. In additional tests with sheet-
fuels in narrow
Table 3. minimum
123
apertures
Maximum velocities of Fames quenched and velocities of frames passed for a perforated block, 1 cm thick
Fuel
Hexane Acetone Ethyl acetate ____ Benzene
The velocities of cool flames obtained with acetaldehyde or diethyl ether as fuel, were similar in value irrespective of the direction of propagation or the run-up length. A summary of the results is given in Table 4.
x
0 -0
O 0
:: ;
<
x 0
0
0
0
0
0
O
0
5-
x
x
0
Flame quenched by sheeting Flame propagated through sheeting 3’ 2
Diameter Figuve
5
-1 2
10-l
of aperture,
cm
2. The quenching of stoichiometric ethyleneair flames by perforated metal sheeting
ing of aperture diameter 0.100 cm, through which ethylene flames propagated at lower velocities than the flames of other fuels (Table 2), flames propagating at velocities up to 200 cm/s relative to the unburnt gas ahead of the flame were quenched. The quenching of flames of several liquid fuels by a perforated metal block is shown in Table 3; the results for ethylene flames are given in Figure 2. Additional tests with ethylene, in which the flame velocity was measured relative to the unburnt gas ahead of the flame as well as relative to the tube, showed that blocks of thickness 1, 3 and 5 cm would quench flames propagating at velocities up to 250, 270 and 260 cm 1 s respectively.
5,
I
I
100
6
Thickness
of
2 perforated
block,cm
Figure 2. The quenching of stoichiometvic air frames by perforated metal blocks. diameter O-175 cm
ethyleneAperture
Discussion Stoichiometric
flames
from different fuels In previous work, using propane as fuel, two limiting cases of the quenching of flames by per-
124
K. N. Palmer and P. S. Tonkin Table 4.
The quenching of cool flames by perforated (aperture diameter: 0.56 cm)
Per cent
Fuel
Direction propagation flame
by
volume of
fuel in air Acetaldehyde
35
Diethyl
28
ether
18 12
8
i-
Range of flame velocities cm/s
Result All Fames quevached
Horizontal
29-36
Horizontal
27-3 1
All flames quenched __All flames quenched
26
All flames quenched
28
Flame propagated through sheeting
‘Horiz.
Horiz.
and. vevt.
and. vert.
and. vert.
forated metals were considered’. In the first the whole of the flame front was reckoned to be quenched by the walls of the apertures, and none by the blank parts of the face of the metal; this would be so when the apertures are close together. The following equation was derived, relating the dimensions of the perforated metal to the critical flame velocity, and was in reasonable agreement with the results for apertures of diameter 0.175 cm. (v+V)=g.eKPy(T-T,)/Hd’.
metal sheeting
28-33
Hoviz.
._____
of of
Vol. 7
. . . [I]
where d is the diameter of aperture, H is the heat abstracted from unit volume of flame for K is the thermal conductivity of the quenching, flame gases, P is the area of aperture per unit area of sheeting, T is the mean axial temperature of flame gases in the aperture, T, is the temperature of the perforated metal, Y is the velocity of unburnt gas during explosion, V is the flame velocity relative to the unburnt gas, and y is the thickness of the perforated metal. In the second limiting case, only that portion of the flame front directly opposed to the aperture was reckoned to be quenched by the walls, the remainder being quenched at the blank face of the metal. This would occur when the apertures are far apart and do not interact. Equation 2 was derived for these conditions and the results for perforated sheeting, of aperture diameter 0.100 cm and less, were intermediate between the values predicted by equations 1 and 2 but tended toward equation 2
at small relatively
22-35
diameters, far apart.
Flame quenched at 22 cm /s only
where
the apertures
(V+?J)=9.6q’(T-T,)/Hd’
were
. . . . [2]
In the present work, flames from the various liquid fuels were all tested against the same perforated metal sheeting and blocks, and so they can be compared directly with each other and with propane flames if allowance is made for the differences in the properties of the flame gases. In equations 1 and 2 the properties of the flame that influence the critical velocity (V + v) were represented by K (T - T,) / H, the values of which are given in the Appendix. The experimental results in Tables 2 and 3 were made comparable by multiplying by the value of K (T - T,) /H for propane and dividing by the value for the fuel. The velocities from TabEe 2 were adjusted in this way and are shown in Table 5, with values for propane flames included for comparison. The velocities for propane flames, corresponding to those in Table 3, were 1 150 and 1500 cm/s for the maximum velocity of quenched flames and the minimum velocity of flames passing through the perforated block respectively. Examination of Table 5 shows that the velocities at which flames were just able to pass through the various perforated sheetings were not the same for all the fuels, as they should be when variation in K (T - T,) / H has been allowed for. The values in Table 5 fell within * 12 per cent of a common value for each size of aperture, and half of this scatter may be
June 1963
The quenching of flames of various fuels in narrow apertures Table 5.
Results in Table 2 adjusted for comparison with those for propane
Diameter of apertures 0,055 cm Max. velocity quenched
Fuel
-
Min. velocity passed
cm/s
cm/s
0.100
I
Max. velocity quenched
in perforated cm
800
1 250
500
650
Hexane
890
1 250
580
630
1 090
1 090
Acetaldehyde
680
1 040
Ethyl
690
850
480
980
1 070
540
1 080
470
720
acetate
Benzene Diethyl
ether
1 080
-~-
attributed to experimental error in the measurement of flame velocities. Likewise, with perforated blocks, the velocity at which the flame just passed was not the same for all the liquid fuels, and the experimental error would again account for only half the scatter. The origin of the remaining scatter may be the assumption that a constant fraction of the heat released in stoichiometric flames of all the fuels must be abstracted for quenching. This assumption may be only approximately true; A. E. POTTER and A. L. BERLAD~ used it for a range of hydrocarbons and for hydrogen, and obtained correlations of experimental data to within k20 per cent but the error due to the assumption cannot be estimated because other variables were involved. Quenching of ethylene flames The results obtained with stoichiometric ethylene flames, Figures 1 and 2, showed that flames passed through perforated sheeting and blocks of aperture diameter 0.100 cm and above at lower critical velocities than with the other fuels (Tables 2 and 3, although with an aperture diameter of 0.055 cm the velocity was similar to that of the other fuels (Table 2 and Figure 1). Equation 1 was in reasonable agreement with the results for several liquid fuels and for propane flames, using perforated blocks of aperture diameter 0.175 cm (Table 3), but the equation overestimated the required velocity of ethylene
660 620
-
0.175 cm
I Max. velocity quenched
passed cm/s
cm/s
___.
sheeting
Min. velocity
Propane
Acetone
125
cm/s
passed cm/s
90
95
125
150
I ~__
Min. velocity
980
1
110
130
730
I
-
-
530
105
105
830
105
115
-_
-
I
flames by a factor of about eight. In addition, with aperture diameters of 0.100 and 0.175 cm the critical velocities of ethylene flames were illdefined (Figures 1 and 2). These differences may have arisen from the aperture diameters being closer to the quenching diameter of stoichiometric ethylene-air flames (0.195 cm)6 than to the quenching diameters of the other fuels (e.g. propane, 0.28 cm). The performance of flame arresters with aperture diameters near to the quenching diameter of the gas mixture may well differ from that when aperture diameters are relatively small. A flame can propagate indefinitely in a tube slightly wider than the quenching diameter, and the process of quenching may be regarded as the removal of sufficient heat from the flame before it can completely regenerate itself by propagating into fresh unburnt gas mixture. With an aperture in perforated metal slightly narrower than the quenching diameter the flame may regenerate before it has propagated the entire length of the aperture; the aperture would then be less effective in quenching flame than would be expected from calculations based on the whole surface area, as in equations 1 and 2. The distance that a flame should propagate through unburnt gas mixture in order to regenerate is x,/n, where x, is the thickness of flame propagating at the standard burning velocity S and n is the expansion ratio on com-
K. N. Palmer and P. S. Tonkin
126
bustion. At the quenching diameter d,, a flame with velocity S is just quenched and hence in
equation 2, which is used because each aperture acts independently, the effective thickness of the perforated metal is x,/n and if there is negligible movement of the unburnt gas, v =0 approximately i.e.
S=9.6K(T-TO)xO/Hd;n
. . . . ]3]
A similar expression is obtained velocity V inside a smaller aperture d. Thus, by division,
for a flame of diameter
V=S (d,/d)”
. . . . [4]
In experiments with ethylene flames and perforated metal of aperture diameter 0.100 cm, movement of the unburnt gas was negligible. With S= 70 cm/s3 and d, =0.195 cm*, equation 4 predicts V ~265 cm/s which is in reasonable agreement with the experimental value of 200 cm/s. If the unburnt gas moves at velocity u through the aperture, the effective thickness of the perforated metal in equation 2 is (x,/n) {(V + u) / V}, and then from equations 2 and 3 V=S{(V+u)/(V+u)}
(d,/d)’
If u and v are large compared with by approximating (V+u)/(V+v)=u/~=l/P equation 5 becomes V=(S/P)
(do/d)?-
. . . [5] V then
. . . . /6]
When ti and v are sufficiently high for the flame to be carried through the perforated metal before there is time for regeneration, then it may be shown that equation 1 would apply. In the experiments with ethylene flames and perforated blocks the flames were often vibratory and unburnt mixture ahead of the flame was usually in motion, so that with d=Os 175 cm and P=0.37 (Table I), equation 6 predicts V=235 cm[s. This value is in reasonable agreement with the experimental values given above of about 260 cm/s, although with blocks 1 cm thick the unburnt gas velocity was small and the use of equation 6 was approximate. Equations 4 and 6, which are only applicable when the regeneration of flame within the apertures is the limiting factor, were in considerably
Vol. 7
better agreement than equations 1 and 2 with the results for the quenching of ethylene flames by apertures close to the quenching diameter. Quenching of cool flames The results in Table 4 showed that cool flames of acetaldehyde and of diethyl ether propagated only slowly, and were quenched by perforated metal sheeting. There was evidence that with ether flames, at least, the maximum velocity of flames that could be quenched increased as the fuel-air mixture became richer and further from the upper flammability limit for normal flame propagation. Conclusions The ease of quenching of flames propagating in a range of stoichiometric vapour-air mixtures was similar, but not identical, to that of propane-air flames propagating at the same velocity. Part of the difference could be attributed to variation in the thermal properties of the flames. When the diameter of the apertures in the perforated metal approached the standard quenching diameter of the gas mixture, the velocities of flames that could be quenched were considerably reduced. The reduction was attributed to only part of the surface of the apertures in the perforated metal becoming involved before the flame regenerated by propagating into further unburnt gas mixture. Cool flames may be quenched by perforated metal sheeting with relatively wide apertures, similar to those required for normal flames propagating at the same slow velocity. The work described in this fluper forms part of the programme of the Joint Fire Research Organization of the Department of Scientific and Industrial Research and Fire O@ces’ Committee; the paper is published by permission of the Director of Fire Research. MY D. K. Freeman, Mrs J. S. Harris and Mr B. Langford assisted in the experimental work.
1 PALMER, K. N. forated sheeting
References ‘The quenching of flames by perand block flame arresters’. Sym-
posium on Chemical Process Hazards with Special
June 1963
The quenching
of flames of various
mated as follows. With propane, the abstraction of 5.0 cal/cm3 of propane was shown by J. P. BOTHA and D. B. SPALDING’ to reduce the burning velocity of a stoichiometric flame to about 4 cm/s. This amount of heat is 0.23 of the total amount released in the flame and its removal was assumed to be sufficient to quench the flame. Potter and Berlad3 have shown that the fraction of the total heat produced in a stoichiometric flame that it must retain is approximately constant for a range of hydrocarbons, including ethylene, in air. They recommended a value of 0.78 for the fraction, which agrees closely with the value of 0.77 ( = 1 - 0.23) taken here. The temperatures of flames in which only 0.77 of the heat released is available are given in Table 6 (column 2). T was then taken as the mean of the values in columns 1 and 2. T,, the temperature of the perforated metal, was taken as 290°K. The value of K, thermal conductivity, was taken from J. HILSENRATH~ using values for nitrogen, the major constituent of the flames, at a temperature of 4 (T + T,). H is the amount of heat to be removed from unit volume of flame in order to quench it. For a 4.0 per cent (stoichiometric) propane-air mixture the value is 0.23 x (488.6/22.4) x 0.04 x (273 / 2 260) x (24.8 / 26.3), because 1 mole propane releases 488.6 kcal on combustion, the adiabatic flame temperature is 2 260”K, and 24.8 mole reactants yield 26.3 mole products after allowing for dissociation. Values of the ratio of reactants to products are given in Table 6 (column 3). Values of K (T - T,)/H for the various fuels are given in Table 6 (column 4).
Reference to Plant Design, pp 51-7. Institution of Chemical Engineers : London, 1961 2 PALMER, K. N. ‘The quenching of flame by wire Seventh Symposium (International) on gauzes’. Combustion, pp 497-503. Combustion Institute (Butterworths) : London, 1959 3 POTTER, A. E. and BERLAD, A. L. ‘The effect of fuel type and pressure on flame quenching’. Sixth (International) on Combustion, pp Symposium 27-36. Combustion Institute (Reinhold) : New York, 1957 4 HOARE, M. F. and LINNETT, J. W. ‘Effect of solid surfaces on the propagation of flame through ethylene-air mixtures’. J. &em. Sot. 1955, 195-202 5 GAYDON, A. G. and WOLFHARD, H. G. Flames, Radiation and Temperature. Their Structure, Chapman & Hall: London, 1960 Pro6 Selected Values of Chemical Thermodynamic perties, Ser. III. U.S. National Bureau of Standards : Washington, 1954 7 BOTHA,J. P. and SPALDING,D. B. ‘The laminar flame speed of propane-air mixtures with heat extraction from the flames’. Proc. Roy. SOC. A, 1954, 225, 71-96 8 HILSENRATH,J. ‘Tables of thermal properties of gases’. Circ. U.S. Nat. BUY. Stand. No. 564. Washington, 1955 APPENDIX Calculation of values of K (T - T,) /H in equations 1 and 2, for various fuels
of the flame T is the mean axial temperature gases in the perforated metal and is the mean of the adiabatic flame temperature and the temperature of the flame after sufficient heat just to quench it has been removed. Adiabatic flame temperatures for stoichiometric fuel-air mixtures were calculated by the method of A. G. GAYDON and H. G. WOLFHARD~, allowing for dissociation of the combustion products, and are listed in Table 6 (column 1). Values of thermodynaThe mic properties were taken from ref. 6. temperature of the flame after sufficient heat has been removed just to quench it was estiTable 6.
Calculated
1 Fuel
Propane Hexane Acetone Acetaldehyde Ethyl acetate Benzene Diethyl ether Ethylene
Adiabatic flame temperature “K 2 2 2 2 2
260 250 230 270 220
2 330
2 260 2 350
127
fuels in narrow apertures
values of stoichiometric
2 Flame temp. fov quenching “K 1 960 1 970 1 930 1 990 1 920 2 060 1 970 2 090
I-
flame properties
3 Mole reactants Mole products 24.8 126.3 46.2149.4 20*5/21.4 12+/ 13.6
24.8 126.9
36.7137.8 29.6 /31.9 15.3/ 15.5
-
4 K (T-To) -___ H 13.70 13.15 12.56 13.23 12.94 14.08 13.32 14.14
Station,
Boreham
Wood, Herts.
(Received August 1962) The quenching of flames propagating in a range of combustible vapours mixed with air has using perforated metal sheeting and blocks with narrow apertures. The been investigated, perforated metal was mounted inside an explosion tube along which the flame propagated. Measurement of flame velocities &owed that the velocities of vapourdir pames just able to propagate through the perforated metal were similar to those of propane-air fames. Experiments with ethylene games showed that. when the diameter of the apertures in the perforated metal was nearly as large as the quenching diameter, the flames were able to propagate through the apertures at considerably reduced velocities. This is discussed on a of cool flames by perforated metal sheeting was also theoretical basis. The quenching investigated.
on the quenching of cool flames, propagating rich fuel-air mixtures.
Introduction PERFORATED metal sheeting and blocks with narrow apertures can be used as flame arresters in pipe systems carrying flammable gas mixtures, to prevent the continued propagation Previous of flame through the gas mixture. experiments with perforated metal1 and wire propane-air gauzes, mainly with stoichiometric flames, showed that the flame propagated through apertures in metal or gauze if the flame velocity exceeded a critical value, which usually depended on the aperture diameter and depth. The flame velocity varied with the composition of the gas mixture and the layout of the pipe system, and to quench flames propagating with high velocities, the apertures in the perforated metal needed to be considerably narrower than the standard quenching diameter for the gas mixture. In the present experiments the critical velocities of stoichiometric fuel-air flames just able to propagate through perforated metals of various aperture diameters and depths were measured to determine whether these critical flame velocities varied with the fuel. As results were already available for propane flames, these were taken as a convenient reference for comparison with the results for the fuels examined. Finally, additional experiments were carried out
in
Experimental Afparatus and materials The sheeting and blocks were of brass and were perforated with circular apertures; the dimensions and spacing of the apertures in the sheeting are given in Table 1 and were accurate to within +_1 per cent. The centres of the apertures were spaced in a rectangular pattern, with an additional aperture in the centre of the rectangle; the spacings given in Table 1 are the lengths of the sides of the rectangles. The perforated blocks were 1 cm thick, drilled with apertures 0.175 cm in diameter in the same pattern as the sheeting, and could be combined in packs with the apertures accurately aligned. Table I.
Characteristics
of perforated
Thickness of sheeting,
Spacings between centres of apertures
Diameter of aperture, d cm
Y cm
0*560
0.175 0.100 0.075 0.055 121
,
O-124 0.073 0.072 0.052 0.046
cm 0.845, 0.285, 0.175, 0.140, 0.110,
1.400 0.465 0.290 0.250 0.180,
sheeting Area of aperture per unit area of sheeting P
0.41 0.37 0.31 0.26 0.24
K. N. Palmer and P. S. Tonkin
122
Table
2.
Maximum
velocities of flames quenched and minimum velocities for various fuels and sizes of perforated sheeting
T Stoichio-
metric mixture (per cent fuel in air)
Fuel, and grade of purity
Hexane (standard) Acetone (analytical) Acetaldehyde (standard) Ethyl acetate (analytical) .___ -. Benzene (analytical) Diethyl ether (analytical)
Vol.
T
Diameter 0.055 Max.
in perforated
850
1 200
1 000
000
550
0.175
cm
Min. velocity
Max. velocity quenched cm/s
l-
-I-
passed
sheeting
0,100 cm
Min. velocity passed cm/s
velocity quenched cm/s
(
._
600
cm/s
cm/s
120
140
1 000
650
800
650
1 000
of apertures
cm
of flames
7
-.-
1 100
.I
I1 050
The explosion tube was horizontal for all experiments with normal flames and for some of those with cool flames, but for the remainder of the cool flame experiments it was vertical. The tube was of Perspex, 6.4 cm in internal diameter and of wall thickness 0.6 cm, and was open at one end and closed at the other. The perforated metal was fixed across the tube, part way along its length, and the distance from the igniter (the run-up) was varied from 11.4 to 313 cm. The gas mixture was ignited at the open end of the tube either by a spark gap or, for cool flames, by a small flame. The six liquid fuels used are listed in Table 2, together with their specified purity. An additional fuel, ethylene, was 98.2 per cent pure. With the liquids the fuel-air mixtures were prepared by vaporizing the fuel, in a metered airstream in a vaporizing vessel. The vessel was a glass tube containing steel balls, to give a large heat exchange surface; and was heated externally by a thermostatically controlled heating tape. The fuel was delivered into the vaporizer by a pump, which could be set by a micrometer control to deliver at the required The fuel-air mixture was cooled in a rate. water-bath before passing into the explosion tube.
Procedure
Circular discs of the perforated metal were made to the same diameter as that of the outside of the explosion tube, and were sealed between two plane ends of the tube with transparent adhesive tape. The tube was filled with fuel-air mixture, the supply of which was then cut off, and the mixture ignited. From a photograph of each explosion in the stoichiometric mixtures, the velocity of approach of the flame to the perforated metal was measured and observation was made of whether the flame propagated through the metal. Cool flames were too dim to be photographed in this manner, but as they propagated slowly their velocities were measured by visual observation. Results
With stoichiometric fuel-air mixtures the flame velocity varied with the run-up length between the igniter and the perforated metal; when the flame velocity of liquid fuels exceeded a critical value, the flame propagated through the perforated metal. The maximum velocities at which flames of liquid fuels were quenched, and the minimum velocities at which they propagated through perforated metal sheeting, are given in Table 2; in some cases the maximum velocity
June
1963
The quenching
of flames of various
for quenching was appreciably lower than the minimum velocity for propagation, but intermediate flame velocities were not obtained in the limited series of experiments carried out. The results for ethylene flames with perforated metal sheeting were more erratic and are given in Figure 1. In additional tests with sheet-
fuels in narrow
Table 3. minimum
123
apertures
Maximum velocities of Fames quenched and velocities of frames passed for a perforated block, 1 cm thick
Fuel
Hexane Acetone Ethyl acetate ____ Benzene
The velocities of cool flames obtained with acetaldehyde or diethyl ether as fuel, were similar in value irrespective of the direction of propagation or the run-up length. A summary of the results is given in Table 4.
x
0 -0
O 0
:: ;
<
x 0
0
0
0
0
0
O
0
5-
x
x
0
Flame quenched by sheeting Flame propagated through sheeting 3’ 2
Diameter Figuve
5
-1 2
10-l
of aperture,
cm
2. The quenching of stoichiometric ethyleneair flames by perforated metal sheeting
ing of aperture diameter 0.100 cm, through which ethylene flames propagated at lower velocities than the flames of other fuels (Table 2), flames propagating at velocities up to 200 cm/s relative to the unburnt gas ahead of the flame were quenched. The quenching of flames of several liquid fuels by a perforated metal block is shown in Table 3; the results for ethylene flames are given in Figure 2. Additional tests with ethylene, in which the flame velocity was measured relative to the unburnt gas ahead of the flame as well as relative to the tube, showed that blocks of thickness 1, 3 and 5 cm would quench flames propagating at velocities up to 250, 270 and 260 cm 1 s respectively.
5,
I
I
100
6
Thickness
of
2 perforated
block,cm
Figure 2. The quenching of stoichiometvic air frames by perforated metal blocks. diameter O-175 cm
ethyleneAperture
Discussion Stoichiometric
flames
from different fuels In previous work, using propane as fuel, two limiting cases of the quenching of flames by per-
124
K. N. Palmer and P. S. Tonkin Table 4.
The quenching of cool flames by perforated (aperture diameter: 0.56 cm)
Per cent
Fuel
Direction propagation flame
by
volume of
fuel in air Acetaldehyde
35
Diethyl
28
ether
18 12
8
i-
Range of flame velocities cm/s
Result All Fames quevached
Horizontal
29-36
Horizontal
27-3 1
All flames quenched __All flames quenched
26
All flames quenched
28
Flame propagated through sheeting
‘Horiz.
Horiz.
and. vevt.
and. vert.
and. vert.
forated metals were considered’. In the first the whole of the flame front was reckoned to be quenched by the walls of the apertures, and none by the blank parts of the face of the metal; this would be so when the apertures are close together. The following equation was derived, relating the dimensions of the perforated metal to the critical flame velocity, and was in reasonable agreement with the results for apertures of diameter 0.175 cm. (v+V)=g.eKPy(T-T,)/Hd’.
metal sheeting
28-33
Hoviz.
._____
of of
Vol. 7
. . . [I]
where d is the diameter of aperture, H is the heat abstracted from unit volume of flame for K is the thermal conductivity of the quenching, flame gases, P is the area of aperture per unit area of sheeting, T is the mean axial temperature of flame gases in the aperture, T, is the temperature of the perforated metal, Y is the velocity of unburnt gas during explosion, V is the flame velocity relative to the unburnt gas, and y is the thickness of the perforated metal. In the second limiting case, only that portion of the flame front directly opposed to the aperture was reckoned to be quenched by the walls, the remainder being quenched at the blank face of the metal. This would occur when the apertures are far apart and do not interact. Equation 2 was derived for these conditions and the results for perforated sheeting, of aperture diameter 0.100 cm and less, were intermediate between the values predicted by equations 1 and 2 but tended toward equation 2
at small relatively
22-35
diameters, far apart.
Flame quenched at 22 cm /s only
where
the apertures
(V+?J)=9.6q’(T-T,)/Hd’
were
. . . . [2]
In the present work, flames from the various liquid fuels were all tested against the same perforated metal sheeting and blocks, and so they can be compared directly with each other and with propane flames if allowance is made for the differences in the properties of the flame gases. In equations 1 and 2 the properties of the flame that influence the critical velocity (V + v) were represented by K (T - T,) / H, the values of which are given in the Appendix. The experimental results in Tables 2 and 3 were made comparable by multiplying by the value of K (T - T,) /H for propane and dividing by the value for the fuel. The velocities from TabEe 2 were adjusted in this way and are shown in Table 5, with values for propane flames included for comparison. The velocities for propane flames, corresponding to those in Table 3, were 1 150 and 1500 cm/s for the maximum velocity of quenched flames and the minimum velocity of flames passing through the perforated block respectively. Examination of Table 5 shows that the velocities at which flames were just able to pass through the various perforated sheetings were not the same for all the fuels, as they should be when variation in K (T - T,) / H has been allowed for. The values in Table 5 fell within * 12 per cent of a common value for each size of aperture, and half of this scatter may be
June 1963
The quenching of flames of various fuels in narrow apertures Table 5.
Results in Table 2 adjusted for comparison with those for propane
Diameter of apertures 0,055 cm Max. velocity quenched
Fuel
-
Min. velocity passed
cm/s
cm/s
0.100
I
Max. velocity quenched
in perforated cm
800
1 250
500
650
Hexane
890
1 250
580
630
1 090
1 090
Acetaldehyde
680
1 040
Ethyl
690
850
480
980
1 070
540
1 080
470
720
acetate
Benzene Diethyl
ether
1 080
-~-
attributed to experimental error in the measurement of flame velocities. Likewise, with perforated blocks, the velocity at which the flame just passed was not the same for all the liquid fuels, and the experimental error would again account for only half the scatter. The origin of the remaining scatter may be the assumption that a constant fraction of the heat released in stoichiometric flames of all the fuels must be abstracted for quenching. This assumption may be only approximately true; A. E. POTTER and A. L. BERLAD~ used it for a range of hydrocarbons and for hydrogen, and obtained correlations of experimental data to within k20 per cent but the error due to the assumption cannot be estimated because other variables were involved. Quenching of ethylene flames The results obtained with stoichiometric ethylene flames, Figures 1 and 2, showed that flames passed through perforated sheeting and blocks of aperture diameter 0.100 cm and above at lower critical velocities than with the other fuels (Tables 2 and 3, although with an aperture diameter of 0.055 cm the velocity was similar to that of the other fuels (Table 2 and Figure 1). Equation 1 was in reasonable agreement with the results for several liquid fuels and for propane flames, using perforated blocks of aperture diameter 0.175 cm (Table 3), but the equation overestimated the required velocity of ethylene
660 620
-
0.175 cm
I Max. velocity quenched
passed cm/s
cm/s
___.
sheeting
Min. velocity
Propane
Acetone
125
cm/s
passed cm/s
90
95
125
150
I ~__
Min. velocity
980
1
110
130
730
I
-
-
530
105
105
830
105
115
-_
-
I
flames by a factor of about eight. In addition, with aperture diameters of 0.100 and 0.175 cm the critical velocities of ethylene flames were illdefined (Figures 1 and 2). These differences may have arisen from the aperture diameters being closer to the quenching diameter of stoichiometric ethylene-air flames (0.195 cm)6 than to the quenching diameters of the other fuels (e.g. propane, 0.28 cm). The performance of flame arresters with aperture diameters near to the quenching diameter of the gas mixture may well differ from that when aperture diameters are relatively small. A flame can propagate indefinitely in a tube slightly wider than the quenching diameter, and the process of quenching may be regarded as the removal of sufficient heat from the flame before it can completely regenerate itself by propagating into fresh unburnt gas mixture. With an aperture in perforated metal slightly narrower than the quenching diameter the flame may regenerate before it has propagated the entire length of the aperture; the aperture would then be less effective in quenching flame than would be expected from calculations based on the whole surface area, as in equations 1 and 2. The distance that a flame should propagate through unburnt gas mixture in order to regenerate is x,/n, where x, is the thickness of flame propagating at the standard burning velocity S and n is the expansion ratio on com-
K. N. Palmer and P. S. Tonkin
126
bustion. At the quenching diameter d,, a flame with velocity S is just quenched and hence in
equation 2, which is used because each aperture acts independently, the effective thickness of the perforated metal is x,/n and if there is negligible movement of the unburnt gas, v =0 approximately i.e.
S=9.6K(T-TO)xO/Hd;n
. . . . ]3]
A similar expression is obtained velocity V inside a smaller aperture d. Thus, by division,
for a flame of diameter
V=S (d,/d)”
. . . . [4]
In experiments with ethylene flames and perforated metal of aperture diameter 0.100 cm, movement of the unburnt gas was negligible. With S= 70 cm/s3 and d, =0.195 cm*, equation 4 predicts V ~265 cm/s which is in reasonable agreement with the experimental value of 200 cm/s. If the unburnt gas moves at velocity u through the aperture, the effective thickness of the perforated metal in equation 2 is (x,/n) {(V + u) / V}, and then from equations 2 and 3 V=S{(V+u)/(V+u)}
(d,/d)’
If u and v are large compared with by approximating (V+u)/(V+v)=u/~=l/P equation 5 becomes V=(S/P)
(do/d)?-
. . . [5] V then
. . . . /6]
When ti and v are sufficiently high for the flame to be carried through the perforated metal before there is time for regeneration, then it may be shown that equation 1 would apply. In the experiments with ethylene flames and perforated blocks the flames were often vibratory and unburnt mixture ahead of the flame was usually in motion, so that with d=Os 175 cm and P=0.37 (Table I), equation 6 predicts V=235 cm[s. This value is in reasonable agreement with the experimental values given above of about 260 cm/s, although with blocks 1 cm thick the unburnt gas velocity was small and the use of equation 6 was approximate. Equations 4 and 6, which are only applicable when the regeneration of flame within the apertures is the limiting factor, were in considerably
Vol. 7
better agreement than equations 1 and 2 with the results for the quenching of ethylene flames by apertures close to the quenching diameter. Quenching of cool flames The results in Table 4 showed that cool flames of acetaldehyde and of diethyl ether propagated only slowly, and were quenched by perforated metal sheeting. There was evidence that with ether flames, at least, the maximum velocity of flames that could be quenched increased as the fuel-air mixture became richer and further from the upper flammability limit for normal flame propagation. Conclusions The ease of quenching of flames propagating in a range of stoichiometric vapour-air mixtures was similar, but not identical, to that of propane-air flames propagating at the same velocity. Part of the difference could be attributed to variation in the thermal properties of the flames. When the diameter of the apertures in the perforated metal approached the standard quenching diameter of the gas mixture, the velocities of flames that could be quenched were considerably reduced. The reduction was attributed to only part of the surface of the apertures in the perforated metal becoming involved before the flame regenerated by propagating into further unburnt gas mixture. Cool flames may be quenched by perforated metal sheeting with relatively wide apertures, similar to those required for normal flames propagating at the same slow velocity. The work described in this fluper forms part of the programme of the Joint Fire Research Organization of the Department of Scientific and Industrial Research and Fire O@ces’ Committee; the paper is published by permission of the Director of Fire Research. MY D. K. Freeman, Mrs J. S. Harris and Mr B. Langford assisted in the experimental work.
1 PALMER, K. N. forated sheeting
References ‘The quenching of flames by perand block flame arresters’. Sym-
posium on Chemical Process Hazards with Special
June 1963
The quenching
of flames of various
mated as follows. With propane, the abstraction of 5.0 cal/cm3 of propane was shown by J. P. BOTHA and D. B. SPALDING’ to reduce the burning velocity of a stoichiometric flame to about 4 cm/s. This amount of heat is 0.23 of the total amount released in the flame and its removal was assumed to be sufficient to quench the flame. Potter and Berlad3 have shown that the fraction of the total heat produced in a stoichiometric flame that it must retain is approximately constant for a range of hydrocarbons, including ethylene, in air. They recommended a value of 0.78 for the fraction, which agrees closely with the value of 0.77 ( = 1 - 0.23) taken here. The temperatures of flames in which only 0.77 of the heat released is available are given in Table 6 (column 2). T was then taken as the mean of the values in columns 1 and 2. T,, the temperature of the perforated metal, was taken as 290°K. The value of K, thermal conductivity, was taken from J. HILSENRATH~ using values for nitrogen, the major constituent of the flames, at a temperature of 4 (T + T,). H is the amount of heat to be removed from unit volume of flame in order to quench it. For a 4.0 per cent (stoichiometric) propane-air mixture the value is 0.23 x (488.6/22.4) x 0.04 x (273 / 2 260) x (24.8 / 26.3), because 1 mole propane releases 488.6 kcal on combustion, the adiabatic flame temperature is 2 260”K, and 24.8 mole reactants yield 26.3 mole products after allowing for dissociation. Values of the ratio of reactants to products are given in Table 6 (column 3). Values of K (T - T,)/H for the various fuels are given in Table 6 (column 4).
Reference to Plant Design, pp 51-7. Institution of Chemical Engineers : London, 1961 2 PALMER, K. N. ‘The quenching of flame by wire Seventh Symposium (International) on gauzes’. Combustion, pp 497-503. Combustion Institute (Butterworths) : London, 1959 3 POTTER, A. E. and BERLAD, A. L. ‘The effect of fuel type and pressure on flame quenching’. Sixth (International) on Combustion, pp Symposium 27-36. Combustion Institute (Reinhold) : New York, 1957 4 HOARE, M. F. and LINNETT, J. W. ‘Effect of solid surfaces on the propagation of flame through ethylene-air mixtures’. J. &em. Sot. 1955, 195-202 5 GAYDON, A. G. and WOLFHARD, H. G. Flames, Radiation and Temperature. Their Structure, Chapman & Hall: London, 1960 Pro6 Selected Values of Chemical Thermodynamic perties, Ser. III. U.S. National Bureau of Standards : Washington, 1954 7 BOTHA,J. P. and SPALDING,D. B. ‘The laminar flame speed of propane-air mixtures with heat extraction from the flames’. Proc. Roy. SOC. A, 1954, 225, 71-96 8 HILSENRATH,J. ‘Tables of thermal properties of gases’. Circ. U.S. Nat. BUY. Stand. No. 564. Washington, 1955 APPENDIX Calculation of values of K (T - T,) /H in equations 1 and 2, for various fuels
of the flame T is the mean axial temperature gases in the perforated metal and is the mean of the adiabatic flame temperature and the temperature of the flame after sufficient heat just to quench it has been removed. Adiabatic flame temperatures for stoichiometric fuel-air mixtures were calculated by the method of A. G. GAYDON and H. G. WOLFHARD~, allowing for dissociation of the combustion products, and are listed in Table 6 (column 1). Values of thermodynaThe mic properties were taken from ref. 6. temperature of the flame after sufficient heat has been removed just to quench it was estiTable 6.
Calculated
1 Fuel
Propane Hexane Acetone Acetaldehyde Ethyl acetate Benzene Diethyl ether Ethylene
Adiabatic flame temperature “K 2 2 2 2 2
260 250 230 270 220
2 330
2 260 2 350
127
fuels in narrow apertures
values of stoichiometric
2 Flame temp. fov quenching “K 1 960 1 970 1 930 1 990 1 920 2 060 1 970 2 090
I-
flame properties
3 Mole reactants Mole products 24.8 126.3 46.2149.4 20*5/21.4 12+/ 13.6
24.8 126.9
36.7137.8 29.6 /31.9 15.3/ 15.5
-
4 K (T-To) -___ H 13.70 13.15 12.56 13.23 12.94 14.08 13.32 14.14