- Email: [email protected]
Non-ignition of an ejected flame through a gap greater than the quenching distance
Fire Research, 1 ( 1 9 7 8 / 1 9 7 9 ) 3 1 7 - 322 © Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
317
Non-Ignition of an Ejected Flame Through a Gap Greater than the Quenching Distance MASAO MAEKAWA and MASATSUGU TAKEICHI
Department of Mechanical Engineering, Ehime University, Matsuyama, Ehime 790 (Japan)
SUMMARY
When a gap is greater than the quenching distance, the flame from an explosion inside a chamber can pass through the gap, but it will not always ignite an outside combustible mixture. There is a non-ignition gap distance, greater than the quenching distance, below which ignition will not occur, and above which it will. The present paper presents experimental determinations of non-ignition gap distances for different values of several parameters (gap width, number of gaps, and chamber volume) for various rectangular gaps with a wide range of lengths. A rectangular gap has three dimensions. One of these is here referred to as the gap distance, the others being gap width and gap length; diagramme definitions of the terms are given. The gap distance for non-ignition varies with the values chosen for the other two dimensions. It increases as the gap width decreases. But when gap length is varied for gap widths over 4.0 cm, the nonignition gap distance first increases with increasing gap length, and then levels off. A second increase may occur or not, depending on other parameters. In particular, any given sectional gap area (fixed values for gap width and gap distance) may be subdivided, giving rise to multi-layered gaps. Experiment establishes that such subdivision is available as a method of preventing ignition of an outside combustible mixture, since ignition may occur through a single gap of given area and not through a subdivided multi-layered gap of the same area. At multi-layered gaps, one gap flame is far in advance of the others. Other gap flames are either soon extinguished within the gap, or advance a short distance without emerging. It seems permissible to consider the resultant as one flame for analysis. The magnitude of the non-ignition gap distance for multi-layered gaps is proportional to ut, u being the ejection velocity of the flame from
the gap, and t the time taken for the flame to pass from the jet origin to the gap exit.
INTRODUCTION
Experimental determination of the parameters for non-ignition of an outside combustible mixture by an ejected flame from a gap is justified by the relevance of the information to a variety of combustors of engineering importance. It has been established that the flame from an explosion inside a chamber does not pass through the gap when the gap spacing is less than the quenching gap distance [ 1]. When the gap spacing is greater than the quenching gap distance, the flame passes through the gap, but in spite of this, it may not ignite the combustible mixture outside the gap. There is a non-ignition gap distance which is greater than the quenching gap distance. It is important to realize that the non-ignition gap distance, the largest spacing for non-transmission of an explosion, depends on other elements of the geometry of the gap (gap width, gap length, volume of the explosion chamber, and gap subdivision) as well as on the nature of the materials used. It is well known that a Davy safety lamp intercepts the flame and prevents propagation to an outside combustible mixture by using wire screens. Davy found that the non-propagation was due to the absorption of the heat of the flame by the wire screens. It must be recognized, however, that other factors are involved, and that flame non-propagation can be achieved with devices other than metal screens. Heat absorption and conduction by metal are not the only physical processes available to us for the purpose. When non-thermal conductors such as polyethylene wire are used for screens instead of metal, the combustible mixture does not burn, and non-propagation is achieved without metal screens. In these cases, the non-propaga-
318 tion seems attributable not just to absorption by wire, but to the fineness of the screens as well. Even when the flame ejected from a single gap ignites the outside combustible mixture, subdivision of the gap area into multilayered gaps is a m e t h o d available to us to prevent ignition. Such subdivision, as an alternative to wire screens, was investigated by the authors by using a variety of rectangular gaps o f different dimensions to determine some of the parameters (gap width, num be r of gaps, and chamber volume) that affect the non-ignition gap distance. Early investigations bY Phillips [2 - 5] are summarized in the literature. Experimental values for flame nonignition gap distances for an equatorial flange are also available in the literature on industrial safety [ 6 ] . However, no practical engineering results for multi-layered gaps were given. A previous work by Maekawa established the equation for the non-ignition gap distance [ 7 ], but did not offer a theory. Phillips pointed o u t [4] that the non-ignition gap distance is proportional to u t for the equatorial flange gap. In the present investigation, the authors have examined whether or not the non-ignition gap distance for multi-layered gaps is also proportional to ut.
EXPERIMENTAL The experimental apparatus consisted of two combustion chambers, a fuel mixing and metering system, a spark ignition source, and a rectangular gap m o u n t e d on the primary combustion chamber as shown in Figs. l ( a ) and (b). The gap consisted of a series of gaps established by a set of parallel thin plates with a uniform gap distance. The gap lengths were 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 cm, and the gap widths were 2.0, 3.0 and 4.0 cm. Two values for the volume of the primary combustion chamber were used: 4 × 103 cm 3 (10.0 cm long, 20.0 × 20.0 cm in section) and 8.0 × 103 cm a (20.0 cm long, 20.0 × 20.0 cm in section). Two values for the volume o f the secondary or outside chamber were also used: 48.0 × 103 cm 3 (30.0 cm long, 40.0 X 40.0 cm in section) and 20.0 × 103 cm 3 (12.5 cm long, 40.0 × 40.0 cm in section). The ignition source, located at the middle of the primary combustion chamber axis, was a capacitance spark between brass
BURNER ~
-
PAPER CLOSED
~.L:~>~.UL.-LAYERED ;If~ T ~II CHA"BER S E C O D .A R ~
CO"BUSTION
MA~ETER
I
II ~ HH
II
II
GAPS [
OSCILLO"L.
"~
I EL ODES I ......... ~
,,
II
FAN
"P~I~RY COMBUSTIONCHAMBER
('
X.
1
GAS
II
I
I AIR
(a) MULTI-LAYERED GAPS GAP DISTANCE G
i
PRIMARY COMBUSTION CHAMBER
'
G*
GAP WIDTH W G*=GXN (N IS THE NUMBEROF GAPS) A=G*XW(A IS SECTIONALAREA OF GAPS)
(b) Fig. 1. (a) Apparatus for measuring non-ignition gap distance. (b) Detail of gaps. electrodes with a gap spacing 0.02 cm. High spark energies were available from the power supply, and the energy was delivered across the gap within one or two ps. Methane and air were supplied from separately m et ered flow meters. A stoichiometric m e t h a n e - a i r mixture was used at room t em perat ure and pressure at the time of ignition because of the erratic ignition that sometimes occurs with hydrocarb o n - a i r mixtures. For such ignition the precision of measurement was not good, and reasonably accurate values for non-ignition gap distances would not be forthcoming from a procedure of single trials. Repeat trials were therefore made at the various gap distances to achieve greater exactitude. The flame was initiated in the primary combustion chamber first. Flame propagation within the gap was upward. The flame passed through the gap and ignited the secondary combustion chamber mixture when the test gap distance was greater than the non-ignition gap distance, and did not ignite when the gap distance was below the ignition gap distance. Ignition and non-ignition were visually observed. The magnitude
319
o f the non-ignition gap distance indicates a m a x i m u m gap distance, below which ignition does n o t occur and above which it does.
10.0
•
8.0 1 6.0-
4.0-
RESULTS AND DISCUSSION
The non-ignition gap distances defined by use o f this apparatus for a m e t h a n e - a i r mixture are shown in Figs. 2, 4 and 8. The curves o f the non-ignition gap distance for different fixed values o f the parameters gap width, n u m b e r o f gaps, and cham be r volume were d e t e rmin ed for a wide range of gap length. It will be seen th at the curve separates an uppe r region o f flame ignition from a lower region where the flame does n o t ignite the secondary c o m bu s tio n ch amb e r mixture. The b o u n d a r y line between the tw o regions is the non-ignition gap distance, namely the m a x i m u m spacing of the gap obtained experimentally which prevents ignition o f the secondary c o m b u s t i o n c h a m b e r mixture.
Effect of chamber volume Figure 2 shows the relation between nonignition gap distances and gap length as parameters o f chamber volume. Curve A is for a primary co mb u s tio n chamber of 8.0 X 103 cm 3 , and a secondary c o m b u s t i o n cham be r of 20.0 X 103 cm 3 . Curve B is for a primary combustion chamber o f 4.0 X 103 cm s , and a seco n d a r y o f 20.0 × 103 cm 3 . Curve C is for a primary o f 4.0 × 103 cm 3 , and a secondary o f 48.0 X 103 cm s . It will be observed from the Figure th at the non-ignition gap distance varies
W-4.0 cm THREEGAPS
0.8
PRIMARY (XlO 3 cm3) SECONDARY(X103 cm3)
i
0.6
~
0.4 0.2
- - o - -
8.0
20.0
- - A - -
4.0
20.0
~
4.0
48.0
~ ,
c ,
l
2.0
l
l
l
4.0
,
l
W
6.0
0
2.0
4.0
6.0
8.0 (Xl06 cm3)
A (cm2) V (cm3)
Fig. 3. Introduced pressure in primary combustion chamber with gap.
with the primary com bust i on cham ber volume. The volume of the secondary combustion cham ber does not, however, appreciably affect the results. The introduced pressure in the primary combustion cham ber is pl ot t ed against A / V in Fig. 3. F r o m the Figure, the relation between the i nt roduced pressure P and A / V is given by:
P =Po e-kA/v
(1)
where A is the sectional area of the gap, V the volume of the primary com bust i on chamber, Po the i nt roduced pressure in the absence o f a gap {closed}, k a constant, and e t he natural logarithm base. It will be seen t hat the introduced pressure depends on bot h factors A and V. In general, the values of A / V decrease with increasing volume of the primary combustion cham ber in the non-ignition experiments. Hence the i nt roduced pressure in a primary c o m b u s t i o n cham ber of large volume is greater than the pressure in a smaller one. Since a higher primary chamber pressure will p r o d u c e a higher ejection velocity at the gap, and since the non-ignition gap distance is greater for a primary cham ber of 8.0 × 103 cm 3 than it is for one o f 4.0 × 103 cm 3 (see Fig. 2), it may be considered that high ejection velocity enlarges the non-ignition gap distance.
m
8.0
I0.0
GAP LENGTH L an
Fig. 2. Non-ignition gap distances of three gaps with 4.0 cm width at various combustion chamber volumes.
Effect of gap width Figure 4 shows non-ignition gap distance curves for a single gap with various gap widths as the parameter. Three curves are defined,
320 A
1.O
/o..~o~B°--'o ~o.~ o
~---~
~o
0.8
/ -/ / s
0.6
/
0.4
/~/0~
SIDLE
:P2.O cm
- - ~
W=3.0cm
~@~
W=4.0cm
0.2
0
, 0
\
,
l
2.0
,
, , 1 1
4.0
6.0
8.0
tional area of the gaps, instead of the gap distance, as the vertical axis. It will be seen that a smaller gap width has a smaller gap sectional area than a larger gap width has. Smaller gap widths also correspond to higher gap ejection velocities. From Fig. 4, smaller gap widths correlate with larger non-ignition gap distances. Thus it can be concluded that a high ejection velocity enlarges the non-ignition gap distance. Since the gap width 4.0 cm has a large sectional area, the velocity of ejection is low. With low velocity, burning occurs easily within the gap, and the non-ignition gap distance is reduced.
10.0
Shape of ejected flame
GAP LENGTH L cm
Fig. 4. Non-ignition gap distances of single gap with various gap widths.
one for each width. Curve A is for a 2.0 cm gap width, B for 3.0 cm, and C for 4.0 cm. The curves show that gap distances for nonignition increase with increasing gap length within 6.0 cm limits. This m a y be attributed t o the t e m p e r a t u r e drop of the flame caused by contraction as the flame advances into the gap. As shown by curve C for a gap width of 4.0 cm, the authors found that the gap distance for non-ignition first increases with increasing gap length, and then levels off. T hey found further that this pattern always appears for gap widths over 4.0 cm. This is an important characteristic of non-ignition. In Fig. 5, the curves o f Fig. 4 are plotted with the sec-
The shape of flame ejected from rectangular multi-layered gaps varies with the n u m b e r of gaps. The flame of the primary combustion chamber progressing towards the gaps does not invade all gaps equally. Figure 6 shows
(a)
SINGLEGAP
(b)
]'go GAPS
(c)
THREEGAPS
Fig. 6. Shapes of ejected flame. SINGLE GAP
S.O
%
•,~
4.0
~
3.0
~
Z.O
~0~
W=2.0cm
~ ( ) ~
W=3.0cm
~ A ~
W=4.0cm
the shape of flame ejected from a single gap, two gaps, and three respectively. The single gap flame occupies the entire gap. With two gaps, the flame does not invade the whole space. It occupies about half the gap distance. Three gaps are similar to a single gap. In Fig. 7 (c), one gap flame is far in advance of others. Thus it seems permissible to consider the flame at multi-layered gaps as a one gap flame.
1.0
Effect of varying the number of gaps 0
i
0
i
2.0
i
i
i
|
i
,
4.0 6.0 8.0 GAPLENGTHL cm
!
10.0
Fig. 5. Sectional gap areas of single gap with various gap widths.
In Fig. 8, the non-ignition gap distances of curves A and B first increase with increasing gap length, then level off, and finally once again increase. In curve C, however, the second increase does not occur. As in the single gap case, the first increase is due to the tempera-
321
&
L
(a)
(b)
(c)
Fig. 7. Photographs of ejecting flame. (a) Single gap, (b) two gaps (schlieren photograph) and (c) three gaps.
gffi4.0 C11t
0.8
6
0.6
r.9
~,e,~
TMO GAPS
.'~"-'0 ~
THREEGAPS
~ 0
FOUR GAPS
0.4
:j~ .~_~_~__~__~-c ~°-°~°'LB °--°'-°'''°~
0.2
0
1 0
1
. Z.O
1
,
. 4.0
,
.
,
6.0
8.0
10.0
GAP LENGTH L cm
Fig. 8. Non-ignition gap distances of two, three and four gaps with gap width 4.0 cm.
ture drop resulting from contraction of the gas when the flame breaks into the gap. Since four gaps has a larger gap sectional area than
two and three gaps have, four gaps has lower ejection velocity from the gap. The low velocity gas can burn for a longer distance. At this point, the authors used a high speed camera to examine the state of combustion within the gap for each curve. The photographs show t h a t there are different combustion states within the gaps. The burning in the gap of curve A decreases over a length of 5.0 cm, and in the gap of curve B through a length of over 8.0 cm. In curve C, however, the burning does not occur. The physical significance of G* in Fig. 9 is that it is represents an imagined gap distance (G* = G × N, and gap area is proportional to G*). As an example, if G* exists between curve B and C, two gaps are needed to prevent ignition. Any given sectional area of gap can be subdivided to prevent ignition of the secondary combustion chamber mixture by dividing the gap distance; even the single gap ignites.
322 G*
G*
i_
I
I.O
O.S
0.5
f7
¢/A
0.4
~ A _ _
I
TWO GAPS
0.2
THREE GAPS FOUR GAPS ,
, 2.0
,
, 4.0
,
i
,
6.0
, 8.0
,
(a)
(b)
(c)
SINGLE GAP
TWO GAPS
THREE GAPS
,
Fig. 10. Time t a k e n for flame to pass f r o m jet origin t o gap e x i t for single gap, t w o and t h r e e gaps.
I0.0
GAP LENGTH L cm
Fig. 9. G* o f single gap, t w o , t h r e e and f o u r gaps w i t h gap w i d t h 4.0 cm.
R E L A T I O N O F N O N - - I G N I T I O N GAP D I S T A N C E AND u t
Phillips has p o i n t e d o u t t h a t the non-ignit i o n gap distance for a single gap is p r o p o r tional t o ut (u is the ejection v e l o c i t y o f the flame f r o m the gap, and t the t i m e t a k e n for the flame t o pass f r o m the jet origin t o t h e gap exit). In viewing the p o i n t G* in Fig. 9, all gaps have the same sectional area w i t h i n the gap length o f 5.0 cm. T h e r e f o r e , the ejection velocities are the same. As s h o w n in Fig. 10, the angle o f the ejected flame f r o m the gap is 24 - 26 °. Here, the angle is assumed t o be c o n s t a n t at 25 ° . tl > t2 > t3
(2)
and the ejecting velocities are: u1 = u2 = u3
(3)
T h e r e f o r e , f r o m eqns. (2) and (3): Ult I > u2t 2 > u3t 3
larger. B u t for gaps with gap widths over 4.0 cm, the non-ignition gap distance first increases with increasing gap length, t h e n levels off, and undergoes a second increase or n o t d e p e n d i n g o n o t h e r parameters. This is an imp o r t a n t characteristic o f non-ignition, and occurs also with multi-layered gaps. (2) T h e m e t h o d o f subdivision into multilayered gaps is available t o us to p r e v e n t the ignition t h a t m a y o c c u r in a single gap o f equivalent sectional area. (3) At multi-layered gaps, the flame ejected f r o m o n e gap only. Phillips' t h e o r y [ 4 ] , as defined b y ut, is applicable to the non-ignition gap distance o f multi-layered gaps as well as t o a single gap. (4) T h e non-ignition gap distance was aff e c t e d b y the v o l u m e o f the p r i m a r y c o m b u s tion c h a m b e r . But the v o l u m e o f the s e c o n d a r y c o m b u s t i o n c h a m b e r does n o t appreciably affect the results.
REFERENCES
(4)
The non-ignition gap distance is found to be proportional to ut. CONCLUSIONS
(1) F o r a single gap, as the gap w i d t h gets smaller, the non-ignition gap distance b e c o m e s
1 H. G. Wolfhard and A. E. Bruszak, C o m b u s t . Flame 4 (1960) 149 - 159. 2 H. Phillips, C o m b u s t . Flame, 7 ( 1 9 6 3 ) 129 - 135. 3 H. Phillips, C o m b u s t . Flame, 19 (1972) 181 - 186. 4 H. Phillips, C o m b u s t . Flame, 19 (1972) 187 - 195. 5 H. Phillips, C o m b u s t . Flame, 20 (1973) 121 - 126. 6 B. S. 229 (1957), British S t a n d a r d s I n s t i t u t i o n , London. 7 M. Maekawa, C o m b u s t . Sci. T e c h n o l . , 11 (1975) 141 - 145.
317
Non-Ignition of an Ejected Flame Through a Gap Greater than the Quenching Distance MASAO MAEKAWA and MASATSUGU TAKEICHI
Department of Mechanical Engineering, Ehime University, Matsuyama, Ehime 790 (Japan)
SUMMARY
When a gap is greater than the quenching distance, the flame from an explosion inside a chamber can pass through the gap, but it will not always ignite an outside combustible mixture. There is a non-ignition gap distance, greater than the quenching distance, below which ignition will not occur, and above which it will. The present paper presents experimental determinations of non-ignition gap distances for different values of several parameters (gap width, number of gaps, and chamber volume) for various rectangular gaps with a wide range of lengths. A rectangular gap has three dimensions. One of these is here referred to as the gap distance, the others being gap width and gap length; diagramme definitions of the terms are given. The gap distance for non-ignition varies with the values chosen for the other two dimensions. It increases as the gap width decreases. But when gap length is varied for gap widths over 4.0 cm, the nonignition gap distance first increases with increasing gap length, and then levels off. A second increase may occur or not, depending on other parameters. In particular, any given sectional gap area (fixed values for gap width and gap distance) may be subdivided, giving rise to multi-layered gaps. Experiment establishes that such subdivision is available as a method of preventing ignition of an outside combustible mixture, since ignition may occur through a single gap of given area and not through a subdivided multi-layered gap of the same area. At multi-layered gaps, one gap flame is far in advance of the others. Other gap flames are either soon extinguished within the gap, or advance a short distance without emerging. It seems permissible to consider the resultant as one flame for analysis. The magnitude of the non-ignition gap distance for multi-layered gaps is proportional to ut, u being the ejection velocity of the flame from
the gap, and t the time taken for the flame to pass from the jet origin to the gap exit.
INTRODUCTION
Experimental determination of the parameters for non-ignition of an outside combustible mixture by an ejected flame from a gap is justified by the relevance of the information to a variety of combustors of engineering importance. It has been established that the flame from an explosion inside a chamber does not pass through the gap when the gap spacing is less than the quenching gap distance [ 1]. When the gap spacing is greater than the quenching gap distance, the flame passes through the gap, but in spite of this, it may not ignite the combustible mixture outside the gap. There is a non-ignition gap distance which is greater than the quenching gap distance. It is important to realize that the non-ignition gap distance, the largest spacing for non-transmission of an explosion, depends on other elements of the geometry of the gap (gap width, gap length, volume of the explosion chamber, and gap subdivision) as well as on the nature of the materials used. It is well known that a Davy safety lamp intercepts the flame and prevents propagation to an outside combustible mixture by using wire screens. Davy found that the non-propagation was due to the absorption of the heat of the flame by the wire screens. It must be recognized, however, that other factors are involved, and that flame non-propagation can be achieved with devices other than metal screens. Heat absorption and conduction by metal are not the only physical processes available to us for the purpose. When non-thermal conductors such as polyethylene wire are used for screens instead of metal, the combustible mixture does not burn, and non-propagation is achieved without metal screens. In these cases, the non-propaga-
318 tion seems attributable not just to absorption by wire, but to the fineness of the screens as well. Even when the flame ejected from a single gap ignites the outside combustible mixture, subdivision of the gap area into multilayered gaps is a m e t h o d available to us to prevent ignition. Such subdivision, as an alternative to wire screens, was investigated by the authors by using a variety of rectangular gaps o f different dimensions to determine some of the parameters (gap width, num be r of gaps, and chamber volume) that affect the non-ignition gap distance. Early investigations bY Phillips [2 - 5] are summarized in the literature. Experimental values for flame nonignition gap distances for an equatorial flange are also available in the literature on industrial safety [ 6 ] . However, no practical engineering results for multi-layered gaps were given. A previous work by Maekawa established the equation for the non-ignition gap distance [ 7 ], but did not offer a theory. Phillips pointed o u t [4] that the non-ignition gap distance is proportional to u t for the equatorial flange gap. In the present investigation, the authors have examined whether or not the non-ignition gap distance for multi-layered gaps is also proportional to ut.
EXPERIMENTAL The experimental apparatus consisted of two combustion chambers, a fuel mixing and metering system, a spark ignition source, and a rectangular gap m o u n t e d on the primary combustion chamber as shown in Figs. l ( a ) and (b). The gap consisted of a series of gaps established by a set of parallel thin plates with a uniform gap distance. The gap lengths were 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 cm, and the gap widths were 2.0, 3.0 and 4.0 cm. Two values for the volume of the primary combustion chamber were used: 4 × 103 cm 3 (10.0 cm long, 20.0 × 20.0 cm in section) and 8.0 × 103 cm a (20.0 cm long, 20.0 × 20.0 cm in section). Two values for the volume o f the secondary or outside chamber were also used: 48.0 × 103 cm 3 (30.0 cm long, 40.0 X 40.0 cm in section) and 20.0 × 103 cm 3 (12.5 cm long, 40.0 × 40.0 cm in section). The ignition source, located at the middle of the primary combustion chamber axis, was a capacitance spark between brass
BURNER ~
-
PAPER CLOSED
~.L:~>~.UL.-LAYERED ;If~ T ~II CHA"BER S E C O D .A R ~
CO"BUSTION
MA~ETER
I
II ~ HH
II
II
GAPS [
OSCILLO"L.
"~
I EL ODES I ......... ~
,,
II
FAN
"P~I~RY COMBUSTIONCHAMBER
('
X.
1
GAS
II
I
I AIR
(a) MULTI-LAYERED GAPS GAP DISTANCE G
i
PRIMARY COMBUSTION CHAMBER
'
G*
GAP WIDTH W G*=GXN (N IS THE NUMBEROF GAPS) A=G*XW(A IS SECTIONALAREA OF GAPS)
(b) Fig. 1. (a) Apparatus for measuring non-ignition gap distance. (b) Detail of gaps. electrodes with a gap spacing 0.02 cm. High spark energies were available from the power supply, and the energy was delivered across the gap within one or two ps. Methane and air were supplied from separately m et ered flow meters. A stoichiometric m e t h a n e - a i r mixture was used at room t em perat ure and pressure at the time of ignition because of the erratic ignition that sometimes occurs with hydrocarb o n - a i r mixtures. For such ignition the precision of measurement was not good, and reasonably accurate values for non-ignition gap distances would not be forthcoming from a procedure of single trials. Repeat trials were therefore made at the various gap distances to achieve greater exactitude. The flame was initiated in the primary combustion chamber first. Flame propagation within the gap was upward. The flame passed through the gap and ignited the secondary combustion chamber mixture when the test gap distance was greater than the non-ignition gap distance, and did not ignite when the gap distance was below the ignition gap distance. Ignition and non-ignition were visually observed. The magnitude
319
o f the non-ignition gap distance indicates a m a x i m u m gap distance, below which ignition does n o t occur and above which it does.
10.0
•
8.0 1 6.0-
4.0-
RESULTS AND DISCUSSION
The non-ignition gap distances defined by use o f this apparatus for a m e t h a n e - a i r mixture are shown in Figs. 2, 4 and 8. The curves o f the non-ignition gap distance for different fixed values o f the parameters gap width, n u m b e r o f gaps, and cham be r volume were d e t e rmin ed for a wide range of gap length. It will be seen th at the curve separates an uppe r region o f flame ignition from a lower region where the flame does n o t ignite the secondary c o m bu s tio n ch amb e r mixture. The b o u n d a r y line between the tw o regions is the non-ignition gap distance, namely the m a x i m u m spacing of the gap obtained experimentally which prevents ignition o f the secondary c o m b u s t i o n c h a m b e r mixture.
Effect of chamber volume Figure 2 shows the relation between nonignition gap distances and gap length as parameters o f chamber volume. Curve A is for a primary co mb u s tio n chamber of 8.0 X 103 cm 3 , and a secondary c o m b u s t i o n cham be r of 20.0 X 103 cm 3 . Curve B is for a primary combustion chamber o f 4.0 X 103 cm s , and a seco n d a r y o f 20.0 × 103 cm 3 . Curve C is for a primary o f 4.0 × 103 cm 3 , and a secondary o f 48.0 X 103 cm s . It will be observed from the Figure th at the non-ignition gap distance varies
W-4.0 cm THREEGAPS
0.8
PRIMARY (XlO 3 cm3) SECONDARY(X103 cm3)
i
0.6
~
0.4 0.2
- - o - -
8.0
20.0
- - A - -
4.0
20.0
~
4.0
48.0
~ ,
c ,
l
2.0
l
l
l
4.0
,
l
W
6.0
0
2.0
4.0
6.0
8.0 (Xl06 cm3)
A (cm2) V (cm3)
Fig. 3. Introduced pressure in primary combustion chamber with gap.
with the primary com bust i on cham ber volume. The volume of the secondary combustion cham ber does not, however, appreciably affect the results. The introduced pressure in the primary combustion cham ber is pl ot t ed against A / V in Fig. 3. F r o m the Figure, the relation between the i nt roduced pressure P and A / V is given by:
P =Po e-kA/v
(1)
where A is the sectional area of the gap, V the volume of the primary com bust i on chamber, Po the i nt roduced pressure in the absence o f a gap {closed}, k a constant, and e t he natural logarithm base. It will be seen t hat the introduced pressure depends on bot h factors A and V. In general, the values of A / V decrease with increasing volume of the primary combustion cham ber in the non-ignition experiments. Hence the i nt roduced pressure in a primary c o m b u s t i o n cham ber of large volume is greater than the pressure in a smaller one. Since a higher primary chamber pressure will p r o d u c e a higher ejection velocity at the gap, and since the non-ignition gap distance is greater for a primary cham ber of 8.0 × 103 cm 3 than it is for one o f 4.0 × 103 cm 3 (see Fig. 2), it may be considered that high ejection velocity enlarges the non-ignition gap distance.
m
8.0
I0.0
GAP LENGTH L an
Fig. 2. Non-ignition gap distances of three gaps with 4.0 cm width at various combustion chamber volumes.
Effect of gap width Figure 4 shows non-ignition gap distance curves for a single gap with various gap widths as the parameter. Three curves are defined,
320 A
1.O
/o..~o~B°--'o ~o.~ o
~---~
~o
0.8
/ -/ / s
0.6
/
0.4
/~/0~
SIDLE
:P2.O cm
- - ~
W=3.0cm
~@~
W=4.0cm
0.2
0
, 0
\
,
l
2.0
,
, , 1 1
4.0
6.0
8.0
tional area of the gaps, instead of the gap distance, as the vertical axis. It will be seen that a smaller gap width has a smaller gap sectional area than a larger gap width has. Smaller gap widths also correspond to higher gap ejection velocities. From Fig. 4, smaller gap widths correlate with larger non-ignition gap distances. Thus it can be concluded that a high ejection velocity enlarges the non-ignition gap distance. Since the gap width 4.0 cm has a large sectional area, the velocity of ejection is low. With low velocity, burning occurs easily within the gap, and the non-ignition gap distance is reduced.
10.0
Shape of ejected flame
GAP LENGTH L cm
Fig. 4. Non-ignition gap distances of single gap with various gap widths.
one for each width. Curve A is for a 2.0 cm gap width, B for 3.0 cm, and C for 4.0 cm. The curves show that gap distances for nonignition increase with increasing gap length within 6.0 cm limits. This m a y be attributed t o the t e m p e r a t u r e drop of the flame caused by contraction as the flame advances into the gap. As shown by curve C for a gap width of 4.0 cm, the authors found that the gap distance for non-ignition first increases with increasing gap length, and then levels off. T hey found further that this pattern always appears for gap widths over 4.0 cm. This is an important characteristic of non-ignition. In Fig. 5, the curves o f Fig. 4 are plotted with the sec-
The shape of flame ejected from rectangular multi-layered gaps varies with the n u m b e r of gaps. The flame of the primary combustion chamber progressing towards the gaps does not invade all gaps equally. Figure 6 shows
(a)
SINGLEGAP
(b)
]'go GAPS
(c)
THREEGAPS
Fig. 6. Shapes of ejected flame. SINGLE GAP
S.O
%
•,~
4.0
~
3.0
~
Z.O
~0~
W=2.0cm
~ ( ) ~
W=3.0cm
~ A ~
W=4.0cm
the shape of flame ejected from a single gap, two gaps, and three respectively. The single gap flame occupies the entire gap. With two gaps, the flame does not invade the whole space. It occupies about half the gap distance. Three gaps are similar to a single gap. In Fig. 7 (c), one gap flame is far in advance of others. Thus it seems permissible to consider the flame at multi-layered gaps as a one gap flame.
1.0
Effect of varying the number of gaps 0
i
0
i
2.0
i
i
i
|
i
,
4.0 6.0 8.0 GAPLENGTHL cm
!
10.0
Fig. 5. Sectional gap areas of single gap with various gap widths.
In Fig. 8, the non-ignition gap distances of curves A and B first increase with increasing gap length, then level off, and finally once again increase. In curve C, however, the second increase does not occur. As in the single gap case, the first increase is due to the tempera-
321
&
L
(a)
(b)
(c)
Fig. 7. Photographs of ejecting flame. (a) Single gap, (b) two gaps (schlieren photograph) and (c) three gaps.
gffi4.0 C11t
0.8
6
0.6
r.9
~,e,~
TMO GAPS
.'~"-'0 ~
THREEGAPS
~ 0
FOUR GAPS
0.4
:j~ .~_~_~__~__~-c ~°-°~°'LB °--°'-°'''°~
0.2
0
1 0
1
. Z.O
1
,
. 4.0
,
.
,
6.0
8.0
10.0
GAP LENGTH L cm
Fig. 8. Non-ignition gap distances of two, three and four gaps with gap width 4.0 cm.
ture drop resulting from contraction of the gas when the flame breaks into the gap. Since four gaps has a larger gap sectional area than
two and three gaps have, four gaps has lower ejection velocity from the gap. The low velocity gas can burn for a longer distance. At this point, the authors used a high speed camera to examine the state of combustion within the gap for each curve. The photographs show t h a t there are different combustion states within the gaps. The burning in the gap of curve A decreases over a length of 5.0 cm, and in the gap of curve B through a length of over 8.0 cm. In curve C, however, the burning does not occur. The physical significance of G* in Fig. 9 is that it is represents an imagined gap distance (G* = G × N, and gap area is proportional to G*). As an example, if G* exists between curve B and C, two gaps are needed to prevent ignition. Any given sectional area of gap can be subdivided to prevent ignition of the secondary combustion chamber mixture by dividing the gap distance; even the single gap ignites.
322 G*
G*
i_
I
I.O
O.S
0.5
f7
¢/A
0.4
~ A _ _
I
TWO GAPS
0.2
THREE GAPS FOUR GAPS ,
, 2.0
,
, 4.0
,
i
,
6.0
, 8.0
,
(a)
(b)
(c)
SINGLE GAP
TWO GAPS
THREE GAPS
,
Fig. 10. Time t a k e n for flame to pass f r o m jet origin t o gap e x i t for single gap, t w o and t h r e e gaps.
I0.0
GAP LENGTH L cm
Fig. 9. G* o f single gap, t w o , t h r e e and f o u r gaps w i t h gap w i d t h 4.0 cm.
R E L A T I O N O F N O N - - I G N I T I O N GAP D I S T A N C E AND u t
Phillips has p o i n t e d o u t t h a t the non-ignit i o n gap distance for a single gap is p r o p o r tional t o ut (u is the ejection v e l o c i t y o f the flame f r o m the gap, and t the t i m e t a k e n for the flame t o pass f r o m the jet origin t o t h e gap exit). In viewing the p o i n t G* in Fig. 9, all gaps have the same sectional area w i t h i n the gap length o f 5.0 cm. T h e r e f o r e , the ejection velocities are the same. As s h o w n in Fig. 10, the angle o f the ejected flame f r o m the gap is 24 - 26 °. Here, the angle is assumed t o be c o n s t a n t at 25 ° . tl > t2 > t3
(2)
and the ejecting velocities are: u1 = u2 = u3
(3)
T h e r e f o r e , f r o m eqns. (2) and (3): Ult I > u2t 2 > u3t 3
larger. B u t for gaps with gap widths over 4.0 cm, the non-ignition gap distance first increases with increasing gap length, t h e n levels off, and undergoes a second increase or n o t d e p e n d i n g o n o t h e r parameters. This is an imp o r t a n t characteristic o f non-ignition, and occurs also with multi-layered gaps. (2) T h e m e t h o d o f subdivision into multilayered gaps is available t o us to p r e v e n t the ignition t h a t m a y o c c u r in a single gap o f equivalent sectional area. (3) At multi-layered gaps, the flame ejected f r o m o n e gap only. Phillips' t h e o r y [ 4 ] , as defined b y ut, is applicable to the non-ignition gap distance o f multi-layered gaps as well as t o a single gap. (4) T h e non-ignition gap distance was aff e c t e d b y the v o l u m e o f the p r i m a r y c o m b u s tion c h a m b e r . But the v o l u m e o f the s e c o n d a r y c o m b u s t i o n c h a m b e r does n o t appreciably affect the results.
REFERENCES
(4)
The non-ignition gap distance is found to be proportional to ut. CONCLUSIONS
(1) F o r a single gap, as the gap w i d t h gets smaller, the non-ignition gap distance b e c o m e s
1 H. G. Wolfhard and A. E. Bruszak, C o m b u s t . Flame 4 (1960) 149 - 159. 2 H. Phillips, C o m b u s t . Flame, 7 ( 1 9 6 3 ) 129 - 135. 3 H. Phillips, C o m b u s t . Flame, 19 (1972) 181 - 186. 4 H. Phillips, C o m b u s t . Flame, 19 (1972) 187 - 195. 5 H. Phillips, C o m b u s t . Flame, 20 (1973) 121 - 126. 6 B. S. 229 (1957), British S t a n d a r d s I n s t i t u t i o n , London. 7 M. Maekawa, C o m b u s t . Sci. T e c h n o l . , 11 (1975) 141 - 145.