Scale effects on the external combustion caused by venting of a confined explosion

COMBUSTION

AND FLAME

8 3 : 3 9 9 - 4 1 1 (1991)

399

Scale Effects on the External Combustion Caused by Venting of a Confined Explosion C. A. C A T L I N British Gas plc, Midlands Research Station, Wharf Lane, Solihull, West Midlands, England B91 2JW

This article describes a field-scale experimental study of the external jet explosion caused when a confined explosion vents gas into a cloud of the same mixture. By inducing turbulence in the chamber prior to ignition in mixtures of natural gas and oxygen enriched air, it has been possible to study systematicallythe effects of unburned gas exit velocity and fuel reactivity on the peak external overpressures. A direct comparison has been made between explosions with similar exit velocities but with very different fuel re.activities. It has been observed that the external overpressures increase with jet exit velocity up to a critical value of the velocity, above which the overpressures remain roughly constant. This trend has been associated with the onset of turbulence quenching in the external combustion zone. The peak overpressure-jet velocity relationships have been collapsed into a single correlation against an effective Karlovitz number, which is proposed as a way of extrapolating to much larger experimental scales. NOMENCLATURE ao Av

co E

f K L P

p, p* t

tchem teddy gl

ambient sound speed ( a 0 = 350 m/s) vent area (m 2) vent coefficient (chamber cross-sectional area/vent area) contraction coefficient for flow through vent ratio o f unburned and burned gas densities scale-up factor Karlovitz number ( K = tchem/teddy ) integral scale of turbulence (m) absolute pressure ( N / m 2) internal chamber pressure (absolute) (Nlrn 2) atmospheric pressure (absolute) ( N / m z) critical pressure (absolute) ( N / m E) flame speed (m/s) time (s) chemical time scale (tehem = tSl/Ul) (S) turbulence time scale (teddy = X/U) (S) laminar burning velocity (m/s) turbulent burning velocity (m/s)

gt Copyright © 1991 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010

UJ

U

AP

AP v APm~ ), P

7 Pi

[]

rms turbulent velocity (m/s) laminar flame thickness (In) mean unburned gas exit velocity through vent at the time of burnt gas venting (m/s) overpressure ( A P = P - P0) (mbar) internal chamber overpressure at the onset o f burned gas venting (mbar) maximum external overpressnre (mbar) overpressure at the flame (mbar) Taylor microscale of turbulence (m) kinematic viscosity o f gas mixture (m2/s) adiabatic index of flammable gas mixture unburned gas density inside chamber ( k g / m 3) molar fraction

INTRODUCTION When an explosion occurs in a vented chamber the overpressures for the most part are generated within the chamber, except when a jet explosion occurs outside. This external explosion has been referred to as a jet explosion because it is caused

0010-2180/91/$3.50

400 by the combustion of the turbulent head of the transient jet of unburned reactants expelled from the chamber. Ignition of the turbulent head is achieved by the flame once combustion products are vented from the chamber. The jet explosion has long been recognized as an important explosion phenomenon, and the magnitude of the explosion and its dependence upon the chamber vent size have been studied previously in ethylene-air mixtures [1]. The jet explosion can generate a strong pressure wave of its own accord [2] and can provide a "high-energy" ignition for flame acceleration through obstacles [3]. It can also initiate a detonation, provided the mixture is sufficiently reactive and the jet exit velocities and pressures high enough [4]. The external overpressure can also cause the internal chamber overpressure to build up to a higher value than it would have reached had the external explosion not occurred [5]. This is because the external pressure wave, on reaching the vent, can arrest pressure relief by restricting venting. The primary objective of the study was to develop an empirical correlation for the maximum external explosion overpressure. In pracflee, vented explosions can occur both in the case when the flammable gas is initially contained entirely within the chamber and also when the external atmosphere is initially flammable. Although the experimental results have not been reported here, this study established that the rnagnitude of the external explosion was generally less in the former case provided the confined explosion dynamics were similar and the mixture was stoiehiometric. This is consistent with the reactivity of the jet gas being lessened by the air it entrains. This article considers the latter ease when the same flammable mixture is present in both the chamber and the external atmosphere. Because the combustion in a jet explosion is highly turbulent, turbulence quenching processes could reduce the combustion rate. The magnitude of the jet explosion overpressures would therefore be larger in a jet explosion with similar jet dynamics, but at larger scale, because the turbulence strain rates are lower. Large-scale experiments were impractical, and therefore the influ-

C . A . CATLIN ence of scale was indirectly investigated by adding oxygen to increase the laminar burning velocity while preserving a stoichiometric mixture. Increasing the laminar burning velocity has the effect of reducing the chemical reaction time scale relative to the turbulence eddy time scale. By assuming that the ratio of chemical time scale to turbulence time scale is the chief factor controlling quenching [6], it was considered possible to judge the effect of increasing the scale of jet explosions in natural gas by relating the larger scale of interest to an equivalent small-scale experiment in a mixture with a higher laminar burning velocity. It was therefore necessary to generate explosions with similar dynamics in different reactivity mixtures. This was achieved by inducing turbulence in the explosion chamber at the time of ignition. The turbulence enabled the rates of combustion in the less reactive mixtures to be increased so as to generate similar explosion dynamics to those caused by a more reactive, but initially quiescent mixture. In this way the effect of increasing the reactivity could be divorced from the confined explosion dynamics. EXPERIMENTAL The explosion chamber of 0.6 x 0.6-m square cross section, was 1.8 m long, and was mounted on four legs with the base 0.3 m above the concrete floor of the test area. The height was chosen to ensure the turbulent head of the jet of gas expelled from the vent remained as close to the floor as possible without its dynamics being influenced. This was necessary so that when combustion occurred the pressure wave generated was hemispherical and centered on the floor immediately below the jet head. This ensured that the pressure measurements were not complicated by ground reflections. Ignition was always at the closed rear face of the box and the vent was mounted at the opposite end. Four different sized (C~ = 2, 3, 5, 7) square-shaped vents were used. The vent coefficient (C~) is the ratio of the cross-sectional area of the box (0.36 m 2) to that of the vent. A Lexan polycarbonate sheet on one of the side faces

EXTERNAL COMBUSTION CAUSED BY VENTING provided the optical access necessary to film the flame. The explosion chamber was positioned with the vent directed into the metal frame of the external enclosure (Fig. 1), which is covered with polythene to contain the external flammable atmosphere. For all but the tests with the very highest oxygen enrichments the polythene was cut just prior to ignition using a low-energy detonating cord (LEDC) taped to the edges of the external enclosure. The polythene could not be released for the mixtures with highest oxygen content because the LEDC was found to ignite the mixture. Tests were therefore done both with and without cutting the polythene, using lower oxygen enrichments, to prevent the LEDC from causing ignition. These established that the polythene had an insignificant effect on the external overpressures. The flammable mixture was prepared by simultaneously injecting natural gas into the explosion chamber while recirculating gas through the chamber and the external enclosure. Oxygen was injected into the pipework of the valve assembly at the rear of the chamber. The recirculation flow route into the chamber was via four 1-inch-diameter holes in the rear (ignition) face, then out through the vent and back from the far end of the external enclosure through the recirculation duct (Fig. 1). By controlling the flow entering the chamber, the valve and pipe arrangement at the rear of the chamber provided the means of adjusting the initial turbulence field. The holes in the rear face of the chamber were not so large that the gas vented back through them would significantly influence the confined explosion dynamics and yet not so small as to induce a restrictive pressure loss on the recirculation system. The polythene plenum (Fig. 1) provided a link between the fan assembly and the rear face of the chamber. This was designed to rupture at a very low overpressure to prevent the flame from propagating into the valve assembly, which could have caused damage. Overpressure was monitored (Fig. 1) by ten Meclec MQ10 piezoelectric transducers, two mounted within the explosion chamber and eight with 0.4-m separation mounted in a duct in the concrete floor. The signals were transmitted to Meclec M142 charge amplifiers

401

through low loss antimicrophonic cables and recorded on Thorn EMI Datatech (Series 7000A; 20KHz) tape recorders. The internal overpressures were required to calculate the gas exit velocities. The close separation of the external transducers was needed to locate accurately the position of the external maximum overpressure. Cine photography used a Locam high-speed (500pps) camera. Methane and oxygen concentrations were monitored remotely from the rig by drawing samples to Miran-80 infrared and Otox analyzers, respectively. Recording equipment, detonating cord, spark ignition, and vaive operation were all controlled automatically using an electronic timer. The fuel was natural gas with an approximate 95% methane-5% ethane composition and the target oxygen enrichments ([02]/([02] + [N2])) were 21% (air), 23%, and 26%, respectively. For the stoichiometric mixtures these oxygen enrichments corresponded to methane concentrations of 8.7%, 9.5%, and 10.6% and oxygen concentrations of 19.1%, 20.7%, and 23.1%, respectively. THEORY In the following analysis it has been assumed that the combustion in the jet explosion is turbulent and the predominant mechanism controlling the combustion rate is turbulence quenching. Support for this is provided by laboratory studies of "combustion-torch" ignition [7] in a dividedchamber combustion bomb. For sufficiently high prechamber exit velocities the flame was found to extinguish, and fluorescence imaging indicated that the flame was only able to reignite when the level of turbulence in the unburned gas had decayed sufficiently. This suggests that the turbulence strain rates in the jetted unburned gas were initially high enough to quench the combustion. The same mechanism would be expected in the explosion experiments in which much higher jet exit velocities were achieved. It has been shown experimentally that turbulent flame extinction [6] occurs at a fixed value of the product of the Lewis and Karlovitz numbers. For the predominantly methane-oxygen-nitrogen

E +T..1.0

!~

I

external enclosure

3.2m

transducer duct

trap valve

reinforced polythene polythene

9m

II

I

1.8m

_~

~, 2" recirculation line f~el I II i~1 T2 T1

12" recirculation duct

Fig. 1. Plan diagram of rig.

I ! I

I

I

II

purge air

S

BB

1.2m

~1

k

manual valve electric valve sample point Spark

I~, 02 ~/ I flame I~ ,~ i~1_ t r a ~ I

~

('3 >

EXTERNAL COMBUSTION CAUSED BY VENTING mixtures considered in this study, the Lewis number is close to unity [6], and, therefore, the Karlovitz number, simply defined as the ratio of chemical to turbulence time scale, is the predominant parameter controlling quenching. The turbulence time scale is given by the ratio of the Taylor microscale to the rms velocity and the chemical time scale by the ratio of laminar flame thickness to laminar burning velocity:

4O3 2.00 x experimental measurement

/

Interpolation~ E. 1.50

.~ I.oo

i 0.50 J

K = td~m/t~ckly = ~I/Ul " U~'/)k. Oxygen enrichment , % 02/(02+N 2)

Using the approximation for the laminar flame thickness,

(a)

25

61 = i,/u],

2¢

and the following relationship between the Taylor and integral scales,

X~IL =

,lu',

it follows [6] that the Karlovitz number is related to the rms velocity, integral length scale, and kinematic viscosity of the gas mixture via

~ o m

tc

5

.)/(L ,;)] Assuming that turbulence quenching in the jet explosion is characterized by the Karlovitz number, and the integral length is proportional to the size of the experiment, then it follows that an equivalent effect to increasing the experimental scale by a factor f can be achieved by increasing the laminar burning velocity of the mixture by a factor ft/4. The laminar burning velocity of the natural gas-oxygen-enriched air mixture was assumed to have a similar dependence upon oxygen-nitrogen ratio as a stoichiometric methane-oxygen-nitrogen mixture. A smooth interpolation (Fig. 2a) through data obtained at atmospheric pressure and ambient temperature [8] provides a relationship (Fig. 2b) between the scale-up factor and oxygen enrichment. This implies that since the maximum oxygen enrichment used in the experiments was 26% the results are theoretically representative of natural gas-air mixtures on experimental scales up to 8 times larger. The following analysis was used to discount any influence on the Karlovitz number of the

2'2

2'4

2'6

Oxygen enrichment, %

2'6

3'o

021(02+N2)

Co)

Fig. 2. (a) Laminar burning velocities in stoichiometric methane-oxygen-nitrogenmixtures. Co)Oxygenenrichment requiredto achievescale-upby prescribedfactor (f). pressure and temperature rises in the unburned gas, caused by compression waves generated within the combustion zone. The effect on the turbulence parameters can be inferred from rapid distortion theory [9] as u ' 3 / L o:

pl.2933 oc p0.9238,

where the density and pressure changes are related adiabatically with ~ = 1.4. The kinematic viscosity [10] and laminar burning velocity, assuming the oxygen-enriched mixtures to have similar temperature and pressure dependences as for methane-air mixtures [11], are p oc 1//)

X

T 2/3

0¢

p-0.524 and

u I c¢ p-O.5 X T 2 o¢ pO.O714,

404 and hence K oc pO.O571 It follows that, even for overpressures as high as 1000 mbar, the Karlovitz number only increases by 4 %. In addition, it has been assumed that the change in the expansion ratio of the natural gas-air mixture caused by oxygen enrichment also has an insignificant effect. This is reasonable because, even for the maximum oxygen enrichment (26%) used, the expansion ratio is no more than 8% higher than that of a stoichiometric

natural gas-air mixture. This compares with the corresponding 74 % increase in the laminar burning velocity. D A T A ANALYSIS To characterize the rms velocity and integral length scale of the turbulence in the external combustion region, it was assumed that the integral length scale was proportional to the square root of the vent area, and the rms velocity was proportional to the maximum unburned gas exit velocity prior to burned gas venting. Clearly the evolution of the rms velocities and integral scales in a transient jet are very complex and will, at large distances of propagation, be ultimately dominated by the development of the shear layers on the perimeter of the jet "stalk." However, in these experiments the jet head propagated up to ten vent diameters before combustion commenced. It is therefore reasonable to assume the jet is not fully developed and the diameter of the gas jet entering the vortex head is still approximately equal to that leaving the vent. In this case the vent diameter characterizes the curvature of the streamlines in the jet head vortex. This is where the flame first experiences stretch, which is later argued as being the cause of the first, and dominant, overpressure pulse. It was also recognized that the rate of pressure rise within the chamber would affect the jet dynamics. However as in all the experiments the rise rate was short compared to the time taken for the transient jet to propagate across the external enclosure, it was assumed that the turbulence present in the jet head at the time of combustion

C.A. C A T L I N was determined by the maximum venting velocities prior to burned gas venting. Because exit velocities were not measured, it was necessary to calculate them from the internal overpressure of the chamber. The overpressure at the onset of burned gas venting (APbv) was determined by simultaneously comparing film of the flame in the box with the internal pressure trace. Burned gas venting was generally found not to correspond to the peak internal overpressure but was often identified as a sudden reduction in what had previously been a steadily increasing rate of pressure rise (Fig. 3). This sudden change of gradient is because of the abrupt increase in the venting velocities when burned gas moves into the vent and the density of the material being ejected falls. The mean unburned gas exit velocity through the vent (U) was calculated from the atmospheric pressure, unburned gas density, and internal chamber pressure (P~) at that transducer nearest the vent, assuming that it was sufficiently close to the vent plate to be measuring the stagnation pressure. The venting formulas [10] are given below for the subsonic (Pt < P*) and choked (Pi > P*) regimes, where P * is the critical pressure"

P* = Po" [('l' + 1)/2] "d~'-',

U=CD" X

[

('~----1) at

-

~//1

if P~ < P * ,

U=CD" L Or if Pi > P * . Because of venturi contraction the area of the vent ( A v) is effectively reduced ( A v × Co) by the contraction coefficient ((79), which has been taken equal to 0.6 [12] for the vents used in this study. The maximum peak external overpressure (APm~) was taken to be largest of the peaks

EXTERNAL COMBUSTION CAUSED BY VENTING

405

50C 40(3 "~ 30(~ o;

20¢

APbv ~

JX:87

JX'48 • ~ 8.6% CH4/air / /

\

\

~1

\

9.6%CH4/23% 02/(02+N2) C - 5

>~ 100 0 -

0.~

0.%

0.~ 5.

i 0.26

Time, sec.

-100

1 0 . 5 % APbv "

E 1200 o;

JX:50

800[[. 9.4%CH4/ a i r /

__

_

_,_z~"

0.050

JX:91 CH4 /26% 02/(02+N2) CV = 7

~

CV=7

4°°1

Vo.'20

.~,~.

,

0.100

0.150

0.200

Time, sec.

Fig.3. Overpressure-time profilesmeasuredinsidetheexplosionchamberfortestsJX:48, 50,87,91. measured at the eight external transducers and was used to characterize the rate of combustion in the turbulent ball of gas. This is justified by independent experimental and theoretical studies [1, 13]. Measurements of the variation in peak overpressure with distance from the center of the jet explosion [1] indicate that it is approximately uniform in the region of flame propagation and decays as the inverse of distance at larger radii. This is the same spatial distribution in peak overpressure that has been calculated to be generated by the combustion of a spherical cloud of uniform concentration mixture that has been ignited centrally and that supports a spherical flame with constant burning velocity [13]. The theoretical calculations establish that, for a given gas mixture, the peak overpressure at the explosion center is only a function of the total mass burning

rate and is therefore an averaged measure of the burning velocity. A Pm~, as measured by that transducer directly beneath the turbulent ball of gas, has therefore been taken as a measure of the burning velocity alone. In practice the flame in the jet explosion would not be expected to have an exact spherical shape. In particular it will be flattened on the lower side because the flame decellerates as it approaches the ground. However, if it is assumed that the flame shapes in the different experiments are geometrically similar, then A Pn~ , still provides a relative measure of the burning velocity between experiments. RESULTS AND DISCUSSION

The experimental results are summarised in Table 1. The flow rates controlling the level of turbu-

JX46 JX47 JX48 JX49 JX50 JX51 JX52 IX53 JX84 JX85 JX87 IX88 JX89 JX90 JX91

JX45

JX44

JX41

JX39 JX40

Test No. 75 100 120 164 159 175 190 177 193 214 127 145 148 134 118 185 204 170 208 217

Peak Phase H Venting Velocity Overpressure A Pmax Prior to Flame Venting (mbar) (m/s) 68 115 180 180 130 160 140 165 110 100 165 200 250 155 90 330 1000 400 370 1100

1

9.1

8.5 9.4 9.2 8.6 9.0 9.4 9.8 10.5 9.5 10.1 7.8 9.6 10,5 10.5 9.6 10.5

5 5 5 7 7 7 2 2 3 5 5 5 5 3 7 7

9.2 9.0 8.9

Methane Concentration (%CH4)

3

2 2 2

(Co)

Vent Coefficient

Jetted Explosion (JX) Test Data

TABLE

------21 23 21 --21 23 23 21 23

--

--

--

--

Oxygen Concentration (%02)

Air Air Air Air Air Air Air Air Air Air 23 26 23 Air Air 23 26 26 23 26

Oxygen Enrichment (%[02]/[02] -I'- [N2])

Very Rich Very lean

Rich Rich

Rich Rich

Rich

Comments

Z

t') >

EXTERNAL COMBUSTION CAUSED BY VENTING

407

Times from ignition (msec)

'

Fig. 4. Typical flame contours.

lence in the chamber were varied on a trial and error basis to achieve the required values of the internal overpressure at the time of burned gas venting. It was found that, with sufficient turbulence, the burned gas venting pressure in a natural gas-air experiment could be made to correspond closely to that of a quiescent oxygen enriched mixture (Fig. 3), which indicated that the burning rates in the two experiments were closely matched. The duration and rate of pressure rise were also similar. The rate of pressure rise was most closely reproduced in experiments with the same vent size (Fig. 3). When the vent size differed, the smaller vent (JX: 87) generated the faster pressure rise rate (Fig. 3), which is explained by the higher confinement. In general the rise rate of the internal pressure increased up to burned gas venting. This behavior is consistent with the ductlike explosion chamber geometry in which the flame propagates a distance approximately 3 times the cross-sectional diameter of the chamber to reach the vent. In this case the pressure rise rate is controlled by the rapidly increasing flame area caused as the flame elongates towards the vent (Fig. 4). The differences in the external explosion caused by increasing reactivity, as measured at the maximum overpressure transducer, were very marked (Fig. 5). The most reactive (26%) oxygen-enriched mixtures had two very high short-duration peaks (Fig. 5). These reduced in magnitude as the

reactivity of the mixture was lessened and were very much smaller in magnitude, if not completely absent, when no oxygen enrichment was used. However, the overall durations of the external overpressure pulses were very similar. The absence of the spikes in the case of the lower reactivity mixtures, assuming the jet turbulence to be closely reproduced, suggests that the combustion was being quenched in the higher-frequency turbulent structures. The presence of two distinct pressure pulses has been interpreted as being due to two phases of flame propagation. The second pulse was most distinct in the experiments with highest jet exit velocities, for which A Pmax was typically measured near transducer T7, over 2 m away from the front face of the explosion chamber. The relative strength of the second pulse compared to the first discounts it being due to a reflection off the chamber. The two pulses are therefore thought to correspond to phases II and HI shown in Fig. 6, which gives a schematic representation of how the flame may interact with the vortex region at the head of a transient jet. During phase I, the flame area remains approximately constant as the flame is advected on the gas jet toward the turbulent vortex. This phase should not therefore give rise to significant overpressures. Phase II is associated with the very rapid increase in flame area caused when the flame is "wrapped" into the vortex at the jet head and phase IU with the following period of spherical flame propagation

408

C.A. CATLIN 400 x E <~

300

"~ Q;

200

~

10(

JX:48 8.6% CH 4 / air U - 177 m/s; Cv= 7

0.

JX:87 9.6% CH4 / 23% O21(02 U = 185 m/s; C v - 5

~ /

0 50

100

-10C

1

0

2(~0

Time, msec.

1200 E

E

JX:91 10.5% CH4 •26% 02/(02+N2 ) U = 217 m/s; C V= 7

100C

/

8oc

60( 4oc

JX:50 9.4% CH 4 / air U = 214 m/s; C V = 7

0 20C 1,~ -20¢

140

Time, msec.

Fig. 5. Overpressure-time profiles measured directly beneath external explosion for tests JX:48, 50, 87, 91.

through the remainder of the turbulent ball of gas. Unfortunately the films did not provide any clear support for these assumptions. The typical duration of the combined first and second overpressure pulses was 10 ms, and, therefore, was only captured on five film frames. This was insufficient to distinguish the two very short periods of much faster flame propagation that are thought to be responsible for the two high overpressure pulseS,

The duration of the first spike in test JX:91 (Fig. 5) is approximately 2 ms. Assuming the flame to propagate into the vortex at the jet exit velocity (217 m/s), it would travel 0.4 m in the time the pulse is generated. This distance is approximately twice the diameter of the vent and therefore also twice the diameter of the stream of gas entering the jet head. It is therefore similar in size to the stagnation region where the stream

lines diverge on entry to the vortex. This supports the interpretation that the first pulse corresponds to the flame being "wrapped" into the vortex. To study the nature of the second pulse, consider tests JX:48 and 50, with low reactive mixtures, in which the second overpressure pulse is dominant. The duration (10 ms) and peak overpressure (200 mbar) of the pulses are approximately equal in both tests (Fig. 5). For low overpressures ( A P F < 300 mbar) the maximum overpressure at the flame is given approximately [13] in terms of the flame speed by

APF =

20003,[ E L~

1 ][SF] 2 ]['~o J mbar.

For the stoichiometric mixtures being used, E -8, and. hence a peak overpressure of 200 mbar corresponds to a flame speed of 100 m/s. In 10

EXTERNAL COMBUSTION CAUSED BY VENTING

~

~

_

_

=~ 1200I E

StagnationflOw region et jet head

Flameapproachesjet

409 • air x oxygenenriched 10 5/26 " 70 - j %CH4/% 02/(O2-~N2} •

800

no quenching

Phase l 10~5126.0 x

head

400

9"5ee..~

1~0 -

~

]

b

.

m

Flameis

~

stretched

Flame

in stagnationflow region

•

9.6/23.0x 9.6/23.0 ee

e.

1~0

•

2~0

2~0

Jet velocity, m/a

Phase IJ 5

X9.5/23.0 10.5/26.0 no quenching X 10.6/26.O x 10.5/26.0

x =o

3

g

2

i __=

=.

Phase

Flamefront becomes

x10"5/26"0

III

spherical

Fig. 6. Three phases of flame propagation in jet explosion.

ms, therefore, the flame will enclose a sphere of combustion products of radius 1 m that has resulted from the combustion of approximately 0.523 m3 of gas. This volume, neglecting both the gas entrained by the jet head and compression effects, is approximately i7 of the chamber volume (0.684 m3), which would have been expelled from the chamber by the piston action of the flame prior to burned gas venting. This argument supports the interpretation that the second overpressure pulse is caused by combustion of the bulk of turbulent gas expelled from the chamber. Although in practice the flame shape would be distorted away from spherical due to the ground, the argument still provides a reasonably accurate estimate of the volume of unburned gas consumed because the overpressure is essentially a measure of the mass burning rate and not the flame speed. The first peak on the overpressure-time trace has been used to characterize the jet explosion overpressure because it was generally the larger

1

e • air ee x oxygenenriched, % CH4/%02/(02 +N2)

f~o

x

9.6123.0 x

9.6/23.0

• e

1~o 2~o Jet velocity, m/s

21o

Fig. 7. (a) First pulse peak overpressure against jet velocity. Co) Normalized first pulse peak overpressure against jet velocity.

of the two peaks. A detailed interpretation of the second peak was not attempted because the structure of the jet head would have been altered by the previous (phase II) explosion, the analysis of which lay outside the scope of this work. A comparison between the first pulse overpressure and jet velocity is provided in Fig. 7a. It is noted that those experiments with the larger vents (C v = 2) have been omitted because in these the external explosion occurred immediately against the vented face of the chamber vent, and the overpressures are believed to have been significantly amplified as a consequence. The majority of data are for natural gas-air mixtures, which show a trend of decreasing overpressure with increasing jet velocity above of 125 m/s (Fig. 7a). That the overpressures decrease suggests, that the combustion is being quenched: otherwise the combustion rates, and the overpressure, would be expected to increase with jet

410

C.A. CATLIN 5~

tD

X9.5/23.0 10.5/26.0~;

E

x X

10.5/26.0

0o

X

•

no quenching

10.5126.0

•

air

0.

"-

3

x

oxygen enriched

% CH 4 / % 0 2 / ( 0 2 + N 2 ) o]* ffJ

9.6123.0 > 0

X

2

9.6•23.0

•

"0

X m

E

0

ee

1

z

I

I

I

I

I

I

I

I

1000

2000

3000

4000

5000

6000

7000

8000

Kadovitz number, K Fig. 8. Normalized first pulse peak overpressure against Karlovitz number.

velocity. The solid line in Fig. 7a through the data for oxygen-enriched mixtures shows the overpressure to increase strongly (A Pm~ oc U 4'5) with jet velocity and which, of the mixtures studied, is clearly the least affected by quenching. The 23% oxygen-enriched mixtures experience an intermediate level of quenching that is most evident above jet velocities of 185 m/s because the overpressures apparently vary very little with increasing jet velocity (Fig. 7a). If it is assumed that the effect of quenching in the 26% oxygen-enriched mixtures is insignificant it should be possible to separate the effects of turbulence quenching by normalizing the overpressure with respect to U 4"5. The "no-quench" line then appears as a constant and there are three downward branches corresponding to the three different reactivity mixtures (Fig. 7b). Finally, the data are replotted (Fig. 8) against the effective Karlovitz number, as defined previously, to show that all data collapse onto a single correlation. This supports the hypothesis that the Karlovitz number characterizes the effects of turbulence

quenching in the jet explosion and also indicates that the results for the 26% oxygen-enriched mixtures were close to the quench regime. The primary importance of this correlation is that it potentially provides a way of extrapolating the peak jet explosion overpressure measured in these experiments to different fuels and to larger experimental scales simply from a knowledge of the vent size, the laminar burning velocity, and Lewis number of the mixture. CONCLUSIONS 1.

2.

A field-scale experimental study has been performed of jet explosions in natural gas-oxygen-enriched air mixtures caused by venting from a confined explosion into a flammable atmosphere of the same mixture. The jet velocity, vent size, and fuel gas reactivity have been systematically varied to study their influence on the external explosion overpressure. Direct comparison of jet explosions with sim-

EXTERNAL COMBUSTION CAUSED BY VENTING flax venting velocities, but in gases with very different reactivities, has been made possible by inducing turbulence in the explosion chamber gas prior to ignition. Two overpressure pulses axe generated in the external explosion, the durations of which suggest that they arise, respectively, from the flame being "wrapped" into the vortex and flame propagation through the remainder of the turbulent ball of gas at the jet head. The magnitude of these pulses increases with reactivity, suggesting that turbulence quenching limits the combustion rates in the less reactive mixtures. . The data suggest that in the most reactive 26 % oxygen-enricbed mixtures, the peak external overpressure rises monotonically with respect to the maximum unburned jet exit velocity raised to the power 4.5. For the intermediate oxygen enrichments the rate of increase in overpressure with jet exit velocity is less. For mixtures with air the overpressures apparently fall with increasing jet exit velocity. . It has been possible to collapse the data into a single correlation against an effective Karlovitz number by normalizing the overpressures against jet exit velocity to the power 4.5. This correlation suggests the 26% oxygen-enriched mixtures axe not significantly affected by turbulence quenching. The correlation also potentially provides an extrapolation from these experiments to natural gas-air explosions on experimental scales 8 times larger. Because of the limited data, and the

411 uncertainties involved in their interpretation, the correlation is only tentatively proposed as providing an extrapolation to different flammable mixtures from a knowledge of the Kaxlovitz and Lewis numbers of the mixture.

REFERENCES 1. Schildnecht, M., Gieger, W. and Stock, M., Ninth Colloquium (International) on Gasdynamics of Explosions and Reactive Systems, Poitiers, France, July 4-8, 1983, pp. 30-40. 2. Harrison, A. J., and Eyre, J. A., Combust. Sci. Technol. 52:91-106 (1987). 3. Harrison, A. J., and Eyre, J. A., Fifth Symposium (International) on Loss Prevention and Safety Promotion in the Process Industries, Cannes, France, September 1986, pp. 1-18. 4. Ungiit, A., and Shuff, P. J., Combust. Sci. Technol. 63:75-87 (1989). 5. Cooper, "M. G., Fairweather, M., and Tite, J. P., Combust. Flame 65:1-14 (1986). 6. Abdel-Gayed, R. G., and Bradley, D., Combust. Flame 62:61-68 (1985). 7. Cattolica, R. J., Twenty-First Symposium (InternationaO on Combustion, The Combustion Institute, Pittsburgh, 1986, pp. 1551-1559. 8. Vinckier, J., and Van Tiggelen, A., Combust. Flame 12:561-568 (1968). 9. El Tahry, S. H., J. Energy 7(4):345-353 (1983). 10. Bird, R. B,, Stewart, W. E., and Lightfoot, E. N., Transport Phenomena, Wiley, New York, 1960. 11. Andrews, G. E., and Bradley, D., Combust. Flame 19:275-288 (1972). 12. Grose, R. D., J. Fluids Eng. Trans. A S M E 107:36-43 (1985). 13. Strehlow, R. A., Luckritz, R. T., and Adamczyk, A. A., Combust. Flame 35:297-310 (1979). Received 22 November 1989; revised 3 March 1990