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Gas-phase ignition of a solid fuel in a hot stagnation-point flow
Eighteenth Symposium (International) on Combustion
GAS-PHASE
The Combustion Institute, 1981
IGNITION OF A SOLID FUEL STAGNATION-POINT FLOW
IN A HOT
TAKASHI NIIOKA, MAMORU TAKAHASHI AND M U N E O IZUMIKAWA
National Aerospace Laboratory, Kakuda Branch P.O. Box 7, Ohgawara, Miyagi 989-12, Japan
Experimental ignition times of a solid fuel in a hot oxidant stagnation-point flow are obtained. It is found that by allowing flow velocity to change, at constant flow temperature, the ignition time takes on a m i n i m u m value at a certain velocity. The lower velocity range up to the minimum point corresponds to a pyrolysis-controled region. Ignition times, which are nearly equal to gasification times, decrease as flow velocity increases. At high velocities above the minimum point, the pyrolysis-control region changes to a reaction-control region and ignition can not occur rapidly even if gasification is completed. A counterflow field of vaporized fuel gas and hot oxidant gas is formed after rapid vaporization, and in this stretched-flow field the exothermic reaction time in the gas phase lengthens as the flow velocity increases. Experimental ignition times, measured by changing either the oxygen concentration or the gas temperature of a flow, demonstrate the transition from a pyrolysis-control to a reaction-control region. Measured surface temperature histories verify a slow gas-phase reaction period.
1. Introduction In many experimental studies, l7 a diffusion flame established in the stagnation-point boundary layer on a condensed fuel has been used to investigate flame extinction phenomena or to derive overall kinetic parameters from measured regression rates and extinction limits. The counter-flow diffusion flame of gaseous fuels also allows observation of flames of a more fundamental and purer type. Such useful configurations enable researchers to maintain a stable flame in addition to offering a fine control of the convective field, making well-controlled experiments which simulate practical environments possible. Moreover, since a simple one-dimensional treatment is available and the flames in the stretched-flow configurations are convenient for testing interactions between fluid mechanics and chemical kinetics, much theoretical information has been obtained. Virtually all previous studies, however, dealt with a steady-state flame. Studies concerning the transient mechanism of ignition or extinction are rare. Recently Annamalai and Durbetaki s measured ignition times of a porous, thermally thin fabric exposed to a normally impinging flame. Their predicted ignition times are calculated from the quasi-steady 742
gas-phase model and data are limited to a region where the dominant process controlling the ignition time is pyrolysis. Other experimental ignition data in the stagnation-point boundary layer have been reported by Grishin and Isakov. 9 They considered the ignition mode as heterogeneous ignition under their experimental conditions and compared the data with their surface reaction theory. Feng and T'ien,10 and Saitoh and Ishiguro, ~1 utilized the configuration described in the present paper to obtain a transient extinction history of a diffusion flame by a numerical procedure and pointed out that during the flame-out period quasi-steady assumption is not valid because the flame diminishes rapidly within a fraction of the gas-phase characteristic time. The gas-phase ignition transient of a semi-infinite solid fuel suddenly placed in a hot stagnation-point flow is treated here. In earlier works it was found that flames develop interesting characteristics as flow velocity increases (i.e. the stretch rate increases and the Damk~Jhler n u m b e r decreases), and finally are extinguished at a critical velocity because chemical reaction rates become comparable with mass transport processes. Therefore a characteristic variation of ignition time with flow velocity may be expected. In condensed-phase 12'13 and heterogeneous 14
742
COAL FLAMMABILITY
ignition time decreases monotonously as flow velocity increases. Heat-transfer to a solid fuel increases as flow velocity increases, and therefore ignition time decrease. At higher velocities, however, gasphase temperature rise caused by the exothermic reaction of oxidant gas and vaporized fuel gas appears to slow down in the flow configuration. How does the ignition time behave in the higher velocity regions? It is curious that no systematically measured gas-phase ignition data exist for this system in view of the fundamental and practical importance of convective ignition. Although the experimental procedure adopted here is the same as that used for a propellant in our previous paper, L3some polymers were employed here as specimens. Blunt body solid fuels are quickly immersed in the flow of a hot oxidant gas. Ignition times are measured by changing velocity, temperature and oxygen fractions of a flow, then the temperature histories at the stagnation-point are measured. The gas-phase ignition mechanism in stretched-flow fields or with low Damk6hler numbers is discussed.
2. Experimental Method The experimental apparatus ~ is shown in Fig. l(a). Oxygen gas flow diluted by nitrogen is heated by an electric furnace and issues from a 15 mm inner diameter nozzle. The nozzle and a 63 mm diameter rectifying duct are wound with electric wires capable of producing a hot flow of up to about 1150 K at 10 m/sec. The shutter shown in Fig. l(a) receives the initial hot flow and then is quickly removed by spring force after setting the sample at the nozzle center. Since the full opening time of the nozzle outlet is only 6 msec and the characteristic flow time is less than 2 msec in this experiment, the specimen is considered to be exposed
SAMPLE
.-,~
r.\ ,.o~%~.A.=.oRf
instantaneously. Simultaneously, the shutter blocks the light beam applied to the phototransistor and the rapid change of the phototransistor electro-motive force (E.M.F.) is recorded on the photocorder as time zero. Infrared (IR) photoresistance made of lead sulfide is sighted for the stagnation-point and IR emission at ignition is detected. The sensitive wavelength of the IR detector is 0.8-2.85 I~m and the response time is under 300 Ixsec. In Fig. l(b) typical profiles of amplified IR emission intensity are shown as a function of time. Ignition time is defined as the time from the drop of the phototransistor E.M.F. to the rapid rise of IR emission. Solid fuels (with the exception of PMMA) become somewhat carbonized at the surface. In such cases a slight increase of IR emission is observed from the start of heating to ignition, but a distinct increase in IR emission is recorded at ignition. The solid fuels employed are PMMA (polymethyl methacrylate), PVC (polyvinyl chloride), HTPB (hydroxyl-terminated polybutadiene) and CTPB (carboxyl-terminated polybutadiene). The first two are generally used as domestic or industrial materials and the latter two are fuel ingredients of solid propellant. Since the ignition point can not be fixed at the stagnation point if the specimen surface is fiat, solid fuels were formed into a hemisphere of 20 mm diameter. Thus the ignition point is always fixed at the stagnation point, as confirmed by a high-speed camera. The initial temperature of the specimen is 293 K. As shown in the middle curve of Fig. l(b), some surface temperature histories were measured for the PMMA specimen. A 25 I~m diameter ehromel-alumel thermocouple junction is imbedded at the stagnation point and the thermocouple wire is stretched slightly on the hemisphere surface. The thermocouple junction is not visible from the surface, but it can be
I
E,M.F.oF PHOTOTRANSISTOR
I~'START OF ~CONVECTIVE HEATING I INTENSITYOF IR EMISSION SURFACE TEMPERATUREi - ~ 293OK~e.~I~GNITION TIME-~-TIME LAG
|
I
t .or GAs
(a)
,TIME
IGNITION (b)
FIG. 1. (a) Schematic diagram of experimental apparatus. (b) Typical profiles for determination of ignition time.
GAS-PHASE IGNITION OF A SOLID FUEL IN A HOT STAGNATION-POINT FLOW seen from the outside. As the rapid rise of the temperature occurs very soon (at most 30 msec) after the start of IR emission, we can neglect this short time interval. The characteristic flow time and the thermocouple response time are estimated at less than several miliseconds, respectively, and therefore the time lag depends mainly on the condition of the thermocouple junction at the solid surface, It seems likely that time lag can be considered as the time required for the thermocouple junction to emerge from the surface since there is rapid regression of the solid at ignition. Because no time lag is observed in either solid-phase ignition 13or heterogeneous ignition, this suggests that a gas-phase exothermic reaction is responsible for ignition.
3. Experimental Results and Discussion 3.1 Variation with External Flow Velocity The change in ignition time (t,~) with flow velocity is shown in Fig. 2. The initial heat flux (h) and the inverse value of the characteristic flow time (a) taken as an abscissa are proportional to the flow velocity. The value of a is given by the equation a = 3 U J 2 R , where U e and R are the external flow velocity and the radius of the sample, respectively. The data are also arranged by the initial value of the heat flux, because the heat flux is a function of time in convective heating. Initial heat fluxes are obtained by the simple theoretical formula used in Refs. 12---14 and are determined only by U (or a) when the external flow temperature (T~) is kept constant, as in the present experiments. Although radiant heat flux is estimated at about 5% in this experiment, it is neglected in calculating the heat flUX. The data points in all figures are the mean values of several tests. In general, as velocity increases, ignition time decreases, but then increases at higher velocities. Since the stagnation-point heat transfer should become larger as flow velocity increases, the ignition time changes at higher velocities does not appear to be a simple process. A similar experiment for PMMA was made by Grishin and Isakov9 and the same change of ignition time was observed. They suggested that the increase in ignition time at higher velocities might be due to liquid film flow of the melt under the action of tangential friction forces of the flow. PVC specimens, however, are easily carbonized at the surface immediately after exposure to a flow and no liquid film flow of the melt is observed. Despite this, a minimum ignition time exists. According to monochrome films taken by a high-speed camera, an initial white spot which subsequently develops into a final diffusion flame exists in the gas flow, and a very short gap between the spot and the surface can be seen, The time lag
743
of the surface temperature might also be evidence of gas-phase ignition. Consequently, the reasons for minimum ignition time under the conditions of gas-phase ignition must be considered. The low velocity region prior to the minimum was termed the "pyrolysis-control region" and the high velocity region was designated as the (gasphase) "reaction-control region." The schematic explanation is shown in Fig. 3. This figure indicates that in the former region the entire ignition time is determined mainly by vaporization time, since the Damk6hler number is large and the chemical reaction time of vaporized fuel gas and oxygen is quite short. Vaporization is promoted as the hot-flow velocity increases because the stagnation-point heatflux is proportional to the square root of the velocity. Therefore the ignition time decreases as the flow velocity increases in the pyrolysis region. It appears that the monotonous decrease of the ignition time curve ceases at a certain critical velocity above which ignition does not occur. The critical point, however, exists beyond the reaction-control region where the ignition time increases as flow velocity increases. Ignition times are apt to scatter in the reaction-control region, as compared with the pyrolysis-control region, but ignition time increases are always recorded. In the reaction-control region, PMMA data especially have good reproducibility. When the velocity is high, the Damk6hler number is small, while a strong stretched flow field of vaporized fuel gas and external oxidant gas is formed in this counter-flow configuration. In these cases, mass-transfer becomes large and exothermic reaction in the gas-phase becomes relatively weak. t7 Therefore, although gasification is completed within a short time at higher velocities, the subsequent temperature rise in the gas-phase proceeds slowly. This is verified by measurement of surface temperature, as will be shown later. Consequently, the ignition time increases in this region are due to an increase in induction time. Although there is a difference between steady and unsteady ignition, the phenomena of ignition time increase below an ignition limit and temperature decrease of the steady counter-flow diffusion flame with flow velocity below an extinction limit t~ are basically the same. Ignition times obtained by adding gasification time and reaction time have the expected minimum value, as illustrated in Fig. 3. 3.2. Variation with Oxygen Fraction of External
Flow Figures 4 and 5 show the effect of oxygen volume percentage contained in the external flow on ignition time. Bubbles appearing on the fuel surface are shown as data on the dashed lines, and the surface condition is not exactly the same as for the other samples. Experiments for each fuel were conducted
744
COAL FLAMMABILITY I
I
I
l
I
+
I
!
I
4
CTPB
A
G) 113
3
.0_ ,4--.
To = 1 0 0 0 K 1113 K
E 17.
PMMA
2
0 r
Z~
Ol
HTPB 4L
PVC l
..
I
,1
6
.
I
9
E1 Initial I
1
I
I0 heat-flux, I
.
.
.
.
.
.
,l.
,.
1 .
.
.
.
,,I
I
I
0.5
. . . .
I
I Q ( s e c -~) for
I
1.5 Te=lll3
,
,
14xlO 4
I 1.5 2 O (sec -~) for Te=IOOO+K
0.5
I
12 h ( w / m 2) J
2.5 x 103 i
2 x 103
k
FIc. 2. Variation of ignition time with initial heatflux and hot gas flow velocity (31% N~ + 69% Oz).
in the pyrolysis-control region and in the reactioncontrol region of Fig. 2. Ignition time is strongly affected by oxygen concentration in the reactioncontrol region: a = 1910 sec -~ in Fig. 4 and a = 1410 sec-t for PMMA in Fig. 5 generally correspond to the reaction-control region. These ignition times are strongly influenced by an oxygen fraction near
31% Nz + 69% 0 2, which is the experimental condition of Fig. 2. On the other hand, corresponding data in the pyrolysis-control region indicate that it is not so dependent on the amount of oxygen near 31% N 2 + 69% O 2. However even the pyrolysis-control region in Fig. 2 will gradually change to a reaction-control region with smaller amounts
GAS-PHASE IGNITION OF A SOLID FUEL IN A HOT STAGNATION-POINT FLOW
745
T,= 1113 K
--
pyROLYSIS-CONTROL~_
R E G O IN
5
REACTION-CONTROL
(
o PMMA, O = f 4 1 0 sec -~ 9 PMMA, o = 500
REG O IN
~ 3 GAS-PHASE
INDUCTION
2
TIME
uJ
T-
g i
.....
40
J
i
i
I00
p e r c e n t o g e (%)
Fie. 5. Variation of ignition time with oxygen fraction of hot flow for PMMA and PVC.
V A P O R Z IA T O IN T I M E ~ EXTERNAL FLOW VELOCITY
FIG. 3. Schematic explanation for ignition time variation. of oxygen and be affected by the oxygen fraction in the flow. Small oxygen percentages for a = 760 sec -~ of Fig. 4 and for PVC in Fig. 5 show this tendency.
3.3. Variation with External Flow Temperature The e[fect of external flow temperature must be similar to that of the oxygen fraction. Namely, when the ignition time mode shifts from the pyrolysiscontrol region to the reaction-control region or vice versa, characteristic changes must appear. Figure
6 represents the effect of external flow temperature. The PVC data clearly show the characteristic ignition time change at about 1100 K. This point coincides with the minimum point in Fig. 2. The ignition time is strongly affected by the external flow temperature in the reaction-control region below 1100 K. The CTPB and HTPB of the reaction-control region can not be obtained because the surface condition becomes uneven prior to ignition. Also, the pyrolysis-control region of PMMA can not be tested owing to limitations of the experimental apparatus, 3.4. Surface Temperature Histories Following the method described in Section 2, surface temperature histories were measured for PMMA. The reproducibility of Fig. 7 is good. As
~
Ignitoble limit
o=,9,o+-2o.~-, ",'u.CC.. ',,'~', \~(,-~zox,o'./.~)
"..". o
~
80
oxygen volume
B
i
60
31%N~ +69%02 O= 1400 sec -I
~
4 m
*~
~
(h ~ 7 , 6 0 x IO" w / ~ ) 2
2
--0-- + HTPB - e - - --e-- C T P B
"re= I O00K 20
40 oxygen
6O volume
3
CT P B ~
I
80
I00
p e r c e n t o g e (~'a
FIC. 4. Variation of ignition time with oxygen fraction of hot flow for CTPB and HTPB.
o 900
I000
1",
(K)
I100
1200
FIc. 6. Variation of ignition time with gas flow temperature.
746
COAL FLAMMABILITY
8OO 700 ignilion
ignition at a larger stretch rate. Hence it follows that Fig. 8 proves the existence of a reaction-control region.
~ 6c~ a
~
E - 400
(/ f
4. Concluding Remarks
.,!I ...... ,60 ,oc-'~h~o.B.,o'.~.~)
500 ( / / ~ \ ~ o . , "/'/ ~ ~ \
8 9 0 (9.45 xlO `~) 630 (7.92,1r 5 3 0 (7.26x10 "~)
PMMA T~ - 1113 K 31%Nt+ 6 9 % 0 t
) 300 2OO
0
0.5
I
1,5 T i m e (secl
2
2.5
FIG. 7. Surface temperature histories. the flow velocity (i.e. a) increases, the temperature rise becomes steep, However, regardless of how quickly the surface temperature attains a high value, ignition does not take place rapidly in the reactioncontrol region: a = 1160 sec-f distinctly represents the reaction-control region. After the temperature approaches 700 K, it remains constant prior to ignition. This means that there is a steady ablation during this period and suggests that a very slow gas-phase reaction occurs gradually prior to an intense ignition reaction. '~ As mentioned in Section 3.1, this period is the gas-phase reaction delay time which is characteristic of the stretched flow field; this is the reason we call it the reaction-control region. 3.5. Extinction Limit There exists an extinction limit when the flow velocity increases more than the values of Fig. 2. Figure 8 shows the limit for PMMA. Since this limit occurs in the reaction-control region, the external flow temperature for extinction should become higher as flow velocity increases because a gas-phase reaction is necessary at higher temperatures for 1150 P M M A in 3 1 % N 2 + 6 9 % 0 ~
o
I100
\'
9
1050
REFERENCES
flow
Ignition 0
0 ~/
I~
\No i g n i t i o n
I000 I000
1200
1400 0
($ec .1 )
FIc. 8. Ignition limits.
Ignition times of solid fuels in a hot stagnationpoint flow were measured experimentally and it was found that there was a certain flow velocity at which the ignition time had a minimum value, as shown in Fig. 2. Since heat-flux to a solid increases as the hot flow velocity increases, gasification of solids occurs faster at higher velocities. Therefore, if we neglect the chemical reaction time of the vaporized fuel gas and the hot-oxidant gas, vaporization time is nearly equal to ignition time. The region in which the ignition time decreases with velocity is called the "pyrolysis-control region." At higher velocities above the minimum point, however, ignition does not occur promptly after the gasification of solids is completed. The counter-flow field of vaporized fuel gas and hot-oxidant gas, i.e., a strong stretch flow field, is formed and the interaction between chemical reaction and mass transfer becomes a problem. From the viewpoint of thermal explosion theory, it is suggested that the exothermic reaction slows down and the subsequent temperature rise following vaporization does not proceed rapidly, From the viewpoint of chain-reaction theory, it seems likely that the rate of chain carrier generation slows down in this flow field. ~8Therefore, the region in which ignition time increases with velocity was called the "reaction-control" region. A general explanation of experimental results is presented in Fig, 3. Figures 4 to 8 all reinforce the present theory. It would be of interest to measure temperature histories in the gas phase. Furthermore, a reaction-control region must also exist in the stretched flow field of premixed gas and the problem of propagation in stretched flow fields should be addressed.
1600
1. TsuJI, H. AND Y A M A O K A , I.: Thirteenth Symposium (International) on Combustion, p. 723, The Combustion Institute, 1971. 2. HoLvE, D. J. AND SAWW,R, R. F.: Fifteenth Symposium (International) on Combustion, p. 351, The Combustion Institute, 1975. 3. KENT, J. M, ANDWILLIAMS,F. A.: ibid., p. 315, 1975. 4. KRISnNAMUnTHV,L.: Comb. Sci. and Tech., vol. 10, p, 21, 1975. 5. SESHADRI,K. ANDWILLIAMS,F. A.: Halogenated Fire Suppressants (R. G. Gann, Ed.), p. 149, ACS Symposium Series 16, American Chemistry Society, Washington, D, C., 1975.
GAS-PHASE IGNITION O F A SOLID F U E L IN A HOT STAGNATION-POINT FLOW 6. SESFIADRI,K.: Comb. and Flame, vol. 33, p. 197, 1978. 7. T'IEN, J. S., SINGHAL, S. N., H.',aROLO, D. P. AND PRAHL, J. M.; Comb. and Flame, vol. 33, p. 55, 1978. 8. ANNAMALAI, K. ANn DURBETAKI, P.: Comb. and Flame, vol. 27, p. 253, 1976. 9. GalSH1N,A. M. ANnISAKOV,G. N.: Fizika Goreniya i Vzryva, vol. 12, p. 366, 1976. 10. FENG, C. C, AND T'mN, J. S.: Comb. Sci. and Tech., vol. 18, p. 119, 1978. 11. SAITOH,T. AND ISHIGURO, S,: Preprints o f Seventeenth ]apanese S y m p o s i u m on Combustion, p. 200, 1979. (in Japanese).
747
12. NIIOKA, T. AND WILUAMS, F. A.: Comb. and Flame, vol. 29, p. 43, 1976. 13. NIIOKA, T., TAKAHASHI, M. ANO IZUMIKAWA,M.: ibid., vol. 35, p. 81, 1979. 14. NIIOKA, T.: Comb. Sci. and Tech., vol. 18, p. 207, 1978. 15. OTSUKA,Y. ANn NIIOKA, T.: Comb. and Flame, vol. 21, p. 163, 1973. 16. KITANO,M. AND OTSUKA,Y.; Transactions of the Japanese Society of Mechanical Engineers, vol. 45, p. 1902, 1979. (in Japanese). 17. NnOKA,T.: Eighteenth Symposium (International) on Combustion, p. 1813, The Combustion Institute, 1981.
COMMENTS P. Durbetaki, Georgia Institute o f Technology, USA. In your studies, I have noted that 21200 K were the maximum temperatures used for the hot gas let impinging on the solid. Did you make any measurements with hot jet temperatures in excess of 1200 K? It appears that the ignition time reaches an asymptotic constant value near these temperatures.
be possible to use Linfin's analysis of the ignition occurring in the premixed-flarne regime or in the ignition regime, which he developed in analyzing the eounterflow diffusion flame, to calculate the limiting condition of infinite ignition time? In Acta Astronautica (1976), Kishnamurthy has extended Linfin's analysis to include the presence of condensed-phase fuel.
Author's Reply. The temperature limit of our experimental apparatus was about 1200 K. At higher temperatures, ignition time must approach a certain minimum value in Fig. 6. We do not have the theretieal proof for the gas-phase ignition, but asymptotic values for the solid-phase and heterogeneous ignition mode in this flow system are given in Refs. 12 and 14, respectively.
Author's Reply. Yes, it is quite possible. The ignitable limit for the eounterflow configuration is illustrated in Fig. 3 of the other theoretical presentation in this proceedings. L i ~ n ' s ignitable condition obtained from the steady problem shows a good agreement with the infinite ignition time condition calculated from the present unsteady analysis. Therefore, in the stagnation-point boundary layer on a condensed material, Krishnamurthy's analysis can be utilized to compare with our experimental result shown in Fig. 8.
F. A. Williams, Ames U.C.S.D., USA. Would it
GAS-PHASE
The Combustion Institute, 1981
IGNITION OF A SOLID FUEL STAGNATION-POINT FLOW
IN A HOT
TAKASHI NIIOKA, MAMORU TAKAHASHI AND M U N E O IZUMIKAWA
National Aerospace Laboratory, Kakuda Branch P.O. Box 7, Ohgawara, Miyagi 989-12, Japan
Experimental ignition times of a solid fuel in a hot oxidant stagnation-point flow are obtained. It is found that by allowing flow velocity to change, at constant flow temperature, the ignition time takes on a m i n i m u m value at a certain velocity. The lower velocity range up to the minimum point corresponds to a pyrolysis-controled region. Ignition times, which are nearly equal to gasification times, decrease as flow velocity increases. At high velocities above the minimum point, the pyrolysis-control region changes to a reaction-control region and ignition can not occur rapidly even if gasification is completed. A counterflow field of vaporized fuel gas and hot oxidant gas is formed after rapid vaporization, and in this stretched-flow field the exothermic reaction time in the gas phase lengthens as the flow velocity increases. Experimental ignition times, measured by changing either the oxygen concentration or the gas temperature of a flow, demonstrate the transition from a pyrolysis-control to a reaction-control region. Measured surface temperature histories verify a slow gas-phase reaction period.
1. Introduction In many experimental studies, l7 a diffusion flame established in the stagnation-point boundary layer on a condensed fuel has been used to investigate flame extinction phenomena or to derive overall kinetic parameters from measured regression rates and extinction limits. The counter-flow diffusion flame of gaseous fuels also allows observation of flames of a more fundamental and purer type. Such useful configurations enable researchers to maintain a stable flame in addition to offering a fine control of the convective field, making well-controlled experiments which simulate practical environments possible. Moreover, since a simple one-dimensional treatment is available and the flames in the stretched-flow configurations are convenient for testing interactions between fluid mechanics and chemical kinetics, much theoretical information has been obtained. Virtually all previous studies, however, dealt with a steady-state flame. Studies concerning the transient mechanism of ignition or extinction are rare. Recently Annamalai and Durbetaki s measured ignition times of a porous, thermally thin fabric exposed to a normally impinging flame. Their predicted ignition times are calculated from the quasi-steady 742
gas-phase model and data are limited to a region where the dominant process controlling the ignition time is pyrolysis. Other experimental ignition data in the stagnation-point boundary layer have been reported by Grishin and Isakov. 9 They considered the ignition mode as heterogeneous ignition under their experimental conditions and compared the data with their surface reaction theory. Feng and T'ien,10 and Saitoh and Ishiguro, ~1 utilized the configuration described in the present paper to obtain a transient extinction history of a diffusion flame by a numerical procedure and pointed out that during the flame-out period quasi-steady assumption is not valid because the flame diminishes rapidly within a fraction of the gas-phase characteristic time. The gas-phase ignition transient of a semi-infinite solid fuel suddenly placed in a hot stagnation-point flow is treated here. In earlier works it was found that flames develop interesting characteristics as flow velocity increases (i.e. the stretch rate increases and the Damk~Jhler n u m b e r decreases), and finally are extinguished at a critical velocity because chemical reaction rates become comparable with mass transport processes. Therefore a characteristic variation of ignition time with flow velocity may be expected. In condensed-phase 12'13 and heterogeneous 14
742
COAL FLAMMABILITY
ignition time decreases monotonously as flow velocity increases. Heat-transfer to a solid fuel increases as flow velocity increases, and therefore ignition time decrease. At higher velocities, however, gasphase temperature rise caused by the exothermic reaction of oxidant gas and vaporized fuel gas appears to slow down in the flow configuration. How does the ignition time behave in the higher velocity regions? It is curious that no systematically measured gas-phase ignition data exist for this system in view of the fundamental and practical importance of convective ignition. Although the experimental procedure adopted here is the same as that used for a propellant in our previous paper, L3some polymers were employed here as specimens. Blunt body solid fuels are quickly immersed in the flow of a hot oxidant gas. Ignition times are measured by changing velocity, temperature and oxygen fractions of a flow, then the temperature histories at the stagnation-point are measured. The gas-phase ignition mechanism in stretched-flow fields or with low Damk6hler numbers is discussed.
2. Experimental Method The experimental apparatus ~ is shown in Fig. l(a). Oxygen gas flow diluted by nitrogen is heated by an electric furnace and issues from a 15 mm inner diameter nozzle. The nozzle and a 63 mm diameter rectifying duct are wound with electric wires capable of producing a hot flow of up to about 1150 K at 10 m/sec. The shutter shown in Fig. l(a) receives the initial hot flow and then is quickly removed by spring force after setting the sample at the nozzle center. Since the full opening time of the nozzle outlet is only 6 msec and the characteristic flow time is less than 2 msec in this experiment, the specimen is considered to be exposed
SAMPLE
.-,~
r.\ ,.o~%~.A.=.oRf
instantaneously. Simultaneously, the shutter blocks the light beam applied to the phototransistor and the rapid change of the phototransistor electro-motive force (E.M.F.) is recorded on the photocorder as time zero. Infrared (IR) photoresistance made of lead sulfide is sighted for the stagnation-point and IR emission at ignition is detected. The sensitive wavelength of the IR detector is 0.8-2.85 I~m and the response time is under 300 Ixsec. In Fig. l(b) typical profiles of amplified IR emission intensity are shown as a function of time. Ignition time is defined as the time from the drop of the phototransistor E.M.F. to the rapid rise of IR emission. Solid fuels (with the exception of PMMA) become somewhat carbonized at the surface. In such cases a slight increase of IR emission is observed from the start of heating to ignition, but a distinct increase in IR emission is recorded at ignition. The solid fuels employed are PMMA (polymethyl methacrylate), PVC (polyvinyl chloride), HTPB (hydroxyl-terminated polybutadiene) and CTPB (carboxyl-terminated polybutadiene). The first two are generally used as domestic or industrial materials and the latter two are fuel ingredients of solid propellant. Since the ignition point can not be fixed at the stagnation point if the specimen surface is fiat, solid fuels were formed into a hemisphere of 20 mm diameter. Thus the ignition point is always fixed at the stagnation point, as confirmed by a high-speed camera. The initial temperature of the specimen is 293 K. As shown in the middle curve of Fig. l(b), some surface temperature histories were measured for the PMMA specimen. A 25 I~m diameter ehromel-alumel thermocouple junction is imbedded at the stagnation point and the thermocouple wire is stretched slightly on the hemisphere surface. The thermocouple junction is not visible from the surface, but it can be
I
E,M.F.oF PHOTOTRANSISTOR
I~'START OF ~CONVECTIVE HEATING I INTENSITYOF IR EMISSION SURFACE TEMPERATUREi - ~ 293OK~e.~I~GNITION TIME-~-TIME LAG
|
I
t .or GAs
(a)
,TIME
IGNITION (b)
FIG. 1. (a) Schematic diagram of experimental apparatus. (b) Typical profiles for determination of ignition time.
GAS-PHASE IGNITION OF A SOLID FUEL IN A HOT STAGNATION-POINT FLOW seen from the outside. As the rapid rise of the temperature occurs very soon (at most 30 msec) after the start of IR emission, we can neglect this short time interval. The characteristic flow time and the thermocouple response time are estimated at less than several miliseconds, respectively, and therefore the time lag depends mainly on the condition of the thermocouple junction at the solid surface, It seems likely that time lag can be considered as the time required for the thermocouple junction to emerge from the surface since there is rapid regression of the solid at ignition. Because no time lag is observed in either solid-phase ignition 13or heterogeneous ignition, this suggests that a gas-phase exothermic reaction is responsible for ignition.
3. Experimental Results and Discussion 3.1 Variation with External Flow Velocity The change in ignition time (t,~) with flow velocity is shown in Fig. 2. The initial heat flux (h) and the inverse value of the characteristic flow time (a) taken as an abscissa are proportional to the flow velocity. The value of a is given by the equation a = 3 U J 2 R , where U e and R are the external flow velocity and the radius of the sample, respectively. The data are also arranged by the initial value of the heat flux, because the heat flux is a function of time in convective heating. Initial heat fluxes are obtained by the simple theoretical formula used in Refs. 12---14 and are determined only by U (or a) when the external flow temperature (T~) is kept constant, as in the present experiments. Although radiant heat flux is estimated at about 5% in this experiment, it is neglected in calculating the heat flUX. The data points in all figures are the mean values of several tests. In general, as velocity increases, ignition time decreases, but then increases at higher velocities. Since the stagnation-point heat transfer should become larger as flow velocity increases, the ignition time changes at higher velocities does not appear to be a simple process. A similar experiment for PMMA was made by Grishin and Isakov9 and the same change of ignition time was observed. They suggested that the increase in ignition time at higher velocities might be due to liquid film flow of the melt under the action of tangential friction forces of the flow. PVC specimens, however, are easily carbonized at the surface immediately after exposure to a flow and no liquid film flow of the melt is observed. Despite this, a minimum ignition time exists. According to monochrome films taken by a high-speed camera, an initial white spot which subsequently develops into a final diffusion flame exists in the gas flow, and a very short gap between the spot and the surface can be seen, The time lag
743
of the surface temperature might also be evidence of gas-phase ignition. Consequently, the reasons for minimum ignition time under the conditions of gas-phase ignition must be considered. The low velocity region prior to the minimum was termed the "pyrolysis-control region" and the high velocity region was designated as the (gasphase) "reaction-control region." The schematic explanation is shown in Fig. 3. This figure indicates that in the former region the entire ignition time is determined mainly by vaporization time, since the Damk6hler number is large and the chemical reaction time of vaporized fuel gas and oxygen is quite short. Vaporization is promoted as the hot-flow velocity increases because the stagnation-point heatflux is proportional to the square root of the velocity. Therefore the ignition time decreases as the flow velocity increases in the pyrolysis region. It appears that the monotonous decrease of the ignition time curve ceases at a certain critical velocity above which ignition does not occur. The critical point, however, exists beyond the reaction-control region where the ignition time increases as flow velocity increases. Ignition times are apt to scatter in the reaction-control region, as compared with the pyrolysis-control region, but ignition time increases are always recorded. In the reaction-control region, PMMA data especially have good reproducibility. When the velocity is high, the Damk6hler number is small, while a strong stretched flow field of vaporized fuel gas and external oxidant gas is formed in this counter-flow configuration. In these cases, mass-transfer becomes large and exothermic reaction in the gas-phase becomes relatively weak. t7 Therefore, although gasification is completed within a short time at higher velocities, the subsequent temperature rise in the gas-phase proceeds slowly. This is verified by measurement of surface temperature, as will be shown later. Consequently, the ignition time increases in this region are due to an increase in induction time. Although there is a difference between steady and unsteady ignition, the phenomena of ignition time increase below an ignition limit and temperature decrease of the steady counter-flow diffusion flame with flow velocity below an extinction limit t~ are basically the same. Ignition times obtained by adding gasification time and reaction time have the expected minimum value, as illustrated in Fig. 3. 3.2. Variation with Oxygen Fraction of External
Flow Figures 4 and 5 show the effect of oxygen volume percentage contained in the external flow on ignition time. Bubbles appearing on the fuel surface are shown as data on the dashed lines, and the surface condition is not exactly the same as for the other samples. Experiments for each fuel were conducted
744
COAL FLAMMABILITY I
I
I
l
I
+
I
!
I
4
CTPB
A
G) 113
3
.0_ ,4--.
To = 1 0 0 0 K 1113 K
E 17.
PMMA
2
0 r
Z~
Ol
HTPB 4L
PVC l
..
I
,1
6
.
I
9
E1 Initial I
1
I
I0 heat-flux, I
.
.
.
.
.
.
,l.
,.
1 .
.
.
.
,,I
I
I
0.5
. . . .
I
I Q ( s e c -~) for
I
1.5 Te=lll3
,
,
14xlO 4
I 1.5 2 O (sec -~) for Te=IOOO+K
0.5
I
12 h ( w / m 2) J
2.5 x 103 i
2 x 103
k
FIc. 2. Variation of ignition time with initial heatflux and hot gas flow velocity (31% N~ + 69% Oz).
in the pyrolysis-control region and in the reactioncontrol region of Fig. 2. Ignition time is strongly affected by oxygen concentration in the reactioncontrol region: a = 1910 sec -~ in Fig. 4 and a = 1410 sec-t for PMMA in Fig. 5 generally correspond to the reaction-control region. These ignition times are strongly influenced by an oxygen fraction near
31% Nz + 69% 0 2, which is the experimental condition of Fig. 2. On the other hand, corresponding data in the pyrolysis-control region indicate that it is not so dependent on the amount of oxygen near 31% N 2 + 69% O 2. However even the pyrolysis-control region in Fig. 2 will gradually change to a reaction-control region with smaller amounts
GAS-PHASE IGNITION OF A SOLID FUEL IN A HOT STAGNATION-POINT FLOW
745
T,= 1113 K
--
pyROLYSIS-CONTROL~_
R E G O IN
5
REACTION-CONTROL
(
o PMMA, O = f 4 1 0 sec -~ 9 PMMA, o = 500
REG O IN
~ 3 GAS-PHASE
INDUCTION
2
TIME
uJ
T-
g i
.....
40
J
i
i
I00
p e r c e n t o g e (%)
Fie. 5. Variation of ignition time with oxygen fraction of hot flow for PMMA and PVC.
V A P O R Z IA T O IN T I M E ~ EXTERNAL FLOW VELOCITY
FIG. 3. Schematic explanation for ignition time variation. of oxygen and be affected by the oxygen fraction in the flow. Small oxygen percentages for a = 760 sec -~ of Fig. 4 and for PVC in Fig. 5 show this tendency.
3.3. Variation with External Flow Temperature The e[fect of external flow temperature must be similar to that of the oxygen fraction. Namely, when the ignition time mode shifts from the pyrolysiscontrol region to the reaction-control region or vice versa, characteristic changes must appear. Figure
6 represents the effect of external flow temperature. The PVC data clearly show the characteristic ignition time change at about 1100 K. This point coincides with the minimum point in Fig. 2. The ignition time is strongly affected by the external flow temperature in the reaction-control region below 1100 K. The CTPB and HTPB of the reaction-control region can not be obtained because the surface condition becomes uneven prior to ignition. Also, the pyrolysis-control region of PMMA can not be tested owing to limitations of the experimental apparatus, 3.4. Surface Temperature Histories Following the method described in Section 2, surface temperature histories were measured for PMMA. The reproducibility of Fig. 7 is good. As
~
Ignitoble limit
o=,9,o+-2o.~-, ",'u.CC.. ',,'~', \~(,-~zox,o'./.~)
"..". o
~
80
oxygen volume
B
i
60
31%N~ +69%02 O= 1400 sec -I
~
4 m
*~
~
(h ~ 7 , 6 0 x IO" w / ~ ) 2
2
--0-- + HTPB - e - - --e-- C T P B
"re= I O00K 20
40 oxygen
6O volume
3
CT P B ~
I
80
I00
p e r c e n t o g e (~'a
FIC. 4. Variation of ignition time with oxygen fraction of hot flow for CTPB and HTPB.
o 900
I000
1",
(K)
I100
1200
FIc. 6. Variation of ignition time with gas flow temperature.
746
COAL FLAMMABILITY
8OO 700 ignilion
ignition at a larger stretch rate. Hence it follows that Fig. 8 proves the existence of a reaction-control region.
~ 6c~ a
~
E - 400
(/ f
4. Concluding Remarks
.,!I ...... ,60 ,oc-'~h~o.B.,o'.~.~)
500 ( / / ~ \ ~ o . , "/'/ ~ ~ \
8 9 0 (9.45 xlO `~) 630 (7.92,1r 5 3 0 (7.26x10 "~)
PMMA T~ - 1113 K 31%Nt+ 6 9 % 0 t
) 300 2OO
0
0.5
I
1,5 T i m e (secl
2
2.5
FIG. 7. Surface temperature histories. the flow velocity (i.e. a) increases, the temperature rise becomes steep, However, regardless of how quickly the surface temperature attains a high value, ignition does not take place rapidly in the reactioncontrol region: a = 1160 sec-f distinctly represents the reaction-control region. After the temperature approaches 700 K, it remains constant prior to ignition. This means that there is a steady ablation during this period and suggests that a very slow gas-phase reaction occurs gradually prior to an intense ignition reaction. '~ As mentioned in Section 3.1, this period is the gas-phase reaction delay time which is characteristic of the stretched flow field; this is the reason we call it the reaction-control region. 3.5. Extinction Limit There exists an extinction limit when the flow velocity increases more than the values of Fig. 2. Figure 8 shows the limit for PMMA. Since this limit occurs in the reaction-control region, the external flow temperature for extinction should become higher as flow velocity increases because a gas-phase reaction is necessary at higher temperatures for 1150 P M M A in 3 1 % N 2 + 6 9 % 0 ~
o
I100
\'
9
1050
REFERENCES
flow
Ignition 0
0 ~/
I~
\No i g n i t i o n
I000 I000
1200
1400 0
($ec .1 )
FIc. 8. Ignition limits.
Ignition times of solid fuels in a hot stagnationpoint flow were measured experimentally and it was found that there was a certain flow velocity at which the ignition time had a minimum value, as shown in Fig. 2. Since heat-flux to a solid increases as the hot flow velocity increases, gasification of solids occurs faster at higher velocities. Therefore, if we neglect the chemical reaction time of the vaporized fuel gas and the hot-oxidant gas, vaporization time is nearly equal to ignition time. The region in which the ignition time decreases with velocity is called the "pyrolysis-control region." At higher velocities above the minimum point, however, ignition does not occur promptly after the gasification of solids is completed. The counter-flow field of vaporized fuel gas and hot-oxidant gas, i.e., a strong stretch flow field, is formed and the interaction between chemical reaction and mass transfer becomes a problem. From the viewpoint of thermal explosion theory, it is suggested that the exothermic reaction slows down and the subsequent temperature rise following vaporization does not proceed rapidly, From the viewpoint of chain-reaction theory, it seems likely that the rate of chain carrier generation slows down in this flow field. ~8Therefore, the region in which ignition time increases with velocity was called the "reaction-control" region. A general explanation of experimental results is presented in Fig, 3. Figures 4 to 8 all reinforce the present theory. It would be of interest to measure temperature histories in the gas phase. Furthermore, a reaction-control region must also exist in the stretched flow field of premixed gas and the problem of propagation in stretched flow fields should be addressed.
1600
1. TsuJI, H. AND Y A M A O K A , I.: Thirteenth Symposium (International) on Combustion, p. 723, The Combustion Institute, 1971. 2. HoLvE, D. J. AND SAWW,R, R. F.: Fifteenth Symposium (International) on Combustion, p. 351, The Combustion Institute, 1975. 3. KENT, J. M, ANDWILLIAMS,F. A.: ibid., p. 315, 1975. 4. KRISnNAMUnTHV,L.: Comb. Sci. and Tech., vol. 10, p, 21, 1975. 5. SESHADRI,K. ANDWILLIAMS,F. A.: Halogenated Fire Suppressants (R. G. Gann, Ed.), p. 149, ACS Symposium Series 16, American Chemistry Society, Washington, D, C., 1975.
GAS-PHASE IGNITION O F A SOLID F U E L IN A HOT STAGNATION-POINT FLOW 6. SESFIADRI,K.: Comb. and Flame, vol. 33, p. 197, 1978. 7. T'IEN, J. S., SINGHAL, S. N., H.',aROLO, D. P. AND PRAHL, J. M.; Comb. and Flame, vol. 33, p. 55, 1978. 8. ANNAMALAI, K. ANn DURBETAKI, P.: Comb. and Flame, vol. 27, p. 253, 1976. 9. GalSH1N,A. M. ANnISAKOV,G. N.: Fizika Goreniya i Vzryva, vol. 12, p. 366, 1976. 10. FENG, C. C, AND T'mN, J. S.: Comb. Sci. and Tech., vol. 18, p. 119, 1978. 11. SAITOH,T. AND ISHIGURO, S,: Preprints o f Seventeenth ]apanese S y m p o s i u m on Combustion, p. 200, 1979. (in Japanese).
747
12. NIIOKA, T. AND WILUAMS, F. A.: Comb. and Flame, vol. 29, p. 43, 1976. 13. NIIOKA, T., TAKAHASHI, M. ANO IZUMIKAWA,M.: ibid., vol. 35, p. 81, 1979. 14. NIIOKA, T.: Comb. Sci. and Tech., vol. 18, p. 207, 1978. 15. OTSUKA,Y. ANn NIIOKA, T.: Comb. and Flame, vol. 21, p. 163, 1973. 16. KITANO,M. AND OTSUKA,Y.; Transactions of the Japanese Society of Mechanical Engineers, vol. 45, p. 1902, 1979. (in Japanese). 17. NnOKA,T.: Eighteenth Symposium (International) on Combustion, p. 1813, The Combustion Institute, 1981.
COMMENTS P. Durbetaki, Georgia Institute o f Technology, USA. In your studies, I have noted that 21200 K were the maximum temperatures used for the hot gas let impinging on the solid. Did you make any measurements with hot jet temperatures in excess of 1200 K? It appears that the ignition time reaches an asymptotic constant value near these temperatures.
be possible to use Linfin's analysis of the ignition occurring in the premixed-flarne regime or in the ignition regime, which he developed in analyzing the eounterflow diffusion flame, to calculate the limiting condition of infinite ignition time? In Acta Astronautica (1976), Kishnamurthy has extended Linfin's analysis to include the presence of condensed-phase fuel.
Author's Reply. The temperature limit of our experimental apparatus was about 1200 K. At higher temperatures, ignition time must approach a certain minimum value in Fig. 6. We do not have the theretieal proof for the gas-phase ignition, but asymptotic values for the solid-phase and heterogeneous ignition mode in this flow system are given in Refs. 12 and 14, respectively.
Author's Reply. Yes, it is quite possible. The ignitable limit for the eounterflow configuration is illustrated in Fig. 3 of the other theoretical presentation in this proceedings. L i ~ n ' s ignitable condition obtained from the steady problem shows a good agreement with the infinite ignition time condition calculated from the present unsteady analysis. Therefore, in the stagnation-point boundary layer on a condensed material, Krishnamurthy's analysis can be utilized to compare with our experimental result shown in Fig. 8.
F. A. Williams, Ames U.C.S.D., USA. Would it