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Ignition of sprays by an incident shock
COMBUSTION A N D F L A M E 25, 177-186 (1975)
177
Ignition of Sprays by an Incident Shock KENJI MIYASAKA and YUKIO MIZUTANI Department of Mechanical Engineering, School of Engineering, Osaka University, Yamada-kami, Suita, Osaka 565, Japan
A shock-tube technique was developed aiming at obtaining pure ignition delay data of sprays independent of the processes of atomization and mixing. A spray column, which was injected by an ultrasonic atomizer and freely falling through the low-pressure section of a shock tube placed horizontally, was ignited by an incident shock. This technique gave ignition delay periods as short as only a few tenths of those obtained by any conventional technique. This may be partly because the ignition delay data obtained by this technique include neither the period of atomization nor that of mixing, and partly because ignition took place in the micromist generated by the shattering of the original droplets in the spray.
Introduction
Most ignition delay data for fuel sprays have so far been obtained either by using a Diesel engine with a window on the cylinder head [1] or a pistoncatching combustion device [2], or by injecting fuel into an electric furnace or a high-temperature burned gas stream [3, 4]. In those techniques, however, it is not possible to separate the atomizing or mixing process from that of ignition, so that pure ignition delay data free from the former processes can not be obtained. The ignition delay data obtained, therefore, differ from each other depending on the apparatus or procedure employed. This inconvenience may be avoided by employing the shock-tube technique where premixed sprays are ignited by an incident or reflected shock or a piston-catching combustion rig in which premixed sprays are adiabatically compressed and ignited. In the present study, an attempt was made to obtain pure ignition delay data free from atomizing and mixing processes by introducing an incident shock into a spray column freely suspended in the low-pressure section of a shock tube and igniting it. There are Lu and Slagg's investigation [5] for a single drop and that of Kauffman and Nicholls [6] for droplet arrays as examples of the application of the shock-tube technique to the study on the ignition of liquid fuels. It is, however, only Mullaney's investigation [7] that the shock-tube
technique has been applied to the spray-ignition investigation. In his experiments, the ignition delays observed still contain the periods of atomization and mixing, as in conventional techniques, since a spray was injected into the high-temperature region behind a reflected shock from a pressure atomizer placed on the end plate of the shock tube in the direction of the axis, which means that his technique has no essential difference from conventional ones such as the electric-furnace technique. Such a lack of spray-ignition investigation utilizing the shock-tube technique may come from the difficulty in suspending a spray in the lowpressure section of a shock tube where the pressure is 0.1 arm or below. This difficulty was successfully overcome by employing an ultrasonic atomizer that shows excellent atomizing characteristics independent of pressure. The shock-tube technique has the advantage of the possibility that the ignition process is separated from the atomizing and mixing processes, while it has several disadvantages. They are (1) the existence of interaction between drops and the incident shock resulting in the shattering of drops and high stagnation temperature, (2) different degrees of acceleration between drops of different sizes or between drops and vapor, (3) collision of drops in the spray resulting in agglomeration or breakup of drops, and (4) high fuel vapor-air ratio even for
Copyright © 1975 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.
178
KENJI MIYASAKA and YUKIO MIZUTANI
fuels of low volatility resulted from the initial low pressure in a shock tube, which put restrictions on the choice of fuel (see Table 2). In the present investigation, the ignition delay was determined for cetane (n-hexadecane) whose vapor pressure is low enough at room temperature, the results of which were compared with the ignitiondelay data obtained by conventional techniques. In addition, the ignition delay was determined also for fuel vapor-air mixtures and for fuel vaporspray-air systems using tetralin (tetrahydronaphthalene) whose saturation pressure was so high at room temperature that a flammable vapor-air mixture was formed in the low-pressure section of a shock tube. An incident shock was utilized for igniting a spray or vapor.
Description of Experiment The schematic diagram of the experimental apparatus is illustrated in Fig. 1. An ordinary heliumdriven shock tube of 64 mm i.d. was employed whose driver and driven sections were 1.5 m and 5.7 m long, respectively. The pressures of driver gas (helium) and driven gas (air) were around 30 atm and 30 to 100 Torr, respectively. The rupture diaphragm was made of aluminum sheet 0.3 to 0.5 mm thick. Three photo-transistor devices were installed on one side of the test section between the fuel atomizer and the end plate, while another one was placed on the end-plate window. These photo-
EIoeuCt(eOrn ,(~ Electronic
transistor devices detected the luminescence due to the ignition of a spray, whose output signals were recorded by a four-channel oscilloscope. A photo-transistor of high sensitivity (PD-32H) was used for the device on the end-plate window, whose output signal was amplified by 60 dB by an amplifier. Pressure was detected by piezoelectric pressure transducers (Kistler 201B5) 1 and 2. The velocity of an incident shock was determined from the period for the shock to travel from pressure transducer t to 2. The output signal of pressure transducer 2 was recorded by a pen-recorder after being stored in a digital memorizer. Temperature behind the incident shock was estimated from its velocity using Rankine-Hugoniot relation. Air was taken as a semi-ideal gas, whose thermodynamic properties were obtained from JANAF tables of thermodynamic properties [8]. The ignition delay was determined from the interval between the time when the incident shock passed transducer 2 and the time when the phototransistor on the end-plate window detected the luminescence of ignition, subtracting the period for the shock to travel from presstire transducer 2 to the atomizer. An ultrasonic atomizer [9] was employed for the purpose of making a drop cloud suspended in the test section of low pressure. The ultrasonic atomizer system is illustrated schematically in Fig. 2. The resonant frequency of the atomizer, which was driven at the electric power of 15 watts, was
[ p. . . . corder
memorizer
_lL [ J
•/'••' ~
Ultrason,c atomizer
" transdu~r 1~ ~
r ~ t ran_sduc,e_r(,2,)
\
~
~
~
Fig. 1. Schematic diagram of experimental apparatus.
Photo~'r~nsistor (,.4)
IGNITION OF SPRAYS BY AN INCIDENT SHOCK
179
18 kHz. The diameter of its horn tip was 8 mm. The rate of injection was varied in steps of 0.25, 0.5 and 1.0 cc/min. The performance of the atomizer at reduced pressure is shown in Figs. 3 and 4. The Sauter mean diameter d is plotted against pressure P~ for three steps of injection rate of cetane in Fig. 3 whereas the numerical fraction Ani/n X 100% of drops belonging to the size class of nominal diameter d. is illustrated in Fig. 4. It is l noticed from these figures that the mean diameter is 83 to 90 ~m for pressure of 30 to 100 Torr and iniection rates of 0.25 to 1.0 cc/min. The mean diameter is almost independent both of pressure and of injection rate, and sprays have rather good uniformity in drop size. The spray angle is roughly zero for a small injection rate, so that the shape of spray whose diameter is close to that of the horn tip of tile atomizer is almost cylindrical. The change in injection rate, therefore, results only in the change in population density of drops. The overall equivalence ratio of the spray column was determined at atmospheric pressure by chopping off a part of the spray column and weighing the drops contained in it, the results of which are shown in Table 1. The fuel: air equivalence ratio for the saturated vapor of typical hydrocarbons in air is tabulated in Table 2. These values are estimated at 20°C. In the range of total pressure o f the present experiments (30 to 100 Tort), the cetane vapor is below
95 Ceta ne
E90 :3.
85
"-" ° ~
80
20
° O" 2 " ~ cc/mirl "
L
1
40
60 P1
I
80 mmHg
I
100
Fig. 3. Atomizing characteristics of the ultrasonic atomizer. the lean flammable limit, the DECH (dietyl cyclohexane) and tetralin vapors are within the flammable range, and iso-octane vapor is above the rich flammable limit. In the present experiment, therefore, cetane was used to study the ignition process of pure sprays, whereas tetralin was used to examine the effect of the coexistence of vapor on the ignition process of a spray. The shock tube was cleaned carefully and blown by a fan to scavenge the unburned fuel and combustion products of the last run.
High-frequency wattmeter Power amp(ifier tube
u~-
reservoir
Electronic counter Ultrasonic atomizer
--
Stepped Etectro-magnetic
i
Fig. 2. Ultrasonic atomizer.
horn
Udil a , .
180
KENJI MIYASAKA and YUKIO MIZUTANI TABLE1 Over~lFuel:AirEquivalence RatiosofSpray Columns
PI Torr
Cetane Injection rate cc/min 0.25 0.5 1.0
Tetralin Injection rate cc/min 0.25 0.5 1.0
20 40 60 80 100
132 66 44 33 27
121 60 40 30 24
760
202 101 67 50 40
3.5
5.3
310 155 103 77 62 8.1
184 92 61 46 37
3.2
281 14l 94 70 56
4.8
7.4
T] = 20°C TABLE 2 Fuel:Air Equivalence Ratios of Saturated Vapors el Torr
Cetane
Tetralin
DECH
Iso-octane
20 40 60 80 100
0.00779 0.00389 0.00260 0.00194 0.00156
0.867 0.433 0.289 0.217 0.173
9.76 4.88 3.25 2.44 1.95
39.7 29.7 23.9
760
0.000204
0.0228
0.257
3.14
T1 = 20°C
2.0 Injection rate V N o injection o 0.25 cc.Jmin • 0.5 cc/min A 1.0 cc4"nin
1.0
25
Cetane E 0.5
20
0.1 10
AOV
0.0~ J 0.8
I
I
I
1.0
1.2
1./..
1000/T2 5
J-
.S"
o15 x t-
o-ev ~/ •/'~
2
0.2 O
A J
K-1
~
Fig. 5. Ignition delays of tetralin. Results
40
60
80 d
100 Nm
120
Fig. 4. Size distribution of drops. Pressure 60 Torr and injection rate 0.5 cc/min.
Ignition Delays of Volatile Fuel Figure 5 illustrates t h e i g n i t i o n d e l a y s o f t h e v a p o r and sprays o f t e t r a l i n . In t h e case o f zero inject i o n , t h e i g n i t i o n d e l a y o f t h e vapor was d e t e r -
IGNITION OF SPRAYS BY AN INCIDENT SHOCK mined by generating an incident shock at 30 sec after a drop of tetralin was dropped from the atomizer into the drain tube. Vapor was generated by the evaporation of the drop and diffused upwards out of the drain tube. The diffusion of vapor in the axial direction was neglected. It is noticed from Fig. 5 that the ignition delay of a tetralin spray is independent of tile injection rate and coincides with that of tetralin vapor. This fact implies that the fuel vapor existing in the inter-drop space is ignited by the incident shock before the drops are ignited, and that the interaction between drops and the incident shock has negligible effects on the process of ignition of fuel vapor. An empirical relation between the ignition delay tig (msec) and static temperature 7'2 (°K) behind the incident shock is obtained from Fig. 5 for the vapor and sprays of tetralin as follows. tt.g -- 0.0005 exp (5,680/T2) [msec].
(1)
The apparent activation energy is 11.3 kcal/mole. Subsequently, a drop of cetane was dropped into the drain tube and a trial was made to ignite cetane vapor by an incident shock. In this case, however, no ignition was observed even for initial pressure as low as 10 Torr of the low-pressure section. This fact confirms that the concentration of cetane vapor is low enough for spray-ignition studies.
181 Ignition Delays of Nonvolatile Fuel Sprays The ignition delays of cetane sprays were determined for injection rates of 0.25, 0.5 and 1.0 cc/min, the results of which are shown in Fig. 6. The ignition delay is ca. 1 msec for 750 °K and ca. 0.27 msec for 1,200 °K in case of injection rate of 0.25 cc/min, which is smaller by a few factors than those obtained by injecting cetane into an electric furnace or into the combustion chamber of a piston-catching combustion rig [2, 3]. This may be partly because the shock-tube data contain neither atomizing nor mixing period, differing from the other data, and partly because interactions occur between drops and the incident shock. The injection rate has only slight effects on the ignition process at low temperature around 750 °K, whereas it has prominent effects at high temperature around 1,200 °K, with the ignition delay decreasing in steps of 0.27, 0.19 and 0.12 msec as the injection rate is increasing in steps of 0.25, 0.5 and 1.0 cc/min. The following empirical relations have been developed:
tig = 0.032
exp(2,540/T2) [msec]
for q=0.25 [cc/min], tig = 0.013 exp(3,220/T2) [msec]
for q=0.5 [cc/min], and tt.g = 0.0038 exp(4,130/T2) [msec]
for q=l.0 [cc/min],
2.0 O
1.0 E 0.5 -
"~ 0.2
_
o
./
I
j
z~f.%
Injection rate - - a - - 1.0 c c l m i n - - o - - 0.5 c c l m i n .--o.-- O. 2 5 c c l m i n
/i 0.1
-
0.05 0.8
I
1
I
1.0
1.2
I.~
10001T 2
K-I
Fig. 6. Ignition delays of cetane sprays.
where q is the injection rate of cetane. The apparent activation energies are 5.0, 6.4 and 8.2 kcal/mole, respectively, for those three steps of injection rate. Kauffman et al. [10] have reported the ignition delay of a single drop of cetane of 930/am diameter observed behind an incident shock. Although they have correlated the ignition delays with the stagnation temperature instead of the static temperature behind the incident shock, comparison with the present data has been made with reference to the static temperature. This comparison shows that the ignition delays plotted in Fig. 6 are
182
KENJI MIYASAKA and YUKIO MIZUTANI
longer than those reported by Kauffman et al. by a few factors, and that, however, the activation energies estimated on the basis of the static temperature are in close agreement with each other. The differences in ignition delay may result from the differences in diameter and population density of drops as well as in partial pressure of oxygen (Kauffman et al. employed pure oxygen). The comparison of the ignition delay of a cetane spray with that of tetralin (Fig. 5) shows that both have similar ignition delays at the low temperature of around 750 °K and no effect of the injection rate is observed, whereas the ignition delay of cetane spray is longer than that of tetralin at 1,200 °K and the effects of the injection rate become prominent, asymptotically approaching the latter as the injection rate increases. Pressure and Luminescence Records
Figure 7 shows a typical example of pressure record monitored with pressure transducer 2 (see Fig. 1) in the case o f a cetane spray injected at a rate of 0.5 cc/min and ignited by an incident shock traveling at a velocity of 1.18 m/msec. The shock wave reaches the pressure transducer 2 at point A and ignition occurs at point B after a constantpressure period 0.7 msec long. Pressure rises hereafter toward point C due to combustion reaction, then gradually decreases because of the arrival of the rarefaction fan. Pressure rises abruptly at point D due to the arrival of the reflected shock.
D 6 to
AB
C
0 0
2
4
I
I
6
8
10
msec
Fig. 7. Pressure record: Cetane spray, injection rate 0.5 cc/min and velocity of the incident shock 1.l 8 m/msec.
Typical examples of luminescence records observed through the end-plate window are illustrated in Fig. 8 for the ignition processes of cetane sprays. The origin of time is taken arbitrarily, and peaks higher than 7 volts are truncated. For a low injection rate around 0.25 cc/min (Fig. 8(a)), the luminescence is kept at a high level for several milliseconds after ignition, followed by extinction in the rai-efaction fan, and then reignition by the reflected shock. For a high injection rate around 1.0 cc/min (Fig. 8(b)), on the other hand, the period of intense luminescence succeeding ignition is elongated, and reignition by a reflected shock takes place before the extinction process by the rarefaction fan goes to completion. This may be because a dense spray is not easily extinguished by a rarefaction fan. Figure 9 shows the luminescence histories detected through windows 1,2, 3 and 4 (hereafter represented by Wl, W2, W3 and W4, respectively) and recorded by a four-channel oscilloscope (see Fig. 1). The signal W4 starts to rise first, then the other signals follow it in order of Wl, W2 and W3. No luminescence, however, is observed through Wi at an injection rate of 0.25 cc/min (Fig. 9(a)). These facts suggest that for such a low injection rate ignition occurs between W~ and W2 at the moment when W4 starts to rise while it occurs between the atomizer and W~ for a high injection rate around 1.0 cc/min. The traveling velocity of the flame is estimated to be 1.1 m/msec from the interval between the rising-up times of W2 and W3, which is 96% of the gas-particle velocity of 1.14 m/msec. This fact implies that ignition takes place in the micromist generated by the shattering and breaking up of the original drops. Another luminescence wave traveling in the opposite direction from W3 to W2 is observed after the first luminescence wave has traveled toward the end plate. This may be because the drop cloud once extinguished by the rarefaction fan is reignited by the reflected shock. The x - t diagram corresponding to Fig. 8(b) and Fig. 9(b) is illustrated in Fig. 10. Horizontal line iJK represents the time when ignition takes place. The loci of drops initially located at the atomizer have been calculated by the method to be described in the Discussion section ignoring breakup of drops. Since drops of 50 to 110/~m diameter are included
IGNITION OF SPRAYS BY AN INCIDENT SHOCK
183
10
® 5 C C~
0
20
10 t
3O
40 0
10
20 t
msec
(a) Injection rate 0.25cc/min
30
40
mSec
(b) Injection r a t e 1.Occ/min
Fig. 8. Luminescence records observed through the end-plate window. Cetane spray and T2= 1,200°K.
6
w2
/
w3
~
3
J
d 0
I
I
I
I
1
2
3
4
t
msec
5 0
I
i
I
I
1
2
3
4
t
msec
t
R
I
5
Aton'~zer 1 2 3
Diaphragm
(a)Injection rate 0.25cc/min
5/
(b)Injection rate 1.0cc/rnin
Fig. 9. Luminescence records observed through windows 1,2, 3 and 4. in the spray, theoretically all the loci of drops are to be included in the hatched area S in Fig. 10. In practice, however, the loci of some drops may exist outside this area because of the effect of the boundary layer and breakup of drops. The spray column meets the incident shock at point A, being carried down along the locus bundle S of drops and broken up into a micromist on the way. Ignition takes place somewhere on line segment JK. The flame travels downstream at a velocity slightly lower than the particle velocity (which corresponds to the gradient of AB ). The second luminescence is observed through W2 and W3 at the moments when the reflected shock passed by W2 and W3, respectively, which proves that the second luminescence is due to the re-ignition of fuel drops by tile reflected shock. The second luminescence is not observed through W~ partly because tile rarefaction fan reaches W~ before the reflected shock does (see Fig. 10), and partly
x
m
Fig. 10. x-t diagram. Cetane spray, injection rate 1.0 cc/min and T2 = 1,200 °K. because the spray has been carried down so that no drops exist at W1 when the reflected shock arrives. The second ignition by the reflected shock takes place behind tile contact surface. Ignition can occur in this region since oxygen exists there due to the mixing zone ca. 1.5 m thick formed around the contact surface because of tile nonideality in the rupture process of the diaphragm and of the development of a boundary layer [11 ]. Discussion
In order to examine the ignition delay data obtained, the histories of the velocity, liquid-phase temperature and stagnation temperature were calculated for a drop contained in the spray. The equation of motion of a drop is
dUa cD P2 ( U d - U 2 )
2A d =m d ~f
,
(3)
184
KENJI MIYASAKA and YUKIO MIZUTANI
where cD denotes the drag coefficient,/)2 and U2 are the density and velocity, respectively, of the surrounding gas, Ud, A d and m d are the velocity, projected area and mass of the drop, and t denotes time. The empirical relation of Rabin et al. [12]
¢D
2O0O 1000
IJ =
5OO
0.271 Re °'21v for 80 < Re < 104
was adopted, where Re denotes Reynolds number. For the heating-up period of a drop between the arrival of the incident shock and the moment when the drop temperature T d reaches the boiling point Tb, the following relation is obtained from the heat balance, ignoring the effect of radiative heat transfer and assuming that the density, Pl' and consequently the diameter, d, of the drop are kept constant. 7r d 3
rr k Nu d (T2 - T d) = -6-
dTd Pl el d t '
(4)
where )~ and T2 are the mean heat conductivity and temperature, respectively, of the surrounding gas and c l is the specific heat of the liquid fuel. The Nusselt number Nu is estimated from the empirical relation of Yuge [13]
20O t-
IO0 5O
20 10
I 20
i 50 d
I lO0
200
I~m
Fig. 11. Heating-up period. T 2 = 1,000 °K
and U2 = 0.973 m/msec. ca. 1,000/am diameter, which are considerably larger than the mean diameter of the present sprays, the initial Weber number
We = ~-p2 u? a_ O
(s)
Nu = 2 + 0.55 Pr 1/3 Re 1/2, where Pr is the Prandtl number. The heating-up period of a drop t h calculated numerically from Eqs. (3) and (4) is illustrated in Fig. 11 for P1 = 60 Tort, T2 = 1,000 °K and U2 = 0.973 m/msec, where the deformation, breakup and collision of drops are ignored. It is noticed from Fig. 11 that the heating-up period t h is 450 /~sec even for a drop of 50/am diameter, which is the smallest one existing in the spray. This value is considerably longer than the ignition delay of the spray, which is between 250 t~sec and 410 /~sec. This fact implies that it is not the original drops, but the micromist generated by breakup of the original drops that is ignited at the early stage of the ignition process. In the meanwhile, Kauffman et al. [10] have found that a micromist of a few micron diameter is formed in the wake of an original drop due to the interaction between the drop and a shock wave. Though their observation is related to drops of
is 826 for the case in which an incident shock hits a drop of 80/am diameter with a speed of 1.26 m/msec, which is much larger than the critical Weber number (-~20) for the breakup of a drop. Here a denotes the surface tension. This fact suggests that the breakup of drops and the generation of a micromist can take place also under the present condition. It may therefore be concluded that the main reason why ignition delays considerably shorter than those obtained by any conventional technique were obtained is that ignition occurred in the micromist. The factors other than the generation of a micromist that affect the ignition process may be the stagnation temperature appearing on the upstream surface of drops and the secondary shock waves formed by the interactions between drops and the incident shock. In fact, Kauffman et al. [6] employed the stagnation temperature as the reference temperature instead of the static temperature behind the incident shock. The
IGNITION OF SPRAYS BY AN INCIDENT SHOCK stagnation temperature Tst estimated from the relation,
Tst =1+
7-1
T2
2a 2
(U2 - Ud)2
(6)
and Eq. (3) is shown in Fig. 12 along with the relative velocity (U2 - Ud), where 7 and a are the heat capacity ratio and the sound velocity, respectively. It is noticed that Tst/T2 is 1.18 even at 100/~sec from the collision against the shock wave for a drop of 80/sm diameter. Since, however, the time required for the drop to break up is supposed to be a fraction of 100/lsec as is to be mentioned later, the period of high stagnation temperature may be only a small fraction of the ignition delay. In the meanwhile, Tst/T 2 is only 1.06 at 50/asec for a droplet of 10/am diameter, which is to be produced by the breakup of the original drops.
1.5
1.0
d 80
i~m
0.8 -q
0.67 1.2
,
O.a
,
0.2
1.0
I 10
20
"'"t""'~ 50
100 t
200 ~sec
I
0
500 1000
Fig. 12. Stagnation temperature and relative velocity. T2 = 1,000 °K and U 2 = 0.973 m/mscc.
It may be concluded from the above discussion that the high stagnation temperature and the secondary shock waves, which are to be generated when the relative velocity between gas and drops exceeds the velocity of sound have effects only on the very initial stage of the ignition process, and that, therefore, their effects over the whole ignition process are not significant. If ignition takes place in the micromist, the ignition delay might depend on the concentration of the micromist. Since the mean diameter and the diameter of spray column are only slightly affected by the injection rate, the increased in-
185 jection rate results in the increased population density of drops and subsequently in the increased density of micromist. Therefore, the assumption that ignition takes place in the micromist is well consistent with the fact that the increased injection rate results in the reduction of the ignition lag. This inference is confirmed also by the fact that the traveling velocity of the flame is very close to that of a gas particle. The ignition delay should contain the period required for generating a mist (shattering time) if ignition takes place in the micromist. Ranger and Nicholls [14] have found that a micromist is generated a few microseconds after collision when an incident shock of Mach 2.7 hits a single water drop of 750/am diameter. In the meanwhile, Kauffman et al. [10] have observed a shattering time around 35 #sec for a single cetane drop of 930/am diameter hit by an incident shock of Mach 3.79. If the shattering time does not differ much from the above data even in the present experiment where the mean diameter of drop is about 80/am, the shattering time might be only a small fraction of the ignition delay period since the ignition delay is a few hundred microseconds. Conclusions Spray columns suspended in the low-pressure section of a shock tube were ignited by an incident shock aimed at obtaining pure ignition delay data free from atomizing and mixing processes. In addition, the effects of the interaction between drops and the incident shock on the ignition or combustion process of sprays were examined. The conclusions obtained are as follows. (1) Ignition delay data for single drops, drop arrays or sprays obtained by the shock-tube technique should be carefully examined, since the saturated vapor of fuel often makes a flammable mixture with air at low pressure. (2) In the case of tetralin, which makes a flammable vapor-air mixture at low pressure, the presence of drops in the mixture has no effect on the ignition delay. (3) The injection rate has negligible effects on the ignition delay of a cetane spray at the relatively low temperature of around 750 °K, whereas the increased injection rate shortens the ignition delay at higher temperatures.
186 (4) The ignition delay data obtained by the incident-shock ignition technique is only a fraction of those obtained by any conventional technique. Such short delays may be attributed to the fact that ignition takes place in the micromist generated by the shattering of original drops. (5) The transient high temperature appearing at the stagnation points of drops and the secondary shock waves generated by the interaction of drops with an incident shock do not seem to have significant effects on the ignition process. (6) Flame travels at a speed close to that of a gas particle behind the incident shock. References 1. Sitkei, G., Kraftstoffaufbereitung und Verbrennung bei Dieselmotoren, Springer, Berlin (1964), p. 149 [in German]. 2. Ogasawara, et ai.,Preprints of JSME No. 734-6, JSME, Tokyo (1973), p. 101 [in Japanese].
KENJI MIYASAKA and YUKIO MIZUTANI 3. Iinuma, K., and Yamazaki, K., Trans. JSME 26, 1662 (1960) [in Japanese]. 4. Mullins, B. P.,Fuel 32,211,234,343,363, 45l, 467 and 481 (1953). 5. Lu, P. L., and Slagg, N.,Astron. Acta 17,693 (1972). 6. Kauffman, C. W., and Nieholls, J. A.,AIAA J. 9, 880 (1971). 7. Mullaney, G.J.,Ind. Eng. Chem, 51,779 (1956). 8. JANAF Tables of Thermodynamic data, Dow Chemical Company, Midland, Michigan, PB-168-370, PB168-370- 1 and PB-168-370-2. 9. Mizutani, Y., Uga, Y., and Nishimoto, T., Bulletin JSME 15,620 (1972).
10. Kauffman, C. W., Nicholls, J. A., and Olzmann, K. A., Combust. ScL Technol. 3, 165 (197l). 11. Hooker, W. J., Phys. Fluids4, 1451 (1961). 12. Rabin, E., Schallenmuller, A. R., and Lawhead, R. B., AFOSR 60-75 (1960). 13. Yuge, T., Trans. ASME, Ser. C82,214 (1960). 14. Ranger, A. A., and Nichoils, J. A.,AIAA J. 7, 285
(1969). Received 3 January 1975; revised 12March 1975
177
Ignition of Sprays by an Incident Shock KENJI MIYASAKA and YUKIO MIZUTANI Department of Mechanical Engineering, School of Engineering, Osaka University, Yamada-kami, Suita, Osaka 565, Japan
A shock-tube technique was developed aiming at obtaining pure ignition delay data of sprays independent of the processes of atomization and mixing. A spray column, which was injected by an ultrasonic atomizer and freely falling through the low-pressure section of a shock tube placed horizontally, was ignited by an incident shock. This technique gave ignition delay periods as short as only a few tenths of those obtained by any conventional technique. This may be partly because the ignition delay data obtained by this technique include neither the period of atomization nor that of mixing, and partly because ignition took place in the micromist generated by the shattering of the original droplets in the spray.
Introduction
Most ignition delay data for fuel sprays have so far been obtained either by using a Diesel engine with a window on the cylinder head [1] or a pistoncatching combustion device [2], or by injecting fuel into an electric furnace or a high-temperature burned gas stream [3, 4]. In those techniques, however, it is not possible to separate the atomizing or mixing process from that of ignition, so that pure ignition delay data free from the former processes can not be obtained. The ignition delay data obtained, therefore, differ from each other depending on the apparatus or procedure employed. This inconvenience may be avoided by employing the shock-tube technique where premixed sprays are ignited by an incident or reflected shock or a piston-catching combustion rig in which premixed sprays are adiabatically compressed and ignited. In the present study, an attempt was made to obtain pure ignition delay data free from atomizing and mixing processes by introducing an incident shock into a spray column freely suspended in the low-pressure section of a shock tube and igniting it. There are Lu and Slagg's investigation [5] for a single drop and that of Kauffman and Nicholls [6] for droplet arrays as examples of the application of the shock-tube technique to the study on the ignition of liquid fuels. It is, however, only Mullaney's investigation [7] that the shock-tube
technique has been applied to the spray-ignition investigation. In his experiments, the ignition delays observed still contain the periods of atomization and mixing, as in conventional techniques, since a spray was injected into the high-temperature region behind a reflected shock from a pressure atomizer placed on the end plate of the shock tube in the direction of the axis, which means that his technique has no essential difference from conventional ones such as the electric-furnace technique. Such a lack of spray-ignition investigation utilizing the shock-tube technique may come from the difficulty in suspending a spray in the lowpressure section of a shock tube where the pressure is 0.1 arm or below. This difficulty was successfully overcome by employing an ultrasonic atomizer that shows excellent atomizing characteristics independent of pressure. The shock-tube technique has the advantage of the possibility that the ignition process is separated from the atomizing and mixing processes, while it has several disadvantages. They are (1) the existence of interaction between drops and the incident shock resulting in the shattering of drops and high stagnation temperature, (2) different degrees of acceleration between drops of different sizes or between drops and vapor, (3) collision of drops in the spray resulting in agglomeration or breakup of drops, and (4) high fuel vapor-air ratio even for
Copyright © 1975 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.
178
KENJI MIYASAKA and YUKIO MIZUTANI
fuels of low volatility resulted from the initial low pressure in a shock tube, which put restrictions on the choice of fuel (see Table 2). In the present investigation, the ignition delay was determined for cetane (n-hexadecane) whose vapor pressure is low enough at room temperature, the results of which were compared with the ignitiondelay data obtained by conventional techniques. In addition, the ignition delay was determined also for fuel vapor-air mixtures and for fuel vaporspray-air systems using tetralin (tetrahydronaphthalene) whose saturation pressure was so high at room temperature that a flammable vapor-air mixture was formed in the low-pressure section of a shock tube. An incident shock was utilized for igniting a spray or vapor.
Description of Experiment The schematic diagram of the experimental apparatus is illustrated in Fig. 1. An ordinary heliumdriven shock tube of 64 mm i.d. was employed whose driver and driven sections were 1.5 m and 5.7 m long, respectively. The pressures of driver gas (helium) and driven gas (air) were around 30 atm and 30 to 100 Torr, respectively. The rupture diaphragm was made of aluminum sheet 0.3 to 0.5 mm thick. Three photo-transistor devices were installed on one side of the test section between the fuel atomizer and the end plate, while another one was placed on the end-plate window. These photo-
EIoeuCt(eOrn ,(~ Electronic
transistor devices detected the luminescence due to the ignition of a spray, whose output signals were recorded by a four-channel oscilloscope. A photo-transistor of high sensitivity (PD-32H) was used for the device on the end-plate window, whose output signal was amplified by 60 dB by an amplifier. Pressure was detected by piezoelectric pressure transducers (Kistler 201B5) 1 and 2. The velocity of an incident shock was determined from the period for the shock to travel from pressure transducer t to 2. The output signal of pressure transducer 2 was recorded by a pen-recorder after being stored in a digital memorizer. Temperature behind the incident shock was estimated from its velocity using Rankine-Hugoniot relation. Air was taken as a semi-ideal gas, whose thermodynamic properties were obtained from JANAF tables of thermodynamic properties [8]. The ignition delay was determined from the interval between the time when the incident shock passed transducer 2 and the time when the phototransistor on the end-plate window detected the luminescence of ignition, subtracting the period for the shock to travel from presstire transducer 2 to the atomizer. An ultrasonic atomizer [9] was employed for the purpose of making a drop cloud suspended in the test section of low pressure. The ultrasonic atomizer system is illustrated schematically in Fig. 2. The resonant frequency of the atomizer, which was driven at the electric power of 15 watts, was
[ p. . . . corder
memorizer
_lL [ J
•/'••' ~
Ultrason,c atomizer
" transdu~r 1~ ~
r ~ t ran_sduc,e_r(,2,)
\
~
~
~
Fig. 1. Schematic diagram of experimental apparatus.
Photo~'r~nsistor (,.4)
IGNITION OF SPRAYS BY AN INCIDENT SHOCK
179
18 kHz. The diameter of its horn tip was 8 mm. The rate of injection was varied in steps of 0.25, 0.5 and 1.0 cc/min. The performance of the atomizer at reduced pressure is shown in Figs. 3 and 4. The Sauter mean diameter d is plotted against pressure P~ for three steps of injection rate of cetane in Fig. 3 whereas the numerical fraction Ani/n X 100% of drops belonging to the size class of nominal diameter d. is illustrated in Fig. 4. It is l noticed from these figures that the mean diameter is 83 to 90 ~m for pressure of 30 to 100 Torr and iniection rates of 0.25 to 1.0 cc/min. The mean diameter is almost independent both of pressure and of injection rate, and sprays have rather good uniformity in drop size. The spray angle is roughly zero for a small injection rate, so that the shape of spray whose diameter is close to that of the horn tip of tile atomizer is almost cylindrical. The change in injection rate, therefore, results only in the change in population density of drops. The overall equivalence ratio of the spray column was determined at atmospheric pressure by chopping off a part of the spray column and weighing the drops contained in it, the results of which are shown in Table 1. The fuel: air equivalence ratio for the saturated vapor of typical hydrocarbons in air is tabulated in Table 2. These values are estimated at 20°C. In the range of total pressure o f the present experiments (30 to 100 Tort), the cetane vapor is below
95 Ceta ne
E90 :3.
85
"-" ° ~
80
20
° O" 2 " ~ cc/mirl "
L
1
40
60 P1
I
80 mmHg
I
100
Fig. 3. Atomizing characteristics of the ultrasonic atomizer. the lean flammable limit, the DECH (dietyl cyclohexane) and tetralin vapors are within the flammable range, and iso-octane vapor is above the rich flammable limit. In the present experiment, therefore, cetane was used to study the ignition process of pure sprays, whereas tetralin was used to examine the effect of the coexistence of vapor on the ignition process of a spray. The shock tube was cleaned carefully and blown by a fan to scavenge the unburned fuel and combustion products of the last run.
High-frequency wattmeter Power amp(ifier tube
u~-
reservoir
Electronic counter Ultrasonic atomizer
--
Stepped Etectro-magnetic
i
Fig. 2. Ultrasonic atomizer.
horn
Udil a , .
180
KENJI MIYASAKA and YUKIO MIZUTANI TABLE1 Over~lFuel:AirEquivalence RatiosofSpray Columns
PI Torr
Cetane Injection rate cc/min 0.25 0.5 1.0
Tetralin Injection rate cc/min 0.25 0.5 1.0
20 40 60 80 100
132 66 44 33 27
121 60 40 30 24
760
202 101 67 50 40
3.5
5.3
310 155 103 77 62 8.1
184 92 61 46 37
3.2
281 14l 94 70 56
4.8
7.4
T] = 20°C TABLE 2 Fuel:Air Equivalence Ratios of Saturated Vapors el Torr
Cetane
Tetralin
DECH
Iso-octane
20 40 60 80 100
0.00779 0.00389 0.00260 0.00194 0.00156
0.867 0.433 0.289 0.217 0.173
9.76 4.88 3.25 2.44 1.95
39.7 29.7 23.9
760
0.000204
0.0228
0.257
3.14
T1 = 20°C
2.0 Injection rate V N o injection o 0.25 cc.Jmin • 0.5 cc/min A 1.0 cc4"nin
1.0
25
Cetane E 0.5
20
0.1 10
AOV
0.0~ J 0.8
I
I
I
1.0
1.2
1./..
1000/T2 5
J-
.S"
o15 x t-
o-ev ~/ •/'~
2
0.2 O
A J
K-1
~
Fig. 5. Ignition delays of tetralin. Results
40
60
80 d
100 Nm
120
Fig. 4. Size distribution of drops. Pressure 60 Torr and injection rate 0.5 cc/min.
Ignition Delays of Volatile Fuel Figure 5 illustrates t h e i g n i t i o n d e l a y s o f t h e v a p o r and sprays o f t e t r a l i n . In t h e case o f zero inject i o n , t h e i g n i t i o n d e l a y o f t h e vapor was d e t e r -
IGNITION OF SPRAYS BY AN INCIDENT SHOCK mined by generating an incident shock at 30 sec after a drop of tetralin was dropped from the atomizer into the drain tube. Vapor was generated by the evaporation of the drop and diffused upwards out of the drain tube. The diffusion of vapor in the axial direction was neglected. It is noticed from Fig. 5 that the ignition delay of a tetralin spray is independent of tile injection rate and coincides with that of tetralin vapor. This fact implies that the fuel vapor existing in the inter-drop space is ignited by the incident shock before the drops are ignited, and that the interaction between drops and the incident shock has negligible effects on the process of ignition of fuel vapor. An empirical relation between the ignition delay tig (msec) and static temperature 7'2 (°K) behind the incident shock is obtained from Fig. 5 for the vapor and sprays of tetralin as follows. tt.g -- 0.0005 exp (5,680/T2) [msec].
(1)
The apparent activation energy is 11.3 kcal/mole. Subsequently, a drop of cetane was dropped into the drain tube and a trial was made to ignite cetane vapor by an incident shock. In this case, however, no ignition was observed even for initial pressure as low as 10 Torr of the low-pressure section. This fact confirms that the concentration of cetane vapor is low enough for spray-ignition studies.
181 Ignition Delays of Nonvolatile Fuel Sprays The ignition delays of cetane sprays were determined for injection rates of 0.25, 0.5 and 1.0 cc/min, the results of which are shown in Fig. 6. The ignition delay is ca. 1 msec for 750 °K and ca. 0.27 msec for 1,200 °K in case of injection rate of 0.25 cc/min, which is smaller by a few factors than those obtained by injecting cetane into an electric furnace or into the combustion chamber of a piston-catching combustion rig [2, 3]. This may be partly because the shock-tube data contain neither atomizing nor mixing period, differing from the other data, and partly because interactions occur between drops and the incident shock. The injection rate has only slight effects on the ignition process at low temperature around 750 °K, whereas it has prominent effects at high temperature around 1,200 °K, with the ignition delay decreasing in steps of 0.27, 0.19 and 0.12 msec as the injection rate is increasing in steps of 0.25, 0.5 and 1.0 cc/min. The following empirical relations have been developed:
tig = 0.032
exp(2,540/T2) [msec]
for q=0.25 [cc/min], tig = 0.013 exp(3,220/T2) [msec]
for q=0.5 [cc/min], and tt.g = 0.0038 exp(4,130/T2) [msec]
for q=l.0 [cc/min],
2.0 O
1.0 E 0.5 -
"~ 0.2
_
o
./
I
j
z~f.%
Injection rate - - a - - 1.0 c c l m i n - - o - - 0.5 c c l m i n .--o.-- O. 2 5 c c l m i n
/i 0.1
-
0.05 0.8
I
1
I
1.0
1.2
I.~
10001T 2
K-I
Fig. 6. Ignition delays of cetane sprays.
where q is the injection rate of cetane. The apparent activation energies are 5.0, 6.4 and 8.2 kcal/mole, respectively, for those three steps of injection rate. Kauffman et al. [10] have reported the ignition delay of a single drop of cetane of 930/am diameter observed behind an incident shock. Although they have correlated the ignition delays with the stagnation temperature instead of the static temperature behind the incident shock, comparison with the present data has been made with reference to the static temperature. This comparison shows that the ignition delays plotted in Fig. 6 are
182
KENJI MIYASAKA and YUKIO MIZUTANI
longer than those reported by Kauffman et al. by a few factors, and that, however, the activation energies estimated on the basis of the static temperature are in close agreement with each other. The differences in ignition delay may result from the differences in diameter and population density of drops as well as in partial pressure of oxygen (Kauffman et al. employed pure oxygen). The comparison of the ignition delay of a cetane spray with that of tetralin (Fig. 5) shows that both have similar ignition delays at the low temperature of around 750 °K and no effect of the injection rate is observed, whereas the ignition delay of cetane spray is longer than that of tetralin at 1,200 °K and the effects of the injection rate become prominent, asymptotically approaching the latter as the injection rate increases. Pressure and Luminescence Records
Figure 7 shows a typical example of pressure record monitored with pressure transducer 2 (see Fig. 1) in the case o f a cetane spray injected at a rate of 0.5 cc/min and ignited by an incident shock traveling at a velocity of 1.18 m/msec. The shock wave reaches the pressure transducer 2 at point A and ignition occurs at point B after a constantpressure period 0.7 msec long. Pressure rises hereafter toward point C due to combustion reaction, then gradually decreases because of the arrival of the rarefaction fan. Pressure rises abruptly at point D due to the arrival of the reflected shock.
D 6 to
AB
C
0 0
2
4
I
I
6
8
10
msec
Fig. 7. Pressure record: Cetane spray, injection rate 0.5 cc/min and velocity of the incident shock 1.l 8 m/msec.
Typical examples of luminescence records observed through the end-plate window are illustrated in Fig. 8 for the ignition processes of cetane sprays. The origin of time is taken arbitrarily, and peaks higher than 7 volts are truncated. For a low injection rate around 0.25 cc/min (Fig. 8(a)), the luminescence is kept at a high level for several milliseconds after ignition, followed by extinction in the rai-efaction fan, and then reignition by the reflected shock. For a high injection rate around 1.0 cc/min (Fig. 8(b)), on the other hand, the period of intense luminescence succeeding ignition is elongated, and reignition by a reflected shock takes place before the extinction process by the rarefaction fan goes to completion. This may be because a dense spray is not easily extinguished by a rarefaction fan. Figure 9 shows the luminescence histories detected through windows 1,2, 3 and 4 (hereafter represented by Wl, W2, W3 and W4, respectively) and recorded by a four-channel oscilloscope (see Fig. 1). The signal W4 starts to rise first, then the other signals follow it in order of Wl, W2 and W3. No luminescence, however, is observed through Wi at an injection rate of 0.25 cc/min (Fig. 9(a)). These facts suggest that for such a low injection rate ignition occurs between W~ and W2 at the moment when W4 starts to rise while it occurs between the atomizer and W~ for a high injection rate around 1.0 cc/min. The traveling velocity of the flame is estimated to be 1.1 m/msec from the interval between the rising-up times of W2 and W3, which is 96% of the gas-particle velocity of 1.14 m/msec. This fact implies that ignition takes place in the micromist generated by the shattering and breaking up of the original drops. Another luminescence wave traveling in the opposite direction from W3 to W2 is observed after the first luminescence wave has traveled toward the end plate. This may be because the drop cloud once extinguished by the rarefaction fan is reignited by the reflected shock. The x - t diagram corresponding to Fig. 8(b) and Fig. 9(b) is illustrated in Fig. 10. Horizontal line iJK represents the time when ignition takes place. The loci of drops initially located at the atomizer have been calculated by the method to be described in the Discussion section ignoring breakup of drops. Since drops of 50 to 110/~m diameter are included
IGNITION OF SPRAYS BY AN INCIDENT SHOCK
183
10
® 5 C C~
0
20
10 t
3O
40 0
10
20 t
msec
(a) Injection rate 0.25cc/min
30
40
mSec
(b) Injection r a t e 1.Occ/min
Fig. 8. Luminescence records observed through the end-plate window. Cetane spray and T2= 1,200°K.
6
w2
/
w3
~
3
J
d 0
I
I
I
I
1
2
3
4
t
msec
5 0
I
i
I
I
1
2
3
4
t
msec
t
R
I
5
Aton'~zer 1 2 3
Diaphragm
(a)Injection rate 0.25cc/min
5/
(b)Injection rate 1.0cc/rnin
Fig. 9. Luminescence records observed through windows 1,2, 3 and 4. in the spray, theoretically all the loci of drops are to be included in the hatched area S in Fig. 10. In practice, however, the loci of some drops may exist outside this area because of the effect of the boundary layer and breakup of drops. The spray column meets the incident shock at point A, being carried down along the locus bundle S of drops and broken up into a micromist on the way. Ignition takes place somewhere on line segment JK. The flame travels downstream at a velocity slightly lower than the particle velocity (which corresponds to the gradient of AB ). The second luminescence is observed through W2 and W3 at the moments when the reflected shock passed by W2 and W3, respectively, which proves that the second luminescence is due to the re-ignition of fuel drops by tile reflected shock. The second luminescence is not observed through W~ partly because tile rarefaction fan reaches W~ before the reflected shock does (see Fig. 10), and partly
x
m
Fig. 10. x-t diagram. Cetane spray, injection rate 1.0 cc/min and T2 = 1,200 °K. because the spray has been carried down so that no drops exist at W1 when the reflected shock arrives. The second ignition by the reflected shock takes place behind tile contact surface. Ignition can occur in this region since oxygen exists there due to the mixing zone ca. 1.5 m thick formed around the contact surface because of tile nonideality in the rupture process of the diaphragm and of the development of a boundary layer [11 ]. Discussion
In order to examine the ignition delay data obtained, the histories of the velocity, liquid-phase temperature and stagnation temperature were calculated for a drop contained in the spray. The equation of motion of a drop is
dUa cD P2 ( U d - U 2 )
2A d =m d ~f
,
(3)
184
KENJI MIYASAKA and YUKIO MIZUTANI
where cD denotes the drag coefficient,/)2 and U2 are the density and velocity, respectively, of the surrounding gas, Ud, A d and m d are the velocity, projected area and mass of the drop, and t denotes time. The empirical relation of Rabin et al. [12]
¢D
2O0O 1000
IJ =
5OO
0.271 Re °'21v for 80 < Re < 104
was adopted, where Re denotes Reynolds number. For the heating-up period of a drop between the arrival of the incident shock and the moment when the drop temperature T d reaches the boiling point Tb, the following relation is obtained from the heat balance, ignoring the effect of radiative heat transfer and assuming that the density, Pl' and consequently the diameter, d, of the drop are kept constant. 7r d 3
rr k Nu d (T2 - T d) = -6-
dTd Pl el d t '
(4)
where )~ and T2 are the mean heat conductivity and temperature, respectively, of the surrounding gas and c l is the specific heat of the liquid fuel. The Nusselt number Nu is estimated from the empirical relation of Yuge [13]
20O t-
IO0 5O
20 10
I 20
i 50 d
I lO0
200
I~m
Fig. 11. Heating-up period. T 2 = 1,000 °K
and U2 = 0.973 m/msec. ca. 1,000/am diameter, which are considerably larger than the mean diameter of the present sprays, the initial Weber number
We = ~-p2 u? a_ O
(s)
Nu = 2 + 0.55 Pr 1/3 Re 1/2, where Pr is the Prandtl number. The heating-up period of a drop t h calculated numerically from Eqs. (3) and (4) is illustrated in Fig. 11 for P1 = 60 Tort, T2 = 1,000 °K and U2 = 0.973 m/msec, where the deformation, breakup and collision of drops are ignored. It is noticed from Fig. 11 that the heating-up period t h is 450 /~sec even for a drop of 50/am diameter, which is the smallest one existing in the spray. This value is considerably longer than the ignition delay of the spray, which is between 250 t~sec and 410 /~sec. This fact implies that it is not the original drops, but the micromist generated by breakup of the original drops that is ignited at the early stage of the ignition process. In the meanwhile, Kauffman et al. [10] have found that a micromist of a few micron diameter is formed in the wake of an original drop due to the interaction between the drop and a shock wave. Though their observation is related to drops of
is 826 for the case in which an incident shock hits a drop of 80/am diameter with a speed of 1.26 m/msec, which is much larger than the critical Weber number (-~20) for the breakup of a drop. Here a denotes the surface tension. This fact suggests that the breakup of drops and the generation of a micromist can take place also under the present condition. It may therefore be concluded that the main reason why ignition delays considerably shorter than those obtained by any conventional technique were obtained is that ignition occurred in the micromist. The factors other than the generation of a micromist that affect the ignition process may be the stagnation temperature appearing on the upstream surface of drops and the secondary shock waves formed by the interactions between drops and the incident shock. In fact, Kauffman et al. [6] employed the stagnation temperature as the reference temperature instead of the static temperature behind the incident shock. The
IGNITION OF SPRAYS BY AN INCIDENT SHOCK stagnation temperature Tst estimated from the relation,
Tst =1+
7-1
T2
2a 2
(U2 - Ud)2
(6)
and Eq. (3) is shown in Fig. 12 along with the relative velocity (U2 - Ud), where 7 and a are the heat capacity ratio and the sound velocity, respectively. It is noticed that Tst/T2 is 1.18 even at 100/~sec from the collision against the shock wave for a drop of 80/sm diameter. Since, however, the time required for the drop to break up is supposed to be a fraction of 100/lsec as is to be mentioned later, the period of high stagnation temperature may be only a small fraction of the ignition delay. In the meanwhile, Tst/T 2 is only 1.06 at 50/asec for a droplet of 10/am diameter, which is to be produced by the breakup of the original drops.
1.5
1.0
d 80
i~m
0.8 -q
0.67 1.2
,
O.a
,
0.2
1.0
I 10
20
"'"t""'~ 50
100 t
200 ~sec
I
0
500 1000
Fig. 12. Stagnation temperature and relative velocity. T2 = 1,000 °K and U 2 = 0.973 m/mscc.
It may be concluded from the above discussion that the high stagnation temperature and the secondary shock waves, which are to be generated when the relative velocity between gas and drops exceeds the velocity of sound have effects only on the very initial stage of the ignition process, and that, therefore, their effects over the whole ignition process are not significant. If ignition takes place in the micromist, the ignition delay might depend on the concentration of the micromist. Since the mean diameter and the diameter of spray column are only slightly affected by the injection rate, the increased in-
185 jection rate results in the increased population density of drops and subsequently in the increased density of micromist. Therefore, the assumption that ignition takes place in the micromist is well consistent with the fact that the increased injection rate results in the reduction of the ignition lag. This inference is confirmed also by the fact that the traveling velocity of the flame is very close to that of a gas particle. The ignition delay should contain the period required for generating a mist (shattering time) if ignition takes place in the micromist. Ranger and Nicholls [14] have found that a micromist is generated a few microseconds after collision when an incident shock of Mach 2.7 hits a single water drop of 750/am diameter. In the meanwhile, Kauffman et al. [10] have observed a shattering time around 35 #sec for a single cetane drop of 930/am diameter hit by an incident shock of Mach 3.79. If the shattering time does not differ much from the above data even in the present experiment where the mean diameter of drop is about 80/am, the shattering time might be only a small fraction of the ignition delay period since the ignition delay is a few hundred microseconds. Conclusions Spray columns suspended in the low-pressure section of a shock tube were ignited by an incident shock aimed at obtaining pure ignition delay data free from atomizing and mixing processes. In addition, the effects of the interaction between drops and the incident shock on the ignition or combustion process of sprays were examined. The conclusions obtained are as follows. (1) Ignition delay data for single drops, drop arrays or sprays obtained by the shock-tube technique should be carefully examined, since the saturated vapor of fuel often makes a flammable mixture with air at low pressure. (2) In the case of tetralin, which makes a flammable vapor-air mixture at low pressure, the presence of drops in the mixture has no effect on the ignition delay. (3) The injection rate has negligible effects on the ignition delay of a cetane spray at the relatively low temperature of around 750 °K, whereas the increased injection rate shortens the ignition delay at higher temperatures.
186 (4) The ignition delay data obtained by the incident-shock ignition technique is only a fraction of those obtained by any conventional technique. Such short delays may be attributed to the fact that ignition takes place in the micromist generated by the shattering of original drops. (5) The transient high temperature appearing at the stagnation points of drops and the secondary shock waves generated by the interaction of drops with an incident shock do not seem to have significant effects on the ignition process. (6) Flame travels at a speed close to that of a gas particle behind the incident shock. References 1. Sitkei, G., Kraftstoffaufbereitung und Verbrennung bei Dieselmotoren, Springer, Berlin (1964), p. 149 [in German]. 2. Ogasawara, et ai.,Preprints of JSME No. 734-6, JSME, Tokyo (1973), p. 101 [in Japanese].
KENJI MIYASAKA and YUKIO MIZUTANI 3. Iinuma, K., and Yamazaki, K., Trans. JSME 26, 1662 (1960) [in Japanese]. 4. Mullins, B. P.,Fuel 32,211,234,343,363, 45l, 467 and 481 (1953). 5. Lu, P. L., and Slagg, N.,Astron. Acta 17,693 (1972). 6. Kauffman, C. W., and Nieholls, J. A.,AIAA J. 9, 880 (1971). 7. Mullaney, G.J.,Ind. Eng. Chem, 51,779 (1956). 8. JANAF Tables of Thermodynamic data, Dow Chemical Company, Midland, Michigan, PB-168-370, PB168-370- 1 and PB-168-370-2. 9. Mizutani, Y., Uga, Y., and Nishimoto, T., Bulletin JSME 15,620 (1972).
10. Kauffman, C. W., Nicholls, J. A., and Olzmann, K. A., Combust. ScL Technol. 3, 165 (197l). 11. Hooker, W. J., Phys. Fluids4, 1451 (1961). 12. Rabin, E., Schallenmuller, A. R., and Lawhead, R. B., AFOSR 60-75 (1960). 13. Yuge, T., Trans. ASME, Ser. C82,214 (1960). 14. Ranger, A. A., and Nichoils, J. A.,AIAA J. 7, 285
(1969). Received 3 January 1975; revised 12March 1975