5.7
3
DRIVEN GAS SECT ION (SQUARE TUBE)
DRIVER G A S SECTION (ROUND TUBE 1
WINDOW SECTION (SQUARE 1
I F U E L NOZZLE HOLDER
I
DIAPHRAGM
I
GEORGE
Shock tube for combustion studies
J. MULLANEY
General Electric Co., Research laboratory, Schenectady, N. Y.
Shock Tube Technique for Study of Autoignition of Liquid Fuel Sprays A window section in the shock tube permits study of the autoignition process with 'high speed photography
THE
status of knowledge on the autoignition of liquid fuel sprays is such that experimental data can provide useful information for theoretical models which add to the understanding of the processes involved. Attacking the many interesting problems in liquid spray combustion requires an apparatus which to the greatest extent possible will permit separation of variables in this already complicated heterogeneous system. The shock tube technique holds promise as a new method for a more fundamental approach to the study of the combustion of liquid fuel sprays. By its use air can be provided a t a known high temperature and pressure without, for example, the complications of design of a static bomb for high temperatures. With the shock tube technique a liquid fuel spray can be injected into a volume large enough so that wall effects are not important. In principle, the range of air pressures and temperatures obtainable is very wide. Both can be rapidly adjusted from one experiment to the next. The shock tube makes convenient the use of high speed photography for autoignition studies. Included here are some of the considerations which led to the specific tube dimensions and geometry used, data on shock wave attenuation as it travels down the tube, some pressure-time measurements on shock waves, and a description of the system for fuel injection assembled for the combustion studies. Temperature determination by the velocity of sound technique for the air behind the reflected shock wave is described. Finally, typical measurements of evap-
*
FLANGE FOR ATTACHMENT TO SHOCK TUBE
F U E L NOZZLE T I P
Window section for combustion shock tube
9 5 METERS
____-
OSCILLOSCOPE EXTERNAL TRIGGER
Arrangement pressure measurements at end of shock tube
-*
AMPLIFIER
TIME DELAY CHASSIS
-SPARK PLUG POWER SUPPLY
[]
E X T E R N A L GAP r
FIRING CIRCUIT
i
I -
VOL. 50, NO. 1
JANUARY 1958
53
Sound wave sensing element was made of barium titanate disks stacked electrically in parallel and mounted in a holder
oration time and ignition lag for nhexadecane are presented. Shock Tube Design
Selection of the shock tube length was correlated with the process time associated with autoignition in Diesel combustion. Depending on the type of fuel injection system used, the injection period for Diesel operation may be in the range of 1 to 6 milliseconds. After the beginning of fuel injection, an ignition delay occurs and may last from a fraction of a millisecond up to several milliseconds in an engine (9), when air temperatures after compression but prior to fuel injection are in the range of 1000O K. The actual ignition delay also depends on the air pressure, fuel, and probably other factors. Following the ignition delay, the burning process may last about 6 milliseconds or more in an engine. The whole Diesel combustion process is usually over in no more than 15 milliseconds. Shock tubes have been described in detail (7, 3. 4). The shock tube used here (see diagram) for combustion studies has a driver section 6 meters long. This part of the tube is pressurized initially, and the high pressure gas is separated from the driven gas of the shock tube by means of a diaphragm. The portion of the driven section of the tube away from the burst diaphragm has a window section (shown) added to it with a provision for a Diesel fuel injection nozzle on one end. The driven part of the shock tube is 10 meters long. The shock tube was used in the following manner. The diaphragm separating the driver and driven section was broken by a sharp probe after a selected driver gas pressure had been set. Either helium or air was used as driver gas. After the incident shock wave reflected from the end of the driven tube in which the fuel nozzle was set, fuel was permitted to enter the high pressure, high temperature air which was, at least theoretically, not in motion. The introduction of the liquid fuel spray could be timed from the voltage pulse to a solenoid which drives a pointed rod into the diaphragm. If the diaphragm was broken by manually operating the pointed rod, a pressure
54
pickup which signals the passage of the incident shock wave initiated the fuel injection. One-dimensional calculations were made to estimate the initial driver and driven gas pressures required to achieve the range of pressures and temperatures obtained at that time in the compression stroke of a Diesel engine when fuel injection begins. They were based on ideal behavior without considering friction losses as the incident shock wave moved down the tube. Results of these calculations with helium as the driver gas are shown in the time-wave position plot (Figure 1). With an initial pres-
INDUSTRIAL AND ENGINEERING CHEMISTRY
sure in the driving section of 170 pounds per square inch absolute, a reflected shock pressure of 260 pounds per square inch absolute at 810' K. is obtained for 5 milliseconds before the rarefaction wave, which has reflected from the end of the driver tube, reached the end of the driven tube. A similar calculation with air used in the driver section was made, and the time-wave position plot for this arrangement is also shown (Figure 1). To obtain the same reflected shock pressure of 260 pounds per square inch absclute and reflected shock temperature of 810' K., the initial driver pressure must now be 660 pounds per square inch absolute. However, the time available before 'the interference of the rarefaction wave is now approximately 30 milliseconds. Shock Tube Performance Pressure-Time at End of D r i v e n Section. A number of experiments werc
made to investigate the pressure and. temperature DS. time that can actually be obtained near the point in the shock tube where the fuel would be injected. Figure 2 is a pressure OS. time trace obtained with the experimental arrangement shown. The beginning of the time in LIGHT SOURCE
1 ; ', , i
MA5K
'
!\
' t
--
HIGH SPEED
ACTERA Photographic system for ignition delay measurements
v)
n
0
45 40
: 35 cn 5 30 J
25 W
E
20
15 10 5
n " - 6 - 5 - 4- 3 - 2 - I
0 I 2 3 4 5 6 7 8 9 DISTANCE FROM D I A P H R A G M M E T E R S Figure 1. Time vs. position for various wave fronts in shock tube
-
Reflected shock pressure, 260 Ib./rq. inch obs.; temperature behind reflected shock, 810' Air driver Helium driver
---
10
K.
A U T O I G N I T I O N OF FUEL SPRAYS
Figure 2. tube
Figure 3. tube
Pressure variation with time near end of shock
Pressure variation with time near end of shock
Driver gas, air; time scale, 10 milliseconds per large division; pressure scale, 75 Ib./sq. inch per large division A. Beginning of sweep B. Incident shock wave C. Reflected shock wave
Driver gas, helium; time scale, 10 milliseconds per large division; pressure scale, 75 Ib./sq. inch per large division A. Beginning of sweep 6. Incident shock wave C. Reflected shock wave
wave would have risen to 1280' K. The temperature immediately behind the reflected shock wave was calculated from the measured pressure ratios by using the Rankine-Hugoniot relation and found to be 815 O K. Finally the pkessure began to fall when the rarefaction wave reached the measuring station (20 milliseconds from the beginning of the oscilloscope sweep). At 50 milliseconds from the beginning of sweep, the shock wave had traveled up to the driver end of the tube and returned. Another cycle of shock wave motion is seen at 85 milliseconds. Helium was used as the driver gas only for the preliminary studies. The rest of the experiments described were made using air as the driver gas. With air instead of helium as the driver gas the actual pressure time curve a t the same measuring point is considerably different (Figure 3). Because of the lower velocity of sound in air than in helium, the rarefaction wave takes a considerably longer time to reach the end of the expansion tube. When air is used as the driver
Figure 2 corresponds to passage of the incident shock wave across the barium titanate trigger gage. In this experiment helium was used as the driver gas a t a pressure of 565 pounds per square inch absolute. Pressure PI in the driven end of the tube was initially 1 atm. It took 10 milliseconds for the incident shock wave a t 75 pounds per square inch absolute to reach the pressure gage after it passed the trigger gage. After a short delay, the wave traveled to the end of the tube, reflected, and returned a t a pressure of 275 pounds per square inch absolute. The incident shock pressure ratio P2/P1 is about 54% of that calculated for the initial driver and driven pressure actually used, without considering friction losses. After the reflected shock wave arrived a t the pressure gage station, the pressure continued to rise for 4 milliseconds, Without friction effects in the tube, the pressure would be constant a t 690 pounds per square inch absolute immediately behind the reflected shock wave and the temperature behind the reflected shock
gas at 565 pounds per square inch absolute it takes 14 milliseconds for the incident shock wave to reach the pressure gage aftec passing the trigger for the oscilloscope sweep. The reflected shock pressure is not so high as previously obtained, and the friction pressure loss not nearly so great. As with the helium driver pressure experiments, the pressure behind the reflected shock wave is rising because of pipe friction and probably other causes; the pressure variation with time (Figure 4) depends on the shock strength selected. About 22 milliseconds from the time the incident shock wave has triggered the oscilloscope sweep an air pressure oscillation is seen pt the pressure measuring station near the end of the tube (Figure 5). A calculation of the time of return of shock wave reflection from the surface of discontinuity shows that the small amplitude pressure pulsation noted a t 22 milliseconds may be due to the return of a wave reflected from the surface of discontinuity. For the operating conditions used to date, the surface of discontinuity
60C
500
9 400
e v)
a 300
v)
W v)
E 200 100
0
~
5
IO
15
20
25
30
35
40
TI ME- MILLISECONDS
Figure 4. Pressure vs. time for several initial driver to driven air pressure ratios
Figure 5. wave
Pressure oscillation following reflected shock
Time scale, 5 milliseconds per large division; pressure scale, 67 Ib./sq. inch per large division A. Beginning of sweep 6. Ait' pressure oscillation
VOL. 50, NO. 1
JANUARY 1958
55
l.O--
DRIVEN GAS SECTION LENGTH
9.5
0 1 20
METERS
/ 1 1 / 1 30 40 50
1 70
60
00
PRESSURE BEHIND INCIDENT SHOCK WAVE P2 - P S I A
Figure 6. lncident shock wave attenuation vs. pressure behind incident shock wave
Figure 7. A.
does not come closer than 1 meter from the end of the shock tube. Many of the experiments have been made using a driver pressure, Pq, of 565 pounds per square inch absolute. Initial driven gas pressure, P I , was varied from atmospheric to 2 pounds per square inch absolute. The lower the initial driven gas pressure, the larger the pressure ratio obtained across the incident shock wave, and the higher the temperature behind the shock wave for a given initial air temperature in the driven ges section. Since fuel injection was to occur after the shock wave had reflected from the end of the tube, the temperatures behind the reflected shock wave are of interest. The time of fuel injection was to be at least 15 milliseconds after the shock wave had reflected. Therefore the temperature would be somewhat changed from that calculated just behind the reflected shock-the pressure is actually rising behind the reflected shock wave which would tend to increase the temperature, but heat is being conducted to the walls which would tend to reduce the temperature a t the tube end. In view of this uncertainty it was decided to determine the temperature behind the shock wave. lncident Shock Wave Attenuation Measurements. Information was available on the incident shock wave attenuation to be expected in a long shock tube (6). These data were obtained with a shock tube driven section 4 meters long and with a more favorable density condition in the driven section than in the author’s apparatus. Therefore, extrapolation required by the author was too extensive to be reliable. Even so, it was doubtful whether more than 60% of the incident shock pressure rise could be expected to be available starting with the driven section pressure near 1 atm., ini-
56
Variation of sound wave pickup signal with time Time scale, 20 microseconds per large division Spark discharge 6. Electrical disturbance C. Arrival of sound wave
tially. This would be especially true, since for the authors apparatus the shock tube wall surface was used as received without special treatment to minimize surface roughness. Consequently much higher pressures would have to be used to charge the driver section to obtain a desired reflected shock pressure than one would expect to use assuming no boundary layer development. Early experimental work, however, showed that the pressure continued to rise behind the reflected shock wave and that this region could be used in the planned combustion studies. Incident shock wave pressure measurements were made using flush mounted pressure gages whose frequency response was satisfactory to 10,000 cycles per second. Although they could not accurately show the rise of the shock front, they could resolve the pressure “plateau” immediately behind the shock front. Figure 6 is a curve showing the ratio of actual pressure ratio P2/P1 (y actual) across the incident shock wave to theoretical pressure ratio P ~ / P I(y theoretical) us. pressure behind the incident shock wave, Pp, for constant P ~ / P I . As Pa decreases the actual pressure ratio across the incident shock wave (when it reaches the end of the tube) approaches the theoretical ratio. The study of attenuation of shock waves in tubes (5, 6, 8 ) is a program in itself and has been pursued to a very limited extent in this investigation. In general, the actual pressure rise ratio obtained across the reflected shock wave, y’, agreed well with that calculated from shock tube theory when the pressure rise ratio, y, actually obtained from the incident shock wave was used. Following the preliminary experiments on shock attenuation, part of which are reported here, plans were made to obtain the temperature of the air a t the end of the shock tube a t predetermined times after the reflected shock wave had passed.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Temperature Determination in Air Behind Reflected Shock Wave b y Velocity of Sound Technique
Calculation of temperature by velocity of sound measurement is a method of pyrometry used by numerous investigators for many years ( 2 ) . This approach seemed particularly well suited for this application for several reasons. First the composition of the medium is fixed and well known. Behind the reflected shock wave the boundary layer should be very small, since theoretically, at least, the velocity of the gas is zero. This also means there should be no correction for air velocity effects because the pulse sending and receiving stations are stationary. For these experiments the instrumentation setup shown schematically was used. A high response, strain gage type of pressure gage was used to generate a signal when the incident shock wave caused by bursting of the diaphragm between driver and expansion chambers passed across its face. This signal was amplified and then entered a time delay chassis. After a predetermined time delay, a signal was sent to a triggering chassis whose output was a high voltage pulse (15 kv.) originally designed for triggering a photo flash bulb. The high potential lead was attached to the external gap glass envelope of an aircraft gas turbine spark plug ignition system. A voltage of 3.2 kv. just below the gap breakdown voltage was applied continuously to the condenser of the gas turbine spark system. The 15-kv. additional pulse fired the aircraft gas turbine spark plug a set time after the incident shock wave passed the pressure gage in the shock tube. h vollage signal on the spark plug side of the external gap started the sweep of the oscilloscope. When the pulse generated at the spark plug reached the opposite side of the shock tube it was picked up by a sound
A U T O I O N I T I O N OF FUEL S P R A Y S
- I4
1000
8 00 0
2
I
W
8
- 8
0
a
w w
600
g
- 7
7a
- 6
g
- 5 a w - 4
5001
-
4 00
t
300l
'
I
70
I
80
'
90
I
"
'
I
'
110 120 - MICROSECONDS
100
TIME
'
130
140
T = -M Wc2 rR
T = temperature, ' K. M W = molecular weight = 28.9 c = velocity of sound (measured), cm. per second y = specific heat ratio = 1.4 a t
x
lo7
ergs/' K.
O
150
0
= 8.31
- mole
I
I
I
I
I
I
I
1
2
3
4
5
6
7
AIR DENSITY
8 9 1 0
x lo3 - G R A M S / C C .
1 Error in sound wave travel time vs. air
Figure 9. density
A plot of calculated temperature us. time for a sound wave to cross the shock tube (path length 5.2 cm.) is shown in Figure 8. When the air pressure was 1 atm. and the temperature 300' K. the measured time of passage of the wave was 135 microseconds, instead of the calculated 148 microseconds. Rapid decay of a shock wave signal from a spark was noted earlier by Hollyer (6). It was thus believed that the wave initiated a t the spark plug actually began as a shock wave, becoming weaker as it progressed across the tube, Thus, incqeasing the air density in the tube would be expected to decrease the shock strength until it had degenerated to a sound wave. A series of experiments was made in which the air density in the tube was raised to a measured level and the time of wave tra-
wave sensing element made of three barium titanate disks a/,-inch in diameter, 1/16 inch thick, stacked electrically in parallel and mounted in a simple holder as shown. A typical oscillogram showing the beginning of sweep and barium titanate signal output when the sound wave arrives is shown Figure 7 . The temperature of air may be expressed as
300' K. R = universal gas content
- I
I
I
Figure 8. Air temperature vs. time for sound wave to traverse 5.1 -cm. path
where
3t
- 2
vel measured. For 300' K. the difference between calculated and measured traverse time us. density is shown in Figure 9. I t appears that above 0.009 gram per cc. this error h e r o . Air densities ranged from 0.007 to 0.012 gram per cc. when the actual fuel injection into high pressure, high temperature air 0 occurred. Results of temperature calculations from sound velocity measurements are shown in Figure 10. Here both calculated temperatures and measured pressure us. time a t the end of the driven tube are shown. The calculated temperature from the measured pressure rise of incident and reflected shock wave is also plotted. If one is interested in investigating autoignition over a range of air temperatures, the shock strength for a
Y
w
TEMPERATURE
a 2
l-
a
a W
a I w
500
I
Figure 1 1 . Typical film showing evaporation of liquid fuel and early stages of burning
oh
llo
Figure 10.
do
I
310 4/0 50 TIME - MILLISECONDS
20
1 70
Temperature calculation results
A. I
6. C. D.
E.
Beginning of fuel injection n-Hexadecane inlo air at 655' K. Illumination from combustion n-Hexadecane into nitrogen at 850' Complete evaporation
VOL. 50, NO. 1
K.
JANUARY 1958
57
-500
600
700
800
900
A I R TEMPERATURE
1000
1100
OK.
Figure 12. Ignition delay and evaporation time for n-hexadecane vs. air temperature
given test can be increased by decreasing the initial driven gas pressure; temperatures can be calculated from sound wave travel measurements at the planned times for fuel injection. Method of Fuel Injection
The fuel nozzle was an accumulator type designed by the General Electric Co. for a high speed Diesel engine. A manual hydraulic pump was used to pressurize the fuel nozzle system to 10,000 pounds per square inch. Fuel was discharged when the pressure of the supply line feeding oil to the nozzle was rapidly dropped. A high pressure, quick opening valve held closed by a heavy spring was released by a solenoid. Time delay from a voltage pulse to the solenoid to the beginning of fuel injection was 23 milliseconds. Fuel injection was planned to occur after the incident shock wave had reflected from the end of the tube. The fuel nozzle was aligned so that fuel would be injected along the axis of the tube in the direction of the driver gas chamber (see diagram of window section). I n a typical experiment, when the incident shock wave passed across the face of a transducer, the output signal, after external amplification, was brought to a time delay chassis. After a preset delay a signal was sent to a power supply which operated the solenoid valve mentioned above. For the experiment described, technical grade n-hexadecane was used. About 108 mg. of fuel was injected through a 40mil hole 25 milliseconds after the incident shock wave passed the pressure transducer. The fuel spray duration was 1.7 milliseconds. Typical Autoignition Experiment
With the system described above a series of experiments with n-hexadecane
58
fuel injection was completed. Air driver pressure Pq was held constant at 565 pounds per square inch absolute and the initial driven air pressure PIwas varied to obtain the desired air temperature. Photographing the combustion zone using the arrangement shown provided the most satisfactory method of ignition delay measurement. The light source illuminates 7 cm. of the 14-cm. total width for the camera field of view. By direct photography at film speeds approaching 6000 frames per second the fuel spray can be observed as it enters the high pressure, high temperature air. At the time of fuel injection (28 milliseconds from the beginning of sweep in Figure 3) the air pressure is rising at the rate of about 3.5 pounds per square inch per millisecond. Figure 11 shows a section of 8-mm. film a t approximately 6000 frames per second in which is displayed the sequence of events from the beginning of fuel injection through the spray evaporation period u p to the occurrence of luminosity from combustion in the field of view. Illumination from combustion usually begins simultaneously from one or more points a few centimeters from the fuel nozzle. Included in Figure 11 is an 8-mm. film which shows fuel evaporation without oxidation. Here nitrogen instead of air was used in the driven gas section of the shock tube. The masking arrangement was eliminated, and the spray penetration distance along the tube axis was found to be 13 cm. for a nitrogen pressure of 17 atm. and nitrogen temperature of 850’ K. What appears to be complete evaporation of the fuel takes place 2.5 milliseconds from the beginning of fuel injection. This technique has been used to study the autoignition of liquid fuel over an air temperature range of 600’ to 1000’ K. and a t an air pressure of about 250 pounds per square inch absolute (Figure 12). At least two important kinetic processes are involved (7) : evaporation of fuel and its oxidation. At high air temperatures the evaporation process is usually believed to control because of the low activation energy indicated from the slope of the ignition lag-air temperature curve. According to Jost (7), ignition delay data at low air temperature reflect control by the chemical reaction. The data presented here for n-hexadecane (Figure 12) are in qualitative agreement with data presented for cetene (7). Acknowledgment
A number of the author’s colleagues, including G. E. Moore, W. E. Kaskan, D. R. White, and C. P. Fenimore, generously provided advice on many occasions. Olaf Brusdal assisted in the shock tube design details and C. W. Moon helped in the experimental work.
INDUSTRIAL AND ENGINEERING CHEMISTRY
The fuel nozzle used in the experiments to date was developed by R. J. Hooker, General Engineering Laboratory, General Electric Co. Literature Cited
(1) Bleakney, W., Weimer,D. K., Fletcher, C. H., Rev. Sci. Znstr. 20, 807 (1949). (2) Charyk, J., “Physical Measurements in Gas Dvnamics and Combustion.” pp. 198, ’372, Princeton University Press, Princeton, h-.J., 1954. (3) Geiger, F. W., Mantz, C. W., “Shock T i b e as an Instrument for Investigation of Transonic and Supersonic Flow Patterns,” Univ. Michigan Rept. Project M72-4 (1949). (4) Glass, I. I., Patterson, G. N,,J . Amonaut. Scz. 22, 73 (1955). (5) Hollyer, R. N., J . Appl. P h p . 27, 254 (1956). (6) Hollyer, R. N., “Study of Attenuation in the Shock Tube,” Univ. Michigan Rept. Project 720-4, pp. 122, 127 (1953). ( 7 ) Jost, W., “Explosion and Combustion Processes in Gases,?’ p. 583, McCraw-Hill, New York, 1946. ( 8 ) Trimpi, R. L., Cohen, 3’.B., “Theory for Predicting Flow of Real Gases in Shock Tubes with Exoerimental Verification,” Natl. Advisory Comm. Aeronaut. T.N. 3375 (1955). ( 9 ) Wakil, M. M., Myers, P. S., Ugehara, 0. H., “Fuel’ Vaporization and Ignition Lag in Diesel Combustion,” preprint SOC. Automotive Engrs., November 1955. RECEIVED for review November 12, 1956 ACCEPTED May 6, 1957
Correction Reaction Rate of Solid Sodium with Air In the article on “Reaction Rate of Solid Sodium with Air” [W. H . Howland and L. F. Epstein, IND.ENG. CHEM. 49, 1931 (1957)], the following added reference and changes should be made in the Literature Cited : (3) Jackson, C. B., ed., “Liquid Metals Handbook, Sodium-NaK Supplement,” in “Liquid Metals Handbook,” NAVEXOS, P-733 (Rev.), pp. 7-9, 94-113: U. S. Government Printing Office. Washington, D. C. (July 1,1955). (4) Lyon, R. Y., ed., “Liquid Metals Handbook,” NAVEXOS, P-733 (Rev.), p. 114, U. S. Government Printing Office, IYashington, D. C. (1952). Reference (4) should become (5). On page 1931, in column 1, reference (2) of the first paragraph should be changed to (7). In column 2, the third line, reference (4)should be changed to (5). I n column 3, paragraph 2, reference ( 3 ) at the end of paragraph should be changed to (3, 4).
