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A study of fast reactions in fuel-oxidant systems
196
PROPELLANT BURNING
11
A STUDY OF FAST REACTIONS IN FUEL-OXIDANT SYSTEMS Anhydrous Hydrazine with 100 per cent Nitric Acid a By MARTIN KILPATRICK AND LOUIS L. BAKER, JR. Introduction Certain liquid fuels when brought into contact with strong oxidizing agents will ignite spontaneously. The ignition of such systems may take place instantaneously when the liquids are first brought into contact or there may be a time delay. Most of the studies of the ignition delay in such systems have employed methods designed to simulate conditions met in a rocket motor. In a typical rocket motor the two reactants are contacted by the impingement of a number of fuel and oxidant streams in a relatively unconfined area. Observation of the time required for ignition under such conditions does not differentiate between the delay created by the time required to achieve efficient mixing and the delay caused by an actual chemical induction period. Methods have been devised to achieve the complete mixing of useful amounts of two hqmds in a very short time. The work of Hartridge and Roughton, 1 Chance, 2 and McKinney and Kilpatrick 3 has proven conclusively that it is possible to effect the complete intermixing of two liquids and begin observation of the resulting mixture only a few milliseconds after the initial contact. These methods have not previously been applied to the study of the ignition of fuel-oxidant mixtures. The present research is an attempt to apply the mixing methods of Roughton, Chance, and McKinney and Kilpatrick to the problem of the self-ignition of fuel-oxidant systems and to study the associated reactions.
Ignition Delay Studies Most of the impetus for the study of the ignition delay time of liquid fuel-oxidant systems is due to the potential use of such systems in rocket a This research is part of the work being done in the Department ot Chemistry of Illinois Institute of Technology, supported by the Office of Naval Research under Contract N7onr-32913. This paper is abstracted from part of the dissertation submitted by Louis L. Baker, Jr. to the Graduate School of Illinois Institute of "Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
motors. Systems which do not ignite spontaneously can be used successfully in rocket motors if some external means of ignition can be found. From a practical standpoint, however, it is preferable to employ a system that will ignite spontaneously. In such a rocket motor it is imperative that ignition take place in a very short time after contact of the reactants. An excessive delay will cause the unreacted materials to accumulate in the motor chamber; ignition could then cause a destructive explosion. For this reason the study of hypergolic ignition (self-ignition) is of great practical importance in the field of rocket technology. Some of the experimental variables which must be controlled for the precise measurement of ignition delay time include: (1) the reactant ratio, (2) the time required to effect complete mixing, (3) the chemical composition of the reactants, (4) the ambient temperature, and (5) the ambient pressure. Studies of the ignition delay of the systems hydrazine-nitric acid, furfuryl alcohol-nitric acid, and liquid ammonia-hydrazine mixtures-nitric acid have been carried out in a small rocket motor. 4 The reactants were contacted by the impingement of a single fuel stream with a single oxidant stream. The nominal values obtained for optimum test conditions are given in Table 1. The effect of reactant ratio on ignition delay time, however, is not clear under the uncertain mixing conditions resulting from the impingement of two streams in an unconfined space. If the complete intermixing could be effected in a time very short compared to the ignition delay time, then the measured delay would be the true chemical delay or induction period, and a change of the reactant ratio should lead to a different chemical delay time. When ignition takes place before mixing is completed, and the remainder of the fluid ignites, reactant concentrations of this igniting mixture cannot be known. Table 2 makes evident the delay due to incomplete mixing by comparing the results obtained for furfuryl alcohol-white fuming nitric acid by a refined open-cup apparatus due to
197
FAST REACTIONS IN FUEL-OXIDANT SYSTEMS
Gunn 5 with those of the impingement method. The impingement method would be expected to yield smaller ignition delay times because of more efficient mixing than the open-cup method due to greater turbulence and impact associated with impinging streams. The present study was undertaken on the premise that a fundamental approach to the phenomenon of igl~ition delay was possible only if complete mixing could be accomplished in a time shorter than the ignition delay time. Studies in this laboratory 3 had already shown that 1 ml of one reactant could be contacted and effectively mixed with 25 ml of a second reactant in less than 10 msec. I t was, therefore, expected that a few milliliters of each of the reactants could be mixed in much less time than ignition delay times such as those reported in Tables 1 and 2.
Rapid Mixing Studies The problem of rapid and efficient mixing o f dilute solutions has been recently reviewed by Roughton and Chance, 6 and the mixing of liquids for reactions evolving gases investigated by McKinney and Kilpatrick) In general, the mixers are of two shapes, T and Y, the T type being preferred. In the T-type mixer, the jets may be directly opposed or arranged so that the two liquids stream tangentially into the mixing area. The tangential arrangement was found to be better at low flow rates but both methods of contacting the liquids gave about equally efficient mixing at high flow rates. The method of opposing jets was adopted in our earlier studies?, 7. s Our findings that the time for mixing by the use of the streaming method cannot readily be reduced below 0.1 msec seem to be confirmed by the fact that recent discussions of fast reactions9 have emphasized relaxation methods for measuring fast reactions where mixing of the reactants is not involved. Such methods may well be developed for measurement of reactions having halftimes of microseconds. Experimental Methods APPARATUS
The apparatus; which tins been described previously,9, 19 is referred to as Reactor I I to distinguish it from an apparatus employed earlier in this laboratory for the study of fast reactions. Reactor II, in combination with the pneumatic injector of Neas, Raymond and Ewing, 3, n is capable of contacting and mixing a few cubic
centimeters of two liquids in a few milliseconds. The operation of the apparatus will be briefly reviewed. A diagram of the fluid mechanical sysTABLE 1. IGNITION DELAY IN A SMALL ROCKET MOTOR 4 Reactants
Hydrazine, 96 per cent--RFNA* Hydrazine, 96 per cent--WFNAt Furfuryl alcohol --WFNA Furfuryl alcohol-WFNA Hydrazine, 71.5 per cent--WFNA Hydrazine, 71.5 per cent--RFNA Ammonia, 14.1 per cent hydrazine-RFNA Ammonia, 9.5 per cent hydrazine-RFNA Ammonia, 5.0 per cent hydrazine-RFNA
Temperature
Ignition Delay
~ 21
3.1 4- 1.4
21
5.0 4- 1.7
21
16.6 4- 2.4
--29
rasec
22 to 40
25
ca. 37
--48
ca. 37
--36
6 to 10
--35 to --40
ca. 14
--37 to --47
37J;
* RFNA, red fuming nitric acid, 24 per cent NO2 . t WFNA, white fuming nitric acid, 96 per cent HNO~ . :~ Sporadic ignition. TABLE 2. IGNITION DELAY FOR FURFURYL ALCOHOL-WHITE FUMING NITRIC ACID Ignition Delay Time Initial Temp. Open-cup~
Impingement4
~
msec
msec
Room --15 --29
36 180 --
16.6 22-40
tern of the reactor is shown in Figure 1. The pneumatic injector, not shown .in the figure, is located above the reactor. I t functions as a quick opening valve discharging nitrogen gas at pressures up to 2000 psi into the area P behind the driving piston. As the driving piston descends,
198
PROPELLANT BURNING
the two smaller pistons F1 and FI' are driven downward. This design was an improvement over that of Reactor I, giving a substantial hydraulic advantage in having the large piston drive the two smaller ones and insuring simultaneous descent of the two small pistons during injection. The liquids to be contacted are initially contained below the smaller pistons in the areas designated as P1 and PI'. During injection, the liquids flow through the lead tubes P2 and p t, the jets P3 and
[
The reactions under study were followed by several techniques, but mainly by measurement of the transient pressure. The pressure sensitive element was a diaphragm type strain gage manufactured by the Statham Laboratories, Inc. The strain gage is in the form of a balanced bridge, the output being supplied by an AC carrier wave, and frequencies of the order of 1000 cps were used. The unbalance of the bridge was amplified and applied to the vertical plates of a cathode-ray oscilloscope. I t was found by calibration against known static pressures that the amplitude of the oscilloscope pattern was accurately proportional to the applied pressure. The natural frequency of the strain gage was stated by the manufacturer to be in excess of 2000 cps so that pressure events occurring in a time of the order of a millisecond should be accurately reproduced. The horizontal plates of the oscilloscope were connected to a single sweep generator which was triggered automatically by the closing of a contact located in the penumatic injector. The oscilloscope sweep normally began between 1 and 1.5 msec before the beginning of injection. The record of the transient pressure was obtained by photographing the screen of the oscilloscope. Another method of following the reactions was by the measurement of light emission. The output of a vacuum phototube-amplifier combination was connected to the vertical plates of a second oscilloscope. The horizontal plates were driven by the same sweep generator used for the transient pressure recording system. In some cases, the time distribution of the light emission was obtained; in others, only the time corresponding to the onset of light emission. Simultaneous measurements of the transient pressure and the light emission were made possible through the use of an additional camera. The progress of the injection could be followed by means of an electronic timer. The timer consisted of a serrated brass rod connected to the moving piston system. A light beam passing between the serrations on the rod was focussed on a phototube cathode. During injection, the serrations on the moving rod interrupted the beam and caused a varying light intensity to fall on the phototube, and the amplified output from t h e phototube was applied to the vertical plates of an oscilloscope. An analysis of the resulting oscilloscope trace gave the time corresponding to a number of positions of the pistons with respect to the stationary light beam. The timer was operated simultaneously with the
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IF[G. 1. Schematic diagram of fluid mechanical system of Reactor II. P / , and mix in the exit tube P4 9 The mixed solution is then ejected into a closed volume of 339 cc where reaction occurs. The reactor was designed to allow for immersion in a low temperature bath. The volume ratio of the reactants could be varied over a range from approximately 4:1 to 1:1 by the use of fittings and additional pistons. The mixing pattern could be varied by using additional mixing plates of differing design. For this study, a T-type mixing plate designed in accordance with the principles set forth by Roughton and Chance was used. t h e simultaneous descent of the two small pistons insured uniformity of the mixing.
199
FAST REACTIONS IN FUEL-OXIDANT SYSTEMS
transient pressure measuring system to give a record of both the injection process and the pressure rise. The final pressure of the product gases was measured by venting the reactor to a glass reservoir by means of a needle vave and measuring the pressure on a mercury manometer. The ignition delay of the fuel-oxidant systems was determined by simultaneous injection and transient pressure measurements. The time between the beginning of the injection process (first contact of the reactants) and the beginning of the pressure rise was obtained with an accuracy of about -4-0.5 msec. The observed delay had to be corrected for the time required for the first pressure pulse to travel from the reactor to the diaphragm of the strain gage, a distance of about 18 in. If the initial pressure pulse was propagated at the speed of sound in ordinary air, it would have required 1.3 msec. This value was used for the correction of the observed data. Even if the pulse were propagated as a shock wave, it it was unlikely that the pulse would have reached the gage in a significantly shorter time because of the irregular path. Simultaneous light emission and transient pressure measurements were made to establish the fact that the beginning of the pressure rise and the onset of light emission occurred simultaneously. Again the experimental results had to be corrected for the transit time of the initial pressure pulse. RATE OF INJECTION
A number of measurements were made of the injection rates obtainable when water was injected into the mixing area from both cylinders of the reactor. The runs were made with the 1 : 1 ratio, the stroke of the piston system being set at 7~ in, and approximately 1.i cc of water was ejected from each cylinder. Some of the data for different driving pressures are shown in Figure 2. The linearity of the plots indicated that the rate of descent of the piston system and hence the injection rate was constant throughout injection. The linearity further indicated that the moving parts accelerated very rapidly to the equilibrium velocity. The data obtained for the injection of water are shown in Table 3. The rate of injection is expressed as the time required for the pistons to descend 7/~ in. Runs 114 to 120 inclusive were made with loosely fitting neoprene piston gaskets and runs 123 and 124 with very tight gaskets, in order to test the effect of gasket resistance. The
gasket resistance had only a small effect on the measured injection rate. Two runs, 73 and 98, were made using the 4:1 injection ratio. In this case, 2.84 cc of water was injected from the large cylinder and 0.74 cc from the small cylinder of the reactor. O.875
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R e a c t o r II
T A B L E 3. INJECTION R A T E MEASUREMENTS WITH WATER Run
Reactant Ratio
Pressure of Driving Gas
Injection Time
114 119 123 124 117 116 120
1:1 1:1 I:I 1:1 1:1 1:1 1:1
1950 1900 1700 1700 1400 1000 600
1.79 1.71 1.91 1.91 1.96 2.21 2.84
73 98
4:1 4:1
1800 1700
4.27 4.31
A rough calculation of the injection rates was made based upon empirical relations found in standard engineering treatisesY. ,s The values calculated for the 1:1 ratio for the water injection are shown in Figure 3 as the dotted line. I t is evident that the calculations predict a much more rapid injection rate than that observed. The calculations indicate t h a t the flow rate should be roughly proportional to the square root of the driving gas pressure, and Chance 2 obtained in-
PROPELLANT BURNING
200
jection rates that were precisely proportional to the square root of the pressure drop using identical mixing systems but at much smaller pressure drops. I t seemed likely, in view of the curvature of the observed plot, t h a t the resistance to gas flow in the pneumatic injector might be slowing the injection. If this were the case, there would have been a pressure drop due to the nitrogen flow from the gas storage reservoir of the pneumatic injector to the driving cylinder, and the pressure
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FIo, 3. Linear flow rate in the exit tube of Reactor I I as a function of the square root of the driving gas pressure. actually driving the large piston would then have been less than the value recorded. A rough calculation of the pressure drop was made according to a method outlined by Beale and Docksey, z4 and the measured driving pressures for the runs plotted in Figure 3 were corrected for the pressure drop. The corrected d a t a show much better proportionality between the injection rate and the square root of the driving gas pressure, although the corrected data still show a slower flow rate than those predicted b y the fluid mechanical calculations. These calculations were made on the basis t h a t the injection
rate was determined entirely by the resistance to fluid flow and not by the gasket resistance. RATE O F MIXING
The rate at which two reactants are contacted is not necessarily equal to the rate at which they are mixed. Reactor I I has been applied to the study of the reaction of sodium-potassium alloy and water. 8 For this reaction it was found t h a t the pressure rise, corresponding to hydrogen evolution, was completed in a time nearly equal to the measured time of completion of the injection. This would suggest that the mixing rate was nearly equal to the injection rate. A number of previous investigations have been made of the efficiency obtainable for the mixing of equal volumes of reactant solutions in T - t y p e mixers} These studies indicate that the reactants should be thoroughly mixed in our reactor. I n our reactor the mixing plate used has two jets of 2.5 mm diameter discharging into an exit tube of 3.9 mm diameter. A T-type mixer described by Roughton ~ and due to Trowse 15 also employs two jets. The diameter of the jets and the exit tube (observation tube) was 0.7 mm. Trowse found that the reactant solutions were 97 per cent mixed at a point 5.7 mm downstream from the jets. His measurements were made at a linear flow velocity at the exit tube of 440 cm/sec. Millikan1~ studied the extent of mixing in another T - t y p e mixer as a function of the flow velocity. He found that the distance downstream to the point of 97 per cent mixing rapidly moved toward the point of first contact as the flow velocity increased. The linear flow velocities in the exit tube of our reactor ranged from 5,000 to 11,000 cm/sec so that mixing should be complete within the 8 mm length of the exit tube of the reactor used in the present study. To further insure the completeness of mixing, the stream emerging from the exit tube impinged on a baffle plate only 25 mm from the exit tube. The time of residence of the stream in the exit tube of the reactor for the total range of flow velocities varied between 0.07 and 0.16 msec. For the study of ignition delay, the injection rate is of interest only as it affects the mixing efficiency. I t is only the time between the contact and the completion of mixing of a given fluid element that is of importance, since presumably the first element of fluid to be injected is the one t h a t will first ignite.
FAST PREPARATION
REACTIONSIN FUEL-OXIDANT SYSTEMS Experimental Results
OF MATERIALS
Nitric acid The acid was prepared by the vacuum distillation of an equal volume mixture of commercial 95 per cent nitric and sulfuric acids in an allglass apparatus. The mixture was cooled to 0~ before reducing the pressure to 10-4 mm Hg, and the anhydrous nitric acid was collected in a trap cooled by a mixture of dry ice and trichloroethylene at a rate of 2 cc per hour. The acid, which consistently analyzed between 100.2 and 100.8 per cent HN03 by titration, was stored at dry ice temperature to avoid decomposition.
Hydrazine The hydrazine was prepared from commercial anhydrous hydrazine, about 95 per cent N2H4, by refluxing in an all-glass distilling apparatus with an equal quantity of calcium or barium oxide at approximately 50 mm Hg. The hydrazine was then fractionally distilled, the first half collected, and analyzed by direct titration with HC1 and by the iodate method using wool red as an internal indicator. The material was found to be 97.7 to 99.0 per cent N2H4.
Aniline Aniline was purified by distillation from zinc dust. The product was colorless and distilled at 182~ under a pressure of 750 mm Hg. The refractive index was found to be 1.4791 at 32.5~
Hydrogen peroxide High strength hydrogen peroxide was analyzed by titration with ceric sulfate and found to contain 84.5 per cent H~02 by weight.
Ammonia-hydrazine mixtures Anhydrous hydrazine was weighed into a small Erlenmeyer flask fitted with a ground glass joint, attached to the vacuum line, and the system evacuated after freezing in a liquid nitrogen bath. Anhydrous ammonia was transferred from a tank to a flask of known volume, the amount being determined by the gas pressure, and condensed into the flask with the hydrazine. The procedure was repeated until the desired amount of ammonia had been added and th~ mixture was stored at the temperature of dry ice.
201
T H E H Y D R A Z I N E - N I T R I C ACID R E A C T I O N
A typical simultaneous measurement of injection and transient pressure for a near stoichiometric mixture of hydrazine and nitric acid is shown in Figure 4. The pressure rise began 2.2 msec after the beginning of injection. Correcting by the 1.3 msec required for the pressure impulse
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4.
TBANSIENT
1V[EASUREMENTS.
PRESSURE
AND I N J E C T I O N
HYDRAZINE-NITRIC
ACID
REACTION Volume HNO,
'olume N~H4
cc
cc
2.84 1.59 1.11 1.11 0.74 0.74
0.74 0.74 0.74 1.11 1.11 1.59
Mole [ MaxiRatio mum HNO3: Pressure N~H4 Rise
Final Pressure
Overall Injection Time
Ignition Delay
arm
ra se c
mJec
2.87 [ 23.4 1.61 I 31.6 1.12 31.7 0.75 46.7 0.51 43.2 0.352 51.3
2.66 2.21 1.80 3.35 3.57 5.26
9.2 5.4 4.9 4.7 4.6 6.4
0.2 0.5 0.9 0.1 0.2 0.4
/
to reach the diaphragm of the strain gage, the ignition delay was 0.9 msec. The average ignition delays for all of the ratios studied of the hydrazine-nitric acid reaction are given in the last column of Table 4. The maximum pressure rises obtained from the transient pressure record are given in the fourth column and the final pressures calculated from the manometer reading, in the fifth column. The plot of piston displacement vs. time in Figure 4 showed a definite break in the slope. No such break was observed in any of the results obtained for the injection of water. The sudden slowing of the injection rate may be attributed to
202
PROPELLANT BURNING
the ignition of the reaction, and the constancy of the injection velocity near the completion of
.~
overall injection times are given in the sixth column of Table 4. The simultaneous measurements of the transient pressure and the light emission showed that the light emission preceded the pressure rise by 0.2 to 1.6 msec. After correction for the transit time of the initial pressure pulse, it was concluded that the beginning of the pressure rise coincided, within experimental error, with the beginning of visible light emission. A typical result of the simultaneous measurement of light emission and transient pressure is shown in Figure 5 for the reaction of nitric acid with a slight excess of hydrazine.
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THE HYDRAZINE-HYDROGENPEROXIDE REACTION
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Fro. 5. Pressure rise and light intensity in reaction of two volumes of hydrazine with one volume of nitric acid, run 152, Reactor II. TABLE
5.
IGNITION
PRESSURE
DELAY
BY T R A N S I E N T
MEASUREMENTS Mole o
Run
Fuel
Oxidant [ant-toFuel
Max imur Pres sure
Ignition Delay
arm
157 Aniline 158]
Nitric acid
8.18
42 25
2.3 1.2
163 Hydrazine 165 162
Hydrogen peroxide
1.15
42 55 58
0.1 1.1 2.2
166 Hydrazine 167
Hydrogen peroxide
2.46
32 41
1.3 1.7
injection suggests t h a t a stable flame front was produced within the exit tube before the injection was completed. Because of this slowing of the injection rate, the overall injection time was substantially longer than the values obtained for the injection of water. The average values of the
The results for the hydrazine-hydrogen peroxide reaction are reported in Table 5, and the results of the simultaneous measurement of injection and transient pressure for two runs at oxidant-to-fuel ratio of 1.15 are shown in Figures 6 and 7. In run 163 the change in the injection rate is again apparent, but was not found in run 162, and this is interpreted to mean that here the ignition did not occur until injection was completed. For the other run made at this same oxidant4ofuel ratio the results were not conclusive. As shown in Table 5, the ignition delays reported for the 1.15 oxidant-to-fuel ratio were less consistent than those for the 2.46 ratio. In the latter case, ignition occurred during the course of injection. Simultaneous measurements of the light emission and the transient pressure for both reactant ratios showed that the beginning of visible light emission coincided with the beginning of the pressure rise. THE ANILINE-NITRICACID REACTION Table 5 also shows two runs for the reaction of aniline with nitric acid. The mixing plate was damaged in both of these runs. I t is likely t h a t during ejection, ignition was delayed and the flame may have flashed back with detonation velocity to the exit tube. THE REACTION BETWEEN LIQUID AMMONIAHYDRAZINE MIXTURES AND NITRIC ACID For these studies the reactor was immersed in a low temperature bath, and the temperature before injection was measured by a thermocouple located close to the reactant solutions. Since injection rate measurements could not be made
FAST
REACTIONS
iN
with the reactor immersed in the bath, ignition delay data were obtained from simultaneous measurements of the transient pressure and the light emission. The results are reported in Table
FUEL-OXIDANT
203
SYSTEMS
tained. This value is of the order of magnitude of the minimum ignition energy for gaseous mix~ t~
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I t was necessary to utilize the fact that injectiou began from 1.0 to 1.5 msec after the contact closed in the pneumatic injector. This was established from observations made under conditions where injection measurements could be made. An average value of 1.2 msec was taken as the time interval. It can be seen from the data that ignition occurred with a delay of only 2.9 msec at a concentration of only 2.7 per cent hydrazine although in two other runs no ignition occurred. A run was made of the reaction of nitric acid with pure ammonia and no ignition occurred.
t 75 ~: as ~ ~ 5O ~ = ~ 2s ~s /(, I I I [ 0 2 4 Q 8 IO 12 o TIME - MSEC. FIG. 6. Simultaneous measurement of injection rate and pressure rise in the reaction of equal volumes of hydrogen peroxide and hydrazine, run 163, Reactor II.
o
Discussion
The longest ignition delay time observed for any of the reactions studied was 2.9 msec. This is so much shorter than the results by other methods (see Tables 1 and 2) that one can only conclude that the other methods do not give efficient mixing. From our results it cannot be concluded that we are measuring the rate of a homogeneous, thermal chemical reaction as this type of reaction should require an energy of activation, and the igntion delay time appears to be independent of the initial temperature of the reactants, for the range - 4 0 ~ to 30~ The actual temperature just before ignition may be of the order of several hundred degrees and a difference of seventy degrees in the initial temperature may not result in sufficient difference in the temperature of the reactants just before ignition when the mixing is efficient. An unignited mixture of a fuel and oxidant such as hydrazine and nitric acid may be likened to a liquid explosive such as nitroglycerine. Nitroglycerine can be ignited by impact alone. Bowden et alY have studied the ignition of nitroglycerine by impact. They found that thin sheets of the material on an anvil would not ignite even under the influence of a rather strong hammer blow unless gas bubbles were present in the liquid. These authors concluded that the high temperatures produced by the adiabatic compression of the bubbles were responsible for the ignition. It was possible to estimate crudely the minimum ignition energy required to ignite the material by estimating the size of the bubbles and the compression ratio, is A value of 10-8 calorie was ob-
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FIG. 7. Simultaneous measurement of injection rate and pressure rise in the reaction of equal
volumes of hydrogen peroxide and hydrazine, run 162, Reactor II. T A B L E 6. IGNITION D E L A Y BY L I G H T EMISSION MEASUREMENTS. E Q U A L VOLUMES OF NITRIC ACID WITH H Y D R A Z I N E - L I Q u I D AMMONIA MIXTURES Run
Hydrazine in Mixture W I per cent
Temp.
Ignition Delay
~
mser
Maximttm Pressure aim
174 175
9.0 9.0
--35 --33
1.0 1.4
41 41
176 177 178
2.7 2.7 2.7
--38 --41 --41
2.9 --* --*
31 3.5 3.5
*No ignition. tures found by more precise methods involving spark ignition,is It is thus reasonable to suppose that the impact and turbulence associated with the mixing process in Reactor I I could have re-
204
PROPELLANT BURNING
sulted in immediate or nearly immediate ignition. This conclusion also results from the failure to observe a systematic increase of the ignition delay as the hydrazine concentration was decreased in the hydrazine-ammonia mixtures. The ignition delay increased to only 2.9 msec for the mixture containing 2.7 per cent hydrazine. The runs made on this mixture that failed to ignite indicated that the impact energy was just insufficient to ignite this mixture. Instead of observing a longer and longer delay as the hydrazine concentration or the temperature was decreased, a point was reached where the mixture simply failed to ignite. Such behavior is more indicative of ignition by mechanical shock than it is of thermally initiated ignition showing an induction period due to the rate of a chemical reaction. A study of the ignition delay of gaseous mixtures of hydrocarbons and oxygen was made by Jost and Teichmann 19 by a rapid compression method. They obtained a straight line when the logarithm of the ignition delay was plotted against the reciprocal of the reaction temperature. The plot was linear from delay values of a few milliseconds to over five seconds. The fact that the data could be represented by an Arrhenius equation indicates that the delay-determining factor was due to the rate of a chemical reaction. The experimental results obtained in the present study show, nevertheless, that fuel-oxidant systems previously thought to have long ignition delays will ignite in a very short time under violent mixing conditions. The measured explosion pressures shown in Table 4 are much lower than those calculated by the thermodynamic method outlined in a treatise by Lewis and yon Elbe. The calculations indicated an equilibrium pressure of the order of 90 atm. Analysis of the reaction products showed quantities of nitrogen oxides greatly exceedir~g that expected from thermodynamic considerations alone. These facts indicate that the reaction was quenched through contact with the metal parts of the reactor. Conclusions
(1) A constant volume reactor was designed to rapidly contact and mix two liquid reactants in a volume ratio ranging from 4:1 to 1:4. (2) The reactor was easily adapted to a change of the mixing pattern and could be immersed in a thermostatic bath. Satisfactory operation was obtained at temperatures as low as -40~
(3) A photoelectric method was devised to follow the course of the injection. The injection of 2.2 cc of water was achieved in 1.7 msec. A somewhat longer time was required for the injection of reacting fuel-oxidant systems because of the thrust created by the reaction process. (4) Reactions were studied in the reactor by following the transient pressure and the light emission. (5) The rate of mixing in the apparatus was shown to be very rapid by reference to measurements made by other investigators who employed geometrically similar mixing patterns. (6) The ignition delay for certain reactant ratios of several fuel-oxidant systems was measured and found to be less than 3 msec in every case. The systems included: (a) Hydrazine vs. nitric acid; (b) Aniline vs. nitric acid; (c) Hydrazine vs. hydrogen peroxide; (d) Hydrazineliquid ammonia mixtures vs. nitric acid. The delays were significantly lower than those observed by other investigators who employed mixing methods where the efficiency of mixing was most uncertain. (7) The very short ignition delays were attributed to (a) the rapid rate of mixing in the apparatus, and (b) the impact and turbulence associated with the mixing process. (8) There was a negligible effect of the initial temperature of the reactant or of the hydrazine concentration on the ignition delay time of the reaction with liquid ammonia-hydrazine mixtures with nitric acid. This result was more indicative of ignition by impact than it was of ignition controlled by the rate of a homogeneous chemical reaction. REFERENCES 1. HARTRIDGE, H., AND ROUGHTON, F. J. W.: Proc. Roy. Soc. (London), IO~A, 376 (1923), et seq. 2. CHANCE, B.: J. Franklin Inst., ~9, 455, 613, 737 (1940). 3. McKINNEY, C. D., JR., AND KILPATRICK, M.: Rev. Sci. Insts., ~$, 590 (1951). 4. KELLOGGCo.,M.W.: Preliminary Experimental
Studies of Liquid Fuel Systems, Final Report. Report No. SPD 236, May 20, 1949, pp. 88, et seq. 5. GuNs, S. V.: J. Am. Rocket Soc., ~$, 33 (1952). 6. ROUGHTON,F. J. W., ANn CHANCE,B.: Investi-
gations of Rates and Mechanisms of Reactions, Technique of Organic Chemistry, Volume VIII, ed. A. Weissberger, p. 669. New York, Interscience Publishers, Inc., 1953.
MECHANISM OF BURNING OF NITRATE ESTERS
205
13. WALKER, W. H., LEwis, W. K., MCADAMS, W. H., AND GILLILAND, E. R.: Principles of Chemical Engineering, pp. 74 et seq. New York, McGraw-Hill Book Co., 1937. 14. BEALE, E. S. L., AND DOCKSEY, P.: J. Institute of Petroleum, 35, 602 (1948). 15. TROWSE, F. W.: Dissertation. Leeds, 1952. 16. ROUGHTON, F. J. W., AND MILLIKAN, G. A.: Proc. Roy. Soc. (London), 155A, 258 (1936). 17. BOWDEN, F. P., MULCAHY, M. F. R., VINES, R. G., AND YOFFE, A.: Proc. Roy. Soc. (London), 188A, 291 (1947). 18. LEWIS, B., AND VON ELBE, G.: Combustion, Flames and Explosions of Gases, p. 440. New York, Academic Press Inc., 1951. 19. JOST, W., AND TEICHMANN, H.: Naturwissenschaften, 27,318 (1939).
7. KILPATRICK, M., AND McKINNEY, C. D., JR.: J. Am. Chem. Soc., 72, 5474 (1950). 8. KILPATRICK, M., BAKER, L. L., JR., AND McKINNEY, C. D., JR.: J. Phys; Chem., 57, 385 (1953). 9. Discussions of the Faraday Society, No. 17, 1954. 10. BAKER, L., JR., Ignition Delay with Rapidly Mixed Liquid Reactants. Tech. Rept. No. 7, Contract N7onr-32913, August 1954. 11. NEAS, C. C.~ RAYMOND, M. W., AND EWING, C. O.: M. I. T. Report No. 11, DIC 6351, November 1, 1946. 12. PERRY, J. H.: Chemical Engineers' Handbook, 2nd. ed., pp. 800 et seq. New York, McGrawHill Book Co., 1941.
12
MECHANISM OF BURNING OF NITRATE ESTERS By RUDOLPH STEINBERGER ~ Introduction This paper represents part of an effort to determine the mechanism of burning of double-base propellants. These are highly complex mixtures, consisting of nitrocellulose, nitroglycerin, stabilizer, inert plasticizer, and usually other adulterants such as carbon black or inorganic salts. I t has, therefore, been found to be more satisfactory to study simple liquid nitrate esters as models for the more complex system. These compounds exhibit burning behavior very similar to that of double-base powders; burning rate levels, pressure coefficients, and temperature coefficients are all comparable. The chemistry of the combustion process of a nitrate ester is necessarily very involved. The compound must go through many stages of degradation, before the final products are formed. However, the system has one property which is highly useful from the point of view of establishing the general outline of the reaction sequence. This is the presence of a liquid-gas interface. Like any a From the Allegany Ballistics Laboratory, a facility owned by the U. S. Navy and operated by the Hercules Powder Company under Contract NOrd 10431.
other phase change, the transport of material across this liquid surface imposes some very definite demands on the system as regards chemical and physical properties. I t is possible to postulate several different mechanisms by which the phase transition can occur. Each mechanism has different requirements as to temperature, so that, at least in principle, one can distinguish between them on the basis of temperature profiles, with special attention to the temperature in the liquid phase up to the surface. The first postulated mechanism involves volatilization of the liquid, as suggested by Belajev 1 and others. 2, 3 In this case no chemical reaction takes place in the liquid phase, the liquid merely serving as a reservoir of combustible vapor. Under such conditions the temperature reached by the surface is simply a function of volatility. To vaporize the most volatile liquid studied by us, ethyl nitrate, would require a surface temperature of approximately 250~ at 1000 psi. Other less volatile nitrate esters would require surface temperatures up to 500~ at the same pressure, If this mechanism is correct, then no conclu~'toms as to chemical reaction sequence can be drawn from liquid phase temperature profile data. I t
PROPELLANT BURNING
11
A STUDY OF FAST REACTIONS IN FUEL-OXIDANT SYSTEMS Anhydrous Hydrazine with 100 per cent Nitric Acid a By MARTIN KILPATRICK AND LOUIS L. BAKER, JR. Introduction Certain liquid fuels when brought into contact with strong oxidizing agents will ignite spontaneously. The ignition of such systems may take place instantaneously when the liquids are first brought into contact or there may be a time delay. Most of the studies of the ignition delay in such systems have employed methods designed to simulate conditions met in a rocket motor. In a typical rocket motor the two reactants are contacted by the impingement of a number of fuel and oxidant streams in a relatively unconfined area. Observation of the time required for ignition under such conditions does not differentiate between the delay created by the time required to achieve efficient mixing and the delay caused by an actual chemical induction period. Methods have been devised to achieve the complete mixing of useful amounts of two hqmds in a very short time. The work of Hartridge and Roughton, 1 Chance, 2 and McKinney and Kilpatrick 3 has proven conclusively that it is possible to effect the complete intermixing of two liquids and begin observation of the resulting mixture only a few milliseconds after the initial contact. These methods have not previously been applied to the study of the ignition of fuel-oxidant mixtures. The present research is an attempt to apply the mixing methods of Roughton, Chance, and McKinney and Kilpatrick to the problem of the self-ignition of fuel-oxidant systems and to study the associated reactions.
Ignition Delay Studies Most of the impetus for the study of the ignition delay time of liquid fuel-oxidant systems is due to the potential use of such systems in rocket a This research is part of the work being done in the Department ot Chemistry of Illinois Institute of Technology, supported by the Office of Naval Research under Contract N7onr-32913. This paper is abstracted from part of the dissertation submitted by Louis L. Baker, Jr. to the Graduate School of Illinois Institute of "Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
motors. Systems which do not ignite spontaneously can be used successfully in rocket motors if some external means of ignition can be found. From a practical standpoint, however, it is preferable to employ a system that will ignite spontaneously. In such a rocket motor it is imperative that ignition take place in a very short time after contact of the reactants. An excessive delay will cause the unreacted materials to accumulate in the motor chamber; ignition could then cause a destructive explosion. For this reason the study of hypergolic ignition (self-ignition) is of great practical importance in the field of rocket technology. Some of the experimental variables which must be controlled for the precise measurement of ignition delay time include: (1) the reactant ratio, (2) the time required to effect complete mixing, (3) the chemical composition of the reactants, (4) the ambient temperature, and (5) the ambient pressure. Studies of the ignition delay of the systems hydrazine-nitric acid, furfuryl alcohol-nitric acid, and liquid ammonia-hydrazine mixtures-nitric acid have been carried out in a small rocket motor. 4 The reactants were contacted by the impingement of a single fuel stream with a single oxidant stream. The nominal values obtained for optimum test conditions are given in Table 1. The effect of reactant ratio on ignition delay time, however, is not clear under the uncertain mixing conditions resulting from the impingement of two streams in an unconfined space. If the complete intermixing could be effected in a time very short compared to the ignition delay time, then the measured delay would be the true chemical delay or induction period, and a change of the reactant ratio should lead to a different chemical delay time. When ignition takes place before mixing is completed, and the remainder of the fluid ignites, reactant concentrations of this igniting mixture cannot be known. Table 2 makes evident the delay due to incomplete mixing by comparing the results obtained for furfuryl alcohol-white fuming nitric acid by a refined open-cup apparatus due to
197
FAST REACTIONS IN FUEL-OXIDANT SYSTEMS
Gunn 5 with those of the impingement method. The impingement method would be expected to yield smaller ignition delay times because of more efficient mixing than the open-cup method due to greater turbulence and impact associated with impinging streams. The present study was undertaken on the premise that a fundamental approach to the phenomenon of igl~ition delay was possible only if complete mixing could be accomplished in a time shorter than the ignition delay time. Studies in this laboratory 3 had already shown that 1 ml of one reactant could be contacted and effectively mixed with 25 ml of a second reactant in less than 10 msec. I t was, therefore, expected that a few milliliters of each of the reactants could be mixed in much less time than ignition delay times such as those reported in Tables 1 and 2.
Rapid Mixing Studies The problem of rapid and efficient mixing o f dilute solutions has been recently reviewed by Roughton and Chance, 6 and the mixing of liquids for reactions evolving gases investigated by McKinney and Kilpatrick) In general, the mixers are of two shapes, T and Y, the T type being preferred. In the T-type mixer, the jets may be directly opposed or arranged so that the two liquids stream tangentially into the mixing area. The tangential arrangement was found to be better at low flow rates but both methods of contacting the liquids gave about equally efficient mixing at high flow rates. The method of opposing jets was adopted in our earlier studies?, 7. s Our findings that the time for mixing by the use of the streaming method cannot readily be reduced below 0.1 msec seem to be confirmed by the fact that recent discussions of fast reactions9 have emphasized relaxation methods for measuring fast reactions where mixing of the reactants is not involved. Such methods may well be developed for measurement of reactions having halftimes of microseconds. Experimental Methods APPARATUS
The apparatus; which tins been described previously,9, 19 is referred to as Reactor I I to distinguish it from an apparatus employed earlier in this laboratory for the study of fast reactions. Reactor II, in combination with the pneumatic injector of Neas, Raymond and Ewing, 3, n is capable of contacting and mixing a few cubic
centimeters of two liquids in a few milliseconds. The operation of the apparatus will be briefly reviewed. A diagram of the fluid mechanical sysTABLE 1. IGNITION DELAY IN A SMALL ROCKET MOTOR 4 Reactants
Hydrazine, 96 per cent--RFNA* Hydrazine, 96 per cent--WFNAt Furfuryl alcohol --WFNA Furfuryl alcohol-WFNA Hydrazine, 71.5 per cent--WFNA Hydrazine, 71.5 per cent--RFNA Ammonia, 14.1 per cent hydrazine-RFNA Ammonia, 9.5 per cent hydrazine-RFNA Ammonia, 5.0 per cent hydrazine-RFNA
Temperature
Ignition Delay
~ 21
3.1 4- 1.4
21
5.0 4- 1.7
21
16.6 4- 2.4
--29
rasec
22 to 40
25
ca. 37
--48
ca. 37
--36
6 to 10
--35 to --40
ca. 14
--37 to --47
37J;
* RFNA, red fuming nitric acid, 24 per cent NO2 . t WFNA, white fuming nitric acid, 96 per cent HNO~ . :~ Sporadic ignition. TABLE 2. IGNITION DELAY FOR FURFURYL ALCOHOL-WHITE FUMING NITRIC ACID Ignition Delay Time Initial Temp. Open-cup~
Impingement4
~
msec
msec
Room --15 --29
36 180 --
16.6 22-40
tern of the reactor is shown in Figure 1. The pneumatic injector, not shown .in the figure, is located above the reactor. I t functions as a quick opening valve discharging nitrogen gas at pressures up to 2000 psi into the area P behind the driving piston. As the driving piston descends,
198
PROPELLANT BURNING
the two smaller pistons F1 and FI' are driven downward. This design was an improvement over that of Reactor I, giving a substantial hydraulic advantage in having the large piston drive the two smaller ones and insuring simultaneous descent of the two small pistons during injection. The liquids to be contacted are initially contained below the smaller pistons in the areas designated as P1 and PI'. During injection, the liquids flow through the lead tubes P2 and p t, the jets P3 and
[
The reactions under study were followed by several techniques, but mainly by measurement of the transient pressure. The pressure sensitive element was a diaphragm type strain gage manufactured by the Statham Laboratories, Inc. The strain gage is in the form of a balanced bridge, the output being supplied by an AC carrier wave, and frequencies of the order of 1000 cps were used. The unbalance of the bridge was amplified and applied to the vertical plates of a cathode-ray oscilloscope. I t was found by calibration against known static pressures that the amplitude of the oscilloscope pattern was accurately proportional to the applied pressure. The natural frequency of the strain gage was stated by the manufacturer to be in excess of 2000 cps so that pressure events occurring in a time of the order of a millisecond should be accurately reproduced. The horizontal plates of the oscilloscope were connected to a single sweep generator which was triggered automatically by the closing of a contact located in the penumatic injector. The oscilloscope sweep normally began between 1 and 1.5 msec before the beginning of injection. The record of the transient pressure was obtained by photographing the screen of the oscilloscope. Another method of following the reactions was by the measurement of light emission. The output of a vacuum phototube-amplifier combination was connected to the vertical plates of a second oscilloscope. The horizontal plates were driven by the same sweep generator used for the transient pressure recording system. In some cases, the time distribution of the light emission was obtained; in others, only the time corresponding to the onset of light emission. Simultaneous measurements of the transient pressure and the light emission were made possible through the use of an additional camera. The progress of the injection could be followed by means of an electronic timer. The timer consisted of a serrated brass rod connected to the moving piston system. A light beam passing between the serrations on the rod was focussed on a phototube cathode. During injection, the serrations on the moving rod interrupted the beam and caused a varying light intensity to fall on the phototube, and the amplified output from t h e phototube was applied to the vertical plates of an oscilloscope. An analysis of the resulting oscilloscope trace gave the time corresponding to a number of positions of the pistons with respect to the stationary light beam. The timer was operated simultaneously with the
~~ DRIVING L PI
~
PIS'TON \ ~
GAS ESCAPE
i
', !
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IF[G. 1. Schematic diagram of fluid mechanical system of Reactor II. P / , and mix in the exit tube P4 9 The mixed solution is then ejected into a closed volume of 339 cc where reaction occurs. The reactor was designed to allow for immersion in a low temperature bath. The volume ratio of the reactants could be varied over a range from approximately 4:1 to 1:1 by the use of fittings and additional pistons. The mixing pattern could be varied by using additional mixing plates of differing design. For this study, a T-type mixing plate designed in accordance with the principles set forth by Roughton and Chance was used. t h e simultaneous descent of the two small pistons insured uniformity of the mixing.
199
FAST REACTIONS IN FUEL-OXIDANT SYSTEMS
transient pressure measuring system to give a record of both the injection process and the pressure rise. The final pressure of the product gases was measured by venting the reactor to a glass reservoir by means of a needle vave and measuring the pressure on a mercury manometer. The ignition delay of the fuel-oxidant systems was determined by simultaneous injection and transient pressure measurements. The time between the beginning of the injection process (first contact of the reactants) and the beginning of the pressure rise was obtained with an accuracy of about -4-0.5 msec. The observed delay had to be corrected for the time required for the first pressure pulse to travel from the reactor to the diaphragm of the strain gage, a distance of about 18 in. If the initial pressure pulse was propagated at the speed of sound in ordinary air, it would have required 1.3 msec. This value was used for the correction of the observed data. Even if the pulse were propagated as a shock wave, it it was unlikely that the pulse would have reached the gage in a significantly shorter time because of the irregular path. Simultaneous light emission and transient pressure measurements were made to establish the fact that the beginning of the pressure rise and the onset of light emission occurred simultaneously. Again the experimental results had to be corrected for the transit time of the initial pressure pulse. RATE OF INJECTION
A number of measurements were made of the injection rates obtainable when water was injected into the mixing area from both cylinders of the reactor. The runs were made with the 1 : 1 ratio, the stroke of the piston system being set at 7~ in, and approximately 1.i cc of water was ejected from each cylinder. Some of the data for different driving pressures are shown in Figure 2. The linearity of the plots indicated that the rate of descent of the piston system and hence the injection rate was constant throughout injection. The linearity further indicated that the moving parts accelerated very rapidly to the equilibrium velocity. The data obtained for the injection of water are shown in Table 3. The rate of injection is expressed as the time required for the pistons to descend 7/~ in. Runs 114 to 120 inclusive were made with loosely fitting neoprene piston gaskets and runs 123 and 124 with very tight gaskets, in order to test the effect of gasket resistance. The
gasket resistance had only a small effect on the measured injection rate. Two runs, 73 and 98, were made using the 4:1 injection ratio. In this case, 2.84 cc of water was injected from the large cylinder and 0.74 cc from the small cylinder of the reactor. O.875
o,Ts0 -
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DRIVING PRESSURELB. PER SGt. IN. --
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FIG. 2. I n j e c t i o n r a t e m e a s u r e m e n t s ,
R e a c t o r II
T A B L E 3. INJECTION R A T E MEASUREMENTS WITH WATER Run
Reactant Ratio
Pressure of Driving Gas
Injection Time
114 119 123 124 117 116 120
1:1 1:1 I:I 1:1 1:1 1:1 1:1
1950 1900 1700 1700 1400 1000 600
1.79 1.71 1.91 1.91 1.96 2.21 2.84
73 98
4:1 4:1
1800 1700
4.27 4.31
A rough calculation of the injection rates was made based upon empirical relations found in standard engineering treatisesY. ,s The values calculated for the 1:1 ratio for the water injection are shown in Figure 3 as the dotted line. I t is evident that the calculations predict a much more rapid injection rate than that observed. The calculations indicate t h a t the flow rate should be roughly proportional to the square root of the driving gas pressure, and Chance 2 obtained in-
PROPELLANT BURNING
200
jection rates that were precisely proportional to the square root of the pressure drop using identical mixing systems but at much smaller pressure drops. I t seemed likely, in view of the curvature of the observed plot, t h a t the resistance to gas flow in the pneumatic injector might be slowing the injection. If this were the case, there would have been a pressure drop due to the nitrogen flow from the gas storage reservoir of the pneumatic injector to the driving cylinder, and the pressure
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ROOT OF PRESSURE" LB. PER $Q. IN.
FIo, 3. Linear flow rate in the exit tube of Reactor I I as a function of the square root of the driving gas pressure. actually driving the large piston would then have been less than the value recorded. A rough calculation of the pressure drop was made according to a method outlined by Beale and Docksey, z4 and the measured driving pressures for the runs plotted in Figure 3 were corrected for the pressure drop. The corrected d a t a show much better proportionality between the injection rate and the square root of the driving gas pressure, although the corrected data still show a slower flow rate than those predicted b y the fluid mechanical calculations. These calculations were made on the basis t h a t the injection
rate was determined entirely by the resistance to fluid flow and not by the gasket resistance. RATE O F MIXING
The rate at which two reactants are contacted is not necessarily equal to the rate at which they are mixed. Reactor I I has been applied to the study of the reaction of sodium-potassium alloy and water. 8 For this reaction it was found t h a t the pressure rise, corresponding to hydrogen evolution, was completed in a time nearly equal to the measured time of completion of the injection. This would suggest that the mixing rate was nearly equal to the injection rate. A number of previous investigations have been made of the efficiency obtainable for the mixing of equal volumes of reactant solutions in T - t y p e mixers} These studies indicate that the reactants should be thoroughly mixed in our reactor. I n our reactor the mixing plate used has two jets of 2.5 mm diameter discharging into an exit tube of 3.9 mm diameter. A T-type mixer described by Roughton ~ and due to Trowse 15 also employs two jets. The diameter of the jets and the exit tube (observation tube) was 0.7 mm. Trowse found that the reactant solutions were 97 per cent mixed at a point 5.7 mm downstream from the jets. His measurements were made at a linear flow velocity at the exit tube of 440 cm/sec. Millikan1~ studied the extent of mixing in another T - t y p e mixer as a function of the flow velocity. He found that the distance downstream to the point of 97 per cent mixing rapidly moved toward the point of first contact as the flow velocity increased. The linear flow velocities in the exit tube of our reactor ranged from 5,000 to 11,000 cm/sec so that mixing should be complete within the 8 mm length of the exit tube of the reactor used in the present study. To further insure the completeness of mixing, the stream emerging from the exit tube impinged on a baffle plate only 25 mm from the exit tube. The time of residence of the stream in the exit tube of the reactor for the total range of flow velocities varied between 0.07 and 0.16 msec. For the study of ignition delay, the injection rate is of interest only as it affects the mixing efficiency. I t is only the time between the contact and the completion of mixing of a given fluid element that is of importance, since presumably the first element of fluid to be injected is the one t h a t will first ignite.
FAST PREPARATION
REACTIONSIN FUEL-OXIDANT SYSTEMS Experimental Results
OF MATERIALS
Nitric acid The acid was prepared by the vacuum distillation of an equal volume mixture of commercial 95 per cent nitric and sulfuric acids in an allglass apparatus. The mixture was cooled to 0~ before reducing the pressure to 10-4 mm Hg, and the anhydrous nitric acid was collected in a trap cooled by a mixture of dry ice and trichloroethylene at a rate of 2 cc per hour. The acid, which consistently analyzed between 100.2 and 100.8 per cent HN03 by titration, was stored at dry ice temperature to avoid decomposition.
Hydrazine The hydrazine was prepared from commercial anhydrous hydrazine, about 95 per cent N2H4, by refluxing in an all-glass distilling apparatus with an equal quantity of calcium or barium oxide at approximately 50 mm Hg. The hydrazine was then fractionally distilled, the first half collected, and analyzed by direct titration with HC1 and by the iodate method using wool red as an internal indicator. The material was found to be 97.7 to 99.0 per cent N2H4.
Aniline Aniline was purified by distillation from zinc dust. The product was colorless and distilled at 182~ under a pressure of 750 mm Hg. The refractive index was found to be 1.4791 at 32.5~
Hydrogen peroxide High strength hydrogen peroxide was analyzed by titration with ceric sulfate and found to contain 84.5 per cent H~02 by weight.
Ammonia-hydrazine mixtures Anhydrous hydrazine was weighed into a small Erlenmeyer flask fitted with a ground glass joint, attached to the vacuum line, and the system evacuated after freezing in a liquid nitrogen bath. Anhydrous ammonia was transferred from a tank to a flask of known volume, the amount being determined by the gas pressure, and condensed into the flask with the hydrazine. The procedure was repeated until the desired amount of ammonia had been added and th~ mixture was stored at the temperature of dry ice.
201
T H E H Y D R A Z I N E - N I T R I C ACID R E A C T I O N
A typical simultaneous measurement of injection and transient pressure for a near stoichiometric mixture of hydrazine and nitric acid is shown in Figure 4. The pressure rise began 2.2 msec after the beginning of injection. Correcting by the 1.3 msec required for the pressure impulse
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i.
10
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FIO. 4. Simultaneous measurement of injection rate and pressure rise in the reaction of one and one-half volumes of nitric acid with one volume of hydrazine, run 142, Reactor II. TABLE
4.
TBANSIENT
1V[EASUREMENTS.
PRESSURE
AND I N J E C T I O N
HYDRAZINE-NITRIC
ACID
REACTION Volume HNO,
'olume N~H4
cc
cc
2.84 1.59 1.11 1.11 0.74 0.74
0.74 0.74 0.74 1.11 1.11 1.59
Mole [ MaxiRatio mum HNO3: Pressure N~H4 Rise
Final Pressure
Overall Injection Time
Ignition Delay
arm
ra se c
mJec
2.87 [ 23.4 1.61 I 31.6 1.12 31.7 0.75 46.7 0.51 43.2 0.352 51.3
2.66 2.21 1.80 3.35 3.57 5.26
9.2 5.4 4.9 4.7 4.6 6.4
0.2 0.5 0.9 0.1 0.2 0.4
/
to reach the diaphragm of the strain gage, the ignition delay was 0.9 msec. The average ignition delays for all of the ratios studied of the hydrazine-nitric acid reaction are given in the last column of Table 4. The maximum pressure rises obtained from the transient pressure record are given in the fourth column and the final pressures calculated from the manometer reading, in the fifth column. The plot of piston displacement vs. time in Figure 4 showed a definite break in the slope. No such break was observed in any of the results obtained for the injection of water. The sudden slowing of the injection rate may be attributed to
202
PROPELLANT BURNING
the ignition of the reaction, and the constancy of the injection velocity near the completion of
.~
overall injection times are given in the sixth column of Table 4. The simultaneous measurements of the transient pressure and the light emission showed that the light emission preceded the pressure rise by 0.2 to 1.6 msec. After correction for the transit time of the initial pressure pulse, it was concluded that the beginning of the pressure rise coincided, within experimental error, with the beginning of visible light emission. A typical result of the simultaneous measurement of light emission and transient pressure is shown in Figure 5 for the reaction of nitric acid with a slight excess of hydrazine.
as
THE HYDRAZINE-HYDROGENPEROXIDE REACTION
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15
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20
25
Fro. 5. Pressure rise and light intensity in reaction of two volumes of hydrazine with one volume of nitric acid, run 152, Reactor II. TABLE
5.
IGNITION
PRESSURE
DELAY
BY T R A N S I E N T
MEASUREMENTS Mole o
Run
Fuel
Oxidant [ant-toFuel
Max imur Pres sure
Ignition Delay
arm
157 Aniline 158]
Nitric acid
8.18
42 25
2.3 1.2
163 Hydrazine 165 162
Hydrogen peroxide
1.15
42 55 58
0.1 1.1 2.2
166 Hydrazine 167
Hydrogen peroxide
2.46
32 41
1.3 1.7
injection suggests t h a t a stable flame front was produced within the exit tube before the injection was completed. Because of this slowing of the injection rate, the overall injection time was substantially longer than the values obtained for the injection of water. The average values of the
The results for the hydrazine-hydrogen peroxide reaction are reported in Table 5, and the results of the simultaneous measurement of injection and transient pressure for two runs at oxidant-to-fuel ratio of 1.15 are shown in Figures 6 and 7. In run 163 the change in the injection rate is again apparent, but was not found in run 162, and this is interpreted to mean that here the ignition did not occur until injection was completed. For the other run made at this same oxidant4ofuel ratio the results were not conclusive. As shown in Table 5, the ignition delays reported for the 1.15 oxidant-to-fuel ratio were less consistent than those for the 2.46 ratio. In the latter case, ignition occurred during the course of injection. Simultaneous measurements of the light emission and the transient pressure for both reactant ratios showed that the beginning of visible light emission coincided with the beginning of the pressure rise. THE ANILINE-NITRICACID REACTION Table 5 also shows two runs for the reaction of aniline with nitric acid. The mixing plate was damaged in both of these runs. I t is likely t h a t during ejection, ignition was delayed and the flame may have flashed back with detonation velocity to the exit tube. THE REACTION BETWEEN LIQUID AMMONIAHYDRAZINE MIXTURES AND NITRIC ACID For these studies the reactor was immersed in a low temperature bath, and the temperature before injection was measured by a thermocouple located close to the reactant solutions. Since injection rate measurements could not be made
FAST
REACTIONS
iN
with the reactor immersed in the bath, ignition delay data were obtained from simultaneous measurements of the transient pressure and the light emission. The results are reported in Table
FUEL-OXIDANT
203
SYSTEMS
tained. This value is of the order of magnitude of the minimum ignition energy for gaseous mix~ t~
~o
6.
I t was necessary to utilize the fact that injectiou began from 1.0 to 1.5 msec after the contact closed in the pneumatic injector. This was established from observations made under conditions where injection measurements could be made. An average value of 1.2 msec was taken as the time interval. It can be seen from the data that ignition occurred with a delay of only 2.9 msec at a concentration of only 2.7 per cent hydrazine although in two other runs no ignition occurred. A run was made of the reaction of nitric acid with pure ammonia and no ignition occurred.
t 75 ~: as ~ ~ 5O ~ = ~ 2s ~s /(, I I I [ 0 2 4 Q 8 IO 12 o TIME - MSEC. FIG. 6. Simultaneous measurement of injection rate and pressure rise in the reaction of equal volumes of hydrogen peroxide and hydrazine, run 163, Reactor II.
o
Discussion
The longest ignition delay time observed for any of the reactions studied was 2.9 msec. This is so much shorter than the results by other methods (see Tables 1 and 2) that one can only conclude that the other methods do not give efficient mixing. From our results it cannot be concluded that we are measuring the rate of a homogeneous, thermal chemical reaction as this type of reaction should require an energy of activation, and the igntion delay time appears to be independent of the initial temperature of the reactants, for the range - 4 0 ~ to 30~ The actual temperature just before ignition may be of the order of several hundred degrees and a difference of seventy degrees in the initial temperature may not result in sufficient difference in the temperature of the reactants just before ignition when the mixing is efficient. An unignited mixture of a fuel and oxidant such as hydrazine and nitric acid may be likened to a liquid explosive such as nitroglycerine. Nitroglycerine can be ignited by impact alone. Bowden et alY have studied the ignition of nitroglycerine by impact. They found that thin sheets of the material on an anvil would not ignite even under the influence of a rather strong hammer blow unless gas bubbles were present in the liquid. These authors concluded that the high temperatures produced by the adiabatic compression of the bubbles were responsible for the ignition. It was possible to estimate crudely the minimum ignition energy required to ignite the material by estimating the size of the bubbles and the compression ratio, is A value of 10-8 calorie was ob-
~
~so
r. ?.5
o
I$
o
I
0
2
/I
I
4
I
6
TIM•
8
I
10
~,2
- MSEC.
FIG. 7. Simultaneous measurement of injection rate and pressure rise in the reaction of equal
volumes of hydrogen peroxide and hydrazine, run 162, Reactor II. T A B L E 6. IGNITION D E L A Y BY L I G H T EMISSION MEASUREMENTS. E Q U A L VOLUMES OF NITRIC ACID WITH H Y D R A Z I N E - L I Q u I D AMMONIA MIXTURES Run
Hydrazine in Mixture W I per cent
Temp.
Ignition Delay
~
mser
Maximttm Pressure aim
174 175
9.0 9.0
--35 --33
1.0 1.4
41 41
176 177 178
2.7 2.7 2.7
--38 --41 --41
2.9 --* --*
31 3.5 3.5
*No ignition. tures found by more precise methods involving spark ignition,is It is thus reasonable to suppose that the impact and turbulence associated with the mixing process in Reactor I I could have re-
204
PROPELLANT BURNING
sulted in immediate or nearly immediate ignition. This conclusion also results from the failure to observe a systematic increase of the ignition delay as the hydrazine concentration was decreased in the hydrazine-ammonia mixtures. The ignition delay increased to only 2.9 msec for the mixture containing 2.7 per cent hydrazine. The runs made on this mixture that failed to ignite indicated that the impact energy was just insufficient to ignite this mixture. Instead of observing a longer and longer delay as the hydrazine concentration or the temperature was decreased, a point was reached where the mixture simply failed to ignite. Such behavior is more indicative of ignition by mechanical shock than it is of thermally initiated ignition showing an induction period due to the rate of a chemical reaction. A study of the ignition delay of gaseous mixtures of hydrocarbons and oxygen was made by Jost and Teichmann 19 by a rapid compression method. They obtained a straight line when the logarithm of the ignition delay was plotted against the reciprocal of the reaction temperature. The plot was linear from delay values of a few milliseconds to over five seconds. The fact that the data could be represented by an Arrhenius equation indicates that the delay-determining factor was due to the rate of a chemical reaction. The experimental results obtained in the present study show, nevertheless, that fuel-oxidant systems previously thought to have long ignition delays will ignite in a very short time under violent mixing conditions. The measured explosion pressures shown in Table 4 are much lower than those calculated by the thermodynamic method outlined in a treatise by Lewis and yon Elbe. The calculations indicated an equilibrium pressure of the order of 90 atm. Analysis of the reaction products showed quantities of nitrogen oxides greatly exceedir~g that expected from thermodynamic considerations alone. These facts indicate that the reaction was quenched through contact with the metal parts of the reactor. Conclusions
(1) A constant volume reactor was designed to rapidly contact and mix two liquid reactants in a volume ratio ranging from 4:1 to 1:4. (2) The reactor was easily adapted to a change of the mixing pattern and could be immersed in a thermostatic bath. Satisfactory operation was obtained at temperatures as low as -40~
(3) A photoelectric method was devised to follow the course of the injection. The injection of 2.2 cc of water was achieved in 1.7 msec. A somewhat longer time was required for the injection of reacting fuel-oxidant systems because of the thrust created by the reaction process. (4) Reactions were studied in the reactor by following the transient pressure and the light emission. (5) The rate of mixing in the apparatus was shown to be very rapid by reference to measurements made by other investigators who employed geometrically similar mixing patterns. (6) The ignition delay for certain reactant ratios of several fuel-oxidant systems was measured and found to be less than 3 msec in every case. The systems included: (a) Hydrazine vs. nitric acid; (b) Aniline vs. nitric acid; (c) Hydrazine vs. hydrogen peroxide; (d) Hydrazineliquid ammonia mixtures vs. nitric acid. The delays were significantly lower than those observed by other investigators who employed mixing methods where the efficiency of mixing was most uncertain. (7) The very short ignition delays were attributed to (a) the rapid rate of mixing in the apparatus, and (b) the impact and turbulence associated with the mixing process. (8) There was a negligible effect of the initial temperature of the reactant or of the hydrazine concentration on the ignition delay time of the reaction with liquid ammonia-hydrazine mixtures with nitric acid. This result was more indicative of ignition by impact than it was of ignition controlled by the rate of a homogeneous chemical reaction. REFERENCES 1. HARTRIDGE, H., AND ROUGHTON, F. J. W.: Proc. Roy. Soc. (London), IO~A, 376 (1923), et seq. 2. CHANCE, B.: J. Franklin Inst., ~9, 455, 613, 737 (1940). 3. McKINNEY, C. D., JR., AND KILPATRICK, M.: Rev. Sci. Insts., ~$, 590 (1951). 4. KELLOGGCo.,M.W.: Preliminary Experimental
Studies of Liquid Fuel Systems, Final Report. Report No. SPD 236, May 20, 1949, pp. 88, et seq. 5. GuNs, S. V.: J. Am. Rocket Soc., ~$, 33 (1952). 6. ROUGHTON,F. J. W., ANn CHANCE,B.: Investi-
gations of Rates and Mechanisms of Reactions, Technique of Organic Chemistry, Volume VIII, ed. A. Weissberger, p. 669. New York, Interscience Publishers, Inc., 1953.
MECHANISM OF BURNING OF NITRATE ESTERS
205
13. WALKER, W. H., LEwis, W. K., MCADAMS, W. H., AND GILLILAND, E. R.: Principles of Chemical Engineering, pp. 74 et seq. New York, McGraw-Hill Book Co., 1937. 14. BEALE, E. S. L., AND DOCKSEY, P.: J. Institute of Petroleum, 35, 602 (1948). 15. TROWSE, F. W.: Dissertation. Leeds, 1952. 16. ROUGHTON, F. J. W., AND MILLIKAN, G. A.: Proc. Roy. Soc. (London), 155A, 258 (1936). 17. BOWDEN, F. P., MULCAHY, M. F. R., VINES, R. G., AND YOFFE, A.: Proc. Roy. Soc. (London), 188A, 291 (1947). 18. LEWIS, B., AND VON ELBE, G.: Combustion, Flames and Explosions of Gases, p. 440. New York, Academic Press Inc., 1951. 19. JOST, W., AND TEICHMANN, H.: Naturwissenschaften, 27,318 (1939).
7. KILPATRICK, M., AND McKINNEY, C. D., JR.: J. Am. Chem. Soc., 72, 5474 (1950). 8. KILPATRICK, M., BAKER, L. L., JR., AND McKINNEY, C. D., JR.: J. Phys; Chem., 57, 385 (1953). 9. Discussions of the Faraday Society, No. 17, 1954. 10. BAKER, L., JR., Ignition Delay with Rapidly Mixed Liquid Reactants. Tech. Rept. No. 7, Contract N7onr-32913, August 1954. 11. NEAS, C. C.~ RAYMOND, M. W., AND EWING, C. O.: M. I. T. Report No. 11, DIC 6351, November 1, 1946. 12. PERRY, J. H.: Chemical Engineers' Handbook, 2nd. ed., pp. 800 et seq. New York, McGrawHill Book Co., 1941.
12
MECHANISM OF BURNING OF NITRATE ESTERS By RUDOLPH STEINBERGER ~ Introduction This paper represents part of an effort to determine the mechanism of burning of double-base propellants. These are highly complex mixtures, consisting of nitrocellulose, nitroglycerin, stabilizer, inert plasticizer, and usually other adulterants such as carbon black or inorganic salts. I t has, therefore, been found to be more satisfactory to study simple liquid nitrate esters as models for the more complex system. These compounds exhibit burning behavior very similar to that of double-base powders; burning rate levels, pressure coefficients, and temperature coefficients are all comparable. The chemistry of the combustion process of a nitrate ester is necessarily very involved. The compound must go through many stages of degradation, before the final products are formed. However, the system has one property which is highly useful from the point of view of establishing the general outline of the reaction sequence. This is the presence of a liquid-gas interface. Like any a From the Allegany Ballistics Laboratory, a facility owned by the U. S. Navy and operated by the Hercules Powder Company under Contract NOrd 10431.
other phase change, the transport of material across this liquid surface imposes some very definite demands on the system as regards chemical and physical properties. I t is possible to postulate several different mechanisms by which the phase transition can occur. Each mechanism has different requirements as to temperature, so that, at least in principle, one can distinguish between them on the basis of temperature profiles, with special attention to the temperature in the liquid phase up to the surface. The first postulated mechanism involves volatilization of the liquid, as suggested by Belajev 1 and others. 2, 3 In this case no chemical reaction takes place in the liquid phase, the liquid merely serving as a reservoir of combustible vapor. Under such conditions the temperature reached by the surface is simply a function of volatility. To vaporize the most volatile liquid studied by us, ethyl nitrate, would require a surface temperature of approximately 250~ at 1000 psi. Other less volatile nitrate esters would require surface temperatures up to 500~ at the same pressure, If this mechanism is correct, then no conclu~'toms as to chemical reaction sequence can be drawn from liquid phase temperature profile data. I t