Cellular structure of detonation in nitromethane

Combustion and Flame

117

Cellular Structure of Detonation in Nitromethane* P. A. Urtiew, A. S. Kusubov, and R. E. Dufft Lawrence Radiation Laboratory, University of California, Livermore, Calilbrnia. U.S.A.

Wave-trace records presented here arc of detonations in nitrometbane-acetone mixtures and are identical to traces lbund in gaseous detonations on carbon-soot-covered sidewalls. The method of obtaining records is described, and results are illustrated and compared with those available in existing literature. Having duplicated oneof the most useful tools o~ experimental ga~ynamies, we made a direct comparison between the processes taking place in liquid and in gaseous dctonabl¢ mixtures. Conclusions are that the processes in the two media are indeed similar, and that the structure of detonation in netromethane is cellular.

Introduction O u r investigation shows beyond any reasonable d o u b t the qualitative similarity between detonation waves in gaseous mixtures and in homogeneous liquid explosives. As a result o f recent efforts the structure o f gaseous detonations has been resolved and described as a nonuniform and inherently unstable w a v e front [ I ] . In nitromethane, the nonuniform n a t u r e o f detonation was observed first by C a m p b e l l et al. [2] and then by Dremin [3] and his co-workers and by Mallory [4]. Shchelkin [ 5 ] was the first to suggest that processes in condensed explofives should be very similar to those found in gaseous mixtures; * Work performed under the auspices of the U. S. Atomic Energy Commission. ~; Present address: Systems, Science. and Software. La Jolla. California, U.S.A.

that is, the detonation wave should be an unstable shock-wave-initiated combustion process locally intensified by collisions o f oblique waves. A l t h o u g h the investigations mentioned seem to prove Shchelkin's postulate by showing clear evidence o f ripples on the wave front and nonuniform burning o f the mixture throughout the cross section, none o f the restilts presented is similar to that obtained in gaseous detonations. T h u s one cannot say unambiguously that processes in condensed explosives and gaseous mixtures are identical. Z i m m e r ' s [ 6 ] recent observation o f indentations on the sidewalls o f a polyethylene container in which a low-velocity detonation prc.pagated in nitroglycerin indicates nonunilbrm distribution o f pressure. But here again the traces were very irregular and did not resemble those found in gaseous detonations, Comlm,~tio~' & Ftunw, 14. I I7- [22(1970) Copyright 1970by The Combu~;tioaInstitule Published by AmericanElsevierPublishingColapany, Itlc

118 During a seri¢~ of recovery experiments we discovered that diluted nitromethane detonations can produce "wall trace" records that are identical to the soot records found in gaseous mixtures. Since a direct comparison between similar records for the two media was possible, it is reasonable to assert that processes in both cases ~lre indeed similar, and that the structure of detonation in the condensed explosives is cellular.

Experiment To discern the details of the detonation wave's complexity, it is usual to work with marginal detonations in which various details in the process are far enough apart to permit their resolution with available instrumentation. In gaseous media, marginal detonation is easily obtained by varying initial pressures, composition, or geometry. The apparatus is seldom, if ever. damaged in an experiment. In condensed explosives, however, pressure is always atmospheric and a new container must be built every time, thus slowing the rate at which meaningful data can be obtained. This, in part, is the reason why work on condensed explosives is still rather I;mited. As mentioned, in the experimental technique we employed the recovered sidewalls of the container display traces of the detonation wave. The choice of material and the surface prepara• tion of the tube proved very important. The best results were obtained with grade 304 stainless steel. Polished surfaces of softer materials such as aluminium and brass provided some evidence on nonuniformities, in the form of pitting on the surface, but did not leave any visible fish-scale patterns. Matted surfaces, however, recorded the progress of detonation by trapping some of the available carbon, thus forming a nice contrast to the white traces• The four sides of the tube were carefully machined to a fine finish (0.4 p). then hydrohoned (sandblasted by a stream of water) to

P. A. Urtiew.A. S, Kusubov.and R. E. Duff achieve a matted appearance, and finally glued into a square tube with an epoxy resin. One end of the tube was sealed with a 2 × 10 -2 em aluminum foil. while the other end was left open for filling. Initiation was accomplished by placing a small charge o f C-4 solid explosive against the aluminum foil and ignitir,g it with an electric detonator. To recover the sidewalls, the best results were achieved by surrounding the whole test chamber with a layer of lead bricks or, even better, by encapsulating the tube with a layer of molten Seraban (Wood's metal). The confining materials act as a momentum trap, thus protecting the tube from excessive defm'mation and dispersion from the firing point.

Observation The record shown in Fig. I is a typical sidewall trace for a 75 : 25 nitromethane-aeetone mixture in a square tube of 2-em cross section and 25-cm length. Its gaseous counterpart, shown in Fig. lb* for comparison, is a carbon-soot record of detonation propagating through a rectangular tube of 2.54x3.81-cm cross section filled initially with equimolar hydrogen-oxygen mixture to a pressure 0f88 m m Hg at room temperature. Both records display an identical eriss-cross pattern o f diagonal traces. In the liquid detonation (Fig. la), however, the traces appear somewhat fuzzier than those in the soot record in Fig. lb. This may be due to the difference in the mechanism of inscribing the pattern, in one case on ~1thin layer of loosely deposited carbon soot, and in the other on a solid steel surface. The similarity ,of the traces that appear on the steel surface with those found on carbon soot is somewhat better illustrated in Fig. 2. Here the initial pattern of the traces can be ascribed to a particular phenomenon known to have taken * By courtesy of Professor A, K. Oppenheim. in whose laboratory this record was obtained by one of the authors.

Cellular Structure of Detonation in Nitromcthane

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(a)

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Figure I. Wall ~raccs of a detonation wave propagating through (a) nitromethane-acctonc mixture (75:25"/r~ by volume);(b) equimolar hydrogen-oxygen mlxturc. Pu= 88 mm Hg place during the initiation transient. In this case, the area of the C-4 explosive (2~-cm in diameter) was smaller than the 2~cm square cross-section area of the tube, Thus the nitromethane-acetone mixture (80:200/0 by volume) located in the corners was initiated by the diffracted shock wave from the booster pellet. Upon reflection in the corners, the diffracted wave became a prominent transverse disturbance, which, in

analogy with known situations in the gaseous media, leads to formation o f a Mach wave. The traces secn in Fig. 2 are quite similar to those known to be paths of triple-wave intersections, which are well known in gasdynamics, and whose ability to leave traces on the sootcovered walls has also been proved E7J. Although not as clearly visible as in Fig. la, the process recorded in Fig. 2 has also reached a

Figure 2. Wall tracesinscrlbcdby triplc-wavcintersections,

which forrncd in the corners duringthe transitionof detonation l¥omthe C-4 explosiveigniter(2,S-era.diam. cylinder) inlo a 2,5-crn-squure tube filledwithan g0:20°/i)nitromethane.acctone mlxturc.

120 steady state near the end of the tube and has left traces forming cells of 1.5-2.5 ram, which are characteristic of the initial mixture and the internal dimension of the tube. As the initial conditions are varied by decreasing the tube cross section or increasing the amount of acetone dilutant, the cell size grows bigger. For this reason one would expect that eventually a critical case may be reached in which only one trace, traversing the sidewall in a zigzag pattern, is present. Although such a limiting condition is easily attainable in most detonable gaseous media, a steady-state, singlemode detonation in the nitromethane-acetone mixture has not been observed. This is probably due to the I~ck of a one-dlmerJsional confinement characteristic of the detonation in the condensed explosives. A similar difficulty has

Figure 3. Wall traces of u chokingdetonation propagating through a I-era-square tube of iron filledwith an 80 :20%, nitromethane-acetonemixture.

P. A. Urticw,A. S. Kusubov,and R. E, Duff'

also been observed in maintaining a spinning gaseous detonation wave in weak glass tubes 1"8], and it is quite consistent with the theories of Manson [9] and Fay [10]. Marginal detonation, however, was observed in a transient state when the initial conditions and the confinement were such that the process decayed to extinction. A typical record of a decaying wave is shown in Fig. 3. Here the composition was the same as in Fig. 2 (200/0acetone with 800/0 nitromethane), but the tube cross section was decreased to 1.0 cm square. The C-4 explosive pellet used to initiate the process overdrove the detonation wave, which then had to attenuate rapidly toward its own steady state. This state was not attained, and the process literally died out. O f interest, however, is the fact that in all such cases, just before the extinction occurred, the cell size became comparable to the tube width. This observation suggested that a container with a variable width would help in determining the size of the nonuniformity in pure nitromethane. For this purpose, a wedge was constructed in which the explosive thickness varied from 3 mm to zero over a distance of 25 cm. Its width was held at 25 cm to avoid the wall effect from the other two sidewalls. The explosive was detonated at the 3-ram side o f the wedge so that the detonation wave would propagate through a converging two-dimensional chamber and find its own region of extinction, The results are shown in Fig. 4, where the traces marking the extinction begin to show about 7 em from the end with zero width. Inscribed lines were introduced to mark a I-cm grid for reference. If it is correct that the extinction occurred when the cell size became comparable to the tube size, and if there were no special effects related to the two-dimensional nature of this experiment or to confinement, the size of the nonuniformity in pure nitromethane is of the order of 0.6.-0.7 ram.

Cellular Structure of Detonation in Nitrometh~ne

121

Figure 4. Choking of a pure nitromethane detonation propagating through a wedge of variable height 3-.0 ram. Mttrginaltraces belbre extinctionare visible5-7 ¢m £rom lilt:end. Correlation One of he reasons for using nitromethaneacetone mixtures in this investigation was to permit comparison of these results with those published previously by Dremin [3]. However. an exact correlation is not possible because different observation techniques and experimental geometries were used in the two investigations. Dremin used round tubes 32 mm in diameter, and larger, plus a smear camera to photograph luminous nonuniformities of the detonation front through a thin slit in the end wall of the test chamber. This gave him the necessary resolution on the film and permitted him to use a large range of charge sizes,

Because the essential part of the present investigation was the recovery of the sidewulls. the amount of explosive was rather importunl and the tube cross section was. thereJbre, hckl below 3 em square. The sqtture geometry of the cross section was chosen primarily for prattle:it reasons, mainly because it simpfifies surface preparation. Nevertheless. u qualitative correlation of lhe comparable results is feasible and informative. Figure 5 plots cell size v.'rsus charge size, which in the present investigatinn is the dimension of the side in ihe square c~'oss section, alld ill Dremin's experiments is the inside tube diumeter. Cell slze is defined as the distance between the

P, A. Urtiew,A. $. Ku~uL,ov,and R. E, Duff

122 i~nt:

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d'62mm

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¢ho~ ,rz, -- em Figure5. Variationof cell sizewiththe internaldimensionof the tube and the amount of acetone in nitromethane. Dremln's results are included for comparison.

tWO parallel lines measured pelpendicular to the tube axis For the multiheaded detonation this corresponds exactly to Dremin's definition. din. which is the ratio of the tube diameter to the number of bright or d,~rk bands appearing on the film. We estimated the dashed line corresponding to the 20% acetone concentration from information given by Dremin for the 62mm-diameter charge, as shown in the insert in Fig. 5. The region of uncertainty, which in the present case is the variation of cell size found on the same record, is indicated by error flags, Aside from this small spread of cell sizes, which may very well be due to the wall effects not present in Dremin's experiments, there is good agreement of results. Conclusions From these preliminary findings, which yield clear evidence of the similarity between detonation processes in gaseous and homogeneous liquid explosives, one car~ deduce that in nitromethane the detonation wave is indeed nortuniform and inherently unstable. The structure of the wave is similar to that of gaseous detonations and is cellular. At any instant of time. each individual cell is bounded by ~vuvediscontinuities that are in continuous transverse motion and collision. On the two-dimensional plane, such as Our tube sidewalls, these bounda-

aries appear as triple-wave inter':~ections whose trajectories are inscribed at, traces on the wall surface. Although the present work does not contain complementary measurements of wave velocities or local pressures, the traces alone seem to indicate that continuous changes of localstate properties are brought about by constant interaction of the transverse waves. These waves also seem to have a significant effect on the overall sustenance of the process. This is even more evident in the extinction record, where the wave fails 'when the transverse wave spacing becomes comparable with the tube width. Shchelkin's hypothesis has once more been proved, but this time through the duplication of the well-known features of the carbon-soot record. It is now felt that a more quantitative analysis of the wave pattern is needed, not only because it will provide a better understanding o f the detonation process and its instability. which seems to be inherent in the exothermic reactive hydrodynamics, but also because it will answer the main outstanding questions of how and why the transverse waves develop as they do. The authors wish to thank Mr. Douglas E. 8 a k k e r , f o r his technical skill and valuable help in preparhlg the material attd assemhlblg the shots.

References • i. STRI~ILOW.R. A,. Combustion & Flame. 12. 81 (1968). 2. CAMPtlI!LL,A, W.. HOLLANI),T, E.. MALIN.M, E,, and CoTrI!R,T. P.. Nature. 178,38 (1956}. 3, DREMIN,A. N., ROZANOV,O. K,. and TROFIMOV,V. S,. Combusrkm & Flame, 7, 153 (1963). 4. MAI.LORY.H, 0,. J. Appl. Phr,~..38. No. f3. 5302(1967). 5. SII('IIELK/N.K, J,* Zh. I~A.~lterim. i 7(,or. Fi:.. 36, 600 ( 1959}. 6, ZIMMER.M., Combu.vtion & Flame. 12, I (1968), 7. DOFF.R, E.. Phys. Fluid*, 4. No. 1I. 1427(1961), 8. Duty. R. E.. and KNIGrlT,H. T.. J. Client. Ptt),s., 20. 1693(1952). 9. M^NSON.N., "Propagation des d~tonationset des d~flagrations dans les m~langesgazeux," L'qlhce Nat'l, d'Etudes et de Recherehes Aeronautiques. Divl,rse I. Paris (1947). Translated in ASTIAAD 132-808. I0. FAY,J, A., J. Chem, Phy,v,. 21). No. 6. 142 (1952).

( Received J u l y 1969)