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Proceedings of the Combustion Institute 31 (2007) 2437–2443
Combustion Institute www.elsevier.com/locate/proci
Effect of debris fragments on direct initiation of spherical detonation waves in stoichiometric oxygen/hydrogen mixtures Makoto Komatsu a, Kazuyoshi Takayama Tsutomu Saito c b
b,* ,
Kiyonobu Ohtani b,
a Iwate Medical School, Department of General Culture, 3-16-1, Honcho Morioka, Japan Tohoku University Biomedical Engineering Organization, 2-1-1, Katahira Aoba, Sendai, Japan c Muroran Institute of Technology, 27-1, Mizumotocho, Muroran, Japan
Abstract Paper reports a result of experiments of spherical shock waves generated by explosions of micro-explosives weighing from 1 to 10 mg ignited by the irradiation of Q-switched laser beam and direct initiation to a spherical detonation wave in stoichiometric oxygen/hydrogen mixtures at 10–200 kPa. We visualized the interaction of debris particles ejected micro-explosives’ surface with shock waves by using double exposure holographic interferometry and high-speed video recording. Upon explosion, minute inert debris launched supersonically from micro-charge surface precursory to shock waves initiated spherical detonation waves. To examine this effect we attached 0.5–2.0 lm diameter SiO2 particles densely on micro-explosive surfaces and observed that the supersonic particles, significantly promoted the direct initiation of spherical detonation waves. The domain and boundary of detonation wave initiations were experimentally obtained at various initial pressures and the amount of micro-charges. 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Direc initiation; Holographic interferometry; High-speed video recording; Micro-explosive; Spherical shock waves
1. Introduction Detonation database [1], based on a thorough survey of existing detonation papers appeared before 1999, summarized the characteristics of detonation waves in various physical and chemical conditions and indicated that detonation experiments in the past were performed mostly
*
Corresponding author. Fax: +81 22 217 5324. E-mail address: [email protected] (K. Takayama).
in detonation tubes, in which the presence of solid wall boundaries strongly affected the initiation of detonation waves. In previous experiments, detonation waves in detonable gas mixtures in threedimensional space were generated by means of laser beam focusing [2–4], electric discharge [5] and over high speed projectiles [6] etc. We became interested in the initiation and propagation of spherical detonation waves in O2/2H2 mixtures by point explosion of silver azide pellets AgN3 weighing 1–10 mg suspended at the center of a test chamber filled with stoichiometric O2/2H2 mixtures at 10–200 kPa by the irradiation
1540-7489/$ - see front matter 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2006.08.111
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M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
of a Q-switched Nd:YAG laser beam [7]. We succeeded to visualize spherical shock wave generations and its to detonation waves by using double exposure holographic interferometry [8] and high-speed video recording based on conventional shadowgraph. Very small non-reactive particles spontaneously ejected from silver azide pellets’ surface at explosion flew at much faster speed than spherical shock waves. We then observed that deflagration fronts were accelerated along the supersonic debris’ wakes catching up shock fronts and that eventually detonation waves were initiated. Hence, to clarify the effect of supersonic debris fragments on the direct initiation of detonation waves, we artificially attached on explosive surface with 0.5–2.0 lm diameter silicone dioxide particles and then ejected then at ignition. Regarding the interaction of inert solid particle cloud with detonation waves, a vertical detonation tube [9] was used and the promotion of deflagration to detonation was reported. In this paper, we visualized initiation of detonation waves for SiO2 particle attached ignitions and simple ones and observed a remarkable difference between these cases and succeeded to the domain and boundary of the direct initiation of spherical detonation waves in stoichiometric O2/2H2 mixtures. 2. Experiments 2.1. Detonation chamber Figure 1a shows a detonation chamber of 290 mm i.d., 340 mm o.d., and 270 mm in width made of carbon steel (JIS S45C). Its two side ends were covered with 25 mm thick and 360 mm o.d.
acrylic windows 2 or 5 mm thick acrylic plates were inserted to protect the observation windows from damages made by directly impingement of small particles and high-temperature exposure behind detonation waves at each shot. This test chamber was designed to withstand against an impulsive high-pressure loading up to 50 MPa under the initial pressure from 10 to 100 kPa. While testing at higher initial pressures from 120 to 200 kPa, we employed another compact test chamber of 100 mm diameter and 100 mm in width placed inside the larger one as seen in Fig. 1a [7]. As seen in Fig. 1b, a micro-explosive was glued on a 0.2 mm diameter cotton thread and positioned at the center of the larger chamber. For the test in the small one we used 25 lm diameter copper line for supporting micro-explosives at its center. The ignition sensitivity of uncoated silver azide pellets was 0.6 lJ/mm2 [10]. In both larger and small chambers, a He–Ne laser beam illuminated a micro-explosive surface to exactly point out the center of the chambers. Then we could accurately target it with a Q-switched Nd:YAG laser beam of 25 mJ/pulse and 7 ns pulse duration (Class IV laser). 2.2. Visualization Double exposure holographic interferometric setup was used for the large chamber tests. The object beam is based on shadowgraph and the reference beam taking nearly equal light path length as that of the object beam is superimposed on a holofilm [7,8]. Light source was a Q-switch ruby laser (Apollo Lasers Inc. 22HD) and holofilms were 100 · 125 mm Ilford sheet films. An iris was placed near the focus of film side image focus
Fig. 1. Detonation chamber.
M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
lens and hence intense flash behind reaction fronts were effectively reduced. The first exposure was performed before the event and the second one was synchronized with the arrival of shock waves or detonation waves at a specified position in the test chamber. Holograms were reconstructed on 100 · 125 mm sheet films and resulting images were stored and processed on PC. Interferomtric fringes describing three-dimensional density fields correspond to density integral along individual collimated light rays and hence, in the case of point symmetric flows, we can determine the spatial density distribution if fringe orders are known. This method can measure density the most accurately among any other visualization methods. In the small chamber tests, we used a high-speed video camera offered by Shimazu Co., which was a test model of one million frames/s and 16 frames to complete commercial prototype model [7]. 2.3. Experimental condition The micro-explosive was a silver azide pellet supplied by Chugoku Kayaku Co. Ltd. 10 mg charges had a cylindrical shape of 1.5 mm diameter and 1.5 mm in height and its total energy was about 15 J, however, approximately the one third or quarter of the total energy contributed to the shock formation [11]. To make smaller microexplosives, we divided a 10 mg pellet into small pieces with a bamboo knife and measured their weight with a scale, Sartrias Microstar with accuracy of 0.1 lg [7,11]. However, because the laser beam diameter was about 2.5 mm and the micro-explosive’s projected surface area was just about 2 mm2, only a fraction of laser beam energy was used for ignition. Fitting shock overpressure data against the scaled distance for silver azide pellets to the known TNT data plot [10], the TNT equivalence factor of silver azide was readily determined to be about 0.40. The detonation speed of the present silver azide of density about 2.2 kg/m3 is 3.34 km/s [10,11]. It was ignited very sensitively to Q-switched Nd: YAG laser beam irradiation and its critical energy was about 3.5 mJ/cm2. This indicates the total energy needed to ignite the pellets is only 63 lJ [10], which means that the additional laser energy barely contributes to the shock formation. The ignition delay time is not more than 80 ns. In this paper, we compared the detonation wave formed with micro-explosives artificially attached with 0.5–2.0 lm diameter silicone dioxide particles and with spontaneous explosions and try to interpret such a remarkable effect for promotion from deflagration to detonation waves. Silicone dioxide particles of 0.5–2 mm in diameter were uniformly and relatively weakly coated with glue made of cellulose/acetone solution on micro-explosive surfaces. It is noted that due to
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its high sensitivity such a surface coating did not affect the laser ignition process [11]. We filled firstly oxygen into the evacuated test chamber at a given partial pressure, then filled hydrogen immediately after, and kept for several minutes to achieve hopefully stoichiometric and homogeneous mole ratio. We did not use any special pre-mixing process but simply expect homogeneous mixing by above mentioned method. At first we tested explosion of 10 mg charges in N2/2H2, varied initial pressures of O2/2H2 mixtures and compared results of inert gas case with detonable one. For high-speed video recording we tested at the initial pressures of 120, 140, 160, 180 and 200 kPa. Initial pressures were varied from 10 to 100 kPa at every 10 kPa and the amount of micro-explosives from 1 to 10 mg at every 1 mg. Interferograms were taken sequentially for combinations of initial pressures and amount of explosives and then those taken at 30 and 50 ls were compared. Measuring areas of waves’ projected images on individual interferograms and assuming slightly perturbed wave shapes to be spherical in shape, we can then determine waves’ averaged radii at a given time instant and then the averaged wave speed is estimated. 3. Results and discussion At first the process of shock formation driven by micro-explosions in a non-reactive N2/2H2 mixture was observed. This gas mixture consists nitrogen and hydrogen of 1:2 mole ratio and has the same specific heat ratio of c = 1.4 and sound speed of about a = 480 m/s at 293 K as those of O2/2H2 of c = 1.4 and a = 440 m/s at 293 K. Figure 2a and b show shock waves generated in it at 60 kPa: (a) it was driven by a simple 10 mg silver azide pellet visualized at 40 ls from the ignition; and (b) driven by the explosion of a SiO2 coated charge and visualized at 30 ls. SiO2 fragments were shattered at supersonic speed and formed conical shock waves of about 50 and hence their shattering speed was about 1200 m/s. As seen later in Figs. 3 and 4, the fragment shattering speed in O2/2H2 mixtures was nearly identical with this value. From sequential observations, the shock speed at earlier stage before 30 ls from laser ignition was estimated to be about 850 m/s at very early stage. In air, the dimensionless relationship between peak overpressure-scaled distance agrees with generalized the blast wave similarity law [10] and hence the shock wave motion in N2/2H2 and O2/2H2 will also be interpreted with blast wave similarity law. The shock speed in N2/2H2 propagated much slowly than shattering fragment speed, whereas in O2/2H2, as later see, on the detonation wave initiation, the detonation wave gradually caught up the debris fragments.
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Fig. 2. Shock waves in N2/2H2 with 10 mg micro-charges at initial pressure 60 kPa: (a) spontaneous explosion; and (b) SiO2 attached.
Fig. 3. Wave propagation in 60 kPa O2/2H2 with 10 mg charges: (a) at 5 ls; (b)10 ls; (c) 40ls; and (d) 70 ls.
Fig. 4. Wave propagation in 100 kPa O2/2H2 with 10 mg charges: (a) at 5 ls; (b) 30 ls; and (c) 45 ls. Notice detonation initiated already at 30 ls.
M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
Figure 3a–d show sequential interferograms of shock waves or detonation waves generated by ignition of 10 mg explosives at initial pressure of 60 kPa: (a) 5 ls after ignition; (b) 10 ls; (c) 40 ls; and (d) 70 ls. Upon ignition, weakly attached debris fragments on the explosive surface were ejected, spontaneously from the explosive surface typically to the due or reverse directions to the laser irradiation. In Fig. 3a, the deflagration front was accelerated even at very early stage and was immediately caught up the shock over wide area of the shock front. At later stage, the detonation was quenched and the shock and deflagration fronts were apparently decoupled. This trend was discussed later. The transition from deflagration to detonation is not always a deterministic phenomenon and then we repeated experiments under the same initial conditions at least twice. In Fig. 4a–c, sequential observations at 100 kPa and 10 mg charges are shown: (a) 5 ls after ignition; (b) 30 ls; and (c) 45 ls, respectively. In Fig. 4a, shattering debris fragments are followed by detonation wave but deflagration driven shock fronts still remain. The transition to detonation is completed already at 30 ls in Fig. 4b and detonation front is only locally spherical in shape but as seen in Fig. 4b and c two nearly spherical wave fronts having individual centers at different points appear to intersect. This trend was already observed in laser-induced detonation [3]. It is noticed that fringe numbers or the image contrast in Fig. 4 is improved better than those in Figs. 2 and 3, because the initial pressure is even higher. The relationship between averaged wave radii against the elapsed time is presented in Fig. 5. Abscissa designates the elapsed time in ls and ordinate averaged radius in mm. Filled circles indicate 10 mg charge explosions at 100 kPa in O2/2H2 (Fig. 4), open circles indicate those at 60 kPa (Fig. 3), and filled squares indicate those at 30 kPa, whereas filled triangles indicate 10 mg
Fig. 5. Wave trajectories measured at 30 ls: solid line, C–J line; 100 kPa, 10 mg; d 10 mg, 100 kPa O2/2H2; s 10 mg, 60 kPa O2/2H2; m 10 mg, 30 kPa O2/2H2; and · 10 mg, 60 kPa N2/2H2.
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charge in N2/2H2 at 60 kPa (Fig. 2a). A thin line in Fig. 5 shows an empirical one corresponding to open circles and filled squares and a dashed line to filled triangles of N2/2H2 at 60 kPa. A thick line in Fig. 5 corresponds to the C–J detonation trajectory calculated under the present condition, whose inclination gives to the C–J detonation velocity of 2818 m/s. Filled circles are distributed nearly parallel to the C–J line after the elapsed time of 20 ls. This implies that immediately after 20 ls from the ignition, the detonation wave was already initiated at the initial pressure of 100 kPa, whereas the wave trajectories at 30 and 60 kPa are only slightly deviated from the N2/ 2H2 shock front trajectory appear to be nearly identical with each other. This indicates that waves remain still shock waves and the transition to detonation will take longer time or even no transitions will take place. The transition to detonation depends strongly on the initial pressure and the amount of micro-explosives. On explosions in O2/2H2, SiO2 particles attached on 10 mg charge surfaces and visualized at 30 ls after ignition at various initial pressures were shattered, which are similarly to Fig. 2b precursory to the shock wave. Figure 6a–d show typical results: (a) the initial pressure of 20 kPa; (b) 40 kPa; (c) 60 kPa; and (d) 80 kPa. In Fig. 6a, the shock wave is still decoupled with deflagration front, while in Fig. 6b at 60 kPa, detonation wave was already in part initiated and in Fig. 6d at 80 kPa, completely established. It is clear that the transition to detonation is obviously promoted with increase in the initial pressure. This trend agrees with the fact that the critical energy depends on the initial pressure [1]. The Reynolds number of a 1 lm free flight particle at about 1 km/s and 100 kPa is in the order of 100. In assuming homogenous mixture, vortices in the wake behind the free flight particle are increased. Deflagration fronts are then accelerated locally along the wake toward the shock wave. When the deflagration front is coupled locally with the shock wave, the detonation wave is initiated. For higher initial pressure and larger micro-charge mass and with larger diameter fragments [3,4], this procedure, in an extreme case, leads the direct initiation. In Fig. 7a and b, we summarize the effect of initial pressure and explosive mass on the initiation of detonation in terms of the ratio of apparent wave speed to the C–J speed of O2/2H2 mixture: (a) without SiO2 particles; and (b) with SiO2 particles attached on the micro-explosive surface. The x-axis denotes the initial pressure in kPa, the y-axis explosive mass in mg and the z-axis color scale display of wave speeds normalized by the C–J speed. The results are deduced from the interferograms at 30 ls. Red color indicates overdriven detonation, whereas blue color corresponds to the shock speed. In Fig. 7a, absence of SiO2 particles, the initiation to detonation
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Fig. 6. Explosion with SiO2 attachment at 10 mg, visualized at 30 ls: (a) at 20 kPa; (b) 40 kPa; (c) 60 kPa; and (d) 80 kPa.
Fig. 7. Effect of initial pressures and charge mass: wave velocity is non-dimensionalized with C–J speed color scale indicates wave speed at 30 ls: (a) spontaneous explosion; and (b) SiO2 attachment.
M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
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The trend of quenching may suggest that these options are erroneous and spherical detonation waves may not remain stable. 4. Conclusions
Fig. 8. Wave trajectories at higher initial pressure of 140 and 200 kPa with 10 mg charge: solid line, C–J line; s 140 kPa, SiO2 attachment; d 140 kPa, spontaneous explosion; n 200 kPa, SiO2 attachment; and m 200 kPa, spontaneous explosion.
As a preliminary study of the initiation spherical detonation waves, we reported results of a detonation chamber experiments in which micro-charges were ignited in stoichiometric oxygen/hydrogen mixtures at 10–200 kPa. The effect of debris fragments ejected from explosive surfaces on the direct initiation of detonation waves are visualized by double exposure holographic interferometry. Shattering micro-fragments significantly promoted the initiation of detonation waves. In the presence of micro-fragments, the critical energy is a minimum. Acknowledgments
presented in red color occurs only in the neighborhood of 10 mg and 100 kPa, and the blue region representing decoupled shock and deflagration appears in a wide area. In Fig. 7b, with SiO2 particle attachment, it occurs in a range of 7–10 mg charge at 100 kPa and with a 10 mg charge at 70–100 kPa and blue region appears in a very small area. Wave velocities not necessarily vary monotonously but singular islands appear. Figure 8 shows a summary of high-speed video recordings at 140 and 200 kPa and micro-charge mass of 10 mg. Two cases, the presence and absence of SiO2 particles, are compared. Abscissa designates elapsed time in ms and ordinate averaged radius in mm. Thick line shows the C–J detonation trajectory. Using the small test chamber, we have a limited view field up to 100 mm in diameter. Filled and open triangles designate the detonation wave trajectories at 10 mg charge, initial pressure of 200 kPa and in the absence and presence of SiO2 particles and filled and open circles designate those at 10 mg charge, 140 kPa and in the absence and presence of SiO2 particles. At initial pressure of 200 kPa, the detonation is initiated instantaneously and appears to be an overdriven detonation up to 7 ls from initiation, but starts to quench, whereas at 140 kPa, the attenuation started already at 3.5 ls. The quenching process is more significant in the case of SiO2 particle attachment. This trend agrees with [3]. Regarding the evolution of spherical detonation waves, if truly spherical detonation waves can exist and propagate at constant C–J velocity, can the cell size be enlarged with propagation in keeping the cell number constant or can it increase, while the cell size remains constant?
Authors acknowledge to Professor Emeritus K. Terao and Professor T. Tsuboi of Yokohama for their discussion and to Mr. H. Ojima for his assistance to throughout the course of the present experiments. This project is in part supported by the Grant-in-Aid for Scientific Research, 12COE2003 offered by the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References [1] M. Kaneshige, J. Shepherd, Explosion Dynamics Laboratory report, FM97-8, 1999. [2] J.H. Lee, B.H.K. Lee, I. Shanfield, Proc. Combust. Inst. 10 (1965) 805–815. [3] J.H. Lee, R. Knystautas, AIAA J. 7 (1969) 312–317. [4] V.F. Klimkin, R.I. Soloukhin, P. Wolansky, Combust. Flame 21 (1973) 111–117. [5] G.G. Bach, R. Knystautas, J.H. Lee, Proc. Combust. Inst. 12 (1969) 853–864. [6] J.H. Lee, On the initiation of detonation by a hypervelocity projectile, in: Zeldovich Mem. Conf. Combust. Voronovo, 1994. [7] M. Komatsu, Experimental Study of Generation of Spherical Detonation Wave, Master Thesis, Graduate School of Tohoku University, 1999. [8] K. Takayama, Proc. SPIE 398 (1983) 174–180. [9] M. Wolinski, P. Wolanski, Archium Combustionis 7 (1987) 353–370. [10] T. Mizukaki, Quantitative Visualization of Shock Wave Phenomena, Doctor Thesis, Graduate School of Tohoku University, 2000. [11] N. Nagayasu, Generation and Motion of Microexplosion Induced Shock Wave and its Application, Doctor Thesis, Graduate School of Tohoku University, 1998.
Proceedings of the Combustion Institute 31 (2007) 2437–2443
Combustion Institute www.elsevier.com/locate/proci
Effect of debris fragments on direct initiation of spherical detonation waves in stoichiometric oxygen/hydrogen mixtures Makoto Komatsu a, Kazuyoshi Takayama Tsutomu Saito c b
b,* ,
Kiyonobu Ohtani b,
a Iwate Medical School, Department of General Culture, 3-16-1, Honcho Morioka, Japan Tohoku University Biomedical Engineering Organization, 2-1-1, Katahira Aoba, Sendai, Japan c Muroran Institute of Technology, 27-1, Mizumotocho, Muroran, Japan
Abstract Paper reports a result of experiments of spherical shock waves generated by explosions of micro-explosives weighing from 1 to 10 mg ignited by the irradiation of Q-switched laser beam and direct initiation to a spherical detonation wave in stoichiometric oxygen/hydrogen mixtures at 10–200 kPa. We visualized the interaction of debris particles ejected micro-explosives’ surface with shock waves by using double exposure holographic interferometry and high-speed video recording. Upon explosion, minute inert debris launched supersonically from micro-charge surface precursory to shock waves initiated spherical detonation waves. To examine this effect we attached 0.5–2.0 lm diameter SiO2 particles densely on micro-explosive surfaces and observed that the supersonic particles, significantly promoted the direct initiation of spherical detonation waves. The domain and boundary of detonation wave initiations were experimentally obtained at various initial pressures and the amount of micro-charges. 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Direc initiation; Holographic interferometry; High-speed video recording; Micro-explosive; Spherical shock waves
1. Introduction Detonation database [1], based on a thorough survey of existing detonation papers appeared before 1999, summarized the characteristics of detonation waves in various physical and chemical conditions and indicated that detonation experiments in the past were performed mostly
*
Corresponding author. Fax: +81 22 217 5324. E-mail address: [email protected] (K. Takayama).
in detonation tubes, in which the presence of solid wall boundaries strongly affected the initiation of detonation waves. In previous experiments, detonation waves in detonable gas mixtures in threedimensional space were generated by means of laser beam focusing [2–4], electric discharge [5] and over high speed projectiles [6] etc. We became interested in the initiation and propagation of spherical detonation waves in O2/2H2 mixtures by point explosion of silver azide pellets AgN3 weighing 1–10 mg suspended at the center of a test chamber filled with stoichiometric O2/2H2 mixtures at 10–200 kPa by the irradiation
1540-7489/$ - see front matter 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2006.08.111
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M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
of a Q-switched Nd:YAG laser beam [7]. We succeeded to visualize spherical shock wave generations and its to detonation waves by using double exposure holographic interferometry [8] and high-speed video recording based on conventional shadowgraph. Very small non-reactive particles spontaneously ejected from silver azide pellets’ surface at explosion flew at much faster speed than spherical shock waves. We then observed that deflagration fronts were accelerated along the supersonic debris’ wakes catching up shock fronts and that eventually detonation waves were initiated. Hence, to clarify the effect of supersonic debris fragments on the direct initiation of detonation waves, we artificially attached on explosive surface with 0.5–2.0 lm diameter silicone dioxide particles and then ejected then at ignition. Regarding the interaction of inert solid particle cloud with detonation waves, a vertical detonation tube [9] was used and the promotion of deflagration to detonation was reported. In this paper, we visualized initiation of detonation waves for SiO2 particle attached ignitions and simple ones and observed a remarkable difference between these cases and succeeded to the domain and boundary of the direct initiation of spherical detonation waves in stoichiometric O2/2H2 mixtures. 2. Experiments 2.1. Detonation chamber Figure 1a shows a detonation chamber of 290 mm i.d., 340 mm o.d., and 270 mm in width made of carbon steel (JIS S45C). Its two side ends were covered with 25 mm thick and 360 mm o.d.
acrylic windows 2 or 5 mm thick acrylic plates were inserted to protect the observation windows from damages made by directly impingement of small particles and high-temperature exposure behind detonation waves at each shot. This test chamber was designed to withstand against an impulsive high-pressure loading up to 50 MPa under the initial pressure from 10 to 100 kPa. While testing at higher initial pressures from 120 to 200 kPa, we employed another compact test chamber of 100 mm diameter and 100 mm in width placed inside the larger one as seen in Fig. 1a [7]. As seen in Fig. 1b, a micro-explosive was glued on a 0.2 mm diameter cotton thread and positioned at the center of the larger chamber. For the test in the small one we used 25 lm diameter copper line for supporting micro-explosives at its center. The ignition sensitivity of uncoated silver azide pellets was 0.6 lJ/mm2 [10]. In both larger and small chambers, a He–Ne laser beam illuminated a micro-explosive surface to exactly point out the center of the chambers. Then we could accurately target it with a Q-switched Nd:YAG laser beam of 25 mJ/pulse and 7 ns pulse duration (Class IV laser). 2.2. Visualization Double exposure holographic interferometric setup was used for the large chamber tests. The object beam is based on shadowgraph and the reference beam taking nearly equal light path length as that of the object beam is superimposed on a holofilm [7,8]. Light source was a Q-switch ruby laser (Apollo Lasers Inc. 22HD) and holofilms were 100 · 125 mm Ilford sheet films. An iris was placed near the focus of film side image focus
Fig. 1. Detonation chamber.
M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
lens and hence intense flash behind reaction fronts were effectively reduced. The first exposure was performed before the event and the second one was synchronized with the arrival of shock waves or detonation waves at a specified position in the test chamber. Holograms were reconstructed on 100 · 125 mm sheet films and resulting images were stored and processed on PC. Interferomtric fringes describing three-dimensional density fields correspond to density integral along individual collimated light rays and hence, in the case of point symmetric flows, we can determine the spatial density distribution if fringe orders are known. This method can measure density the most accurately among any other visualization methods. In the small chamber tests, we used a high-speed video camera offered by Shimazu Co., which was a test model of one million frames/s and 16 frames to complete commercial prototype model [7]. 2.3. Experimental condition The micro-explosive was a silver azide pellet supplied by Chugoku Kayaku Co. Ltd. 10 mg charges had a cylindrical shape of 1.5 mm diameter and 1.5 mm in height and its total energy was about 15 J, however, approximately the one third or quarter of the total energy contributed to the shock formation [11]. To make smaller microexplosives, we divided a 10 mg pellet into small pieces with a bamboo knife and measured their weight with a scale, Sartrias Microstar with accuracy of 0.1 lg [7,11]. However, because the laser beam diameter was about 2.5 mm and the micro-explosive’s projected surface area was just about 2 mm2, only a fraction of laser beam energy was used for ignition. Fitting shock overpressure data against the scaled distance for silver azide pellets to the known TNT data plot [10], the TNT equivalence factor of silver azide was readily determined to be about 0.40. The detonation speed of the present silver azide of density about 2.2 kg/m3 is 3.34 km/s [10,11]. It was ignited very sensitively to Q-switched Nd: YAG laser beam irradiation and its critical energy was about 3.5 mJ/cm2. This indicates the total energy needed to ignite the pellets is only 63 lJ [10], which means that the additional laser energy barely contributes to the shock formation. The ignition delay time is not more than 80 ns. In this paper, we compared the detonation wave formed with micro-explosives artificially attached with 0.5–2.0 lm diameter silicone dioxide particles and with spontaneous explosions and try to interpret such a remarkable effect for promotion from deflagration to detonation waves. Silicone dioxide particles of 0.5–2 mm in diameter were uniformly and relatively weakly coated with glue made of cellulose/acetone solution on micro-explosive surfaces. It is noted that due to
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its high sensitivity such a surface coating did not affect the laser ignition process [11]. We filled firstly oxygen into the evacuated test chamber at a given partial pressure, then filled hydrogen immediately after, and kept for several minutes to achieve hopefully stoichiometric and homogeneous mole ratio. We did not use any special pre-mixing process but simply expect homogeneous mixing by above mentioned method. At first we tested explosion of 10 mg charges in N2/2H2, varied initial pressures of O2/2H2 mixtures and compared results of inert gas case with detonable one. For high-speed video recording we tested at the initial pressures of 120, 140, 160, 180 and 200 kPa. Initial pressures were varied from 10 to 100 kPa at every 10 kPa and the amount of micro-explosives from 1 to 10 mg at every 1 mg. Interferograms were taken sequentially for combinations of initial pressures and amount of explosives and then those taken at 30 and 50 ls were compared. Measuring areas of waves’ projected images on individual interferograms and assuming slightly perturbed wave shapes to be spherical in shape, we can then determine waves’ averaged radii at a given time instant and then the averaged wave speed is estimated. 3. Results and discussion At first the process of shock formation driven by micro-explosions in a non-reactive N2/2H2 mixture was observed. This gas mixture consists nitrogen and hydrogen of 1:2 mole ratio and has the same specific heat ratio of c = 1.4 and sound speed of about a = 480 m/s at 293 K as those of O2/2H2 of c = 1.4 and a = 440 m/s at 293 K. Figure 2a and b show shock waves generated in it at 60 kPa: (a) it was driven by a simple 10 mg silver azide pellet visualized at 40 ls from the ignition; and (b) driven by the explosion of a SiO2 coated charge and visualized at 30 ls. SiO2 fragments were shattered at supersonic speed and formed conical shock waves of about 50 and hence their shattering speed was about 1200 m/s. As seen later in Figs. 3 and 4, the fragment shattering speed in O2/2H2 mixtures was nearly identical with this value. From sequential observations, the shock speed at earlier stage before 30 ls from laser ignition was estimated to be about 850 m/s at very early stage. In air, the dimensionless relationship between peak overpressure-scaled distance agrees with generalized the blast wave similarity law [10] and hence the shock wave motion in N2/2H2 and O2/2H2 will also be interpreted with blast wave similarity law. The shock speed in N2/2H2 propagated much slowly than shattering fragment speed, whereas in O2/2H2, as later see, on the detonation wave initiation, the detonation wave gradually caught up the debris fragments.
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Fig. 2. Shock waves in N2/2H2 with 10 mg micro-charges at initial pressure 60 kPa: (a) spontaneous explosion; and (b) SiO2 attached.
Fig. 3. Wave propagation in 60 kPa O2/2H2 with 10 mg charges: (a) at 5 ls; (b)10 ls; (c) 40ls; and (d) 70 ls.
Fig. 4. Wave propagation in 100 kPa O2/2H2 with 10 mg charges: (a) at 5 ls; (b) 30 ls; and (c) 45 ls. Notice detonation initiated already at 30 ls.
M. Komatsu et al. / Proceedings of the Combustion Institute 31 (2007) 2437–2443
Figure 3a–d show sequential interferograms of shock waves or detonation waves generated by ignition of 10 mg explosives at initial pressure of 60 kPa: (a) 5 ls after ignition; (b) 10 ls; (c) 40 ls; and (d) 70 ls. Upon ignition, weakly attached debris fragments on the explosive surface were ejected, spontaneously from the explosive surface typically to the due or reverse directions to the laser irradiation. In Fig. 3a, the deflagration front was accelerated even at very early stage and was immediately caught up the shock over wide area of the shock front. At later stage, the detonation was quenched and the shock and deflagration fronts were apparently decoupled. This trend was discussed later. The transition from deflagration to detonation is not always a deterministic phenomenon and then we repeated experiments under the same initial conditions at least twice. In Fig. 4a–c, sequential observations at 100 kPa and 10 mg charges are shown: (a) 5 ls after ignition; (b) 30 ls; and (c) 45 ls, respectively. In Fig. 4a, shattering debris fragments are followed by detonation wave but deflagration driven shock fronts still remain. The transition to detonation is completed already at 30 ls in Fig. 4b and detonation front is only locally spherical in shape but as seen in Fig. 4b and c two nearly spherical wave fronts having individual centers at different points appear to intersect. This trend was already observed in laser-induced detonation [3]. It is noticed that fringe numbers or the image contrast in Fig. 4 is improved better than those in Figs. 2 and 3, because the initial pressure is even higher. The relationship between averaged wave radii against the elapsed time is presented in Fig. 5. Abscissa designates the elapsed time in ls and ordinate averaged radius in mm. Filled circles indicate 10 mg charge explosions at 100 kPa in O2/2H2 (Fig. 4), open circles indicate those at 60 kPa (Fig. 3), and filled squares indicate those at 30 kPa, whereas filled triangles indicate 10 mg
Fig. 5. Wave trajectories measured at 30 ls: solid line, C–J line; 100 kPa, 10 mg; d 10 mg, 100 kPa O2/2H2; s 10 mg, 60 kPa O2/2H2; m 10 mg, 30 kPa O2/2H2; and · 10 mg, 60 kPa N2/2H2.
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charge in N2/2H2 at 60 kPa (Fig. 2a). A thin line in Fig. 5 shows an empirical one corresponding to open circles and filled squares and a dashed line to filled triangles of N2/2H2 at 60 kPa. A thick line in Fig. 5 corresponds to the C–J detonation trajectory calculated under the present condition, whose inclination gives to the C–J detonation velocity of 2818 m/s. Filled circles are distributed nearly parallel to the C–J line after the elapsed time of 20 ls. This implies that immediately after 20 ls from the ignition, the detonation wave was already initiated at the initial pressure of 100 kPa, whereas the wave trajectories at 30 and 60 kPa are only slightly deviated from the N2/ 2H2 shock front trajectory appear to be nearly identical with each other. This indicates that waves remain still shock waves and the transition to detonation will take longer time or even no transitions will take place. The transition to detonation depends strongly on the initial pressure and the amount of micro-explosives. On explosions in O2/2H2, SiO2 particles attached on 10 mg charge surfaces and visualized at 30 ls after ignition at various initial pressures were shattered, which are similarly to Fig. 2b precursory to the shock wave. Figure 6a–d show typical results: (a) the initial pressure of 20 kPa; (b) 40 kPa; (c) 60 kPa; and (d) 80 kPa. In Fig. 6a, the shock wave is still decoupled with deflagration front, while in Fig. 6b at 60 kPa, detonation wave was already in part initiated and in Fig. 6d at 80 kPa, completely established. It is clear that the transition to detonation is obviously promoted with increase in the initial pressure. This trend agrees with the fact that the critical energy depends on the initial pressure [1]. The Reynolds number of a 1 lm free flight particle at about 1 km/s and 100 kPa is in the order of 100. In assuming homogenous mixture, vortices in the wake behind the free flight particle are increased. Deflagration fronts are then accelerated locally along the wake toward the shock wave. When the deflagration front is coupled locally with the shock wave, the detonation wave is initiated. For higher initial pressure and larger micro-charge mass and with larger diameter fragments [3,4], this procedure, in an extreme case, leads the direct initiation. In Fig. 7a and b, we summarize the effect of initial pressure and explosive mass on the initiation of detonation in terms of the ratio of apparent wave speed to the C–J speed of O2/2H2 mixture: (a) without SiO2 particles; and (b) with SiO2 particles attached on the micro-explosive surface. The x-axis denotes the initial pressure in kPa, the y-axis explosive mass in mg and the z-axis color scale display of wave speeds normalized by the C–J speed. The results are deduced from the interferograms at 30 ls. Red color indicates overdriven detonation, whereas blue color corresponds to the shock speed. In Fig. 7a, absence of SiO2 particles, the initiation to detonation
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Fig. 6. Explosion with SiO2 attachment at 10 mg, visualized at 30 ls: (a) at 20 kPa; (b) 40 kPa; (c) 60 kPa; and (d) 80 kPa.
Fig. 7. Effect of initial pressures and charge mass: wave velocity is non-dimensionalized with C–J speed color scale indicates wave speed at 30 ls: (a) spontaneous explosion; and (b) SiO2 attachment.
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The trend of quenching may suggest that these options are erroneous and spherical detonation waves may not remain stable. 4. Conclusions
Fig. 8. Wave trajectories at higher initial pressure of 140 and 200 kPa with 10 mg charge: solid line, C–J line; s 140 kPa, SiO2 attachment; d 140 kPa, spontaneous explosion; n 200 kPa, SiO2 attachment; and m 200 kPa, spontaneous explosion.
As a preliminary study of the initiation spherical detonation waves, we reported results of a detonation chamber experiments in which micro-charges were ignited in stoichiometric oxygen/hydrogen mixtures at 10–200 kPa. The effect of debris fragments ejected from explosive surfaces on the direct initiation of detonation waves are visualized by double exposure holographic interferometry. Shattering micro-fragments significantly promoted the initiation of detonation waves. In the presence of micro-fragments, the critical energy is a minimum. Acknowledgments
presented in red color occurs only in the neighborhood of 10 mg and 100 kPa, and the blue region representing decoupled shock and deflagration appears in a wide area. In Fig. 7b, with SiO2 particle attachment, it occurs in a range of 7–10 mg charge at 100 kPa and with a 10 mg charge at 70–100 kPa and blue region appears in a very small area. Wave velocities not necessarily vary monotonously but singular islands appear. Figure 8 shows a summary of high-speed video recordings at 140 and 200 kPa and micro-charge mass of 10 mg. Two cases, the presence and absence of SiO2 particles, are compared. Abscissa designates elapsed time in ms and ordinate averaged radius in mm. Thick line shows the C–J detonation trajectory. Using the small test chamber, we have a limited view field up to 100 mm in diameter. Filled and open triangles designate the detonation wave trajectories at 10 mg charge, initial pressure of 200 kPa and in the absence and presence of SiO2 particles and filled and open circles designate those at 10 mg charge, 140 kPa and in the absence and presence of SiO2 particles. At initial pressure of 200 kPa, the detonation is initiated instantaneously and appears to be an overdriven detonation up to 7 ls from initiation, but starts to quench, whereas at 140 kPa, the attenuation started already at 3.5 ls. The quenching process is more significant in the case of SiO2 particle attachment. This trend agrees with [3]. Regarding the evolution of spherical detonation waves, if truly spherical detonation waves can exist and propagate at constant C–J velocity, can the cell size be enlarged with propagation in keeping the cell number constant or can it increase, while the cell size remains constant?
Authors acknowledge to Professor Emeritus K. Terao and Professor T. Tsuboi of Yokohama for their discussion and to Mr. H. Ojima for his assistance to throughout the course of the present experiments. This project is in part supported by the Grant-in-Aid for Scientific Research, 12COE2003 offered by the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References [1] M. Kaneshige, J. Shepherd, Explosion Dynamics Laboratory report, FM97-8, 1999. [2] J.H. Lee, B.H.K. Lee, I. Shanfield, Proc. Combust. Inst. 10 (1965) 805–815. [3] J.H. Lee, R. Knystautas, AIAA J. 7 (1969) 312–317. [4] V.F. Klimkin, R.I. Soloukhin, P. Wolansky, Combust. Flame 21 (1973) 111–117. [5] G.G. Bach, R. Knystautas, J.H. Lee, Proc. Combust. Inst. 12 (1969) 853–864. [6] J.H. Lee, On the initiation of detonation by a hypervelocity projectile, in: Zeldovich Mem. Conf. Combust. Voronovo, 1994. [7] M. Komatsu, Experimental Study of Generation of Spherical Detonation Wave, Master Thesis, Graduate School of Tohoku University, 1999. [8] K. Takayama, Proc. SPIE 398 (1983) 174–180. [9] M. Wolinski, P. Wolanski, Archium Combustionis 7 (1987) 353–370. [10] T. Mizukaki, Quantitative Visualization of Shock Wave Phenomena, Doctor Thesis, Graduate School of Tohoku University, 2000. [11] N. Nagayasu, Generation and Motion of Microexplosion Induced Shock Wave and its Application, Doctor Thesis, Graduate School of Tohoku University, 1998.