Study on low velocity detonation phenomena in Nitromethane

In detonation of an explosive, there are two forms, high velocity detonation (HVD) and low velocity detonation (LVD). In a liquid explosive, the highest pressure of LVD changes with explosive and conditions, its pressure is a few GPa and has destructive power equivalent to HVD. It is important also about security to get to know the actual condition of LVD. Moreover, it is important that the performance of explosives is grasped completely to control HVD and LVD and deflagration and to demonstrate an expected function. 1) Nitromethane (NM: density 1131~1137kg/m) is typical liquid explosive and the substance which has the explosion power exceeding TNT. Since it is comparatively insensible, it is used as an organic composition material in an industrial field, only as an explosive. And it is homogeneous compared with a heterogeneous solid explosive and the character is simple. And it is suitable to take photographs since it is transparent. So, in this research, we observe optically using a high speed camera and discuss the results in order to grasp HVD and the LVD characteristic of NM. Experiment Optical observation by a high speed camera was performed experiment. IMACON468 (product made from HADLAND PHOTONICS) and HPV-1 (product made from Shimadzu Corp.) were used for the high speed camera. Schematic illustration of optical observation is shown in Fig.1. PVC pipe(30mm of diameters of inner, density 1360-1540 kg/m 3 ) with a height of 50mm was loaded with 50g of high explosives SEP (made in Asahi Chemical Chemicals, detonation velocity about 7,000m/s, density 1310 kg/m 3 ) as booster. A container is a path 30mm x 30mm box form inside, and an order side is constituted from PMMA (the density of 1185kg/m 3 ). NM is poured into it. Booster is put on a container through PMMA Gap, and it explodes with a No. 6 electric detonator. Photography is performed using the shadow graph method. Variation in velocity by PMMA Gap length It is experimented by changing the length of PMMA Gap with 0, 1, 2, 5, 10, 20, 50, 100 and 200mm. NM is poured in a container and it compared what kind of relation between the length of PMMA Gap and shock wave velocity, detonation velocity there is. Next, water is poured in a container, and it is experimented on same condition. Variation in velocity by material of side wall It is experimented by changing the material of the side wall of a container into steel and aluminum and copper. The influence on the shock wave and detonation velocity by material of a side wall is considered. As for the length of PMMA Gap, it is experimented by 5mm. Results and Discussions A framing photograph is shown in Fig.2 and a streak photograph is shown in Fig.3. When the shock wave which precedes the inside of a container wall enters into NM, the shock wave collides and cavity occurs in the central part. The reaction in the liquid containing this cavitation causes LVD. As a role of a cavitation, it’s generally understood that the reaction by the shock of the mutual collision at the time of the micro jet of a liquid jumping into cavity bubbles. 1) The framing photographs in the case of that PMMA Gap length is 0mm and 200mm are shown in Fig 4 and Fig5. In the case of that PMMA Gap length is 0mm, since the shock wave produced by detonation of SEP entered into NM directly and high pressure was applied, HVD (the usual detonation) was happened. In the case of that PMMA


Experiment
Optical observation by a high speed camera was performed experiment.IMACON468 (product made from HADLAND PHOTONICS) and HPV-1 (product made from Shimadzu Corp.) were used for the high speed camera.Schematic illustration of optical observation is shown in Fig. 1.PVC pipe(30mm of diameters of inner, density 1360-1540 kg/m 3 ) with a height of 50mm was loaded with 50g of high explosives SEP (made in Asahi Chemical Chemicals, detonation velocity about 7,000m/s, density 1310 kg/m 3 ) as booster.A container is a path 30mm x 30mm box form inside, and an order side is constituted from PMMA (the density of 1185kg/m 3 ).NM is poured into it.Booster is put on a container through PMMA Gap, and it explodes with a No. 6 electric detonator.Photography is performed using the shadow graph method.

Variation in velocity by PMMA Gap length
It is experimented by changing the length of PMMA Gap with 0, 1, 2, 5, 10, 20, 50, 100 and 200mm.NM is poured in a container and it compared what kind of relation between the length of PMMA Gap and shock wave velocity, detonation velocity there is.Next, water is poured in a container, and it is experimented on same condition.

Variation in velocity by material of side wall
It is experimented by changing the material of the side wall of a container into steel and aluminum and copper.
The influence on the shock wave and detonation velocity by material of a side wall is considered.As for the length of PMMA Gap, it is experimented by 5mm.

Results and Discussions
A framing photograph is shown in Fig. 2 and a streak photograph is shown in Fig. 3.When the shock wave which precedes the inside of a container wall enters into NM, the shock wave collides and cavity occurs in the central part.The reaction in the liquid containing this cavitation causes LVD.As a role of a cavitation, it's generally understood that the reaction by the shock of the mutual collision at the time of the micro jet of a liquid jumping into cavity bubbles. 1) The framing photographs in the case of that PMMA Gap length is 0mm and 200mm are shown in Fig 4 and Fig5.In the case of that PMMA Gap length is 0mm, since the shock wave produced by detonation of SEP entered into NM directly and high pressure was applied, HVD (the usual detonation) was happened.In the case of that PMMA  Gap length is 200mm, since the pressure of incidence is low, LVD is not led.Precedence shock wave velocity and low velocity detonation are found from framing photographs and streak photographs.The relation between velocity and the distance from PMMA Gap are shown in Fig. 6.From this graph, the velocity of precedence shock wave is about 2,700 -2,900m/s, and the velocity of detonation wave is about 1,600 -1,700m/s.The detonation wave propagates at a velocity higher than the sound velocity of NM (1350 m/s).So this is low velocity detonation (LVD).
Variation in velocity by material of side wall A framing photograph in case of using metal for the side wall is metal is shown in Fig. 7, Fig. 8.And the comparison of velocity by material is ＳＨＯＷＮ in Fig. 9. From Fig. 9, it is shown that each precedence shock wave is different by the material of side wall.This is because each sound velocity is different by the material of side wall.Shock wave is a wave of the strong pressure change transmitted in the velocity exceeding sound velocity.That is if the sound velocity transmitted in the inside of a substance is fast, propagation of shock wave will also become fast.And from Fig. 9, it seems that there is no relation between precedence shock wave velocity and detonation velocity.

Conclusion
This time, generation of the cavitation which is an important factor for LVD generating was clarified by optical observation.And in the optical observation experiment, the action of a precedence shock wave and detonation wave in NM was observed, and those velocities were found with the streak photograph and the framing photograph.From those results, it turned out that the distance which detonation wave maintains becomes short as Gap length becomes long i.e. incidence pressure becomes small.Moreover, when the material in the sidewall was changed, the change in the detonation velocity was not seen.So it seems that there is no relation between precedence shock wave velocity and detonation velocity.

Fig. 6 Fig. 9
Fig.6 Relation between velocity and distance from PMMA Gap