Initiation of detonation during gap testing of liquids.
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Streak-camera observations have shown that wall interactions are frequently responsible for shock initiation of nitromethane in gap-test geometry. Therefore, pressure calibrations of the gap test for liquids made at the attenuator surface along the charge axis should not be used to interpret the test results. Uncritical acceptance of gap-test values for a series of liquids with different properties may lead to erroneous sensitivity ordering. On the other hand, modified gap-test experiments can be used to obtain fundamental initiation data if appropriate instrumentation shows that wall effects are absent. A valid series of reaction time measurements is presented for nitromethane. Experimentally valid initiation data must be interpreted in terms of divergent reactive flow. G AP tests 1 for determining the sensitivities of liquids to shock initiation of detonation have excellent reproducibility, and, moreover, they are simple and economical. The excellence of the reproducibility suggests that it is worthwhile to interpret gap-test data in terms of shock pressure of initiation. Shock pressures, in principle, can be utilized to compute shock temperatures if additional thermodynamic data are available25; temperatures, in turn, allow one to discuss chemical reaction rates in initiating shocks. Such information, rapidly available from an experiment as simple as the gap test, would be very useful in studies of the mechanism of shock initiation and ultimately in studying the relation between shock sensitivity and chemical structure. However, there is reason to suspect that a fundamental interpretation of gap-test data can be made only if detailed three-dimensional processes occurring in this somewhat complicated geometry are completely understood. Sensitivity data from gap-test experiments are often reported in terms of the number of cards (usually cellulose acetate sheets 0.010 in. thick) making up the attenuator. The number of cards or the total thickness of plastic is reported as the 50% failure thickness. Thickness is converted to pressure in the attenuator by measuring the shock and free-surface velocities along the axis of the donor-attenuator system, or by measuring only the shock velocity if the Hugoniot of the attenuator material is already known. With some difficulty, attenuator pressure may be measured directly with a gage. 6'7 The entering pressure in the liquid can be calculated from the attenuator pressure by solving the interface equations if the shock velocity in the unreacted liquid is known. The experimental measurements, on which the entering shock pressure depends, are made at the center of the attenuator; thus, it is implicitly assumed that whether the liquid detonates is decided along the charge axis. In this note, experiments are reported which show that conditions on the charge axis do not necessarily control initiation; on the contrary, when the initial shock is too weak to cause detonation directly, initiation occurs at the container walls. The assumption that the axial peak pressure entering the liquid initiates a homogeneous thermal explosion leads to confusing inconsistencies that preclude fundamental interpretation, and in some cases, result in erroneous sensitivity ordering of liquid explosives. The liquid explosive used in these experiments was technical grade nitromethane (NM) manufactured by CommercialCite
Deflagration to Detonation Transition (DDT) phenomena of liquid fuel octane droplets in air in detonation tube are simulated here. The numerical formulation is described, on which code-CPTD is developed. Basic properties of liquid-fueled detonation structure together with the effects of partial preevaporation and droplet amount on detonation structure and development are investigated in current study. Simulations reveal that the presence of some amount of initial fuel vapor in the tube will substantially expedite transition to detonation, on the other hand, the existence of some fuel droplets may exhibit a suppression to detonation wave, and increasing amount or concentration of fuel droplets will delay deflagration to detonation transition. The calculations show that the numerical results are in good agreement with experimental ones, which implies that a feasible numerical method has been provided here to simulate pulse detonation engine operation processes including DDT phenomena.
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Inert
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To verify the three current international views on the generating condition of over-detonation during gaseous deflagration-to-detonation transition(DDT),the DDT process of gaseous hydrogen-oxygen mixture was experimentally studied by using pressure sensors.The whole pressure histories of gaseous DDT from detonation to DDT and then to stable detonation were obtained.The experimental results showed that the generation of DDT requires certain physico-chemical conditions,and for certain initial pressure conditions,the transformation time(or distance) decreases firstly and then increases with increasing concentration of hydrogen.The peak pressure in over-detonation is about 1.5～2 times as much as that in the stable detonation.
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The effective producing detonation is an essential technology of pulse detonation engine.The deflagration to detonation transition progress of pulse detonation engine was researched using two-dimensional numerical simulation aimed at detonation engine tube with three different sub-chamber structures.Studies indicate that:(1)the energy of electric sparks is too weak to directly trigger a detonation;the detonation is fully established through a series of interactions and impacts of reflected shock waves.(2)Because of different sub-chamber,the deflagration to detonation transition(DDT)is also different;the simulation results of three kinds of situations are contrasted,and the best sub-chamber design is obtained whose SDT time and length is the shortest.
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According to the characteristic of pulse detonation inside the pulse detonation engine,the three-dimensional models for two-phase detonation were built.The CE/SE method was used to calculate the deflagration to detonation.The cal-culation results show that the pressure field inside the tube is three-dimensional variation during the deflagration to detonation.When it is detonation wave,three-dimensional effect degenerates to be two-dimensional.The pressure data agree well with the experimental data.
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To study the influence of jet flow on the propagation of detonation wave,the deflagration-to-detonation transition(DDT)process were numerically simulated aiming at two cases:physical obstacle and jet flow in detonation tube.The results show that detonation was achieved under the two conditions.Jet flow undergoing a complex process with the flame in the detonation tube can accelerate the flame velocity in the pre-mixed combustible mixture;thus the DDT process can be strengthened.Compared with the case of utilizing physical obstacle,the case of adopting jet flow in the detonation tube can shorten the DDT run-up distance by 15.4%.The results provide theoretic guidance for experiment of pulse detonation engine.
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Motivated by the current interest in the mechanism of the deflagration to detonation transition(DDT),the DDT process and detonation wave structure of aluminum-air mixture are investigated experimentally by a large scale tube with length of 32.4 m and inner diameter of 0.199 m.The overall DDT process can be divided into slow reaction compression stage,pressure wave speed-up and shock wave formation stage,transfer from shock reaction to critical shock reaction,transfer from critical shock to overdriven detonation,and detonation stage.The optimal concentrations of mixtures in this experimental tube are obtained,and the critical concentration of DDT is also studied.Eight pressure gauges are well-distributed at each periphery of four certain sections in the 1.4 m long detonation testing tube for detonation wave testing.According to the test results,the detonation wave structure of aluminum-air mixture is analyzed,which shows single head mode.
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Efficient initiation of detonation waves is the key to the pulse detonation engine(AB.PDE) operation.In the process of deflagration to detonation transition(AB.DDT),detonation waves were successfully triggered by controlling the shock wave reflection.Interaction between flame and shock is strengthened by putting obstacles in detonation chamber with reasonable arrangement,which can efficiently organize reflection of shock wave and initiate detonation timely.Thus,DDT distance was shortened comparatively.Gasoline/Air detonation waves propagate successfully in the combustor.These results are valuable to the investigation on DDT process of two-phase pulse detonation engine at high frequency.
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