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performamance of transient tempertureature in hot spot ignition

By Scott Knight,2014-09-06 22:00
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performamance of transient tempertureature in hot spot ignition

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    performamance of transient tempertureature in hot spot

    ignition

    Zhang Qi

    5 (State key Lab. of Explosion Science and Tech., Beijing Institute of Technology,

    Beijing 100081)

    Abstract: This work presents a method to estimate probability initiating combustion and detonation by

    temperature load in void in energetic material. When a void is compressed its volume decreases and the

    temperature in the void increases. The local thermal source due to the void deformation in energetic 10 material forms a temperature load which may initiate combustion and detonation. The temperature-time

    history in the void was called temperature load in this work. Its peak value and holding time are the

    necessary and sufficient condition initiating combustion or detonation of energetic material

    surrounding the void. Comparing with the energy acted on the unit area of hot spot, the performance of

    transient temperature which can better serve to reflect the physical essence in ignition. The main object 15 of this work is to study transmitting process of temperature load from void to energetic material and

    probability initiating combustion and detonation by temperature load in void.

     Keywords: ignition; temperature load; energetic material

    0 Introduction

    20 Voids within energetic material can generate hot spots that have the potential to start local

    burning leading to partial reaction or detonation. Several thermomechanical mechanisms that have

    been suggested as potential hot-spot sources include: friction between adjacent grains, jetting of

    material fragments across voids, hydrodynamic pore collapse, viscous heating and internal shear,

    and shock interactions at density discontinuities [1-2]. Much of the experimental data and 25 analytical modeling of hot spots indicates that the specific mechanisms responsible for hot spot

    formation depend on the physical and thermodynamic properties of the heterogeneous materials,

    as well as the means by which the energy is transmitted to the material from an external source. In

    general, there are three kinds of mechanisms in the theory referring to hot spot formation to

    transition to detonation in inner voids of energetic materials. The first is the compression of any 30 gaseous or vapour content in a void to produce high temperature [3-5].The second is the action of

    the plastic flow and the high-speed jet in the collapse of a void . The third is the ignition energy

    from viscoplastic heating of the condensed-phase material surrounding a collapsing cavity in the

    process of void compression. Many works have investigated those conditions corresponding to

    each one or two of three mechanisms. It is widely agreed that under the low loading rate the void 35 with large size prefers the first mechanism.

    In analyzing the hot spot ignition, the energy per unit of area on the surface of hot spot is

    often taken as the critical condition of ignition. It means that the critical ignition value is

    determined by the total energy of the hot spot divided by its surface area of boundary. However, in

    the experiment of hot spot ignition that the authors carried out, it was found that the hot spots with 40 the same dimension and energy, obviously having an identical energy per unit of area for each

    other, caused different ignition phenomena in practice. When the holding time or the duration is

    longer, the hot spots with an identical energy can lead to ignition, while when the holding time is

    shorter they cannot. It showed that in the theory taking the energy per unit of area as the critical

    condition of ignition is not complete and correct. In order to comprehensively understand this

     45 phenomenon, it is necessary to study response of energetic material around a void to the

     Foundations: Foundation for Doctor Dissertation of China20111101110008

    Brief author introduction:Zhang Qi, (1956-), Prof. Explosion Mechnics. E-mail: qzhang@bit.edu.cn

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    豆丁网地址!/msn369

    temperature inside the void so as to obtain the knowledge of different modes initiating combustion

    or detonation by local thermal source.

    When a void is compressed its volume decreases and the temperature in the void increases.

    The temperature-time history in the void was called temperature load in this work. The 50 temperature load is exerted on the surrounding energetic material around the void, and makes its

    temperature rise and chemical reaction start. If the chemical reaction can release enough energy to

    make shock wave to keep in self-sustained state, the ignition occurs. Whether the chemical

    reaction can cause shock wave self-sustained is dependent on the reaction rate and the width of

    reaction zone under the temperature load inside the void. Generally, the ignition is a process in 55 which the energy of reaction zone increases with distance and the width of reaction zone decreases

    with distance gradually till the width of reaction zone tends to a constant in state of stable

    detonation. Although a large number of reports dealt with the experimental results of the width of

    reaction zone in the process of stable detonation, few works focused on its varying in the process

    of ignition. By analysis above, it is known that when the width of reaction zone close to the void is 60 less than that in state of stable detonation, combustion or detonation can not be initiated.

    Width of reaction zone is the span from start section to finish section of chemical reaction

    along to propagation direction. Since the chemical reaction close to void originates from heat

    transfer of the temperature load in void, the responding zone width to temperature in void caused

    by heat transfer can be considered to be the critical parameter. By using numerical simulation, the 65 response characteristics of energetic materials around void to the temperature load was discussed

    in this work, the probability initiating combustion or detonation by temperature load in void was

    analyzed, and a method to estimate the critical parameter initiating combustion or detonation was

    proposed and proved by the designed experiment.

    Ignition process was considered in the model. A void exists in energetic material and a 70 temperature load acts on the interface between void and energetic material. The temperature load

    is ignition source. The ignition source in the void is called hot spot in the model. It is different

    from traditional thermal explosion theory in which hot spot refers to a small volume of energetic

    material with high temperature.

    Two stages existing in the process initiating combustion or detonation were assumed in the 75 model. The first is transmitting of temperature load from void to energetic material. This is base

    initiating combustion or detonation. The second is chemical reaction in energetic material

    surrounding the void. The effect of chemical reaction surrounding the void on temperature inside

    void was ignored in the model. That chemical reaction surrounding void occurs in a span of delay

    time was assumed in the model.

80 1 Computational model of temperature response

     Governing equation of heat transfer is given by 2? ?? 2 ? T T ?T & ? ? ++ q= ρ c λ2?r r ?r ?t ? ?

    Where T is the temperature; ρ is the density of energetic material which is equal to 3 1470kg/mfor given energetic material in this work; c is the specific heat of energetic material

    & 85 which is equal to 300 Jkg?; qis the heat produced by chemical reaction in unite mass of

    energetic material in unite time. From the supposition of the model the effect of chemical reaction

    in process of transmitting of temperature load from void to surrounding region was ignored, hence, 豆丁网地址!/msn369

    豆丁网地址!/msn369

    ?1?1?1& q= 0; λ is the heat conductivity which is equal to 0.21 JmsK. Boundary and initial conditions for Equations (1) are

     90 r = aT , =f (t)

     t = 0,T = T 0 Where a is the radius of spherical void, f(t) is a function of t, shown in Fig.1,Tis the initial0 temperature in energetic material. onsideration. The temperature In calculating, a spherical void of diameter 1mm is taken into c

    load is exerted on the boundary of void. The aim is to clarify the responding regularity of 95 energetic material to temperature load in the void and probability initiating combustion or

     detonation by the temperature load. 2 Response of energetic material around void to temperature The temperature acted on the boundary of void varying with time was shown in Fig. 1. Under

    this boundary condition, the response process of energetic material around the void to temperature 100

     was analyzed, and the temperature variations in energetic material around the void were given as shown in Fig. 2. From Fig. 2, it can be obtained that the maximum width of temperature distribution in the near region of void is about 10μm if the flash point for energetic material is equal to 230?C. Fig.1 Temperature load on boundary of void 105