Characteristics of the Development of a Chain Thermal Explosion when Burning Gas Mixtures under Atmospheric Pressure
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USTION, EXPLOSION, AND SHOCK WAVES
Characteristics of the Development of a Chain Thermal Explosion when Burning Gas Mixtures under Atmospheric Pressure S. N. Kopylova, b, *, P. S. Kopylova, I. P. Eltysheva, and T. V. Gubinab aAll-Russian
Research Institute for Fire Protection, Balashikha, Moscow oblast, 143903 Russia Research Nuclear University MEPhI, Moscow, 115409 Russia *e-mail: [email protected]
bNational
Received April 30, 2015; revised July 15, 2019; accepted August 20, 2019
Abstract—An experimental study of the features of the critical transition between two kinetic combustion regimes of gas-air mixtures reacting by a chain mechanism is carried out. The presence of two radically different combustion modes is experimentally confirmed by the example of the combustion of propane–air mixtures, both containing an inhibitor (trifluoromethane) and without it, and in complete agreement with the predictions of the theory of A chain thermal explosion (CTE). It is shown that the pattern observed in this work is characteristic not only for slow-burning but also for fast-burning gas mixtures of various compositions. The two distinct and radically different regions observed on the pressure–time variation curve during the combustion of gas mixtures are a manifestation of the general laws which combustible gas mixtures reacting by the chain mechanism follow in a wide range of pressures. It is found that fluorine-containing hydrocarbon is capable of induced oxidation in rich combustible gas mixtures. Trifluoroethane CF3H has an inhibitory effect on the chain combustion process of the propane–air mixture before it enters the CTE and promotes combustion in the CTE mode. If the transition to the CTE is not possible during the propane–air mixture combustion, the fluorinated agent has only an inhibitory effect on combustion. Keywords: chain thermal explosion, inhibition, promotion, induced oxidation DOI: 10.1134/S1990793120040077
INTRODUCTION As is known [1, 2], the condition for thermal ignition (thermal explosion) consists of the simultaneous fulfillment of the following relations, ensuring the accumulation of thermal energy in a system with progressive self-acceleration:
q+ ≥ q−,
(1)
dq+ dT ≥ dq− dT ,
(2)
where q+ is the heat arrival rate, q− is the heat removal rate, and T is temperature. This condition is general; i.e., it is not limited by the nature and mechanism of the reactions. The condition for branched-chain ignition is the excess of the reproduction rate of active intermediate particles over the rate of their death [2, 3]: (3) f ≥ g – 2 ( w0k ) , where w0 is the chain nucleation rate and k is the reaction rate constant of the nonlinear branching of the chains. The author of [4] first drew attention to the fact that, since quantity q+ is equal to the product of the rate of a chemical reaction and its thermal effect, the 0.5
extremely strong non-Arrhenius temperature dependence of the rate of the chain process creates particularly favorable conditions for the development of a thermal explosion. It was a
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