The Wave Theory of Ignition

Abstract

The concept of the wave mechanism of formation of a quasi-stationary zone of a chemical reaction is introduced for determination of critical conditions and a delay time of ignition with heat losses. It is shown that the connection of the depth of burnup and the velocity of an intermediate wave with the value of heat losses determines the specifics of ignition under nonadiabatic conditions. The wave theory of ignition, which has shown high efficiency when determining temporary characteristics of ignition in systems with the complex mechanism of chemical interaction (consecutive, parallel, independent reactions) is presented. Main characteristics of the ignition process are determined on the basis of the wave mechanism of formation of a chemical reaction zone, namely the time of inert warming up, establishment and failure of a thermal balance; the physical meaning of these characteristics is also clarified. It is shown that the development of a heating process depends on a ratio of characteristic times of formation of a chemical reaction zone capable of independent propagation, and complete burnout on a sample surface. Critical values of a heat loss coefficient are calculated. It was shown that critical ignition conditions coincide with limiting conditions of occurrence of an intermediate combustion wave ICW. The main characteristics of ignition process for a first-order reaction are determined. The results of numerical calculations validated the main assumptions as well as the main conclusions of the approximate analysis. It is suggested to find the scale temperature (ignition temperature) from the equality of an external thermal flux to the heat flux from a reaction zone in an intermediate combustion wave, which maximum temperature is equal to ignition temperature. It is shown that it is possible to consider warming up as inert at surface temperatures smaller than ignition temperatures; the time of this stage makes the main part of a delay time of ignition τ ig. It allows expressing τ ig through characteristics of an intermediate combustion wave. The analytical method of calculation of temporary characteristics of ignition of a porous body under conditions of a counter nonstationary gas filtration is created. It is found that the rate of heating of a substance during ignition is close to the rate of heating of this substance with an intermediate wave of filtration combustion (IWFC) with the maximum temperature equal to ignition temperature. The analytical method of calculation of temporary characteristics of ignition of a porous sample with a flow of hot gas (cocurrent nonstationary filtration) is also suggested.