The Ignition Process

When heat is supplied to a gaseous mixture of oxidizer and fuel components, i. e., a premixed gas, an exothermic reaction occurs and the temperature increases. The reaction may conhnue and proceed into the unreacted portion of the mixture even after the source of the heat is removed. The amount of heat that has to be supplied to the mixture to achieve this is defined as the ignition energy. If, however, the reac-hon terminates after removal of the heat source, ignition of the mixture has failed. This is because the heat generated in the combustion zone is not sufficient to heat the unreacted portion of the mixture from the initial temperature to the ignihon temperature. 

Ignition is dependent on various physicochemical parameters, such as the type of reactants, reaction rate, pressure, the heat transfer process from the external heat source to the reactants, and the size or mass of the reactants. The rate of heat production is dependent on the heats of formation of the reactants and products, the temperature, and the activation energy. As the process of ignition includes an external heating and an exothermic reaction of the reactants, there is a non-steady heat balance during these phases. 

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Side view of the ignition process in an SCSI engine showing the spark arc and early flame development. 

Aqueous cyanide effluent containing a little methanol in a 2 m3 open tank was being treated to destroy cyanide by oxidation to cyanate with hydrogen peroxide in the presence of copper sulfate as catalyst. The tank was located in a booth with doors. Addition of copper sulfate (1 g/1) was followed by the peroxide solution (27 1 of 35 wt%), and after the addition was complete an explosion blew off the doors of the booth. This was attributed to formation of a methanol vapour-oxygen mixture above the liquid surface, followed by spontaneous ignition. It seems remotely possible that unstable methyl hydroperoxide may have been involved in the ignition process. 

The ignition process for FRC materials can be described in terms of the relationship between time to ignition and heat flux. A technique developed by FMRC using its Small-Scale Flammability Apparatus (2-6) was used for the quantification. 

In many practical systems, one cannot distinguish the two stages in the ignition process since To> t, thus the time that one measures is predominantly the chemical induction period. Any errors in correlating experimental ignition data in this low-temperature regime are due to small changes in rt. 

The ignition process was modeled as an energy release in a relatively small volume inside the vessel with the power as a given function of time. 

Fig. 12.7 Light attenuation and pressure profile of the ignition process of a micro-rocket motor with the BK igniter.

Since initiation of the decomposition is dependent on the heat flux supplied by the high-temperature gas flow, the ignition process is dependent on the various gas-flow parameters, such as temperature, flow velocity, pressure, and the physicochemical properties of the gas. 

When the ignition process occurs under conditions of constant pressure, the momentum equation is expressed by... 

Equations (13.7)-(13.13) are used to evaluate the ignition processes of energetic materials with appropriate initial and boundary conditions. In general, the conditions in the thermal field for ignition are given by... 

Tg, and 3 -are the surface temperatures at t , tg, and tp respectively. During the time tp < t < tjs, ignition is completed, combustion occurs without external heating, the surface temperature reaches T, and the bunting rate attains a steady-state value with regression velocity at t, Fig. 13.4 shows a schematic diagram of the ignition process expressed by Eqs. (13.17)-(13.21). 

The ignition process is basically the same for any tubed firework, but when the sparks start to fly, the chemistry becomes very different. Sparks are self-luminous but they need atmospheric oxygen to sustain the high temperature oxidising reaction with the steel particles or other emitters that are present, as described in the previous chapter. 

The ignition process initiates a self-propagating, high-temperature chemical reaction at the surface of the mixture. The rate at which the reaction then proceeds through the remainder of the composition will depend on the nature of the oxidizer and fuel, as well as on a variety of other factors. "Rate"... 

Quantitative development of the above model of the ignition process is overwhelmingly complicated. Lewis Von Elbe therefore chose to attempt correlation of experimental minimum ignition energies with some energy functions computed from minimal flame parameters... 

Reactions (5) and (6) are endothermic, but reaction (7) is strongly exothermic. Reactions (1)—(7) constitute the ignition process. According to Blackwood and Bowden the chief reaction when the powder begins to burn is the oxidation of charcoal by potassium nitrate. 

Lugovskoi et al. (28) in a qualitative study experimentally observed the ignition process on suspended catalyst particles (H2 + 02) and showed the relations between the local values of the transport coefficients and spots where the ignition process starts. 

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