Abstract
Critical conditions are usually obtained for ignition in a self-heating solid system consisting of two components generating heat independently, one component being inexhaustible and the other exhaustible by either simple first order or autocatalytic reaction. Ignition depends upon whether the exhaustible component can cause a temperature rise in excess of the upper stationary, but unstable, value possible for the inexhaustible component reacting alone. The system provides a theoretical model for some commonly occurring examples of self-heating and ignition in porous solids containing oxidisable oils. It is shown that: (a) the ignition criterion of the model, which involves a nonarbitrary critical temperature increase, has a high degree of physical reality; (b) the model is, in principle, capable of predicting ignition from primary kinetic and thermal data; (c) it is likely to be possible often to make a reliable prediction of critical size for self-ignition in a two-component system at ordinary atmospheric temperatures by a simple extrapolation from small-scale ignition data, obtained at higher temperatures, in the same way as for ignition due to a single reaction. Examination of both adiabatic and non-adiabatic flame theories showed that a 'steady state' exists only under the special condition that a heat sink exists at the initial temperature. For the general case of freely propagating, non-adiabatic flames only a quasi-steady state can be achieved.
Highlights
Critical conditions are usually obtained for ignition in a self-heating solid system consisting of two components generating heat independently, one component being inexhaustible and the other exhaustible by either simple first order or autocatalytic reaction
We offer the suggestion that their relationship may be regarded in this light an alternative view is helpful, viz. that both treatments are legitimate but refer to physically different situations
We have examined some simple criteria for spontaneous ignition suggested by various authors in order to determine whether any of them are capable of explaining Aljerf’s ignition delay results and those of Mullins [18] for calor gas
Summary
The papers on ignition cover too wide a range of subjects to permit detailed discussion on average seven minutes allotted for flame analysis. We assume that the reaction rate is determined by fuel concentration, oxygen concentration and temperature At this point one asks the question 'When does ignition occur?' Mullins [18] has suggested that it occurs after a definite fraction of the reaction has been completed while Bragg prefers the idea of a constant temperature rise between the mixing of the combustibles and the appearance of flame. If one could explain results over the entire range of mixture strengths by a second order equation one should, as Bragg suggests, find that, on adding small amounts of oxygen to a stream of hot fuel and nitrogen, the ignition delay was independent of oxygen concentration. Whilst it has been suspected for some time that convection may be very important in determining the limits in this type of system, the interplay between convection and quenching, and the two different types of limit, have not, so far as we are aware, previously been shown
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