Autoignition delay time

Griffiths [85] and Schrieber et al. [86] have shown how the Muller scheme can be modified to incorporate these features by slightly expanding the low temperature part of the mechanisms and adding a further intermediate. While mass and energy balances were intrinsic in the formulation, an empirical approach was adopted both for the form of the additional reaction and for all the rate constants. The latter, for instance, included pressure dependent terms. Good fits were obtained to rapid compression machine and shock-tube autoignition delay-times for heptane, iso-octane, and their mixtures. 

Fig. 7.27. Calculated autoignition delay-times (solid lines) and excitation times (dashed lines) for stoichiometric methane, acetylene, ethylene and ethane with air at (a) 1 atm, (b) 1500 K. Initial radius of the centre is 1 mm. Geometry is cylindrical. From [194].

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Inspection of the numerical solutions of the equations shows that, with the exception of Es= 0 kcal/mole, the rate of surface temperature increase with time is very large once the surface temperature reaches approximately 420°K—on the order of 108°K/sec. Because typical autoignition temperatures are of the order of 625°K for composite propellants, the particular value of the ignition temperature does not affect the computed numerical value of the ignition-delay time. 

Fig. 7.17. Influence of NO2 on ignition delay-times in a rapid compression machine [141], Fractional ignition delays are shown for various post-compression temperatures. Note that at the higher temperatures NO2 enhances autoignition at small concentrations while hindering it at greater concentrations. Stoichiometric mixture of 90% iso-octane/10% n-hep-tane, total concentration 3.2 x 10 mol cm . ...

This can be understood from the concept of an autoignition wave that can occur in a mixture with temperature gradients. Successive points along a temperature gradient have slightly different delay-times and their succes-... 

When a reaction mixture is under the same conditions of temperature and pressure as in Figure 2, where an autoignition is likely to develop, there is a certain time lapse, called the autoignition delay, before this becomes effective. This delay is important in the Diesel engine but also in the safety of industrial combustion reactions. The measurement of these delays is carried out mainly using batch reactors, rapid compression machines and shock tubes. 

Measurements of ignition delay times for methyl and ethyl acrylates and methyl and ethyl crotonates indicated that methyl and ethyl esters had similar reactivity but acrylates autoignited more easily than crotonates. Detailed kinetic models were proposed to reproduce these results correctly. The difference of reactivity between C(3) and C(4) unsaturated esters is suggested to be due to easier unimolecular initiations involving the production of branching agents. ... 

Ignition of a mixture above its autoignition temperature is not instantaneous. The ignition time delay may be a fraction of a second for temperatures well in excess of the autoignition temperature a few minutes when the material is just above its autoignition temperature. 

The autoignition temperature (AIT) or tlte maximum spontaneous ignition temperature is defined as the maximum temperature at which combustion occurs in a combustible bulk gas mixture when tlie temperature of a flammable gas-air mixture is raised in a uniformly heated apparatus. The AIT represents a tlircshold below which chemicals and combustibles can be handled s ely. (The AlTs of selected substances arc available in the literature. ) The AIT is strongly independent on tlie nature of hot surfaces. The AIT may be reduced by as mudi as 100-200°C when the surfaces arc contaminated by dust. When tlie temperature of a flanuiiable mi.xturc is raised to or above the autoignition temperature, ignition is not spontaneous. Most notably in liquids, there is a finite delay before ignition lakes place, i.e., a lapse between the time tlicrc is a flammable mixture reaches its flame temperature and tlie first appearance of a flame. An equation tliat correlates with the ignition temperature is also available in the literature. ... 

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