Autoignition calculations

Before the size of the flammable portion of a vapor cloud can be calculated, the flammability limits of the fuel must be known. Flanunability limits of flammable gases and vapors in air have been published elsewhere, for example, Nabert and Schon (1963), Coward and Jones (1952), Zabetakis (1965), and Kuchta (1985). A summary of results is presented in Table 3.1, which also presents autoignition temperatures and laminar burning velocities referred to during the discussion of the basic concepts of ignition and deflagration. 

Equations (21)-(24) permit the temperature history of the propellant grain during ignition to be calculated. Ignition of any point on the surface is assumed to occur when the propellant autoignition temperature is reached at that point. The propagation rate can then be predicted from the different times at which the different positions on the propellant surface are ignited. The important basic assumption in this approach is the applicability of the... 

Semi-empirical formulae, based only on molecular structure, have been derived which allow flammability limits to be calculated for hydrocarbons and alcohols. Flash points, autoignition temperatures and boiling points may also be calculated from molecular structure for these classes. Quoted examples indicate the methods... 

Calculation of hot surface ignition temperature is complex and depends on many variables. From a safety perspective, it is often safest to assume that a surface with a normal temperature slightly below the autoignition temperature is a potential ignition source. 

In contrast, the pressures of adiabatic (isentropic, according to Poisson) compression which are necessary to bring an explosive mixture to autoignition are 19.5-23.9 atm according to Vant Hoff s calculations, and 25-40 according to Falk s [23] data quoted by Nernst. 

The duration of the compression stroke in the calculated times (Table 6.14) corresponded to approximately 50 ms, signifying that autoignition occurred after top dead centre, as observed experimentally. The choice of bdc as the initial condition is essential to permit the development of spontaneous combustion during the course of the compression stroke as the gas pressure and temperature increase. This definition of the time for ignition differs from that given by, (i) the admission of reactants to a hot, closed reaction vessel, (ii) the end of compression in an RCM, or (iii) the passage of a shock through the reactants in a shock tube. 

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].

Flammable liquid flash point (closed cup) —5°C (22°F) vapor density 3.52 (air = 1) flashback fire hazard vapor pressure (calculated) 113 torr at 40°C (104°F) vapor forms explosive peroxides when anhydrous DOT Label Flammable Liquid, UN 1179 Combustible liquid flash point (open cup) 57°C (135°F) autoignition temperature 170°C (338°F) may form unstable peroxides... 

Flammable liquid flash point (closed cup) 25°C (77°F) vapor pressure (calculated) 218 torr at 100°C (212°F) vapor density 1.1 (air = 1) at 100°F flashback fire hazard autoignition temperamre 194°C (381°F) vapor forms explosive mixtures with air in the range... 

In Figure (9), temperature and species profile plots obtained in an investigation of autoignition in the end-gas of an Sl-engine, known as knock, as presented by Soyhan et al. ( yhan et al., 2000), are shown. The calculations are obtained in employing a two-zone model (burnt and unburnt zones), the detailed mechanism for iso-octane and n-heptane mixtures... 

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