Ignition by heat conduction

A remarkable qualitative conclusion which may be experimentally verified is that at the ignition limit the amount of heat released by the heated surface to the gas being ignited by heat conduction vanishes since the temperature gradient is equal to zero. 

Ignition by heat radiation 6.3 Ignition by heat conduction... 

The temperature instability of a two-dimensional reactive fluid of N hard disks bounded by heat conducting walls has been studied by molecular dynamics simulation. The collision of two hard disks is either elastic or inelastic (exothermic reaction), depending on whether the relative kinetic energy at impact exceeds a prescribed activation barrier. Heat removal is accomplished by using a wall boundary condition involving diffuse and specular reflection of the incident particles. Critical conditions for ignition have been obtained and the observations compared with continuum theory results. Other quantities which can be studied include temperature profiles, ignition times, and the effects of local fluctuations. 

The more favorable start-up times for cordierite are mainly attributed to its lower thermal conductivity. Before ignition, axial heat conduction in the solid is less pronounced for the ceramic material, due to its lower k. Heat generated on the surface cannot diffuse away from the reaction front located near the channel exit at a fast enough rate this leads to the formation of a spatially confined reaction zone (see in Fig. 8.9a the more pronounced hot spot at the reactor rear-end), which in turn promotes faster fuel consumption and leads to faster light-off. This faster light-off is also attributed to the fact that less heat is accumulated in the cordierite compared to the FeCr alloy. In Fig. 8.10, thermal power generated by surface... 

The physical properties of a flaimnable solid, such as hardness, texture, waxiness, particle size, melting point, plastic flow, tiiennal conductivity, and heat capacity, impart a wide range of cliaracteristics to tiie flanmiability of solids. A solid ignites by first melting and tiien producing sufficient vapor, which in turn mixes witii air to fonn a flaiimiable composition. 

In solid form, Mg is difficult to ignite because heat is conducted rapidly away from the source of ignition it must be heated above its mp before it will bum. However, in finely divided form it may be ignited by a spark dr the flame of a, match. Mg fires do not flare up violently unless there is moisture present. Therefore it must be kept away from w, moisture, etc. It m y. be ignited by a spark, match flame, or even spontaneously when the Mg is finely divided and damp, particularly with w-oil emulsion. Also, Mg reacts with moisture, acids, etc to evolve H2 which is a highly dangerous fire arid explosion hazard (Ref 23)... 

Finally, we estimate the order of magnitude of the time of the heated solid to achieve Tpy at the surface. This is primarily a problem in heat conduction provided the decomposition and gasification of the solid (or condensed phase) is negligible. We know that typically low fuel concentrations are required for piloted ignition (XL 0.01-0.10) and by low mass flux (mv 1-5 g/m2 s) accordingly. Thus, a pure conduction approximation is satisfactory. A thermal penetration depth for heat conduction can be estimated as... 

Let us return to our discussion of the prediction of ignition time by thermal conduction models. The problem reduces to the prediction of a heat conduction problem for which many have been analytically solved (e.g. see Reference [13]). Therefore, we will not dwell on these multitudinous solutions, especially since more can be generated by finite difference analysis using digital computers and available software. Instead, we will illustrate the basic theory to relatively simple problems to show the exact nature of their solution and its applicability to data. 

For long heating times, eventually at t —> oo, the temperature just reaches Tig. Thus for any heat flux below this critical heat flux for ignition, gig crit, no ignition is possible by the conduction model. The critical flux is given by the steady state condition for Equation... 

The flammability of solid substances is determined by burning rate tests [10,134,135]. From a mold, a pile of the substance under investigation is placed on a noncombustible, nonporous, and low heat-conducting base plate. One end of the pile is ignited by a hot gas flame or a hot platinum wire (temperatures above 1000°C). The burning rate is established and measured. 

Conceptually, Mallard and Le Chatelier stated that the heat conducted from zone II in Fig. 4.4 is equal to that necessary to raise the unbumed gases to the ignition temperature (the boundary between zones I and II). If it is assumed that the slope of the temperature curve is linear, the slope can be approximated by the expression (7 r 7 i)M, where 7 is the final or flame temperature, 7-... 

Pyrolysis commences at bed surface temperatures in the range of 150-300°C [22,23]. Almost simultaneously, flaming combustion takes place in the combustion system above the fuel bed (see Figure 58C). At t2 the pyrolysis is sustained by heat from over-bed flames. The heat is transported by radiation. At times ts to t4 the dominant heat source has changed to the char combustion zone (ignition front) instead. The heat from the ignition front is also transported by means of conduction and radiation. 

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