Ignitibility limits

Figure A3.14.4. P-T ignition limit diagram for H2 + O2 system showing first, second and third limits as appropriate to a closed reactor. The first and second limits have similar positions in a typical flow reactor, for which there is also a region of oscillatory ignition as indicated.

Figure A3.14.6. P-T ignition limit diagram for CO + system in a flow reactor showing location of...

Hull, Ignition Limits of IPN Vapour/Air Bubbles in Tubes of 25.5mm Diameter , DRIC BR-41608, Rocket Proplsn Est, Westcott (Engl) (1973)... 

U-shaped curve, we have mixtures that can be ignited for a sufficiently high spark energy. From Equation (4.25) and the dependence of the kinetics on both temperatures and reactant concentrations, it is possible to see why the experimental curve may have this shape. The lowest spark energy occurs near the stoichiometric mixture of XCUi =9.5%. In principle, it should be possible to use Equation (4.25) and data from Table 4.1 to compute these ignitability limits, but the complexities of temperature gradients and induced flows due to buoyancy tend to make such analysis only qualitative. From the theory described, it is possible to illustrate the process as a quasi-steady state (dT/dt = 0). From Equation (4.21) the energy release term represented as... 

In practice there is only a certain range of compositions where premixed gases will bum There is obviously a great deal of interest in this composition because it sets limits of operation of many processes. The ignition limit is only loosely defined because it depends sensitively on temperature, flow conditions, and the presence of trace chemicals in the fuel, which can act as flame initiators or inhibitors by enhancing or slowing either r or rt. 

The combustion of HCN has been studied extensively. Ignition limits in air are ca 42.5 ... 

Mixtures of (H202)gas/air will not detonate at subatmospheric press. However, a 35 mol% mixt will detonate at 6700 ft/sec at 1 atm. The ignition limit under these conditions is about 30 mol% H202 vapor (Ref 21)... 

Fig. 14.5 Ignition limits for an equimolar mixture of propane and oxygen [241]. The temperatures and pressures correspond to the initial reaction conditions in the 500 cm3 quartz vessel containing the reactants.

Ignition temperature was used with partial success (52, 57, 74, 135,189). Results depended critically on the method used—e.g., adiabatic compression, ignition limits in tubes, or crucible method. Jackson (70, 86, 87) determined ignition temperatures for a large number of hydrocarbons by the crucible method and developed correlations with structure. 

Curve 0 turns out for large to be the asymptotic envelope of the family and, consequently, gives the limiting relation between distance and the temperature of the cold wall at the ignition limit. 

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. 

We present a table which illustrates the various orders of magnitude at the ignition limit for a fictitious reaction whose rate is equal to... 

In the absence of catalysis on the surface, similarity of the concentration and temperature fields is achieved precisely at the ignition limit if the coefficients of diffusion and thermal diffusivity are equal, since in this case both the diffusion gradient and the temperature gradient at the igniting surface are equal to zero, and the equations of diffusion and thermal conductivity with the chemical reaction may be reduced to the form of an identity (see our work on flame propagation [3]). 

The temperature gradient and conductive thermal flux at the igniting surface disappear at the ignition limit. 

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