Experimental results physical and chemical effects

The Mallard-Le Chatelier development for the laminar flame speed permits one to determine the general trends with pressure and temperature. When an overall rate expression is used to approximate real hydrocarbon oxidation kinetics experimental results, the activation energy of the overall process is found to be quite high—of the order of 160kJ/mol. Thus, the exponential in the flame speed equation is quite sensitive to variations in the flame temperature. This sensitivity is the dominant temperature effect on flame speed. There is also, of course, an effect of temperature on the diffusivity generally, the dif-fusivity is considered to vary with the temperature to the 1.75 power. 

The pressure dependence of flame speed as developed from the thermal approaches was given by the expression 

As briefly mentioned earlier, with the background developed in the detailed studies of hydrocarbon oxidation, it is possible to explain this pressure trend more thoroughly. Recall that the key chain branching reaction in any hydrogen-containing system is the following reaction (3.15)  

Any process that reduces the H atom concentration and any reaction that competes with reaction (4.62) for H atoms will tend to reduce the overall oxidation rate that is, it will inhibit combustion. As discussed in reaction (3.21), reaction (4.63) 

Even though SL decreases with increasing pressure for the conditions described, m0 increases with increasing pressure because of the effect of pressure on iJr,. And for higher 02 concentrations, the temperature rises substantially, 

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