Page 337 - Advanced thermodynamics for engineers

326 CHAPTER 15 COMBUSTION AND FLAMES

15.2.2 PROCESSES OCCURRING IN COMBUSTION
The thermodynamics of combustion generally relate to the gas in isolation from its surroundings.
However, the surroundings, and the interaction of the gas with the walls of a container, etc. can have a
major effect on the combustion process. The mechanisms by which the combusting gas interacts with
its container are

1. transport of gaseous reactant to the surface – diffusion;
2. adsorption of gas molecules on surface;
3. reaction of adsorbed molecules with the surface;
4. desorption of gas molecules from the surface;
5. transport of products from the surface back into the gas stream – diffusion.
These effects might occur in a simple container, say an engine cylinder, or in a catalytic converter.
In the first case the interaction might stop the reaction, while in the second one it might enhance the
reactions.

15.3 EXPLOSION LIMITS
The kinetics of reactions was introduced previously, and it was stated that reactions would, in general,
only occur when the atoms or ions of two constituents collided. The reaction rates were derived from
this approach, and the Arrhenius equations (see Section 14.3) were introduced. The tendency for a
mixture to spontaneously explode is affected by the conditions in which it is stored. A mixture of

hydrogen and oxygen at 1 bar, and 500 C will remain in a metastable state, and will only explode if
ignited. However, if the pressure of that mixture is reduced to around 10 mm Hg (about 0.01 bar) there
will be a spontaneous explosion. Likewise, if the pressure was increased to about 2 bar there would
also be an explosion. It is interesting to examine the mechanisms which make the mixture become
hypergolic. The variation of explosion limits with state for the hydrogen–oxygen mixture is shown in
Fig. 15.2.

10000

Third limit
1000
Pressure / (mm Hg) 100 Second limit No explosion

10
Explosion
First limit

1
390 410 430 450 470 490 510 530 550 570 590
Temperature / ( C)
FIGURE 15.2
Explosion limits for a mixture of hydrogen and oxygen (from Lewis and von Elbe (1961)).