4. Conclusions
Steady-steady simulations are performed using computational fluid
dynamics. The fluid viscosity, specific heat, and thermal conductivity
are calculated from a mass fraction weighted average of species
properties, and the specific heat of each species is calculated using a
piecewise polynomial fit of temperature. Natural parameter continuation
is performed by moving from one stationary solution to another.
Knowledge of critical parameters gains a fundamental understanding of
the essential factors affecting the stability of the combustion process.
Particular emphasis is placed upon the sustained combustion of at least
a portion of fuel under essentially adiabatic conditions at a rate which
surmounts the mass transfer limitation. The major conclusions are
summarized as follows:
- It is possible to achieve essentially adiabatic combustion in the
presence of a catalyst at a reaction rate many times greater than the
mass transfer limited rate.
- Flammable mixtures of carbonaceous fuels normally burn at relatively
high temperatures, and substantial amounts of nitrogen oxides
inevitably form if nitrogen is present.
- Complete catalytic combustion of a target species can only occur when
oxygen gas is found in molar stoichiometric excess; a condition which
is easily met when the target species is present in trace quantity in
air.
- In combustion systems utilizing a catalyst, there is little or no
nitrogen oxides formed in a system which burns the fuel at relatively
low temperatures.
- In the mass transfer limited zone, the reaction rate cannot be
increased by increasing the activity of the catalyst because catalytic
activity is not determinative of the reaction rate.
- Among the unique advantages of the catalytically supported thermal
combustion in the presence of a catalyst is the fact that mixtures of
fuel and air which are too fuel-lean for ordinary thermal combustion
can be burned efficiently.