Tutorials Manual

Chapter 2: Combustion in Gas-phase Processes
Tutorials Manual
When chemical kinetics is the limiting factor of the reacting system under
investigation, we would model the reactor as a perfectly stirred reactor (PSR). The
perfect mixing of reactants and products inside a PSR is usually accomplished by
using mixers or multiple-jet injections. However, a gas-turbine combustor normally
does not have these mixing mechanisms and has to rely on fluid motions, i.e., largescale
eddies and turbulence, to provide the necessary mixing actions. Local
turbulence is particularly important as it promotes micro-scale mixing among the gas
species.
Although it is adequate in many cases to treat the gas-turbine combustor as a PSR, 20
this PSR approach does not always yield proper predictions for the combustor outlet
condition. One of the factors that can cause the failure of a PSR approach is the
interaction between the turbulence (micro-mixing) and chemical kinetics. If the
turbulence is too weak to provide fast mixing among the gas species, the micromixing
process will interfere with the chemical kinetics. In some cases, when the
reactants (non-premixed cases) or the reactants and the products (premixed cases)
fail to mix microscopically before they are blown out of the combustor, no combustion
zone can be established inside the combustor. The partially stirred reactor (PaSR)
model is a tool that can be used to assess the extent of turbulence-kinetics interaction
in a gas-turbine combustor or to provide information on how turbulence intensity will
affect the combustor.
2.6.3.2 Problem Setup
The project file for this sample problem is located in the samples41 directory and is
called pasr__ch4_air.ckprj. A skeletal methane/air combustion mechanism is used
to speed the calculation, since we are interested in knowing whether combustion can
be sustained inside the combustor by turbulence mixing.
Molecular mixing is important to this problem despite the fact that the fuel and air are
premixed before entering the combustor. For premixed problems, good and fast
mixing between the fresh reactants and the burned products is required to anchor the
combustion zone inside the combustor. To provide a good starting point for the back
mixing, we need to initialize the PaSR with the burned state. This is similar to starting
a gas-turbine combustor with a pilot flame. We can use the equilibrium model or the
steady state PSR model to obtain the burned state of the premixed fuel-air mixture.
Effects of the initial condition on the solutions will be minimal once the simulation time
passes the residence time of the combustor.
20. T. Rutar and P.C. Malte, J of Engineering for Gas Turbines and Power, 124:776-783
(2002).
RD0411-C20-000-001 65 © 2007 Reaction Design