Tutorials Manual

Chemkin 4.1.1
Chapter 2: Combustion in Gas-phase Processes
2.6.2 Jet Flame Network
2.6.2.1 Problem Description
This user tutorial describes the mechanics of constructing a PSR (perfectly stirred
reactor) cluster to represent a non-premixed jet flame.
Often, chemical kinetics is either omitted entirely or greatly reduced to make CFD
simulations possible for chemically reactive flow systems. When pollutant emissions
are to be predicted, the assumption of local chemical equilibrium is not appropriate
and the utilization of detailed reaction mechanism is needed. For example, the
characteristic chemical time scale of NO is much larger than that of a typical flame
species and is compatible to the characteristic time scale, or residence time, of the
flow system. Consequently, the local NO concentration level is far from its equilibrium
value and depends on the chemical state, the age, and the history of the gas mixture.
For simple flow fields, the exit concentration of the species of interest can be obtained
by simply integrating its production rates along streaklines (or streaktubes). In this
case, the detailed reaction mechanism is used to calculate production rates according
to local chemical states and can be a component of the post-processing utility.
However, this approach is not suitable for complex flow fields as strong mixing actions
and re-circulations make tracking streaklines difficult. Building a reactor network from
“cold” CFD solutions is a plausible approach under this situation as it can utilize the
detailed reaction mechanism while preserving some key fluid dynamic features that
are important to emission predictions such as the residence time. Discussions on how
to identify the reactors and their connectivity are beyond the scope of this tutorial.
General guidelines on deriving reactor networks from CFD solutions can be found in
papers done by Bhargava et al. 16 (see p. 57) and Faravelli et al. 18
2.6.2.2 Problem Setup
For this problem, we assume that a seven-reactor network is derived from local gas
composition and temperature solutions of a CH 4 -air diffusion jet flame simulation. The
residence times of these reactors and the connectivity and mass flow rates among
them are also obtained from the velocity solutions of the CFD simulation. The PSR
network representing the diffusion jet flame is given in Figure 2-38.
18. Faravelli et al., Computers and Chemical Engineering 25:613-618 (2001).
© 2007 Reaction Design 60 RD0411-C20-000-001