Abstracts - KTH Mechanics

Comparative Study of Turbulence Modeling in variable
density Jets and Diffusion Flames
F. Tabet-Helal a, B. Sarh a and I. Gökalp a
In this study, the performances of the Reynolds Stress model (RSM) and the
k- model with Pope correction in their turbulence dissipation rate equation, ,
are investigated in many configurations of variable density jet and nonpremixed
flames. The predictions are compared to the available experimental
data.
It is known that the k- model and the RSM model over-predict the
spreading and decay rate of the round jet flow by 40 % 1. To improve the
accuracy of these turbulence models for solving round jet flows and jet flames,
the turbulence constants (C1 and C2) of the turbulence dissipation rate
transport equation may be modified, as they are responsible for the
generation/destruction of the turbulence energy dissipation. Modifications to
the turbulence constants have been suggest in the past by McGuirk and Rodi 2,
Morse 3, Launder et al. 4 and Pope 1. All modifications involve the turbulence
constants becoming functions of the velocity decay rate and jet width. For
complex flows, a more convenient way is to introduce a correction like that
proposed by Pope 1 taking into account the vortex stretching. Also, this
modification is more generally applicable.
The variable density jets studied are respectively methane, air and CO2. The
corresponding density ratio between the jet flow and the air coflow are
respectively 1.8, 1 and 0.66. The diffusion flames considered are respectively,
H2/Air, CH4/Air and CH4-H2-N2/Air flames. A flamelet library approach has
been applied to account for chemistry and turbulence interaction effects. This
work is focused on the applicability of Pope correction to a wide range of
variable density jets and diffusion flames.
A Pope correction model is implemented in the commercial CFD code
Fluent 6.1.22. User-define subroutine function of the flow variables is written
for this purpose.
The results obtained compare favourably with the experimental data. The
two turbulence models predictions including Pope correction in equation are,
in general, very close in all the configurations considered. However, the RSM
model calculations needs more refine mesh and consequently more computing
time than the k- model.
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a LCSR-CNRS, 1 C avenue de la recherché scientifique, Orléans 45071, France.
1 Pope, AIAA J. 16, 3 (1978).
2 Mcguirk and Rodi, 1 st Symp. On Turbulent Shear Flows,71-83, (1979)
3 Morse, PhD Thesis, London University, 1977.
4 Launder et al., NASA SP-311, 1972.
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