Citation

Liu F, Wen JX. Fire Safety J. 2002; 37(2): 125-150.

Copyright

(Copyright © 2002, Elsevier Publishing)

DOI

unavailable

PMID

unavailable

Abstract

For buoyant diffusion flames, both thermal and mechanical forces affect the turbulence mixing and combustion processes. The computation of individual turbulent heat flux and temperature variance are necessary and important. This raises questions about the use of traditional two equation k-[var epsilon] type turbulence models in such applications. The present study is aimed at demonstrating the significant effects of turbulence modelling on the CFD simulation of buoyant diffusion flames. Two different turbulence models are used to compute McCaffrey's flame data. The first model is based on Hanjalic's (Proceedings of the 10th International Heat Transfer Conference, Vol. 1, 1994, p. 1) four-equation turbulence model, which has been modified by the present authors to account for turbulence anisotropy and coupled with an algebraic formulation for Reynolds stresses. The second is the low-Reynolds-number (LRN) k-[var epsilon] model of Ince and Launder (Int. J. Heat Fluid Flow 10 (1989) 110). The predictions of both models for temperature gradients and buoyancy generation of turbulence are examined. It is found that the generalised gradient diffusion hypothesis formula in the LRN k-[var epsilon] model severely under-predicts the buoyancy production of turbulent kinetic energy. Considering that the simple gradient diffusion formula in the standard k-[var epsilon] model with buoyancy modification predicts even lower turbulence production due to buoyancy, fire modellers are cautioned about the use of the two equation k-[var epsilon] type turbulence models in such applications. Furthermore, it is found that the velocity and temperature predictions by the two turbulence models differ as much as 20% along the centreline. The LRN k-[var epsilon] model under-predicts the radial spreading rate of the flame, the temperature rise in the persistent flaming region and the lower portion of the intermittent flaming region. For the rest of the intermittent flaming region and the thermal plume, it over-predicts the temperature rise. The modified version of Hanjalic's turbulence model has achieved quantitatively good agreement with the experimental data on the predictions of velocity and temperature distributions. It has also given better predictions for the radial spreading rate of vertical flames and the flame shapes.