Numerical modeling of turbulent jet diffusion flames in the atmospheric surface layer

The evolution of turbulent jet diffusion flames of natural gas in air is predicted using a finite-volume procedure for solving the flow equations. The model is three dimensional, elliptic and based on the conserved-scalar approach and the laminar flamelet concept. A laminar flamelet prescription for temperature, which is in agreement with measurements in methane/air flames and accounts for radiative heat losses, has been modified and adapted to natural-gas flames. The k-ε-g turbulence model has been used. Different probability-density functions for the conserved scalar and an alternative method which does not require the use of a pdf are employed. The model has been applied to flames in the buoyancy-momentum transition regime, in both cases where the fuel jet is immersed in a co-flowing or in a cross-flow air stream whose properties correspond to the atmospheric surface layer. Experiments have been carried out for a horizontal flame in a wind tunnel with simulated atmospheric boundary layer, and measurements of temperature distributions are compared with the numerical results; a good agreement is found. The influence of wind properties on flame shape has been investigated. For horizontal flames, a correlation is proposed for the stoichiometric flame length as a function of the Froude number and the wind to jet velocity ratio. Flame length predictions have been compared with available experimental data and correlations proposed in the literature.