Large eddy simulations of wake-stabilised flares

• LES has been used to predict major properties in a natural gas flare.
• Both low and high momentum-flux-ratio flames have been investigated.
• Potential benefits and limitations of using the LES technique are discussed.
• LES predictions are in closer agreement with experimental data.

This paper investigates the potential benefits, as well as limitations, of using the LES technique to predict the combustion efficiency, emissions and temperatures of wake-stabilised flares. LES calculations have been performed for two jet-to-cross-flow momentum flux ratios, with the results being compared with experimental data, where available, and with predictions of a closed RANS model using a second-moment turbulence closure. The flame fragmentation and the secondary flame zone attached to the release pipe at high jet-to-cross-flow momentum flux ratios, and their influence on the evolution of the counter-rotating vortex pair found in such flames, are discussed in detail, with realistic predictions being produced by the LES. The global mixing characteristics and the combustion efficiency of such flames are also compared with available data with reasonable agreement found. In all cases, LES predictions are found to be superior to equivalent RANS results, although the extra computational effort required in predicting species concentrations and flare efficiencies, in particular, may not be warranted.