This paper presents a numerical investigation of natural gas combustion in an OPRA OP16 tubular DLE combustor operating in premixed mode. The aim of this study is to compare different CFD models for the accurate prediction of emissions. The numerical results are validated against experimental results obtained in an atmospheric combustion test rig.
A CFD workflow using AVL FIRE software has been devised enabling accurate CO and NOx emission calculations. Different turbulence models have been employed for adiabatic flow simulations at nominal load: the k-ζ-f turbulence model, PANS and LES-CSM model. Two detailed reaction mechanisms have been used to model combustion chemistry and thermodynamic properties: the DRM19 mechanism and the GRI 3.0 reaction mechanism. In the adiabatic simulations both the k-ζ-f and LES-CSM turbulence models perform well in combination with the GRI 3.0 reaction mechanism. In the non-adiabatic simulations heat exchange between combustor liner and cooling air is taken into account by using a wall temperature profile derived by scaling the experimentally obtained temperature profile such that the requirement of conservation of energy is satisfied.
Reactive flow simulations at nominal load using the k-ζ-f turbulence model and GRI 3.0 reaction mechanism resulted in accurate predictions for time-averaged combustor pressure drop, CO and NO emissions: the predicted CO value deviates less than 20% from its experimentally obtained value, for NO the deviation is less than 10%. Inclusion of heat exchange between combustor and cooling air liner reduces this deviation to less than 10% for CO and less than 2% for NO. At a load equal to 106% of its nominal value, non-adiabatic simulations result in less satisfactory CO and NO predictions: NO is over predicted by approx. 20% while CO is overpredicted by nearly a factor of four. The latter is caused by the fact that experimentally obtained CO emissions reduce significantly when the load is increased from 100% to 106%, while numerically obtained CO emissions just change by a few ppm.