A stochastic model for aircraft gas turbine combustor emissions
The emission of NOx from aero-gas turbine combustors, which in the present generation of designs consists mainly of thermal NOx ' is of great concern due to its potential damage to the stratospheric ozone layer. Soot production in gas turbine combustors is also undesirable since it is both the major source of exhaust smoke and, more importantly, the principal agent in thermal radiation to the combustor liner. Furthermore thermal radiation from the soot redistributes energy in the combustor, modifying the temperature field. This consequently affects the production of other pollutants, notably that of thermal NOx > since the production rate is especially sensitive to temperature. Mathematical models for predicting gas turbine combustor emissions can be divided, in general terms, into two main groups, Methods based on zonal (or modular) approach and on CFD modelling. CFD modelling allows the use of computation intensive multi-dimensional Navier-Stokes codes but cannot account for detailed chemistry which is responsible for emissions. On the other hand, although the modular approaches make significant assumptions about the mean flowfield and mixing, they employ detailed chemical kinetics. The work reported in this thesis seeks to develop a model for emission predictions in the gas turbine combustor which combines the advantage of both the modular approach and CFD modelling. The strategy was based on a pdf calculation using the Monte-Carlo simulation technique because the chemical source term is in closed form for the approach and the solution procedure requires a CFD based calculation. Averaging of the particle properties was on an extended zonal or planar basis in order to reduce computational effort. The predictions are evaluated against available experimental results and other predictions employing more conventional approaches. Since the pdf method allows the modelling of slow chemistry and simultaneous influence of multiple scalars, the thermal NO x production rate was implemented considering the effect of NO concentration itself. Predicted exit NOx concentration was higher than the measured exit level. It has been thought that this discrepancy is mainly due to neglecting radioactive heat loss for temperature calculations. The modelling of soot formation and oxidation has proved more problematic since the assumption that soot is simply perturbation to the gaseous field, analogous to the NO concentration, and temperature may be accurately described by single adiabatic flamelet are no longer valid at elevated pressure and temperature conditions. Soot bum-out is under-predicted. The computed mean soot oxidation is less than 10% of the maximum production levels, even when OH is considered to be oxidising species in addition to O2 •• Although high soot formation rate was predicted as a result of neglecting radioactive loss and using single perturbed flamelet calculation, the main uncertainties come from instantaneous soot oxidation rate and the particle size effect which influence the particle surface area.