Numerical Investigation of Lean Blow Out of a Model Gas Turbine Combustion Chamber Using a Presumed JPDF-Reaction Model by Taking Heat Loss Processes Into Account

Due to their excellent behaviour within the scope of mixing, ignition and burnout, swirl-flames are used within quite a manifold of scientific and industrial applications. The development of a swirl-induced inner recirculation zone, which provides heat and active chemical species to the ignition domain of the flame, plays an important role for stabilisation of these highly turbulent flames. Modern concepts for reducing thermal NO_x emissions require high ignition stability even if very lean fuel/air-mixtures are in use. Therefore, there is a great demand for models which are able to predict lean blow out of turbulent, aerodynamically stabilised flames. In contrast to the integral approach of many stability models which mostly are based on global quantities, numerical models offer highest possible flexibility aiming at variation of geometry, operating conditions and further parameters. For solving the convective-diffusive problem, a RANS (Reynolds Averaged Navier Stokes) method based on a finite volume approach is applied, using the standard k-ε turbulence model. A joint-probability-density model with an assumed shape of the probability-density-function (presumed shape JPDF-model) describes the interaction of turbulence and chemical reaction. The latter is based on one single variable, describing the mixing state and one single variable, describing the state of reaction progress. The demand, to apply a chemical reaction mechanism, which is based on one single reaction progress variable, is solved by using the concept of the semi-global 2-domain-1-step chemical kinetics scheme. To predict lean blow out for confined diffusive swirl-flames makes it necessary to take into account the convective and radiative heat loss processes. To consider the influence of heat loss on the chemical reaction, the 2-domain-1-step chemical kinetics scheme had to be extended. The local distribution of heat loss inside the flow field is covered by a variable named “enthalpy-index”, which describes the normalised ratio of the local enthalpy and local enthalpy under adiabatic conditions for a given mixture composition. With this combined model LBO (Lean Blow Out) limits have been deduced for a Methane/Air-flame in a model gas turbine combustor. The results confirm, that lean blow out is predicted at much lower thermal loads if taking heat loss processes into account.