steady-state turbulent premixed flames

This work deals with experimental investigations and numerical simulations of the process of combustion under the conditions of gas turbine combustors, especially for the lean premixed combustion concept with low thermal NOx-formation. Particular emphasize is ascribed to the influence of flow turbulence on reaction rates. Proper description of the interaction of turbulence and kinetics may be investigated on the basis of the turbulent burning velocity. The turbulent burning velocity was determined by contour analysis of turbulent bunsen-type flames. Therefore, a special model bumer system (matrix burner) including four geometrically similar burners was used. The experiments were performed at an atmospherical test plant where the fuel gas/air mixture was preheated up to 400°C by an in-series connected air heater. Corresponding to gas turbine application with lean premixed combustion, air/fuel equivalent ratios in the range 1.75 < air factor < 2.25 (0,47< Damkoehler number <2.41) were investigated in order to obtain moderate adiabatic flame temperatures. From the measurements followed a nearly constant degree of turbulence Tu=8% in the reaction zone. Fuel gas was natural gas.

For the calculation of the flame the 2-dimensional CFD-code CATS-2d applying the k-e turbulence model and the turbulent flame speed closure of Schmid was used. Since the aim of the model was primarily to describe heat release, the integral one step reaction CH4+2O2 = CO2+2H2O has been considered as adequate for modelling methan/air combustion. The overestimation of adiabatic flame temperature was avoided by introducing an empirical dissociation enthalpy. No radiation model has been used. Like in the experiment the flame had to be stabilized in order to blow-off.

The first picture shows a photography of the flame.

The second picture shows the comparison of measurement and calculation by means of the main species. The Damkoehler number of flame 1 was 1.82 and the one of flame 2 was 0.47. The quantitative comparison between experimental and numerical results is excellent.