Modelling Issues of Lean High-Pressure Turbulent Premixed Hydrogen-enriched Hydrocarbon Combustion at Gas Turbine Conditions

(Abstract) It is widely recognized that stationary gas turbine combustors under lean turbulent premixed conditions offers the advantages of low temperature operation and thus low NOx emissions. At such ultralean conditions, hydrocarbon flames are inherently unstable, with low range of extinction limits, hampering wide range operability especially at high pressures. However, addition of small amounts of hydrogen, characterized by high burning velocities, has the potential to extend this extinction limit. Hydrogenated fuels offers higher turbulent flame speed ST compared to pure hydrocarbons under identical conditions. These issues are addressed using the existing Algebraic Flame Surface-Wrinkling reaction subclosure in which the influence of high-pressure and the Lewis number were explicitly included [1]. The fuel effects were derived from the flame ball concept by Zel'dovich [2]. In the first instance, comparative studies between calculations and experiments are based on turbulent flame speed of lean high-pressure premixed turbulent pure methane flames obtained on a typical sudden-expanding dump combustor [3]. The pure methane mixtures were preheated to 673 K with maximum operating pressure of 10 bar, for a range of equivalence ratios. Simulation studies carried out using the AFSW model predicts an increase in ST for pure methane mixtures in line with experiments. In the second step, reaction model correlation results for hydrogen enriched methane flames were compared with the corresponding measured data obtained on the same burner. Griebel et al. [4] observed a nonlinear increase in ST with hydrogen addition, especially for equivalence ratio 0.50. The maximum hydrogen weightage for mixed fuel mixtures is 50 % by volume. In this part of the study, for the hydrogen blended methane mixtures, the differences between and analytical results from the AFSW model in its basic form and measurements are found to be significant. This increase in S T is explained using critical chemical time scale taken from a numerical study of outwardly propagating spherical flames by Lipatnikov and Chomiak [5]. This time scale characteristic of the critically curved laminar flamelets based on leading point concept [5] addresses the combined preferential-thermo-diffusive effects.