Modelling of combustion processes and NO formation with reduced reaction mechanisms

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Comparison of conventional and low-dimensional manifold methods to reduce mechanisms

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Modelling of the flow field within a generic aero-engine combustor

A generic combustor has been modelled using Reynolds Averaged Numerical Simulations (RANS) and Large Eddy Simulations (LES). The combustor is representative of a sector of an aero-engine combustor, including one fuel injector and several dilutions ports. To enable optical measurements the combustor has been equipped with air-cooled quartz windows. Both RANS and LES computations have been performed, and two grids have been applied. One grid represents the combustor only, while the other also includes the burner swirler passages. The difference between applying the boundary conditions at combustor inlet and including the swirlers in the CFD model are studied. The computations are performed with a Rolls-Royce in-house code. The code is block-structured and parallelised using MPI (Message Passing Interface). Both isothermal and combustion computations have been carried out. The combustion process has been modelled with a conserved scalar flamelet model, but also a finite rate chemical model has been applied. Detailed experimental data are available (LDA, PIV, DGV, Raman, CARS) with the primary and dilution zone of the combustor. The measurements have been performed by DLR Cologne within the EC funded research project MOLECULES. The cold flow field in the primary zone is represented well by both the RANS and LES model, but if the injector is modelled, the combustor inlet profiles are better represented by the LES model. In the dilution zone there are larger differences between LES and RANS. The RANS computations underestimate the jet penetration. The combusting simulations using the conserved scalar model compare not very well with the experimental results close to the injector: the mixture burns too fast. The finite rate reaction model, combined with RANS gave better results. It is concluded that in the vicinity of the injector the chemistry cannot be assumed to be infinitely fast

Mathematically reduced reaction mechanisms applied to adiabatic flat hydrogen/air flames

Several hydrogen/air reaction systems are reduced mathematically to one-step schemes, using the method introduced by Maas and Pope [1]. The reduction is obtained by assuming fast reaction groups of the reaction system to be in steady state. We developed a method to apply the reduced schemes to adiabatic flat flames. The results are compared with those of detailed chemistry calculations. The accuracy of the results of reduction of the most simple reaction system, (which does not include the species HO2 and H2O2) is quite well. The other reaction systems, however, give appreciable errors in burning velocity. This is mainly caused by large deviations in HO2 mole fractions between reduced and full scheme calculations, while the reaction rate and the burning velocity are sensitive to variations in the mole fraction of HO2. Considering the time scales of the reaction system and the time scales of convection and diffusion it is shown that at low temperatures, where the mole fraction of HO2 reaches its maximum, the basic assumptions applied to reduce the mechanisms are not justified. It is concluded that the hydrogen/air system can only be reduced to an accurate one-step reduced scheme, if reaction schemes without HO2 and H2O2 are used. This reduction technique also indicates, in accordance with conclusions of Peters et al. [3], that a two-step reduced scheme has to be used if more realistic hydrogen/air schemes, including HO2 and H2O2, are considered

Post-processing method for predicting NO formation in one-dimensional and two-dimensional premixed methane-air flames

A post-processing method to predict NO formation in one- and two-dimensional flames is developed. A flame calculation with a reduced number of species and reactions is performed first. The remaining species are computed in a post-processing step. The computational effort is reduced further by introducing steady-state assumptions for intermediates. The results of the post-processing method applied to adiabatic flat flames agree well with complex calculations. Computations of burner-stabilized flames are compared with results of measurements on a ceramic foam surface burner, and the agreement is satisfactory. The post-processor method makes it possible to perform two-dimensional detailed NO computations within a reasonable computing time. As an example, results for a two-dimensional slot burner in a confined environment with cold walls are presented