Predictive tools for turbulent reacting flows : a comparison between FLUENT and GENUS

Two different CFD codes, FLUENT 5.5 and GENUS, are compared with independent experimental data. FLUENT is a widely used commercial software package while GENUS is a newly developed modular CFD prediction tool of Continuum Computational Mechanics. The GENUS turbulence model library features non-linear closures at the second moment level for both velocity and scalar fields whereas FLUENT allows closures at this level only for the velocity field. The GENUS code will be used for turbulent reacting flow calculations in combustor-related geometries at ALSTOM Power. FLUENT and GENUS are here compared on the basis of two test cases relevant to flow in gas turbine combustors. The first test case features an axisymmetric isothermal swirling flow with central air injection in a dump combustor. Experimental data are provided by the International Flame Research Foundation (IFRF) and include mean and RMS velocities in the axial and tangential (swirl) directions at various cross-stream planes. This case is computed using a 2D axisymmetric and a 3D sector model with FLUENT and only the 3D sector model with GENUS. Both the Standard k-e Model and second moment based closures are used to represent turbulent transport. A grid dependency test has been performed in the context of the 2D calculations and no substantial differences were observed. A comparison between 2D and 3D calculations featuring a Reynolds Stress Model and FLUENT revealed significant solution-convergence and prediction improvements in favour of the latter - the k-e Model solution remained almost unaffected. A comparison between results obtained by FLUENT and GENUS using the k-e Model reveals significant differences in favour of the latter package. When FLUENT and GENUS are compared using the Reynolds Stress Model the results are similar but FLUENT is found to predict an unphysically large internal recirculation zone. The second test case features a range of planar, stoichiometric methane/air, premixed flames propagating in a frozen uniform turbulence field. The latter is in each case obtained by assigning to the turbulence intensity a constant value for the whole domain. The integral length scale is assigned the value of 10 mm. The calculation is transient and integration proceeds until a steady state in flame-space is obtained. Calculated turbulent burning velocities as a function of the turbulence intensity are compared against experimental data provided by Abdel-Gayed et al. This case is in principle simple to calculate and well suited for the evaluation of turbulent combustion models. Using FLUENT with the Eddy-Dissipation concept based closure for the mean reaction rate, extreme difficulties were observed with respect to flame front stabilisation. In consequence a combination of Arrhenius expressions and the Eddy-Dissipation concept were used as a means to alleviate the above difficulties. In the latter case FLUENT was found to yield substantially under-predicted values for the turbulent burning velocity and over-predicted burnt gas temperature. Surprisingly, calculations were found to require an inordinate amount of CPU time of the order of days. Using GENUS with an algebraic fractal flame surface based model for the flamelet regime of combustion predictions were found to be in excellent agreement with experimental data while all calculations were performed in a few hours.Validerat; 20101217 (root