Large Eddy Simulations Of The Small-hole Injection Flow Within The PRECCINSTA Gas Turbine Combustor

In technical combustion devices, fuel is usually injected into the air flow or the combustion chamber through small holes, which may result in non-uniform distribution of the equivalence ratio of reactant mixture. Compared to the widely used Reynolds averaged(RANS) method, large eddy simulation(LES) method is more accurate in predicting the turbulent phenomena. For investigating the compact of upstream small hole injection on the downstream mixing process, and the degree of unmixedness of mixture before entering into the combustion chamber, high fidelity LES method was utilized to compute the small hole injection problem in the well-documented PRECCINSTA industrial gas turbine model combustor.The high quality block-structured grid about the complex geometry of the PRECCINSTA burner, consisting of 12 jet holes with the diameter of 1 mm, was generated with the aid of ICEM-CFD software. The in-house LES code, LESOCC2 C,was then used to simulate the mixing of methane and air within the radial swirl channel.In total, four isothermal flow cases with different combinations of global equivalence ratios(0.7 or 0.83) and grids(1.2 or 1.8 million) were simulated. The results show that the fluctuation range of the equivalence ratio predicted by the two grids is almost the same as [0.4, 1.0] at global equivalence ratio of 0.7. At global equivalence ratio of 0.83, the range predicted by fine grid is [0.59, 1.01], while it is[0.6, 1.15] by coarse grid. It means that high quality grid is preferred to describe the development of turbulence and the mixing progress between air and methane. For both global equivalence ratios, large fluctuation of local equivalence ratio was found before the mixture entering into the combustion chamber, therefore it leads to a strong partially premixed combustion mode. Due to this, a sub-grid scale combustion model capable at dealing with partially premixed combustion is needed to simulate the PRECCINSTA combustor under consideration.Additionally, the application range of two reduced reaction mechanisms for methane/air combustion, 2s CM2 and 2S_CH4_BFER, were also checked bysimulating fifteen one-dimensional laminar premixed flames at different equivalence ratios. When the equivalence ratio varies between 0.5 and 1.2, the computational results obtained with the two reduced mechanisms are in good agreement with that obtained with detailed methane-air mechanism, in terms of adiabatic flame temperature, laminar flame speed and laminar flame thickness. For equivalence ratio greater than 1.2, the 2s CM2 mechanism shows a large error, while for equivalence ratio greater than 1.3 both mechanisms are disabled. Considering the results obtained in LES of the small-hole injection, it can be concluded that both mechanisms are applicable in describing the combustion process with the global equivalence ratio as0.7 and 0.83. However, the 2S_CH4_BFER mechanism is preferred because it has a wider application range for equivalence ratio.