Numerical Simulation Study On Performances Of Swirl Burners In Aero-engine Combustors

In recent years,swirl burners have been widely researched and developed for being applied in aero-engine combustor.A recirculation zone is created at the outlet of the swirler,which can promote rapid mixing of fuel/air then achieve sustained combustion.The burner structural parameters,such as angle of swirler vanes,stage number of swirler and outlet contraction ratio of the chamber,are critical to affect the shape of the recirculation zone and velocity distribution in the combustion chamber,and then determine the temperature and pollutants emissions.The influences of burner structure on flow fields actually attribute to the vortex breakdown and the Precessing Vortex Core(PVC),which have complex impact on flame structure and combustion process.In the research presented in this thesis,both Reynolds Averaged Navier-Stokes(RANS)and Large Eddy Simulation(LES)methods have been used to study effects of burner structures on flow fields and instability of coherent vortex structures.As LES has used to solve the large-scale vortex structure in the turbulent flow field directly,the instantaneous pressure iso-surface can show the PVC created by the swirling flow and the Central Vortex Core(CVC)created by contraction of the burner outlet.In order to research the turbulence instability of flow fields,Root Mean Square(RMS)of velocity fluctuation and spectrum analysis methods are used respectively.Results show that,with the increase of the swirling intensity,the flow field becomes more and more unstable and the center of the recirculation zone moves to the upstream gradually.The spiral vortex structure in the flow field is more obvious under strong swirling flow condition and shows more powerful precessing characteristic.Meanwhile,the longitudinal cross-sectional streamline of the burner shows that the PVC rotates around its own vortex axis while rotating around the central axis of combustion chamber.From the power spectral density map,it is found that the peak of the spectrum near the central vortex nucleus is high,which may be due to the instability of the CVC movement created by contraction structure.With RANS method,effects of burner configuration on mean velocity flow fields and emission characteristics have been studied.For optimizing the simulation,turbulence model has been used to form close cycle of multi-equations and Eulerian-Lagrangian method has been used to simulate two-phase spray flows.The non-premixed equilibrium hypothesis probability density function(PDF)combustion model was employed to describe the interaction between turbulence and chemical reactions.Results suggests that increasing the swirl intensity will enlarge the area of the recirculation zone and reverse flow velocity,and that the flow velocity of the inlet shear layer will be accelerated due to the squeeze of recirculation zone.With regard to emissions,NO emissions will firstly increase and then decrease with the increase of swirl number at upstream of chamber.By comparing the outlet temperature distributions of the burners,it is found that as the swirl number increases,the outlet temperature distribution becomes heterogeneous.The counter swirling promotes the fuel burn more completely,and a larger recirculation zone is beneficial to stabilize the flame.However,NO emissions of co swirl burner are much lower than the counter swirl burner.It is found that the existence of contraction structure changed the pressure of combustion chamber and eliminated the reverse pressure gradient caused by the swirling flow.The flow switches its direction at the outlet of chamber when the contraction diameter shrinks to a critical radio.In addition,the influence of contraction radio on different chamber position,it was found that there is a huge difference between the upstream and downstream velocity distributions.Therefore,a reasonable exit structure size is vital to the swirling flow field.