Large Eddy Simulation Of Dynamics Of Lean Premixed Swirling Combustion

With increasing awareness of environmental protection,the development of clean and efficient gas turbine has drawn world wide attention.Low emission combustion technology has been widely developed to reduce NOx emissions.Lean premixed combustion appears to be the most promising low emission combustion technology,however it is susceptible to thermoacoustic instability,which will lead to decline in the quality of combustion.Combustion process will become unstable and be prone to flashback and local extinction,which greatly reduce the life of gas turbine.In this paper,the CFD model was adopted to study the flow and combustion process of swirling premixed combustion, The main contents and conclusions of this paper were as follows:(1) The model of premixed combustion was built up on the basis of chemical reaction kinetics and fluid dynamics and the validity of the model was verified by comparison with the experimental data.(2) The cold flow process was numerical studied,results showed that there existed stable central recirculation zone and corner recirculation zone and precessing vortex core was situated on the boundary of the central recirculation zone. The strong vortical motion in the chamber can produces acoustic waves propagating.With increasing of swirl number,the acoustic oscillation frequency of the combustor also increased.The study of fuel and air mixing process showed that the turbulence intensity and degree of vortex breakdown was enhanced with increasing of swirl number which was good for mixing.(3) The thermoacoustic coupling oscillation under combustion condition was calculated and analyzed by large-eddy simulation.The results showed that with decreasing of equivalence ratio, pressure oscillation amplitude gradually increased, but the flame temperature as well as CO2, CO, NOx emissions decreased significantly, which indicated lean premixed combustion had advantages of low pollution and disadvantage of combustion instability. When the mixture inlet temperature was below650K,the amplitude of pressure oscillation was low and combustion could keep stable,with increasing of inlet temperature, the amplitude of pressure oscillation increased gradually and combustion became unstable. When swirl number was low,the length of the flame was long and combustion was stable.With increasing of swirl number,the flame became shorter gradually and combustion became unstable.When swirl number was increased to1.3,the flame propagated upstream into the premixing tube and flashback occurred.(4) The phase analysis of thermoacoustic coupling oscillation showed that under stable combustion condition,coupling degree of pressure oscillation and overall heat release oscillation was low and phase relationship was not obvious;When combustion became unstable,the waveform of pressure oscillation and heat release oscillations was similar and the phase difference was less than90degrees.(5) Flame surface variation during one oscillation period showed that flame position was substantially unchanged under stable combustion, in contrast, the flame shaked when thermoacoustic oscillation occurred, leading to the elongation and shortening of the flame.By analyzing the interaction between the flow field and flame, the vortex motion on the boundary of central recirculation zone caused the stretch and fracture of the flame which was the major factor to excited combustion instability.