A Species-weighted Flamelet/progress Variable Model For Large Eddy Simulations Of Turbulent Jet Oxy-fuel Combustion And Its Flame Instability

Oxy-fuel combustion is one of the most promising technologies for large-scale carbon capture.It has wide application prospects in the power,steel,cement,refining and chemical industries.Flame instability is a key difficulty limiting the technology development and industrial application of oxy-fuel combustion due to the significant difference between the physical and chemical properties of CO₂ and N₂,as well as the reduced stream momentum ratio of oxidant to fuel.Large eddy simulation（LES）coupled with flamelet/progress variable（FPV）model is able to obtain detailed information on turbulent combustion with an acceptable computational cost.It has become an important method for the study of flame instability in recent years.However,most of the existing combustion models are based on the assumption of equal diffusion,which cannot correctly capture the flame instability due to differential diffusion of CO₂ in the oxy-fuel combustion.In this thesis,the hydrodynamic and thermo–diffusive instability is investigated in a round jet oxy-fuel turbulent diffusion flame（Sevault et al.2012）based on model development,code implementation,and numerical simulation.First,a high-fidelity large eddy simulation（LES）program is developed for turbulent diffusion flame based on the FPV model with the assumption of equal diffusion.An explicit filtering method based on the finite volume framework is employed to reduce the commutation error caused by the standard LES approach with implicit filtering.Moreover,the tabulation strategy of the flamelet database has been optimized.Compared with the classical LES-FPV model,the deviation on the predicted time-averaged temperature from the experimental value of Sandia flame D（Barlow et al.1998）is reduced from 15%to10%.Large eddy simulations were also performed on the boundary layer flashback in a swirling stratified flame（Ranjan et al.2016）.The results show that the proposed model captures the interaction between flame and flow well.The deviation of predicted flashback distance from the experimental value is less than 5%.Following that,a species-weighted FPV（SWF）model is proposed to describe the multi-diffusion characteristics in oxy-fuel combustion,which includes the effect of turbulent and molecular diffusion based on equal diffusion FPV and differential diffusion FPV models.The differential diffusion FPV model is based on tabulations with the detailed molecular diffusion of the mixture,which correctly captures the effect of differential diffusion on the combustion characteristics in a CO₂-rich environment.For verification,large eddy simulations are performed on the turbulent round jet oxy-fuel diffusion flame.The results show that the SWF model captures the effect of multi-diffusion on combustion characteristics well.Compared with the existing combustion model with differential diffusion in the literature,the prediction deviation on the time-averaged species mass fraction from the experimental data is reduced from 10%to 5%.To get more insights on the effect of hydrodynamic instability on the stability of oxy-fuel flames,simulations of turbulent oxy-fuel flames with different Reynolds numbers（Re）are conducted.The results show that with the increase of Re from 12,000 to 18,000,the magnitude of flame local extinction increases.The minimum temperature in the local extinction region decreases from 780 K to 450 K.The hydrodynamic instability is enhanced with the increase of Re,and the extinction process changes from the process of transport of fresh reactants into the burning region governed by the roll-up of small-scale vortexes to the process of flamelet distorted by resolved-scale vortexes.Finally,based on the LES-SWF model and fictitious species approach,the effects of thermophysical,chemical,and diffusion properties of CO₂ on the flame instability of oxy-fuel flame with Re=18,000 are studied in detail.The results show that the diffusion properties of CO₂ provide the greatest effect on the flame instability in oxy-fuel conditions,followed by the chemical properties of CO₂.In contrast,the thermophysical properties have the weakest effect on flame instability.The diffusion properties of CO₂ significantly increase the magnitude and frequency of extinction,while the chemical properties of CO₂increase the extinction duration.The diffusion and chemical properties of CO₂ enhance the thermo–diffusive instability of the flame.With the combined effect of the diffusion and chemical properties,the extinction process is changed to the process of flamelet distorted by resolved-scale vortexes.