Numerical Investigation Of NOx Emission Of Lean Premixed Combustor Using Chemical Reactor Network Model

Pollutant control is one of the key design goals for the combustor.Developing a numerical method for prediction of NOx emissions is an important way to pre-estimate the low-pollution combustor.The Computational Fluid Dynamics-Chemical Reactor Network(CFD-CRN)method has great advantage in fast and accurate prediction of NOx emissions for combustor.The thesis presents a numerical study where a hybrid CFD-CRN approach is established to predict pollutant emissions of a lean premixed model burner for gas turbine applications based on post-processing of CFD results such as characteristic distribution of flow,temperature and species field.Several criteria are studied for separating the combustor into chemically and physically homogeneous zones.The combustor is separated into several regions including the preheating region,the flame front region,the flame transition region,the post flame region,the central recirculation region and the corner angular recirculation region based on the fuel-air mixing characteristics,velocity field,temperature field,OH distribution,and Damkohler number.A complex CRN model is established to characterize the flow characteristics and flame structure inside the combustion chamber.The influence of CRN zone-dividing criteria and difference between CRN models on predicting NOx emission characteristics of combustor is investigated.The results show that the complex CRN model has the advantage of predicting NOx emission characteristics under higher adiabatic flame temperature compared with the simple CRN model,which indicates that the CFD-CRN method can quickly and validly predict the NOx emissions of the lean premixed swirl combustor under high equivalence conditions.The difference between CRN zone-dividing criteria mainly causes the changes of residence time distribution between adjacent regions,but has small effects on total residence time of combustor and the distribution ratio of total flue gas recirculation,so the difference between NOx emissions predicted by CRN models with different CRN zone-dividing criteria is not significant.The effects of several crucial parameters including residence time and flue gas recirculation ratio on NOx emission are studied through sensitivity and uncertainty analysis based on validity of the CFD-CRN method.The results show that one-sample statistical analysis and the Levene test indicate that the residence time disturbances of the preheating zone and the flame front zone have more significant effects on emissions and stability of NOx prediction of CRN models.The flue gas recirculation ratio of CFD-CRN method should be accurately calculated under high adiabatic flame temperature,which has significant effect on NOx formation.The lean premixed flame combustion characteristics under different pressure and fuel/air unmixedness are simulated by CFD method.The difference of CRN zone-division and the crucial parameters including the residence time and the flue gas recirculation ratio are studied.The results show that the CRN region distribution is similar when pressure and fuel/air unmixedness increase.However,the high flame temperature resulted from high pressure and unmixedness induces that the high temperature region gradually moved to the upstream of the combustor,which has significant effects on morphological distribution of the preheating zone,the flame front zone and the central recirculation zone.The difference in hydrodynamic characteristics represented by the residence time and the flue gas recirculation ratio has no significant influence on NOx emission predictionThe influence mechanism of the fuel/air unmixedness of lean premixed burners on NOx formation and reaction path under different pressure and equivalence ratio conditions is studied using CRN model.The results show that the high pressure causes the decrease of O radical concentration,which leads to the gradual decrease of NOx formation in combustor under the condition of low adiabatic flame temperature and good fuel/air mixing.The increasing of pressure causes the first decrease and then increase of NOx formation when the air-fuel unmixedness increases.In addition,under the higher adiabatic flame temperature,even if the premixing uniformity is better,the NOx emissions gradually increase with high pressure;the negative impact of high pressure on NOx formation is also increasing as the premixing uniformity deteriorates,and the trend of NOx generation become more significant under high pressure.The higher adiabatic flame temperature and pressure cause that the sensitivity of fuel/air unmixedness to NOx emission gradually increase.The tendency of NOx production increasing with fuel/air unmixedness is more significant.In summary,this thesis has established a CFD-CRN simulation method for the NOx emission prediction through the comparison with the experimental data of the lean premixed burner.The applicability of CFD-CRN method in NOx emission prediction of the lean premixed combustor under different pressure and fuel-air unmixedness was investigated and discussed.The CFD-CRN method is convenient and effective for estimating pollutant emissions,and has important value for the design and optimization of low-pollution combustors.