Study And Application Of NO_x Prediction Model And Flamelet Tabulation Method For Aeroengine Combustion Chamber

As the emission standards of aeroengine are increasingly stringent,low emission becomes one of the important design goals of aeroengine combustion chamber,especially NO_x emission.In order to reduce the NO_x emission in combustion chamber,new combustion methods are required,and therefore studies on combustion methods and NO_x emission in advanced combustion chamber of low emission need to be carried out.It is of great significance on revealing the generation mechanism of NO_x emission,exploring the combustion methods and guiding the design of combustion chamber to carry out studies of numerical method on turbulent combustion and NO_x emission in combustion chamber.In current thesis,studies on the combustion models and NO_x emission prediction models based on flamelet method are carried out for the numerical simulation of turbulent combustion and NO_x emission in aeroengine combustion chamber.In combustion simulation,researches on tabulation methods are carried out including that construction method of laminar flamelet database,the management of the PDF of reaction progress variable andβ-PDF and so on.Flamelet/Progress Variable（FPV）model and Flamelet Generated Manifolds（FGM）model are established with different PDFs of reaction progress variable ofδ-PDF andβ-PDF of which efficient processing method is established,respectively.In the numerical prediction of NO_x,FPV and FGM models are extended by introducing enthalpy defect into laminar flamelet database,and nonadiabatic FPV and FGM models are established for the strong correlation between formation of NO and temperature.The simplified radiative model of Optically Thin Model（OTM）is developed to complete the resolution of nonadiabatic flow field while high computational efficiency is maintained.Because the formation of NO is a slow process and the amount of NO is extremely small,the NO_x prediction is accomplished by separately solving the transport equation of NO mass fraction,of which there are two ways to deal with the source term:one is the net reaction rate is from the flamelet lookup table directly（NO-NET）,and the other is the NO mass fraction from transport equation is used to correlate the reversed reaction rate（NO-PC）.Two types of turbulent flame are simulated using combustion models and NO_x emission prediction model based on flamelet method above,to validate the accuracy and applicability of the turbulent combustion models and NO_x emission prediction models.By the simulation of Cabra partially premixed lifted flame,the effect of tabulation methods including the construction method of laminar database and PDF of reaction progress variable on the prediction of flame lift is studied,indicating that in the prediction of flame lift,FPV and FGM with reaction progress variable PDF ofβ-PDF have a good precision,but FPV and FGM with reaction progress variable PDF ofδ-PDF have a great deviation and Steady Laminar Flamelet Model（SLFM）is failure.By the simulation of bluff-body stabilized flame HM1,several factors affecting the precision of flow,mixing,combustion and NO_x prediction are studied,including construction method of laminar flamelet database,PDF of reaction progress variable,radiation and chemical reaction mechanism,and the effect of NO_x prediction models on the precision of NO_x prediction is also studied.These factors have a little effect on the major thermochemical parameters.The construction method of laminar flamelet database and the NO_x precidion model have a great effect on the prediction of NO_x formation together.The NO-PC NO_x prediction model based on FGM model has a good precision but the NO-NET NO_x prediction model has a great overprediction.The NO_x prediction models based on FPV model all overpredict the prediction of NO_x formation.The PDF of reaction progress variable has a little effect on the prediction NO_x.The chemical reaction mechanism and radiation have a great effect on the precision of NO_x prediction,and hence appropriate selection of chemical reaction mechanism accounting for the NO_x formation correctly is necessary for improving the precision of NO_x prediction and accounting for the radiation of high-temperature gas can improve the precision of NO_x prediction.In current thesis,the flow and combustion in two types of dual-swirler combustion chambers are studied by means of serveral optical measurement methods.The temperature and relative concentration of major species（H₂O、CO₂、O₂ and N₂）in a dual-swirler combustion chamber with swirler of discrete jets and radial swirler and with film cooling are measured by means of Coherent Anti-Stokes Raman Scattering（CARS）,Tunable Diode Laser Absorption Spectroscopy（TDLAS）and Spontaneous Vibration Raman Scattering（SVRS）.In addition,the velocity distribution in dual-swirler combustion chambers with swirler of discrete jets and radial swirler and with mutiholes cooling is measured by means of two-dimensional Particle Image Velocimetry（PIV）to study the influence of intake area and swirl strength of swirler on the structure of flow field and turbulent characteristics,indicating that the intake area and swirl strength of swirler primarily affect the structure of flow field in secondary zone and the turbulent characteristics around swirl flow and primary jets.In current thesis,a set of numerical software are established to model the three-dimensional two-phase turbulent combustion and NO_x emission in aeroengine combustion chamber.Non-equilibrium Langmuir-Knudsen model which can account for the non-equilibrium effect in evaporation process is developed on the basis of sheet breakup model of LISA and droplet breakup model of KHRT,to model the atomization and evaporation process in combustion chamber.The three-dimensional two-phase turbulent combustion in the dual-swirler combustion chamber with swirler of discrete jets and radial swirler and with film cooling is simulated,and the results are compared with that of CARS,TDLAS and SVRS to validate the precision and applicability in combustion chamber,showing that the combustion models with reaction progress variable PDF ofβ-PDF can better capture the process that the unburned mixture is ignited by the burned recirculated gas and the flame lift,and FGM with reaction progress variable PDF ofβ-PDF（FGM-β）has a good precision.The FGM-βis applied to model the turbulent combustion field and performance of dual-swirler combustion chambers with swirler of discrete jets and radial swirler and with multiholes cooling to study the effect of intake area and swirl strength of swirler on the combustion field and performance,indicating that both intake area and swirl strength of swirler have an important effect on the combustion field and OTDF.In current thesis,the three-dimensional two-phase turbulent combustion and NO_x emission are simulated in a Lean Direct Injection（LDI）combustion chamber,and the results are compared with that of experiment to validate the precision and applicability of combustion models and NO_x prediction models in LDI combustion chamber,indicating that the NO-PC NO_x prediction model based on nonadiabatic FGM model has a good precision and the error is below 2%in the prediction of NO_x emission of LDI combustion chamber.In addition,accounting for the radiation can reduce the 20%of outlet averaged EINO,improving the prediction precision,and accounting for the detailed mechanism of NO_x formation is helpful to improve the NO_x prediction precision.In addition by numerically studying the coupling relativeship among flow,mixing,combustion and NO_x emission,the the mechanism of NO_x formation in LDI combustion chamber is revealed and the correction between NO_x emission and the recess depth of central element LDI is obtained.