A Study Of Numerical Methods For The Ignition Process In Aeroengine Combustors

Combustor is a key component of aero-engines.The ignition reliability is a basic require-ment of combustor design,which is crucial for the safety of flight.From ignition experiments of laboratory-scale combustors,the numerical method for the ignition issue of aeronautical combus-tors is investigated in this thesis.Firstly,the Taylor-Galerkin and Discontinuous-Galerkin schemes are realized in the numerical method,meanwhile the numerical accuracy of the schemes are verified and compared.For precise geometry description in engineering problems,a NURBS integrated numerical method with curved elements is developed.The ETAU scheme is extended to the NETAU scheme,which is suitable for curved elements.Furthermore,the mathematical expression of NURBS integrated Taylor-Galerkin scheme is derived.Secondly,a high-order and low-dissipation numerical method of large eddy simulation(LES)is developed for the turbulent combustion problem during the ignition process of combustor.The high-order Taylor-Galerkin scheme is applied to solve the governing equations.Two-step reduced mechanism or analytically reduced chemistry(ARC)can be used for reaction.The dynamically thickened flame model(DTFM)is applied as the turbulent combustion model.The results of 2D laminar flame with various conditions show the accuracy of ARC is close to the detailed chemistry and better than the two-step reduced mechanism.Meanwhile,3D jet flames are calculated and discussed.The computational cost of ARC is about 2.5 times higher than that of the two-step reduced mechanism.Parallel scaling tests show the computational code has good strong-scaling and weak-scaling.Thirdly,the flow structure of single swirling injector is investigated using our LES method and compared with the experimental measurements.Then,the ignition process and flame propa-gation characteristics of two ignition modes in the laboratory-scale combustor developed by our group are investigated with experiments.These two ignition modes are determined by the con-ditions in scheduling fuel delivery and igniter sparking,which are the FFSL(Fuel First,Spark Later)and SFFL(Spark First,Fuel Later)modes,respectively.Systematic ignition experiments have been done for various bulk velocity,equivalence ratio and thermal power,which provide data base for simulation validation.These two ignition modes exhibit different patterns of injector-to-injector flame propagation during the light-round process:an " arc" flame is shown in the FFSL mode;and a“sawtooth”pattern is found in the SFFL mode.For the FFSL mode,a estimation model is proposed to approximate the mean circumferential flame speed,which agrees well with experimental values.Considering the computational scale of the ignition process in the annular combustor,the commercial code CONVERGE CFD with adaptive mesh refinement(AMR)and detailed chemistry is applied to investigate the light-round process of the annular combustor under the FFSL ignition mode.The calculated flame propagation has an overall agreement with experi-ment;and the heat release rate is close to the experimental value;The calculated light-round time is 23%larger than experiment value.This may be explained as the effect of wall temperature on the flame propagation,which is not considered in the simulation.Finally,the numerical method developed in present work is applied to investigate the ignition process of an aeronautical combustor using large eddy simulation.The computational geometry of the combustor has complicate structures of the real aeronautical combustor,including the diffuser,dome,flame holder,primary and dilution holes,etc.The average flow field of large eddy simula-tion has an overall agreement with the RANS result of ANSYS CFX,but the LES results present more detailed flow structures in the diffuser,domain between dome and casing,and swirler outlet,which are qualitatively reasonable.With two-step reduced mechanism of kerosene and dynami-cally thickened flame model,the ignition process of the combustor is simulated using LES.The interaction between flame and flow field during the ignition process is investigated.