Study And Application Of Unsteady Flamelet/progress Variable Model For Aeroengine Combustor

As the demands for the new generation of military and civil aeroengine are increasing,the requirements for combustor are greatly increasing,including combustion efficiency,outlet temperature distribution,combustion stability,emissions,ignition/blowout limits,et al.To the demands for the new generation of aeroengine combustor of high performance,the researches on the new combustion organization need to be carried out,and also the researches on chemical reaction and the interaction between turbulence and combustion are required to be carried out.The aviation kerosene is a macromolecular hydrocarbon fuel,the combustion process of which has thousands of elementary reactions in combustor.Combustion is divided into nonpremixed and premixed combustion,and the combustion process is quite complicated.Numerical simulation is one of the important methods in the study of combustion mechanism and aeroengine combustor design,and combustion model is a key technical problem in the numerical simulation.In this paper,taking the complex turbulence combustion in aeroengine combustor as the background,the unsteady flamelet/progress variable model based on flamelet concept is studies,and the unsteady flamelet/progress model and the module of probability density function integration for laminar flamelet library are established,self-developing a three-dimensional two-phase turbulent combustion numerical software for aeroengine combustor.Then the unsteady combustion process in a aeroengine model combustor is simulated in parallel,revealing the unsteady characteristics of different locations in combustor,and the rationality of combustion model and computational method is verified by the comparison with the results of optical measurement,including velocity,temperature,relative mole fraction of main components and hydroxyl.In this work,three typical turbulent flames are simulated with unsteady flamelet /progress variable model,steady flamelet/progress variable model and steady flamelet model based on flamelet concept,comparing the applicability of the three models.The three typical turbulent flames include “Sandia” diffusion attached flame,“Owen” coaxial diffusion flame with a local lifted phenomenon and “Cabra” partially-premixed lifted flame,and they are characterized by a gradual enhancement of unsteady combustion.The researches have shown that:(1)steady flamelet model has a high precision in modeling the pure diffusion flame,while it has a low precision in modeling the flame with unsteady combustion effects;(2)steady flamelet/progress variable model can only capture the unsteady combustion phenomenon to a certain extent,and however it has a low precision in modeling flame with strong unsteady combustion effects;(3)the steady and unsteady combustion characteristics of flame can be captured simultaneously with unsteady flamelet/progress variable model with high accuracy in the simulation of pure diffusion and partial premixing combustion flame,and the unsteady combustion characteristics of flame can be captured,such as local extinguishing,completely quenching,reigniting,etc.Some key fundamental problems in the unsteady flamelet/progress variable model are discussed in this paper.For example,the definition of the progress variable and chemical reaction kinetics are studied systematically here.Studies have shown that:(1)the progress variable represents the degree of chemical reaction,and different definitions of the progress variables have a large effect on the results.When the prediction of one species in flow field is concerned,the definition of process variable should contain this species,improving the proportion of this species in the process variable;(2)When the mass fraction of one species has a high proportion in fuel or oxidant,this species cannot be included in the definition of process variable,avoiding the simulated location of combustion too close to the front and larger calculaton error;(3)chemical reaction kinetics have little effect on temperature and mass fraction of the main species,but have large effect on intermediate products of combustion,such as carbon monoxide and hydroxyl and the results with detailed chemical reaction kinetics has a higher precision than that with reduced chemical reaction kinetics.Particle Image Velocimetry(PIV),Coherent Anti-stokes Raman Scattering(CARS),Tunable Diode Laser Absorption Spectroscopy(TDLAS),Spontaneous Vibrational Raman Scattering(SVRS),Plane Laser Induced Fluorescence(PLIF)are used to measure cold flow field and combustion process in a aeroengine model combustor in this paper respectively,and the distribution of velocity,temperature,mass fraction of main species and hydroxyl are obtained.In current paper,an unsteady method based on URANS,and unsteady flamelet/process variable model with detailed chemical reaction kinetics of kerosene are studied in the aeroengine model combustor,and three-dimensional two-phase turbulent combustion numerical simulation of the aeroengine combustor is accomplished.The numerical results are compared with that with PIV,CARS,TDLAS,SVRS and PLIF technique.LISA model and KH-RT model are used as the primary and second atomization model respectively,and standard evaporation model is used to model the evaporation process,so atomization,evaporation mixing and turbulent combustion are considered synthetically.The study on the distribution of the combustion thermodynamic scalar in the mixture fraction space reveals the unsteady combustion characteristics of flame at different position in aeroengine combustor,and the unsteady combustion phenomenon of local extinguishing and flame lifting near the nozzle of the aeroengine combustor.The results of numerical simulation are in good agreement with experimental measurement results,including velocity,temperature,mass fraction of main species and hydroxyl.So the unsteady flamelet/progress variable and numerical simulation method in this paper can simulate the unsteady turbulent combustion characteristics and combustion performance in aeroengine combustor.