Fully Compressible Numerical Simulation Study Of Mechanisms And Influencing Factors Of Combustion Instability

Combustion instability is a complex multiphysics and chemical processes,including the interactions between turbulent flow,fuel/air mixing,chemical reaction and acoustic propagation.In the present thesis,combustion instabilities of laminar premixed flames,turbulent premixed flames in single and dual combustors,and spray flames in realstic aero-engine combustor are investigated using high-accuracy fully compressible numerical simulation methods.The purpose is twofolds: one is to reveal the undelying mechanisms and influencing factors of combustion instability,and the other is to validate the accuracy of large eddy simulation(LES)method.The self-excited combustion instability modes in laminar premixed flames are first investigated using fully compressible direct numerical simulation(DNS)method.The flame anchor positions and flow fields show different characteritics in different flame holder temperatures,which results in different flame/acoustic interactions.When the flame holder temperature is 300 K and 773 K,the peak frequencies of flame transfer functions,intrinsic thermo-acoustic(ITA)modes and pressure fluactuations in the selfexcited flames are very closed.The instability processes are mainly dominated by the ITA modes.When the flame holder temperature is 400 K and 1500 K,the peak frequencies of the coupling modes between system acoustic modes and ITA modes,and pressure fluactuations in the self-excited flames are very closed.The instability processes are mainly dominated by the coupling modes between the system acoustic modes and ITA modes.Then,fully compressible LESs are performed to investigate the nonlinear flame response of turbulent premixed flames in a single combustor.Walls are set as isothermal and adiabatic walls respectively to investigate the effect of wall temperature on the nonlinear flame response.At moderate frequency,both amplitudes and phases of nonlinear flame response and mushroom-shaped flame dynamics in the osciallation cycle are well reproduced by LES in the isothermal case.At high frequency,LES results from the isothermal case provide reasonable agreements with the experimental results in the reproduction of linear increasing trend of flame response and two mushroomshaped flame dynamics in the oscillation cycle.While LES results from the adiabatic case give good reproduction of amplitudes of flame response and flame dynamics at moderate frequency,but failed in the reproduction of flame response at high frequency.Wall boundary conditions affect the flame structures and the spatial distributions of heat release fluctuations by changing wall temperature,then affect the nonlinear flame response.At moderate frequency,heat release fluctuations in different parts of the combustor are in phase,and wall temperature has limited influences on the global flame response.While at high frequency,wall temperature has significant influences on the global flame response since the heat release fluctuations in different parts of combustor are out of phase.Accurate reproduction of flame response at high frequency needs accurate calculation of wall temperature.Based on the study of turbulent flames in single combustor,LESs are performed to investigate the nonlinear flame response of two interacting flames in a dual combustor.LES gives good agreement with experiments in the reproduction of amplitudes and flame dynamics at moderate frequency,but failed in the reproduction of phase.At high frequency,the flame dynamics are well reproduced,but the amplitude and phase from LES still have large difference with experiments.Flame/flame interactions in the dual combustor have significant influences on the nonlinear flame response of turbulent flames.In the merging zone,the collisions of two fluids decrease the value of vorticity and weaken the effect of vortex on the flame,then decrease the amplitudes of flame response.At high frequency,the formation and annihilation of burning pockets have significant influence on the combustion instability.Finally,the fully compressible LES method is applied to the simulation of spray combustion instability in a realistic aero-engine combustor.LES reproduces the spray combustion instability process in the realistic aero-engine combustor successfully.The pressure and heat release oscillate with the same frequency and phase,which shows that the combustion instabilitise are mainly caused by thermo-acoustic interaction.In the spray combustion instability processes,flame surfaces collapse and merge with the adjacent flame surfaces to form burning pockets.The formation and annihilation of burning pockets cause the fluctuations of flame surface area and heat release rate.The fluctuations of heat release rate cause the fluctuations of pressure and velocity,and affect the flow field and temperature field,which results in the fluctuations of droplet evaporation rate and equivalence ratio,then affect the combustion instability process in reverse,a feedback loop is formed.The oscillation amplitudes are affected by the initial droplet diameter and wall boundary condition.When the initial averaged droplet diameter increases from 15 ?m to 20 ?m,the frequency and amplitude of combustion instability decrease.When the cooling air is removed from the combustor,the oscillation frequency increase and the amplitudes decrease.