Investigations On The Mechanism Of The Acoustic-excited Flame Dynamics And Combustion Instability Suppression

The interaction between an acoustic wave and a flame can cause thermoacoustic instabilities in many aeronautics and industrial engines.Many complex phenomena are involved in the thermoacoustic instabilities,and the mechanism of the thermoacoustic instability is still unclear.Aiming at the mechanism of thermoacoustic instability in the engine,a set of experimental platforms for acoustic wave/flame interaction have been designed in this thesis to systematically study the fundamental mechanism for the acoustic wave/flame interaction by using high-speed camera imaging,high-speed schlieren imaging,PIV and direct numerical simulation（DNS）.First of all,this study experimentally investigates the interaction process of acoustic wave and laminar flame,and reveals the response characteristics of the methane/air premixed flames excited by acoustic waves.Compared with high-frequency acoustic waves,the flame structure shows more significant oscillation under the excitation of the low-frequency acoustic waves.As the frequency increases,the flame structure becomes insensitive.During the process of flame structure evolution,local extinction of the flame can be observed.The PIV results show weak relevance between the flame local extinction and the vortex structure around the flame.The main reason for the local extinction is attributed to the speed difference between the upper and lower parts of the flame.Even without the external acoustic excitation,the low frequency oscillation of the flame can still be obtained.The results of schlieren images and proper orthogonal decomposition indicate that this phenomenon is caused by the buoyant structure around the flame.Moreover,increasing the amplitude of the acoustic excitation will cause the flame blow-off.For the present combustion chamber,the flame can be easily blown off when the excitation frequency reaches 260 Hz.The results of the numerical simulation show that when the acoustic wave with a frequency of 260 Hz is applied to the combustion chamber,the flame locates at the trough of the acoustic wave.Theoretical analysis reveals that large velocity disturbance occurs at the low sound pressure level region of the combustion chamber and the higher amplitude of the velocity is the main reason for the flame blow-off at 260 Hz.The effects of flow velocity,equivalence ratio and sound pressure level on the flame of the bluff body are experimentally studied.The results show that,the higher flow velocity weakens the acoustic response of the flame structure.When the flow velocity reaches 20 m/s,one should increase the excitation frequency to at least 100 Hz to witness significant response of the flame.When the flow velocity is low,the sound wave excitation will produce vortical and multilayered structure around the flame surface.The results of high-speed photography and high-speed schlieren show that the flame stratification phenomenon is mainly caused by the expansion of the flame bottom.Under the excitation of sound waves,the turbulent flame presents periodic changes in the height and periodic expansion at the bottom.The spectrum of the flame luminous intensity shows that the response frequency of the turbulent flame is consistent with the sound wave frequency,and the nonlinear phenomenon is very weak.Based on the simultaneous CH₂O/OH PLIF technology,the flame composition distribution and flame front curvature variation under acoustic excitation are studied.The PLIF successfully captured the development process of vortex structure on the flame surface within a period,and its composition distribution indicates that,under a higher speed flow,the application of acoustic excitation would widen the distribution range of CH₂O radical around the flame.Finally,the boundary of the OH-PLIF images are extracted,and the flame curvature,progress variables and flame surface density are calculated,the results show that in some cases,applying acoustic excitation will make the flame surface smoother.The effects of velocity perturbation and equivalence ratio perturbation on the laminar flame are investigated using direct numerical simulation.The numerical results show that velocity perturbation can induce flame local extinction,and the flame local extinction is mainly caused by the speed difference between the upper and lower parts of the flame.Equivalence ratio perturbation causes flame extend to both sides,and the flame extending is related to the fuel mass fraction and heat release rate.The influence of velocity perturbation and equivalence ratio perturbation on the flame become weaker when the excited frequency is high.A semi-enclosed combustion chamber with a cavity is designed to study the acoustic field of the high-speed jets in a confined space,and the influence of jet velocity on the acoustic field is experimentally investigated.For the subsonic cold jet,the combustion chamber with a low jet velocity is dominated by the oscillation at frequency range of300Hz-600Hz.With the increase of the jet velocity,the oscillation in the combustion chamber is mainly dominated by the high frequency.When the geometry of the combustion chamber is fixed,the increase of flow velocity improves the amplitude of the high-frequency oscillation,while the dominant frequency stays the same.Comparing with the cold jet at the same jet velocity,the main frequency of oscillation of the reacting jet in the combustion chamber is evidently reduced,and the oscillation intensity in the combustion chamber is significantly enhanced.In the case of supersonic cold jet flow,the change of the combustion chamber geometry shows less influence on the dominant frequency.However the amplitude of the dominant oscillation is greatly influenced by the combustion chamber geometry.At last,the influence of flame front fluctuation is considered to analyze the factors affecting the intrinsic thermoacoustic instability.The results show that,as the ratio of combustion temperature to unburned gas temperature increases,the gain of the intrinsic thermoacoustic instability will gradually decrease.However,the increase of the pressure drop ratio will lead to an increase of the intrinsic thermoacoustic instability gain.Extending the length of the combustion chamber will reduce the volume of the stabilization zone for the three oscillation modes.With a shorter combustion chamber,a larger stabilization zone can be obtained by adjusting the inlet reflection coefficient.