Study Of Aeroacoustic Characteristics Of Bluff-body And Influence On Flame Instabilities

Bluff-body is widely used in industrial combustors to stabilize flames.Acoustic wave fluctuation is one of the key factor that may induce the combustion instabilities,which always couple with the vortex structures to influence the dynamic responses of premixed flames.The study on the flow structures,acoustic propagation characteristics of bluff-body and their effects on the premixed flame instabilities can help to control the combustion instabilities.In this thesis,we investigated the aeroacoustic characteristics of bluff-bodies and the dynamics of bluff-body stabilized premixed flames,so as to analyze the interactions between the acoustic wave,vortex structures and flames.The space-time conservation element/solution element(CE/SE)scheme was applied into the monotonically integrated large eddy simulation(MILES)to simulate the flow and acoustic characteristics of bluff-body structures.This method unifies the space and time dimensions,and simultaneously soles the flow and acoustic information.It is physically conservative,and has high resolution with low numerical dissipation.Acoustic forcing of different frequencies were imposed,and the bluff-body flow field,vorticity field and shear layers were analyzed.Results show that with existence of high-frequency external acoustic forcing,the recirculation zone behind bluff-body is moved up.The vortex shedding frequency in the shear layer is same with the forcing frequency.The field upstream of the bluff body is dominated by duct-flow,while the field far downstream is dominated by acoustical propagation.The cross-section averaged multi-microphone method was used to capture the acoustic wave components,and the reflection coefficient,transmission coefficient,scattering matrix and the end correction length were analyzed.The results show that,the amplitudes of the reflection coefficient are similar while the phases differ.The acoustic wave and flow field interacts stronger when St number is near1,where the end correction length decrease slightly.The dynamic instabilities of the bluff-body stabilized premixed flames were simulated,for which a method coupling the premixed flame G-equation and discrete vortex method was established to simulate the vortex and flame movement behind bluff-body structures.The local level set and re-initialization method was used to solve the flame G-equation,and the WENO scheme was applied to calculate the spatial derivatives.The discrete vortex method was used to supply velocity field to sole the kinematic G-equation.It describes the vortex generation,motion,and dissipation behind the bluff-body,and conformal mapping was used to simplify calculations for complicated geometries.The Simulation results of the bluff-body stabilized M flames show that,external forcing with higher amplitude induces more intense vortex movements and more wrinkled flames.With the increase of forcing amplitude,the amplitude of flame transfer function gradually decreases,while the phase increases.For the bluff-body stabilized ducted V flames,the effects of amplitude and frequency of inflow forcing,Reynolds number and bluffbody structure on the premixed flame dynamics are examined.The flow field and vortex movement of forced bluff-body flow are dominated by both the forcing frequency and selfinstability frequency.The flame transfer function amplitude of the bluff-body stabilized ducted V flame decreases as the forcing frequency increases,and the flame transfer function phase increases linearly with the forcing frequency.The higher forcing amplitude,as well as the sharper bluff-body corner can increase the flame transfer function amplitudes.