Investigation On The Mechanism Of Combustion Oscillation In A LPP Combustor

The oscillating combustion is a self-oscillation phenomenon,which is generated because of the intercoupling of the heat release fluctuation and the pressure fluctuation.When the oscillating combustion occurs,the pressure fluctuation might be powerful to cause the combustor and the aero-engine vibrating,even the huge combustion noise will be generated,which will affect the performance of the aero-engine.When the oscillating conbution is more serious,the aero-engine will be destroyed,and the pollution emissions will increase.The mechanism of combustion oscillation in a Lean Premixing Prevaporizing(LPP)low emission combustor is studied by using both experiment and numerical simulation.Combustion flow field characteristic,acoustic characteristic and flame structure during the combustion oscillation process are experimentally studied under different working conditions.And the oscillating combustion process is also numerically simulated by LES method.The main research work in this paper are as follows:Firstly,in order to investigate the influence of different inlet air flow rates and fuel-air ratios on the combustion flow field velocity,fluctuating velocity and Reynolds stress,PIV is used to measure the cold and combustion flow field.Experimental results show that:(1)For the cold flow field,the recirculation zones can be found obviously,the area of the primary recirculation zone(PRZ)is big enough to full of the whole measurement field,and the PRZ area of cold flow field is bigger than that of combustion flow field.(2)With the increasing of the fuel-air ratio,the width of the PRZ increases obviously,but the length of the PRZ decreases.At the same time,the axial velocity increases,the turbulent intensity is strengthened,and the fluctuating velocity and Reynolds stress increase.(3)With the increasing of the inlet air flow rate,the axial velocity increases,the length of the PRZ increases but the width of the PRZ will decrease.Besides,the fluctuating velocity and Reynolds stress increase accordingly.Secondly,in order to study the influence of different inlet air flow rates and fuel-air ratios on the pressure fluctuation,the high frequency pressure sensor and NI data acquisition system are used to capture the pressure fluctuation signals in the combustor when the oscillating combustion occurs.It can be discovered from experimental results that:(1)The main frequency of the pressure fluctuation which is generated by the compressor and fuel supply system for the cold field disappeares when the oscillating combustion occurs.(2)With the increasing of the fuel-air ratio,the main frequency of combustion oscillation remains unchanged,but the amplitude of combustion oscillation becomes more and more bigger.(3)With the increasing of the inlet air flow rate,the main frequency of combustion oscillation remains unchanged,and the amplitude of combustion oscillation becomes more and more bigger as well.Thirdly,in order to investigate the influence of different inlet air flow rates and fuel-air ratios on the flame structure,a High Speed Camera is used to capture the flame structure when the combustion oscillation occurs,at the same time,a new image processing method based on video FFT is proposed to find the mechanism of the combustion oscillation.Experimental results illustrate that:(1)When the combustion oscillation occurs,the flame structure changes dramatically,the area of the flame decreases and increases periodically.(2)With the increasing of the fuel-air ratio,the color of flame becomes more and more blue,the brightness of flame decreases,and the flame leaps more tempestuously.(3)The acoustic energy source comes from the oscillation of flame front which is located at the strong shear layer between the primary swirling jet and the pilot swirling jet,the interaction between the flame and the unstable swirling flow seems to be the mechanism of the combustion oscillation in LPP combustor.Finally,the FLUENT of ANSYS 14.0 software is applied to simulate the combustion oscillation process in the LPP combustor compared with the experimental results,the numerical simulation results reveal that:(1): The main frequency of combustion oscillation for the numerical simulation results is close to that of the experimental results(2)The simulated instantaneous temperature contour on the central cross-section of LPP combustor aggreed well with the flame structure.(3)The mathematical models and calculation methods in this paper can be used to simulate the combustion oscillation process,and the numerical results can be applied to provide the guide for the further discovering the mechanism of combustion oscillation in LPP combustor.