| Combustion noise is an inevitable phenomenon in the combustion chamber of aeroengines,propulsion systems,gas turbines and other industry devices,which has adverse impact on human health and equipment operation safety.The formation mechanism and control technology of combustion noise are of great importance for the safe operation of propulsion systems and the advancement of technology.The production of combustion noise is a result of the coupling of turbulent and heat release processes,and is closely related to the boundary layer,vortex dynamics,heat release rate fluctuations and other factors.This thesis focuses on experimental and theoretical research on cold aerodynamic noise and combustion noise produced by jet and swirl nozzles,aiming at revealing the trends and characteristics of combustion noise.A prediction model is developed for the confined flame combustion noise.For the research in cold aerodynamic noise,this thesis carries out a series of experiments to investigate the characteristics of jet and swirl aerodynamic noise under different Mach numbers,measurement angles and nozzle diameters.The influences of swirl number and premix length on swirl noise are also analyzed.To improve the accuracy of measurement,the disturbance of upstream incoming noise is significantly reduced by choosing the effective rectifier materials.The experimental results indicated that the farfield sound pressure level spectrum of jet noise agrees well with the theoretical prediction before the peak frequency.The directivity of sound field is simultaneously affected by the Mach number and nozzle diameter.The sound pressure level of swirl noise is great higher than that of jet flow at the same Mach number in all frequency bands.The swirl vortex forms a swirl peak in the low frequency range of noise spectrum,which represents the characteristics of the specific swirl nozzle.It can be revealed that swirl noise is dominated by two competing sound sources,shear layer and swirl vortex.There is a specific value of swirl number and premix length,which makes the overall sound pressure level in the far field reach the maximum value.For the research in combustion noise,this thesis experimentally analyzes the combustion noise of open flames with different equivalence ratios and thermal powers using the noise measurement system.The empirical correlation between overall sound pressure level and equivalent ratio and thermal power is extracted.Additionally,the combustion noise of confined flames is studied in a swirl model combustor and the low-order network model is used to predict its combustion noise.It is shown in the experiments that the sound pressure level spectra have obvious peaks at the system acoustic modes for the combustion noise of confined flames.The energy distribution between different modes is related to thermoacoustic instabilities.As a conclusion,the low-order network model can accurately predict the peak frequencies of the thermoacoustic transfer function,although it may have some deviations in the prediction of the peak amplitudes.The study of direct combustion noise in this thesis reveals that both of swirling vortex and shear layer dominate the far-field noise of the swirling flow.Empirical correlations are established between the overall sound pressure level of the open flame direct combustion noise and the equivalence ratio and thermal power.A low-order network model method is developed for predicting confined flame combustion noise,which indicates a possible connection between combustion noise and thermoacoustic instability.These findings provide insight into the formation and control of direct combustion noise. |
