Self-Excited Instability And Control Of Liquid Fuel Swirl Combustion

Combustion instability has been prevalent for decades in oil/gas furnaces,ground-based gas turbines and propulsion systems.Severe combustion instability can cause severe vibrations in combustion equipment and even the entire system,which can lead to component damage and system failure.In order to study the self-excited instability of liquid fuel swirl combustion,a liquid fuel swirl combustion instability test system was independently designed and built.The designed combustor could spontaneously generate stable and continuous thermoacoustic oscillations under certain conditions.Pressure disturbances during combustion were monitored and recorded by dynamic pressure sensors.The global CH* chemiluminescence of the flame was recorded by a filter-equipped photomultiplier tube.The perturbation pressure and transient CH* chemiluminescence data were simultaneously saved through a high-response data acquisition card.Instantaneous flame images were recorded using a high-speed camera.In addition,in order to test the control effects of the acoustic damping devices on the self-excited combustion instabilities,an acoustic impedance test system was independently designed and built.The system was used to test the sound absorption characteristics of perforated plates and Helmholtz resonators.An off-axis single-mirror schlieren system was also built to study the flow fields of the jets.Firstly,the effects of fuel properties and combustor structure on combustion stability were studied.It was found that when the pressure pulsation and the unsteady heat release were in phase（Rayleigh criterion）,combustion instability was prone to occur.The effect of combustor structure on the combustion instabilities of ethanol and n-heptane was discussed,and the combustion instability contour maps of ethanol and n-heptane were drawn.Flow field temperature measurements indicated that fuel properties altered the temperature distribution in the combustor,potentially affecting the oscillation strength.Secondly,a single-layer perforated plate with an adjustable back cavity was used to mitigate the liquid fuel spray combustion instability.The acoustic properties of plates with different porosity and hole sizes were first tested in an impedance tube.Then,the perforated plates were fitted to the intake end of the combustor one by one to stabilize the combustion.In addition,the sound absorption characteristics of multi-aperture perforated plates and the plates with non-uniform distributed holes working under bias flow were studied,and the attenuation effects of these plates on the liquid spray combustion instabilities were confirmed by experiments.Thirdly,subsequent studies confirmed that the double-layer perforated plates with bias flow could greatly broaden the sound absorption frequency bandwidth.Installing dual perforated plates at the combustor inlet reduced pulsating pressure and CH* intensity within the combustion chamber by up to 14 d B and 19 d B,respectively.When the primary air flow rate deviated from the optimal value,the double-layer perforated plates could still reduce the pulsating pressure amplitude in the combustion chamber by about 80%.In addition,the effects of the frequency and amplitude of the incident sound,cavity depth,and bias flow rate on the acoustic attenuation effect of the multi-aperture combined double-layer perforated plates were further investigated.Then,the inhibitory effects of CO₂-O₂ and CO₂-Ar jets on combustion instability in the liquid fuel combustor were experimentally investigated.The jets could significantly attenuate pressure and heat release rate fluctuations.The high flow rate jets could also significantly reduce the flame temperature and the NOx emission concentration.But it was also easier to blow out the flame and cause a dramatic increase in CO emissions.Finally,the sound absorption performance of the improved Helmholtz resonator（HR）was experimentally investigated under bias flow conditions.The larger the bias flow rate,the wider the sound absorption frequency band.Appropriate bias flow rate also increased the maximum sound absorption coefficient of HR.The smaller size of the improved HR is conducive to saving space.Compared to the conventional HR,the improved HR suppressed pressure and heat release pulsations within the combustor more effectively.