Characteristics And Control Study Of Combustion Thermoacoustic Instabilities In Non-premixed Swirl Burners

Recently,the utilization of oil and natural gas as fuels presents an upward trend with China's oil and natural gas development.Based on our energy distribution pattern of more coal,less natural gas and poor oil,and more stringent pollutant emission demand,the gas-steam combined cyclel and the integer gasification combined cyclel technologies are widely adopted.Gas turbine is the most critical component,but the lean premixed combustion in gas turbines is proved to be notoriously susceptible to combustion thermoaoustic instabilities.Moreover,non-premixed combustion boiler burning oil and natural gas also has the potential of thermoaoustic instabilities in the combustion process.The mechanism and control technology aimed at premixed combustion have been fully studied over the years,but less concerned with non-premixed combustion equipped with different types of flame holders in boilers particularly.Our work focused on the characteristics and the controlling of combustion thermoacoustic instabilities in non-premixed swirl burners.First,Jet in Cross-Flow?JICF?was explored in non-premixed swirl combustion instabilities by varing JICF gases,fuel composition and flue gas recirculation effect.The pressure fluctuation,flame transfer function,flame images and schlieren images were employed to study the combustion instabilities.With N2 as JICF working medium,the effects of jet-to-crossflow momentum flux ratio?J?and equivalence ratio perturbation on the coupling of the pressure and heat release fluctuation depended on the appropriate operation condition.When JICF gas was CO₂,the heat release fluctuation kept almost the same with different equivalence ratio,the fitted curves of the corresponding heat release fluctuation first gradually increased and then saturate nonlinearly.It was observed that the corresponding critical J increased with the equivalence ratio when the heat release approached saturation and the acoustic velocity perturbation came to respond on the flame,meanwhile another critical J also existed when vortex shedding and heart release fluctuation had the same peak frequency with the acoustic forcing.When JICF gas was CH4,as J increases the pressure amplitude had a valley under lower equivalence ratio.With different JICF gases the combustion pressure amplitude had the best reduction efficiency,accompanied by different operation conditions.When the combustion experiments of CH4/C3H8 mixtures were carried out under different J,the pressure amplitude decreased with J and ?CH4?the percentage of CH4 volume flowrate?,but as the percentage of CO volume flowrate??CO?in CO/C3H8 mixtures increases,the pressure amplitude reduced to a certain value and then kept a constant level,featuring the minimum at ?CO 10%.When CO₂ was added in the inlet air,the best control effect of the thermoacoustic instabilities appeared at J=3.6 and ?CO₂=10%.Second,the combustion thermoaoucstic instability was analyzed in the non-premixed swirl burner with acoustic exciation seated at combustion chamber and inlet line respectively.To evaluate the combustion instabilities,the pressure fluctuation,flame transfer function,flame images and schlieren images,and Poincare map were employed.When acoustic exciation was added to the combustion chamber,jet-to-crossflow momentum flux ratio had little impact on the pressure peak amplitude.The flame heat release rate fluctuation had a nonlinear response on acoustic forcing amplitude.When the forcing frequency was higher than 270Hz,the flame heat release rate was perturbated dominantly by JICF effects.And the phenomenon of heat release rate saturation with the forcing amplitude still existed but depended on the forcing frequency.The J corresponding to the strongest heat release rate perturbation under each exciation frequency was different.When acoustic exciation was added to the inlet line,JICF had limited impact on the pressure perturbation of the combustion chamber.The equivalence ratio fluctuation by acoustic exciation had great influence on the flame response,featuring good in-phase coupling between the heat release rate and pressure fluctuation.The flame transfer function for the exciation frequency of 50 and 135 Hz under lower JICF perturbation presented a nonlinear response as forcing amplitudes varied,and for the exciation frequency of 100 Hz,the flame response had the linear behavior with JICF perturbation.Poincare map had different harmonic wave features under different acoustic exciation location.Third,based on the analysis of Karman vortices and swirling vortices effects on the combustion instabilities,a non-premixed swirl combusor operated on the power station boiler was conducted to measure thermoacoustic instabilities.With the laboratory scaled combustor the influence of combustor geometry and JICF perturbation on the combustion instabilities was studied,such as changing air injection location and inlet line length,removing the flame baffles,moving the primary swirler.It was found that thermal-drivern thermoacoustic instability was not the major vibration cause of combustion instabilities.When removing the flame baffles,the pressure amplitude had a small drop,but the couple of pressure and heat realease fluctuation of the system can occur potentially,depending on the interaction of flame and vortex.When moving the primary swirlers,azimuthal velocity perturbation generated by swirlers and the flame/vortex interaction had a great influence on the combustion thermoacoustic instabilities.The controlling of JICF perturbation on the combustion instabilities in the non-premixed swirl combustor showed few effects,contributed to higher JICF location disturbing the vortices of the flame weakly.By analyzing the burner and combustor chamber system in full scale and laboratory scale combustors,the phase lag of the pressure and heat release fluctuation was the dominant mechanism controlling the boiler vibration,and the time lag of ms order will induce strong thermoacoustic instabilities by the coupling of the pressure and heat release fluctuation.Changing combustor geometry to reduce the turbulence intensity can be a way to control combustion thermoacoustic instabilities.