Research On The Dynamic Characteristics And Control Of Thermoacoustic Instability In Swirl Premixed Combustion

Premixed combustion technology has been widely used in gas turbine industry due to its excellent performance in reducing NO_x emissions.However,premixed combustion technology is prone to combustion instability(or thermoacoustic instability)while achieving clean combustion.With the continuous demand for clean,efficient,durable and reliable gas turbines in industry,combustion instability has become the key factor that hinder the development of advanced combustors.Combustion instability is caused by the coupling of sound waves and flame heat release rate in the unstable burners.The interaction between flame and acoustic waves will cause unsteady flow oscillations in the combustion chamber.If the amplitude of thermoacoustic instability increases continuously,the structure of the combustion chamber will be damaged or the normal operation of the burner will be affected.Combustion instability is related to acoustic oscillation,fluid flow or mixing oscillation and flame heat release rate oscillation.The interdisciplinary and nonlinear characteristics make the analysis and control of combustion instability very challenging.At the same time,in the premixed combustion chamber,combustion instability and NO_x emissions are contradictory,reducing NO_x emissions,the probability of combustion instability will increase,so how to realize the synchronously control of combustion instability and NO_x emission is very important.Up to now,the coupling mechanism of thermoacoustic oscillation has not been fully studied.The research on the nonlinear dynamic characteristics of thermoacoustic oscillation and its active or passive control is very crucial,especially for heavy-duty gas turbine.In order to explore the characteristics of premixed swirling flame under thermoacoustic oscillation,and carry out the corresponding active or passive control strategies,a laboratory scale swirling premixed combustion test-bed is built,which can be used for the theoretical and practical study of thermoacoustic oscillation.Various types of transverse jet structures are designed to study the control effect of jet in cross flow parameters on thermoacoustic oscillation and NO_x emission.In order to optimize the effect of transverse jet control,this paper also innovatively proposes the use of oxygen enriched and superheated steam jet in cross flow,which could achieve a better thermoacoustic oscillation and pollutant emissions suppression effect.Firstly,the macrostructures and NO_x emission characteristics of premixed flame under thermoacoustic instability were studied.This paper mainly studies the influence of two variables of methane flame:fuel flow rate and equivalence ratio.The results show that when the equivalence ratio of flame increases,the macrostructure of flame changes,the length of flame front first decreases and then increases,and the average length of flame root gradually decreases.With the increase of thermal power and equivalence ratio of the combustor,the flame length increases.The evolution diagram of thermoacoustic instability shows that there is a mode transfer of flame—acoustic interaction in the combustion chamber.Under the condition of thermoacoustic instability,the temperature and velocity fields of flame front or flame root affect the NO_x emissions.Along the radial direction of the burner,the peak temperatures of the inner or outer circulation zones gradually decrease.Secondly,the influence of jet in cross flow parameters on combustion thermoacoustic instability is studied.The jet in cross flow method is simple and easy to operate.It can change the chemical reaction process of the flame and the flow field of the combustion chamber at the same time,and affect the mixing or vortex shedding of the combustion reactants.In this study,the effects of flow rate,jet height,jet direction,jet medium and jet temperature on thermoacoustic oscillation were investigated.The suppression ratio of thermoacoustic oscillation can reach 90%by setting reasonable transverse jet parameters.It is also found that the damping effect of carbon dioxide with high molecular is better than that of nitrogen,argon and helium.In addition,the amplitude and frequency switching of unsteady flame occurred during the experiment.The length of flame decreases with the increase of jet velocity,and the length of flame front or flame root decreases with the increase of flow rate.Thirdly,the simultaneously control effect of oxy medium and superheated steam is studied,and the simultaneously control of combustion thermoacoustic oscillation,NO_x emissions and thermal power is realized.The transverse jet can change the temperature field of the combustor thus reduce the NO_x emission concentration.Under the condition of oxygen rich medium and superheated steam transverse jet,the acoustic wave and the fluctuations of heat release rate of premixed flame experienced mode transition,but the turning point of mode transition is different.Under high velocity transverse jets,the premixed flame becomes dispersed and flat.It is also found that the combustion instability is related to oxygen concentration,but the specific physical and chemical mechanism of jet in cross flow that controlling the combustion thermoacoustic instability still needs further study.Then,the nonlinear response of non-premixed flame excited by sound waves is studied.The dynamic response of non-premixed flame under acoustic excitation was studied on a laboratory scale burner.The experimental data were analyzed by nonlinear time series analysis,and several different flame acoustic resonance modes were obtained.It is found that the coupling resonance of flame and acoustic occurs only at a certain frequency.The acoustic resonance characteristics of the combustion chamber and the inlet section of the non-premixed burner are different,showing quasi-periodic oscillation and limit cycle oscillation,respectively.The results show that the flame acoustic resonance can lead to mode shifting of the oscillation frequency and amplitude.When varies the burner inlet length and air volume,the intermittent jitter of the flame heat release is observed.The vibration intensity increases as the burner inlet length increases.Finally,the damping effects of different active control strategies are well studied.By establishing the coupled thermoacoustic model of flame and sound,the suppression ratio of the active controller for the thermoacoustic limit cycle oscillation and transient process is studied,and the decay time of the thermoacoustic oscillation signal in the active control process is compared.In order to evaluate the effect of active control,the amplitude of actuator voltage under four control strategies is investigated.