| The increase in environmental awareness has made the reduction of pollutant emissions in the combustion process one of the main challenges of modern industrial combustion equipment.Especially in the main power generation devices such as boilers,gas turbines and internal combustion engines,policies and industry regulations for low emissions are becoming more stringent.However,due to the high volumetric thermal load of the large oil and gas power station boilers,the high combustion rate and the high rate of nitrogen oxides are high.At the same time,the complex characteristics of heavy oil,turbulence and combustion interactions make it difficult to optimize the coupling relationship between the parameters of the combustion system.The international research on combustion vibration mainly focuses on the flame structure,thermoacoustic instability of premixed combustion in compact confined spaces such as gas turbines and aeroengines,and the use of passive control methods to suppress combustion vibration.The research on the combustion vibration in the large boiler is rare,especially the diagnosis and control of the combustion vibration after the large boiler adopts the low nitrogen combustion system.In order to study the response of non-premixed flame under acoustic excitation,a nonpremixed combustion test rig was built.In order to study the thermoacoustic instability and nitrogen oxide emissions of heavy oil boilers,the linear thermoacoustic characteristics of the furnace were numerically and experimentally analyzed.In order to study the sound absorption characteristics of the perforated plate,an impedance tube test bench was built,and the influence of the chamber length and the bias flow velocity on the sound absorption characteristics of the perforated plate was analyzed.Firstly,the influence of the geometry change of the combustion chamber on the sound field response of the non-premixed flame is studied.The acousitc response of different flame heat release rates were studied at the lengths of the inlet sections of 0.245 m,0.345 m and 0.445 m.Different acoustic modes of the combustion chamber at different inlet section lengths were obtained by acoustic simulation.Through experiments and simulation analysis,it can be obtained that the heat release rate response of the flame at different inlet section lengths is affected by the acoustic modes of different combustion chambers,including the inlet section mode and the fuel tube mode.The effect of the flame divider on the sound field response of the flame was explored.The nonlinear excitation effect of different sound field frequencies on the flame response is analyzed.Secondly,a one-dimensional thermoacoustic analysis method for heavy oil boilers was established.By analyzing the thermoacoustic linear growth rate at different latitudes of the furnace,the influence of model parameters,the length of the inlet section and the flame separation plate on the combustion thermoacoustic instability were obtained.The results show that the unstable mode is close to the natural first-order longitudinal mode of the furnace.In the unstable mode,the furnace exhibits a dependence on the flame index and time delay.After removing the six flame dividers,the time delay of the flame is reduced.In addition,in conjunction with the reduction in the length of the inlet,the furnace becomes stable near the first-order longitudinal mode.These improvements are applied to the actual boiler and eliminate the possibility of vibration.The extended one-dimensional thermoacoustic analysis method was applied to the Saudi Yanbu oil and gas boiler,and the influence of the length of the inlet section and the position of the oil gun on the linear thermoacoustic instability growth rate was predicted and analyzed.Thirdly,the influence of the flame separator on the NOx emission and thermoacoustic instability of the heavy oil boiler furnace was analyzed.When combustion instability occurs,three-dimensional acoustics are used to predict the pressure mode and linear growth rate of the furnace.As the number of installed flame separators increases,the amount of carbon oxide emissions decreases.Simulations and experimental results show that geometric changes at the exit of the burner can significantly reduce carbon oxide emissions.The presence of six flame dividers affects the combustion instability of the boiler.The comparison between the one-dimensional thermoacoustic analysis method and the three-dimensional finite element analysis and the cylindrical flame and the spatially distributed time delay show that there is a negative growth rate region near the same time delay.Then,the sound absorption curves of different perforated plates designed at the same sound absorption frequency at different bias flow velocities under different chamber lengths were compared.The sound absorption characteristics of the double-layer perforated plates were studied experimentally and theoretically.The bias velocity not only greatly affects the sound absorption performance of the two-hole plate structure in a linear state,but also affects the nonlinear effect caused by the high-voltage amplitude.The ratio limit between the two bias flow velocities is given by 0.25-4.Experimental and theoretical analysis results show that the two-hole plate method can achieve a wide attenuation range by adjusting the bias flow velocity.With this double-plate arrangement,the bias flow velocities at the two plates can be individually adjusted and the damper operated at a selective target absorption frequency.Finally,the damping properties of the perforated plate at the inlet portion of the combustion chamber were investigated experimentally and numerically.Experimental results of the flame response under acoustic excitation indicate that the installation of a perforated plate at the inlet serves two effects: one is acoustic attenuation;the other is redistribution of pressure mode.Furthermore,the results show that for non-premixed flames,the change of the shape of the acoustic mode is more effective when the combustion instability is caused by inlet pressure fluctuations. |
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