Research On Acoustic Radiation And Propagation Characteristics Of Leakage Sources In Boiler Tube Arrays

In boiler,the "four tube" leakage accident is an important factor affecting the safe and economic operation of power station boilers.Acoustic monitoring of tube leakage has become one of the most promising tools in power plant boiler fault diagnosis technology.However,due to the limitations of acoustic theories and technical difficulties in the tube array,the research on the leakage detection mainly focus on the theory and method of the water wall leakage detecting and locating in the boiler.The problem,which is the leakage source detecting and locating in a more prominent two-dimensional tube array,has not been able to break through in theory and technology,and this problem has not been solved.The acoustic radiation generated by the leak jet and its acoustic propagation are complex in the tube array.Acoustic methods for leak location require an understanding of the characteristics of the sound source and the sound propagation characteristics.Therefore,it has important academic and theoretical significance and engineering application value to study the leakage source acoustic radiation and its acoustic propagation in the heat exchanger tube array.Based on the mechanism of the leakage sound,this dissertation studies the acoustic spectrum characteristics and radiation characteristics of the leakage source.In this study,it is mainly divided into two parts:1.Leakage jet flow field and direct noise research(no tube condition);2.When there is a steel pipe in front of the leak source,the flow field change caused by the leak jet impacting the tube wall,and the effect of secondary sound on direct noise(in case of tube).The study combines the simulation calculation with the experimental method to obtain the flow field distribution and acoustic spectrum characteristics for different inlet pressures and leakage apertures:1.For the pressure inside the tube is the same with no tube condition,when the leakage aperture is increased,the length of the core area of the jet increases,the coverage of the acoustic spectrum is basically unchanged,but the sound pressure level increases.The main vocalization zone is where the turbulent velocity gradient changes most,and the directivity direction is 20°in the range of 0?180°.And the sound in front of the leaking nozzle is higher than the sound behind the leaking nozzle,which is basically consistent with the results of the predecessors;2.For the study of secondary sound,when there is a tube in front of the spout,the leaking jet impinges on the tube which causes the flow field to change drastically in the vicinity of the tube wall.The flow field velocity is rapidly attenuated after the jet impinges on the tube,the greater the inlet pressure,the more the leakage noise covers the low frequency region,and the main vocalization zone is concentrated near the impacted tube wall;in the case of the same aperture and pressure,comparing the no tube condition,the leakage noise covers more low frequency regions,and the sound pressure level in the high frequency region is reduced,and the energy distribution is more even;for the pressure is the same in tube,the larger the aperture is,the direction of sound pressure is more,and finally a sound pressure level enhancement occurs every 20°.In actual operating conditions,the leakage noise spectrum can be extended to the entire audible frequency band(20 Hz-20 kHz),and a stable electroacoustic signal can be selected as the reference source setting for subsequent research.In the study of acoustic propagation characteristics in tube arrays,it is divided into two parts:1.Sound pressure propagation changes in the tube array,including sound propagation characteristics in different directions,sound propagation characteristics when temperature changes,the radiation characteristics of the sound source-in the tube array,the "sound diode effect" of the sound waves through various arrays,etc.;2.The variation of sound propagation velocity in the tube array,the speed of sound changes directly affects the accuracy of the delay positioning,the blockage of the tube wall has a great influence on the speed of sound in the array.The array of heat exchanger tubes in boiler has a regular periodic arrangement,which is consistent with the structure of the phononic crystal.Therefore,the phononic crystal theory is applied to the acoustic propagation characteristics of the tube array.The temperature near the nozzle decreases with the leakage,forming a "cold point"defect for the high temperature array environment,and simulates the influence of the"cold point" position on the sound field characteristics.For the influence of sound propagation direction,temperature,etc.on the characteristics of sound field in an array environment,the radiation characteristics of sound waves in the array with different frequencies in the non-uniform temperature field are studied.The programming of the plane wave expansion method based on MATLAB is compared with the simulation results and the experimental results.The results obtained by the three are basically consistent:1.For N?4(N represents the number of tube rows through which sound waves pass in a certain direction),it can be considered as the condition for band gap formation;through different tube array structure,there is a single-pass phenomenon of sound waves in some frequency bands;the phenomenon of acoustic wave diffraction occurs when there is a cold point;the bandgap frequency changes with the angle deflection at 0°-45°;2.The phononic crystal does not only weaken the sound wave,but also enhances it(acoustic waveguide phenomenon);3.When the temperature field changes,the acoustic directivity changes accordingly.In the non-uniform temperature field,the acoustic radiation characteristics are combined by the array and the temperature distribution.Based on the wave front difference between the acoustic radiation characteristics of the open environment and the array environment,the apparent(effective)sound velocity of the array environment is proposed.In this dissertation,the fundamental equation of acoustic wave is processed by the finite difference time domain method(FDTD),and the perfect matching layer boundary(PML)is used for programming calculation at the boundary to establish the calculation model of array and open environment.The processing of the signal uses the cross-correlation algorithm to obtain the arrival time.In the calculation,the sound source is set separately on the outside and inside of the array.The receiving points are set in different propagation directions to process the single-frequency signals,and the calculation results of the open environment are compared.In order to verify the feasibility of the calculation,we conducted an experiment,and the obtained sound velocity was consistent with the calculated trend:in the no-array environment,the delay algorithm using the cross-correlation algorithm is completely feasible.In the presence of the array,the cross-correlation of the spectrum cannot be performed because the acoustic spectrum of the white noise changes after passing through the tube array.The sound wave velocity is related to the band gap frequency of the phononic crystal.The sound wave velocity in the band gap is relatively slow,and the sound wave velocity in the conduction band is relatively fast,but the overall sound velocity is slow relative to the open environment,and different frequencies of sound waves have different effective speeds in the tube array.The five-element positioning model of the open environment is established,and experimental research is carried out.The position obtained by the positioning is basically consistent with the actual position of the sound source.Placing the sound source in the array environment for experiments,measure the apparent(effective)sound velocity of the sound wave multiple times,and place the sound source inside the array to measure the apparent(effective)sound velocity in different directions:This positioning method is relatively accurate in an open environment.When there is a tube array,the error is large and the positioning cannot be achieved.The velocity of sound waves in the tube array is anisotropic,and the sound velocity in different directions is different,which is the main reason for the failure of sound source localization;The anisotropy of the sound velocity and the radiation characteristics of the sound pressure in the array can be combined,and the sound propagation direction of the single frequency is used to judge the sound propagation direction.The obtained results have important reference value for sound source localization in the array.