Detection Of Multiphase Gas Temperature And Concentration Distribution Based On Acoustic Velocity And Relaxation Attenuation Tomography

Mixed gas flow process widely exists in modern life and industrial production.With the complexity of production process and equipment structure,as well as the importance of the energy conversion efficiency,safety performance and environmental protection,the real-time monitoring of mixed gas parameters in the production becomes particularly significant.For example,the concentration and temperature of COz,NOx and other gases in the boiler furnace of a power station should be monitored in real time;the composition of mixed gases in oxygen generation device of aero craft should be detected to ensure the safety and stability of equipment.However,the flow field is complex and gas parameters are changed incessantly due to the changes of flow state,heat exchange,fluid compression and other factors in the flow process.It is difficult to monitor in real time.As a nondestructive detecting technology,ultrasonic detection has the advantages of non-intrusion,real-time and low cost.At present,scholars mainly focus on temperature measurement via acoustic velocity.However,the acoustic velocity is not only related to the temperature,but also the concentration of mixed gas,which is also an important parameter in mixed gases flow analysis.In order to obtain more accurate parameters,the combination measurement of acoustic velocity and acoustic relaxation attenuation,which is less studied,is adopted in this paper.The acoustic relaxation attenuation has the property which is related to acoustic frequency and the information of concentration of each gas can be obtained.In this paper,a new reconstruction algorithm is proposed to solve the key problems in acoustic Computed Tomography(CT).And the acoustic velocity-temperature and acoustic velocity-concentration tomography with high accuracy are realized.Moreover,the acoustic relaxation attenuation-concentration tomography is developed innovatively.The main research contents of this paper are as follows:1.Basic acoustic theory,emphasis on ultrasonic relaxation attenuation theory.In this paper,the basic theory of ultrasonic is studied,and the formula for calculating the acoustic velocity in multiphase gas is presented.In the classical attenuation part,the formulae of viscous attenuation and heat conduction attenuation are given.In acoustic relaxation attenuation part,the mechanism of attenuation,the external and internal energy of gas molecules and the energy transfer model of molecular collision are studied.And the formula of acoustic relaxation attenuation coefficient for mixed gas is also derived.However,the relaxation time is an important parameter which is difficult to be derived.In order to obtain the relaxation time,the probability of molecular collision energy transfer is necessary and it can be calculated by specific iterative formulae.And the required parameters in these iterative formulae can be calculated by matching L-J and biased exponential function.Based on that,the matrix solution formula of the differential equation of vibration temperature is derived Finally,the acoustic relaxation attenuation coefficient can be solved in the form of plane wave.This calculation process is the foundation for acoustic tomography.2.Acoustic tomography theory and reconstruction algorithm.In this paper,the acoustic relaxation attenuation tomography method and reconstruction algorithm are studied based on acoustic velocity and acoustic relaxation attenuation which is the innovation point of this study.In imaging theory,the hardware system of acoustic tomography is designed.And The direct and inverse problem formulae of sound velocity and relaxation attenuation tomography are derived respectively.The sensitivity field matrix of different pixels is calculated,and the propagation path which is close to or located in the grid line is processed.Such process can reduce the error of the model,and the sensor can be arranged in any position.In the reconstruction algorithm part,matrix M construction algorithm based on neighborhood constraints is proposed.In acoustic CT,the sensitive field is a sparse matrix,and the number of pixels is required to be less than the number of propagation paths,which causes extreme difficulty in imaging.The algorithm proposed in this paper greatly improves the accuracy of solution.Moreover,the number of pixels can far exceed the number of propagation paths,and the M matrix is not affected by the measured space.Regularization parameters and error estimation of model can be obtained in simulation calculation.The fast online calculation could be achieved.It can be universally applied to acoustic velocity,acoustic relaxation attenuation tomography and even optical tomography.And it is extremely valuable in application.3.Simulation of acoustic velocity and relaxation attenuation in mixed gasesIn this paper,acoustic velocity-gas temperature tomography,acoustic velocity-gas concentration tomography and acoustic relaxation attenuation-gas concentration tomography are simulated respectively.The common algorithm and the M matrix construction algorithm based on neighborhood constraints are both used to reconstruct the images of different pixels,and the comparison shows that the new algorithm has a good imaging result,and the optimal number of pixels is selected.Various parameters of different kinds gases in acoustic relaxation attenuation are calculated in simulation,and the effective relaxation frequency is obtained in the frequency spectrum of acoustic relaxation attenuation of three gas concentrations which is also calculated.The optimal frequency is selected for imaging calculation in following.Taking two groups of mixed gases as examples,the reconstructed image can be simulated by the acoustic relaxation attenuation coefficients with one gas concentration changed.According to the frequency characteristics of acoustic relaxation attenuation,the detection scheme for multiple concentration changes in the mixed gases is presented.Finally,the simulation of decoupling of acoustic velocity and acoustic relaxation attenuation is earied out.More comprehensive and accurate information of mixed gases' temperatue and concentration can be obtained by combination with the acoustic velocity tomography.It is showed in simulation results that both acoustic velocity and acoustic relaxation attenuation tomography can obtain highly accurate reconstructed images by adopting the new algorithm.The realization of this method provides more possibilities for acoustic detection.4.Development of an acoustic detection system based on FPGA and design of experimentAn acoustic detection system is designed and built.The FPGA is used as the central processing unit to control the circuits of the transmitting part and the receiving part(amplification,filtering and analog-to-digital conversion)of the detection system,and the PCB is made.The control program in FPGA is designed in VHDL language to realize multiplex simultaneous measurement.In order to realize the communication between the upper computer and the core board,a serial port program in C language for multi-channel data reading is compiled.Finally,an experimental scheme for detecting acoustic velocity and acoustic relaxation attenuation is designed and the relevant tests are implemented in experimental platform.It is proved that the platform can be adopted to detect the flow process of mixed gases.In this paper,the detection of temperatue and concentration of mixed gases based on acoustic velocity and relaxation attenuation tomography is achieved.A new reconstruction algorithm is proposed to solve the key problems in acoustic tomography.Moreover,acoustic relaxation tomography opens up a new field of acoustic CT.