Study On The Scale Resolution And Cluster Identification Of Optical Signals In Circulating Fluidized Bed

Gas-solid fluidization has been widely used in the fields of petrochemical,metallurgy,energy,materials,environmental technology and so on.Circulating fluidized bed(CFB)has the advantages of high operating gas velocity,high solid circulation flow rate,fast mass transfer heat and transfer rate,and large processing capacity.As well,CFB has been widely applied in various industrial fields.The gas-solid two-phase flow in the CFB has non-uniform characteristics.It shows inhomogeneous distribution over both axial and radial directions.It is also manifested by the continuous formation and dissipation of the particle clusters in the local.As the highly non-uniform flow structure and nonlinear behavior of the gas-solid two-phase flow in a CFB,it is difficult to describe its essential characteristics by statistical averages.Generally,multi-scale analysis can effectively reveal the gas-solid flow inherent reaction mechanism.The gas-solid two-phase flow in a CFB can generally be decomposed into three scales: the micro-scale describes the movement of the dispersed particles;the meso-scale describes the interaction between the dense phase and the dilute phase at the cluster size;the macro scale describes the size of the fluidized bed device,the entire system within the gas-solid flow boundary.There is still no reasonable criterion or method to classify the three scales accurately and effectively in a CFB.In this thesis,considering the correlation and time-delay characteristics of the three scales between the adjacent signals from a two-channel fiberoptic probe in the CFB,a criterion of scale resolution of the CFB signals was proposed.After the scale resolution of fiber optical signals,this thesis proposed a method to select cluster identification threshold for the meso-scale signal,and takes it as the cluster identification method of the particle concentration signal.Through the cluster identification method,characteristics of clusters at different radial positions of the CFB riser were figured out.The correlations among the cluster characteristics and the correlations between the radial distribution of solid concentration signals and cluster characteristics were analyzed.In this thesis,it is found that the signals of two adjacent measurement points in a circulating fluidized bed have the following characteristics on three scales: at macro-scale,the pair of signals has a high correlation and poor time-delay character;at meso-scale,the pair of signals has a good correlation and obvious time-delay;at micro-scale,the pair of signals is completely irrelevant,and accordingly has no time-delay character.According to these fetures,this thesis proposes the scale resolution criterion of the fiber optical signal in the circulating fluidized bed,the correlation-time-delay function f?-r,and the scale distributions of the particle concentration signals at different radial positions were also compared.In this thesis,the source and characteristics of micro-scale signals were explored by analyzing the Shannon entropy and spectrum distribution of low scale decomposition signals.It is concluded that the variation of Shannon entropy is different between micro-scale and meso-scale signals,the similarity of spectral distribution is high between experimental signal and noise at micro-scale.The current mainstream cluster identification methods were deeply studied and analyzed.It was concluded that the wavelet analysis method based on multi-scale characteristics of signals was more suitable for the identification of clusters.At the same time,it was found that the existing wavelet analysis method is equivalent to taking the meso-scale signal as the cluster fluctuation signals directly,without removing the background concentration in the meso-scale signal.Therefore,it proposed that on addition to scale resolution of the original signal,a cluster identification threshold for the mesoscale signal is necessary,which will finally identify the cluster.The selection of the identification threshold for the meso-scale signal was mainly conducted through analyzing the thresholds of a simulated ideal meso-scale signal and some actual meso-scale signals with different particle concentration distributions.It was concluded that the variations of the average cluster duration time and the probability density distributions of particle concentration with assumed identification thresholds,could be used as a basis for selecting the cluster identification threshold.According to the scale resolution and cluster identification method proposed in this thesis,the hydrodynamic characteristics of the clusters in the riser have been studied.The distributions of particle concentration and cluster properties at different radial positions of the riser in CFB were analyzed.The properties of different clusters which appear at different times in the same time series have been studied.It was found that there is a significant correlation between cluster duration time,cluster frequency and cluster velocity.When the average cluster velocity increases,the cluster frequency will increase and the average cluster duration time will decrease.