Characterization And Recognition Of Flow Regimes In Gas-solid Fluidized Beds

Gas-solid fluidized beds are widely used in energy,chemical and environmental protection fields.Flow regim dominates the heat and mass transfer and chemical reaction process in the bed.Therefore,accurate characterization and identification of flow regimes are the key to safe and efficient operation of fluidized bed.At present,flow regimes characterization and identification based on pressure fluctuations are most feasible in industrial applications,and are also research hotspot and frontier problem.Firstly,in order to excavate the gas-solid flow information hidden in pressure fluctuation signals,researchers have proposed a large number of analytical methods to establish the accurate link between pressure fluctuation characteristic parameters and gas-solid flow phenomena,so as to achieve flow regimes characterization.Due to the complexity of pressure fluctuations and gas-solid flow,there is still no consensus on which methods can exactly relate to the phenomenon of gas-solid flow.Secondly,the characteristic parameters extracted from pressure fluctuations are faced with reliability problems because of the non-stationarity of gas-solid flow.The effectiveness of the analytical method characterizing the flow regimes depends mainly on subjective judgment.Finally,researchers are also trying to build mechanism models of pressure fluctuation formation,so as to deepen the understanding of the gas-solid flow process in fluidized bed.However,due to the complexity of gas-solid two-phase flow phenomena and the limitations of measurement methods,the understandings in these areas are far from sufficient.In this thesis,the characterization and identification of gas-solid fluidized bed flow regimes,reliability and effectiveness evaluation of charactering methods and the formation mechanism of pressure fluctuations are studied in depth by experimental methods.The software and hardware systems for high-speed camera and high-precision multi-point pressure signal acquisition are constructed to realize the simultaneous measurement of pressure and image signals.A joint analysis method combining signal shape and flow image has been developed and the time-space correlation law between pressure fluctuations and gas-solid flow phenomenon is established.The creative method provides a powerful analytical tool for the follow-up study of this thesis.The time-domain,frequency-domain and state-space characteristics of pressure fluctuations are systematically studied.Based on the aforementioned measurement method,the deep gas-solid flow mechanism behind the above three classes of characteristics is explored,and the similarities,advantages and disadvantages of flow regimes characterization methods based on the above three classes of characteristics are clarified;The S-transform method is introduced for the first time to analyze the pressure fluctuations,accurate identification and evaluation of gas-solid phase motion frequency and motion non-stationarity are realized.It is further found that the time-frequency plane of the S-transformation can reflect the complexity difference of the gas-solid motion between the bubbling bed and the turbulent bed clearly.A new characteristic parameter is proposed based on the difference of time-frequency plane.Based on the analysis of combined pressure signal morphology and gas-solid flow image,the formation mechanism of pressure fluctuations and the mechanism of flow regime transformation in the bubbling bed and the turbulent bed are studied.The six sources of pressure fluctuations are identified,simple connections between the rising,falling,local peak,local valley,peak propagation of the pressure fluctuations and the causing phenomenon are established,and the difference of generation mechanism of pressure fluctuations in different flow regimes are studied;Based on the Reynolds transport theorem,a mathematical model of pressure fluctuation in the plenum is proposed to explain the negative correlation between pressure and bed hegith;The variation law of differential pressure signal during the transition from bubbling bed to turbulent bed is found and its deep mechanism is clarified.Based on this,a characteristic parameter reflecting the flow regime transition mechanism is designed.Three quantitative indicators of reliability(i.e.S_l,S_n and S_p)are proposed.The reliability of 23 methods of flow regime characterization and the influence of measurement position,particle size and static bed height on the reliability of characterization methods are studied.An evaluation framework was constructed based on three reliability indicators,and a more reliable method set was selected.The effectiveness of the flow regime characterization is defined,and the effectiveness calculation is realized by the silhouette index algorithm in this thesis.It is found that using differential pressure signals can improve the effectiveness of flow regimes characterization.Based on the reliability and effectiveness of the characterization method,a flow regimes recognition system with high accuracy and good extension is constructed.