Research On Multi-scale Characteristics Of Circulating Fluidized Bed Based On Pressure Signal

Pressure fluctuations have been shown to be closely related to the dynamic behavior of gas-solids multi-scale structures.In order to fully understand the multi-scale characteristics in circulating fluidized bed(CFB),experimental work is carried out in a comprehensive CFB system,The high-density operation conditions were reached with the superficial gas velocity(U_g)of 5-9 m/s and the solids circulation rate(G_s)of 100-800kg/m~2s in riser,U_g=1-9 m/s and G_s=100-1000 kg/m~2s in downer.Firstly,the effect of sampling frequency(f_s)on pressure fluctuations was studied and a suitable f_s was determined.The operating conditions that only reflect the characteristics of the macro(meso and micro)scales are simulated from the experimental point,then the main frequency bands that cause the corresponding pressure fluctuations are clarified,and it is used as the basis for the multi-scale decomposition of pressure fluctuations.Further,the multi-scale structure was identified by comparing the dynamic characteristics of the decomposed signals in CFB.The pressure signals representing the characteristics of multi-scales were quantitatively obtained from the frequency domain and the flow characteristics of the multi-scale structure were studied.Finally,The similarities and differences of the multi-scale flow characteristics of riser and downer under the same conditions were analyzed.The time-domain fluctuation characteristics of the pressure signals in riser and downer are closely related to the sampling frequency(f_s).When f_s is low,the pressure fluctuation is relatively slow.fluctuation form of slow change and rapid vary coexist in time-domain fluctuation curve with the increase of f_s,showing that pressure fluctuations are caused by various gas-solid dynamic behaviors.Furthermore,the research on the fluctuation characteristics of pressure signal in different frequency domains shows that the low-frequency components(f<5 Hz)under different f_s all show the periodic fluctuation characteristics of the wave shape.While the high-frequency component(f>5Hz)can gradually fully reflect the behavior of meso and micro scale with the increase of f_s,so intensity of fluctuation becomes larger,and standard deviation(Sd)of high-frequency component increases from 10 Pa to 30 Pa in riser and increases from 20Pa to 31 Pa in downer.Moreover,the fluctuation curve of high-frequency component is more dense and noisy,indicating that f_s has an effect on the complexity of the high-frequency component.Finally,the approximate entropy(Ap En)is used to analyze the complexity of the high-frequency component of pressure signals.When f_s=50-400Hz,the Ap En of high-frequency component in riser and downer gradually decreases,while when f_s=400-1000 Hz,the Ap En of the high-frequency component remains constant,indicating that the high-frequency component can fully reflect the dynamic characteristics of micro-scale behavior when f_s=400 Hz,and may be least affected by noise.Therefore,the appropriate f_s for the pressure signal is 400 Hz.Experimental work prove that multi-scale structure can indeed cause pressure fluctuations in different frequency bands.When there is only macro gas flow in the bed,the amplitude of pressure power spectrum density at f<1 Hz is relatively high,indicating that low frequency fluctuations are the main factor causing macro pressure fluctuations.The briquette particles with millimeter-level diameter have multiple peaks in pressure power spectrum within 2-10 Hz due to movement and collision behaviors in downer.Therefore,the fluctuations due to the meso-scale behaviors in pressure signal are concentrated in the intermediate frequency.The collision of discrete particles in downer disturbs the gas,causing power spectrum to have multiple broad-spectrum peaks within50-200 Hz.Therefore,the fluctuations due to the micro-scale behaviors in pressure signal are concentrated in the high-frequency.The above results provide a reliable evaluation standard for the quantification of multi-scale structures in CFB at the frequency domain of pressure signals.Further,pressure fluctuations in CFB are decomposed at 9 scales through wavelet analysis and recurrence analysis,and multi-scale structure is identified by characterizing and comparing the dynamic characteristics of the wavelet decomposition signals:detail signals at 1-2 scale(50-200Hz)reflect micro-scale characteristics,detail signal at 3-6 scale(3.125-50 Hz)reflects meso-scale characteristics,and detail signal at 7-9 scale and approximate signal at 9scale(0-3.125 Hz)reflect macro-scale characteristics.Then the sub-signal is reconstructed to obtain pressure signals at macro,meso and micro scales.Through the energy of the macro,meso and micro scale pressure signals,the axial distribution characteristics of multi-scale structures in riser and influence of operating conditions were investigated.The axial distribution of multi-scale structures is closely related to solids holdup.It gradually decreases with the increase of the axial position.Thus the pulsation degree of macro solids holdup,the agglomeration and coalescence intensity of particle cluster,and the collision intensity of discrete particle gradually decrease along the axial position.The maximum signal energy of the macro,meso and micro scales at the bottom can reach 290488,1191,18577 Pa~2,and gradually attenuates along the axial.Under high-density conditions,the distribution of multi-scale signal energy along the axial in riser has changed,and the gas-solid multi-scale behavior has also become more intense.The increase of G_s or decrease of U_g will increase solids holdup.At the axial height of 14.06 m,the energy increase of macro-scale signal can reach up to 2290%,and the pulsation ability of macro solids holdup greatly increases;the energy increase of meso-scale signal can reach up to 2366%,the agglomeration and coalescence behavior of particle cluster to become greater intensity;the energy increase of micro-scale signal can reach up to 1973%,and contact opportunity of particles increases,causing the particle collision behavior to become more intense.The energy of pressure signals at various scales in downer exhibits an exponential or"C-shaped"distribution along the axial.The signal energy of macro and meso scales increases with the increase of G_s,the maximum increase of signal energy at macro and meso scales can reach 5458 and 1846%respectively at the axial height of 3.58 m,but the change trend with the increase of U_g differs under different G_s.The signal energy of macro-scale rises with the increase of U_g when G_s<500 kg/m~2s,while in G_s≥500 kg/m~2s decreases with the increase of U_g.The signal energy of meso-scale increases with the increase of U_g when G_s≤600 kg/m~2s,and decreases with increase of U_g when G_s≥800kg/m~2s.The signal energy of micro-scale is not sensitive to changes in U_g and G_s.The flow characteristics of multi-scale structures in riser and downer were compared under the same conditions.When G_s=100-300 kg/m~2s,the pulsation capacity of macro solids holdup in downer is gradually close to that of the riser with the increase of U_g,and the severity of the particle agglomeration and coalescence in downer gradually approaches and exceeds the riser.As far as the full development region of the two reactors are concerned,the collision strength of discrete particles between riser and downer is equivalent when G_s≤200 kg/m~2s,while particle collision behavior in downer is more intense when G_s≥300 kg/m~2s.