The Flow Regimes And Analysis Of Pressure Difference Fluctuation In Gas-solid Micro Fluidized Beds

The gas-solid micro-fluidized bed combines the advantages of traditional gas-solid fluidized beds and micro-reactor,which has the advantages of excellent transfer performance,high wall flux and safe and controllable process,has been widely used in a great deal of research fields.Due to the polymorphism and complexity of gas-solid two-phase flow,accurately and quantitatively description of gas-solid flow in gas-solid micro-fluidized beds is the basis for the design and optimization of this kind of reactor.In this paper,the flow characteristics of a gas-solid micro-fluidized bed with a bed diameter of 3mm～20 mm have been experimentally studied and analyzed.The main results are as follows:(1)The fluidization characteristics of Geldart group A and Geldart group B particles were investigated in four gas-solid micro fluidized beds with different inner diameters ranging from 3 mm to 20 mm,respectively.At the same time,the variation rules of the minimum fluidization velocity affected by some important factors,such as bed geometry,operating conditions and physical-phase properties were studied.The result showed that the bed pressure drop characteristics in the gas-solid micro fluidized bed were closely related to the used particle type,and the flow characteristics of two types of particles were significantly different under various flow states.In the fixed bed stage,compared with Geldart group B particles,the interaction between the Geldart group A particles and the wall was stronger,leading to greater deviation of the experimental pressure drop value from the calculated value derived from the pressure formula of traditional fluidized beds.While in the fluidized bed stage,the larger particle size and density of Geldart group B particle showed higher bubble coalescence and rupture degree in the fluidized bed,which intensified the collision among different particles and increases the energy loss,as a result the pressure drop of the whole fluidized beds showed a higher experimental value.Among the factors affecting the minimum fluidization velocity,the influence of bed inner diameter and static bed height on the minimum fluidization velocity could not be ignored.An empirical correlation to predict the minimum fluidization velocity of the micro fluidized bed was proposed within the scope of the experimental investigation under the condition of some important influencing factors.(2)The flow regimes in gas-solid micro fluidized beds were studied by using time domain analysis and visualization techniques.The results showed that,with the increase of gas velocity,the fluidized beds experienced four flow regimes:particulate,bubbling,slugging and turbulent flow for Geldart group A particles.While the fluidized bed entered bubbling regime directly after the minimum fluidized velocity was reached for Geldart group B particles.In a fluidized bed with a bed diameter of less than 10 mm,the static bed height had a significant effect on the transition velocity of the slugging state and the turbulent state.The stronger cohesion of Geldart group A particles inhibited the coalescence of bubbles and made the operation area of bubbling regime larger.Large voids between Geldart group B particles could stabilize or promote the coalescence of bubbles,making slug regime more likely to occur.When D_h/d_p was small,the fluidized beds would directly enter turbulent regime from bubbling regime for Geldart group B particle.(3)The Hurst index of Geldart group A and B particles was greater than 0.5 under different flow regimes,indicating that the time series of each flow regime had a long-term correlation.The frequency domain analysis method showed that the dominant frequencies of the power spectra for the two kinds of particles were all located at low frequencies,and with the increase of gas velocity,the power spectral density increased firstly and then decreased.The autocorrelation coefficients and reconstructed three dimensional attractor phase diagrams of time series signals under different flow regimes were studied respectively,and the autocorrelation variation trend and attractor characteristics all prove that the gas-solid micro fluidized beds have chaotic characteristics.By analyzing the correlation dimension of attractor,it could be seen that the chaotic characteristic was the strongest under bubbling regime.