Study On Ultrafine Particles Fluidization Characteristics In Gas-Solid Fluidized Bed Intensified By Physical Fields Coupling Of Acoustic (Electric) Fields And Mechanical Vibration

The ultrafine particles can agglomerate easily, and the effect of fluidization can be weakened since some particles exist in channel form. To intensify mass and heat transfers, the experimental measurement and simulation were carried out in this work to investigate systematically the ultrafine particles fluidization characteristics in gas-solid fluidized bed intensified by physical fields coupling of acoustic (electric) fields and mechanical vibration.First, the effect of acoustic field on ultrafine particles flow characteristics and sub-aggregate features in gas-solid fluidized bed were studied. Air and Pcaco3, class C, lime carbonate were used as gas and solid phases. The sound pressure level (SPL) and sound wave frequency used in this experiment were 90-150 dB and 30-8000 Hz, respectively. The experimental results showed that sound wave frequency decreased with increasing added amount of ultrafine particles when SPL was constant. The relationship of particle diameter of sub-aggregate with sound wave frequency was not monotonous variation at frequency of uniformity freedom bubbling fluidization. The maximum response frequency of sound wave corresponding minimum sub-aggregate diameter was about 120 Hz. The maximum response frequency of sound wave was independent on SPL and added amount of ultrafine particles.The models of gas-solid two-phase flow and physical fields coupling acoustic fields and flow field were established by means of analyzing affect theory of acoustic field on ultrafine particle. To study the acoustic field on formation of large size aggregate for ultrafine particles and on sub-aggregate segregated from large size aggregate further, the aggregate/sub-aggregate oscillation model was developed by analyzing dynamics stress due to the gas flow and viscous force between aggregates. This model was verified by the experimental data. The influence of acoustic field on sub-aggregate size was predicted. The effect of coupling with acoustic field and fluid on fluidization characteristics of gas and solid two phases was simulated using fluid dynamics software FLUENT. The simulation results showed that acoustic energy increased evidently with increasing superficial gas velocity, and the critical velocity corresponding regime from fixed bed to fluidized bed was 0.057 m·s-1. The minimum fluidization velocity increased obviously with increasing particle diameter. Nevertheless, the minimum fluidization velocity increased inconspicuous with increasing SPL as the particle diameter was constant. The particle concentration was parabolic distribution, and it was decreased gradually from wall region to central area. The particle concentration increased with increasing SPL far from wall region. Effect of acoustic field on particle concentration at wall region was the small. The comparison was carried out between the experimental results of radial particle concentration along different axial heights with the simulation data under the acoustic field condition. The experimental results were in agreement with the simulation data.The effect of electric field on ultrafine particles flow characteristics in a gas-solid fluidized bed was studied. The experiment was carried out using air and Pcaco3, class C, lime carbonate as gas and solid phases, respectively. The electric field strength is 0-400 kV·m-1, and the electric field vibration frequency is 0-1000 Hz. Three different field configurations have been tested:co-flow electric field, cross-flow electric field, and variable electric field configurations. The particle volume fraction of a fluidized bed was studied under the influence of electric field strength, electric field vibration frequency, and superficial velocity using three different field configurations. The application of these electric fields increased the bed expansion and uniformity of the particle fluidization obviously.The mathematical model coupling electric field with flow field was established analyzing the equation of motion of ultrafine particle, the equation of fluid force, the equation of contact force and the equation of viscous force. The fluidization behavior of electric fluidized bed of ultrafine particle marked Pcaco3, class C, lime carbonate was studied and the motion trajectory of ultrafine particle was simulated by using fluid dynamics software FLUENT. The static electric model of external electric field was proposed using TFM method based on the model proposed by Rokkam. The potential distribution and particle motion in external electric field fluidized bed was calculated. The simulation data showed that particles motioned near original position of bottom of fluidized bed at lower electric field vibration frequency. When the electric field vibration frequency reached 20 Hz, the particle motion was at freely up-and-down pattern. With increasing the electric field frequency further, the particle motion was more restricted and in chaos phenomenon evidently. The comparison between the experimental results of relative variation of the particle volume fraction and the simulation data was carried out with three different field configurations. The experimental results were in agreement with the simulation data. This was illustrated that the model established for coupling electric field and flow field was valid for predicting relative variation of the particle volume fraction.The effect of coupling acoustic (electric) field with mechanical vibration on ultrafine particles flow characteristics was also studied in a gas-solid fluidized bed. The experiment was carried out using air and four different solid powders as gas and solid phases, respectively. The four different powders were Pa-sio2 silica, Pt-sio2 silica, Pcaco3 lime carbonate and Pt-caco3 lime carbonate, respectively. The average diameter of four powders is 1.2-74.7μm. The experiments were used six different SPLs. The mechanical vibration frequency was reached 300 Hz, and the amplitude was 1-30 mm. The experiments of fluidized bed coupling electric field with mechanical vibration field were carried out using air and Pcaco3 lime carbonate as gas phase and solid phase. The horizontal electric field strength was 4 kV—cm-1 and the five different mechanical vibration frequencies were used. The effects of coupling acoustic field with mechanical vibration field and electric field with mechanical vibration field on ultrafine particles flow characteristics were systemically studied in gas-solid fluidized bed. The minimum fluidization velocity obtained using acoustic field and mechanical vibration simultaneously was lower than that using mechanical vibration independently. The minimum fluidization velocity was reached minimum as dimensionless mechanical vibration strength and dimensionless acoustic strength were 5.64 and 0.53, separately. The particle aggregate size decreased obviously with increasing acoustic field strength as horizontal mechanical vibration strength was constant for Pt-sio2 silica. The effect of mechanical vibration strength on particle aggregate size was not obvious when acoustic field strength was constant for Pt-sio2 silica. The effect of acoustic field strength on the minimum fluidization velocity and particle aggregate size was the larger than that of horizontal mechanical vibration strength for Pcaco3 lime carbonate. The horizontal mechanical vibration strength and acoustic field strength were both in favour of reducing particle aggregate size for Pt-caco3 lime carbonate. The range of aggregate size of Pt-caco3 lime carbonate was 90-590 μm. The experimental results showed that the bed of Pcaco3 lime carbonate was brought about collapse using static electric field alone and the application of horizontal mechanical vibration alone was in favour of the bed expansion. The horizontal mechanical vibration can neutralize bed collapse induced by static electric field and the electric field can neutralize bed collapse induced by horizontal mechanical vibration as the electric field and the horizontal mechanical vibration were simultaneously applied. The induction of electric field and horizontal mechanical vibration together can cause unstable flow of aggregates.