Numerical Simulation And Experimental Study On The Gas-Solid Flow In Internally Circulating Fluidized Beds

Internally circulating fluidized beds (ICFB) own their unique advantages in the field of municipal solid waste incineration. Gas-particles flow behaviors in fluidized beds are systemically studied in this dissertation, using discrete element method (DEM) and experimental approaches. Based on the discrete treatment of the particle phase, various kinds of characteristic information are employed to quantitatively analyze fluidization mechanism, including gas-particle velocity fields, particle acceleration fields, pressure fields and pressure fluctuations, and et al. Also, detailed experimental investigations are carried out in this paper, aiming at visual measurements of the bubbles, spatial segregation of the tracer particles, and their residence time distribution (RTD). All these benefit us with more accurate understanding of the fluidization phenomena, provide the first-hand evidence for the mathematical models, and lend bases of favorable industrial applications.Present research mainly consists of following sections: (i) visual measurements of fluidization phenomena using CCD camera and DEM simulations; (ii) gas flow behavior through the bed and its division, microscopic characters of particle movement; (iii) visual analysis of the bubble movement in the ICFB with uneven gas feed, and thereof, investigations on particle streaming, quantitative evaluations of the dynamic mixing process, and numerical predictions of the heat transfer concerning materials in the beds; (iv) experimental researches on the segregating phenomena and RTD of tracers in multi-component ICFBs.CCD experiments are carried out to validate the numerical predictions of DEM under the same condition. Comparisons indicate that numerical simulations successfully reproduce the complicated bubble phenomena, e.g., the arising, escaping, enlarging and bursting of a bubble, et al. Force analysis show that, at the initial stage of a bubble, drag and pressure gradient forces push particles outward and build a small cave near the jetting point. As the time proceeds on, such caves expand. Meanwhile, the directions of the pressure gradient force, exerted on particles at the root of the cave, shift from outward to inward gradually. Consequently, particles fill the bottom of the cave,which finally escapes from the distributor in the shape of a round bubble. Moreover, the predicted bubbles'periods are nearly identical with the experimental results. Fast Fourier transfer (FFT) analysis of the pressure fluctuations manifest that the faster the jetting velocity at the inlet, the higher the frequency of the bubble. Further more, under the faster air jetting velocities, fluctuations of high frequency and small amplitude increase.During DEM calculations, voidage directly counts on local particle densities, and the existence of the wake varies with the bubble development. Therefore, the simulated pressure fields around a bubble agree more with the reported experimental results. Isobars on the top and at the bottom of the bubble are not symmetrical. And, there is pressure gradient within the bubble. In gas velocity fields, it can be vividly found that, being regions of low flow resistance, bubbles serve as a short cut for gas flow, and there is intensive gas exchange between the emulsion phase and the bubble phase. Furthermore, DEM results also give the visual description the patterns of streamlines in the fluidized column. Basically, gas streamlines are normal to isobars, which rationally reflect the flow's instinct of traveling through paths of the lowest resistance. Also, the regular streamline pictures implicate that fluidizing air embodies laminar flow patterns.Using results of DEM simulations, the gas flux division in the fluidized columns is quantitatively investigated. It is found that, within the excess flux, the proportion of the visible flow, the through-flow, and the interstitial flow in emulsion phase is 7%, 36%, 57%, respectively. Under the effect of through-flow, excess gas flows through the bubble into the above emulsion, in stead of in the form of more visible bubbles. The higher fluidizing air velocity, the more proportion of the excess gas flows into the emulsion. Whereas, it is assumed in two-phase theory that emulsion maintains the status of critical fluidizing condition. In fact, there is a close relationship among the visible bubble flow, through-flow and the interstitial flow, and it is inappropriate to attribute excess flow to one of theses three components.By the means of particle velocity fields, acceleration fields, and CCD snapshots, particle motions are also discussed in detail in this paper. (i) particles moving downward along the boundary of the rising bubble, and collide against each other and extrude at the bottom of the bubble, forming the wake of certain volume; dragged by the jet flow, particles below the bubble tend to penetrate into it, which partially provide the source of the bubble wake, as well. (ii) when bubbles approaching the bed surface, their explosions result in three kinds of particle ejection, including bulge-burst, wake-spike eruption, and jet-spray mechanism. The most common is the bulge burst mechanism due to single bubble bursts, where the drag force on particles is 6 times of the pressure gradient force. As the gas velocity increases, the wake escape from the bed into the freeboard, which is the so-called wake-spike mechanism. At even higher gas supply, the jet-spray mechanism occurs when the trailing bubble elongates, and entrains the solids in the bulge of the leading bubble up above the bed surface. (iii) Consecutive vector fields and streamline pictures of (u-umf) indicate that there are two local swing circles in the freeboard before the bubble eruption, with the left one clockwise and the right anticlockwise. During the eruption, such local circle streams have the opposite direction. Periods of such switch equal to that of the bubble.Visual snapshots of fluidized beds with uneven gas supply and inclined air distributor show that rising bubbles earns lateral displacement, due to different bed depth along the bed width. Particle velocity fields directly illustrate that there are regular granule circulating streams in such beds. With higher gas velocity, the higher velocity side is a bubble active zone, where there is an ascending solids flow carried by the bubbles, consequently. Because of the gradient in particle concentration between the two sides, particles in the right side move to the left. The design of inclined distributor enhances such a descending flow over the distributor as a result of gravity. Hence, an overall convective particle circulation is set up. When gas flow rate in the higher velocity zone increase, the carriage of the bubble are enhanced and results in a more intensive particle circulation. Whereas, change in the lower velocity zone causes reverse effect. The inner circulating flow rate is sensitive to the gas velocities in both zones, suggesting that it is important to configure the ratio of their flux.A statistical mixing index, Ashton index, is firstly introduced to assess the mixing process in fluidized beds in detail, with the aid of tracer particle fields. The mixing process mainly involves the three following microscopic mechanism: fast convective mixing, slow diffusive mixing, and local shear mixing. It is concluded that the scale of inner particle circulation is crucial in determining the mixing patterns within the beds. Comparisons with reported experiment prove that the mixing time scale, evaluated with Ashton mixing index, deserves its practical meaning.In the case of fluidized beds with horizontal distributor and even gas supply, due to bubbles'pronounced ascending movement and the inherent direction of gravitation, particle circulation penetrates through the whole bed axially, which induces a faster convective blending mode along the bed height. On the other hand, bubbles'lateral motion is restrained by their neighbors, and particle circulations are localized. Therefore, particles'horizontal activities are reduced, then the diffusive mixing is the prominent mode in the lateral direction. Under the same condition, the axial mixing rate is 3.5 times of the lateral. For the case of fluidized beds with uneven gas supply and inclined air distributor, the time averaged particle flow rate between the two lateral part of the bed is far larger than the case of horizontal distributor and even gas injection. For the large scale particle circulating stream in ICFBs, materials in different lateral parts enjoys a rapid convective exchange, and the mixing rate is obviously accelerated, with the magnitude nearly equal to the axial mixing rate.DEM heat-transfer models are established for fluidized beds, taking the gas- particle and particle-particle heat exchange into account. Under condition of even gas supply, particles are confined to undergoing convective heat transfer with local gas, and their temperature distribution depends more on the gas phase temperature field in the bed. The regular particle streams improve transverse diffusion performance of the solid phase. As a result, particles are transferred between the high temperature zone and the cold of the gas phase frequently, carrying the heat from the left side to the right. Additionally, turbulent particle flow helps to increase the particle-particle heat transfer. All these benefit the fluidizing system with a better ability to recover homogenous temperature field of particles from external disturbing effects.In terms of the changeable physical properties of the municipal solid waste, segregation and mixing characteristics are also experimentally researched in a multi-component ICFB setup.Results of layer sampling experiments show that there are no significant variations of the distributions of bulky materials within a wide range of air velocities in the higher gas flow rate zone. On the other hand, the distributions of bulky tracers are closely related to the gas velocity in lower gas flow rate zone, since such increase reduces the particle circulation and enhances local mixing performance. Compared with other physical parameters, density of tracer particles plays an important role in determining their axial segregation.The averaged residence time of the bulky materials initially decreases with the increasing flow rate in higher velocity zones, and then gains in value. For the deslagging operations, there is a most favorable lateral velocity gradient configuration of the fluidizing air. The averaged residence time increases positively with the air velocities in the lower gas flow rate zone, due the strengthening recirculation of the particles. With respect to the tracers'physical characters, larger particles of higher density tend to leave the bed quickly. On the other hand, larger particles of lighter materials have a longer stay in the fluidized bed. Additionally, the shape has a prominent effect on the tracers'RTD. Being more spheric and smooth, tracers retain in the bed for a shorter averaged residence time, vice versa. It is found that incombustible materials are relatively separable from the combustible ones and the bed materials, which ensures the complete combustion and desirable deslagging performance.