Study On Solid Flow And Heat Transfer Of Granular In Moving Beds

Granular materials are widely used in the chemical industry with particulate reaction engineering, design of powders, storage of grain flows, separation and granulation. Roughly one-half of the products and about three-quarters of the raw materials of chemical industry are in form of granular materials. Considerable importance has been given in industries these days for the handling of granular materials. But the understanding of the behaviors in granular materials is not very established. It has been estimated in interrelated industries that about 40% of equipment capacity is wasted in conveying granular materials. It is far from the aim of optimization design and saving energy. Therefore, it is the frontal and full of challenge in chemical engineering research. The thesis focused on the behaviors of granular moving bed, and investigated the solid flow and heat transfer by means of particle kinematic model (PKM), discrete element model (DEM), and particle contact heat transfer model (PCHM). Not only the novel conceptions and theories were established, but also discover many new phenomena and then theory established.The experimental cost becomes more and more expensive, while the cost of computer decreases rapidly. More investigations have been completed by computer simulation with high accuracy and low expense. The thesis investigated the transfer phenomena in granular materials by computational fluid dynamics (CFD). The creative works can be summarized as following:1. PKM considers that the granular flow velocities are determined by pure kinematic effects. Only one coefficient is included in PKM and is only related to particle diameter, so it is very convenient to engineering application. In the thesis PKM was used to simulate the particles flow. For center discharging the simulation results accorded with experimental one very well, while for eccentric discharging the simulation couldn't show the effects of boundaries on particles flow. An analytic solution of kinematic model was obtained by Lie transformation group. It showed that the velocity of particles flow was maximum when x=0. Through localization of max particles velocity distributing lines and making sure that the value of x on the lines always equal 0, the modified kinematic model was proposed and could simulate the eccentric discharging well.2. The main advantage of DEM is that highly complex systems can be modeled with basic data without oversimplifying assumptions. DEM can obtain the real trajectory of particle motion, so it can reflect intrinsic characters of granular materials. In the thesis, DEM was used to investigate the flow pattern of the particles discharging and continuously flowing in two-dimensional silos (flat-bottomed silo and wedge-shaped hopper silo). The accuracy was confirmed by comparing calculated results with experimental one. The effects of key parameters such as the shape of silos, the width of outlet, and the shape of inserts on particle flow were clarified. The particles flowed as "slug flow" in the top part of any silos, while "funnel flow" in the low part. In batch discharging, the particles on the surface of materials in silos flowed together to the silo center. 3 kinds of inserts were used to change the flow field of silos. The placement of inserts improves the flowing performance. Comparing with traditional conical insert, the 2# insert (inverse hopper insert) invented in the thesis could improve granular flow better.3. There are many factors can effect granular motion, such as particle diameter, density, shape, angle of repose, character of particle group (diameter distribution, void fraction and so on), discharging rate. Three granular materials (Polyethylene, Polystyrene, Colophony) flow characteristics ware studied experimentally. The results showed that discharging rate has little effects on flow pattern, while the repose angle, diameter and shape of particles have great effects on flow pattern. The time line becomes more sharp-angled with increasing the repose angle, and the dead zoon becomes greater. With the increasing of sphericity degree and decreasing of particle diameter, the flow performance becomes better and particles mix more easily.Four kinds of diameter distribution (random distribution, normal distribution, binary mixing and uniform diameter) were studied by DEM. The spatial distribution of velocity, void fraction and stress were obtained, while probability distribution of these variables by Mathematical Statistics. The results showed that:a) The distribution of particle diameter has obvious effects on particle flow, and the mixing of different particle diameter is propitious to particles flow. The effects were more remarkable in flat-bottom silo than in wedge-shaped hopper silo.b) Linear equations can be used to describe the relationship between discharge rate and orifice size by G215 vs. Do for the same distribution of particles diameter. Do has least effects on discharge rate for uniform diameter distribution, while greatest effects for normal distribution. The effects become little in hopper silo.c) The force structures of particles in hopper are spatially non-uniform. There are arched stress chains above orifice in flat-bottomed silo, but the arched stress chains are not evident in hopper silo.d) The stresses between particles and bed walls are spatially non-uniform too. The shape of silo, the shape of inserts and the installation position of inserts all can influence the stresses distribution.e) The probability distributions of each variables are influenced by diameter distribution, shape of silo, insert and so on. The effects of diameter distribution are greater in flat-bottomed silo than in hopper silo.4. The heat transfer in granular materials can be found in many industry processes. But the understanding for it is very poor. In the thesis, a particle contact heat transfer model (PCHM) was established. PCHM includes four independent mechanisms: ?thermal conduction through the solid, ?thermal conduction through the contact area between particles, ?thermal conduction through the fluid film near the contact surface, (Dheat transfer by convection solid-fluid-solid. The model combined with DEM was used to simulate heat transfer between particles and heating wall. At the same heat transfer temperature difference, the relationship of each mechanism was: Q4 , and Q2 has greatest effect on Qeff. The keyfactors effecting on ï¿¡>eff include: the number of particles contacting with heating wall, the contact time, and the contact area. When the average diameter is same, the diameter distribution has little effects on heat transfer. When the average diameter is different, the effective heat transfer coefficient decreases with the increasing of particle diameter. The quantity of heat transfer is spatially non-uniform.