Experimental And Numerical Study On Heat And Mass Transfer Of Moving Granular Media

A variety of granular particles in different shapes is widely exist in the natural and industrial applications,such as chemical engineering,coal,mine,environment,energy,medicine,food and light industry.Drying process is a critical part in production,which directly determines the output quality of particles.Heat and mass transfer between particles and gas flow occurs during drying process.The complexity of particle-gas two phase flow greatly increases the difficulty of that research.However,it lacks of work on heat and mass transfer of moving granular particle so far,especially for a large number of particles.It is difficult to obtain information on heat and mass transfer of particles because of limitations of experimental environment and measurement methods.Studies focused on a single particle or a small amount of particles by numerical simulation method.The,present work is devoted to revealing complex heat and mass transfer characteristics of moving granular particles by experimental and simulated approaches.Dynamic and heat and mass transfer characteristics model were established.Focusing on the problem of large particle number,the definition of virtual particle was built.The discrete element model was adopted to track the movement of each virtual particle and soft sphere model was used to study the process of collisions among virtual particles.Based on heat transfer and mass transfer theories,the heat and mass transfer model on moving particles was built.Therefore,the above models were applied to study the heat and mass transfer characteristics of flexible filamentous particles during drying process in a rotary dryer.Mean residence time of particles was calculated and drying behaviors were researched.The relationships of temperature and moisture content and operating parameter have been revealed.A pilot test system was established to study the dynamic behaviors of flexible filamentous particles.Mean residence time and residence time distribution of particles in a cold pilot rotary dryer were measured.The feasibility and rationality of numerical model on dynamic characteristics of particles were verified by comparing with experimental results.Then,dynamic behaviors of a larger number of particles have been researched on by numerical simulation method.The relations of an important parameter mean residence time and operating conditions such as rotational speed,drum slope,gas velocity and mass flow rate of particle were presented.Experimental and simulation results presented that residence time of filamentous particles showed normal distribution under different operational conditions.Based on static drying experiments in the fixed furnace,results indicated that temperature and moisture content were significantly non-uniform distributed within granular particles,and drying efficiency was quite low as well.It could be observed that wall temperature and gas velocity had significant influence on the heat and mass transfer behavior of granular particles.Therefore,a pilot rotary dryer system was established in order to study heat and mass transfer characteristics of wet granular particles.Flexible filamentous particles were taken as operating material.The temperature and wet moisture content of particles were real time measured under several operating conditions.These conditional parameters including wall temperature,rotational speed,inlet gas temperature and velocity.Relations of heat and mass transfer characteristics and operating conditions were discussed by experimental and simulation methods.Based on above experimental and numerical simulation study,a heat and mass transfer model on a large amount of granular particles in industrial rotary dryers was established.Results on outlet temperature and moisture content of particles obtained by simulation method were in well agreement with experimental data.Then,based on single factor analysis method,the effects of heat and mass transfer characteristics and operating parameters,which included wall temperature,gas phase parameters,rotary dryer conditions and inlet mass flow rate of particles,have been revealed.A general mathematical model for prediction of granular particles and gas phase parameters was proposed.