Numerical Simulation Of Particle Movement And Heat Transfer Enhancement In Externally Heated Rotary Kiln

Due to the advantages of stable operation and easy temperature control,external heated rotary kilns are widely used in industrial fields such as soil thermal desorption,food drying and waste plastic pyrolysis.Most of the traditional external heat rotary kilns are cylindrical,the effect on promoting the heat transfer process of the internal particle bed is not obvious.Changing the wall structure and adding internal lifters are the two methods to enhance the particle heat transfer in the external heat rotary kiln.In this paper,the petal drum and the lifter-petal drum are developed,and the direct numerical simulation method is used to simulate the movement,mixing and heat transfer process of the particles in the externally heated rotary drums(cylinder drum,petal-drum,lifter-petal drum).The effects of structural parameters and operating parameters on particle flow and heat transfer are analyzed with emphasis,and the conclusions are drawn as follows:(1)Firstly,the DEM model and the physical model of the rotary drum are established,and the accuracy of the model is verified by comparing with the experimental results;The effects of different rotational speeds and filling rates on the particle movement and mixing quality in the cylinder drum,the petal drum and the lifter-petal drum are studied respectively;The results show that the contact number between particles and wall is increased in petal drum and lifter-petal drum,thereby the heat transfer area is larger;The velocity distribution in the particle bed is significantly influenced by the petal side wall and the lifters,but the degree of influence varies under different working conditions;The active layer thickness within the particle bed is obviously reduced by the addition of lifters,thereby the particle exchange coefficient between the active and static region is decreased;The fluctuation of particle concentration and mixing variance in each area is weakened by lifters,and the particles in each local region can reach a uniform mixing state earlier;In most working conditions,the mixing efficiency of particles in the lifter-petal drum is higher than that in the cylinder drum and petal drum.(2)On the basis of the above research results,the influence of the lifter parameters on the particle movement and mixing in the lifter-petal drum is mainly studied.The results show that the number of lifters has little effect on the mixing level of the particles with the same physical properties,and the difference of the average moving distance between the two groups of particles with different densities is enlarged by the increment of the lifter number,and the size segregation is also inhibited;Increasing the length of the lifters will significantly improve the average moving distance of the particles and reduce the difference of the moving distances between the two groups of particles.And with the increment of the lifter length,the size segregation will be significantly weakened;The L-shaped lifters have the greatest effect on promoting the mixing process of the particles with the same physical properties,while the straight lifters have the worst effect.(3)Based on the above two-dimensional studies,the particle motion,mixing and heat transfer process in the rotary drum are simulated from a three-dimensional perspective.And the results show that the particle heat transfer in the three-dimensional rotating cylinder will be weakened with the increment of rotation speed and inclination angle,and the residence time of the particles in the rotating cylinder will decrease.In addition,at the same rotational speed,the particle temperature at the drum outlet,in the heating region and at each axial position are the lowest in cylinder drum and highest in lifter-petal drum;The petal-shaped wall boundary can increase the residence time of particles in the drum and the temperature of the overall particle bed.The movement trend of the particles in the petal-shaped drum to the outlet of the three-dimensional rotary cylinder is weakened.