Research On Mixing And Heat Transfer Characteristics Between Particles In Internally Heated Rotary Devices

Internally heated rotary devices are widely used in various industrial processes,such as the pyrolysis of fuel particles and the heat treatment of granular materials.In actual processing,the degree to which the granular particles can be mixed and the efficiency of heat transfer in the particle bed have a great influence on how well the materials can be utilized.The mixing and heat transfer between particles significantly depend on the properties of materials and operating conditions.A review of the current literature shows that comprehensive research on mixing and heat transfer in particle beds has been limited,and studies on various heat transfer mechanisms in internally heated rotary devices have been insufficient.Therefore,an investigation into the correlation between mixing and heat transfer in particle beds with different material properties,looking into the intricacy of heat transfer mechanisms such as heat conduction and heat radiation,and exploration of measures to improve mixing and heat transfer quality could be of great reference value and scientific significance for the practical application of relevant industries.This paper numerically simulates the mixing and heat transfer between particles in internally heated rotary devices by coupling the discrete element method(DEM)with a heat transfer model.The effects of the physical properties(particle size and density),thermal properties(particle thermal diffusivity),and operating conditions(rotation speed,filling rate and placement and length of baffles)on the mixing and heat transfer characteristics of granular materials are systematically studied.The main research contents are as follows:(1)The heat transfer mechanism and heating process of oil shale particles in a horizontal rotary retort.The heat transfer characteristics in the rotary retort under different particle sizes,rotation speeds and filling rates are analysed.The research results show that as the particle size increases,the heat flow absorbed by the oil shale particles through five heat transfer mechanisms increases significantly.The proportion of heat flow generated by heat conduction decreases,while the proportion of radiant heat flow increases greatly.Either increasing the rotation speed or reducing the filling rate shortens the heating time of oil shale,but the heat flow generated by different heat transfer mechanisms and the corresponding thermal contribution remain basically unchanged.The main heat transfer mechanism in the rotary retort is particle-fluid-particle heat conduction,followed by particle-particle heat radiation,and finally particle-particle heat conduction.(2)The interrelationship of mixing and heat transfer between granular materials with different properties in an internally heated rotary drum.The evolution of mixing and heat transfer characteristics with rotation speeds is analysed from the perspective of time and number of revolutions,respectively.The research results indicate that due to the different physical and thermal properties of particles,the heat transfer effect cannot be completely evaluated by the mixing quality.Increasing the rotation speed is conducive to promoting the mixing and heat transfer at the same time,but the quality of the mixing and heat transfer decreases at the same number of revolutions.The main heat transfer mechanism in a particle bed depends on the ratio of the thermal conductivity of the granular material to that of the fluid.(3)The performance of central cross baffles and peripheral baffles in particle beds with different properties.The effects of central cross baffles and peripheral baffles with different lengths on the mixing characteristics and heat transfer characteristics of granular materials are analysed.The research results show that a central cross baffle is beneficial to promote the mixing and heat transfer between particles,and a central cross baffle of an appropriate length is the best choice for improving the quality of mixing and heat transfer.However,the peripheral baffle at most lengths weakens the mixing and heat transfer between particles,and only a sufficiently long peripheral baffle significantly improves the quality of mixing and heat transfer.Baffles only affect the heat transfer process in particle beds,and do not change the thermal contribution of heat transfer mechanisms.