Study On Heat Transfer Characteristics Between The Vertical Rotating Disk And Loose Material

As an important development direction of indirect drying, rotating disk dryer has gained rapid promotion and broad application as a result of high efficiency and energy saving. Heat transfer characteristics between the rotating disk and the material were studied in this work. The result of this work is meaningful for the rotating disk dryer optimizing design and operating.With the support of National Science and Technology Major Project of China (Water Program) (NO.2010ZX07319-002), the heat transfer characteristics between vertical rotating disk and the loose material was investigated through experiment, theoretical analysis and numerical simulation by utilizing the rotating disk dryer as prototype.An experimental setup was built up to investigate the heat transfer between the vertical rotating disk and loose materials. The temperature variation of material layer was measured using construction sand measure as sample. Besides, the effect of rotating speed, particle size and filling rate on the heat transfer characteristics was also experimentally studied.Based on the approach of dealing with mixing process in Particle heat transfer model, unsteady heat transfer physical model of porous media was built by consideration of fixed bed thermal resistance and gas film thermal resistance between the agitator and heat disk. The analysis solution and numerical solution were employed to solve the temperature distribution of material layer and the average heat transfer coefficient between vertical rotating disk and loose material. Based on the results from numerical solution, a simplified formula was proposed to solve the average heat transfer by analysis solution and then corrected by experimental results to derive a semi-empirical formula. The maximum relative error between the results from semi-empirical formula and those from experiments was only 5.6%, indicating its applicability and accuracy.The factors influencing on the average heat transfer coefficient between vertical rotating disk and loose material were analyzed employing semi-empirical formula. The results show that the main factors are rotating speed, space between agitator and disk, particle radius, thermal conductivity of particles. The space of disk and porosity of particulate material have slight effect but the influence of material and thickness of disk can be neglected. The average heat transfer coefficient increases with disk speed exponentially, while as the rotating speed is larger than 20 rpm, the increasing slows down greatly. The most effective measure to enhance heat transfer is to reduce space between agitator and disk in industrial application, as it is about 0.01 m. The average heat transfer coefficient was decreased by 61% when the particle radius was increased from 0.1 mm to 1.1 mm. The average heat transfer coefficient was increased linearly with the thermal conductivity of particle. When heat oil was used as heat medium, the average heat transfer coefficient is only 52%～70% of that using steam as heat medium. For hot flue gas, it is only 6%～10%.