间接加热式列管回转干燥机传热系数模型构建

Modeling for heat transfer coefficient in indirect-heating tube rotary dryer

传热系数是列管回转干燥机设计和热工计算所必须的至关重要的设计参数之一,其精度的高低决定了干燥机尺寸、结构设计以及操作参数的合理性。目前还没有一种能够确切描述其加热管与物料颗粒传热过程的可靠而实用的传热模型。该文在对列管回转干燥机传热机理分析的基础上,提出了列管与颗粒间换热的基本构成为:列管管壁与气体介质间对流、气体介质与颗粒间的导热以及列管管壁与颗粒间的辐射换热;通过对列管回转干燥机内料层膨胀的试验研究,分析了颗粒对列管气膜边界层的影响;在此基础上,建立了预测列管外壁与颗粒间总传热系数的数学模型,并以2mm直径的陶瓷球为物料,在6个转速条件下测量了管壁与颗粒间的换热系数,对模型进行验证;试验结果表明,模型预测的误差小于13%,可满足工程计算的精度要求。研究结果可为列管回转干燥机传热机理的深入研究提供参考。

Heat transfer coefficient is one of the most crucial parameters in thermal calculation and design for a tube rotary dryer. The dimension, structure and operating parameters of a suitably designed dryer rely on the accuracy of the employed heat transfer coefficient. Because of the existence of tubes, particles＇ motion behavior and heat transfer mechanism in a tube rotary dryer are more complicated than in a conventional rotary dryer. So far, there is no reliable heat transfer model to describe the heat transfer process between the tubes＇ surface and particles in a tube rotary dryer. As a result, the main approach of heat transfer coefficient determination is still an experimental test. The main reason is the insufficiency of understanding on the mechanism of heat transfer between heating tube＇s surface and particles. Our experimental investigation showed that heat transfer between tubes＇ surface and particles obeyed different mechanisms in different material cases of fine powder, grain and block. This paper aims at the material case of grain. In this case, the main influence factor on heat transfer was the gas film on the surface of tubes. Based on the analysis of heat transfer mechanism, this paper redeemed that heat transfer between tubes surface and particles consisted of heat convection between tubes and gas film, heat conduction between gas film and particles, and, heat radiation between tubes surface and particles. By experimenting on traces of particle layer expansion in the dryer, the influence of particle on the gas boundary layer on tube surface was also investigated. Finally, a mathematical model was carried out for the prediction of heat transfer coefficient between tubes surface and particles. In order to validate the developed model, a series of experimental tests were performed. Ceramic spherical grains with a diameter of 2mm were used as testing particles. 6 heat transfer coefficients corresponding to 6 rotational speeds were carded out. Comparison of the experimental results and predictions showed that the maximum relative error （ema~） was -12.14%, while the minimum error （emin） was -9.78%. According to the engineering design experience, the model was able to well meet engineering requirements, and offer guidance for drying process calculation. The results also showed that the fraction of radiation heat transferred from tubes＇ surface to particles was nearly as high as 8% of the total heat transfer. While, in case of this experiment, the temperature of heating tubes＇ surface was only in the range of 75-85℃. As a result, the heat radiation transferred to particles should be taken into consideration of the model, because in practice, the tubes＇ surface temperature can be at a relative high level （generally 150-300℃）. The error analysis showed that, disregarded insufficient study of the thickness determination of gas boundary layer on the tube surface, the model still brought a fixed error at a level of about 10%. However, as our investigation went on, more understanding on performances of boundary layer and motion behavior of particles and gas media were to be obtained and, a more accurate heat transfer coefficient model for tube rotary dryer would be hopefully carried out.