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

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 carried out. Comparison of the experimental results and predictions showed that the maximum relative error (emax) 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°C. 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°C). 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.