Analysis And Experimental Research On Heat Transfer Process In An Indirect-heating Tube Rotary Dryer

Drying is an important unit operation process in the industrial areas. The substantial process of drying is heat and mass transfer. An in-depth research on heat and mass transfer process is of great value for conserving energy and reducing emissions. As one of the advanced drying equipment, the indirect-heating tube rotary dryer is of many advantages, such as energy conservation, higher heat efficiency, safety, lower pollution and so on. Because of the worldwide energy shortage and environmental requirements, the tube rotary dryer is of more and more widely applications at present. However, due to the complex mechanisms of particle movement and heat transfer, understandings of heat transfer process between granular flow and moving solid wall are still at the level of experimental test and engineering experience. Most of related literatures are only concerned with the operating experience and equipment construction improvement. Deficiency of heat transfer mechanism research in a tube rotary dryer has seriously restricted the design and application of large-scale equipment. So, for the theory and application development, heat transfer mechanism analysis is essential for the development of tube rotary dryer. This paper selected the tube rotary dryer as the object to perform investigation of heat transfer mechanism between immersed tube’s surface and material bed by means of theoretical analysis, experimental study and design calculation, then the heat transfer coefficient calculation model and computing platform for engineering design and application were built up.Analysis showed that heat transfer coefficient between tube’s surface and material bed was mainly affected by the influence of particle on gas film boundary layer. So, in this paper, analysis of heat transfer processes and heat transfer coefficient calculation models for both fine powder and grain materials were carried out, according to the comparison of particle’s size and the thickness of gas single-phase boundary layer.In the fine powder model, material bed was considered as a mixture fluid composed of fine powder and gas, and the physical parameters were determined by mass concentration and volume fractions of gas and solid. Heat transfer between the mixture flow and the tube’s surface satisfied Zhukauskas’s convective heat transfer relationship of flow crossing the tubes. For the grain material, the heat transfer between tube’s surface and material bed consists of heat transfer between tube’s surface and both particles and gas in the material bed. When the calculation model of grain material was built up, referring to the fluidized theory, the conception of material bed’s "bulk expansion ratio" was introduced to evaluate the occupation of gas and also the gap between particles. On the basis of heat transfer mechanism analysis, it was proposed that the total heat transfer between tube’s surface and material bed consisted the following four parts:convective heat transfer between the particle and tube’s surface through gas film when the particle doesn’t reach the tube, contact heat transfer during the collision between the particle and tube’s surface, radiation heat transfer between the particle and surface, and convective heat transfer between the tube’s surface and the gas within the material bed. Finally, on the basis of the particle’s and gas’s area coverage of the tube’s surface, the total heat transfer coefficient between tube’s surface and grain material bed was calculated from weighted average of the four parts of heat transfer coefficient. In the model, the convective heat transfer between tube’s surface and particle was analyzed on the basis of gas boundary theory, and the coefficient was derived by the consideration that the particle influences the gas film boundary layer thickness. The way of particle intensifying heat transfer is by means of decreasing thickness of gas film boundary layer.In order to validate the proposed heat transfer coefficient calculation model, this paper presented an experimental device system to realize the purposes of observation of moving status of material bed and the measurements of bulk expansion ratio and heat transfer coefficient between the immersed tube’s surface and material bed in an indirect-heating tube rotary dryer. The system utilized carbon brush to connect the heater of the rotary tube and electric source. A wireless transmitter was employed to emit tube’s surface temperature signal. With consideration of the tube’s unsteady heat transfer characteristic, a simple, accurate and reliable data processing method to derive heat transfer coefficient from the measured parameters was developed. The error analysis results showed that the developed measuring approach yielded high accuracy of heat transfer coefficient with relative error less than3.5%. Ceramic spherical grains and PTA (Pure Terephthalic Acid) fine powder were used as testing material, a large number of experimental researches on both bulk expansion ratio and heat transfer coefficient between tube’s surface and material bed were carried out by means of this measuring system and method. Effects of such parameters as rotary speed, material filling ratio, particle diameter and tube rotating radius on heat transfer coefficient are investigated. Measurement results showed that average heat transfer coefficient increases with rotary speed and tube’s rotating radius, but decreases with particle’s diameter; average heat transfer coefficient for15%,20%and25%filling ratio are almost unchanged, while coefficient for30%filling ratio is a little higher than the low filling ratio ones.Comparison of the experimental results and model predictions showed that the maximum relative error of the proposed heat transfer coefficient calculation model for grain material is less than9%, and the maximum relative error for powder material is less than10%. According to the engineering design experience, the models are able to well match engineering requirement, so the predicted heat transfer coefficients are able to provide references for both drying process calculation and equipment design. Using the validated heat transfer calculation model, this paper analysis the four components of heat transfer between tube’s surface and grain material bed. Analysis results showed that the heat transfer from the tube’s surface to material bed is mainly composed by means of convective and contact heat transfers between surface and particles; with increase of the rotary speed, convective heat transfer coefficient decreases while contact heat transfer coefficient increases; heat transfer coefficient between tube’s surface and the gas within material bed contributes a neglectable part to the total heat transfer coefficient. In practice, the tubes’ surface temperature can be at a relative high level, so radiation heat transfer is ought to be considered in engineering calculation.In order to further confirm the validity and practicability of the heat transfer coefficient calculation model, on the basis of Eular-Eular two-fluid model, a computing platform for flow field and heat transfer calculation in a tube rotary dryer was established in this paper. Combined the heat transfer characteristic in a tube rotary dryer, the heat transfer coefficient calculation model was used in the platform to define the thermal boundary condition of tube’s surface. Then aided by CFD simulation, the predictions of particle concentration distribution, velocity and temperature fields in the tube rotary dryer were performed. In order to verify the precision of the computing platform, comparisons between the simulation and testing results of particle concentration distribution and tube’s surface temperature were carried out. The results showed that the computing platform is able to predict the particle’s movement and heat transfer processes in a tube rotary dryer with a high accuracy, and provide a high efficient tool for engineering design, calculation and prediction.This paper aims at gaining the development of heat transfer coefficient calculation model between tube’s surface and material bed in a tube rotary dryer, and performs thorough investigation of heat transfer mechanism between immersed tube’s surface and material bed by means of theoretical analysis, experimental study and design calculation. The research wok can offer a valuable reference to equipment design, engineering calculation and prediction for the tube rotary dryer.