Heat and Mass Transfer Effects in a U-Bend in Pneumatic Drying
Doctoral thesis, 2005
Pneumatic conveying drying is a widely used process in many engineering applications. Due to the high velocity of the drying medium and the limited conveying distance, the drying times of theses dryers are typically short, in the order of seconds. Consequently, it is important to keep the suspended materials dispersed under good heat and mass transfer conditions in the system. However, the most effective drying takes place near the feed point (Perry et al., 1997) and in the U-bends (Fyhr and Rasmuson, 1997), where the velocity difference (slip) between gas and solid particles is the greatest. To improve the performance of pneumatic drying, better understanding of the gas and solid local conditions is important.
The present study investigates the phenomena of pneumatic drying in a U-bend. The investigation is divided into four steps, i.e. gas flow, gas-solid flow, non-isothermal gas-solid flow and non-isothermal gas-solid flow involving a mass transfer of moisture (a drying effect). The calculations of the first and the second steps use the commercial software Fluent 6.0 and the calculations of the third and the fourth steps use the commercial software Fluent 6.1. In the drying calculations, two UDSs are inserted into the solution to take into account the solid particle moisture content and the mass fraction of water in the gas phase, respectively. The moisture evaporation rate of solid particles and the humidification rate of gas are treated in the source terms of the corresponding UDS equations. The evaporation energy of moisture is treated in the source term of the solid energy equation.
Each step is sequentially validated. Calculation results generally show good agreement with experimental data. However, experimental data show a faster dispersion of particles than calculation results. The gas and solid particles velocities are in the same ranges as those of the experiment data. The moisture content is somewhat slightly under-predicted. A major reason for this may be due to the absence of the cyclone in the simulation and another reason may be due to the lack of detailed modeling of the saw-dust properties.
The effects of several important parameters are studied numerically. An increase in the solid loading ratio causes a decrease in the drying rate and an increasing gas velocity basically augments the drying rate. Different bend radius ratios slightly alter the mass-weighted average values of the drying rate and moisture content in and after the U-bend. Lower feed moisture content causes a lower drying rate and different positions along a U-bend slightly affect drying behaviour. Due to curvature created by a U-bend, solid particles experience centrifugal forces which cause solid particles to collide with the outer bend wall, thus causing solid particles to decelerate and accumulate in this region. Deceleration of solid particles causes an increase in the slip velocity between gas and solid particle, which is desired. However, accumulation of solid particles also causes undesired results, such as a decrease in the mass-weighted average values of the Nu number and the drying rate. The presence of a disperser in a U-bend is needed to disperse the solid particles at the outer bend wall.
Keywords: pneumatic conveying drying, U-bend, computational fluid dynamics (CFD)