Some aspects on gas-solid flow in a U-bend: Numerical investigation

Pneumatic drying is widely used in many engineering applications. It has been shown in earlier research [Fyhr C. and Rasmuson A., Mathematical model of a pneumatic conveying dryer, AIChE Journal, Vol. 43, pp 2889-2902, 1997.] that the U-bends in the pneumatic conveying dryer system significantly influence drying behavior since they create enhanced slip velocities between suspended material and the drying medium. On one hand, this slip will increase external heat and mass transfer rates, thereby enhancing drying conditions. On the other hand, increasing the number of bends will cause an increase in pressure drop. Use of the suitable mean gas velocity and the suitable bend radius ratio will result in a better design and improved operating conditions.Two-phase CFD calculations, using a Eulerian-Eulerian model and commercial program Fluent 6.0, were employed to calculate the gas and particle flows in a U-bend. Variables studied include: particle diameter, particle density, particle volume fraction, gas velocity and bend radius ratio. The numerical calculations were validated against experimental data from the literature. The density and diameter of particle vary from 600 up to 1000 kg/m³ and from 0.00025 up to 0.001 m, respectively. The gas velocity and particle volume fraction vary from 10 up to 25 m/s and from 0.001 up to 0.01 m³/m³, respectively. The bend radius ratio varies from 3 up to 8 m/m. The slip velocity is affected by all the studied parameters, in particular, particle diameter, gas velocity and bend radius ratio; whereas the total pressure drop is strongly affected by gas velocity and bend radius ratio. A low mean gas velocity will give a lower total pressure drop and longer particle residence time. A small bend radius ratio will produce a faster dispersion of particles, which benefits drying, but on the other hand, will increase the total pressure drop. Thus, optimizing gas velocity and bend radius ratio is important in reducing energy consumption.