Computational fluid dynamics (CFD) modeling of grain-water suspensions in tube

Abstract

In this study of solid-liquid flow, rice and cassava starch particles were used as the dispersed media and water was used as the carrier fluid. Experiments were carried out on slurries with solid concentrations of 5%, 10% and 15% w/w which flowed in a 13mm ID and 3 m long tube –in - tube heat exchanger. Steam was used as the heating medium. Calculated convective film to particle heat transfer coefficient (hfp) values ranged from 11 to 32 kW m -2 K -1 for cassava and rice particles with uncertainty of ± 2 kW m -2 K -1. A decrease in heat transfer coefficient values was found as a result of short residence time at the higher flow rates. To investigate the solid-liquid two-phase flow Eulerian multiphase model was adopted in simple axisymmetric geometry. Velocity profiles of the liquid and solid phases with different particle fractions were estimated from the simulated results. The respective velocities of both phases were higher in the upper part of the tube than in the lower portion because of settling caused by gravity. The slip velocity of particles was estimated from the simulations and it ranged from 6.68 cm/s to 9.80 cm/s for the cassava particles and 13.83 cm/s to 19.38 cm/s for the rice particles. The rice grains always lagged the liquid phase where as the cassava starch globules moved faster than the carrier liquid in the central region and very close to the wall. The particle volume concentration profile was also investigated and it was observed that a high particle concentration formed a core around tube centreline.