Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re-U), axial flow Reynolds number (Re-W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers. (c) 2006 Elsevier Ltd. All rights reserved.

High pressure induced water uptake characteristics of Thai glutinous rice

Glutinous rice (or sticky rice) has to be soaked in water over an extended period of time before cooking. Soaking provides some of the water needed for starch gelatinisation to occur during cooking. The extent of water uptake during soaking is known to be influenced by temperature. This paper explores the use of very high pressures up to 600 MPa to accelerate water uptake kinetics during soaking. Changes occurring in length, diameter and moisture content were determined as a function of soaking time, pressure and temperature. The results show that length and diameter are positively correlated with all three parameters. However, the expansion ratios are not very high: the maximum length expansion ratio observed was 1.2, while the maximum diameter expansion ratio was 1. 1. Given these low values, it was possible to model water uptake kinetics by using the well-known Fickian model applied to a finite cylinder, assuming uniform average dimensions and effective diffusion coefficient. The results showed that the overall rates of water uptake and the equilibrium moisture content increased with pressure and temperature. The effective diffusion coefficient, on the other hand, did not follow the same trend. Temperature influenced the effective diffusion coefficient below 300 MPa, but had a marginal effect at higher pressures. Moreover, the effective diffusion coefficient increased with temperature between 20 and 50 degrees C, but dropped at higher temperatures. This drop can be attributed to the gelatinisation of starch, which restricts the transport of water. Regardless, it is possible to increase the quantity of water absorbed by rice and the rate at which it is absorbed, by using high pressures and temperatures. (c) 2004 Elsevier Ltd. All rights reserved.