Drag models comparison by single injection in vacuum fluidised beds

Single bubble injection simulations inside a minimally fluidized bed have been studied widely and are often used to validate the accuracy of different numerical models. Bubble shape, size and voidage distribution are the important parameters that are validated from the experiments. In the present work, the most widely used drag model (Gidaspow’s drag model) is compared to a new proposed slip flow drag model which takes into account the presence of the slip flow regime, often encountered in vacuum fluidized beds and characterised by Knudsen no. (Kn). Shape and size prediction of the bubble evolution inside the bed is carried out numerically by using the two fluid model, comparing the results predicted by the drag models. It is seen that the predictions are different for the two drag models only under high vacuum conditions corresponding to Kn in slip/transition flow regime. The predictions are also found sensitive to pressure gradient in the bed and fluid velocity.

Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31

The evolution of hot working flow stress with strain is examined in torsion, uniaxial compression and channel die compression. The flow stress was found to be strongly dependent on texture and deformation mode. At low strains this dependency accounted for a difference in flow stress of up to a factor of two. At higher strains the influence of texture and deformation mode was less marked. The stresses corresponding to an equivalent strain of 0.5 were modelled using a power law expression with an activation energy of 147 kJ/mol and a strain rate exponent of 0.15. The influence of texture and deformation mode on flow stress is rationalised in terms of the influence of prismatic slip, twinning and dynamic recrystallisation on deformation stress and structure.

A semianalytical Sachs model for the flow stress of a magnesium alloy

A semianalytical Sachs-type equation for the flow stress of magnesium-base alloys is developed using the Schmid law, power law hardening, and a sigmoidal increase in the twinning volume fraction with strain. Average Schmid factors were estimated from electron backscattered diffraction (EBSD) data. With these, the equation provides a reasonable description of the flow curves obtained in compression and tension for samples of Mg-3Al-1Zn cut in different orientations from rolled plate. The model illustrates the general importance of basal slip and twinning in magnesium alloys. The significance of prismatic slip in room temperature tension testing is also highlighted. This is supported with EBSD slip line trace analysis and rationalized in terms of a possible sensitivity of the critical resolved shear stress for prismatic (cross) slip to the stress on the basal plane.

The hot working flow stress and microstructure in magnesium AZ31

The evolution of flow stress and microstructure for wrought magnesium alloy AZ31 was characterised using torsion and compression testing. Temperatures ranging between 300°C and 450°C and strain rates between 0.001s^-1 and 1s^-1, were employed. Constitutive equations were developed for the flow stress at a strain of 1.0 for torsion, and 0.6 for compression. The flow stress was found to be strongly dependent on deformation mode at low strains. This can be explained in terms of the influence of the deformation accommodating processes of prismatic slip and dynamic recrystallisation (DRX). At higher strains, when the change in flow stress with strain is lower, the flow stress was relatively insensitive to deformation mode. Optical microscopy carried out on torsion samples quenched after twisting to strains between 0.2 and 2 revealed dynamically recrystallised (DRX) grains situated on pre-existing grain boundaries. The average grain size was reduced from 22.5μm down to 7.3 μm after a strain of 2, with the initial grain size being halved after a strain of 0.5.