Shu-San Hsiau

Stagnant zones in granular moving bed filters for flue gas cleanup

The flow patterns in three symmetric and two asymmetric louver-walled granular moving bed systems are studied experimentally. The flow pattern histories of granular solids in moving beds were recorded by videocamera throughout the whole test period. The image processing system including a frame grabber was used. Selected discrete images showing the flow pattern development were digitized and stored in files. Individual granular flow frames show a development of quasi-stagnant zones close to the louvered walls, the shear zones and a central flow region with width depending on the louver angle. The vertical shift of one or the other louvered wall in asymmetric configuration considerably influences the history of quasi-stagnant zones.

Velocities in moving granular bed filters

Abstract This paper concerns the velocity fields of three symmetric louver-walled moving-bed systems. The louver angles were 10°, 20° and 50° in the three experimental tests. The particle-tracking method and image-processing technology were employed to measure the granule velocity fields in the louver sections. The horizontal and vertical velocity distributions extracted from the velocity fields at five levels of the louver section were plotted and discussed. The velocity profiles of filter granules are influenced by the louver geometry. In the steeper louver–angle system, the velocity profile of filter granules is similar to plug flow. The stagnant zones exist along the louver walls in the filter system with shallow louver sections.

Hot gas granular moving bed filters for advanced power systems

In advanced coal-fire cycles it is important to remove the fine particles from high temperature and high pressure gas streams, to satisfy gas turbine fuel quality requirements. J Smid, S S Hsiau, C Y Peng and H T Lee assess granular moving bed hot gas particulate clean-up systems for advanced IGCC and PFBC, under development in the USA.

Moving bed filters for hot gas cleanup

In the December issue of Filtration+Separation, J Smid, S.S Hsiau, CY Peng and HT Lee assessed granular moving bed hot gas particulate clean up systems for advanced IGCC and PFBC advanced power stations, under development in the USA. They now turn their attention to the development of the particulate collection devices themselves…

Effect of solid fraction on fluctuations and self-diffusion of sheared granular flows

Abstract The influences of solid fraction on the fluctuations and self-diffusion of sheared granular flows were experimentally studied in a shear cell device. Employing the image processing technology and particle tracking method, the local displacements and velocities of particles were measured. From the measured velocities, the fluctuations, granular temperatures and shear rates could be determined. The self-diffusion coefficients were determined from the history of particles’ diffusive displacements. The periodic cell concept was used for long-time tracking purpose. Because of the gravitation force, the flows consist of both a solid-like region with higher and more uniform velocities in the lower test section and a fluid-like region in the upper part. The velocity fluctuations and self-diffusions were anisotropic with greatest components in the streamwise direction. The flow rheology was very sensitive to the mean solid fraction of the granular flow. The dependence of the diffusion coefficients on the velocity fluctuations and the shear rate were discussed.

Density effect of binary mixtures on the segregation process in a vertical shaker

Abstract The segregation process of binary mixture with different density ratios in a granular bed is investigated in this paper. Particles that were 14% (by volume) heavier and 86% lighter put in a container subjected to an external vertical vibration. The velocity fields of both species were measured by an image-processing system. The segregation intensity was determined from the recorded images. It was found that the heavier particles tended to move towards the centers of the two convectioncells of the lighter particles. The mixture with the greater density ratio could be segregated more completely. The increase in the vibration acceleration helps the segregation process.

Effective thermal conductivities of a single species and a binary mixture of granular materials

Abstract The effective thermal conductivity is developed by employing the dense-gas kinetic theory. The free path used in the theory varies with the particle velocity. The analytical results can be used for the whole range of the product of Biot number and Fourier number provided that the Biot number is less than 0.1. For the very small Biot–Fourier numbers, the conductivities are found to increase with the particle diameters and the square root of granular temperatures. For the limit of very large Biot–Fourier numbers, the effective thermal conductivity is found to be linearly proportional to the granular temperature. The effective thermal conductivities for the binary mixtures are also derived. The influences of the species concentration, the total solid fraction and the Biot–Fourier number on the thermal conductivity are investigated. Increasing the concentration of the smaller particles or reducing the size of the smaller species can increase the thermal conduction in the binary mixture system.

Numerical simulation of flow patterns of disks in the symmetric louvered-wall moving granular filter bed

Abstract Employing the stack disk method in the louvered-wall configuration described in the Appendix, the initial coordinates of the disks center and the filter boundary were defined. The flow pattern histories of granular solids, velocity field and development of quasi-stagnant zones close to the louvered walls of three kinds of two-dimensional (2D) symmetrical moving granular filter beds and 2D hybrid moving granular filter beds were studied numerically. The discrete element method was used in this study. The numerical results reported here and experimental results provide fundamental and important information for designing moving granular bed for high-temperature flue gas clean-up filters.

Effect of moisture content on the convection motion of powders in a vibrated bed

Abstract The cohesive forces between grains, such as capillary forces, van der Waals forces and electrostatic forces, play significant roles in the motion of particulate solids. This paper intends to investigate the influence of moisture content on the convection motion of wet granular materials in a vibrated bed. The rising velocity of a larger bead in the background of smaller beads in a vibrated bed was used to characterize the strength of the convection cells. It was found that the moisture content in the bed influenced the rising velocity of the larger bead. Under some specific vibration conditions, the rising velocity of the larger bead increased with the addition of a small amount of water. In these cases, the small amount of water produced a lubrication effect which resulted in the enhancement of the convection motion of the bed. With greater amount of water, the rising velocity of the larger bead decreased due to the viscous force and liquid bridge force between the water and beads.

Heaping phenomenon of particles in a shaker

According to the levels of vibrational acceleration amplitude, different motion states may appear in a vibrated granular bed, including heaping, coherent, expansion, wave and arching. The heaping phenomena of sands or granular materials are well-known, but the internal physics is still unclear. This paper experimentally studies the heaping phenomenon in a vertical shaker. Heaping phenomenon occurs in a shaker when the vibrational acceleration amplitude is greater than 1.2 gravitational accelerations. In addition to the vibrational acceleration amplitude, particle properties (diameter, density and repose angle) and container sizes (width and bed initial height) are used as controlling parameters to investigate the heaping phenomenon.