S.C. Saxena

Hydrodynamic characteristics of a magnetically stabilized air fluidized bed of an admixture of magnetic and non-magnetic particles

Fluidization characteristics of a bed consisting of an admixture of iron (90 mass%, 1416 μm) and copper (10 mass%, 935 μm) shots stabilized by an external uniform magnetic field collinear with the gas velocity vector are investigated over an extended range of magnetic field intensity (up to 17 430 A/m) and air velocity (up to a maximum of 8.3 m/s) at ambient conditions of temperature and pressure. The fluidization behavior is observed at eight magnetic field intensity (H) values. Minimum fluidization (Ums) and minimum bubbling (Umh) velocities are reported as a function of (H). Umf) is independent of H and the variation of Umb with H is expressed by: Umb = Umf exp (0.103H) where H is in kA/m. Bed voidage is measured as a function of superficial fluidizing air velocity at each H and is found to increase slowly with increase in air velocity for all those H values where the bed integrity is preserved.

Effect of distributors on Gas—Solid Fluidization

Abstract The efficient operation of a fluidized bed is very much dependent upon distributor performance, which in turn depends on its design parameters. The work reported here deals with the characteristics of such distributors as are commonly employed in laboratories, pilot plant and large scale operations. Specifically a porous plate distributor, two bubble cap distributors of different geometries and four Johnson screen distibutors of different percent open area have been investigated in a 30.5 cm by 30.5 cm square fluidized bed as a function of air fluidizing velocity and bed height. The pressure drop data for all the distributors have been correlated by a single equation with two unknown constants. The ratio of distributor pressure drop to bed pressure drop is found to increase rapidly with increase in fluidization velocity. The bed expansion ratio is found to increase with increase in excess fluidization velocity and distributor pressure drop but decreases with increase in bed height or weight.

Some hydrodynamic investigations of a magnetically stabilized air-fluidized bed of ferromagnetic particles

Abstract To gain fundamental understanding about the structure of a magnetically stabilized bed, an experimental facility has been designed, fabricated and tested. In particular, experimental data on spherical steel shots of three different sizes are reported as a function of increasing and decreasing fluidization velocities. Experimental data of bed pressure drop, bed voidage and mass minimum bubbling velocity reveal that bed behavior and its structure can be understood in terms of three broad categories of the applied uniform magnetic field intensity. The bed properties and its distinguishing features in the weak, moderate and strong magnetic field regions are reported. Earlier in the literature, somewhat similar classifications have been proposed on the basis of heat transfer and theological measurements.

GAS HOLDUP AND HEAT TRANSFER FROM IMMERSED SURFACES IN TWO- AND THREE-PHASE SYSTEMS IN BUBBLE COLUMNS

Two experimental slurry bubble column facilities comprising of 10.8 and 30.5 cm diameter columns and appropriate for conducting hydrodynamic and heat transfer studies are described. The average and local gas holdup data are reported for the air-water system as a function of air velocity. The holdups for the three phases are also reported for the air-water-glass beads system over a range of air velocity values. The air holdup data are compared with the predictions of some of the commonly used correlations. The heat transfer coefficient for a 19 mm diameter cylindrical probe and the two- and three-phase dispersions are measured as a function of air velocity. Most of these hydrodynamic and heat transfer data correspond to the churn turbulent regime and the values obtained on the two columns differ appreciably from each other under similar operating conditions. This fact indicates that the scaleup of slurry bubble columns could be quite difficult on the basis of data obtained on the bench and pilot-plant scal...

Heat transfer to immersed surfaces in gas-fluidized beds of large particles and powder characterization

Abstract A particle classification scheme is proposed for fluidized beds by considering simultaneously the hydrodynamic and thermal properties. The powder characterization is obtained by considering Archimedes number together with Reynolds number at minimum fluidization. The powders are classified in three groups and the validity of the scheme is demonstrated by considering heat transfer data from fluidized beds of sands (dp = 0.794 and 1.225 mm) and fire clay (3.0 mm) at ambient temperature and pressure in conjunction with heat transfer correlations and models developed for ‘large-particles’.

Hydrodynamic characteristics of gas-fluidized beds over a broad temperature range

Abstract Minimum fluidizing velocity and incipient bed voidage are reported for beds of four sand particles of average diameters 559, 751, 1225 and 3778,μm as a function of temperature in the range 300 – 1250 K. The experiments are conducted in a 0.152 dia. cylindrical bed equipped with external band heaters. Different temperatures are obtained by electrically preheating the fluidizing air and by the combustion of air-propane mixtures in the bed. The bed voidage at minimum fluidization ϵmf is found to exhibit a characteristic variation with the corresponding Reynolds number Remf, and Archimedes number Ar. It is shown that this variation is due to the changing fluidflow field around the particles, and hence to the inter-particle forces. The qualitatively different observed behaviors of ϵmf with Remf are in accord with the powder classification scheme of Saxena and Ganzha. The experimental values of minimum fluidizing velocity Umf for the four systems cannot be represented by any of the eight recommended Wen and Yu-type correlations based on the Ergun equation. This is discussed and explained on the basis of the observed variation of σmf with temperature, which must be accounted for in the development of a general correlation for Umf. The present experimental results are in agreement with the findings of earlier workers concerning the variation of ϵmf with temperature.

Rheology of a magnetically stabilized bed consisting of mixtures of magnetic and non-magnetic particles

Abstract Hydrodynamic fluidization characteristics of an admixture bed consisting of 10 mass % iron (1416 μm) and 90 mass % copper (935 μm) are investigated under the influence of an external uniform magnetic field collinear with the gas velocity vector. The experiments are conducted at ambient conditions for superficial air velocity (Ug) varying in the range 0–4.0 m/s, and for ten values of the magnetic-field intensity (H) in the range 4000-22 000 A/m. The magnetic field is created by a Helmholtz electromagnet and its spatial uniformity is established by detailed direct measurements. Bed-pressure-drop is measured as a function of increasing and decreasing Ug, over a range of H values. These data are employed to determine the minimum fluidization (Umf) and bubbling (Umb) velocities as a function of H; and for the calculation of gross bed voidage (ϵ) as a function of Ug. ϵ is found to increase from approximately 0.4 to 0.8 as Ug is increased but does not exhibit any pronounced dependence on H. Umf is independent of H, but Umb is a weak function of H and increases with increase in H. Of particular interest is the observed bed fluidization behavior and its structural changes as Ug is increased at a given H, and the changes in the nature of bed response as H is increased. At low H (5400 A/m) and Ug values, iron particles form stalagmites at the bed surface but these become unstable as Ug is increased. Increase in H increases stalagmite stability, number and penetration in the bed. Iron particles gravitate toward the central region, channels are formed in the bed, and iron clusters are formed in the outer bed region at larger Ug values. At H = 9000 A/m, stalagmites are replaced with clusters of constant size, and at still greater H values larger stalagmites appear; the majority of iron particles are consumed in this structure with copper particles contributing to major agitation. The stability of the stalagmites increases with H, and at 22 000 A/m stalagmite formation wobbles in the bed and fountain type bubbling occurs from the central bed region.

A two-dimensional gas fluidized bed for hydrodynamic and elutriation studies

Abstract A two-dimensional fluidized bed having the dimensions of 52.1 cm (20.5 in) by 2.00 cm (0.787 in) is designed and tested for its use in hydrodynamic and elutriation studies. The fluidization column is provided with a calming section and freeboard which are 45.7 cm (18.0 in) and 129.5 cm (51.0 in) high respectively. A porous distributor plate is provided whose pressure drop is found to vary linearly with air velocity in the range of current interest. Fluidization experiments with three sand particles (788, 488 and 167 μm), glass beads (427 μm), millet (2064 μm) and green peas (4578 μm) are reported. Bed expansion and bubble growth characteristics are examined in some detail. Variations of bed height and pressure drop with fluidization velocity are analyzed to establish bed voidage as a function of gas velocity, and minimum fluidization velocity. The latter is also measured for three particles in a 0.305 m square fluidized bed. These studies reveal that two-dimensional fluidized beds are great tools for making novel qualitative investigations for mechanistic details of processes taking place in three-dimensional fluidized beds. Currently, investigations are underway for elutriation phenomenon.

Hydrodynamic behavior of magnetically stabilized fluidized beds of magnetic particles

A theoretical analysis of beds consisting of magnetic particles and exposed to an external uniform magnetic field is presented. The external magnetic-field intensity may be strong enough so that the magnetized bed particles create significant field which distorts the external uniform field, and exert appreciable interparticle force. This complicates the hydrodynamic bed behavior much more than that observed in an uniform magnetic field in the absence of any appreciable interparticle magnetic force. This phenomenon is examined by considering the force balance on a bed particle comprising of gravitational, buoyancy, drag and magnetic forces. This has resulted in a relationship involving the minimum bubbling velocity, the corresponding voidage value and other parameters. Available and new experimental data are discussed on the basis of this theoretical analysis. This treatment leads to an enhanced understanding of the magnetically stabilized and partially stabilized magnetofluidized beds over a wide gas velocity and magnetic-field intensity ranges.

Particle classification scheme of saxena and ganzha and high-temperature bed voidage data at minimum fluidization

Etude des donnees hydrodynamiques a haute temperature relatives aux lits fluidises de particules de differents tailles, publiees recemment, dans la perspective du schema de classification des poudres de Saxena et Ganzha, en vue detablir son applicabilite aux systemes a haute temperature