Minimum Bubbling Velocity Research Articles

Heat-transfer characteristics of magnetofluidized beds of pure and admixtures of magnetic and nonmagnetic particles

The hydrodynamic behavior of an iron and iron-sand beds is investigated in the presence of an externally applied uniform magnetic field collinear with gas flow. In addition heat-transfer measurements from a horizontal electrically heated probe are conducted. The gas velocity is varied over a wide range to bring about different regimes for different magnetic-field intensity values in the range 0–9641 A m −1. The minimum bubbling velocity and heat-transfer coefficient values are discussed and correlated. It is found that bed voidage and segregation of admixture components play a very important role in establishing the hydrodynamic and heat-transfer characteristics of the magnetofluidized beds.

Heat-transfer rate variations from the surface of a heater probe in a magnetofluidized bed

A cylindrical strip heater heat-transfer probe is designed and employed to measure heat-transfer coefficients in the horizontal configuration at three angular positions (0, 90 and 180° relative to gas flow while submerged in a magnetically stabilized fluidized bed of 1086 μm iron shots. The superficial air velocity is varied up to about 3.8 ms −1, and measurements have been conducted for five values of magnetic-field intensity up to 5662 A m −1. In each case, pressure drops across the bed and a section of it are determined to obtain values of superficial minimum fluidization and minimum bubbling velocities, and gross bed voidage. The heat-transfer coefficient is found to be dependent on the angular position at the horizontal cylindrical probe immersed in the bed. The magnetic field destabilizes the gas pocket at the upstream side of the probe and this characteristically influences the h w values. The magnetic interparticle forces augment the gas flow in the equatorial lateral zones by the formation of a channel at the probe surface leading to gas bypassing. At the downstream side, the variation is somewhat similar to the variation observed for the total heat-transfer coefficient. Experimental minimum bubbling velocities are well corrected by a semitheoretical expression proposed by the authors.

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

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 ( U g) 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 U g, over a range of H values. These data are employed to determine the minimum fluidization ( U mf) and bubbling ( U mb) velocities as a function of H; and for the calculation of gross bed voidage (ϵ) as a function of U g. ϵ is found to increase from approximately 0.4 to 0.8 as U g is increased but does not exhibit any pronounced dependence on H. U mf is independent of H, but U mb 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 U g 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 U g values, iron particles form stalagmites at the bed surface but these become unstable as U g 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 U g 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.

Effect of an acoustic field on bubbling in a gas fluidized bed

Very fine particles in the Geldart C range, are difficult to fluidize because of interparticle forces. When a bed of fine particles is fluidized in the presence of an acoustic field, the sound waves agitate the bed material, helping to disrupt the large clusters of particles formed by interparticle forces, thus promoting more uniform fluidization and bed expansion. The present paper reports on the combined effects of gas velocity and frequency and intensity of the sound waves on bubbling behavior. Data on minimum bubbling velocity, bubbling frequency and bed expansion were obtained in a shallow batch fluidized bed with fine fly ash particles at room temperature.

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 ( U mh) velocities are reported as a function of ( H). U mf) is independent of H and the variation of U mb with H is expressed by: U mb = U mf 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.

On the origin of bubbles and Geldart's classification

Void propagation equations based only on conservation of mass and the Ergun equation for flow in porous media are sufficient to show that bubbles, which are continuity shocks, will form in a fluidized bed near minimum fluidization for small particles and that voids will disperse for large particles, as for Geldart's Type B and D powders. This is due to the fact that the void propagation velocity increases with the square of the porosity for Type B particles and decreases with porosity for Type D particles. A further approximate analysis involving the elastic solids stress between the particles shows the existence of a minimum bubbling velocity and hence provides a rational explanation for the boundary between Type A and B particles. This approximate analysis is further extended to the study of bubbling regimes at low and high fluidizing pressures.

微粉流動層における層見かけ粘度と流動性との関係

The apparent viscosity was measured in a fluidized bed with fine particles. The measurement was performed in the bed expanded uniformly at gas velocities between minimum fluidizing velocity and minimum bubbling velocity by means of the falling-sphere method. As the apparent viscosity obtained decreased with the diameter of the falling spheres, the viscosity value was determined by extrapolation. The apparent viscosity obtained was correlated with particle diameter, density and bed voidage. An equation for predicting the apparent viscosity in the emulsion phase in a bubbling fluidized bed was introduced by combination with the correlation of the voidage of the emulsion phase. Comparison of the values predicted from this equation with the data in the literature shows good agreement. To investigate the influence of the apparent viscosity on the fluidization quality, deviations of pressure fluctuations and bubble diameter were correlated with the apparent viscosity. It was shown that the proposed correlation for the apparent viscosity was useful for predicting the fluidization quality.

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

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.

Effect of pressure on the minimum bubbling velocity of polydisperse materials

The effect of pressure on thephenomenon of formation of gas bubbles in a polydisperse bed of granular materials has been investigated. The experiments have been carried out at pressures of 0.1, 2.2, 4.2 and 6.3 MN/m 2 for 13 mixtures of particles of coal and char, differing in their granular composition. The effect of the granular composition of mixtures upon the minimum bubbling velocity has been presented in the form of phase diagrams and a correlation for minimum bubbling velocity, valid in the pressure range investigated, has been proposed. The suitability of available correlation equations, concerning the transition from particulate fluidization to aggregative fluidization, has also been discussed.

Fluidization of fine powders with air in the particulate and the bubbling regions

Several fractions of a silica—alumina catalyst and an impregnated catalyst were fluidized with air to check published bubble point correlations and to determine the expansion of the dense phase in the bubbling region. The minimum bubbling velocities were fairly close to those reported by Geldart and others, but U mb varied with only the 0.6 power of the particle size. The dense phase expansion was significant even at velocities much above U mb and corresponded to relative velocities up to 2 – 3 times U mf .

Behaviour of gas-fluidized beds of fine powders part I. Homogeneous expansion

The minimum bubbling velocity has been correlated from the results of experiments on 48 gas/solid systems and is a function of the density and viscosity of the fluidizing gas, the mean sieve size of the powder and the fraction of fines less than 45 μm. An improved equation is presented to predict U mb/U mf ; this parameter also correlates well with the maximum non-bubbling bed expansion ratio H mb/H mf which is used to calculate the maximum dense phase voidage ϵ mb . An equation based on the Carman-Kozeny theory can be used to predict bed voidages between incipient fluidization and bubbling. An accurate value of particle density is essential in these equations and a simple comparative method has been used to determine particle densities of fine porous powders.

Behaviour of gas-fluidized beds of fine powders part II. Voidage of the dense phase in bubbling beds

The collapse rate technique has been used to evaluate the average dense phase properties in vigorously bubbling beds of fine powders. The results of experiments on 13 air/solid systems are used to correlate the average dense phase voidage, ϵ D , in terms of the physical properties of the gas and powder, and our predictive equations fit literature data. ϵ D increases as the particle density and mean particle size decrease, and as the fraction of fine <45 μm, gas viscosity and gas density increase. The gas velocity, U D , through the dense phase of a bubbling fluidized bed has been calculated from ϵ D assuming Darcy's law and can now be predicted. Since it is less than the minimum bubbling velocity, the improved performance of fluidized bed reactors when the gas/solid properties are changed so as to increase ϵ D can be attributed more to smaller bubbles splitting and coalescing frequently rather than to the small amount of extra gas passing through the dense phase.

Bubble sizes in a fluidized bed at elevated temperatures

Observations were made of the minimum bubbling and minimum slugging velocities, and bubble sizes at 20, 200 and 300/sup 0/C in a bed of spherical steel shot. Under equivalent conditions of bed height and excess gas velocity at 200 and 300/sup 0/C, bubbles were 75 to 85 percent of the sizes at room temperature. This trend was confirmed by the finding that higher excess gas velocities were achieved before the bed started to slug. Minimum bubbling velocity appears less sensitive to temperature than minimum fluidization velocity values reported in the literature.