Fluidized Bed Jet Research Articles

Investigating the particle dispersion in a spout-fluid bed using particle trajectory

Abstract Particle dispersion in a spout-fluid bed was investigated using time series of particle trajectory by evaluation of dispersion coefficient based on Einstein’s expression. A series of experiments were performed in a lab-scale spout-fluid bed using microwave heating and infrared thermal imaging (MH-ITI) technique for recording the position of a tracer. The influence of gas velocity on the particle dispersion in different flow regimes has been taken into account. The results show that the particle dispersion behavior depends on the flow regime. In the flow regime of internal jet, the increase of gas velocity improves the axial dispersion. In the case of jet in fluidized bed with bubbling, increasing fluidizing gas velocity promotes the radial dispersion at the lower part of bed and the axial dispersion at the upper part of bed. In the flow regime of jet in fluidized bed with slugging, the effect of fluidizing gas is related to the slug pattern. The result also indicates that the transition of flow regime can be evaluated from the particle trajectory.

Experimental Investigation on Flow Patterns of Cylindrical-Conical Spout-Fluidizing Bed

Experimental investigations on hydrodynamic characteristics of cylindrical pressurized spout-fluidizing bed were carried out. Two kinds of millet were used as bed materials. The operational pressure is 0.1MPa~0.4MPa (absolutely pressure). Five distinct flow patterns, i.e, fixed bed(FB), jet in fluidized bed with bubbles(JFB), jet in fluidized bed with slugging(JFS), spout with aeration(SA) and spout-fluidizing bed(SF) were identified. Effects of the static bed height and operational pressure on the flow pattern map were particularly studied. Typical flow pattern images obtained by a high- resolution digital CCD camera were presented for classifying these flow patterns. Typical flow pattern maps were plotted for describing the transitions between flow patterns with operating conditions

Hydrodynamic behaviour of upflowing jet in fluidized bed: Velocity profiles of sand particles

The hydrodynamic behaviour of the submerging jet from nozzle in particulate medium by determining the velocity profiles is investigated in a 500 mm × 250 mm column filled with sand particles of 230 μm mean diameter and 2576 kg/m 3 density. The upflow jet air emerges from a nozzle with 15 mm diameter. Three nozzles with lengths of 80, 40 and 0 mm from above the distributor are used. The experiments show that the jet fundamental properties and the self-similarity of the reduced velocity profiles remain constant in the process. The self-similarity of the reduced velocity profiles is thus an intrinsic property in the steady-state flow of the jets in a particulate medium, whatever the longitudinal axis, the length of the nozzle, the jetting velocity and the bed state (aerated or fluidized).

Prediction of flow regimes in spout-fluidized beds

Five main flow regimes in spout-fluidized bed were identified in this study, namely, fixed bed, spout with aeration, spout-fluidization, jet in fluidized bed and slugging, together with their corresponding major frequencies translated from pressure signals. The empirical equation A = aB b , in which A = Fr */( H′/ D i ) and B = ( Fr */( H′/ D i ))/( u gt/ u mf) are respectively the spout-geometry and spout-geometry-fluidization dimensionless numbers, was proposed to distinguish these flow regimes.

Flow pattern and transition of rectangular spout–fluid bed

Experimental study on the flow patterns and transitions in a rectangular spout–fluid bed (cross-section of 300 mm × 30 mm and height of 2000 mm) packed with Geldart group D particles was carried out. Six distinct flow patterns, i.e. internal jet, spouting, fluidizing, jet in fluidized bed with bubbling, jet in fluidized bed with slugging and spout–fluidizing were identified. Criteria as well as schematic diagrams and typical flow pattern images obtained by a high-resolution digital CCD camera were presented for classifying these flow patterns. Typical flow pattern maps were plotted for describing the transitions between flow patterns with operating conditions. In addition, two kinds of flow instabilities were observed. Based on the flow pattern images recorded by the digital CCD camera, the mechanism of flow instabilities was preliminarily discussed. It was found that the flow instabilities are caused by stochastic visible large bubbles in the boundary of spout jet and near the fluidizing gas distributor. The growth of surface disturbances might exacerbate these flow instabilities.

Modeling of wet jet in fluidized bed

A wet jet zone is established in many applications wherever feeding and dispersing a liquid, solution or slurry into fluidized bed by gases is needed. In the present study, a simple mathematical model has been developed to simulate the wet jet in fluidized bed. The different stages involved inside the jet zone have been estimated and analyzed. The evaporation stage of traveling droplets through the jet flare has been treated. The rates of evaporation of each size at all positions along the jet flare have been estimated according to the velocities and surrounding conditions. The final droplet sizes have been determined. Moreover, the total evaporation rate from traveling droplets, before collision either with entrained sand particles or flare boundaries, has been estimated. The traveling droplets, partially evaporated, may collide and settle on entrained sand particles. The model predicts the settlement rates of liquid droplets on entrained sand particles. The total part evaporated from settled liquid has been estimated as well. The study has been applied to the pneumatic feeding of liquid fuel into fluidized bed combustors operating at 850 ∘ C . The model has been utilized to predict the ratio of fuel vapor that releases inside the jet flare. The remaining part is assumed to evaporate inside the emulsion phase. Three different liquid fuels have been considered: a heavy oil, diesel fuel and gasoline. The main independent variables are those related to the injection conditions including the initial velocity of dispersing air, u 0 , and air-to-liquid mass ratio, ALR. The model results demonstrate that only very small droplets completely evaporate inside the flare. The liquid settling over the entrained sand particles plays an essential role in the fuel evaporation inside the flare. The phenomenon is dominant at conditions that result in generation of droplets of larger sizes, i.e., heavier fuel, lower u 0 , and greater ALR. The ratio of vapor fuel released in jet flare increases with lighter fuel, higher u 0 and lower ALR. At u 0 = 200 m / s and ALR = 1.0 nearly all-liquid fuel evaporates inside the flare.

Hurst’s analysis to detect fluctuation of the jet in a two-dimensional fluidized bed

The experimental study of vertical jet in a two-dimensional fluidized bed is presented. The time series of vertical jet penetration depth are acquired by image processing. The fluctuation of jet which is studied by means of Hurst’s analysis becomes weaker with increase in jet velocity, fluidization number, particle diameter and static bed height. They are found that the signal is persistent and the fractal dimensional of vertical jet is between 1. 19 and 1. 64.

Temperature distribution around heated horizontal jet in fluidized bed

In a previous study (Chen and Weinstein, 1993) in which the literature was reviewed, the authors determined the shape and extend of the void formed by a horizontal jet using X-ray imaging. The void fraction distribution around the jet and the tracks of the bubbles formed by the jet gas leaving the void were described. In the present study using the same experimental facility, the horizontal jet was heated and temperature surveys were obtained in the bed. These data provide a comparison of the picture of the jet structure obtained from the spread of the heated gas with that previously obtained from the spread of the jet mometum. In addition, in the present study the superficial velocity of fluidizing gas (through the grid) was varied to determine its effect on the jet penetration and dispersion.