Local Voidages Research Articles

Direct numerical simulation of particle clustering in gas–solid flow with a macro-scale particle method

Particle clustering has long been a focus in the study of gas–solid flow. Detailed flow field information below the particle scale is required to understand the mechanism of its formation and the statistical properties of its dynamic behavior, but is not easily obtained in both experiments and numerical simulations. In this article, a meshless method is used to reveal such details in the destabilizing of a suspension with hundreds of particles. During the process, doublets, quadruplet and larger clusters are seen to form and disintegrate dynamically, showing a tendency to minimize local voidages. At the same time, single vertical streams, pairs of parallel streams and many irregular streams appear and disappear between particle clusters alternatively, exhibiting a tendency to suffer lowest resistance. Globally, the spatio-temporal compromise between these two tendencies results in a configuration of large clusters separated by fast flow streams. In the clustering process, the inter-phase slip velocity is seen to increase long after the forces on each phase have stabilized, suggesting that inter-phase friction is not a function of local voidage and Reynolds number only, as commonly considered. The article concludes with prospects on the sub-grid scale models for continuum description of gas–solid flow that can be established upon such simulation results.

Measurement and modeling of axial distribution of adsorbent particles in expanded bed: taking into account the particle density difference

Based on the previous work (Yun et al., Chem. Eng. Sci. 59 (2004) 449–457), after the analysis of adsorbent structure, bed expansion and in-bed fluid hydrodynamics, a model was developed to describe the axial variations of the mean particle size and local bed voidage for adsorbents with a broad size distribution and different densities in expanded bed. In the model, the different-size particles were assumed to have different densities while the equal-size particles have the same density. Particle densities were described by a function that considered two parts of each particle: the resin-core part consisting of resin and inert core-particulates, and the thin resin film part without inert core-particulates. With UpFront FastLine SP adsorbents as interest test particles, the particle size and local voidage changes with bed height were measured in a 20 mm I.D. column and then predicted by the model. It was shown that the mean particle size decreased, while the local bed voidage increased with increase of bed height. These trends are similar to those for particles with a broad normal size distribution and nearly uniform density. More obvious increase trends of voidage with increase of bed height exist in expanded beds with UpFront FastLine SP particles compared with those of Streamline particles. The present model gave a good prediction of bed expansion, the mean particle sizes and local voidages in most parts of bed compared with experimental data.

Flow Characteristics in Slot‐Rectangular Spouted Beds with Draft Plates

AbstractAn experimental investigation was conducted of slot‐rectangular spouted beds with air entry slots spanning the full thickness of the column and vertical draft plates intended to help control the solids circulation rate. With increasing superficial gas velocity, the flow between the draft plates changed from bubbling to slugging and then to spouting with dilute pneumatic between the plates and moving‐bed downward motion on both sides. However, there was difficulty maintaining stability and symmetrical flow on the two sides. Once spouting is established, pressure drops and local voidages vary with gas velocity, particle size and gas entry size in broadly similar manners as for conventional spouted beds

Gas and solids between dynamic bubble and emulsion in gas-fluidized beds

The gas and solid distributions of the dynamic two-phase structure in fluidized beds were studied by statistic analysis of experimental results from an optical fiber probe. The FCC (Geldart A) and irregular sand particles (Geldart B) were used as bed materials, respectively. Such two types of particle led to extremely different dynamic flow structures in the bed. However, for both particles, the dynamic flow structure exhibited the continuous double-peak probability density functions of the local voidages from ε mf to 1, one peak for the emulsion and another for the bubbles. A probability model was thus proposed to describe and simulate such dynamic gas–solid distribution and its complex evolution from bubbling to turbulent regimes. The gas and solid distributions in the two phases were further investigated and were found to depend strongly on gas velocity, bed position and particle properties. Both gas and solid distributions in the emulsion decrease, whereas those in the bubble phase increase with increasing superficial gas velocity. The ratio of gas distributions in the two phases is heterogeneous over the cross section of the column and exhibits a core–annulus structure, which is considerably different for the FCC and sand particles. The bubbles contain more solids while closer to the bed centre and at higher gas velocity. Compared to the sand, for the FCC, more particles distribute into the bubble phase.

Further measurements of flow dynamics in a high-density circulating fluidized bed riser

Local voidages were determined for conditions corresponding to dense suspension upflow and fast fluidization in a riser of diameter 76.2 mm and height 6.1 m using a reflective-type optical fiber probe at superficial air velocities between 4 and 8 m/s and solids circulation fluxes up to 425 kg/m 2 s. High-density flow of cross-sectional mean volumetric solids concentration of about 0.2±0.05 was achieved in the riser with fluid catalytic cracking (FCC) particles of mean diameter 70 μm and density 1600 kg/m 3. Local time-mean voidages were nearly as low as ε mf at the wall and as high as 0.9 at the axis. As in our other recent work, solids refluxing near the wall of dilute CFB risers no longer existed for high-density conditions. Statistical analysis of local voidage fluctuations was used to characterize the local flow dynamics. The core behaves as a relatively uniform dilute flow, interspersed with infrequent particle structures. The number and density of these structures increases with radius. Maximum heterogeneity for high-density conditions occurs at some distance from the wall, unlike dilute risers where the flow is least uniform at the wall. Intermittency index vs. local voidage plots are “bell-shaped”, with a maximum where the local time-mean voidage is approximately 0.75.

Characteristics of Gas-Fluidized Beds in Different Flow Regimes

Hydrodynamic characteristics among different flow regimes of gas fluidized beds are compared on the basis of experiments with fluidized catalytic cracking particles in a 76.2 mm diameter riser. Pressure and local voidage fluctuations were analyzed using both statistical and chaotic tools. The standard deviations of local voidage fluctuations are much lower in a high-density circulating fluidized-bed riser than in the bubbling and turbulent flow regimes, even for identical local time-mean voidages. A chaotic time series analysis can distinguish flow structures of the various flow regimes. Bifractal structures, characterized by two Hurst exponents, two correlation dimensions, and two Kolmogorov entropies, characterize the motions of the dilute and emulsion phases in the bubbling and turbulent flow regimes. The two-phase structure becomes less distinguishable with increasing gas velocity, eventually disappearing on reaching dilute phase transport. However, for high solids concentrations, the bifractal character persists, suggesting that particles may travel in two different forms. Radial profiles of chaotic parameters are relatively flat in the bubbling and turbulent flow regimes, but significantly nonuniform in dense suspension upflow. Flow behavior in the high-density riser and in the dense bottom region of a low-density riser operated in the fast fluidization regime differ from the bubbling and turbulent flow regimes, even when compared where there are equal local voidages.

Two distinctive variational regions of radial particle concentration profiles in circulating fluidized bed risers

The radial profiles of particle concentration in circulating fluidized bed (CFB) risers experience actually a sequential variation as `uniform (dense)–nonuniform–uniform (dilute),' in response to the flow evolution from minimum fluidization to pneumatic transport. Through measuring the lateral distributions of solid concentration in the range ε mf∼1.0 of the cross-sectionally averaged voidage, it was found that the first variational stage, from dense uniform (minimum fluidization) to nonuniform, is completed via the rapid decrease of the particle concentration in the bed core, and the succeeding second stage, from nonuniform to dilute uniform (pneumatic transport), is implemented via the quick decrease of the particle concentration near the bed wall. Based on this observation, general correlations were proposed to predict the local voidages on the bed wall (specifically, at the point where the dimensionless radius is equal to 0.99) and at the bed center, which can be used for the entire spectrum of gas–solid fluidization regimes prevailing in CFB risers. Further analyses demonstrated that such two variational regions are related also to the regime transition and the lateral distribution features of the transient solid concentration fluctuations.

Instantaneous local heat transfer and hydrodynamics in a circulating fluidized bed

Local heat transfer mechanisms and hydrodynamics are studied in a 9.3 m tall, 152 mm i.d. transparent cold model circulating fluidized bed for 171 μm Ottawa sand. Instantaneous measurements at the wall are made with platinum-coated heat transfer probes. For some conditions, simultaneous local voidages are determined using a capacitance probe. Results show that the sudden and dramatic peaks in the measured instantaneous heat transfer coefficients are directly caused by the arrival of strands of particles at the heat transfer surface. Analyses of the capacitance probe signals indicate that these strands possess wide distributions of voidages which vary with the local time-averaged area-averaged suspension density. Simultaneous heat transfer probe measurements further suggest the existence of characteristic residence lengths for these strands. The average falling velocity of the strands is 1.26 m s−1 using high-speed cinematography.

A Review of Combustion of Single Coal Particles in Fluidized Beds

The state of the art of the theory of the process of combustion of single coal particles in fluidized beds is reviewed. The process of combustion of coal is divided into two stages: devolatilisation, and burning of the char. The devolatilisation and the subsequent combustion of the volatile matter takes an order of magnitude less time than that for burning of the char. Therefore, the burning of char dominates the characteristics and performance of the combustor. Generally, the combustion of char is found to be controlled by the mass transfer to and reaction on the external surface if the area of pores in the char is less than 0.3 × 106m2/m3. Existing information suggests that in a fluidized bed oxygen diffuses to the carbon surface and it enters into heterogeneous reaction with carbon producing mainly CO. The carbon monoxide burns to CO2 in a diffusion flame surrounding the char particle. The surface temperature of the char, which is always higher than the bed temperature, is found to increase with decreasing size of the char, but it may start decreasing again if the site of CO oxidation moves away from the char surface for very small char particles. The mass transfer rate to the char particles may be calculated by considering the radial variation of local voidages, but this effect is significant only in limited cases.