Spout Region Research Articles

Numerical simulations of hydrodynamic behaviors in conical spouted beds

The axial and radial distributions of static pressures and vertical particle velocities of conical spouted beds have been simulated and compared with experimental data. Simulation results show that, among all factors investigated, the Actual Pressure Gradient (the APG term) in conical spouted beds, introduced as the default gravity term plus an empirical axial solid phase source term, has the most significant influence on static pressure profiles, followed by the restitution coefficient and frictional viscosity, while other factors almost have no effect. Apart from the solid bulk viscosity, almost all other factors affect the radial distribution of the axial particle velocity, although the influence of the APG term is less significant. For complex systems such as conical spouted beds where a fluidized spout region and a defluidized annulus region co-exist, the new term introduced in this work can improve the CFD simulation. Furthermore, for other systems with the Actual Pressure Gradient different from either fluidized beds or packed beds, the new approach can also be applied.

Computational fluid dynamics (CFD) modeling of spouted bed: Influence of frictional stress, maximum packing limit and coefficient of restitution of particles

Following the previous article [Du, W., Bao, X., Xu, J., Wei, W., 2006. Computational fluid dynamics (CFD) modeling of spouted bed: assessment of drag coefficient correlations. Chemical Engineering Science 61 (5), 1401–1420], this contribution describes the influences of the frictional stress, maximum packing limit and coefficient of restitution of particles on CFD simulation of spouted beds. Using the two-fluid method embedded in the commercial CFD simulation package Fluent 6.1, the spouting hydrodynamics of a cylindrical–conical spouted bed was simulated and verified with the experimental data of He et al. [He, Y.L., Lim, C.J., Grace, J.R., Zhu, J.X., Qin, S.Z., 1994a. Measurements of voidage profiles in spouted beds. Canadian Journal of Chemical Engineering 72 (4), 229–234; He, Y.L., Qin, S.Z., Lim, C.J., Grace, J.R., 1994b. Particle velocity profiles and solid flow patterns in spouted beds. Canadian Journal of Chemical Engineering 72(8), 561–568]. The results showed that, for coarse particles, the frictional stress is important only for the annulus computation and has no obvious effects on the hydrodynamics of the solids flow in the spout region. The specification of the maximum packing limit could significantly affect the properties of the pseudo-fluid phase of the particles by changing the radial distribution function. The strong dependence of the pseudo-fluid properties of the particle phase, such as pressure, bulk viscosity and shear viscosity, on the granular temperature accounts for the influence of the coefficient of restitution of particles on CFD modeling. The solids volume fraction at loose packing state is suitable for spouted bed simulations, and a pretest of the coefficient of restitution of particles must be conducted when no experimental datum is available.

High Temperature Spouted Bed Paddy Drying with Varied Downcomer Air Flows and Moisture Contents: Effects on Drying Kinetics, Critical Moisture Content, and Milling Quality

This research is an experimental study on paddy drying conducted in a two-dimensional spouted bed batch dryer to investigate the effects of downcomer airflow, drying temperature, and initial moisture content on drying kinetics, milling quality, and thermal energy consumption. The system of spouted bed drying and a comparison of spouted bed drying in the present study, and fluidized bed drying in the related literature, are also discussed. Downcomer air flows of 0, 20, and 30% ([defined as mass flow rate of downcomer air/total mass flow rate of air] × 100%), inlet air temperatures of 110, 130, and 150°C, and initial moisture contents of 18–35% d.b. were used. It was found that moisture transfer did not only occur in the spout region but also took place in the downcomer region. The moisture content and temperature of the paddy dropped as grain moved downward in the downcomer, which resulted from the presence of an evaporative cooling phenomenon. The characterization of drying curves regardless of any drying condition could be described by nearly linear relationships between moisture content and time. Although high downcomer air flow and drying temperature could enhance effective moisture reduction, they caused an adverse decrease in critical moisture content and HRY (head rice yield). Critical moisture content and HRY could also increase with an increase in initial moisture content. The difference in moisture content between the initial and critical moisture contents varied between 4.5 and 8.0%d.b., depending upon drying conditions. No significant effect on color was evident. From the point of view of HRY, the correct management of a two-stage spouted bed drying system could be a suitable and attractive alternative for rice mills. Finally, a comparison between the spouted bed drying and fluidized bed drying showed that the spouted bed had advantages over the fluidized bed in terms of product quality. The specific drying rates (kg water evaporated h−1 m−3) of both techniques were comparable. With respect to energy consumption, spouted bed drying was not as efficient as fluidized bed drying for intermediate initial moisture content, but a contrary result was obtained for low initial moisture content.

Prediction of gas-particle dynamics and heat transfer in a two-dimensional spouted bed

Abstract The aerodynamics of particles and heat transfer of gas-to-particles in a two-dimensional spouted bed (2DSB) with draft plates are investigated by the discrete element method (DEM). The physical properties of the particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the empirical correlations proposed by Kudra et al. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. The draft plates can reduce the minimum spouting velocity and pressure drop. They also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated. The mixing of a 2DSB without draft plates of 10 000 particles is better than that of 26 000 particles. In our simulation, the gas-to-particle heat transfer is investigated. The Ranz–Marshall correlation and the correlation of Sartori et al. are applicable in the spout region and the downcomer region, respectively. The gas-to-particle heat transfer occurs mainly in the central or spout region, as reported by Freitas and Freire.

Investigation of Fluid and Coarse‐Particle Dynamics in a Two‐Dimensional Spouted Bed

AbstractThe aerodynamics of particles and gas flow in a two‐dimensional spouted bed (2DSB) with draft plates is investigated with the aid of the discrete element method. The geometry of the 2DSB with draft plates is set as close as possible to the experimental apparatus of Kudra [1] and Kalwar [2]. The physical properties of the coarse particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the correlations of Kudra [1] and Kalwar [2]. In the spout region, the particle vertical velocities are found to decrease as the height increases. The fluid velocity in the downcomer region decreases as the superficial gas velocity increases. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. At the minimum spouting velocity, the bed height does not affect the particle circulation rate in the 2DSB with draft plates. The draft plates not only reduce the minimum spouting velocity and pressure drop but also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated and the effect of putting in a deflector on the possible breakage of the particles is also estimated.

Soybean Drying by Two-Dimensional Spouted Bed

Urease activity, cracking, and breakage are important factors in considering the quality of raw soybean for feed meal industries. A two-dimensional spouted bed dryer was investigated to determine its capability for thermally inactivating the urease enzyme and maintaining its other qualities. The experimental results have shown that the drying kinetics of soybean in a two-dimensional spouted bed dryer are of the form described in the thin layer drying. The expression for the model parameter in Newton's law of cooling equation accounting for the moisture contents and inlet air temperatures was developed. The initial moisture content and inlet air temperature conditions cause cracks in the kernels. The strong collision between kernels and deflector because of high superficial velocity leads to high percentage of broken soybeans in the spout region. However, the velocity of 15.9 m/s can reduce the breakage below 5%. The inactivation of urease at low-to-moderate moisture content is suitably described by the first order kinetics. The modified Monod equation is applied when the moisture content is higher than 26% dry basis due to the inhibitory effect of water content on the inactivation rate. To complete urease inactivation and maintain protein quality, the temperatures of 150°C should be used.

Mixing and circulation of solids in spouted beds: particle tracking and Monte Carlo emulation of the gross flow pattern

Mixing and circulation of monosized particles in laboratory-scale tapered spouted beds have been characterized experimentally by measuring non-invasively the 3-D trajectory of a single tracer via a radioactive velocimetry technique. Processing the obtained Lagrangian trajectory allowed determination of the mixing dynamics in the longitudinal, radial and circumferential directions, of the return length and return time distributions, and of the mean Eulerian flow fields. A conceptual solids flow structure has been delineated. A four-zone 2-D axisymmetrical Monte Carlo model has been developed for emulating the elementary steps in play in the longitudinal mixing, i.e., the direction of slowest mixing, and in the return (or circulation) time and length of the solids phase. The four-zone solids flow structure is viewed as: (i) a spout region with a constant upward particle velocity, (ii) an annulus region above the conical base with a downward velocity radial profile, (iii) an annulus region within the conical base where the linear velocity, considered to be parallel to the cone wall, is equal to that of the incoming particles, (iv) a fountain in which the particle movement is characterized by the particle residence time, an exiting radius, and an average fountain height. The model proved successful in restoring the measured return time and return length distributions, and the mixing response curves.

SENSITIVITY ANALYSIS ON THE FLUID DYNAMICS OF A DRAFT TUBE SPOUTED BED WITH BOTTOM PARTICLES FEED

ABSTRACT A numerical study was carried out on the parametric sensitivity of the fluid dynamic variables for a draft tube spouted bed (SB) with bottom particles feed. The sensitivities of variables such as the air and particle velocities and voidages in the spout region, as functions of key model parameters, like the spout–annulus air distribution, solids recirculation rates and gas–particle drag coefficient were determined. The model was numerically integrated and sensitivities obtained by the central differences approach. The sensitivities were seen to depend on air inlet flowrates and heights in the bed. For air flowrates close to the minimum spouting velocity the model is very sensitive to all the parameters studied. In most cases, the variables were moderately to highly sensitive to air distribution in the bed. On the other hand, the variables were minimally to moderately sensitive to the drag coefficient and solids recirculation rates. The maps of sensitivity obtained provide useful information for planning of the probe location, hydrodynamic modeling and directing of future work on SB with bottom particles feed.

GAS-TO-PARTICLE HEAT TRANSFER IN THE DRAFT TUBE OF A SPOUTED BED

The gas-to-particle heat transfer coefficients in the spout region of draft tube spouting beds of 2.6 mm glass beads were determined during continuous operation with particle fed through the base. For this purpose, detailed profiles of the gas temperature were measured in both spout and annular regions, as well as the temperature of the particles in the annulus. Fluid dynamic data were used to determine the volumetric solids concentration in the spout region. The effects of the spouting air flow rate, the particles feed rate, the height of the bed and the solids volumetric concentration, β v , on the heat transfer coefficients were also determined. The data obtained was compared with the predictions of equations for spouted beds and vertical pneumatic conveying.

Dynamics of the spout of gas plumes discharging from a melt: Experimental investigation with a large-scale water model

In the present study, the spout region of a gas plume discharging from a melt has been investigated using a water model of 180 cm in height and 160 cm in diameter. The lateral movement of the spout, as measured optically, increases with the gas flow rate and has been found to be ± 20-cm wide or wider, and very fast. The spout height, as measured with video-optical and electrical methods, strongly fluctuates with time. Clear definitions have to be made of the quantities to be determined in the highly dynamic process. Long-time averages of the radial height profiles and momentary maximum height values are reported. It is confirmed that the nondimensional spout height, defined and measured in a certain manner, is independent of the Froude number and of the nondimensional nozzle diameter.

Spout diameters in full and half spouted beds

AbstractA fiber optic probe has been used to measure spout diameters in a half‐column and a full‐column based on the significantly higher counts of output electric pulses in the spout region than in the annulus. The flat wall of a semi‐cylindrical spouted bed column was found to cause considerable distortion of spout shapes which became approximately semi‐elliptical in cross‐section. Caution must therefore be exercised when using spout diameter data from the face of half‐columns for spouted bed modelling and design. The often‐used McNab (1972) equation was found to underestimate the spout diameters in the full‐column, with an average deviation of 35.5%.

Particle flow model in a spouted bed

A simplified mathematical model has been developed to estimate the particle flow rate in a spouted bed. It is based on the mass and momentum conservation laws in the spout region, and on the force balance in the annulus. Then the friction between particles and the vessel wall are considered in addition to the gravity and drag forces. The model equations are solved numerically, and the predicted values of the particle velocities and particle mass flow rates are compared with the authors' previous experimental results with reasonable agreement.

MODELING AND SIMULATION OF AIRFLOW IN SPOUTED BED DRYERS

ABSTRACT This paper presents an axisymmetric three-dimensional semi-empirical model that describes the airflow in both the spout and annular regions. Both the pressure and air velocity distributions can be calculated at any point inside the dryer. Thus, the effect of the fluid dynamics on the heat and mass transfer coefficients and, consequently, on the drying process, can be evaluated. The finite element method was used to solve the model. Pressure distributions were measured for several conditions in which stable spouted bed flow regime was attained. Simulated pressure distributions agreed well with experimental data.

CONVECTIVE HEAT TRANSFER DURING COATING OF PARTICLES IN TWO-DIMENSIONAL SPOUTED BEDS

ABSTRACT This work deals with the evaluation of gas-to-particle convective heat transfer in a process of coating of particles in two-dimensional spouted beds. A detailed calculus routine is presented and two correlations of heat transfer coefficient, for the spout and annular regions are derived. The study is performed with two, two-dimensional spouted beds with different sizes and with two different particles, placebo and soybean.

The spout of air jets upwardly injected into a water bath

The spout region of gas jets in liquids has received little attention, although it has both theoretical and practical significance. In this study, the spout of upwardly injected gas jets in water was characterized experimentally in terms of gas fraction, bubble frequency, and axial velocity distributions for ultimate incorporation into turbulent recirculating flow models. The measurements were made with a two-element electroresistivity probe coupled to a microcomputer. For the turbulent flow conditions prevailing in the jet plume and spout, special hardware and software were developed to analyze the signals generated by contact of the bubbles with the sensor in real time. Correlations of the gas fraction with axial and radial position for different gas flow rates have been established from the measurements. The dimensions of the spout were obtained from time-exposure photographs; when compared with the gas-fraction measurements, the spout boundary always corresponded to values ranging from 0.82 to 0.86. The radial profiles of bubble frequency at different levels in the spout exhibit a bell shape; the bubble frequency decreases with increasing height. The velocity of the bubbles in the spout drops linearly with increasing axial position.

流動層における分散板用多孔板に関する研究 多孔板直上におけるスパウト径およびその高さについて

Spout region in fluidized beds similar to that in spouted beds is important for reaction, mass transfer and so on.In the present paper, the spout diameter Ds and the spout height Hm above the distributor were tested in the semicylindrical fluidized bed by changing some factors, such as the geometry of perforated plates, particle size and operating conditions.At first, an empirical correlation for Hm based on qin has been obtained in the range of U/Umf=1-7.Next, the spout formed at each hole in perforated plate is assumed to be similar to that in spouted beds.According to the assumption, a new empirical correlation for Ds and Hm in the fluidized bed has been presented by modifying the semi-empirical correlation in spouted beds.It was found that Ds and Hm increased with increase of the superficial gas velocity for U/Umf≤5, on the other hand, they were independent of the superficial gas velocity for U/Umf>5.

Some comments on a physical approach to particle-, bubble- or drop-fluid systems

Following the previous article [Du, W., Bao, X., Xu, J., Wei, W., 2006. Computational fluid dynamics (CFD) modeling of spouted bed: assessment of drag coefficient correlations. Chemical Engineering Science 61 (5), 1401–1420], this contribution describes the influences of the frictional stress, maximum packing limit and coefficient of restitution of particles on CFD simulation of spouted beds. Using the two-fluid method embedded in the commercial CFD simulation package Fluent 6.1, the spouting hydrodynamics of a cylindrical–conical spouted bed was simulated and verified with the experimental data of He et al. [He, Y.L., Lim, C.J., Grace, J.R., Zhu, J.X., Qin, S.Z., 1994a. Measurements of voidage profiles in spouted beds. Canadian Journal of Chemical Engineering 72 (4), 229–234; He, Y.L., Qin, S.Z., Lim, C.J., Grace, J.R., 1994b. Particle velocity profiles and solid flow patterns in spouted beds. Canadian Journal of Chemical Engineering 72(8), 561–568]. The results showed that, for coarse particles, the frictional stress is important only for the annulus computation and has no obvious effects on the hydrodynamics of the solids flow in the spout region. The specification of the maximum packing limit could significantly affect the properties of the pseudo-fluid phase of the particles by changing the radial distribution function. The strong dependence of the pseudo-fluid properties of the particle phase, such as pressure, bulk viscosity and shear viscosity, on the granular temperature accounts for the influence of the coefficient of restitution of particles on CFD modeling. The solids volume fraction at loose packing state is suitable for spouted bed simulations, and a pretest of the coefficient of restitution of particles must be conducted when no experimental datum is available.