A NUMERICAL SIMULATION OF SWIRLING FLOW PNEUMATIC CONVEYING IN A HORIZONTAL PIPELINE

ABSTRACT

A numerical simulation for swirling and axial flow pneumatic conveying in a horizontal pipe was carried out with a Eulerian approach for the gas phase and a stochastic Lagrangian approach for particle phase, where particle-particle and particle-wall collisions were taken into consideration. The k-? turbulence model is used to characterize the time and length scales of the gas-phase turbulence. Models are proposed for predicting the particle source and additional pressure loss. The numerical results are presented for polyethylene pellets of 3.1 mm diameter conveyed through a pipeline of 13 m in length with an inner diameter of 80 mm, solid mass flow rate was 0.084 kg/s, and gas velocity was varied from 10 m/s to 18 m/s. The particle flow patterns, the particle concentration and the particle velocity, and additional pressure loss were obtained. It is found that the particle velocity and concentration has almost same value along flow direction in swirling flow pneumatic conveying. The profile of particle concentration for swirling flow pneumatic conveying exhibits symmetric distribution towards the centerline and the higher particle concentration appears in neighbor of wall in the acceleration region. At downstream, the uniform profile of particle concentration is observed. The particle velocity profile, on the other hand, is uniform for both swirling and axial flow pneumatic conveying. A comparison of the calculations with the measured data shows a good agreement within an average error of less than 15 percent.     

CHARACTERISTICS OF A HORIZONTAL SWIRLING FLOW PNEUMATIC CONVEYING WITH A CURVED PIPE

In order to prevent flow blockage phenomenon and to reduce the impact of particles on the wall of the bend, an experimental study of the swirling flow pneumatic conveying system with a horizontal curved pipe was carried out in this work. The experiment was performed in a 90-deg pipe bend with pipe diameter 75 mm and centerline curvature ratio 12. The straight pipes with 75 mm inside diameter at the upstream and downstream of the bend were 1.3 m and 4.0 m in lengths, respectively. The initial swirl number was varied from 0.22 to 0.60, the mean air velocity from 10 to 20 m/s, and the solid mass flow rate from 0.07 to 0.68 kg/s. It is found that in the lower air velocity range, the overall pressure drop of the swirling flow pneumatic conveying shows a lower tendency than that of axial flow pneumatic conveying. The minimum air velocities can be decreased by using the swirling flow pneumatic conveying. From the visualization of particle flow patterns, the impact of particles on the wall of the bend can be reduced using the swirling flow.     

CHARACTERISTICS OF A HORIZONTAL SWIRLING FLOW PNEUMATIC CONVEYING WITH A CURVED PIPE

ABSTRACT

In order to prevent flow blockage phenomenon and to reduce the impact of particles on the wall of the bend, an experimental study of the swirling flow pneumatic conveying system with a horizontal curved pipe was carried out in this work. The experiment was performed in a 90-deg pipe bend with pipe diameter 75 mm and centerline curvature ratio 12. The straight pipes with 75 mm inside diameter at the upstream and downstream of the bend were 1.3 m and 4.0 m in lengths, respectively. The initial swirl number was varied from 0.22 to 0.60, the mean air velocity from 10 to 20 m/s, and the solid mass flow rate from 0.07 to 0.68 kg/s. It is found that in the lower air velocity range, the overall pressure drop of the swirling flow pneumatic conveying shows a lower tendency than that of axial flow pneumatic conveying. The minimum air velocities can be decreased by using the swirling flow pneumatic conveying. From the visualization of particle flow patterns, the impact of particles on the wall of the bend can be reduced using the swirling flow.     

A Numerical Simulation of Swirling Flow Pneumatic Conveying in a Vertical Pipeline

A numerical prediction for the axial and swirling pneumatic conveying in a vertical pipe was performed based on an Eulerian approach for the gas and a stochastic Lagrangian approach for the particles, where κ – ? turbulence model, the model of particle-particle and particle-wall collisions, was adopted. The numerical results are presented for polyethylene pellets of 3.2mm diameter conveyed through a pipeline of 12m in height with an inner diameter of 80mm. The initial swirl number was 0.0 and 0.68, the mean gas velocity varied from 11 to 17m/s, and the solid mass flow rate was 0.03 and 0.084 kg/s. From the numerical analysis, the swirl decay of the swirling gas-solid flow was found to be rapid in the acceleration region and approached the clean swirling flow in a fully developed region. The turbulent kinetic energy and energy dissipation rates of the swirling gas-solid flow increased near the wall and reduced in other regions. The comparison of predicted values with measured data showed a good agreement.     

Measurements of Particle Velocity and Concentration for Dilute Swirling Gas-Solid Flow in a Vertical Pipe

Experimental studies concerning the characterization of a dilute swirling gas-solid flow were carried out in a vertical pipe with a height of 12 m and an inner diameter of 80 mm. Polyethylene pellets, with mean diameter of 3.2 mm, were used as test particles. The initial swirl number varied from 0.0 to 0.94, the mean gas velocity varied from 9 to 25 m/s, and the solid-gas ratio varied from 0.2 to 0.7. In this study, the particle velocity and concentration profiles were measured by the photographic image technique for both nonswirling (axial) and swirling gas-solid flows. It was found that the particle velocity of the swirling flow is lower than that of the axial flow in the range of high gas velocity; however, high particle velocity in the former flow can be obtained in the range of low gas velocity. The particle velocity profiles, on the other hand, were found to be nearly uniform in both the swirling and axial flows. The particle concentration profiles in the swirling flow exhibited symmetric distributions with respect to the pipe axis, and a higher particle concentration appeared in the vicinity of the wall located in the acceleration region.

gas-solid two-phase flow particle concentration particle velocity pipeline swirling flow     

Motion of Large Particles in a Horizontal Pneumatic Pipe

The horizontal pneumatic transport of large particles with particle to pipe diameter ratio of 0.6 and particle densities of 928 kg/m³ and 2193 kg/m³ was examined experimentally and numerically. The pipe diameter and length were 10 mm and 8.8 m, respectively. The mean air velocity was between 14.2 m/s and 23.0 m/s and the number feed rate of particle was almost constant at seven per second. In this study, the method of characteristics was used for the simulation of gas flow, which considered not only the particle-particle collisions but also the particle-wall collisions. It is found that particle transport is possible even when the mean air velocity is smaller than the terminal settling velocity of particle's and that the arrival time intervals at the downstream section are not always uniform although the particles are fed uniformly. Furthermore, the velocity difference between different density particles becomes small as the mean air velocity decreases, because the particle velocities become uniform due to particle-particle collisions, and the ratio of particle velocity to the mean air velocity is almost independent of air velocity. In addition, it is shown that the particle-wall collision at the pipe joint due to pipeline misalignment can be one of the sources of bouncing motion of particles as shown by simulation results.     

Numerical Study of Air Compressibility During Horizontal Pneumatic Transport

Using numerical simulations, the effect of the compressibility of air on the flow pattern of particles and pressure drop in the presence of particles during horizontal pneumatic transport operating under negative pressure was examined. The length and inside diameter of the pipeline were 30 m and 40 mm, respectively, and the chosen particles (4 mm in diameter) had densities of ρ_p = 1000 and 2000 kg/m³. The mean air velocities at pipe the inlet were U_inlet = 19, 22, and 28 m/s, and the range of the mass flow rate ratios of particle to air, μ, was varied up to 2.0. For a given inlet air velocity, the difference in the flow pattern between compressible and incompressible flow calculation is generally small. For ρ_p = 1000 kg/m³ particles the additional pressure drop in compressible flow increases when μ is above 0.5 and U_inlet is 28 m/s, μ is above 1.3 and U_inlet is 22 m/s, and μ is above 1.5 and U_inlet is 19 m/s. In these cases, the particle flow pattern is homogeneous. For ρ_p = 2000 kg/m³ particles, the pressure drop increases only when μ is above 1.5 and U_inlet is 28 m/s. The difference is not noticeable when the particle flow pattern is heterogeneous. Also, the difference in the additional pressure drop is much larger during homogeneous flow than heterogeneous flow.     

Effect of Particle Properties on Slug Flow Conveying in a Horizontal Pneumatic Pipeline

The influence of particle properties on slug flow conveying was experimentally examined by using polyethylene particles of different densities from 825 kg/m³ to 945 kg/m³ in a horizontal pipeline 5.5 m in length, inside diameter of 32 mm, for air speeds below 8 m/s. It was found that hardness affects the slug flow conveying in such a way that for soft particles lower limiting velocity as well as boundary air velocities for suspension flow and slug flow increases. Additionally, it was found that the frictional characteristics of a particle influence its flow pattern. Also, there are two types of slug flow, that is, a solitary slug flow and a consecutive slug flow. In a solitary slug flow, there is at most only one plug in the pipeline. In a consecutive slug flow, the particles are conveyed continuously as slugs. There is always at least one slug in the pipeline.     

Simulation of Gas-Solid Flow in Vertical Pipe by Hard-Sphere Model

This article presents a two-dimensional study of the gas-solid flow in a vertical pneumatic conveying pipe by means of a hard-sphere model where the motion of individual particles can be traced. Simulations were performed for a pipe of height 0.9 m and width 0.06 m, with air as gas phase and particles of density 900 kg/m³ and diameter 0.003 m as solid phase. Periodic boundary conditions were applied to the solid phase in the axial direction. Different cases were simulated to examine the effects of the number of particles used, superficial gas velocity, and restitution coefficient. The results show that the main features of plug flow can be reasonably captured by the proposed simulation technique. That is, increasing the number of particles in a simulation will increase the length of plugs but does not change the velocity of plugs; the solid fraction of a plug is relatively low if the number of particles is small. In particular, it is shown that increasing superficial gas velocity will increase the velocity of plugs and the frequency of plugs, and the pressure drop through a rising plug increases linearly with the plug length, suggesting that the total pressure of a conveying system with a given length can be quantified from the information of plug length and plug frequency. Increasing the restitution coefficient can promote the momentum transfer between particles and hence the raining down of particles from the back of a plug in vertical pneumatic conveying. The simulation offers a useful technique to understand the fundamentals governing the gas-solid flow under pneumatic conveying conditions.     

An Experimental Study on Degradation of Maize Starch During Pneumatic Transportation

Although attrition during pneumatic conveying is a common problem, very few publications can be found in the open literature on this subject. The particle-to-wall impact is perhaps the predominant cause of degradation since the particle impinges the wall surface at high velocities in dilute phase pneumatic conveying. The most important factors appear to be the conveying air velocity and moisture content. This article presents the experimental findings of a study on degradation of maize starch during pneumatic conveying process. The tests were carried out in a conveying setup having a pipe length of approximately 50 m and a pipe inner diameter of 50 mm in order to find out the breakage of particles under various airflow velocity conditions and temperatures. Dehumidified air was used during the experimentation, and the air temperatures used during these test were 100°C and 25°C. The experimental results indicated that for a given air temperature condition, the variation of attrition rate was a complex function of air velocity and solids loading ratio. Further, for any start pressure condition, the attrition rate was found to increase substantially with increase in air temperature.     

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACT

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.     

Wavelet Multi-Resolution Cross-Correlation Analysis of Swirling Gas-Solid Flow in a Horizontal Pipe

A wavelet multi-resolution cross-correlation analysis was developed and applied to experimental pressure-time signals in order to analyze the characteristics of swirling gas-solid flow in both Fourier and physical spaces. The experiment was carried out in a horizontal pipe with a length of 7.5 m and an inner diameter of 76 mm. The initial swirl number based on the total inflow was varied from 0.0 to 0.61, the mean gas velocity was varied from 6 to 28 m/s, and the solid mass flow rate was varied from 0.08 to 0.5. From the wavelet multi-resolution correlation analysis of the fluctuating pressure in the range of low air velocity, the characteristics of swirling gas-solid two-phase flows were extracted at various frequencies. Much stronger correlations were found in the range of low frequency, which implied periodic motion of dunes and sliding clusters. Additionally, it was revealed that the motion of a large cluster sliding flow contains two smaller clusters and the moving velocities of dunes were 1 m/s and 2 m/s, respectively. However, no correlation existed at smaller scales of correlation features, which indicated heterogeneous suspension flow.     

CFD Studies of Indoor Airflow and Contaminant Particle Transportation

This article presents a numerical study of indoor airflows and contaminant particle transportation in three ventilated rooms. The realizable k - ε model is employed to model the air-phase turbulence, while the Lagrangian particle tracking model is utilized for the particle-phase simulation. The predicted air-phase velocities and contaminant particle concentrations are validated against the experimental data obtained from the literature. In the first case, the realizable k - ε model successfully captures the flow trend and reasonably predicts the airflow velocity. The realizable k - ε model under-predicts the vertical air velocities along the vertical inlet jet axis by 11% at x = 0.219 m, which is slightly better than the standard k - ε model error of 17%. In a two-zone room case, the realizable k - ε model, combined with a Lagrangian particle tracking model, predicts the particle concentration decay with the highest normalized difference being 24%. In the third case, the influence of particle size, location of particle resource, and particle-wall collision on the particle concentrations is investigated by the realizable k - ε model and the Lagrangian model. It is found that for relatively small particles (diameter ≤ 10 μm), the particle concentration may be insensitive to the particle diameter. In addition it has been observed that the particle-collision model may have considerable effect on the particle concentration prediction.     

An Investigation into Pressure Fluctuations and Improved Modeling of Solids Friction for Dense-Phase Pneumatic Conveying of Powders

The aim of this paper is to investigate into flow mechanism with the help of pressure signal fluctuations analysis and modeling solids friction in case of solids–gas flows for fluidized-dense-phase pneumatic conveying of fine powders. Materials conveyed include fly ash (median particle diameter 30 µm; particle density 2300 kg m^?3; loose-poured bulk density 700 kg m^?3) and white powder (median particle diameter 55 µm; particle density 1600 kg m^?3; loose-poured bulk density 620 kg m^?3). These were conveyed in different flow regimes varying from fluidized-dense-to-dilute phase. To obtain information on the nature of flow inside pipeline, static pressure signals were studied using technique of Shannon entropy. Increase in the values of Shannon entropy along the flow direction through the straight-pipe sections were found for both the powders. However, drop occurred in the Shannon entropy values after the flow through bend(s). Change in slope of straight-pipe pneumatic conveying characteristics along the flow direction is another factor which provided indication regarding change in flow mechanisms along the flow. A new technique for modeling solids friction factor has been developed using a solids volumetric concentration and ratio of particle terminal settling velocity to superficial air velocity by replacing the conventional use of solids loading ratio and Froude number, respectively. The new model format has shown promise for predictions under diameter scale-up conditions.     

Optical Probe for the In-Line Determination of Particle Shape, Size, and Velocity

A laboratory optical probe was developed to simultaneously determine the following particle characteristics: circularity, particle projection area, equivalent diameter of a circle, length of the particle outline or perimeter, maximum chord length, aspect ratio, and particle velocity. Using the projection area and the perimeter, the particle shape factor circularity can be determined. The aspect ratio was approximated by the ratio of the equivalent diameter to the maximum chord length. The basic measuring principle is multi-point scanning of the particle shadow image by a line of optical fibers. In addition, the particle velocity can be measured by a differential spatial filter of optical fibers. These fibers are step index fibers with a core diameter of 64 µm and cladding of 70 µm. The shadow image of a single particle was generated by a parallel laser beam. The uncertainty of the measured circularity and aspect ratio was investigated by using metal wires with diameters of 0.12 to 0.5 mm as test particles with known circularity and aspect ratio. The standard deviations were 1.9% for the circularity and 15.5% for the approximated aspect ratio. In addition, the optical probe system was investigated by measurements of solid particles with different shapes. As an example, the results of sand, marjoram seed, and metallic oxide particles are shown. Using 1000 sand particles, the correlation between equivalent diameter and particle velocity could be demonstrated. The presented configuration of the optical probe is applicable in the size range of 0.1 to 0.9 mm and up to a particle velocity of 5 m/s.     

Modeling of the Transient Particle Velocity Distribution in the Fluidized Bed Combustor (FBC) Riser

The geometrical distributions of transient particle velocity in a fluidized bed combustor (FBC) riser are critical to FBC design and manufacturing. Particle image velocimetry (PIV) instrumentation was applied to visualize the particle transient movement in the area of interest (AOI) of an experimental cold model (152 mm ID × 610 mm height) of an FBC. Sixteen (16) PIV particle velocity profiles were generated for the AOI in a 1.6-second time period. A ten (columns) × five (rows) grid was set for each profile. Three levels of fluidizing air velocity were set for the experiments at 2.477 m/s, 2.677 m/s, and 2.823 m/s respectively. A knowledge-based regression method was applied to generate the empirical model of the two-dimensional particle transient velocity with the consideration of four independent variables, x (x coordinate), y (y coordinate), V_g (fluidizing air velocity), and t (time). This model had a data fitting accuracy of 88%. In addition, this model had a very good validation performance with the data obtained from other tests at different experimental setups.

The modeling method is applied for the first time to the particle movement in FBC risers. The research work proved that the design of the experiment and regression analysis are very effective and practical for evaluating experimental conditions and analyzing experimental results in FBC systems.     

GAS HOLDUP IN BAFFLED BUBBLE COLUMNS OF DILUTE SLURRIES OF FINE POWDERS AND VISCOUS LIQUIDS

Experimental gas phase holdup data reported by the authors as taken on two baffled bubble columns and involving slurries of fine powders (average particle diameter ≤90 μm) and dilute suspensions (mass fraction ≤ 20 percent) in viscous fluids are re-examined after correction for a small calculation error in superficial gas velocity. The two bubble columns are: a Plexiglas bubble column, 0.108 m in diameter and 2.25 m tall, equipped with seven 19 mm tubes arranged in equilateral triangular configuration with a pitch of 36.5 mm and a Pyrex glass column, 0.305 m in diameter and 3.24 m tall, equipped with thirty-seven 19 mm tubes arranged in the same configuration. Air and nitrogen are used as gas phase, water and Therminol as liquid phase, and iron oxide powders, glass beads and sand as solid phase. The small column data are at ambient temperature while those belonging to the large column extend from ambient to 473K. These data are examined to assess the influence of column diameter (scale-up), temperature, slurry concentration and viscosity, and superficial gas velocity on gas holdup for baffled columns.

It is found that in the 0.108 m diameter column the holdup is about the same for axial probes of different diameters, 19 mm, 31.8 mm and 50.8 mm, for two-phase systems involving liquids of small (water) and large (Therminol) viscosities. However, when a seven-tube bundle is installed, the holdup increases. This is consistent with the bubble dynamics and observed bubble sizes. This qualitative trend is also upheld by three-phase systems involving dilute slurries of fine powders. In the larger column when fully packed with a thirty-seven tube bundle, the holdup is found to be the same as for the small column. A loosely packed bundle gives smaller holdup at temperatures greater than the ambient due to the larger size of bubbles. The gas holdup is greater for a less viscous system and this is again due to the larger size of bubbles in a more viscous system. The influence of temperature is pronounced and is very characteristic of the nature of liquid involved. For example for water and Therminol the variations are in opposite directions with change in temperature. Bubble splitting and foaming control the nature of these dependencies. With the addition of solids the holdup is almost insignificantly altered at all temperatures.     

Experimental and Simulation Studies of Dilute Horizontal Pneumatic Conveying

Dilute horizontal pneumatic conveying has been the subject of this experimental and numerical study. Experiments were performed utilising a 6.5 m long, 0.075 m diameter horizontal pipe in conjunction with a laser-Doppler anemometry (LDA) system. Spherical glass beads with three different sizes 0.8–1 mm, 1.5 mm, and 2 mm were used. Simulations were carried out using the commercial discrete element method (DEM) software, EDEM, coupled with the computational fluid dynamics (CFD) package, FLUENT. Experimental results illustrated that, for mass solid loading ratios (SLRs) ranging from 2.3 to 3.5, the higher the particle diameter and solid loading ratio, the lower the particle velocity. From the simulation investigations it was concluded that the inclusion of the Magnus lift force had a crucial influence, with observed particle distributions in the upper part of the conveying line reproducible in the simulation only by implementing the Magnus lift force terms in the model equations.     

PARTICLE CONCENTRATION MEASUREMENTS BY LASER IMAGING FOR A TURBULENT DISPERSION

A laser imaging system has been developed which can be used for investigating the particle concentration variation in explosive test apparatus such as the Ciba-Geigi and Hartmann Bomb during turbulent dispersions of air-particle mixtures. The pulsed UV (337 nm) laser imaging system using a 500X optical and electronic magnification system has a measurement volume of 900 µm by 675 µm and an in-focus depth of field of 780 µm for a 32 µm diameter particle. Particles in the measurement control volume are imaged every 33 ms during the dispersion process and viewed in real time but stored for later analysis on a video tape system.

This paper presents the results of investigating the lycopodium particle concentration variations during the dispersion process of 0.200 grams of lycopodium particles in the Hartmann Bomb explosive test apparatus. Data were taken at the center line and at a radius ratio of 0.5 at a height of 0.102 m (4 inches) above the base of the Hartmann lucite tube. Twenty-five separate dispersions were made at each radius ratio and were based on a reservoir pressure of 103 kPa (15 psig) and 0.200 g of lycopodium powder. The average number of lycopodium particles based upon 25 dispersions at 33 ms intervals in the 473.9 × 10⁶ m³ control volume are reported for a total elapsed time of 15 seconds. The maximum average particle concentration observed was 6.4 particles at 133 ms for r/R = 0.0 and 6.5 particles in 333 ms for r/R = 0.5. Based upon uniform dispersion model for 0.200 g of lycopodium powder, 6.8 particles per control volume, would be expected. The time averaged data followed a Poisson Distribution for each time increment after 0.73 s for both radius ratios of r/R = 0.0 and 0.5 (based upon 95% confidence interval and Kolmogorov-Smirnov test). Data from 0 to 0.73 seconds could not be assigned confidence levels as the data did not follow a Poisson Distribution or any other known statistical distribution. No significant particle agglomeration was observed for the dispersion of lycopodium particles. In any one dispersion the number of lycopodium particles in the control volume was observed to vary widely during each 033 second measurement cycle.

To further investigate the particle dispersion, the flow pattern characteristics in the Hartmann dispersion apparatus were studied using flow visualization techniques based upon a matched Reynolds number (3.13 × 10⁶) dispersion of fluorescent dye by turbulent water injection. The matched Reynolds number flow visualization work further indicated the Hartmann Bomb dispersion method produces local pockets of nonuniformly-mixed mixtures during initial stages of the dispersion process, and this work further points out the shortcomings of integrating optical probes.     

Large-Eddy Simulation (LES) of Large Hydrocyclones

Fluid dynamics theory applied to the hydrocyclone flow field has progressed in past experimental and theoretical investigations from a two-dimensional Prandtl mixing-length turbulence modeling to the current level of three-dimensional large-eddy simulation (LES) and differential-stress approaches. The LES approach eliminates the explicit empiricism that is imposed in the κ-ε model. Turbulence in hydrocyclones is anisotropic. Since only the subgrid scales are modeled in LES, the anisotropy is largely taken care of in this approach. This article presents validation of the LES model with laser-anemometry data collected on 75 mm and 250 mm hydrocyclones. Verification with experimental data on mass split, axial velocity, tangential velocity, root-mean-squared velocity, air-core profile, and size classification clearly stands as a proof that LES can be applied to even larger hydrocyclones.