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.     

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

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-ε 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.     

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.     

DISTRIBUTIONS OF THE TANGENTIAL VELOCITIES ON THE DUST LADEN GAS FLOW IN THE CYLINDRICAL CYCLONE DUST COLLECTOR

The authors measured the distributions of the tangential velocities of the feed dust concentration C_o = 0. 6 ~15. 0 g/ m³ by the hot wire anemometers. The test dust was fly-ash which had the mean diameter Xpn = 2.03 μm. The used cylindrical cyclone had the diameter D1 = 140 mm, the diameter of the exit pipe D2 = 50 mn and the total length HT = 382 mm. In order to measure the fluctuating velocities and the time mean velocity on the dust Iaden gas flow and on the pure air flow, we had applied two kinds of the hot-wires. The flow Reynolds number Rec was Rec = Qo/HiV = 797~5582. The expert mental results sho-ed that there were no distinct differences of the time mean and fl uctuating velocities between the pure air flow and the dust laden gas flow. Then the authors have tried to explain those expert mental results by the response time of the fine solid particles and that of the scale of the nixing length in the turbulent rotational air flow.     

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.     

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.     

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.     

PREDICTION OF HEAT TRANSFER IN A TURBULENT GAS-SOLID PIPE FLOW USING A TWO-FLUID MODEL

The dynamic and thermal behaviour of a turbulent gas-solid flow in a vertical pipe has been numerically studied by means of an original two-fluid approach which includes the effects of particle-particle and particle-wall collisions as well as particle-turbulence interactions. A k-ε-k_p model for the flow dynamics has been combined with the energy balances of each phase. Comparisons have been carried out with available experimental data, first for the dynamic validation, and then for the thermal portion of the problem, in the case of rather large panicles (200 to 500 µm). Heat transfer from wall to suspension is seen to be strongly affected by particles. As a consequence, a suspension Nusselt number decrease of 20% may be observed at loading ratio smaller than two. Further increase is possible if more panicles are injected. Predictions had excellent agreement with experimental data for 500 µm particles. Slight discrepancies appear for smaller particles, probably due to more complex, partially understood panicle-turbulence interactions.     

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.     

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.     

CONTRIBUTIONS OF ROAD DUST RESUSPENSION TO THE AIRBORNE PARTICLE CONCENTRATIONS IN TAIPEI

The high concentrations of airborne particles have been one of the main air pollution problems in Taipei, Taiwan. In this study, the possible sources of airborne particles were investigated using concentration profile and chemical composition. The vertical concentration profile of TSP was measured at 1.5 m, 11 m, and 38 m above ground. The concentrations of TSP at 1.5 m above ground are always greater than 300 µg/m³; and even up to 1230 µg/m³, while those at 11 m above ground are only half of these values. The concentrations at 38 m above ground are only about half of those at 11 m above ground. Similar concentration profiles are found for Mg, Ca, Pb, Fe, and Zn measured and the enrichment factors with respect to the composition of road dust are generally less than 3. Therefore, there is a not upward flux of airborne particles from ground. The net upward fluxes are estimated to be about 100 ton/yr/km¹ from the vertical concentrations profiles and particle size distributions. The particle resuspension rates due to wind flow from a Teflon filter were also determined at the same time. The resuspension rates of particles by wind were found to be on the order of 10^-6 to 10^-5sec^-1. These particle resuspension fluxes are much smaller than those calculated by concentration profile. Other mechanisms, e.g. traffic-induced resuspension, are needed to be included in further study.     

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.     

Numerical prediction of particle erosion of pipe bends

In the present study the Euler/Lagrange approach in combination with a proper turbulence model and full two-way coupling is applied for erosion estimation due to particle conveying along a horizontal to vertical pipe bend. Particle tracking considers both particle translational and rotational motion and all relevant forces such as drag, gravity/buoyancy and transverse lift due to shear and particle rotation were accounted for Laín and Sommerfeld (2012). Moreover, models for turbulent transport of the particles, collisions with rough walls and inter-particle collisions using a stochastic approach are considered Sommerfeld and Laín (2009). In this work, the different transport effects on spherical solid particle erosion in a pipe bend of a pneumatic conveying system are analysed. For describing the combined effect of cutting and deformation erosion the model of Oka et al. (2005) is used. Erosion depth was calculated for two- and four-way coupling and for mono-sized spherical glass beads as well as a size distribution of particles with the same number mean diameter (i.e. 40?μm). Additionally, particle mass loading was varied in the range from 0.3 to 1.0. The erosion model was validated on the basis of experiments by Mazumder et al. (2008) for a narrow vertical to horizontal pipe system with high conveying velocity. Then a 150?mm pipe system with 5?m horizontal pipe, pipe bend and 5?m vertical pipe with a bulk velocity of 27?m/s was considered for further analysis. As a result inter-particle collisions reduce erosion although the wall collision frequency is enhanced Sommerfeld and Laín (2015); additionally, considering a particle size distribution with the same number mean diameter as mono-sized particles yields much higher erosion depth. Finally, when particle mass loading is increased, bend erosion is reduced due to modifications of particle impact velocity and angle, although wall collision frequency grows.     

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.     

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.     

ROOM-TYPE AIR CLEANER DESIGN CRITERIA AND PROTOTYPE PERFORMANCE

Two rules-of-thumb for minimum performance of a room-type air cleaner have been developed from consideration of a first order model for room air quality. By adopting a criterion that the use of an air cleaner should cause the particle concentration to be at least cut in half, the rule-of-thumb for a room with no smokers is that the product of filter efficiency and flow rate should be ≥.8 m³/min (≥30 cfm). If the particle concentration is dominated by smokers or other sources, the product of filter efficiency and filter flow rate should be = m³ /min (= 100 cfm)

Tests were conducted to determine the efficiencies of candidate filter media. The selected media, Filtrete G-0115, has a fractional efficiency for 1 μm particles of 97 percent when clean, and an efficiency of 78 percent when fully loaded. This drop in efficiency is due to the masking of the electrets on the surfaces of the filter fibers.

A fibrous filter room-type air cleaner was designed to perform in accordance with the rules-of-thumb. When operated with a clean filter, the maximum flow rate is 3.2 m³/min and, when operated with a fully loaded filter, the maximum flow rate is 1.8 m³/min. The system has a multispeed fan which will provide lower flow rates.     

PREDICTION OF VERTICAL LIQUID SOLID PIPE FLOW USING MEASURED CONCENTRATION DISTRIBUTION

The performance of a standard (k-L) turbulence model for predicting slurry pipeline pressure gradients is examined using experimental measurements of velocity and solids concentration distribution for upward vertical flows. Assuming the solids concentration rises abruptly from a value of zero at the pipe wall over a distance corresponding to one half a particle diameter, satisfactory predictions were obtained for sand particles of median diameter up to about 800 um in a 25.8 mm vertical pipeline. No damping of fluid turbulence or additional stresses due to particle-particle interactions were included in the computational model. Velocity distributions are found to be flatter than those of turbulent single-phase flows, because of the variation of mixture properties in the vicinity of the pipe wall.     

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.     

LOCAL EFFICIENCY MEASUREMENTS APPLICABLE TO BOTH AUTOMOTIVE ENGINE AND CABIN FILTRATION

Presented herein are velocity and particle concentration profiles that were measured in planes just upstream and downstream of an engine air intake filter (AF3192) with a Laser Doppler Velocimeter. These detailed measurements allow the computation of local filter efficiency, with overall efficiency being an integral of the local efficiencies. Measurements were performed for nominal air flow rates ranging from 34 m³/hr to 268 m³/hr with monodisperse polystyrene latex spheres of 0.966, 5.3 and 10.2 micron diameters. Two test housings were employed in the research -- the SAE J726 housing (for engine tests) and a small angle diffuser housing that is similar to the SAE J1669 housing (for cabin filtration tests). The SAE housing generally had a higher filtration efficiency than the small angle diffuser housing, and the SAE housing overall efficiency's variation with air flow rate was flatter than that of the diffuser housing. As expected from Stokes number considerations, the large particles had higher filtration efficiencies than the small particles, and the overall filtration efficiencies of 30% to 80% were within the range predicted by theoretical models and gravimetric testing. Work will continue with more particle sizes, SAE dust, various contaminant loading (dirty filters), charged/charge neutralized systems, and housings more representative of actual installed [in the automobile] geometries.     

Design and Evaluation of Four-Stage Low-Pressure Cascade Impactor Using Electrical Measurement System

The low-pressure cascade impactor has been used to collect ultrafine particles that cannot be measured by conventional cascade impactors. Low-pressure cascade impactors resemble ordinary impactors, but are operated at reduced pressures of 0.05 ∼ 0.4 atm. Many kinds of low-pressure impactors have been developed by different researchers. However, it is still difficult to accurately design and evaluate the low-pressure cascade impactor.

In this study, a four-stage low-pressure cascade impactor for measuring the size distribution of submicron aerosol particles was designed and evaluated. To evaluate particle collection efficiency of each stage, an electrical measurement system was used. The cut-point diameters of Stages 1 through 4 were 0.238, 0.173, 0.111, and 0.063 μm in aerodynamic diameter. Stage 2 showed poor steepness of the collection efficiency curve and larger cut-point Stokes number than theory, which may be attributed to high nozzle velocity. The fluorometric method for particle collection efficiency measurement was shown to be unreliable for ultrafine particles.

The solid particle collection efficiency of the designed impactor was examined with different substrate conditioning methods. Porous metal substrate and silicon-coated substrate were tested with NaCl particles. It was shown that silicon coating did not effectively reduce the particle bounce because of high nozzle velocity, whereas the porous metal substrate considerably enhanced the particle collection efficiency.