Investigating Straight-Pipe Pneumatic Conveying Characteristics for Fluidized Dense-Phase Pneumatic Conveying

This article presents results from an investigation into the pneumatic conveying characteristics (PCC) for horizontal straight-pipe sections for fluidized dense-phase pneumatic conveying of powders. Two fine powders (median particle diameter: 30 and 55 µm; particle density: 2300 and 1600 kg m^?3; loose-poured bulk density: 700 and 620 kg m^?3) were conveyed through 69 mm I.D. × 168 m, 69 mm I.D. × 148 m, 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipelines for a wide range of air and solids flow rates. Straight-pipe pneumatic conveying characteristics obtained from two sets of pressure tappings installed at two different locations in each pipeline have shown that the trends and relatively magnitudes of the pressure drops can be significantly different depending on product, pipeline diameter and length and location of tapping point in the pipeline (indicating a possible change in transport mechanism along the flow direction). The corresponding models for solids friction factor were also found to be different. There was no distinct pressure minimum curve (PMC) in any of the straight-pipe PCC, indicating a gradual change in flow transition (change in flow mechanism from dense to dilute phase). For total pipeline conveying characteristics, the shapes of the PCC curves and the location of the PMC were found to be significantly influenced by pipeline layout (e.g., location and number of bends) and not entirely by the dense-to-dilute-phase transition of flow mechanism. Seven existing models and a new empirically developed model for PMC for straight pipes have been evaluated against experimental data.     

Modeling Dense-Phase Pneumatic Conveying of Powders Using Suspension Density

This article presents results of an investigation into the modeling of pressure drop in horizontal straight pipe section for fluidized dense-phase pneumatic conveying of powders. Suspension density and superficial air velocity have been used to model pressure drop for two-phase solids-gas flow. Two applicable models formats (developed by other researchers using two different definitions of suspension density) were used to represent the pressure drop due to solids-gas flow through straight pipe sections. Models were generated based on the test data of conveying power-station fly ash and electrostatic precipitator (ESP) dust (median particle diameter: 30 and 7 µm; particle density: 2300 and 3637 kg m^?3; loose-poured bulk density: 700 and 610 kg m^?3, respectively) through a relatively short length of a smaller diameter pipeline. The developed models were evaluated for their scale-up accuracy and stability by using them to predict the total pipeline pressure drop (with appropriate bend model) for 69 mm I.D. × 168 m; 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipes and comparing the predicted versus with experimental data. Results show that both the models with suspension density and air velocity generally provide relatively better prediction compared to the conventional use of solids loading ratio and Froude number. For fly ash, the two formats result in considerable different predictions, whereas they provide relatively similar results for ESP dust.     

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.     

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.     

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.     

Atmospheric Fine and Coarse Mode Aerosols at Different Environments of India and the Bay of Bengal During Winter-2014: Implications of a Coordinated Campaign

In this paper, we present mass concentrations of particulate matter [PM_2.5, PM₁₀ size fractions and total suspended particulates (TSP)] measured simultaneously over land stations (Kullu, Patiala, Delhi, Ajmer, Agra, Lucknow, Varanasi, Giridih, Kolkata, Darjeeling, Jorhat, Itanagar, Imphal, Bhubaneswar, and Kadapa), mostly distributed across the Indo-Gangetic plain (IGP) of India as well as in the marine atmosphere over Bay of Bengal (BoB) in the period from 20 January to 3 February, 2014. The main objective of this study was to quantify the continental outflow of particulates (PM_2.5, PM₁₀ and TSP) from IGP and associated regions into the BoB along with low level north-east wind flow during winter monsoon period. The present study provides a glimpse of the aerosol loading over the IGP region. During this campaign, the highest average PM_2.5 (187.8 ± 36.5 µg m^?3, range 125.6–256.2 µg m^?3), PM₁₀ (272.6 ± 102.9 µg m^?3, range 147.6–520.1 µg m^?3) and TSP (325.0 ± 71.5 µg m^?3, range 220.4–536.6 µg m^?3) mass concentrations were recorded at Varanasi, Kolkata and Lucknow over middle and lower IGP regions. The PM_2.5 (average 41.3 ± 11.9 µg m^?3; range 15.0–54.4 µg m^?3), PM₁₀ (average 53.9 ± 18.9 µg m^?3; range 30.1–82.1 µg m^?3) and TSP (average 78.8 ± 29.7 µg m^?3; range 49.1–184.5 µg m^?3) loading over BoB were found to be comparable to land stations and suggests possible continental outflow. Over the continental region, the highest PM_2.5/PM₁₀ ratio was recorded at Delhi (0.87). The PM_2.5/PM₁₀ ratio over BoB (0.77) was found to be quite high and comparable to Varanasi (0.80) and Agra (0.79).     

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.     

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.     

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

ABSTRACT

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.     

Solids Friction Factors for Fluidized Dense-Phase Conveying

There have been numerous correlations proposed for determining a solids friction factor ( λs ) for fully suspended (dilute phase) pneumatic conveying. Currently, there are no equivalent correlations that predict λs in nonsuspension dense-phase flows. In dense-phase conveying there are two basic modes of flow: plug/slug flow, which is predominantly based on granular products, and fluidized dense-phase flow, which is more suited to fine powders exhibiting good air retention capabilities. In plug/slug type flow, the stresses between the moving plug of material and the pipe wall dominate the solid-phase frictional losses. In fluidized dense-phase flow the frictional losses are characterized as a mixture of particle-wall and particle-particle losses but are heavily influenced by the gas-solid interactions. In this paper, a series of calculations were performed on experimental data in order to estimate λs for four types of material conveyed in the fluidized dense-phase flow regime. The solids frictional factors were found to be relatively independent of particle properties for varying air and solid mass flow rates and pressure drops. The resultant pressure drop from the empirical model showed good agreement with the experimental data.     

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.     

Evaluation of Scaleup Procedures Using “System” Approach for Pneumatic Conveying of Powders

This article presents results from an ongoing research effort aimed towards developing a validated scaleup procedure for pressure drop for the dense-phase pneumatic conveying of powders. Two existing/popular forms of the “system” approach for scaling up of diameter were evaluated. The validity of the current technique for length scaleup using a “system” approach was also examined. The existing method showed good potential for dilute-phase flow, but resulted in appreciable under-predictions when predicting for dense-phase flow. The effect of bends on the accuracy of the method was also investigated. In this study, steady-state conveying data of four different powders conveyed in various pipes (diameter/lengths) were used for the purpose of scaleup investigations.     

CFD modeling of dilute phase pneumatic conveying in a horizontal pipe using Euler–Euler approach

ABSTRACT

Computational fluid dynamics simulations were performed to investigate the behavior of dilute phase pneumatic conveying of plastic pellets in a horizontal circular pipe. The pellets are 200?µm in diameter and 1000?kg/m³ in density. A parametric study was performed to investigate the effects of turbulence model and model collision parameters on pressure drop, solid’s volume fraction and velocity profiles. Among model collision parameters, specularity coefficient has considerable effect on the pressure drop. Moreover, the results from simulations carried out for different solid loadings and velocities were compared with experimental data found in the literature. The air velocities range from 6 to 15?m/s and solids to air mass flow ratios range from 1 to 3. At higher air velocities, the pressure drops predicted by the standard k-omega turbulence model are higher than the pressure drops predicted by the standard k-epsilon model. In contrast, at lower gas velocities, the standard k-epsilon model predicts higher pressure drops compared to the standard k-omega turbulence model. However, no significant difference in solids and air velocity profiles is observed for the two different turbulence models.     

Modeling Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article results from an ongoing investigation aimed at developing a new validated test-design procedure for the accurate prediction of pressure drop for dense-phase pneumatic conveying of powders. Models for combined pressure drop coefficient (“K”) for solids-gas mixture were derived using the concept of “suspension density” by using the steady-state “straight pipe” pressure drop data between two different tapping locations of the same pipe and also for two different diameter pipes. It was observed that the derived models were different depending on the location of tapping points (for the same pipe) and selected pipe diameters. The derived models were then evaluated by predicting the pressure drop for pipelines with various diameters or lengths (69 mm I.D. × 168 m, 105 mm I.D. × 168 m, 69 mm I.D. × 554 m) for the conveying of power station fly ash. A comparison between the predicted pneumatic conveying characteristics (PCC) and the experimental plots showed that the models resulted in significant over-predictions. In the second part of the article, the “system” approach of scaleup was evaluated. “Total” pipeline pressure drop characteristics for test-rig pipelines were scaled up to predict the PCC for larger/longer pipes. It was found that the “system” approach generally resulted in grossly inaccurate predictions. It was concluded that further studies are needed for a better understanding of the solids-gas flow mechanism under dense-phase conditions.     

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.     

Particle size distribution control and related properties improvements of tungsten powders by fluidized bed jet milling

Fine grinding process of different particle size tungsten powders was carried out by fluidized bed jet milling. The results showed that the jet milling treatment caused deagglomeration of tungsten powders, which led to particles sufficient dispersion and narrow particle size distribution. Grinding gas pressure of 0.70 Mpa made the particles achieve high speed which promoted the particles collision contributing to particle dispersion and shape modification. For tungsten powder with particle size of 3 μm FSSS, a higher packing density with 5.54 g/cm³ was obtained, compared with the 3.71 g/cm³ of the original powder. For tungsten powder with particle size of 1 μm FSSS, the big agglomerates disappeared and the particle size distribution become narrower, while small aggregates about 2–3 μm still exist after the jet milling process. For tungsten powder with particle size of 5 μm and 10 μm FSSS, different medium diameter particle size and narrow particle size distribution of monodisperse tungsten powders can be produced by the optimized jet milling parameters. More important, the effective dispersion, favorable shape modification and precise classification have been achieved in the simple process.     

The Prediction of the Emissivity and Thermal Conductivity of Powder Beds

The particle size distribution and packing (loose bulk and tapped density) of a mixture of ground biomass from Douglas fir wood particles was characterized by different practical methods: sieving, digital imaging and scanning electron microscopy. The ground mixture was analyzed using a set of 14 wire mesh sieves. The calculated mean diameter of mixture was 251 µm. The mixture was divided into four size fractions of mean size ranging from 74 to 781 µm. Particle length measured by imaging technique were 3–4 times larger than the mean diameter determined by sieve analysis. Similarly, particle width was 1.0–2.5 times larger than mean particle diameter. The sphericity of particles in each of the four fractions increased with decreasing size of the sieve indicating that smaller particles also have a smaller aspect ratio. Empirical power law equations were developed to correlate the packing and flow ability of ground particles (HR) to the mean diameter, with R² values of 0.88 and 0.91, respectively. The HR values indicated good flow ability for the large particles and poor flow ability for the smallest particles and the entire mixture. HR and porosity ratio reached an asymptote for particles larger than 400 µm.     

Influence of Particle Size and Heating Rate on Decomposition of BC Dry Chemical Fire Extinguishing Powders

Pyrolysis of BC dry chemical fire extinguishing powders which are useful for Class “B” and Class “C” fires was conducted on a thermogravimetric analyzer with sample loading of 10–25 mg under dynamic air atmosphere. The effect of particle sizes (medium value 48.99, 27.24, 4.93 µm) and heating rates (10, 15, and 20°C min^?1) were examined. The pyrolysis kinetics of the samples was analyzed using a distribution activation energy model. It was found that the decomposition temperature decreased and the pyrolysis rate increased after the samples were milled. The agglomeration of particles during production did not have an appreciable influence on the pyrolysis process of the samples in our experimental conditions. The activation energy value was 77.13?219.78 kJ · mol^?1, 58.18?288.67 kJ · mol^?1, and 44.59?209.17 kJ · mol^?1 for the powder of particle size 48.99, 27.24, 4.93 µm. We should use micro powder in fire extinguishing.     

Thermal Conductivity of Magnesium Alloys in the Temperature Range from −125 °C to 400 °C

Magnesium alloys have been widely used in recent years as lightweight structural materials in the manufacturing of automobiles, airplanes, and portable computers. Magnesium alloys have extremely low density (as low as 1738 kg · m^?3) and high rigidity, which makes them suitable for such applications. In this study, the thermal conductivity of two different magnesium alloys made by twin-roll casting was investigated using the laser-flash technique and differential scanning calorimetry for thermal diffusivity and specific heat capacity measurements, respectively. The thermal diffusivity of the magnesium alloys, AZ31 and AZ61, was measured over the temperature range from ?125 °C to 400 °C. The alloys AZ31 and AZ61 are composed of magnesium, aluminum, and zinc. The thermal conductivity gradually increased with temperature. The densities of AZ31 and AZ61 were 1754 kg · m^?3 and 1777 kg · m^?3, respectively. The thermal conductivity of AZ31 was about 25 % higher than that of AZ61, and this is attributed to the amount of precipitation.     

A Simple Technique for Scaling Up Pneumatic Conveying Systems

A brief survey has shown that although scaling-up techniques in pneumatic conveying systems have generally been based on laboratory-scale test data, there still exists a divergence of opinions about the right choice of certain basic parameters such as solids friction factor and air friction factor. In this article, a simple model for pressure drop calculation has been proposed based on the classical Darcy's equation with some modifications. A parameter K, called pressure drop coefficient, has been shown to be independent of pipe diameter and hence suitable for scaling up to pipe sizes different from those used in laboratory-scale tests. For each of the bulk material and pipe size combinations used in this study, we calculated the standard deviation of predicted pressure values from the experimental values along the central 45° line passing through the origin; it varied from±165 mbar to a maximum±285 mbar. It has been shown that the model can be used for both horizontal and vertical pneumatic conveying.