TURBULENT RESISTANCE OF COMPLEX BED STRUCTURES

The influence of complex packing geometry on its frictional/hydrodynamic resistance was investigated both experimentally and theoretically. The complex bed structures were modelled using the cubic packing of spheres and different number of thin bands inserted between the spheres. The turbulent flow resistance of the model systems was determined experimentally on the basis of pressure drop and air flow rate measurements. In the theoretical approach these systems were regarded as compositions of two Representative Elementary Units (REU's) which contribute to the overall pressure drop within the apparatus. The values of the overall coefficient f₀, characterising resistance of the complex geometry structures were correlated with the values of the local coefficients f_i, describing resistance of the particular REU*s. The latter ones were independently estimated using the Computational Fluid Dynamics (CFD) code.     

Performance of nanofiber/microfiber hybrid air filter prepared by wet paper processing

High-performance air filters composed of a hybrid structure of nanofiber/microfiber were fabricated using wet paper processing. Two types of nanofibers (NF) with average diameters of 180 and 234?nm were mixed with a suspension of microfibers (11.5 and 11.7?µm) in various mixing fractions. Then, the suspension was filtered to fabricate hybridized fiber sheets with a known nanofiber/microfiber composition. The effects of NF diameter and mixing fraction on the performance of the hybrid filters were experimentally investigated. With increasing NF fraction, both the particle collection efficiency and the pressure drop increased. The quality factor (Q_f) was used to evaluate the performance of the prepared filters. As predicted by the single fiber filtration theory, the experimentally obtained Q_f was almost independent of the mixing fraction of the NF. The collection efficiency and pressure drop of the hybrid filters could be controlled by the NF fraction at the same Q_f. Moreover, the inhomogeneity factor of fiber packing (δ) did not significantly affect Q_f over the δ range from 3 to 23 for our filters. This implies that the lower particle capturing efficiency due to heterogeneous packing could be compensated by a decrease in the pressure drop, resulting in the same Q_f value. Therefore, Q_f for particles smaller than 100?nm, which are in the diffusion-controlled regime, can be increased by reducing the NF diameter.

Copyright © 2019 American Association for Aerosol Research     

Wall effect in laminar flow of non-Newtonian fluid through a packed bed

Experiments were conducted on laminar flow of non-Newtonian fluid through a packed bed of low column to packing particle diameter ratio (3.8) to elucidate the wall effect on pressure drop and mass flux. Carboxy methyl cellulose (CMC) at different concentrations was passed through the packed bed and the pressure drop was measured at different CMC concentrations and flow rates. It was found that the pressure drop increases with the increase in CMC flow rate. The pressure drop also increases with the increase in CMC concentration for a given flow rate. The friction factor is plotted against Reynolds number and the data for different CMC concentration are found to be scattered around a line expressed as f=1.03/Re^0.87. The tri-regional model of Cohen and Metzner [1] predicted correctly the mass flux in the packed bed at different pressure drop values and CMC concentrations with parameters K₀ (related to pore geometry) value of 1.5 and L_e/L (related to effective path length) value of 1.2, respectively.     

The behaviour of the dispersed phase in liquid-liquid cocurrent flow through a packed bed

The drop size distributions produced by the cocurrent flow of kerosene dispersed in water through a vertical column packed with spheres have been measured at different levels of flow rate, volume fraction dispersed phase, packing diameter and height of packed bed. The drop size measurements were made by isolating and photographing a portion of the dispersion as it emerged from the packing. The Sauter mean diameter is predicted by: where C is a constant, f(?) is the phase fraction effect, ∈ is the power input per unit volume, τ is the residence time and d_p is the packing diameter.     

Pressure drop in a mobile-bed contactor

The overall pressure drop and the vertical static pressure profile in a 0.14-m. diameter mobile-bed contactor have been investigated for two packing sizes and three packing heights. The gas and liquid flow rates were the two other variables studied. Foam polystyrene spheres were used as packings. The relationship between the static pressure and vertical position of the bed was found to be linear. A correlation of the drag parameter with G_g, G_t and L_p is presented. States of fully-mobile bed operation are discussed.     

Investigation of hydrodynamic performance and effective mass transfer area for Sulzer DX structured packing

The hydrodynamic performance in terms of pressure drop (?P) and liquid holdup (h_L), and tshe effective mass transfer area (a_e) of Sulzer DX structured packing were investigated at 293.15 K and 101.3 kPa. In addition, the flooding velocity (u_F) was also calculated based on the experimental results of liquid holdup, and the effective voidage correction factor (?) was obtained by combining the Billet model and the experimental effective fraction. The liquid volume method and pressure difference from just below to above the column packing approach are used to describe the hydrodynamic performance in a structured packing column. Experimental results showed that the operational conditions in terms of gas flow rate, liquid flow rate, viscosity, and liquid systems strongly affect the hydrodynamic performance. The experimental comparison between the pressure drop profiles in air‐water (polyethylene oxide [PEO]) and MEA‐H₂O‐CO₂ systems indicated that both the reacting MEA and CO₂ partial pressure can enhance the pressure drop value. In addition, the Bain‐Haugen correlation model was developed to predict the flooding velocity data with an acceptable AARD of 8.1%, and a model was also successfully proposed to predict the values of liquid holdup with an AARD of 11.8%, which is lower than 14.7% in Billet model. Furthermore, the effective mass transfer area was found to be increased by increasing both the liquid and gas flow rate by using NaOH‐H₂O‐CO₂ system. A model was also proposed to calculate the experimental a_e with an acceptable AARD% of 19.52, and this built model (Eq. 39) can reasonably explain the experimental phenomenon. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3625–3637, 2018     

Modeling of dry pressure drop for fully developed gas flow in structured packing using CFD simulations

Dry pressure drop in columns equipped with structured packings is considered to involve two components: drag force due to the direction changes near the column walls and in the transition region between two packing layers rotated to each other by 90°, and friction force between the different gas flows inside the crossing triangular channels and with the packing solid walls. It is believed that in a packed bed with compact sheet density and large packing surface area (above 250 m²/m³), the major contribution of the pressure drop is generated by the friction component.In this paper, a model is proposed to determine the dry pressure drop friction component. The gas is assumed to establish a fully developed turbulent flow inside the structured packing channels. The structured packing geometry consists of a combination of periodic elements. It is shown that the reproduction of one periodic element aerodynamics leads to determine the gas distribution and pressure drop inside the packed bed. Therefore, modeling the dry pressure drop through one periodic element is a meaningful representation of the dry pressure drop over the packing.CFD simulations are carried out on periodic elements using different turbulence models: RNG k−ε, realizable k−ε, and SST k−ω. The best results that agree with the experimental data in the literature are obtained with the SST k−ω model. The CFD model proposed is used to study the impact of packing geometry variations on the dry pressure drop and to bring up a correlation for the pressure drop with respect to changes of packing geometry: channel height dimension, channel opening angle, and corrugation angle.     

Comparative study of dry pressure drop and flow behaviour of gas flow through sieve plate packing

Sieve plate packing is a newly developed packing that has been used in several industries due to its simple structure and operating flexibility, and no liquid flooding. In this work, first, systematic experiments were conducted to measure the pressure drop of gas flow through six sieve plate packings. The results indicated that the geometric characteristics of the packing have complicated effects on the pressure drops. Based on this, CFD simulations on the gas flow field were conducted using the realizable k-ε model, and flow behaviours such as the pressure drop, pressure nephogram, and velocity distributions within different packings were obtained. The simulation results clearly showed interesting flow patterns, including the contraction and expansion of the gas stream through the sieve hole, the flow separation on the sharp edge of the hole, and the vortexes formed when gas impacts the downstream plate. By comparing the flow patterns and the pressure drop under different packings operating at different conditions, the effects of the geometric characteristics of the packing on the pressure drop could be clearly distinguished from the flow behaviours, so that the variations in pressure drop with various packing structures were clearly indicated. Finally, based on the experimental data and the simulated results, correlations for the prediction of the pressure drops were proposed. This work will provide a useful basis for understanding the flow behaviour of gas and liquid two-phase flow in sieve plate packing.     

A hybrid method of turbulent flow modelling in packings of complex geometry

In this work, experimental investigations and computational simulations were combined into a hybrid method of complex phenomena modelling. In particular, the thermo-anemometric technique and the multi-scale methodology of modelling were applied to investigate air turbulent flow within a rectangular container filled with spheres in cubic arrangement and different baffles alternatively inserted between the spheres. The model systems formed the complex geometric structures where three length scales were distinguished. Hence, the local fluctuations of air velocity were examined in the micro-scale determined by the size of the anemometric probe. The interstitial flow distributions, in turn, were investigated in the cell scale related to the sphere diameter. At last, the pressure drop changes caused by the superficial flow distributions were analysed in the apparatus scale. In each case, the particular experimental data were approximated by numerical modelling. However, when the information exchange between the complementary models was arranged, the significance of the flow mechanisms dominating in particular length scales could be confirmed in relation to all the experimental data determined in this work. In recapitulation, it was indicated how experimental and numerical investigations can be effectively combined in searching for a “profitable” anisotropy of the packing resistance to flow.     

Experimental and analytical study of countercurrent flow limitation in vertical gas/liquid flows

An experimental and analytical study of adiabatic countercurrent flow limitation (flooding) in single vertical ducts is reported. The experiments were carried out in a rectangular channel using saturated liquid and vapour of Refrigerant 12 (CCl₂F₂). The steady-state liquid delivery (flooding) curves as well as local pressure drop and void fraction distributions in the countercurrent flow were measured in a range of system pressures from p/p_crit = 0.16 to p/p_crit = 0.31, and for various total liquid injection rates and locations. The measured flooding curves j₁ = f(j_g) as well as pressure drop and void fraction during partial liquid delivery (j₁ < j₁ⁱⁿ) were not affected either by the axial liquid feed location or by the excess liquid rate carried upwards by the vapour. Moreover, for given flow conditions during flooding pressure drop and void fraction were essentially the same at different axial positions. Radial void fraction distributions evaluated from optical fibre probe data indicate an annular-type flow pattern. Based on this experimental evidence, a mechanistic core/film flow model was developed for the calculation of flooding. The analytical results are compared with the present high pressure and with comparable atmospheric pressure experimental data, showing reasonable overall predictions not only of the flooding curves, but also of the pressure drop in countercurrent flow.     

Optical monitoring of the injection molding of intercalated polypropylene nanocomposites

Intercalated polypropylene (PP)/clay nanocomposites were produced by twin screw extrusion; afterwards, the optical monitoring of their injection molding was done using a laser sensor. The transmitted light intensity as a function of molding time was measured. The mold and melt temperatures, packing pressure and flow rate were changed. The nanocomposite had higher induction times than the PP, that is, scattering structures were detected later in the nanocomposite than in the PP, which was attributed to a retardation effect promoted by the clay on the PP crystallization growth rate. The morphologies of the injection molded samples were analyzed by polarized light optical microscopy, differential scanning calorimetry and transmission electron microscopy. The nanocomposite samples showed a second core, a thicker skin layer, highly oriented nanoclay's tactoids in the skin region and average spherulites' sizes smaller than the PP. The final light intensity I_f was correlated with the spherulites' sizes: high values of I_f represented samples with large spherulites. The PP sample had average spherulites' sizes larger than the Nano samples. However, the surging of a second core with large spherulites in the Nano samples changed the expected pattern: the PP samples showed I_f lower than the Nano samples. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

A multi-scale approach for CFD calculations of gas-liquid flow within large size column equipped with structured packing

This work has been carried out in the framework of post-combustion CO₂ capture process development. Considering the huge amount of gases to be treated and the constraints in terms of pressure drop, it appears that the absorption column will be equipped with high efficiency high capacity packings such as structured packings. The present paper focuses on the CFD modellisation of the two-phase flow within this complex geometry. For limited computational resources reasons, it is presently impossible to run computations at large scales taking into account the gas-liquid interaction and the real geometry of the packing and original approaches must be developed. In the present work, a multi-scale approach is proposed. It first considers liquid-wall and liquid-gas interaction at small scale via two-phase flow calculations using the VOF method. Second, the latter results are used in three-dimensional calculations run at a meso-scale corresponding to a periodic element representative of the real packing geometry. Last, those results are further used at large scale in three-dimensional calculations with a geometry corresponding to a complete column. Results are compared with experimental data and with other CFD simulations in terms of liquid hold-up, pressure drop and unit operation. Some suggestions are made for further development.     

Effect of liquid addition on the bulk and flow properties of fine and coarse glass beads

The effect of water on the packing and flow properties of fine and coarse particles was experimentally investigated. Four different particle sizes of glass beads, from 5 to 275 μm, were studied with increasing water weight‐percentages. Using a FT4 Powder Rheometer, changes in bulk properties were collected as a function of water content and particle size. The results show that water content plays a significant role on the packing and flow of the particles. Small amounts of water created porous aggregates due to liquid bridging. Greater amounts of water resulted in the filling of the void‐spaces. This was indicated by an increase in basic flow energy, density, and pressure drop, with a decrease in porosity. A greater understanding of bulk properties of wetted material is useful to develop standard systems that can be used to examine the behavior of more complex situations, and implement changes to improve materials handling and processing. © 2015 American Institute of Chemical Engineers AIChE J, 62: 648–658, 2016     

Process kinetics of UASB reactors treating non‐inhibitory substrate

The degradation of a non‐inhibitory substrate (sucrose) in upflow anaerobic sludge bed (UASB) reactors with different superficial flow velocites (u_s) was performed to generate experimental data. Additionally, a kinetic model accounting for the mass fraction of methanogens (f) and granule size distribution in UASB reactors is also proposed. At the volumetric loadings of 2.65–21.16 g COD dm^?3 day^?1, both the COD removal efficiency and granule size of the UASB reactors increase with increasing u_s. The f values determined experimentally increase from 0.13–0.24 to 0.27–0.43 if the volumetric loading is increased from 2.65 to 5.29 g COD dm^?3 day^?1. With a further increase in volumetric loading, the f values decline because of the accumulation of volatile fatty acids (VFAs). The predicted residual concentrations of VFAs and COD are in fairly good agreement with the experimental data. From the calculated effectiveness‐factor values, the influence of mass transfer resistance of the substrate sucrose on the overall substrate removal rate should not be neglected. From parametric sensitivity analyses together with the simulated concentration profiles, methanogenesis is the rate‐limiting step. Copyright © 2003 Society of Chemical Industry     

Investigation of flow parameters and efficiency of mixture separation on a structured packing

Results of experimental study of the effect of initial maldistribution of structured packing irrigation on efficiency of binary mixture separation are presented in this article. The studies were carried out in the experimental distillation column with the diameter of 0.9 m using the R114 and R21 freon mixture. Experiments were performed on the structured Mellapak 350.Y packing of stainless steel 316L, containing 19 layers with the total height of 4.016 m at the ratio of mole liquid and vapor flow rates L/V = 1 and 1.7, respectively, and the pressure in the upper part of the column p_top = 3 bars. Nonuniformity at the packing inlet was generated via the blocking of some holes in the liquid distributor. Here, we present some results on efficiency of mixture separation, pressure drop on the packing, distribution of local liquid flow rate under the packing over the cross‐section and on the column wall within the range of vapor loading factor (0.69 < F_v < 1.61 Pa^0.5), as well as experimental data on distribution of local concentration of the low‐boiling component over the cross‐section and along the height of the structured packing. It is found out that significant maldistribution of mixture concentration and liquid flow rate over the cross‐section slightly changes along the height in the lower part of the column at a change in the degree of packing irrigation nonuniformity at the inlet. It is shown that efficiency of mixture separation depends considerably on the value of parameter L/V, vapor flow factor F_v, and size of the zone underirrigated by liquid at the inlet. In the studied range of liquid and vapor flow rates, the relative pressure drop on the packing does not depend on the ratio of liquid and vapor flow rates L/V and degree of irrigation maldistribution. © 2013 American Institute of Chemical Engineers AIChE J 60: 690–705, 2014     

Fluid dynamics characterization of a pneumatic bed using a spouted bed type solid feeding system

The development of a bed is reported, in which characteristics from spouted beds and pneumatic conveying are combined. The apparatus can be described as a pneumatic transport bed modified by the introduction of a recirculation process for inert materials, which leads to a final configuration akin to that of spouted beds. The dependency of the particle recirculation rates on the air flow rates and on the distance between the air inlet and the lower end of the central tube (z₀) was investigated. Also studied was the variation of the total pressure drop with air flow rate, for different loads of inert materials (glass spheres, d_p = 2.8 mm) and different z₀ values.     

Influence of the turbulence model in CFD modeling of wall-to-fluid heat transfer in packed beds

Computational fluid dynamics as a simulation tool allows obtaining a more detailed view of the fluid flow and heat transfer mechanisms in fixed-bed reactors, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. In this way, this tool permits obtaining of mean and fluctuating flow and temperature values in any point of the bed. An important problem when modeling a turbulent flow fixed-bed reactor is to decide which turbulence model is the most accurate for this situation. To gain insight into this subject, this study presents a comparison between the performance in flow and heat transfer estimation of five different RANS turbulence models in a fixed bed composed of 44 homogeneous stacked spheres in a maximum space-occupying arrangement in a cylindrical container by solving the 3D Navier-Stokes and energy equations by means of a commercial finite volume code, Fluent 6.0^®. Air is chosen as flowing fluid. Numerical pressure drop, velocity and thermal fields within the bed are obtained. In order to judge the capabilities of these turbulence models, heat transfer parameters (Nu_w, k_r/k_f) are estimated from numerical data and together with the pressure drop are compared to commonly used correlations for parameter estimations in fixed-bed reactors.     

Pressure drop through in-bulk flax seeds

The airflow resistance of in-bulk flax seeds (Linum usitatissimum) and the effect of airflow rate, bed depth, type of packing, and presence of foreign material (“fines” and “chaff”) were studied. A good fit of the experimental data (R > 0.993) was obtained through the model DPL = cQ + dQ ² (where DPL is pressure drop per unit of bed depth, Pa/m; Q is airflow rate, m³/s-m²; c and d are constants; and airflow range is 0.011–0.141 m³/s-m²). The airflow resistance increases when bulk density and bed depth increase. In dense packing the pressure drop is 1.3 to 1.5 times the pressure drop in loose packing. The resistance increases with the increase of fines and decreases with the increase of chaff.     

The Single-Fiber Collision Rate and Filtration Efficiency for Nanoparticles I: The First-Passage Time Calculation Approach

We describe an approach to filtration-efficiency calculations as an alternative to the traditional depth filtration theory. The new approach involves linking the single-fiber efficiency to the collision rate coefficient/kernel between nanoparticles and fibers, and correspondingly inferring the collision kernel via dimensionless mean first-passage time (MFPT) calculations. This method has the advantage of easily incorporating the influences of particle diffusion, inertia, and particle size; therefore, all filtration mechanisms can be considered simultaneously. Through non-dimensionalization of the equation of motion for a particle in MFPT calculations (the Langevin equation), it is shown that both the single-fiber efficiency E_f and dimensionless particle-fiber collision kernel, H, are functions of the ratio of particle radius to filter-fiber radius, R, the solid volume fraction in the filter, V_f, the ratio of particle persistence distance to the particle-filter collision distance, Kn_D (the diffusive Knudsen number), and the ratio of the particle translational kinetic energy to the thermal energy χ_f. Using a Kuwabara flow-cell model to define the geometry and flow field, MFPT calculations are used to determine H and E_f for nanoparticles in atmospheric pressure systems, i.e., when particle inertia is negligible but when diffusion and interception act in tandem to collect particles. From MFPT results, regression equations for both H and E_f are developed. A comparison is made between MFPT results and commonly invoked depth-filtration single-fiber efficiency relationships, experimentally measured values, and H equations derived from Sherwood number correlations based upon measurements of heat transfer from a fluid flowing perpendicular to an array of cylinders. Good agreement is found with both measurements and previously developed equations over a wide range of parameter space.

Copyright 2014 American Association for Aerosol Research     

圆管中中性悬浮液流动阻力和流变性的实验测定

对高浓度的液固悬浮液在圆管中的流变特性进行了实验研究,实验体系为聚苯乙烯颗粒在NaCl水溶液中的中性悬浮液。测定了在层流状态下,固体颗粒的体积分数变化（20%～50%）、悬浮液流速变化（0.031～0.22 m&#8226;s^-1）以及颗粒粒度变化对悬浮液压降的影响规律。实验结果表明,固体颗粒的浓度会影响悬浮液的流变性质,颗粒粒径对悬浮液流变性影响微弱。悬浮液的压降随颗粒体积分数和流速的增大而增大,悬浮液的流动特性在较高颗粒浓度范围内符合幂率流体模型。