Numerical simulation of the flow in a rotating disk filtration module

The characterization of the flow inside an experimental flat membrane module with a smooth rotating disk was performed. The module consists of a disk rotating at speeds up to 3000 rpm inside a cylindrical housing equipped with a stationary circular flat membrane. The characterization was carried out by using a finite volume CFD software with the κ-omega turbulence model and results of the range of rotation speeds 300 ≤ Ω ≤ 20000 rpm were compared with experimental and theoretical data reported in previous studies. The simulations suggest high permeate fluxes for the device due to large average shear stresses on the membrane and the absence of stagnant zones inside the module, which are desirable features to avoid membrane fouling processes. The simulations show an overall good agreement with theoretical results based on the main assumption that the wall shear stress on the membrane and on the disk can be predicted using modified correlations for rotating flow over a stationary wall and for flow induced by a rotating disk, respectively and with experimental pressure measurements. It has been found that the flow rate imposed at the inlet of the module has an important effect on the pressure distribution. At the membrane some discrepancies were found between the results obtained with the simulations and with the theoretical approach because of the limitations of the assumptions, especially at low rotating speeds for which the effect of the flow through the module becomes important. The correlations relating the disk rotation rate with the surface averaged pressure and the shear stress on the membrane were determined.     

Coagulation of bentonite suspension by polyelectrolytes or ferric chloride: Floc breakage and reformation

Coagulation process usually involves different hydrodynamic conditions, in particular when it is followed by a filtration step. In this study, coagulation performance was investigated under a wide range of shear stress. Floc behaviour was followed in-line by laser granulometry to determine size distribution and structure. Synthetic suspension of bentonite in tap water was used as a reference for mineral solids in surface water. Three cationic polymers (polyamine based and polyDADMAC) and ferric chloride were tested using different coagulation reactor geometries. Jar-test indicated coagulation performance under mild hydrodynamic conditions and Taylor–Couette reactors were used to create shear stresses up to 8 Pa. Flocs formed with ferric chloride are not able to grow under middle shear stress like 1.5 Pa. On the contrary, polyelectrolytes lead to large flocs, dense (D_f = 2.6) and resistant to shear stress. A qualitative comparison of floc resistance to shear depending on hydrodynamic conditions and coagulant type is given through the calculation of the strength factor. Fractal dimension measurements indicate a mechanism of particle erosion when flocs are subjected to a higher shear stress in Taylor–Couette reactor. Floc re-growth is also investigated, and breakage appears to be non-reversible regardless of coagulant and conditions experimented.     

Simulation of supercritical water–hydrocarbon mixing in a cylindrical tee at intermediate Reynolds number: Impact of temperature difference between streams

The objective of this work is to study the impact of the temperature difference between the streams on the flow dynamics and mixing of supercritical water (SCW) and a model hydrocarbon (n-decane), under fully miscible conditions, in a small-scale cylindrical tee mixer (pipe ID = 2.4 mm), at an intermediate inlet Reynolds number of 500 using 3-D CFD simulations. When the water and n-decane streams enter the mixer at inlet temperatures of 800 K and 700 K respectively (ΔT = 100 K), the flow remains laminar and the variations of density and viscosity with temperature do not have a significant impact on the flow and mixing dynamics. However, when the water inlet temperature is 1000 K (ΔT = 300 K), the water–HC shear layer becomes unstable close to x = 5D downstream of the mixing joint followed by shear-layer rollup and transition to turbulence. This leads to significant enhancement in the mixing rate. However, in a simulation of SCW n-decane mixing with the same inlet conditions but with the physical properties held fixed at the inlet values (no variation with temperature), the shear layer remains stable and steady state is reached. It was found that, the large variation of temperature of 300 K within the mixing layer leads to an increase in the local fluid density and a decrease in the local fluid viscosity within the mixing layer attributed mainly to the cooling of water and the heating of n-decane respectively. These physical property variations result in an increase in the local Reynolds number within the shear layer rendering it unstable to perturbations in the flow. Thus, the variations in mixture density and viscosity with temperature under near-critical conditions were found to have a significant impact on the flow and mixing dynamics in the tee mixer.     

Raman spectroscopy mapping of amorphized zones beneath static and dynamic Vickers indentations on boron carbide

Three-dimensional models of amorphized zones beneath quasistatic and dynamic Vickers indentations on boron carbide were constructed using micro-Raman spectroscopy. The square of amorphized zone depth varied linearly with load and the maximum amorphized area occurred beneath the indentation imprint in accord with the maximum shear stress under Hertzian contact. Reduced measurements of amorphization intensity at loads above 10 N may be due to a loss of subsurface amorphized material through lateral cracks. Utilizing an expanding cavity model with power-law (n = 0.79–0.80) and linear (E_p = 0.39–0.45) strain hardening responses, finite element simulations were conducted to determine the critical values of stress and strain required to cause amorphization. These simulations suggest that amorphization may initiate at von Mises stresses and equivalent plastic strains above 6.6 GPa and 0.026, respectively. These results may be useful for validating computational models of boron carbide under complex loading scenarios (e.g., ballistic impact).     

DESIGN AND TESTING OF AN AEROSOL/SHEATH INLET FOR HIGH RESOLUTION MEASUREMENTS WITH A DMA

A modified aerosol/sheath inlet was designed for a differential mobility analyzer (DMA) for high resolution measurements based on field model calculations which include fluid flow, electric field, and convective/diffusive transport. To avoid the predicted flow recirculation for the current inlet design at an aerosol-to-sheath flow ratio of 0.05, the slit width is reduced and aerodynamically shaped so that the sheath velocity and aerosol velocity more nearly match. Numerical results are presented comparing the fluid flow of the old and new inlet. Problems associated with the old inlet include: flow unsteadiness at a flow ratio of 0.025, voltage shift at the peak particle concentration as a function of the flow ratio, and the historical observation that, while performing tandem differential mobility analyzer measurements (TDMA), the voltage applied on the second DMA for the peak particle concentration is higher than that for the first. Measurements demonstrate that all these problems are reduced or eliminated with the new inlet design. The TDMA measurements include flow ratios of 0.1, 0.05, 0.025 and 0.0125 at sheath flows of 166 and 333 cm³ s^-1 (10 and 20 l min^-1). The challenge of performing measurements at these low flow ratios will be discussed including flow calibration, flow matching, and pressure monitoring. The new inlet is applied to the measurement of the National Institute of Standards and Technology 0.1 μm Standard Reference Material 1963, and it is shown that the DMA can accurately measure the standard deviation of this narrowly distributed aerosol (σ/D_p=0.02).     

Performance evaluation of long differential mobility analyzer (LDMA) in measurements of nanoparticles

Performance of a long differential mobility analyzer (LDMA) in measurements of nanoparticles was evaluated experimentally and numerically. In the evaluation of the LDMA measurements, silver particles in a size range of 5–30 nm were used under an increased flow rate. The numerical calculation method was used for calculating the particle trajectory in the LDMA, and the results were used for a comparison with Stolzenburg's transfer function. Using the CFD method, the flow around the aerosol inlet slit was analyzed, and the resulting particle mobility distribution was compared with that for an ideal flow. The resulting flow effect on the penetration efficiency caused by the inlet and exit slits were negligible when a well-designed system was used. The experimental measurements of mobility distributions were in good agreement with the theoretical prediction of particle size ranges over 10 nm, but some discrepancies were observed when particle size ranges were below 10 nm in size. The numerical calculation estimated the discrepancy found below the 10 nm particle size ranges.     

Design of a new FM01-LC reactor in parallel plate configuration using numerical simulation and experimental validation with residence time distribution (RTD)

The goal of this work was to develop new geometry design of inlet and outlet distributors of the FM01-LC in parallel plate configuration using Computational Fluid Dynamics (CFD). The new distributor geometry was experimentally evaluated with RTD experimental curves using the stimulus-response technique and approximated with axial dispersion model (ADM), plug dispersion exchange model (PDEM) and by solving the hydrodynamic (Reynolds average Navier–Stokes equation for low Reynolds number, RANS-LRN) and mass transport (convection–diffusion equation in transient and turbulent regimen) equations using computational fluid dynamics (F-tracer RTD method). Two sets of RTD experiments (common and new inlet and outlet distributors) in FM01-LC reactors with channel thickness of 0.011 m were carried out. The volumetric flows (Q) employed were from 0.5 to 3.5 L min⁻¹ (U₀ = 0.02-0.15 m s⁻¹). The new FM01-LC reactor had a more homogeneous velocity field in the entire reaction zone, as shown by axial dispersion values lower than those obtained with the common FM01-LC, at different Reynolds numbers. The RTD curves obtained with Comsol Multiphysics 4.3a are in agreement with RTD experimental curves, but deviations are observed at Reynolds numbers greater than “5991”.     

Enhanced electrokinetic,dielectric and electrorheological properties of covalently bonded nanosphere-TiO2/polypyrrole nanocomposite

Nanosphere-TiO₂ was synthesized, surface functionalized with (3-aminopropyl) triethoxysilane (APTS) and then covalently bonded with polypyrrole (PPy) with bottom-up surface engineering strategy to obtain nanosphere-TiO₂/PPy core/shell hybrit nanocomposite. All the materials were subjected to full chemical and morphological characterizations by using various techniques. The presence of NaCl, AlCl₃, cetyltrimethyl ammonium bromide and sodium dodecylsulfate observed to cause high colloidal stabilities of the nanocomposite dispersions by reaching to zeta(ζ)-potential values of ζ > + 30 mV and ζ < − 30 mV. A series of suspensions were prepared by dispersing nanosphere-TiO₂ and nanosphere-TiO₂/PPy particles in insulating silicone oil (SO) and dielectric properties were determined using an LCR meter. Antisedimentation stabilities of these suspensions were determined against gravitational forces and 54% colloidal stability was achieved with the nanocomposite after 30 days. Polarizabilities of the suspended particles were observed using an optical microscope under externally applied electric field strength. Then the suspensions were subjected to electrorheological measurements by investigating the effects of shear rate, particle volume fraction, shear stress, and electric field strength. Non-Newtonian shear thinning behaviors were observed for the samples. Further, vibration damping characteristics of the materials were determined with shear stress and frequency oscillation measurements. Enhanced reversible viscoelastic deformations were observed for the dispersions from creep-recovery tests and 64% creep-recovery was obtained for nanosphere-TiO₂/PPy/SO system under E = 3.5 kV/mm.     

Experiments in a large-scale venturi scrubber: Part I: Pressure drop

The present work details pressure drop measurements in a large-scale venturi scrubber with an inlet and throat diameters of 250 and 122.5 mm, respectively.The flow rates were varied between 0.4835 and 0.987 kg/s for the gas while the liquid was set between 0.013 and 0.075 kg/s. The results show that the pressure drop depends on both the gas and liquid phases flow rates, the droplet fraction and the method of liquid injection (spray and film).The pressure drop in the diverging section is well predicted by a boundary layer model.     

Measurements of the flow of supercritical carbon dioxide through short orifices

This paper describes the methods used to measure flow rate of supercritical and two-phase CO₂ through short orifices. Orifices with diameters of 1 millimeter and orifice length-to-diameter ratios of 3.2 and 5 were tested. Flow rates through these orifices were measured over a broad range of inlet conditions in the supercritical region with orifice inlet pressures ranging from 5 MPa to 11 MPa and inlet densities ranging from 86.5 kg/m³ to 630 kg/m³. The data were compared to the isentropic real gas model for expansion of a fluid through a nozzle in order to observe the behavior of the discharge coefficient. For a given orifice inlet condition, the single-phase discharge coefficient was found to be between 0.81 and 0.87 and was independent of the pressure ratio. The discharge coefficient increased as the pressure ratio decreased when two-phase CO₂ was present with orifice inlet pressures of 7.7 MPa and 9 MPa. The critical mass flow rate and critical pressure ratio were determined for each test. The raw data from this investigation are available on the internet.This paper describes the methods used to measure flow of supercritical and two-phase CO₂ through short orifices. Orifices with diameters of 1 millimeter and orifice length-to-diameter ratios of 3.2 and 5 were tested. Flow rates through these orifices were measured over a broad range of inlet conditions in the supercritical region with orifice inlet pressures ranging from 7.7 MPa to 11 MPa and inlet densities ranging from 111 kg/m³ to 630 kg/m³. The data were compared to the isentropic real gas model for expansion of a fluid through a nozzle in order to observe the behavior of the discharge coefficient. For a given orifice inlet condition, the single-phase discharge coefficient was found to be between 0.81 and 0.87 and was independent of the pressure ratio. The discharge coefficient increased as the pressure ratio decreased when two-phase CO² was present with orifice inlet pressures of 7.7 MPa and 9 MPa. The critical mass flow rate and critical pressure ratio were determined for each test. The raw data from this investigation are available on the internet.     

Numerical study of flow uniformity and pressure characteristics within a microchannel array with triangular manifolds

Flow uniformity among individual channels within a microchannel array can be a significant factor affecting the performance of laminated structured micro-devices. In this study, numerical modeling is used to quantitatively investigate the impact of the geometry of the right triangular manifold and the dimensions of microchannels on desired uniformity and pressure drop. The CFD tool COMSOL is used to perform the simulations within the low-Reynolds number system (5 ≤ Re ≤ 25). In our biomedical application, it is important to have low dead volume and residence time in the manifolds. A methodical approach is introduced to identify a design that balances low manifold volume and maintenance of flow uniformity. It has also been shown that including a short vertical spacing at the corner of manifolds is critical to achieving a high level of flow uniformity. Careful analysis and physical interpretation of trends herein enables a more intuitive approach to array design.     

Coupled CFD–DEM of particle-laden flows in a turning flow with a moving wall

The nature of the particle–solid interactions and particle–fluid interactions in rectangular duct bend geometry with/without a moving wall is studied, taking into account particle collision, colloidal, and hydrodynamic forces, and four way coupling between the fluid flow and particles. The focus is on systems where particles and fluid phase have similar length scales, fluid Reynolds number (Re_f) ∼ 1, and particle's Stokes number (St) ∼ 1. Particles move toward the walls of the channel near the bend, and have long residence times in these regions. Buoyancy force has negligible effect on particle motion, where adhesion and drag forces lead to particle motion and agglomeration patterns. The effect of a free surface on agglomeration sites in the turning flow is elucidated.     

Physico-chemical concept of drag reduction nature in dilute polymer solutions (the Toms effect)

The physicochemical concept of turbulent drag reduction (the Toms effect) integrates physicochemical characteristics of polymer solutions with hydrodynamic and rheological flow parameters into a generalized equation, where the increment in volumetric flow rate Q_P is a function of the external shear stress τ_w, temperature, volume of macromolecular coils with immobilized solvent V_c and a function of their volume fraction Ψ = C · [η]/(1 + C · [η]). The Q_P depends on the coil intrinsic elasticity [G] = kT/V_c as well. This model allows one: (1) to describe the Toms effect in terms of useful elastic work spent by macromolecular coils with immobilized solvent to overcome the frictional forces (i.e. the forces of intermolecular interactions), (2) to forecast the initial conditions of the Toms effect (τ_* ≈ (RT)/(M · [η])) and (3) to explain the unusual temperature dependence of the polymer solutions flow.     

A novel set-up and a CFD approach to study the biofilm dynamics as a function of local flow conditions encountered in fresh-cut food processing equipments

The hydrodynamic conditions as well as design and surface properties within fresh-cut food processing equipment create a complex environment for biofilms. A new experimental approach was thus proposed to identify those physical parameters impacting biofilm development in such conditions. A set-up comprising original mock-ups mimicking generic features of washing tanks (e.g. welds, folds, flat surfaces, air/liquid/wall interface) was designed. The flow pattern therein was characterized using two computational fluid dynamic calculation approaches. Full trials were run for 48 h at 10 °C with a Pseudomonas fluorescens strain to identify the preferential biofilm formation areas. As in current industrial systems, the pilot rig had recirculation areas and low wall shear stress rates (τ_w < 0.1 Pa) in corners and angles. These were identified as critical areas with Surface Microbial Loads (SML) over 5 Log₁₀/cm². However, τ_w alone failed to explain why SML in areas under unidirectional flow was higher than in the mock-ups. Lastly, air/liquid/wall interface conditions were more critical than immersed surfaces. This study validated the possibility of using CFD methods to understand the way in which flow pattern influences biofilm formation. The methodology proposed would be helpful in quantifying equipment components criticality based on biofilm growth parameters.     

Particle image velocimetry for characterizing the flow structure of oil–water flow in horizontal and slightly inclined pipes

The simultaneous flow of oil and water in pipelines is a common occurrence in the chemical and process industry. An experimental investigation of oil–water flow in horizontal and slightly inclined pipes is presented in this paper. The experiments are performed in a 15 m long stainless steel pipe section with internal diameter 56 mm at room temperature and atmospheric outlet pressure. Exxsol D60 oil (density 790 kg/m³ and viscosity 1.64 mPa s) and water (density 996 kg/m³ and viscosity 1.00 mPa s) are used as test fluids. The pipe inclination is changed in the range from 5° upward to 5° downward. The measurements are made for two different mixture velocities, 0.50 and 1.00 m/s at water cut 0.50. The cross-sectional distribution of phase fractions in oil–water flow is measured using a traversable single-beam gamma densitometer. The different flow regimes are determined based on visual observations. The particle image velocimetry (PIV) is utilized in order to obtain non-invasive instantaneous velocity measurements of the flow field. Based on the instantaneous local velocities, mean velocities, root mean squared velocities and Reynolds stresses are calculated. Stratified flow with mixing at the interface is observed at mixture velocity 0.50 m/s. Interfacial waves are observed in upwardly and downwardly inclined flows. At mixture velocity 1.00 m/s, interfacial mixing is increased and dual continuous flows are observed. The degree of mixing largely depends on the pipe inclination. In general, higher water hold-up values are observed for upwardly inclined flows compared to the horizontal and downwardly inclined flows. The slip between the phases increases as the pipe inclination increases. The maximum mean axial velocity is detected in the more viscous oil phase at equal volumetric flow rates of oil and water. In addition, measured mean velocity and turbulence profiles show a strong dependency with pipe inclination. The largest root mean squared velocities and absolute values of the Reynolds stresses are observed close to the pipe wall due to higher mean axial velocity gradients. A damping effect of Reynolds stress is observed around the oil–water interface due to stable density stratification. The presence of interfacial waves enhances turbulence fluctuations in inclined oil–water flows.     

Optimization of a hydrodynamic cavitation reactor using salicylic acid dosimetry

In the present work, optimization of a hydrodynamic cavitation reactor, for maximizing the extent of hydroxyl radical generation, has been investigated using salicylic acid as a dosimeter. The effect of different operating parameters such as inlet pressure into the reactor, shape of the orifice, and concentration of salicylic acid employed was investigated where the extent of hydroxyl radical generation was quantified in terms of concentration of the hydroxylated products 2,5- and 2,3-dihydroxybenzoic acid. With an upstream pressure of <100 psi no hydroxyl radicals were produced but excellent results were obtained with a small circular nozzle at 4000 psi and a salicylate concentration of 750 ppm. The use of a combination of ultrasound along with hydrodynamic cavitation is also reported for the first time and results in a 15% improvement in the hydroxyl radical generation when the distance between the orifice and transducer is 5–10 mm.     

Characterization of particles packing in alumina green tape

Aqueous alumina slurry was prepared with a commercial powder of elongated particles, which has the aspect ratio ranging from 1 to 3.5 with the mean of 1.6, to examine the effect of forming conditions on the particle alignment in green tapes. The slurry appeared pseudoplastic with a yield stress, but showed no thixotropic behavior. Its flow curve fitted very well to the Herschel–Bulkley model approximation, which suggested shear-thinning constant of 0.54. Polarized microscopy with the liquid immersion technique was applied to examine the particle orientation through the direction along the tape thickness. In the absence of coquette flow, randomly oriented particles were noted in the tape. At the top surface, particles were aligned with their long-axes (a-axis) along the casting direction. The variation in the degree of orientation was 6.8 ± 1.2. In the area near the Mylar carrier, a-axis of particle made an angle to the carrier surface with the degree of orientation about 5.8 ± 1.0. As the combination of pressure flow and coquette flow, tape cast with casting velocity of 2.5 and 91.5 cm/min, which respectively resulted in shear rate of 1.38 and 50.8 s^?1, were observed. The orientation was significant near the top surface and was higher than that above the carrier surface. The a-axis of particles above the carrier surface was inclined to the surface at low shear rate (1.38 s^?1), but was nearly parallel at high shear rate (50.8 s^?1). Nevertheless, the orientation varies with the location in the tape prepared at the shear rate of 50.8 s^?1.     

NUMERICAL PREDICTION OF THE PERFORMANCE OF A MANIFOLD SAMPLER WITH A CIRCULAR SLIT INLET IN TURBULENT FLOW

The performance of a bioaerosol manifold sampler with a circular slit inlet in a turbulent flow field was modeled using a 3-D numerical approach. The standard κ–ε turbulence model was used for simulating the mean turbulent flow, and the Lagrangian approach was used for predicting the particle trajectories. The ratios of wind velocities to sampler inlet velocities were from 0.5 to 3.5. Calculations were conducted for particle sizes of 2, 8, 15,and26 μm. The agreement between numerical and empirical sampling efficiencies was good. It was found that lower sampling efficiencies at high R values were associated with increased positive pitch of the velocity vectors generated at the inlet slit. Unbalanced sampling velocities between the upstream and downstream arcs were found only at high R values. At an inlet velocity of 0.8 m/s, sampling efficiencies for 15 μm particles decreased about 24% as R was increased from 0.5 to 3.5. A similar effect was observed at an inlet velocity of 0.4 m/s. Turbulence decreased sampling efficiency and was related to the sum of the magnitudes of the wind and sampling velocity vectors.     

Cold flow experimental study and computer simulations of a compact spouted bed reactor

Spouted beds are a very interesting class of gas–solid contactors that possess excellent heat transfer and mixing characteristics, while they are particularly suited to process coarse particles. Proper design of such beds requires the prediction of various hydrodynamic characteristics, such as the minimum spouting velocity and maximum spoutable height. Contrary to their typical initial applications, spouted beds have been finding recently more frequent use on the one hand at endothermic processes and on the other hand using much finer particle sizes. In the current work, the hydrodynamic characteristics of a laboratory scale spouted bed of 0.05 m diameter have been investigated via cold flow studies using olivine particles of 3.55–5.00 × 10⁻⁴ m size. Hydrodynamic parameters have been measured at this compact geometry and fine particle size and were compared with common literature correlations. An empirical correlation was derived to predict the fountain height for the studied fine particle spouted bed. Computer simulations have been further used to investigate the heat transfer characteristics of the bed under endothermic reactive conditions, using methane reforming as a case study. Given sufficient external heat supply, a spouted bed operating at a well-mixed regime can efficiently drive even highly endothermic reactions.     

Cement particle flocculation and breakage monitoring under Couette flow

Freshly mixed concrete is a suspension of cement and aggregate particles dispersed in water. To secure the desired quality of freshly mixed concrete, understanding its rheological behavior, which depends on its flow rate, is necessary. A number of chemical and physical factors influence the rheology of freshly mixed concrete, and the flocculation of cement particles is thought to cause thixotropy and shear thinning. This study proposes a rheometer coupled with a laser backscattering device, which allows us to simultaneously measure the viscosity and the size distribution of cement clusters in cement paste suspension. The laser backscattering instrument measures the cluster size distribution and monitors its growth or breakdown, while the parallel-plate rheometer measures its rheological properties. As a result, the change of cement grains was continuously observed with the change of shear stress under specific strain rates of 1 s^− 1, 10 s^− 1, and 100 s^− 1.