Pressure drop for single and two‐phase flow of non‐newtonian liquids in helical coils

New experimental results on pressure loss for the single and two‐phase gas‐liquid flow with non‐Newtonian liquids in helical coils are reported. For a constant value of the curvature ratio, the value of the helix angle of the coils is varied from 2.56° to 9.37°. For single phase flow, the effect of helix angle on pressure loss is found to be negligible in laminar flow regime but pressure loss increases with the increasing value of helix angle in turbulent flow conditions. On the other hand, for the two‐phase flow, the well‐known Lockhart‐Martinelli method correlates the present results for all values of helix angle (2.56‐9.37°) satisfactorily under turbulent/laminar and turbulent/turbulent conditions over the following ranges of variables as: 0.57 ≤ n′ ≤ 1; Re′ < 4000; Re_l < 4000; Re_g < 8000; 8 ≤ x ≤ 1000 and 0.2 ≤ De′ ≤ 1000.     

Deposition of magnetite particles from flowing suspensions under flow‐boiling and single‐phase forced‐convective heat transfer

The deposition rate of colloidal magnetite particles was measured under both single‐phase forced‐convective and flow‐boiling conditions. All measurements were made at alkaline pH where both the heat transfer surface and the surface of the magnetite particles appear to be negatively charged. For single‐phase forced convection, the deposition rate constant is lower than the mass transfer coefficient for colloidal particles, and the difference is attributed to the force of repulsion between the negatively charged surfaces of the particle and substrate. The deposition rate measured under flow‐boiling conditions is lower than that reported for the deposition of colloidal particles at neutral pH. The difference is, again, attributed to the force of repulsion between the particle and substrate. Particle removal rates were significantly lower than deposition rates; analysis using the theory of turbulent bursts suggests a removal efficiency of only 10^?9% for each turbulent burst. The low removal efficiency is consistent with the particle diameter being significantly smaller than the thickness of the laminar sublayer in these tests.     

Pilot‐scale investigation and CFD modeling of particle deposition in low‐dust monolithic SCR DeNOx catalysts

Deposition of particles in selective catalytic reduction DeNO_x monolithic catalysts was studied by low‐dust pilot‐scale experiments. The experiments showed a total deposition efficiency of about 30%, and the deposition pattern was similar to that observed in full‐scale low‐dust applications. On extended exposure to the dust‐laden flue gas, complete blocking of channels was observed, showing that also in low‐dust applications soot blowing is necessary to keep the catalyst clean. A particle deposition model was developed in computational fluid dynamics, and simulations were carried out assuming either laminar or turbulent flow. Assuming laminar flow, the accumulated mass was underpredicted with a factor of about 17, whereas assuming turbulent flow overpredicted the experimental result with a factor of about 2. The simulations showed that turbulent diffusion in the monolith channels and inertial impaction and gravitational settling on the top of the monolith were the dominating mechanisms for particle deposition on the catalyst. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1919–1933, 2013     

Application of a modified Eulerian model to study the particle deposition on a vertical surface in turbulent flow

In this study the v2-f model was used with the two-phase Eulerian approach to predict the particle deposition rate on a vertical surface in a turbulent flow. The standard Eulerian particle model was adopted from the literature and modified, considering the majority of particle transport mechanisms in the particle deposition rate. The performance of the modified model was examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model took into account the effects of drag force, lift force, turbophoretic force, electrostatic force, inertia force and Brownian/turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and charged particles under the influence of an electric field were considered. The predictions of the modified particle model were in good agreement with the experimental data. It was observed that when both electrostatic forces are present they are the dominant factor in the deposition rate in a wider range of particle sizes.     

Explicit calculation of the friction factor in pipeline flow of Bingham plastic fluids: a neural network approach

An artificial neural network (ANN) approach was used in this paper to develop an explicit procedure for calculating the friction factor, f, under both laminar and turbulent flow conditions of Bingham plastic fluids in closed conduits and pipe networks. The procedure aims at reducing the computational efforts as well as eliminating the need for conducting complex and time-consuming iterative solutions of the governing implicit equations for calculating the friction factor, f. The ANN approach involved the establishment of an explicit relationship among the Reynolds number, Re, Hedstrom number, He, and the friction factor, f, under both laminar and turbulent flow conditions. Although, an analytical solution of the governing equation under the laminar flow regime was also feasible (such an equation is also provided in this paper), the ANN model is applicable under both laminar and turbulent flow conditions where the analytical approach will have major limitations (especially when considering the implicit equation that govern the turbulent flow regime).     

Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows

Thermophoretic deposition of aerosol particles (particle diameter ranges from 0.038 to 0.498 μm) was measured in a tube (1.18 m long, 0.43 cm inner diameter, stainless steel tube) using monodisperse NaCl test particles under laminar and turbulent flow conditions. In the previous study by Romay et al., theoretical thermophoretic deposition efficiencies in turbulent flow regime do not agree well with the experimental data. In this study, particle deposition efficiencies due to other deposition mechanisms such as electrostatic deposition for particles in Boltzmann charge equilibrium and laminar and turbulent diffusions were carefully assessed so that the deposition due to thermophoresis alone could be measured accurately. As a result, the semiempirical equation developed by Lin and Tsai in laminar flow regime and the theoretical equation of Romay et al. in turbulent flow regime are found to fit the experimental data of thermophoretic deposition efficiency very well with the differences of less than 1.0% in both flow regimes. It is also found that Talbot's formula for the thermophoretic coefficient is accurate while Waldmann's free molecular formula is only applicable when Kn is greater than about 3.0.     

Mist flow visualization for round jets in Aerosol Jet® printing

With the microdroplets of water serving as light scattering particles, the mist flow patterns of round micro-jets can be visualized using the Aerosol Jet^® direct-write system. The visualization images show that the laminar mist jet (with sheath-to-mist ratio Y?=?1:1) appears to extend to more than 20 times the diameter of nozzle orifice D for jet Reynolds number Re?<?600, especially with D?=?0.3?mm and less. For smaller jets (e.g., with D?=?0.15?mm), laminar collimated mist flow might be retained to 40×D for Re?<?600 and for Re ～1500 within 20×D from the nozzle. The laminar part of mist flow associated with larger jets (e.g., with D?=?1.0?mm for Re?<?600) tends to exhibit noticeable gradual widening due to viscous diffusion. For free jets, their breakdown length—the distance from nozzle where transition from laminar to turbulent mist flow takes place as signaled by a rapid widening of mist stream—is shown to decrease with increasing Re. The presence of impingement wall tends to prevent turbulence development, even when the wall is placed further downstream of the free-jet breakdown length for a given Re. The critical Re for an impinging jet to develop turbulence increases as the standoff S is reduced. The mist flow of impinging jet of D?=?1.0?mm seems to remain laminar even for Re?>?4000 at S?=?12?mm.

Copyright © 2018 American Association for Aerosol Research     

Influence of gas velocity on the particle collection and reentrainment in an air-cleaning electrostatic precipitator

Particle deposition and reentrainment experiments were performed in a two-stage electrostatic precipitator (ESP), consisting of positive corona precharger and collecting electrode sections. Attention was focused on studying the indoor air pollution deposition and reentrainment into six size ranges from 0.3 to >10?μm. Tests were performed in an office room (200?m³) for airflow velocities from 1.4 to 8?m/s. The effect of airflow velocity on the collection efficiency of the ESP was investigated both experimentally and analytically to study reentrainment phenomena in a turbulent flow. A stationary two-dimensional analytical model was carried out by modeling the particle transport. The boundary conditions for charged particles on collecting and repelling electrodes were determined by physical considerations, including chaotic and drift motions, the reflection of charged particles from a surface, and the reentrainment of charged particles. A decrease in the experimental collection efficiency for large particle diameters (≥0.5?μm), as compared to the theoretical prediction, was interpreted as the reentrainment of particles. The size-resolved dust reentrainment fluxes from the collecting electrode were evaluated in two limiting cases, considering that either the reentrained particles are not charged or that they are charged as the particles in the deposition flux. Dimensional analysis is applied to these results, introducing the wall friction velocity as a universal parameter that determines the flow character. In general, the particles with diameters <5?μm and >5?μm exhibit different reentrainment behavior.

Copyright © 2018 American Association for Aerosol Research     

Investigation of laminar flow in a stirred vessel at low Reynolds numbers

Many mixing applications involve viscous fluids and laminar flows where the detailed as well as overall flow structures are important. In order to understand the fluid dynamic characteristics of low Re laminar flows in mixing vessels, the flow induced by a Rushton impeller for three Re namely, 1, 10 and 28, was studied both experimentally and computationally. It was found that for the highest Re, the flow exhibited the familiar outward pumping action associated with radial impellers under turbulent flow conditions. However, as the Re decreases, the net radial flow during one impeller revolution was reduced and for the lowest Re a reciprocating motion with negligible net pumping was observed. This behaviour has not been reported in the literature in the past and represents a highly undesirable flow pattern from the standpoint of effective mixing. The CFD results successfully reproduced this behaviour. In order to elucidate the physical mechanism responsible for the observed flow pattern, the forces acting on a fluid element in the radial direction were analysed. The analysis indicated that for the lowest Re, the material derivative of radial velocity near the blade tip is small thus a balance exists between pressure and viscous forces; the defining characteristic of creeping flow. The velocity and pressure forces are in phase because the velocity is driven by the pressure field generated by the rotation of the impeller. Based on these findings, a simplified analytic model of the flow was developed that gives a good qualitative as well as quantitative representation of the flow.     

Copper deposition on micropatterned electrodes from an industrial acid copper plating bath

Copper was deposited on micropatterned electrodes in a parallel plate reactor (PPR) using an industrial acid copper plating bath, and the deposit thickness distributions were measured. The plating bath contained, besides copper sulfate and sulfuric acid as main components, small amounts of sodium chloride and an organic additive LP-1^TM. Copper deposition was carried out under various flow conditions (laminar and turbulent) and applied current densities. Three patterns, each of them consisting of a series of parallel copper microtracks, were manufactured on the electrode surface. The pattern position was chosen to be parallel or perpendicular to the flow direction, corresponding to the two extreme positions for the industrial plating process of patterned electrodes, in casu the round pattern tracks of minicoils. A multi-ion model was used to simulate copper deposition from sulphuric acid solutions, taking into account flow phenomena controlling the mass transfer rate, and the deposition kinetics. The differential equations were solved numerically by use of the multidimensional upwinding method (MDUM). Copper deposition on plane electrodes was investigated and compared to cd distributions obtained from MDUM-simulations. For the case of perpendicular pattern position in laminar flow, the deposit growth in the vias was also modelled and simulated numerically.     

The deposition of haematite particles from flowing water

Deposition of 0.2 μ haematite (α – Fe₂O₃) particles from suspension in turbulently flowing water, Reynolds Number = 11000, onto metal tube surfaces has been studied with particular reference to the effect of pH. Measurements on stainless steel showed trivial deposition outside the pH range 5 to 8. Within this range deposition rose sharply to a peak at pH = 6.2. This is similar to the findings of Kuo and Matijevic (1980) who studied haematite particle deposition onto a packed bed of stainless steel beads under laminar flow conditions. This shows that particle/surface interactions govern deposition in this system and that hydrodynamic and geometric factors are secondary.     

PARTICLE DEPOSITION IN A NEARLY DEVELOPED TURBULENT DUCT FLOW WITH ELECTROPHORESIS

This study is concerned with deposition of neutral and charged particles in nearly developed turbulent duct flows. The cases that the duct is vertical or horizontal and when the particles carry Boltzmann, static electrification, as well as saturation charge distributions are analyzed. The mean turbulent flow field is evaluated with the aid of the FLUENT code, using the Reynolds stress transport model. Deposition rate of particles in the size range of 0.01–100 μm are studied and the effects of electric field intensity on particle deposition velocity are evaluated. The simulation results are compared with the available experimental data, the earlier numerical results and those obtained from empirical equations for fully developed duct flows. It is shown that the electrostatic effect significantly increases the particle deposition rate.     

Particle dispersion in a turbulent natural convection channel flow

Direct numerical simulations of particle dispersion in the turbulent natural convection flow between two vertical walls kept at constant but different temperatures are reported. It is assumed that the particles do not affect the flow (i.e. the dilute phase approximation is adopted). Particles with different levels of inertia, or Stokes numbers (0.843≤St≤17.45), are tracked according to the drag force imposed by the fluid. The gravity force is included for two cases, St=0.843 and St=17.45. The different levels of turbulence near the wall and near the center of the channel produce, as in isothermal turbulent channel or pipe flow, a larger concentration of particles near the wall. This effect becomes more important, and the deposition velocity of particles on the wall increases, as the particle inertia is increased. The simulations at St=8.38 and St=17.45 predict similar concentration profiles and deposition velocities according to the large inertia of these particles. The deposition velocities, obtained when the gravity force is ignored in the particle equations, follow the trend observed and measured for isothermal turbulent channel flows in the diffusion impaction regime. For the conditions considered, the gravity vector imposes a strong descending motion on the particles and this produces the increase of the particle concentration near the wall and a reduction of the deposition velocities in comparison with the results without the gravity force.     

Predicting turbulence modulations at different Reynolds numbers in dilute-phase turbulent liquid–particle flow simulations

The present work examines the predictive capability of two-fluid CFD model based on the kinetic theory of granular flow in capturing the Reynolds number (Re) dependence of fluid-phase turbulence modulations in dilute-phase turbulent liquid–particle flows. The model predictions are examined using turbulent liquid–particle flow data in a vertical pipe at Re=17,000, 48,000, 65,000, and 76,000 in the particle concentration range of between 0.5% and 4.0% (v/v). The experimental data indicate that the fluid-phase turbulence intensities are enhanced with respect to the single-phase flow at Re≤48,000 but are attenuated at Re≥65,000. The simulation results indicate that the CFD model can successfully predict the turbulence modulations at Re=17,000, 65,000, and 76,000 both qualitatively and quantitatively, but not at the intermediate Re of 48,000. In this regard, (1) different drag correlations to describe the fluctuating drag force are needed to accurately predict the trends in the turbulence modulations as a function of Re, and (2) appropriate combinations of the drag correlations and turbulence closure models to describe the long-range fluid–particle interactions must be identified in each phase at different Re in order to accurately predict the turbulence modulation, granular temperature, and particle radial concentration profile.     

Longitudinal gas dispersion in transitional and turbulent flow through a straight tube

Longitudinal dispersion data are reported for a straight tube (internal dia. 1¼′ (2.77 cm)). Three binary gas systems were examined over the regime 3 × 10² < Re < 10⁴ with special emphasis on the transition from laminar to turbulent flow. Effect of Schmidt number is studied in detail over the transition regime, and practical variation with Sc for the lower turbulent regime is compared with the predicted effect based on existing theories of turbulence. In the laminar regime results are reported which diverge from Taylor's theory at Re lower than the normally accepted value for the onset of turbulence.     

A coupled CFD-Monte Carlo method for simulating complex aerosol dynamics in turbulent flows

A coupled computational fluid dynamics (CFD)-Monte Carlo method is presented to simulate complex aerosol dynamics in turbulent flows. A Lagrangian particle method-based probability density function (PDF) transport equation is formulated to solve the population balance equation (PBE) of aerosol particles. The formulated CFD-Monte Carlo method allows investigating the interaction between turbulence and aerosol dynamics and incorporating individual aerosol dynamic kernels as well as obtaining full particle size distribution (PSD). Several typical cases of aerosol dynamic processes including turbulent coagulation, nucleation and growth are studied and compared to the sectional method with excellent agreement. Coagulation in both laminar and turbulent flows is simulated and compared to demonstrate the effect of turbulence on aerosol dynamics. The effect of jet Reynolds (Re_j) number on aerosol dynamics in turbulent flows is fully investigated for each of the studied cases. The results demonstrate that Re_j number has significant impact on a single aerosol dynamic process (e.g., coagulation) and the simultaneous competitive aerosol dynamic processes in turbulent flows. This newly modified CFD-Monte Carlo/PDF method renders an efficient method for simulating complex aerosol dynamics in turbulent flows and provides a better insight into the interactions between turbulence and the full PSD of aerosol particles.

Copyright © 2017 American Association for Aerosol Research     

Deposition of micrometer-sized aerosol particles in infant nasal airway replicas

In vitro measurements of particle filtration were made for nasal geometries of 11 infants aged 3–18 months. The geometries were obtained from computed tomography (CT) scans of seven males and four female infants and replicas were built using rapid prototyping. Particles ranging in aerodynamic diameter from 0.8 to were passed through these replicas with simulated tidal breathing. Filtration was determined from particle counts upstream and downstream of the models using an electrical low pressure impactor (ELPI). Mathematical fits were constructed to predict the measured deposition based on the relevant parameters. The fractional deposition, η, is found to depend on the Reynolds number of the flow (Re), the particle Stokes number (Stk), and an airway dimension D defined as airway volume divided by airway surface area. This dependence is well captured by the formula η=1-(2.164*10⁵/(2.164*10⁵+(Re^1.118Stk^1.057(D/D_avg)^-2.840)))^0.8510. Here, D_avg is the average value of the dimension D for the group studied and is equal to 1.20 mm. Re and Stk also use the dimension D as the length scale in their definitions.     

Vaporizing Microdroplet Inhalation,Transport, and Deposition in a Human Upper Airway Model

Evaluation of injuries from inhalation exposure to toxic fuel requires detailed knowledge of inhaled aerosol transport and deposition in human airways. Focusing on highly toxic, easily volatized JP-8 fuel droplets, the three-dimensional airflow, temperature distributions, and fluid-particle thermodynamics, i.e., droplet motion as well as evaporation, are simulated and analyzed for laminar as well as locally turbulent flow conditions.

Specifically, using a commercial finite-volume software with user-supplied programs as a solver, the Euler-Lagrange approach for the fluid-particle thermodynamics is employed with: (1) a low Reynolds number k-ω model for laminar-to-turbulent airflow, and (2) a stochastic model for random fluctuations in the droplet trajectories with droplet evaporation. Presently, the respiratory system consists of two major segments of a simplified human cast replica, i.e., a representative oral airway from mouth to trachea (Generation 0) and a symmetric four-generation upper bronchial tree model (G0 to G3). Experimentally validated computational fluid-particle thermodynamics results show that evaporation of JP-8 fuel droplets is greatly affecting deposition in the human airway. Specifically, droplet deposition fractions due to vaporization decrease with increasing ambient temperatures and decreasing inspiratory flow rates. It is also demonstrated that assuming idealized velocity profiles and particle distributions in or after the trachea may greatly overpredict particle deposition efficiencies in the upper bronchial tree.     

Characterization of Submicrometer Aerosol Deposition in Extrathoracic Airways during Nasal Exhalation

Submicrometer and especially fine aerosols that enter the respiratory tract are largely exhaled. However, the deposition of these aerosols under expiratory conditions is not well characterized. In this study, expiratory deposition patterns of both ultrafine (<100 nm) and fine (100–1000 nm) respiratory aerosols were numerically modeled in a realistic nasal-laryngeal airway geometry. Particle sizes ranging from 1 through 1000 nm and exhalation flow rates from 4 through 45 L/min were considered. Under these conditions, turbulence only appeared significant in the laryngeal and pharyngeal regions, whereas the nasal passages were primarily in the laminar regime. Exhaled particles were simulated with both a continuous-phase drift flux velocity correction (DF-VC) model and a discrete Lagrangian tracking approach. For the deposition of ultrafine particles, both models provided a good match to existing experimental values, and simulation results corroborated an existing in vivo–based diffusion parameter (i.e., D ^0.5 Q ^?0.28). For fine particles, inertia-based deposition was found to have a greater dependence on the Reynolds number than on the Stokes number (i.e., St^0.1 _kRe^0.9), indicating that secondary flows may significantly influence aerosol deposition in the nasal-laryngeal geometry. A new correlation was proposed for deposition in the extrathoracic airways that is applicable for both ultrafine and fine aerosols over a broad range of nasal exhalation conditions. Results of this study indicate that physical realism of the airway model is crucial in determining particle behavior and fate and that the laryngeal and pharyngeal regions should be retained in future studies of expiratory deposition in the nasal region.     

Combined forced and free convection flow past a horizontal flat plate

The problem of simultaneous forced and free convection flow of a Newtonian fluid past a hot or cold horizontal flat plate is investigated by means of numerical solutions of the full equations of motion and thermal energy subject only to the Boussinesq approximation. These solutions span the parameter ranges 10 ≤ Re ≤ 100, 0.1 ≤ Pr ≤ 10, and –2.215 ≤ Gr/Re^5/2 ≤ 2.215 where Re, Pr, and Gr are based on the ambient free stream fluid properties and the overall plate length l. When Gr > 0, the boundary flow near the plate surface is accelerated relative to the corresponding forced convection flow, with a resulting increase in both the local skin friction and heat transfer coefficients. When Gr < 0, the boundary flow is decelerated, the local skin friction and heat transfer are decreased, and the flow actually separates for Gr/Re^5/2 < –0.8 when Pr = 0.7. In the latter circumstance, an increasing degree of upstream influence is observed as Gr/Re^5/2 is further decreased.