Planar drop-sizing and liquid volume fraction measurements of airblast spray in cross-flow using SLIPI-based techniques

This paper presents planar measurements of Sauter mean diameter (SMD) and liquid volume fraction distributions of airblast spray injected into cross-flow. The experiments are conducted using a combination of structured laser illumination planar imaging with laser sheet drop-sizing (SLIPI-LSD) and particle/droplet imaging analysis (PDIA) techniques. Effect of gas to liquid mass ratio (GLR) and cross-flow velocity (U_cross) is studied. Planar SMD distribution at low GLR improved with increase in U_cross due to secondary atomization of large droplets. Uniform SMD distribution in a range of 10–20 µm is observed for GLR more than 3. The distribution of liquid volume fraction at low GLR condition shows poor dispersion with most of the liquid concentrated near the injector. The liquid volume fraction distribution improves with increase in GLR and better dispersion is observed for GLR more than 3 and two-phase momentum ratio (q₂) greater than 13.06. Spatial bifurcation in liquid fraction is found for high GLR conditions. The SMD in the range of 10–20  µm and uniform distribution of liquid are observed for GLR more than 3 and q₂ > 25.6.     

Atomization of glycerin with a twin-fluid swirl nozzle

Atomization of liquids with high viscosity is always a challenge, especially when small diameter droplets and high liquid flow rates are simultaneously required. In the present research, the performance of a Venturi–vortex twin-fluid swirl nozzle is examined, attending to its capabilities to generate droplets with diameters below 20 µm when atomizing pure glycerin at room temperature. In this nozzle, air is injected tangentially in a central convergent section, and discharges suctioning the liquid fed to a coaxial chamber, here using a gear pump. The resulting spray is visualized and analyzed. Droplet size distributions are measured with a laser diffractometer. As expected, droplet diameter increases with liquid flow rate, and quickly diminishes when air flow rate is increased. Sauter mean diameters (SMD) below 15 µm can be obtained even when atomizing pure glycerin. However, these values are obtained for relatively low glycerin flow rates (∼5 l/h), and with rather wide distributions. For 10 l/h and an air-to-liquid mass flow rate ratio (ALR) of 13.7 more than 26% of the glycerin volume is atomized in droplets smaller than 20 µm. Liquid ligaments are observed near the nozzle exit, but they tend to break up while moving downstream.     

Numerical Solution and Sensitivity Analysis of Pore Liquid Pressure and Cake Porosity to Model Parameters

Filter cake formation is important in groundwater and oil wells where drilling contains suspended mud particles. The accumulation of these mud particles on the borehole wall creates a pressure drop in the well. Furthermore, the migration of colloidal particles into adjacent porous rock could damage the formation and cause productivity decline. In this study, numerical solutions for pore liquid pressure variation across the cake with variable total stress and associated porosity variation are obtained. Mass equations for captured and suspended particles are averaged along the mud cake thickness, taking into account conditions on the cake surface and at the filter septum. The variability of total stress in soil consolidations problem is considered to determine the pore liquid pressure along the mud cake thickness. Then, the relation between porosity and pressure is studied to determine the mud cake porosity. Experimental data obtained by various researchers is used to compare and test the validity of numerical solutions to develop guidelines for model applications. Results show that the pore liquid pressure increases with the decrease of membrane impedance value (i.e. less pervious membrane). Also, the pressure profile has a cubic function of dimensionless cake thickness. The conclusions from the sensitivity analysis conducted in this study agree with earlier conclusions.     

Effects of hydrophilic particles on bubbly flow in slurry bubble column

To investigate the effects of hydrophilic particles on slurry bubble flows in a bubble column, distributions of the local gas holdup and the bubble frequency are measured using an electric conductivity probe. Particles are made of silica and their diameter is 100 μm. The particle volumetric concentration C_S is varied from 0 to 0.40. The measured data imply that the presence of particles promotes bubble coalescence. The film drainage time for two coalescing bubbles in a quasi two-dimensional bubble flow in a small vessel is also measured to quantitatively evaluate the particle effect on coalescence. A particle-effect multiplier is introduced into a coalescence efficiency model by taking into account the data of film drainage time and is implemented into a multi-fluid model. The main conclusions obtained are as follows: (1) the local gas holdup and bubble frequency in slurry bubble flows decrease with increasing the particle concentration, (2) the hydrophilic particles enhance bubble coalescence and the enhancement saturates at C_S ≃ 0.45, (3) the particle effect on coalescence is well accounted for by introducing the particle-effect multiplier to the film drainage time, and (4) the multi-fluid model can give good predictions for the distribution of the local gas holdup in the slurry bubble column.     

Particle behavior in a turbulent pipe flow with a flat bed

Particle behavior in a turbulent flow in a circular pipe with a bed height h = 0.5R is studied at Re_b = 40,000 and for two sizes of particles (5 μm and 50 μm) using large eddy simulation, one-way coupled with a Lagrangian particle tracking technique. Turbulent secondary flows are found within the pipe, with the curved upper wall affecting the secondary flow formation giving rise to a pair of large upper vortices above two smaller vortices close to the pipe floor. The behavior of the two sizes of particle is found to be quite different. The 50 μm particles deposit forming irregular elongated particle streaks close to the pipe floor, particularly at the center of the flow and the pipe corners due to the impact of the secondary flows. The deposition and resuspension rate of the 5 μm particles is high near the center of the floor and at the pipe corners, while values for the 50 μm particles are greatest near the corners. Near the curved upper wall of the pipe, the deposition rate of the 5 μm particles increases in moving from the wall center to the corners, and is greater than that for the larger particles due to the effects of the secondary flow. The maximum resuspension rate of the smaller particles occurs above the pipe corners, with the 50 μm particles showing their highest resuspension rate above and at the corners of the pipe.     

Evaluation of stochastic particle dispersion modeling in turbulent round jets

ODT (one-dimensional turbulence) simulations of particle-carrier gas interactions are performed in the jet flow configuration. Particles with different diameters are injected onto the centerline of a turbulent air jet. The particles are passive and do not impact the fluid phase. Their radial dispersion and axial velocities are obtained as functions of axial position. The time and length scales of the jet are varied through control of the jet exit velocity and nozzle diameter. Dispersion data at long times of flight for the nozzle diameter (7 mm), particle diameters (60 and 90 µm), and Reynolds numbers (10, 000–30, 000) are analyzed to obtain the Lagrangian particle dispersivity. Flow statistics of the ODT particle model are compared to experimental measurements. It is shown that the particle tracking method is capable of yielding Lagrangian prediction of the dispersive transport of particles in a round jet. In this paper, three particle-eddy interaction models (Type-I, -C, and -IC) are presented to examine the details of particle dispersion and particle-eddy interaction in jet flow.     

CFD–DEM simulation of the pneumatic conveying of fine particles through a horizontal slit

Fine particles play a significant role in many industrial processes. To study the dynamic behavior of fine particle and their deposition in rock fractures, the pneumatic conveying of fine particles (approximately 100 μm in diameter) through a small-scale horizontal slit (0.41 m × 0.025 m) was studied, which is useful for the sealing technology of underground gas drainage in coal mining production. The CFD–DEM method was adopted to model the gas-particle two-phase flow; the gas phase was treated as a continuum and modeled using computational fluid dynamics (CFD), particle motion and collisions were simulated using the DEM code. Then, the bulk movement of fine particles through a small-scale horizontal slit was explored numerically, and the flow patterns were further investigated by visual inspection. The simulation results indicated that stratified flow or dune flow can be observed at low gas velocities. For intermediate gas velocities, the flow patterns showed pulsation phenomena, and dune flow reappeared in the tail section. Moreover, periodic flow regimes with alternating thick and sparse stream structures were observed at a high gas velocity. The simulation results of the bulk movement of fine particles were in good agreement with the experimental findings, which were obtained by video-imaging experiments. Furthermore, the calculated pressure drop versus gas velocity profile was investigated and compared with relative experimental findings, and the results showed good agreement. Furthermore, the particle velocity vectors and voidage distribution were numerically simulated. Selected stimulation results are presented and provide a reference for the further study of fine particles.     

Effects of deformed volume, volume fraction and particle size on the deformation behaviour of W/Cu composites

Tungsten/copper (W/Cu) particle reinforced composites were used to investigate the scaling effects on the deformation and fracture behaviour. The effects of the volume fraction and the particle size of the reinforcement (tungsten particles) were studied. W/Cu-80/20, 70/30 and 60/40 wt.% each with tungsten particle size of 10 μm and 30 μm were tested under compression and shear loading. Cylindrical compression specimens with different volumes (D_S = H) were investigated with strain rates between 0.001 s⁻¹ and about 5750 s⁻¹ at temperatures from 20 °C to 800 °C. Axis-symmetric hat-shaped shear specimens with different shear zone widths were examined at different strain rates as well. A clear dependence of the flow stress on the deformed volume and the particle size was found under compression and shear loading. Metallographic investigation was carried out to show a relation between the deformation of the tungsten particles and the global deformation of the specimens. The size of the deformed zone under either compression or shear loading has shown a clear size effect on the fracture of the hat-shaped specimens.The quasi-static flow curves were described with the material law from Swift. The parameters of the material law were presented as a function of the temperature and the specimen size. The mechanical behaviour of the composite materials were numerically computed for an idealized axis-symmetric hat-shaped specimen to verify the determined material law.     

Effects of the mass and volume shrinkage of ground chip and pellet particles on drying rates

The effects of varying the mass and volume of ground chip and pellet particles on the particle drying rate were analyzed. Samples of whole pellets and chips were hammer milled using a 3.2 mm screen and the ground chip and pellet particles were found to have similar size distributions, although the pellet particles were denser and more spherical than the chip particles. Prior to drying, water was added to the particles to obtain 0.10, 0.30, 0.50, 0.70, and 0.90 moisture contents (on a dry mass basis). The moistened particles were subsequently dried in a constant temperature thin layer dryer set at 50, 100, 150, or 200 °C under dry pure nitrogen, dry compressed air, or atmospheric air. The chip and pellet particles exhibited similar degrees of shrinkage, but the pellet particles underwent a greater reduction in their bulk volume during drying. It appears that the more spherical pellet particles are prone to shrinkage in more than one direction, whereas the needle-like chip particle shrink only in one direction. A variable radius first order drying model was found to fit the experimental data better than a fixed radius model.     

Generation and control of tracer particles for optical flow investigations in air

The production of monodisperse tracer particles with suitable properties for optical flow investigations, such as small size, spherical shape, smooth surface, appropriate density and diffraction index, non-evaporating and agglomerating, electrically neutral, non-toxic and easily removable, is a challenging task due to the sensitivity of the particle size distribution to the boundary conditions. In order to obtain general design and operating rules for atomizers which are mostly applied for air flows, this dependence is investigated here. It is shown that high concentrations of narrow band particle size distributions, with a mean diameter below 1 µm, can easily be generated by means of multi-hole nozzles under over-critical pressure conditions, when the kinetic energy, transferred into the seeding liquid through the nozzle exits, is well balanced with the liquid volume inside the atomizer. In addition, flow visualization pictures are presented which permit a useful assessment of the functioning of the nozzles and reveal operating features of the atomizers which were not previously known. In particular, it is shown that existing explanations of the importance of certain design features of the Laskin nozzle are of minor importance for the generation of appropriate tracer particles.     

Numerical solution and sensitivity analysis of filter cake permeability and resistance to model parameters

The migration and capture of solid particles in porous media occur in fields as diverse as water and wastewater treatment, well drilling, and in various liquid-solid separation processes. Filter cakes are formed when a liquid containing solid particles is forced through a pervious surface which allows the liquid transport while retaining solid particles. Following a literature survey, a governing equation for the cake thickness is obtained by considering the instantaneous mass balance. Later, numerical solutions for the cake thickness, cake permeability, cake resistance, solid particle velocity (cake compression rate) and concentration of suspended particles are obtained and a sensitivity analysis is conducted. The sensitivity analysis shows that the cake permeability and cake resistance are more sensitive to the rate constant of cake erosion than they are to the rate constant of particle capture. However, the concentration of suspended solid particles, and the solid velocity are mostly sensitive to the slurry parameter and the rate constant of particle trapping. Moreover, cake permeability, compressibility, concentration of suspended particles, and the solid velocity are very sensitive to the concentration at the filter septum. Finally, as expected, with a thicker slurry, more particles are captured inside the cake, thus forming a thicker and more resistant cake. Also, as more particles are being filtered at the filter septum, a thinner cake is formed and a smaller effluent concentration is achieved.     

Heat transfer and hydrodynamic studies on two- and three-phase fluidized beds of floating bubble breakers

Heat transfer characteristics in three-phase fluidized beds of floating bubble breakers have been studied in a 0.142 m I.D. x 2.0 m high Plexiglas column fitted with an axially mounted cylindrical heater.Effects of the liquid and gas velocities, the particle size, the volume ratio of floating bubble breaker to particles on phase holdup, the vertical bubble length, and the heat transfer coefficient have been determined.In the bubble-coalescing regime, the heat transfer coefficient in three-phase fluidized beds having the volume ratio V_f/V_s of 10–15% produced a maximum increase in heat transfer coefficient of about 20% in comparison to that in the bed without floating bubble breakers. Also, bubble length and gas-phase holdups exhibited their maximum and minimum values at a volume ratio of 10–15%. The heat transfer coefficient in three-phase fluidized beds of floating bubble breakers can be estimated from the surface renewal model with isotropic turbulence theory.Heat transfer coefficients expressed in terms of the Nusselt number have been correlated with the particle Reynolds number and the volume ratio of floating bubble breakers to particles.     

Technique for quantitative mapping of three-dimensional liquid–gas phase boundaries in microchannel flows

A diagnostic technique capable of characterizing interfaces between transparent, immiscible fluids is developed and demonstrated by investigating the morphology of liquid–gas interfaces in an adiabatic two-phase flow through a microchannel of 500 μm × 500 μm square cross section. Water seeded with 0.5 μm-diameter fluorescent polystyrene particles is pumped through the channel, and the desired adiabatic two-phase flow regime is achieved through controlled air injection. The diagnostic technique relies on obtaining particle position data through epifluorescent imaging of the flow at excitation and emission wavelengths of 532 nm and 620 nm, respectively. The particle position data are then used to resolve interface locations to within ±1 μm in the focal plane. By mapping the interface within individual focal planes at various depths within the channel, it is possible to determine the complete liquid–gas interface geometry across the channel cross section in a dynamic flow environment. Utilizing this approach, the liquid–gas phase boundaries of annular flows within a microchannel have been successfully characterized.     

Cold flow experiments in an entrained flow gasification reactor with a swirl-stabilized pulverized biofuel burner

Short particle residence time in entrained flow gasifiers demands the use of pulverized fuel particles to promote mass and heat transfer, resulting high fuel conversion rate. The pulverized biomass particles have a wide range of aspect ratios which can exhibit different dispersion behavior than that of spherical particles in hot product gas flows. This results in spatial and temporal variations in temperature distribution, the composition and the concentration of syngas and soot yield. One way to control the particle dispersion is to impart a swirling motion to the carrier gas phase. This paper investigates the dispersion behavior of biomass fuel particles in swirling flows. A two-phase particle image velocimetry technique was applied to simultaneously measure particle and gas phase velocities in turbulent isothermal flows. Post-processed PIV images showed that a poly-dispersed behavior of biomass particles with a range of particle size of 112–160 µm imposed a significant impact on the air flow pattern, causing air flow decelerated in a region of high particle concentration. Moreover, the velocity field, obtained from individually tracked biomass particles showed that the swirling motion of the carrier air flow gives arise a rapid spreading of the particles.     

Effect of heater size on ultrasonic enhancement of boiling in water and surfactant solutions

Experiments were performed to study enhancement of heat transfer from the wire of d = 50 µm and the tube of d = 1.5 mm in subcooled pool boiling by ultrasonic waves. The working fluids are clean water and Alkyl (8-16) Glucoside surfactant solutions of different concentrations and bulk temperature 30 °C. The wire resistance was translated to the temperature, using the calibration data, the temperature of the tube was measured by thermocouple. The differences between effect of ultrasonic field on boiling in water for heaters of d = 50 µm and d = 1.5 mm may be summarized as follows: for boiling on the wire of d = 50 µm in subcooled water, T_b = 30 °C, enhancement of heat transfer coefficient due to applied ultrasonic field is about 70% and 20% at heat flux q = 620 kW/m² and q = 1350 kW/m², respectively. For boiling in surfactant solutions at the same boiling conditions enhancement of heat transfer coefficient is in the range of 5–10% at heat flux q = 620 kW/m² and 10–16% at heat flux q = 1350 kW/m² depending on solution concentration. For boiling on the tube of d= 1.5 mm in subcooled water, T_b= 30 ℃, enhancement of heat transfer coefficient due to applied ultrasonic field is about 50% and 45% at heat flux q = 500 kW/m² and q = 2500 kW/m², respectively. The same values are obtained for boiling in surfactant solution of concentration C = 250 ppm. For the wire of d = 50 µm the heat transfer enhancement due to acoustic vibrations in surfactant solutions is not as strong as in water. This fact may be considered as evidence of significant role of relationship between jet flow and ultrasonic field.     

Direct numerical simulation of particle dispersion in a turbulent jet considering inter-particle collisions

To investigate the behaviour of inter-particle collision and its effects on particle dispersion, direct numerical simulation of a three-dimensional two-phase turbulent jet was conducted. The finite volume method and the fractional-step projection algorithm were used to solve the governing equations of the gas phase fluid and the Lagrangian method was applied to trace the particles. The deterministic hard-sphere model was used to describe the inter-particle collision. In order to allow an analysis of inter-particle collisions independent of the effect of particles on the flow, two-way coupling was neglected. The inter-particle collision occurs frequently in the local regions with higher particle concentration of the flow field. Under the influence of the local accumulation and the turbulent transport effects, the variation of the average inter-particle collision number with the Stokes number takes on a complex non-linear relationship. The particle distribution is more uniform as a result of inter-particle collisions, and the lateral and the spanwise dispersion of the particles considering inter-particle collision also increase. Furthermore, for the case of particles with the Rosin–Rammler distribution (the medial particle size is set d₅₀ = 36.7 μm), the collision number is significantly larger than that of the particles at the Stokes number of 10, and their effects on calculated results are also more significant.     

Numerical simulation of a supersonic gas–solid flow over a blunt body: The role of inter-particle collisions and two-way coupling effects

A supersonic dusty gas flow over a blunt body is considered. The mathematical model of the two-phase gas–particle flow takes into account the inter-particle collisions and the two-way coupling effects. The carrier gas is treated as a continuum, the averaged flow field of which is described by the complete Navier–Stokes equations with additional source terms modeling the reverse action of the dispersed phase. The dispersed phase is treated as a discrete set of solid particles, and its behavior is described by a kinetic Boltzmann-type equation. Particles impinging on the body surface are assumed to bounce from it. Numerical analysis is carried out for the cross-wise flow over a cylinder. The method of computational simulation represents a combination of a CFD-method for the carrier gas and a Monte Carlo method for the “gas” of particles. The dependence of the fine flow structure of the continuous and dispersed phases upon the free stream particle volume fraction α_p∞ and the particle radius r_p is investigated, particularly in the shock layer and in the boundary layer at the body surface. The particle volume fraction α_p∞ is varied from a negligibly low value to the value α_p∞ = 3 × 10⁵ at which inter-particle collisions and two-way coupling effects are simultaneously essential. Particular attention has been given to the particles of radii close to the critical value r_p1, because in this range of particle size the behavior of the particles and their effect on the carrier gas flow are not yet completely understood. An estimate of the turbulent kinetic energy produced by the particles in the shock layer is obtained.     

Dynamical deformation of a flat liquid–liquid interface

The passage of solid spheres through a liquid–liquid interface was experimentally investigated using a high-speed video and PIV (particle image velocimetry) system. Experiments were conducted in a square Plexiglas column of 0.1 m. The Newtonian Emkarox (HV45 50 and 65% wt) aqueous solutions were employed for the dense phase, while different silicone oils of different viscosity ranging from 10 to 100 mPa s were used as light phase. Experimental results quantitatively reveal the effect of the sphere’s size, interfacial tension and viscosity of both phases on the retaining time and the height of the liquid entrained behind the sphere. These data were combined with our previous results concerning the passage of a rising bubble through a liquid–liquid interface in order to propose a general relationship for the interface breakthrough for the wide range of Mo ₁/Mo ₂ ∈ [2 × 10⁻⁵–5 × 10⁴] and Re ₁/Re ₂ ∈ [2 × 10⁻³–5 × 10²].     

Study of airborne particles produced by normal impact of millimetric droplets onto a liquid film

The aim of this work is to carry out an experimental investigation into the generation of airborne microparticles when millimetric droplets of aqueous solutions impact onto a liquid film. Impact experiments using 3.9 mm diameter droplets were carried out for Weber numbers between 159 and 808, with a fixed Ohnesorge number of 2 × 10⁻³ and film parameters S _f (the ratio between the thickness of the liquid film h _film and the diameter of the impacting droplet d _i) between 0.3 and 1. Observed results show that the deposition/splashing threshold is independent of the parameter S _f in agreement with the data in the literature. The aerosol measurement results demonstrate the production of solid particles from the evaporation of secondary microdroplets with diameters less than 30 μm formed when splash occurs. The median diameter of these microdroplets is around 20 μm, corresponding to a value of d ₅₀/d _i = 5 × 10⁻³. Taken together, the results show that the mass and the number of particles emitted increase as the Weber number increases. Moreover, at a Weber number of 808, the results show that the mass and number of particles emitted increases as the parameter S _f decreases. In this case, the mean number of microdroplets emitted per impact is equal to 14 for S _f = 1 and equal to 76 for S _f = 0.3.     

Yield stress and flow behavior of concentrated ferrofluid-based magnetorheological fluids: the influence of composition

In this paper, the magnetorheological (MR) and magnetoviscous properties of ferrofluid-based iron particle suspensions were investigated. The 2.1-µm mean size Fe particles were dispersed in high-concentration transformer oil-based ferrofluid, the iron particle volume fraction in the resulting nano-micro composite magnetorheological fluid samples varying from Φ _Fe = 5 to 40 %. The ferrofluid carrier has φ _p = 23 % solid volume fraction of magnetic nanoparticles stabilized with chemisorbed oleic acid monolayer and without any excess surfactant. In the absence of the field, the ferrofluid has a quasi-Newtonian behavior with a weak shear thinning tendency. The static yield stress shows an increase of about 3 orders of magnitude for an iron particle content of approx. Φ _Fe = 25 % (Φ _tot = 42.25 %), while above this value, a saturation tendency is observed. The dynamic yield stress (Bingham model) also increases with the magnetic induction and the particle volume fraction; however, the saturation of the MR effect is less pronounced. The relative viscosity change has a maximum at Φ _Fe = (10–15) % due to the accelerated increase of the effective viscosity of the composite for higher Fe content. Addition of micrometer-sized iron particles to a concentrated ferrofluid without any supplementary stabilizing agent proved to be a direct and simple way to control the magnetorheological and magnetoviscous behavior, as well as the saturation magnetization of the resulting nano-micro composite fluid to fulfill the requirements of their use in various MR control and rotating seal devices.