Influence of atomiser design and coaxial gas velocity on gas entrainment into sprays

To study the influence of atomiser design and coaxial air velocity on entrainment of coaxial and confined sprays, the sprays issuing from a number of different atomisers are investigated experimentally under various external flow conditions. Air and liquid velocity profiles in the spray are determined by phase-doppler-anemometry (PDA), liquid mass flux profiles are measured using a mechanical droplet collection device (patternator) with a high spatial resolution. Experiments are performed in a 300 mm diameter vertical wind tunnel at superficial air velocities up to 30 m s⁻¹ at liquid flow rates from 0.083 to 0.278 kg s⁻¹. Experimental results are compared with free spray data and a generalised free jet theory. Comparing the sprays from different atomisers displays high induced air flow rates for high velocity narrow sprays and high entrained air flow rates for wide sprays. The influence of coaxial air velocity depends largely on the width of the spray and may be predicted by a simple model that is developed to determine the entrainment of coaxial and confined sprays from free spray data.     

Solid entrainment rate into gas and gas—solid,two-phase jets in a fluidized bed

A mathematical model for solid entrainment into a permanent flamelike jet in a fluidized bed was proposed. The model was supplemented by particle velocity data obtained by following movies frame by frame in a motion analyzer. The experiments were performed at three nominal jet velocities (35, 48, and 63 m/s) and with solid loadings ranging from 0 to 2.75. The particle entrainment velocity into the jet was found to increase with increases in distance from the jet nozzle, to increase with increases in jet velocity and to decrease with increases in solid loading in the gas—solid, two-phase jet.     

An experimental study of air entrainment at a solid/liquid/gas interface

Some data from an experimental study of air entrainment into a fluid bath by a continuous moving plane tape is presented. The separate effects of surface tension and viscosity are described and the various modes of air entrainment are given in the context of fluid properties. The velocity of air entrainment is found to be a function of surface tension and viscosity for viscosities less than 4.65 poise. For viscosities greater than this value, the air entrainment velocity tended to a constant value of 9.5 cm sec^?1 independent of surface tension. Relationships of the form: We = k Re^a and We = c (Bo + 1)Re^d, are suggested by analogy to describe air entrainment data without and with buoyancy effects. Data from studies on four tapes and nine fluids gave a very high correlation when plotted in the above form. The data is in substantial agreement with that from similar studies, and shows that the condition Ca = We/Re = constant is not a global criteria for air entrainment by a plunging surface. The experimental data shows that air entrainment velocity may be estimated from the relationship V_AF = 67.679 (μ√(g/ρσ))^?0.672 for the normally plunging plane tapes studied.     

Solids entrainment into gas, liquid, and gas-liquid spray jets in fluidized beds

The injection of liquid into a fluidized bed is a crucial step in many processes such as fluid coking, fluid catalytic cracking, or gas-phase polymerization, whose performance greatly depends on good and rapid contact between the injected liquid and the fluidized particles. The liquid spray, created by two-phase (gas-liquid) nozzles, forms a jet, i.e. a gas-rich cavity within the fluidized bed. Past studies have shown that good liquid-solid contact requires a large entrainment rate of particles into the jet, followed by intensive mixing of liquid droplets and entrained particles within the jet. The objective of this study is the experimental measurement of solids entrainment into spray jets. The specific application of interest is the enhancement of solids entrainment under conditions relevant to the fluid coking process.A novel and accurate experimental technique has been developed to measure the solids entrainment from a fluidized bed into two-phase gas-liquid jets, gas jets and liquid jets. The effects of operating conditions of the nozzle (sonic versus subsonic) and of the fluidized bed on the solids entrainment have been investigated. The differences between the mechanisms of solids entrainment for two-phase gas-liquid, gas and liquid jets have been analyzed.This experimental tool has been applied to the design and testing of a mixing chamber consisting of a cylindrical tube placed at a certain distance downstream of the nozzle tip, resulting in a confined, turbulent jet with enhanced liquid-solid mixing properties.     

Turbulent transition in impactor jets and its effects on impactor resolution

A review of a number of widely used impactors suggests that the poorer performance often seen in some of the stages is due to the onset of turbulence in the jet at too large values of either the Reynolds number Re, or the nozzle-to-collector distance L. This phenomenon is studied here by measuring the collection efficiency versus Stokes number curves η(S) of a low-pressure thin-plate-orifice impactor as a function of Re and L (measured in units of the orifice diameter d_n). A drastic broadening of the η(S) curve is observed in the vicinity of a critical Reynolds number $\text{Re}{}^1\text{(L/d}{}_{\mathrm{<mtext>n</mtext>}}\text{)}$. $\text{Re}{}^1$ increases at diminishing L/d_n, taking values near 800 and 400 for L/d_n of 3.1 and 4, respectively. No transition is seen at L/d_n=1 or 2, even at Re as high as 2700. This transition in the jet modifies both the high and the low S tails of the η(S) curve. It should be distinguished from a previously studied turbulent transition of the boundary layer near the collector plate, which arises at much larger Reynolds numbers, changes only the low Stokes number tail of the η(S) curves and disappears when using small collector plates. A specialized experimental apparatus is used to provide an initial jet with very low turbulence level, as well as to isolate incipient turbulence effects from other mechanisms leading to broadening of the η(S) curves. The particles are brought very close to the axis via aerodynamic focusing, while particle capture by Brownian diffusion is offset with a repulsive electric field. Free-stream turbulence ahead of the impactor nozzle is eliminated by passing only a small fraction of the flow through the critical orifice and the focusing lenses. The remaining gas required to attain jet Reynolds numbers up to 3700 is introduced laminarly and axisymmetrically as sheath air through an outer porous wall right before the impactor nozzle. At Re in the range of a few tens, a strong increase of the critical Stokes number with increasing L/d_n is observed.     

Effect of addition of CO2 to the fuel on combustion of gas jets in air

Results of an experimental study of the effect of addition of CO₂ to methane and to a propane-butane mixture on conditions of stabilization of the diffusion flame in air are reported. It is shown that the flame lift-off length substantially increases with increasing fraction of CO₂ in the fuel; the main reason is the changes in the velocity and concentration in the region of the “ignition” points; a secondary factor is an increase in the characteristic combustion time. Data on the influence of the CO₂ admixture in the fuel in amounts greater than the fuel flow rate up to ≈1.5 times on the characteristic combustion time are obtained.     

Effect of secondary gas injection on the particle entrainment rate in a gas fluidized bed

The effect of secondary gas injection on the particle entrainment rate was measured in a cold model fluidized bed (i.d. 0.1 m, height 2.25 m) and discussed. Sand particles below 0.425 mm in screen size were used as bed materials. The particle size (0.128–0.363 mm), the overall superficial gas velocity (0.78–2.76 m/s), the secondary gas fraction (0–0.5), and the static bed height (0.1–0.3 m) were considered as experimental variables. The particle entrainment rate decreased with an increase of the secondary gas fraction. The injection level of the secondary gas was shown to have an important influence on its effect. The effect of the secondary gas was appreciable for over-bed injection; the effect was reduced, however, when the injection level was placed in the splash zone or in the dense phase by means of increasing the bed height.     

Fouling mitigation in crossflow filtration using chaotic advection: A numerical study

Fouling mitigation in a crossflow filtration system using chaotic advection is numerically studied. A barrier-embedded partitioned pipe mixer (BPPM) is selected as a static mixer, creating chaotic advection in a laminar flow regime. Mixing characteristics are controlled via two design parameters, the mixing protocol and the dimensionless barrier height (β). The average dimensionless concentration boundary layer thickness () and the surface-averaged dimensionless wall concentration () dramatically decrease with the introduction of the BPPM, incorporating a chaotic flow system. and decrease as β increases, and the largest reduction of is observed in the counter-rotational protocol. A semi-ring configuration is revealed to be the most appropriate configuration to characterize mixing near the membrane surface. It is found that a filtration system with a globally chaotic flow shows the best mixing performance and the largest reduction of fouling.     

A new approach to jet phenomena gas entrainment and recirculation in a bidimensional spouted fluidized bed

Although gas jets injected into fluid beds fluctuate, previous models assumed that they were steady. A new model separates the bed in a zone in which the jet is never present and a zone in which the jet fluctuates (a point in this zone will alternately be in the jet and in the emulsion phase). Its predictions matched the rate of gas entrainment into the jet and the rate of gas recirculation between emulsion and jet which were observed in a large 2 dimensional bed equipped with a “V” grid and a central jet.     

Further studies on momentum dissipation of grid jets in a gas fluidized bed

Four experimental aspects of momentum dissipation of grid jets in a gas fluidized bed of cracking catalyst are reported. First, an investigation was made into the radial distribution of axial momentum for air jets issuing from vertical long nozzles into a minimally fluidized bed. The theory of Abramovich was compared with these experimental results. Second, data on the distribution of axial momentum along the axis for vertical air jets issuing from single and multi-hole grids were compared with previous work. Third, the axial dissipation of vertical air jets loaded with fluid cracking catalyst issuing into air and also into a fluid bed was studied. Finally, the shape of the boundary of a jet dissipating into a fluid bed was compared to that given by theory.     

Geometrical and hydrodynamical study of gas jets in packed and fluidized beds using magnetic resonance

Magnetic resonance (MR) was used to image the motion of particles and gas just above the distributor of 3D beds of particles fluidized by air. Three different distributors were used: (i) a single‐orifice distributor, with orifice diameters 1.0–4.0 mm, (ii) a plate, drilled with a triangular array of 79 holes, each of 0.35 mm diameter, with a central nozzle containing a single hole of diameter 1.0, 2.5, or 9.0 mm, (iii) distributors with two or three orifices and diameters of 1.0 or 2.5 mm. It proved possible to extract geometrical information, such as the length of a jet, from MR images, each averaged over ～5 min. Also, light was shed on the question of why is there such a discrepancy between reported jet‐lengths. The fluidization state, the “start‐up” procedure and also the number of holes all play a significant role in determining the measured distance a jet penetrates into a bed. The question as to whether the observed voids represent permanent jets or streams of bubbles was considered. The evidence from ultra‐fast MR measurements strongly suggests that only the lower part of a jet from an orifice in a multi‐orifice distributor is permanent; bubbles form at the top of the jet. Consequently, the top of each jet is transient. However, most of the jet from a single orifice is a permanent cavity when the bed of particles is not fluidized. The length of a jet was successfully correlated with operating variables using dimensional analysis. Finally, the flow of particles around a single jet was measured with high resolution MR.     

Numerical experiments in fire science: a study of ceiling jets

In this paper, a numerical experiment, consisting of 90 simulations in Fire Dynamics Simulator (FDS), is conducted for two different purposes. Firstly, numerical experiments are explored as a research method in fire science, and it is demonstrated that numerical experiments could be used as a complement to traditional fire experiments in fire science research. Secondly, an evaluation of previously derived correlations for ceiling jet excess temperatures and velocities is performed with the help of the results from the numerical experiment. The procedure used in this evaluation constitutes an outline for how a numerical experiment can be conducted in fire science. The evaluation indicates that the existing correlations will give a good estimate of the average temperature in a ceiling jet calculated by FDS. However, the correlations do not give a good estimate of the maximum excess temperature. A new correlation to estimate the maximum temperature has therefor been developed and is presented in the paper. Copyright © 2014 John Wiley & Sons, Ltd.     

Clathrate hydrate formation using liquid jets impinging on each other: An observational study using paired water jets or water and methylcyclohexane jets

This paper reports the experimental attempt at visually observing and characterizing the formation and/or growth of clathrate-hydrate crystals accompanying the impingement of two cylindrical jets of either the same liquid, water, or two mutually immiscible liquids, water and methylcyclohexane (MCH), in a difluoromethane (HFC-32) gas phase held at a constant pressure. We observed that the hydrate crystals intermittently emerged on the liquid sheet radially expanding from the point of the jet collision, flowed to the periphery of the sheet, and then engulfed in the droplets when they were disintegrated from the periphery of the sheet. The observations of the droplets falling in the gas phase indicated that even some of the droplets initially containing no hydrate started to be covered by hydrate films presumably as the result of delayed hydrate nucleation on the individual droplets. The measurements of the gas consumption rate due to hydrate formation showed that the ratio of the water-to-hydrate conversion and the efficiency of using the cool energy supplied by the liquid jets for hydrate formation tended to increase as the liquid flow rate per jet increased and thereby intensified the liquid atomization, indicating an increasing proportion of hydrate formation on the droplets with an increase in the liquid flow rate.     

Numerical and experimental investigation on surface air entrainment mechanisms of a novel long‐short blades agitator

We investigate numerically and experimentally the mechanisms of surface air entrainment in the vessels equipped with the long‐short blades agitator. VOF method coupled with LES model is used to visualize the surface air entrainment process. In the case of partial submergence of the long blades (LBs), the interaction of the LBs with the liquid free surface creates a depression behind the LBs. Backfilling of the liquid into the depression leads to gas separation and entrapping into the liquid. The critical tip velocity of the LBs, u_tip,c, for the onset of gas entrainment is measured in vessels with diameters, T = 200～600 mm. It is found that when H/T ≥ 1.0, u_tip,c is determined by the LBs, independent of the liquid level. u_tip,c is also affected by the size of the vessel through the diameter of the sweeping circle of the LBs, but for substantially large vessels, it approaches a constant value. © 2017 American Institute of Chemical Engineers AIChE J, 63: 316–325, 2018     

Experimental data for code validation: Horizontal air jets in a semicircular fluidized bed of Geldart Group D particles

Experiments were conducted with 6 mm plastic beads (Geldart Group D) in a semi‐circular, gas‐fluidized bed with side jets. Attention was paid to particle characterization and bed measurements, making the resulting dataset ideal for CFD‐DEM validation and uncertainty quantification. The bed was operated slightly above and below the minimum fluidization velocity, with additional fluidization provided by one of two pairs of opposing jets located above the distributor near the flat, front face of the unit. Care is taken to report material properties and bed conditions with either measured distribution functions or uncertainty bounds. High‐speed video imaging and particle tracking velocimetry are used to extract bin‐averaged velocity profiles, which are used to extract jet penetration depths. The time‐averaged mean and standard deviation of the bed pressure drop is also reported. Finally, the lower jets are also inserted into the bed until the opposing jets merge to form a spout‐like pattern. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2351–2363, 2018     

A study on mixing mechanism in air spouted beds

The mixing mechanism of solid particles in the air spouted bed was studied by employing an impulse response technique. The particles, german millet and barley, were spouted by air in columns of diameter of 8.4 cm and 12.6 cm. In the proposed theory, it was assumed that the mixing of the particles in the spouted bed occurs when they circulate through spout, fountain and annulus. Also a theoretical model was derived by assuming that the particle flow in the annulus is a combination of many annular plug flows while the flow in the spout as well as in the fountain is a mixed flow The residence time distribution of the particles in the bed was measured by injecting a portion of colored particles into the feed line and analyzing the concentration of the colored particles in the discharge line. The experimental results and the proposed theory were most satisfactorily agreed when the null residence time in the spout and in the fountain was assumed in the theoretical model.     

Experimental study on the air core in a hydrocyclone

The evolution of an air core is an important phenomenon in hydrocyclones, which are sensitive to the operating conditions. The morphology of the air core in a hydrocyclone is studied experimentally, using a high-speed video camera and a noise analyzer. Three stages are observed during the increase of flow rate from 300 to 2,400 L per hour, which could be separated by both the noise measurement and the stability of the air core. The flow rate is found to have a nonmonotonic effect on the diameter of the air core, leading to a peak diameter. The influence of the overflow and underflow on the underflow-to-throughput ratio and air flow patterns is also investigated. It is found that the underflow-to-throughput ratio and air flow patterns are sensitive to the valve openings of both outlets.     

A theoretical and experimental study of the crossflow microfiltration process of silica particles in an aqueous suspension

This work presents a theoretical and experimental analysis of a crossflow microfiltration process of silica particles in suspension. The silica suspensions were 0.001 M of NaCl with a pH of 6 (to maintain a constant ionic force within the medium to produce a stable silica particle suspension) for three different concentrations of silica particles: 100, 300, and 500 mg L⁻¹. The membrane used in the crossflow microfiltration experiments was a commercial polymeric membrane, microporous, asymmetric with a nominal pore diameter of 0.2 µm, manufactured by OSMONICS (Minnetonka, MN). The experiments were performed in a bench scale crossflow microfiltration system with a flat rectangular membrane cell. The permeate flux was obtained as a function of the transmembrane pressure, the crossflow velocities, and the silica particles concentration. The mathematical model describing the process takes into account the variation of the physical properties of the suspension (dynamic viscosity and mass diffusivity) with the silica concentration. The experimental data are used to predict the maximum silica concentration at the membrane surface as a function of the operating conditions.     

A case study on suspended particles in a natural gas urban transmission and distribution network

Suspended particles in the natural gas transmission and distribution network of the city of Kerman, Iran were investigated. Particle concentration and size distribution were measured in different locations of the natural gas pipeline network. Particle samplings were carried out in two seasons: summer, when there is the lowest consumption, and winter, when there is the highest consumption of natural gas. Additional particle characterization was carried out by scanning electron microscopy coupled with energy dispersion X-ray (SEM/EDX) and X-ray diffraction(XRD) analyses. Particle concentration was found to be significantly higher in winter as compared to summer. The range of particle concentrations in summer was from 0.12 mg/Nm³ at the end of the pipeline to 4.7 mg/Nm³ at the network entrance, and from 0.30 mg/Nm³ to 22.1 mg/Nm³ in winter. Particle size distribution showed a higher frequency of smaller particles in winter than in summer. Larger particles were more likely to exist at the network entrance as compared to the exit. The average particle size ranged from 181 μm at the network end to 253 μm at the entrance in summer, and from 74 μm to 209 μm in winter. Particle characterization confirmed the presence of corrosion products in the suspended particles.