Gas–liquid two-phase flow division at a micro-T-junction

The phase distribution of a gas–liquid flow through a 1 mm T junction has been studied. Gas superficial velocities of 2.5 and 4.9 m/s and liquid superficial velocities 0.09–0.42 m/s were investigated. Increasing the liquid superficial velocity was shown to decrease the liquid taken off at the side arm. Increasing the gas superficial velocity was found to affect the phase split by increasing the fractional liquid taken off. It was noticed that pressure has no influence in the phase split when it was increased from 0.13 to 0.18 MPa. From examination of data from different pipe sizes, it was seen that the 1 mm T-junction shared similar split characteristics as those observed for larger diameter junctions. Finally, the gas–liquid flow pattern through the junction was observed to be slug for a range of gas and liquid superficial velocities.     

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.     

CFD simulation of flow pattern and jet penetration depth in gas-fluidized beds with single and double jets

A simple two-fluid model is validated by comparing single-jet fluidization experiments and numerical predictions. Subsequently, flow pattern and jet penetration depth are explored numerically in the bed with double jets under equal and unequal gas velocities. Glass balltoni with a density of 2550 kg/m³ and a diameter of 275 μm is employed as solid phase. The model used in this study considers the effect of the dispersed solid phase on both gas and particle momentum equations of the inviscid model A (Gidaspow, 1994). Numerical simulations are carried out in the platform of CFX 4.4, a commercial CFD code, together with user-defined FORTRAN subroutines. Both jet penetration depth and jet frequency predicted are in good quantitative agreement with measurements in an incipiently fluidized bed with a single jet. By combining solid volume fraction distribution and particle-phase velocity vector profile, three flow patterns (isolated, merged and transitional jets) are identified in the gas-fluidized bed with double jets, which depend more on the nozzle distance than the jet gas velocity. For the equal jet gas velocity, the jet penetration depth decreases with increasing nozzle distance in the merged-jet and transitional-jet regions, then reaches a minimum value in the transitional-jet region, and finally keeps steady in the isolated-jet region. For the unequal jet gas velocity, the merged jet penetration depth increases with increase in the velocity of one jet as the other jet gas velocity is fixed, whilst the jet penetration depths change a little in the transitional-jet region and remain a constant in the isolated-jet region.     

Rotating fluidized bed with a static geometry: Guidelines for design and operating conditions

Computational fluid dynamics (CFD) simulations of the hydrodynamic behavior of rotating fluidized beds in static geometry (RFB-SG) are carried out for gas–solid flows. The rotating motion of the reactor bed is induced by the tangential injection of the gas along the circumference of the fluidization chamber. Steep gradients in the gas velocity fields both in radial and tangential direction generate turbulence. The radial and tangential drag forces fluidize the particle bed in both radial and tangential direction.An Eulerian two-fluid model is used. Gas phase turbulence is accounted for by a k–ε model adapted for rotational flows. The RFB-SG simulations provide guidelines for a design and operation with a high efficiency in gas–solid momentum transfer, excellent gas–solid separation and limited solids losses. Hydrodynamic variables like the centrifugal force, the injection pressure, the radial and tangential slip velocities, solids hold-up are calculated for both polymer particles (300 μm, 950 kg/m³, Geldart Group B) and glass beads (70 μm, 2500 kg/m³, Geldart Group A) to allow for a comparison among different fluidization chamber designs. Unstable bed behavior, like slugging and channeling, is also numerically predicted.     

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.     

Local time-average gas holdup comparisons in cold flow fluidized beds with side-air injection

Fluidized beds are interesting and useful processing systems that are employed in many industries such as processing biomass into biofuels or the coating of pharmaceuticals. Knowledge of fluidized bed hydrodynamics is necessary for the design and scale-up of such devices. This paper describes the local time-average differences of gas holdup in a 10.2 cm and 15.2 cm diameter cold flow fluidized bed that were recorded using 3D X-ray computed tomography. Three different Geldart type B bed materials are studied at various superficial gas velocities and side-air injection flow rates, where the side-air injection simulated the immediate volatilization of a fuel like coal or biomass particles. Variations in side-air injection flow rate have little influence on global bed hydrodynamics, but significantly affects local gas holdup. Axial annular flow dominates over all flow conditions for each material and bed diameter. Wall effects increasingly influence hydrodynamics as bed diameter decreases for all materials.     

Particle deposition velocity onto a face-up flat surface in a laminar parallel flow considering Brownian diffusion and gravitational settling

The Gaussian Diffusion Sphere Model (GDSM) was developed and improved to reflect the effects of gravitational settling as well as Brownian diffusion of aerosol particles on deposition velocity onto a face-up flat surface in a laminar parallel flow. The model improvement also includes the applicability of the GDSM to a flat surface of any shape with finite dimensions. When deposition velocity for a face-up circular flat plate of 45 cm diameter, representing e.g. a semiconductor wafer in a laminar parallel flow, was calculated by the GDSM and compared with that by the theory of Liu and Ahn (1987). Particle deposition on semiconductor wafers. Aerosol Science and Technology, 6, 215–224, the agreement was good for the tested particle sizes ranging 0.003–1 μm and free stream velocities ranging 5–500 cm/s. Based on this result, deposition velocities onto the face-up square flat plates with different orientations in a laminar parallel flow, simulating e.g. photomasks, were predicted.     

Local Flow and Heat Transfer Behaviours in V-L-S Fluidized Bed Evaporator

The fluidized bed evaporator is a subject of considerable interest and can be employed in the process industries. However, the V-L-S flow behavior and heat transfer characteristics are still not well understood. In this work, a vertical fluidized bed evaporator with external natural-circulation flow boiling was established to investigate the local V-L-S flow behaviour and heat transfer characteristics and to reveal the influence of flow on heat transfer performance. It consists of a heated tube with inner diameter of 3.8 × 10⁻² m and height of 1.4 m and a circulating tube with the same inner diameter and height of 0.92 m. Both tubes are made of quartz glass plated with transparent electrically heating film. The solid particles added to the evaporator are glass, ceramic, Teflon™ and poly-formaldehyde beads or cylinders with the diameter ranging from 1.8 × 10⁻³ to 4 × 10⁻³ m. The particle volume changes from 0 to 1 × 10⁻³ m³ and heat flux varies from 5 × 10³ to 1.2 × 10⁴ W m⁻². The local velocity and holdup of solid particles, flow region transition, length of the V-L-S flow boiling, fluid circulating velocity and pressure drop in the V-L-S fluidized bed evaporator were visually investigated with CCD measuring technique. The main results on flow are as following. Axial solid holdup in heated tube of the evaporator decreases obviously for middle-density particle systems against the direction of the gravity. Three flow regions can be found in heated tube of the evaporator: L-S region, transition region and V-L-S region, and in transition region, the radial profile of solid holdups is relatively uniform. The increase of particle volume enlarges the length of V-L-S region and pressure drop, while decreases the circulating velocity of fluid mixture. The average heat transfer film coefficients of V-L-S flow boiling were estimated and a dimensionless correlation was obtained based on 120 sets of experimental data with a maximum relative deviation of 10.8%. The axial variation of the heat transfer coefficients has close relation to the axial distribution of the solid holdups.     

Thermal inefficiency model for determination of the bed-to-gas heat transfer coefficients with effectiveness factor in a fluidised bed

Process operations often involve the physical interaction of a gas and a solid phase. Fluidised bed heat transfer can be characterised by limited space–time (τ) on the basis of particle volume in the bed. As aimed in this study, a thermal inefficiency model (TIM) was developed using a pseudo-steady-state heat balance, i.e., equating the electrical power input to the rate of heat transfers from the bed to the gas. A bench-scale fluidised bed (105 × 200 mm) was operated for obtaining the gas temperature profiles. Temperature data were used for extracting the bed-to-gas heat transfer coefficients (h_BG) with effectiveness factors (η) from the TIM. Fluidised bed experiments at low temperature range (290–473 K) were conducted avoiding excessive instrumentation and time. Compressed dry air entered the bed through a distributor of a 200-mesh brass sieve and fluidised the single charge of alumina particles (1.3 kg) with a mean diameter approximately 250 μm. The superficial gas velocity was changed from 0.085 to 0.469 m s^− 1. The bed-to-gas heat transfer coefficients (h_BG0×η₀) at initial bed hight and thermal inefficiency constants (k_I) were calculated from the intercept and slope of the linear form of the TIM, respectively. The agreement between the experimental and predicted values of gas temperatures confirmed by the TIM. The latter may be successfully used to design fluidised beds for, e.g., drying or combustion.     

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.     

Dynamic behaviour of sewage sludge incineration in a large-scale bubbling fluidised bed in relation to feeding-rate variations

In this paper, a mathematical model is developed for the simulation of a large-scale sewage sludge incineration plant. The model assumes the bed to consist of a fast gas phase, an emulsion phase and a fuel particle phase with specific consideration for thermally-thick fuel particles. The developed model is employed to predict the dynamic response of the bed combustion to fluctuations in sludge feeding-rate. Calculation results indicate that the bed combustion is sensitive to fluctuations with response times greater than 30 min, but severe delays exist for both outlet oxygen level and bed temperatures; from 6 to 13 min for O₂ and 22–45 min for temperatures. Depending on the fluctuation frequency, the corresponding phase shifts are 39–96° for outlet O₂, 138–336° for bed temperature and 80–336° for freeboard temperature.     

Horizontal gas and gas/solid jet penetration in a gas–solid fluidized bed

In this paper, the real time, dynamic phenomena of the three-dimensional horizontal gas and gas/solid mixture jetting in a 0.3 m (12 in) bubbling gas–solid fluidized bed are reported. The instantaneous properties of the shape of the jets and volumetric solids holdup are qualified and quantified using the three-dimensional electrical capacitance volume tomography (ECVT) recently developed in the authors’ group. It is found that the horizontal gas jet is almost symmetric along the horizontal axis during its penetration. As the jet width expands, the total volume of the gas jet increases. A mechanistic model is also developed to account for the experimental results obtained in this study. Comparison of jet penetration length and width between the model prediction and ECVT experiment shows that both the maximum penetration length and the maximum width of the horizontal gas jet increase with the superficial gas velocity. When the horizontal gas jet coalesces with a bubble rising from the bottom distributor, it loses its symmetric shape and can easily penetrate into the bed. For the horizontal gas/solid mixture jet penetration in the bed, the tail of the jet at the nozzle shrinks and the jet loses its jet shape immediately when the jet reaches its maximum penetration length, which are different from the characteristics exhibited by the gas jet. The solids holdup in the core region of the gas/solid mixture jet is higher than that in the gas jet. The penetration length of the horizontal gas/solid mixture jet is also larger than that of the gas jet.     

Catalytic gasification of woody biomass in an air-blown fluidized-bed reactor using Canadian limonite iron ore as the bed material

A Canadian limonite iron ore was tested for the first time as a catalytic bed material for air-blown gasification of pine sawdust at various equivalence ratios (ER, 0.20–0.35) on a pilot-scale fluidized bed gasifier, in comparison to a conventional olivine bed material. Effects of bed materials (iron ore and olivine) on tar formation and gasification efficiencies were comparatively investigated. The use of Canadian limonite iron ore as the bed material was found to be more active than olivine for tar reduction in the fluidized bed gasification of biomass at a small ER (?0.3), leading to a very low tar yield of 15–25 g/kg biomass at ER = 0.30. The yields of combustible gas (carbon monoxide hydrogen, methane and C₂ hydrocarbon gases) and cold gas efficiency were generally the highest at medium values of ER (0.25–0.30) for both bed materials. The iron ore was less active than olivine for producing combustible gases, leading to a lower cold gas efficiency (50% at ER = 0.30) compared to 75% for olivine. However, the use of the iron ore produced a higher yield of hydrogen than that of olivine in the gasification: 5.0 mol hydrogen per kg of biomass with the iron ore at ER = 0.30 which was about 25% higher than that with olivine.     

Biofuel ethanol adulteration detection using an ultrasonic measurement method

Hydrous ethanol is a worldwide used biofuel. According to Brazilian regulations, the concentration of ethanol in hydrous ethanol can be accepted at a maximum concentration of 93.8% and a minimum of 92.6% by mass. The aim of this study is to identify the possible changes in hydrous ethanol fuel using ultrasonic attenuation and propagation velocity. The experiments were performed in the Laboratory of Ultrasound of the Brazilian National Institute of Metrology (Inmetro). The experiments and uncertainties in the methodology were evaluated according to the Guide to the Expression of Uncertainty in Measurement, JCGM 100:2008. The test samples used in this study were mixtures of ethanol and water with ethanol concentrations varying from 89.84% to 93.71% by mass; and a commercial fuel ethanol bought from a local distributor. The correlation coefficient between ethanol concentrations and ultrasonic propagation velocity was 0.99 (in modulus), and the maximum combined uncertainty was 0.60 m s^?1. Considering attenuation, the correlation coefficient was 0.97, and the maximum combined uncertainty was 0.085 dB cm^?1. However, its signal is not stable resulting an unreliable parameter. Within the tested concentration range, the highest concentration that is statistically different (p < 0.002, α = 5%) from 92.60% is 92.25%, considering propagation velocity as parameter. To validate the methodology, a commercial ethanol fuel was tested using the proposed method as well as the gas chromatography analytical method (gold standard). Result was statistically identical for propagation velocity when compared to the gold standard.     

Microstructure and properties of needle punching chopped carbon fiber reinforced carbon and silicon carbide dual matrix composite

Chopped carbon fiber preform reinforced carbon and SiC dual matrix composites (C/C–SiC) were fabricated by chemical vapor infiltration (CVI) combined with liquid silicon infiltration. The preform was fabricated by repeatedly overlapping chopped carbon fiber web and needle punching technique. A geometry model of the pore structure of the preform was built and reactant gas transportation during the CVI was calculated. The microstructure and properties of the C/C–SiC composites were investigated. The results indicated that the CVI time for densification of the preform decrease sharply, and the model showed the permeability of the preform decreased with the increase of its density. The C/C–SiC exhibited good mechanical characteristics, especially excellent compressive behavior, with the vertical and parallel compressive strength reached to 359(±40) MPa and 257(±35) MPa, respectively. The coefficient of friction (COF) decreased from 0.60 (at 8 m/s) with the increase of sliding velocity, and finally stabilized at ~0.35 under the velocity of 20 m/s and 24 m/s, and the variations of COF were not sensitive to the sliding distance. The wear rates were between 0.012 cm³/MJ and 0.024 cm³/MJ under different velocities. These results showed that the chopped carbon fiber preform reinforced C/C–SiC are promising candidates for high-performance and low-cost friction composites.     

Effluent organic matter removal from reverse osmosis feed by granular activated carbon and purolite A502PS fluidized beds

Applying pre-treatments to remove dissolved organic matter from reverse osmosis (RO) feed can help to reduce organic fouling of the RO membrane. In this study the performance of granular activated carbon (GAC), a popular adsorbent, and purolite A502PS, an anion exchange resin, in removing effluent organic matter (EfOM) from RO feed collected from a water reclamation plant located at Sydney Olympic Park, Australia were evaluated and compared through adsorption equilibrium, kinetics and fluidized bed experiments. The maximum adsorption capacity (Q_max) of GAC calculated from the Langmuir model with RO feed was 13.4 mg/g GAC. The operational conditions of fluidized bed columns packed with GAC and purolite A502PS strongly affected the removal of EfOM. GAC fluidized bed with a bed height of 10 cm and fluidization velocity of 5.7 m/h removed more than 80% of dissolved organic carbon (DOC) during a 7 h experiment. The average DOC removal was 60% when the bed height was reduced to 7 cm. When comparing GAC with purolite A502PS, more of the later was required to remove the same amount of DOC. The poorer performance of purolite A502PS can be explained by the competition provided by other inorganic anions present in RO feed. A plug flow model can be used to predict the impact of the amount of adsorbent and of the flow rate on removal of organic matter from the fluidized bed column.     

Film and slug behaviour in intermittent slug–annular microchannel flows

Whilst there are numerous experimental, theoretical and computational studies of Taylor flow in microchannels, the intermittent slug–annular regime has largely been neglected. In this paper time-resolved micro-PIV data are collected and used to study the flow characteristics of a gas–liquid system for flow regimes spanning Taylor to annular flow. The experimental work used a 1.73 mm diameter channel with water and nitrogen as the working fluids, for gas and liquid superficial velocity ranges of 0.35–8.65 m s^?1 (40<Re_G<1000) and 0.071–0.18 m s^?1 (120<Re_L<300), respectively. Time-averaged velocity profiles were obtained in the liquid film surrounding the gas bubbles (or the gas core in the pseudo-annular flow regime) and in the liquid slugs (which changed from regular slugs to annular rings as the gas superficial velocity was increased). These data showed that the velocity in the liquid film relaxed back to an equilibrium value following the passage of each liquid slug or annular ring. In contrast rather flat velocity profiles were observed in the liquid slug. Based on a simple representation of the flow structure, average gas holdups were estimated using independent experimental data obtained by the micro-PIV technique and by direct observation of the flow structure. A phenomenological model of intermittent slug flow, based on the representation of the flow structure as a train of slugs and bubbles moving over a liquid film, is used to interpret the experimental data. The modelling work highlights the different behaviour of the limiting cases of slug and annular flow, in terms of the gas–liquid interfacial shear and its influence on the pressure field.     

Mixing–segregation phenomena of binary system in a fluidized bed

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D., 1.8 m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.     

Characteristics of droplets generated by bubble bursting from chromic acid solutions

The effects of electrolyte concentration and gas flow rate on the characteristics of droplets generated from bubbles bursting on the surface of CrO₃ solution were studied with an experimental bubbling system. The experimental conditions included two electrolyte concentrations, 125 and 250 g l^-1 of CrO₃, and three flow rates of sparging air in the range of 4–8 l min^-1. A cascade impactor collected droplet samples for chemical analysis. A laser aerosol spectrophotometer and an aerodynamic particle sizer were employed simultaneously to measure the number concentration and size distribution of the droplets. A layer of foam formed on the liquid surface under all experimental conditions studied except at the gas flow rate of 4 l min^-1 in 125 g l^-1 CrO₃ solution. Foams had a significant effect on the characteristics of droplets generated from bursting bubbles. At identical gas flow rate and electrolyte concentration, the formation of foams led to a reduction in number concentration of droplets larger than 10 μm in aerodynamic diameter and a lower concentration of airborne Cr(VI). In the ranges of gas flow rate and electrolyte concentration tested, the results showed that the airborne Cr(VI) mass concentration increased significantly with gas flow rate and slightly with electrolyte concentration in the presence of foams. The results obtained in the present study should have applications in the emission control of Cr(VI)-containing droplets in chromium electroplating processes.