An investigation of gas void fraction and transition conditions for two-phase flow in an annular gap bubble column

Gas-liquid flow may be characterised in terms of the gas void fraction, α. This is an important variable in two-phase flow, used in predicting the occurrence of flow regimes, and the associated pressure drop, and mass and heat transfer. The gas void fraction transitions in a two-phase flow system from uniform bubble flow (homogeneous) to churn-turbulent bubble flow (heterogeneous) in an open tube bubble column (OTBC) and an annular gap bubble column (AGBC) have been investigated using a vertical column with an internal diameter of 0.102 m, containing a range of concentric inner tubes which formed an annular gap; the inner tubes had diameter ratios from 0.25 - 0.69. Gas (air) superficial velocities in the range 0.014-0.200 m/s were studied. Tap water and aqueous solutions of ethanol and isopropanol, with concentrations in the range 8 - 300 ppm by mass, were used as the working liquids. Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in the OTBC and the AGBC, when operated at the same gas superficial velocity using a porous sparger. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (i) the presence of the inner tube causes large bubbles to form near the sparger, which destabilize the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles. Moreover, (ii) the shape of the void fraction profiles changes with gap geometry, which affects the distribution parameter in the drift flux model. Radial profiles of the local void fraction were obtained using a two- and four-point conductivity probe, and were cross-sectionally averaged to give mean values that were within 12% of the volume-averaged gas void fractions obtained from changes in aerated level. The presence of alcohol inhibited the coalescence between the bubbles, and consequently increased the mean gas void fraction at a given gas superficial velocity in both the open tube and the annular gap bubble columns. This effect also extended the range of homogeneous bubbly flow and delayed the transition to heterogeneous flow. Moreover, isopropanol results gave slightly higher mean void fractions compared to those for ethanol at the same mass fraction, due to their increased carbon chain length. It was shown that the void fraction profiles in the annular gap bubble column were far from uniform, leading to lower mean void fractions than were obtained in an open tube for the same gas superficial velocity and liquid composition. The chord length measurements in the OTBC for both the tap water and alcohol solutions exhibited two trends with respect to increasing j_g: (i) at low j_g, in the homogeneous flow, an increasing function was obtained; and (ii) with further increase in j_g, a reduction in the chord length was observed. In the presence of the orifice, the results concerning mean chord lengths show a decreasing function of the bubble size with increasing j_g; this was visually demonstrated using photographs. For the AGBC, the chord lengths obtained from the conductivity probe offered evidence of the bubble size decreasing as j_g increased in the heterogeneous regime, which agreed with the findings of the OTBC. This was also confirmed using the results obtained from photographs. A novel approach for bubble size transformation was implemented to process the conductivity probe measurements. An analytical method was used as a forward transform to predict the chord length distribution from the bubble size distribution and an optimisation approach was applied as a backward transform method to obtain the bubble size distribution from the chord length distribution. The challenge was to consider a variable aspect ratio, φ, for the bubble shape, which depended on their size. The model gave excellent and reasonable predictions for the bubble sizes as their trends were identical to the trend of the chord length, and to the bubble size obtained from photographs

Evidence of Plasmonic Induced Photocatalytic Hydrogen Production on Pd/TiO2 Upon Deposition on Thin Films of Gold

H2-production from renewables using sunlight is probably the holy grail of modern science and technology. Among the many approaches for increasing reaction rates, by increasing light absorption, plasmonic materials are often invoked. Yet, most plasmonic metals on semiconductors are also good for Schottky barrier formation. In this work, we are presenting evidences of de-coupling the plasmonic from Schottky effects on photoreaction. To conduct this we have systematically changed the under-layer gold film thickness and associated particle size. On top of the thin film layer, we have deposited the exact amount of a prototypical Schottky-based photo-catalyst (Pd/TiO2). We found up to 4 times increase in the H2-production rate at a critical Au film thickness (8 nm-thick). Below this thickness, the plasmonic response is not too strong while above it, the PR decays in favor of the Drude absorption mode. The reaction requires the presence of both UV (to excite the semiconductor) and visible light (to excite Au particles) in order to obtain high hydrogen production, 800 µmol/gCatal.min (probably the highest direct hydrogen (not current) production rate reported on a performing catalyst). The enhancement origin is quantitatively traced to its computed electric field strength (EFS). Adding a dielectric (SiO2) in between the Au thin layer and the catalyst exponentially decreased the reaction rate and EFS, with increasing its thickness. This work indicates the possibility of making an active and stable photo-catalyst from fundamental concepts yet further progress on the structural (technological) front is needed to make a practical catalyst

Comparing the Reaction Rates of Plasmonic (Gold) and Non-Plasmonic (Palladium) Metal Particles in Photocatalytic Hydrogen Production

Both Pd and Au metal particles are used in photocatalytic hydrogen generation. Yet while both act as electron sink only gold is poised to respond to visible light due to its plasmonic response. In order to quantitatively gauge their relative contribution into the reaction, the photocatalytic H2 production, from Au/TiO2 and Pd/TiO2 catalysts was studied under UV and UV–Vis light. While under UV light excitation, a weak dependence on the work function of the metal is observed, under UV–Vis light, Au is found to be twice more active than Pd. Under identical UV–Vis light irradiation, the turn over frequency calculated from XPS at.% is found to be 2.8 and 1.8 s−1 for Au and Pd, respectively. The effect is far more pronounced when the rates are normalized to the number of particles of each metal. Both the semiconductor TiO2 (UV light) and the plasmonic metal (visible light) need to be excited for the enhancement to occur; visible light alone causes a negligible reaction rate. Photocurrent measurements further confirmed the difference in the photocatalytic activity under UV and UV–Vis light excitation. Moreover, because of the presence of Au particles responding to visible light the reaction rate is enhanced due to “light penetration depth” effect

Destabilisation of a homogeneous bubbly flow in an annular gap bubble column

Destabilisation of a homogeneous bubbly flow in an annular gap bubble colum

An experimental study of gas void fraction in dilute alcohol solutions in annular gap bubble columns using a four-point conductivity probe

The influence of alcohol concentration on the gas void fraction in open tube and annular gap bubble columns has been investigated using a vertical column with an internal diameter of 0.102 m, containing a range of concentric inner tubes which formed an annular gap; the inner tubes had diameter ratios from 0.25 - 0.69. Gas (air) superficial velocities in the range 0.014-0.200 m/s were investigated. Tap water and aqueous solutions of ethanol and isopropanol, with concentrations in the range 8 - 300 ppm by mass, were used as the working liquids. Radial profiles of the local void fraction were obtained using a four-point conductivity probe and were crosssectionally averaged to give mean values that were within 12% of the volumeaveraged gas void fractions obtained from changes in aerated level. The presence of alcohol inhibited the coalescence between the bubbles and consequently increased the mean gas void fraction at a given gas superficial velocity in both the open tube and the annular gap bubble columns. This effect also extended the range of homogeneous bubbly flow and delayed the transition to heterogeneous flow. Moreover, isopropanol results gave slightly higher mean void fractions compared to those for ethanol at the same mass fraction, due to their increased carbon chain length. It was shown that the void fraction profiles in the annular gap bubble column were far from uniform, leading to lower mean void fractions than were obtained in an open tube for the same gas superficial velocity and liquid composition

Destabilisation of a homogeneous bubbly flow in an annular gap bubble column

Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilize the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift flux model

Experimental study of void fraction behaviour in vertical bubbly gas-liquid flow using conductivity and measurements

The void fraction is an important variable in describing gas-liquid two-phase flows, since it is required to predict the heat and mass transfer coefficients and the pressure drop and is an indicator of the flow regime. The contrast in conductivity between water and air is one way to measure the void fraction in gas-liquid flow. This project has examined use of the ring conductivity electrodes to measuring the void fraction in an up-flow bubble column. The conductivity method has potential to be a low cost, safe and accurate method of measuring local void fractions in pipes and other process engineering mass transfer devices. In this project, the void fraction was measured in an air-water system by using conductivity in a 2" pipe equipped with two ring electrodes. Further gas hold-up experiments were conducted in the annular channel formed between 2" and a 4" pipe, using a system of four pairs of electrodes. The data obtained from the experiments agreed fairly well with the Maxwell and Burggeman theories which relate the dimensionless conductance to the void fraction. The measured void fractions were correlated using the drift-flux model, as proposed by Zuber and Findlay. Significant differences were observed between the void fraction measurements obtained for the annular channel and for an empty pipe, when operated at the same gas superficial velocity

PROBAVNI SUSTAV KOŠTUNJAČA PERSORBIRA MIKROČESTICE

The ability of the teleost gut to absorb microparticulate material was examined following rectal intubation (3.5 g kg -1 ) of commercial grade cornstarch (≈21 mm diameter), or potato starch (≈43 mm diameter). Tissue samples were taken from the mid - and hind-gut of control and treated fish 18 h postintubation. Collected samples were processed using standard plastic and staining protocols and resultant photomicrographs examined by computer-assisted image analysis. Cornstarch particles (8-14 mm), were observed to pass from gut lumen to the lamina propria via a paracellular or persorptive route only. No evidence for the like passage of potato starch was found.Mogućnost probavnog sustava koštunjavičastih vrsta riba da apsorbiraju sitne čestice pojedinih tvari bila je istraživana s pomoću rektalne intubacije (3, 5 g kg -1 ) komercijalnoga škroba kukuruza (promjera ≈21 mm) i škroba krumpira (promjera ≈ 43 mm). Uzorci tkiva uzeti su iz srednjeg i stražnjeg dijela crijeva kontrolne i tretirane skupine riba, i to 18 sati nakon intubacije. Sakupljeni su uzorci spremljeni u standardne plastike i obojeni prema protokolu, te fotomikrografski snimljeni na računalu. Čestice škroba kukuruza (8-14 mm) zapažene su da iz lumena crijeva prolaze u laminu propriju samo paracelularnim ili persorptivnim putem. No, nije zapažen prolazak čestica škroba krumpira

Randomised open-label trial of docosahexaenoic acid-enriched fish oil and fish meal on cognitive and behavioural functioning in Omani children

Objective: Examine the effect of docosahexaenoic acid (DHA) enriched fish oil supplement and fish meal on cognitive and behavioural functioning manifested as attention-deficit/hyperactivity disorder in primary school students (9-10 years old) in Muscat, Oman. Methods: Randomised open-label trial involving two types of intervention: fish oil supplement or one serving (100 gram) of fish per weekday for 12 weeks. Red cell total lipid docosahexaenoic acid levels were assessed. Verbal Fluency Test, Buschke Selective Reminding Test, and Trail Making Test were used to measure cognitive functioning. Behavioural functioning was assessed using a standardised Arabic version of the National Initiative for Children’s Health Quality Vanderbilt Assessment Scales. All measurements were carried out before and after intervention. Results: DHA levels increased by 72% and 64% in fish oil (Mean: 3.6% to 6.2%) and fish meal (Mean: 3.4% to 5.6%) group, respectively (p=0.000). The Trail Making Test was the only cognitive test that demonstrated marked differences between groups: median inter-quartile range difference between pre and post intervention in the Trail Making Part B score was 61.5 (SE: 19.3, 103.2) in the fish oil vs. Fish meal group, 24.5 (SE: -15.2, 74.7, p=0.005). The Vanderbilt Assessment Scales also showed significant differences between groups (p<0.001). Conclusion: This study contributes to available evidence on the cognitive and behavioural benefits of DHA in healthy school children. Consideration should be made to expand the food fortification programme as part of broader policy to improve child health