•Effects of column diameter and liquid height on gas holdup are clarified.•Gas holdups at various column diameters and liquid heights are obtained.•A guideline for developing gas holdup correlations ...is obtained.
Experiments on the total gas holdup, αG, in air–water cylindrical bubble columns were carried out to investigate effects of the column diameter, DH, and the initial liquid height, H0, on αG. Ranges of DH and H0 were 160⩽DH⩽2000mm and 400⩽H0⩽4000mm, respectively. The superficial gas velocity, JG, was varied from 0.025 to 0.35m/s. The characteristics of gas holdup showed that all the flows in the present experiments were pure heterogeneous. The following conclusions were obtained for αG in air–water bubble columns: (1) the effects of DH and H0 on αG are negligible when scaling up from small to large bubble columns, provided that αG in the small columns are obtained for DH⩾200mm and H0≳2200mm. The height-to-diameter ratio is useless in evaluation of the critical height, above which αG does not depend on H0, (2) for the above ranges of DH and H0, Akita-Yoshida’s and Koide’s correlations can give good evaluations of αG for a wide range of JG by tuning the model constants, (3) for DH<200mm, the decrease in DH increases the population of large bubbles, which results in the decrease in αG, and (4) for H0≲2200mm and DH⩾200mm, αG at a constant JG decreases with increasing H0 and approaches an asymptotic value, and the Froude number using JG and H0 as the characteristic scales well correlates αG in this regime.
•Impact in slopes (angle θ) form shallow, elongated, asymmetric craters.•Asymmetric ejecta dispersal and avalanche is the origin of these crater shapes.•Topographic maps are used to classify 3 crater ...shape regimes as a function of θ.•Scaled fits of the θ-dependence of the crater scales and ratios are derived.•Profiles of craters formed in the experiments closely resemble those in Vesta.
Low-velocity impact cratering experiments are conducted in sloped granular targets to study the effect of the slope angle θ on the crater shape and its scales. We use two types of granular matter, sand and glass beads, former of which has a larger friction coefficient μs=tanθr, where θr is the angle of repose. Experiments show that as θ increases, the crater becomes shallower and elongated in the direction of the slope. Furthermore the crater floor steepens in the upslope side and a thick rim forms in the downslope side, thus forming an asymmetric profile. High-speed images show that these features are results of ejecta being dispersed farther towards the downslope side and the subsequent avalanche which buries much of the crater floor. Such asymmetric ejecta dispersal can be explained by combining the Z-model and a ballistic model. Using the topographic maps of the craters, we classify crater shape regimes I-III, which transition with increasing θ : a full-rim crater (I), a broken-rim crater (II), and a depression (III). The critical θ for the regime transitions are larger for sand compared to glass beads, but collapse to close values when we use a normalized slope θ^=tanθ/tanθr. Similarly we derive θ^-dependences of the scaled crater depth, length, width and their ratios which collapse the results for different targets and impact energies. We compare the crater profiles formed in our experiments with deep craters on asteroid Vesta and find that some of the scaled profiles nearly overlap and many have similar depth / length ratios. This suggests that these Vestan craters may also have formed in the gravity regime and that the formation process can be approximated by a granular flow with a similar effective friction coefficient.
•Mass transfer from single bubbles in NaCl aqueous solutions in pipes is measured.•Experimental condition covers various bubble shapes.•Increase in electrolyte concentration decreases mass transfer ...rates.•Reliable Sherwood number correlations are presented.
Mass transfer rates, kL, of single carbon-dioxide (CO2) bubbles rising through vertical pipes filled with electrolyte aqueous solutions were measured to investigate the effects of the presence of electrolyte on kL. Sodium chloride (NaCl) was used for electrolyte and its concentration was varied from 3.5 to 14wt.%. The pipe diameters, D, were 12.5, 18.2 and 25.0mm and the bubble diameter, d, ranged from 5 to 25mm. The diameter ratio, λ(= d/D), was varied from 0.2 to 1.7, to cover various bubble shapes, i.e. ellipsoidal, cap, semi-Taylor bubbles and Taylor bubbles. The conclusions obtained are as follows: (1) kL in the NaCl aqueous solutions decreases with increasing the NaCl concentration mainly due to the reduction of the diffusion coefficient of CO2 in the liquid phase, (2) the Sherwood numbers, Sh, of Taylor bubbles of L/D>1 in the NaCl aqueous solutions can be well evaluated using the available Sherwood number correlation for clean Taylor bubbles, where L is the length of a Taylor bubble, (3) Sh of ellipsoidal, cap, semi-Taylor bubbles and Taylor bubbles of L/D≤1 in the clean water and NaCl aqueous solutions can be well correlated in terms of the Peclet number and the dimensionless bubble diameter d∗=d/dT, where dT is the bubble diameter at the transition from the ellipsoidal-cap bubble regime to the semi-Taylor bubble regime, and (4) the Sh correlations give good predictions for long-term bubble dissolution processes in NaCl aqueous solutions.
The heat-discharging kinetics of an iron-substituted Mn2O3/Mn3O4 redox pair subjected to long-term thermal cycling tests using a temperature swing process at high temperatures was investigated for ...next-generation concentrated solar power plants equipped with thermochemical energy storage. The heat-discharge mode kinetics for long-term thermal-cycled samples have never been reported. Additionally, comparisons of the heat-discharge mode kinetics for both long-term thermal-cycled and as-prepared samples have never been discussed. In terms of the reproducibility and sustainability of thermochemical energy storage, kinetic evaluations of samples with thermally stable morphologies subjected to long-term thermal cycling at high temperatures are important for next-generation solar thermal power plants. For the long-term thermal-cycled sample, the A2 model based on the Avrami–Erofeev reaction describes the discharging mode behavior in a fractional conversion range of 0–0.24, the contracting area (R2) model best fits in a fractional conversion range of 0.24–0.50, and the third-order (F3) model matches in a fractional conversion range of 0.50–0.70. For the as-prepared sample, the power-law (P2) model describes the behavior of the first part of the discharging mode, whereas the Avrami–Erofeev (A4) model best fits the last half of the discharging mode. The predicted theoretical models for both samples were compared with previous kinetic data.
Mass transfer from single carbon dioxide bubbles rising through contaminated water in a vertical pipe of 12.5 mm diameter was measured to investigate effects of surfactant. The bubble diameter was ...widely varied to cover various bubble shapes such as spheroidal, wobbling, cap and Taylor bubbles. The gas and liquid phases were 99.9% purity carbon dioxide and a surfactant solution made of purified water and Triton X-100. Comparison of mass transfer rates between contaminated and clean bubbles made clear that the surfactant decreases the mass transfer rates of small bubbles. The Sherwood numbers of small bubbles in the extreme cases, i.e. zero and the highest surfactant concentrations, are well correlated in terms of the bubble Reynolds number, Schmidt number and the ratio, lambda, of the bubble diameter to pipe diameter. The Sherwood numbers at intermediate surfactant concentrations, however, are not well correlated using available correlations. The mass transfer rates of Taylor bubbles also decrease with increasing the surfactant concentration. They however increase with the diameter ratio and approach those of clean Taylor bubbles as lambda increases. The main cause of this tendency was revealed by interface tracking simulations, i.e. the surfactant adsorbs only in the bubble tail region and the nose-to-side region is almost clean at high lambda.
We studied the performance in terms of the long-term cyclic thermal storage and heat-charging kinetics of Fe-substituted manganese oxide for use in thermochemical energy storage at temperatures ...exceeding 550 °C in a next-generation concentrated solar power system in which a gas stream containing oxygen is used for reversible thermochemical processes. The Fe-substituted Mn2O3 was evaluated from the viewpoint of its microstructural characteristics, thermodynamic phase transitions, and long-term cycling stability. A kinetic analysis of the heat-charging mode was performed at different heating rates to formulate the kinetic equation and describe the reaction mechanism by determining the appropriate reaction model. Finally, the kinetics data for the sample obtained after the long-term cycling test were compared and evaluated with those of the as-prepared sample and kinetic literature data tested under different conditions. For the long-term cycled sample, the Avrami–Erofeev reaction model (An) with n = 2 describes the behavior of the first part of the charging mode, whereas the contracting area (R2) reaction model best fits the last half of the charging mode. For the as-prepared sample, except for the early stage of the charging mode (fractional conversion < 0.2), the contracting volume (R3) reaction model fits the charging mode over a fractional conversion range of 0.2–1.0 and the first-order (F1) reaction model fits in the fractional conversion range of 0.4–1.0. The predicted kinetic equations for both the samples were in good agreement with the experimental kinetic data.
► An interface tracking method predicting bubbles with soluble surfactant is proposed. ► Marangoni effect on small bubbles is well predicted with the proposed method. ► Surfactant effect on the ...velocity and shape of a Taylor bubble is investigated. ► Surface tension reduction near the bubble nose increases Taylor bubble velocity. ► Velocity models for a clean bubble can be used if the nose surface tension is given.
Effects of soluble surfactant on the terminal velocity of a Taylor bubble rising through a vertical pipe are investigated using an interface tracking method. A level set method is utilized to track the interface. Transport of surfactant in the bulk liquid and at the interface is taken into account. The amount of adsorption and desorption is evaluated using the Frumkin and Levich model. The normal component of surface tension force is computed using a ghost fluid method, whereas the tangential component, i.e., the Marangoni force, is evaluated by making use of the continuum surface force model. Simulations of small air bubbles contaminated with soluble surfactant are carried out for validation. The Marangoni effects on the bubbles, i.e., the surface immobilization and the increase in drag coefficient, are well predicted. Then Taylor bubbles rising through vertical pipes filled with contaminated water at a low Morton number are simulated for various Eötvös numbers, various bulk surfactant concentrations and two different surfactants, i.e., 1-pentanol and Triton X-100. As a result, the following conclusions are obtained: (1) the reduction of surface tension near the bubble nose is the cause of the increase in terminal velocity, (2) the surfactant does not affect the terminal velocities of high Eötvös number bubbles since the bubbles at high Eötvös numbers are independent of surface tension, (3) the terminal velocity of a low Morton number Taylor bubble can be evaluated by making use of available correlations for clean Taylor bubbles, provided that the degree of contamination near the bubble nose is known and the Marangoni effect in the nose region is negligible, and (4) the Hatta number, which is the ratio of the adsorption velocity to the bubble velocity, is a primary factor governing the degree of contamination in the bubble nose region.
•Effects of the initial liquid height on the gas holdup are investigated.•Wide ranges of the superficial gas velocity and initial liquid height are tested.•Reliable gas holdup databases are offered ...for a rectangular and a cylindrical column.•An accurate gas holdup correlation accounting for the height effect is proposed.•The functional form of the correlation is validated against other data.
Effects of the initial liquid height, H0, of air–water bubble columns on the total gas holdup, αG, were investigated in this study. Systematic databases of αG in a rectangular and a cylindrical column of the hydraulic diameter DH of 200mm were obtained. An image processing method was applied to high-speed video images of the liquid height to obtain accurate gas holdup data. Ranges of the superficial gas velocity, JG, and the dimensionless liquid height, H0∗ (= H0/DH), were 0.025⩽JG⩽0.40m/s and 1.5⩽H0∗⩽5.0, respectively. The bubbly flows observed in these ranges could be classified into either the heterogeneous bubbly flow consisting of bubbles much smaller than DH or that with huge bubbles of the column-width scale. The main parameter governing the flow regime transition was JG. The gradient, dαG/dJG, of αG with respect to JG was of use in flow regime identification. The increase in H0∗ decreased αG because a long bubble residence time at a high H0∗ leads to the increase in the mean bubble diameter due to bubble coalescence. The Froude number using H0 as a characteristic length well correlates αG at various H0. An empirical correlation of αG in terms of the Froude number was then proposed. Comparing the correlation with the αG data showed that the correlation can give good evaluations of αG in the rectangular and cylindrical columns by tuning model parameters.
•Experiments on pressure drops in two-phase flows in U-bends are carried out.•Bend pressure drops in an air-water system are compared with available correlations.•Bubbly, plug, slug and annular flows ...are dealt with.•A Chisholm-type bend pressure drop correlation is presented.
Pressure drops in horizontally aligned U-bends were measured for air-water bubbly, plug, slug and annular flows to examine the applicability of available correlations to the data. The bend disturbed the flow and induced large pressure drops not only within the bend but also in the downstream-side straight pipe. The bend pressure drop was therefore evaluated as the sum of the pressure drop caused within the bend and the extra pressure drop caused within the re-developing region in the downstream-side straight pipe. The experimental data were well correlated using the Lockhart-Martinelli method and the Chisholm correlation gives the most reasonable evaluation for the bend pressure drop among the available correlations tested. The fundamental functional form of the Chisholm correlation was therefore used to develop an empirical correlation applicable to the present data. The empirical correlation gave reasonable evaluations for the air-water two-phase flows.
PDF
