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A compact and efficient microbubble generation method based on subcooled boiling on a thin metal wire is proposed in this study. The generation process of microbubbles was observed using a highspeed camera to evaluate the bubble diameter and the number density. The experiments were conducted using resistance heating by direct current (DC) and alternating current (AC) (60 Hz) in several electric power conditions. The dependence of the bubble number density on the concentration of the dissolved oxygen implies that microbubbles consist of incondensable gases transferred from the liquid phase into vapor bubbles during the bubble generation and growth in boiling and remained in the bubbles during the subsequent condensation. Cyclic heating with AC is more effective for generating microbubbles than constant heating with DC. The number density increases with increasing the time-averaged heat flux, which also increases the mean bubble diameter, though the increasing rate of the diameter is not significant.

This paper presents results for the experimental vector velocity field obtained through spatial filter velocimetry in a triangular tube bundle with tubes of 20 mm O.D. and a transverse pitch of 25.2 mm. The experiments were conducted for single-phase water flows with Reynolds numbers ranging from 447 to 1842, and for air–water two-phase bubbly flow with liquid Reynolds number of 909 and gas Reynolds number of 42. The vector velocity field for each experimental condition was obtained using a variable meshing scheme to improve the spatial resolution in regions of low velocities. The results indicate that the variable meshing strategy is promising to increase the spatial resolution of the spatial filter velocimetry technique and to release from the constraint imposed by the uniform window sizes, especially for experimental data obtained with relatively low frame rates. Details of the flow structure of external flow across tube bundles, such as the near-wall velocity profiles, wake region and the effects of the gas buoyancy on vertical mean velocities are presented in this paper with high temporal and spatial resolution, which can be useful for validation of numerical models.

Mass transfer rates, k(L), of single carbon-dioxide (CO2) bubbles rising through vertical pipes filled with alcohol-electrolyte mixed aqueous solutions were measured to investigate the combined effects of alcohol and electrolyte on k(L). 1-octanol and 1-heptanol, which are known to adsorb to the interface and behave like surfactant, were used. Sodium chloride (NaCl) was used for the electrolyte. The surface tension, σ, decreased with increasing the NaCl concentrations, C-N, while keeping the alcohol concentration, C-A, constant. Three combinations of C-A and C-N having the same values of σ were selected for the experimental conditions to avoid the effect of σ. The pipe diameters, D, were 12.5 and 18.2 mm. A wide range of bubble diameter covered various bubble shapes, i.e. ellipsoidal, cap, semi-Taylor and Taylor bubbles. The conclusions obtained are as follows: (1) the combination of C-A and C-N for the same σ does not affect the aspect ratios of ellipsoidal bubbles and the lengths, L, of Taylor bubbles, (2) the Sherwood numbers, Sh, of bubbles in alcohol-NaCl mixed aqueous solutions depend on the combination of C-A and C-N due to the change in the Schmidt number, Sc, even at the same σ for all the tested combinations of concentrations of impurities, and (3) the modified Sherwood number, Sh(D), of contaminated Taylor bubbles is well correlated in terms of the bubble Reynolds number, Sc, the dimensionless group for surfactant properties and L/d(T), where d(T) is the transition bubble diameter at the transition from the ellipsoidal-cap bubble regime to the transition regime.

This paper presents an analysis of the performance of the Spatial Filter Velocimetry (SFV) technique applied to external flows across tube bundles to evaluate the benefits of using low particle concentration in a complex geometry. The experimental vector velocity fields were obtained in a tube bundle test section made of acrylic with 20 rows of 4 tubes of 20mm O.D. mounted in a normal triangular configuration and transversal pitch per diameter ratio of 1.25. The SFV results reliability was evaluated by comparing the volumetric flow rates estimated by the experimental vector velocity fields with the volumetric flow rates measured by the flowmeter, showing good agreement. Additionally, the experimental results are used to check the validity of numerical results obtained through CFD simulation. The experiments were conducted for Reynolds numbers of 909, 1842 and 3902. The velocity profiles obtained through the SFV technique agree with the CFD simulations results, which indicate consistent results for both analyzes. Therefore, the numerical and experimental results indicate that the SFV technique provides accurate velocity data and is suitable to be applied to complex geometries, especially in regions away of the test section wall due to the good visibility provided by low particle concentration.

This paper presents the experimental vector velocity field obtained through the spatial filter velocimetry technique in a normal triangular tube bundle test section with tubes of 20 mm O.D. and transverse pitch of 25.2 mm. The experiments were conducted for water flows and Reynolds numbers ranging from 447 to 1842. The pressure field was estimated based on the velocity results by solving the Poisson equation. The shear stress around the tubes was analyzed calculating the viscous and Reynolds stresses. The results for turbulence strength and flow vorticity were calculated and discussed. All the analyses were performed using a second-order finite difference scheme to estimate the partial derivatives. The results indicate that the spatial filter velocimetry technique provides accurate velocity data and is suitable to be applied to complex geometries. In addition, the obtained velocity fields show that the SFV technique is capable of capturing asymmetric flow distributions, which switch from time to time, indicating a suitable spatial and temporal resolution for the experimental conditions evaluated in the present study.

Aspect ratios of ellipsoidal bubbles in clean and contaminated systems were measured to investigate the effects of surfactant on the bubble aspect ratio. The bubble diameter ranged from 0.80 to 5.8 mm. Air was used for the gas phase and the glycerol-water solutions were used for the liquid phase. The experimental ranges of the Morton number, M, the bubble Reynolds number, Re, the Eötvös number, Eo, the Weber number, We, and the Tadaki number, Ta, were as follows: 10(-7) < M < 10(-3), 3.6×10(-1) < Re < 1.7×10(2), 1.1×10(-1) < Eo < 5.9, 5.0×10(-3) < We < 3.7 and 5.3×10(-2) < Ta < 3.6. Triton X-100 and 1-octanol were used for surfactant. The terminal velocities of bubbles were confirmed to be independent of the surfactant concentration, and therefore, the bubbles were fully-contaminated from the point of view of the terminal velocity. By making use of the aspect ratio data, the applicability of available aspect ratio correlations was examined and an empirical correlation was proposed. The applicability of the proposed correlation was also examined for sodium dodecyl sulphate (SDS) and 1-decanol. As a result, the following conclusions were obtained: (1) the aspect ratio database, which is of use for validating and developing shape correlations, was obtained, (2) the available aspect ratio correlations are not applicable to the present data for fully-contaminated ellipsoidal bubbles, (3) the aspect ratios are well correlated in terms of a combination of the Eötvös and bubble Reynolds numbers, which proves that all the relevant forces, i.e. the viscous, inertial, buoyant and surface tension forces, should be taken into account to correlate bubble shapes, and (4) the empirical correlation of the aspect ratio developed by using the Triton X-100 and 1-octanol data also gives good evaluations for bubbles contaminated with SDS and 1-decanol in spite of largely different adsorption-desorption characteristics.

Numerical simulations of contaminated bubbles or drops adopt a model of adsorption-desorption kinetics developed for quiescent systems. However, the model has rarely been validated due to the lack of experimental data of surfactant concentration at a moving interface. Hence, we experimentally investigated surfactant concentration at the moving interface of a spherical drop falling in a stagnant liquid containing surfactant using the velocity distributions measured by spatiotemporal filter velocimetry (SFV). The molar flux of surfactant to the interface was also evaluated by substituting the measured velocity and surfactant concentration into the conservation equation of surface concentration of surfactant to examine the applicability of the Frumkin–Levich model to a drop falling in a contaminated system. We confirmed that the evaluation of surfactant concentration using SFV is of great use in understanding adsorption-desorption kinetics at an interface in a contaminated system and for validation of adsorption and desorption models. The measured results showed that the Frumkin–Levich model is not applicable to a moving interface, whereas it is applicable to an immobile interface, that the dependence of the molar flux on the molecular weight of surfactant is not so strong, and that the ratio of the surface concentration of surfactant to the maximum concentration, i.e. the surface coverage, is an appropriate index for judging an applicable range of the Frumkin–Levich model.

Experiments on air–water two-phase swirling annular flows in a vertical pipe of 40 mm diameter were carried out at atmospheric pressure and room temperature to investigate interfacial and wall friction factors, fi and fw. The friction factors were evaluated using measured pressure drops and void fractions. Measurements of liquid film thickness and flow observation were also conducted. The ranges of the gas and liquid volume fluxes, JG and JL, were 12.5 ≤ JG ≤ 20.0 m/s and 0.03 ≤ JL ≤ 0.11 m/s, respectively. The main conclusions obtained are as follows: (1) fi and fw in swirling annular flows are several times larger than those in non-swirling flows, (2) fi is well correlated in terms of the liquid volume fraction and the gas Reynolds number, ReG, (3) ReG and the liquid Reynolds number, ReL, are required for correlating fw, and (4) the liquid film thicknesses in two-phase swirling flows in a one-fifth model of a BWR separator are well predicted using the two-fluid model and the correlations of fi and fw developed based on the experimental data.

Spatiotemporal filter velocimetry (SFV) was extended to Lagrangian measurements with boundary-fitted measurement areas, and was applied to flows about single spherical drops of glycerol-water solution falling in stagnant silicon oil under clean and contaminated conditions to examine its applicability to the estimation of the Marangoni stress and surfactant concentration at a moving interface. Effects of bulk concentration of surfactant on the velocity field, the Marangoni stress and the surface concentration of surfactant were discussed from the measured data. As a result, we confirmed that accurate velocity distribution in the vicinity of the interface measured by SFV enables us to evaluate interfacial velocity and interfacial shear stresses and to estimate the Marangoni stress, interfacial tension and surfactant concentration at the interface with the assumption of negligible surface viscosity. The flow inside the drop and the interfacial velocity become weak due to the Marangoni stress caused by the gradient of surfactant concentration at the interface as the bulk concentration of surfactant increases. These results demonstrate that SFV is of great use in experimental analysis of adsorption and desorption kinetics at a moving interface.

The swirler and the 1st pick-off ring (POR) of a gas-liquid separator are improved to reduce the pressure drop, Δ P T , in the separator while keeping the flow rate of separated liquid, W ST , high. Two swirler designs are presented, i.e. V8 (swirler with eight vanes) and V6 (swirler with six vanes), whose diameters of the hubs supporting the vanes are smaller than that of the normal swirler, N8, and the nose shape of vanes are coned. The swirlers are installed into a one-fifth scale separator with three PORs. The gap width of the 1st POR is reduced to the value of the liquid film thickness in the barrel of the separator and the rear shape of the 1st POR is smoothed. The performance of the downscaled separator with the proposed swirlers and the 1st POR are evaluated under the nominal operating condition of the Hyper BWR and a higher quality condition. As a result, the following conclusions are obtained in the present experimental range: (1) Δ P T in the separator is reduced by 23% using V8, and V8 gives the same W ST as N8, confirming that the decrease in the hub diameter and the coned-nose shape of vanes are effective, (2) V6 can realize further reduction of Δ P T (38%) without deteriorating the liquid-separation rate, (3) the reduced gap width of the 1st POR and a tapered shape of the downstream side of the 1st POR are also effective to decrease the pressure drop, i.e. the improved POR decreases Δ P T by 42% and 54% by combining with V8 and V6, respectively, and (4) the improved separators also show good performances in Δ P T and W ST at the higher quality condition.

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 14 wt.%. The pipe diameters, D, were 12.5, 18.2 and 25.0 mm and the bubble diameter, d, ranged from 5 to 25 mm. 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.

We investigate electrokinetic behavior of colloidal particles in the vicinity of a solid-liquid interface. Colloidal liquids are expected to be used as thermal transport media for heat transfer applications such as nanofluids and phase change emulsions. They contain submicrometer-sized particles in liquid, and electrokinetic behavior of the solute particles should play an important role in the heat transfer between solid-liquid interfacing boundaries. However, experimental investigation of the behavior remains difficult due to the required spatial resolution beyond diffraction limit. We developed a measurement system based on laser Doppler principle using an interference of evanescent waves generated at total internal reflections of incident lasers at a solid wall. The system was developed for the measurement of velocities of colloidal particles at an interfacing boundary of colloidal liquid and a solid wall. The system has a unique advantage of a high spatial resolution in the direction perpendicular to the boundary due to the short penetration depth of an evanescent wave in the range of a few hundred nanometers. The principle and performance of the measurement system were investigated using a scanning probe in the measurement volume. We experimentally confirmed the validity of the measurement and characterized the uncertainty of velocity measurement. The system was further applied in a series of measurements of alumina particles dispersed in water in a square-shaped cell under induced electric fields. The measured velocities are proportional to the field strengths at different particle concentrations. The linear relationship is consistent with theoretical predictions, which demonstrates the feasibility of the system for the measurement of velocities of colloidal particles in the near wall region.

Experiments on air-water two-phase swirling flows in a one-fifth scale model of a steam separator with three pick-off rings (PORs) are carried out to investigate characteristics of two-phase swirling flows and separator performance. In addition to flow observation, liquid film thickness, flow rates of separated liquid and pressure drops are measured. The ranges of the gas and liquid volume fluxes, J G and J L , tested are 12.0 ⩽ J G ⩽ 17.8 m/s and 0.05 ⩽ J L ⩽ 0.11 m/s, respectively. The main conclusions obtained are as follows: (1) the liquid film thicknesses at the PORs are smaller than the gap widths of the PORs under the nominal operating condition, which results in a high liquid-separation rate, (2) the increase in J G decreases the flow rate of unseparated liquid, whereas the flow rate of unseparated liquid is independent of J L except for low J G conditions, under which the liquid film thicknesses at the 2nd and 3rd PORs tend to be larger than the gap widths of PORs, (3) the presence of the 2nd and 3rd PORs realizes high liquid-separation rates by capturing the liquid unseparated at the 1st POR and the pressure drops at the 2nd and 3rd PORs are much smaller than that at the 1st POR, and (4) the pressure drops at the swirler and the 1st POR, which are the main source of the pressure drop of the separator under the nominal operating condition, increase with J G , whereas J L increases only the latter.

Improvement of efficiency of a gaslift pump is required for lifting submarine resources economically. We propose a gaslift pump utilizing phase change of working fluid to improve the efficiency by avoiding air compression. A working fluid, which has a boiling point lower than surrounding temperature, is supplied as liquid to an evaporator near the bottom of a riser pipe through an adiabatic pipe. The working fluid boils at the evaporator due to heating from the surrounding liquid. The generated gas is injected into the riser pipe instead of compressed air and forms gas–liquid two-phase flow, by which the water is drawn from the bottom opening of the riser pipe. The working fluid is separated from the drawn water at the top of the pipe, and it flows into a cooler to supply the condensed liquid into the adiabatic pipe again. This pump needs the energy only to cool the working fluid, i.e. no energy is required for compressing gas. The feasibility of the pump is confirmed through the experiments using a prototype gaslift pump using a hydro-fluorocarbon as the working fluid. Prediction of the flow rate of drawn water is also carried out.

Aspect ratios E of ellipsoidal bubbles in infinite stagnant clean liquids are measured for log M = −6.6, −5.5, −4.9 and −3.9, where M is the Morton number. An empirical correlation of E applicable to a wide range of the Morton number is proposed by making use of the present data and Sugihara's data at log M = −11 (2007). The aspect ratio in this correlation is expressed in terms of the combination of the Eotvos number and the bubble Reynolds number to account for the effects of the inertial, viscous, buoyant and surface tension forces on E . Terminal velocities of ellipsoidal bubbles are accurately predicted by using the proposed correlation and a drag correlation proposed by Rastello et al. (2011).

We measured the radial and axial temperature distributions of a saturated steam-air mixture in a vertical circular pipe (diameter, 49.5 mm; cooling height, 610 mm) in our previous study. Those of a superheated steam-air mixture in the same experimental setup were measured in this study. The condensation heat flux qc and heat transfer coefficient hc for the superheated mixture were evaluated, and they were compared with those for the saturated mixture. There was no large difference in qc between the saturated and superheated mixtures. The heat transfer coefficient hc (which was evaluated for the saturation temperature) for the superheated mixture agreed with that for the saturated mixture. hc computed with the diffusion layer model for saturated temperature agreed well with the hc measured for the superheated mixture.

Mass transfer from single carbon-dioxide (CO2) bubbles rising through vertical pipes filled with surfactant−electrolyte mixed aqueous solutions was measured to study the combined effects of surfactant and electrolyte on the mass transfer. Triton X-100 and sodium chloride (NaCl) were used for surfactant and electrolyte, respectively. The surface tension, σeq, at an equilibrium state decreased with increasing the concentration of NaCl at a constant concentration of Triton X-100. The pipe diameters were 12.5 and 18.2 mm. A wide range of bubble diameter covered various bubble shapes from ellipsoidal to Taylor bubbles. The combined effects of Triton X-100 and NaCl on the Sherwood number, Sh, showed a different trend from those of alcohol and electrolyte, i.e. bubbles in the Triton X-100−NaCl mixed aqueous solution were not fully-contaminated even with the same σeq as in the condition fully-contaminated with Triton X-100, due to the difference in distributions of surfactant concentrations at the interface. The Sh of bubbles fully-contaminated with Triton X-100 were not affected by the presence of NaCl. The Sh of contaminated Taylor bubbles can be expressed as a combination of functional forms of available Sh correlations for clean and fully-contaminated Taylor bubbles.

Mass transfer from single fully contaminated carbon dioxide bubbles rising through vertical pipes was measured to investigate the effects of surfactants. The diameter ratio of the bubble diameter to the pipe diameter was varied to cover various bubble shapes from ellipsoidal to Taylor bubbles. Triton X-100 and 1-octanol served as surfactants. The mass transfer rates, kL, of contaminated bubbles in the ellipsoidal regime were smaller than those of clean bubbles due to the interface immobilization. The kL of Taylor bubbles contaminated with Triton X-100 and 1-octanol showed varying trends because of the difference in the surfactant distributions at the bubble interfaces which strongly depend on the Hatta number. The Sherwood numbers of contaminated bubbles were well correlated by introducing the bubble diameter, at which the bubble shape transits from ellipsoidal to Taylor bubbles.

In this study, we measured counter-current flow limitation (CCFL) characteristics in an inverted U-tube (18.4 mm diameter and 1.0 m straight-part length) simulating steam generator (SG) U-tubes under conditions of steam condensation at pressures of 0.1–0.14 MPa. Differential pressure ΔP between the top of the inverted U-tube and the lower tank was measured, and the flow patterns wave estimated by comparing the waveforms of ΔP with those in air–water experiments. As a result, we classified the flow patterns under CCFL conditions into CCFL-P, CCFL-L and CCFL-T. The falling water flow rate under CCFL conditions slightly increased as the pressure increased and the cooling water temperature decreased (subcooling of cooling water increased). In the case of CCFL-L, CCFL characteristics in the inverted U-tube were between those in air–water and saturated steam–water experiments at 0.1 MPa. Furthermore, we derived a Wallis type CCFL correlation and its uncertainty from CCFL data, including previously measured data...

The purpose of this study is to derive a counter-current flow limitation (CCFL) correlation and evaluate its uncertainty for steam generator (SG) U-tubes in a pressurized water reactor (PWR). Experiments were conducted to evaluate effects of the liquid viscosity on CCFL characteristics using air–40 wt% or air–60 wt% glycerol water solution and saturated steam–water at atmospheric pressure with vertical pipes simulating the lower part of the SG U-tubes. The steam–water experiments confirmed that CCFL characteristics could be expressed in terms of the Wallis parameters (JG* and JL*) for the pipe diameters of D = 14, 20, and 27 mm. A CCFL correlation was derived using the ratio μG/μL of the viscosities of the gas and liquid phases, μG and μL, as a correction term representing effects of fluid properties, where JG*1/2(μG/μL)−0.07 was expressed by a cubic function of JL*1/2(μG/μL)0.1. In the correlation, the constant C indicating the value of JG*1/2(μG/μL)−0.07 at JL* = 0 was (1.04 ± 0.05), and this uncertaint...