In relation to the development of the interfacial area transport equation, axial developments of void fraction profile, bubble number density, interfacial area concentration and Sauter mean diameter of adiabatic nitrogen-water bubbly flows in a 9 mm-diameter pipe were measured by using a stereo image-processing method under normal- and micro-gravity environment. The flow measurements were performed at four axial locations (axial distance from the inlet normalized by the pipe diameter, z/D = 5, 20, 40 and 60) at various flow conditions of superficial gas velocity (0.00823-0.0303 m/s) and superficial liquid velocity (0.147-0.907 m/s). The effect of gravity on radial distribution of bubbles and axial developments of two-phase flow parameter was discussed in detail based on the obtained data and the visual observation.
Spatial evolution of radial void and bubble size distributions in CO2-water gas-liquid bubbly flows in a vertical pipe were measured using a conductivity probe and an image processing method. Numerical simulations were carried out using a three-dimensional one-way bubble tracking method with a mass transfer model, which takes into account the accumulation of surfactants on bubble surface and the absorption and desorption of gas components. As a result, the following conclusions were obtained: (1) the decreasing rate of the total gas volume decreases in the flow direction due to the desorption of N2 and O2, the accumulation of surfactants and the bubble volume expansion due to the decrease in static pressure, and (2) the bubble tracking method gave good predictions for the bubble dissolution process in CO2-water bubbly pipe flows.
The effects of a surfactant on a single ascending bubble motion and its surrounding liquid motion were investigated using 1-pentanol and purified water systems. An ellipsoidal bubble (2.8 mm in equivalent diameter) was examined in a quiescent liquid. The bubble motion was visualized using a high-speed video camera. As a result, changes of the amplitude and frequency of the bubble trajectory were observed at a 1-pentanol concentration of 150ppm. At 500ppm, “small initial deformation” due to the strong damping effect was caused by adsorption of the surfactant on the bubble surface. The bubble velocity became low as reported by the other researchers. Furthermore, the liquid motion in the vicinity of the bubble was visualized via PIV method (Particle Image Velocimetry) with recursive cross correlation PIV algorithm. As a result, the difference in vorticity associated with the strength of the shed vortex was observed at the inversion points at which the bubble changes the direction of the zigzag motion. The difference affects the change of the amplitude and frequency of the bubble trajectory. An effect of the boundary condition changing from free-slip to no-slip in the contaminated system yields this difference.
In the present investigation, detailed observations on the flow patterns and measurements for pressure drop characteristics affected by the curvature effect in a micro-channel were carried out. The test channel has a square cross section that has 2.0 mm on a side, and the radii of the curvature were 1.0 mm and 3.0 mm. It was cleared that the pressure fluctuation intensity became larger in the case of the curvature 1.0mm compared with the case of the curvature 3.0 mm. Furthermore, in order to investigate the influence of the wall wettability, we applied the water repellent micro-channel. In the case of the water repellent micro-channel, the flow pattern tended to easily transit to Annular flow side compared with the normal micro-channel without the water repellent, and we can see a new flow regime like Rivulet flow due to applying water repellent. In addition, the time-averaged pressure drop was decreased in the case of water repellent compared with the case of normal wall. The reason of this is considered that the liquid slug could move easily due to the lower surface tension force at the solid-liquid interface.
In this study, a new micro bubble generator, i.e. nozzle, with a very simple construction is proposed newly and the behavior, such as velocity distribution and spread, of micro bubble jet flow is made clear by PIV and LDA measurements of velocity distribution. The new nozzle uses a very high velocity water jet flow having a large velocity gradient near the nozzle exit. The shearing force shears the gas bubbles and makes considerable small bubbles or rather micro bubbles. The aeration characteristics of this micro bubble generator, i.e. nozzle, are also discussed.
The effect of microbubble washing was examined for removing oil from the “nori” net. The quantity of oil in the “nori” net after the washing experiments had considerably decreased. 83% or more of oil in the “nori” net was reduced at 25°C. To clarify the washing mechanism of microbubbles, surface tension measurement has been carried out on aqueous solution exposed to microbubbles. The surface tension of aqueous solution decreases with the treated time of microbubbles. These phenomena are attributed to behavior of microbubbles in solution. It was found that shrinking phenomenon of microbubbles greatly decreases the hydrogen bonding causing decreased surface tension. Microbubbles contributed to weakening the hydrogen bonding. When the detergent molecules were introduced into water, the replacement of the water molecules by the detergent molecules results in a decrease in the surface tension of water. However, the detergent solution exposed to microbubbles greatly increases the hydrogen bonding causing increased surface tension. It was also examined that degradation of 4-ethylphenol had occurred.
It is known that water-jet with cavitation clouds shows unsteady behavior and causes high impact. The study on unsteady cavitation clouds is useful for not only the water jet technology but also various engineering fields. The unsteadiness of cavitation clouds is one of the most interesting problems for the use of the cavitating water-jet. In this study, we tried to estimate the behavior of the cavitation clouds at the impingement. Especially, the behavior of the cavitation clouds was observed using a high-speed video camera triggered by the cavitation impact, and analyzed with an image processing technique. As a result, we showed the behavior on two kinds of cavitation clouds on the impinging wall and related the collapse of cavitation clouds with the erosion region through the image analysis of cavitation clouds.
Trajectories of single silicon drops in linear shear flows of glycerol-water solutions were measured to evaluate the transverse lift forces acting on the drops. Experiments were conducted under the conditions of logM = -6.3 and -4.8, 0 < ReD ≤ 23 and 3.2 ≤ ω ≤ 6.0 s-1, where M is the Morton number, ReD the drop Reynolds number, and ω the magnitude of the velocity gradient of the linear shear flow. The lift coefficient CL was calculated from the measured drop trajectory. As a result, the following conclusions were obtained: (1) CL of spherical drops steeply decreases to zero with increasing ReD, (2) CL of deformed drops decreases from positive to negative values with increasing ReD, and (3) when the shear rate is large and CL is positive, CL takes a larger value for smaller M.
An electrostatic levitation technique is a promising method to measure physical properties of extremely high melting temperature materials, since it can avoid the effect of container. Previously, surface tension and viscosity have been calculated by an oscillating drop method based on linear approximations. However, the actual levitating drop includes nonlinear effects on the oscillation. For highly precise measurements, nonlinear effects should be taken into account. In the present study, the oscillatory motion and rotating drop of levitated drop is experimentally investigated to clarify the nonlinear effect. From the experimental results, it is clarified that the resonance frequency becomes smaller as the oscillation amplitude is larger. But, on the rotating drop, resonance frequency becomes larger as the rotation speed is increased. The result indicates that the large-amplitude oscillation includes the effect of nonlinear oscillation.
The containerless processing conducted by droplet levitation techniques makes it possible to manufacture new materials and conducts non-contact measurement of material properties under the microgravity condition. It is pointed out that surface deformation and internal flow of the large levitated droplet influence the containerless processing. There are few studies about surface deformation and internal flow of the large droplet levitated by the acoustic wave. In the present study, the characteristics of the surface deformation and the internal flow of the droplet levitated by the ultrasonic wave are investigated under the normal gravity condition and microgravity condition by using the aircraft. Under the microgravity condition, it is possible to levitate a larger and more spherical droplet in comparison with under the normal gravity condition. When the droplet surface oscillates in the second mode, the internal flow corresponding to the surface oscillation and the asymmetrical complex flow are observed.
Recently, particle laden high speed jet flow is used widely for fine machining of brittle material. However, the machining characteristics, particularly the flow characteristics of particle laden high speed jet flow are not well understood because of its complexity. In this study, the flow characteristics of particle laden high speed free jet flow issued from a jet machining round nozzle are examined experimentally by PIV measurement for various mass loading ratio and supplied air pressure. Major results are (1) the particle velocity increases with increasing supplied air pressure and decreasing of mass loading ratio, (2) the spread of particle increases with increasing mass loading ratio and decreasing supplied air pressure, (3) the spread width of particle laden jet flow to the downstream can be expressed by a linear function, and (4) the particle velocity profile on the jet-centerline can be divided into three regions.
In-flight behavior of micro-nano particles and the interaction between shock wave and particles in a supersonic jet which impinges onto the substrate in a micro space are clarified in detail by computational simulation. The optimal particle diameter for maximum impinging particle velocity exists. Particles smaller than the optimal diameter are decelerated drastically prior to the impact on the cavity as a result of unavoidable particle-shock interaction. The larger deceleration through shockwave is observed with decrease in particle diameter. The particle less than 100 nm cannot penetrate the shockwave and carried by wall jet without impinging on the tooth cavity. The electrostatic force in the vicinity of tooth cavity acts effectively for the acceleration of submicron particles even in the presence of unavoidable shockwave in micro space.
Turbulence modulation by high-inertia particles having the same response time and same volume loading is investigated. Particles slightly larger than Kolmogorov micro-scale are released into a steady isotropic homogeneous turbulence. The magnitude of turbulence attenuation increases as the particle becomes smaller. In the small attenuation case, vortex tubes can dodge around the particles and develop. In the large attenuation case, the vortex tubes collapse and high dissipation region which is responsible for turbulence attenuation is formed near the particle surface.
Utilization of ultrasound is considered as an alternative technique for the separation of particles. The authors studied particle behavior in ultrasonic waves in water with frequencies of 23 kHz or 97 kHz. As a result, the particle aggregation types of bands, clump and points are observed, depending on the frequency, power of ultrasound and particle diameter. It was found that some of the aggregation locations for the clump and the point aggregations are reproducible, respectively.
We investigated the heat transfer during tube quenching for developing an on-orbit cryogenic fluid management system. This paper describes the experimental results of tube quenching for two flow directions (upflow and downflow) under terrestrial conditions. Liquid nitrogen is used as the test fluid, and it is injected into a transparent heated tube at a mass velocity of 100-300 kg/m2s. The tubes are made of Pyrex glass, and have two kinds of inner diameter (10mm and 13.6mm). The thermal data reveals that the quenching time during which the inner wall temperature became equal to the liquid nitrogen temperature under upflow conditions is less than that under downflow conditions. The flow visualization data reveals that the droplet size and flow derangement under upflow conditions were greater than those under downflow conditions. This flow behavior leads to an increase in the cooling rate of the tube wall under upflow conditions.
To develop compact and high-performance cooling systems, experiments on the increase of CHF for flow boiling in narrow channels by improved liquid supply were conducted. A large surface of 150mm in heated length and 30mm in width with grooves of an apex angle of 90 deg, 0.5mm depth and 1mm in pitch was employed. A structure of narrow heated channel between parallel plates with unheated auxiliary channel was devised and tested by using water in different gap sizes and in different combinations of flow rates. The auxiliary channel was installed for the purpose of additional liquid supply to the heating surface from the transverse direction perpendicular to the flow in the main channel. In the present paper, the effect of reduction in liquid flow rate applied to the auxiliary channel was investigated, where the ratios of liquid velocity at the inlet of auxiliary channel to that at the inlet of main heated channel were varied as 1, 2/3 and 1/3. Almost no change in both CHF values and in heat transfer coefficients were observed despite of the reduction in the flow rate for the auxiliary channel. In the case of the same total volumetric flow rate, values of CHF for gap size of 5mm were higher by around 1.5 times than those for gap size of 2mm. CHF values larger than 1×106W/m2 was obtained for mass velocity G= 33kg/m2s at s= 5mm and for G= 82kg/m2s at s= 2mm, where the mass velocities were calculated by hypothetical inlet liquid velocities under the assumption that the total flow rate was distributed at the same velocity to both of heated and unheated channels.
The structure of the flow around a rising bubble in silicone oil is experimentally studied by flow visualization using photochromic dye. The first experimental support for the existence of the standing eddy at the rear of the bubble, as predicted by a previous study by Ryskin and Leal [J. Fluid Mech. 148, 19-35 (1984)], was obtained in the case of high bubble aspect ratio χ . The drag coefficient of small bubble rising in this solution was compared with theoretical one derived by Mei [Phys. Fluids, 6, 418-420(1994)], and it is found that the bubble rising in this solution behaves as the “clean bubble". It is also shown that both the degree of the fore-aft asymmetry of the bubble shape and the size of standing eddy increases as the discrepancy in both rising velocity and drag coefficient between experimental results and Moore's theory [J. Fluid Mech. 23, 749-766 (1965)] increases.
The present paper describes a new measuring technique of bubble velocity inair-magnetic fluid two-phase flow for flow regimes of slug flow and churn flow. The proposed technique using the electromagnetic induction achieves a non-contact measurement. In the present method, the measurement was carried out by analyzing two signals, which are the induced electromotive forces obtained in two sets of pick-up coils, with an alternating magnetic field. Inorder to verify the credibility of the measurement, an alternative experiment for measuring bubble velocity was carried out together with a theoretical comparison with an existing model. As a result,it was proved that the proposed measurement particularly gave good estimations of bubble velocity particularly in slug and churn flows observed in vertical two-phase flow.
An image velocimetry technique for measuring fluid motion of liquid-solid flow with high particle density is developed and presented. The new technique, which obtains velocities and directions by connecting path lines of tracers in sequence images captured by high speed cameras, is classified into a kind of particle tracking velocimetry. Detail of the algorithm of the developed image velocimetry, called path line connecting velocimetry (PLCV), is presented. Some results of measuring liquid-solid flows induced by dispersion phase with high concentration are presented.
The bubble plume around a square-section cylinder is simulated. The two-dimensional vortex method for gas-liquid two-phase flow, proposed by the authors in a prior paper, is employed. The small air bubbles are injected into still water from a nozzle below a square-section cylinder of which side length is equivalent to the nozzle width. The water velocity on the horizontal cross-sections satisfies the similarity distribution around the plume centerline. The cylinder affects mainly the water flow in the upper region. The water velocity is lower, but the vertical component of the water turbulent intensity is two times higher than those for the plume without the cylinder. The water flow spreads more toward the horizontal direction.
The lattice Boltzmann method for two-phase flow is applied to the simulations of binary droplet collisions with the size ratio of 0.5 for various Weber numbers of 30 < We < 83, impact parameters of 0 ≤ B ≤ 0.75 and Reynolds numbers of 3000 < Re < 4900. The density ratio of the liquid to the gas is fixed at 50. The results are classified into the three collision formations such as coalescence, reflexive separation, and stretching separation. The boundaries between the three types of collisions are compared with existing theoretical predictions. The mixing processes are also simulated for various Weber numbers at B=0 to investigate the mixing rate during separating collisions, and the relation between the mixing rate and the Weber number is obtained.
In this paper, numerical simulations of a contaminated water drop sinking in silicone oil were performed by a Front-tracking (FT) method. In order to achieve high spatial resolution near the interface, an Adaptive Mesh Refinement (AMR) method was employed. A finite difference scheme suitable for AMR grid systems, which are independent from vortex structure, was proposed and evaluated. Importance of accuracy of the finite difference scheme for viscous term at interface between different size cells was confirmed. Effect of the calculative thickness of interface in FT simulations was evaluated by comparing the results with and without AMR. It was found that the proposed AMR-FT method taking the contamination effect into account can reproduce the experimentally observed water drop very precisely.
An interface-tracking method based on a phase-field modeling is evaluated through numerical simulation of thermal liquid-vapor flows with phase change in micro-fluidics devices. In the modeling, the interface with a finite width between phases is autonomously formed while satisfying the free-energy theory. The phase-field method therefore needs no conventional elaborating algorithms for advection and reconstruction of the interfaces. The numerical results of two-dimensional van-der-Waals fluid flows around a critical point prove that the method has a potential to capture liquid-vapor motions in the non-ideal fluid with heat and mass transfer across the interfaces, such as bubble nucleation on a heater and cavitating flows around a solid body.
The collision between a vortex ring and glass particles is simulated by the three-dimensional vortex method. The vortex ring, convecting with the self-induced velocity in a quiescent air, collides with particles. The Reynolds number for vortex ring is 2600, while the Stokes number for particle is 0.74. Just after the collision with the vortex ring, the particles surround the vortex ring, forming a dome. It is parallel with the preferential distribution of the particle with Stokes number unity that have been measured and simulated numerically in various free turbulent flows. The collision reduces the circulation and core diameter of the vortex ring.
At Research Institute for Applied Mechanics (RIAM) of Kyushu University, a Cartesian-grid method in combination with CIP-based interface capturing schemes is being developed for computing extremely nonlinear wave-body interactions. To confirm validity of this calculation method, careful and systematic experiments of both two- and three-dimensions are also carried out. In this paper, following a brief description of the calculation method, some recently obtained numerical results are shown for violent water-wave behavior around the ship's deck and resultant impulsive pressure and ship motions, which are compared with corresponding experiments.