The behavior of flows including dense solid particles is complex especially in near-wall regions due to existence of interactions between particles, particle-wall and particle-fluid. Due to the difficulty of experimental observations, the effect of a wall is not known well at present. In the present study, new boundary condition on a flow field including dense moving particles is proposed and the influence of solid wall on a flow field including dense moving particles is investigated in detail by means of microscopic direct numerical simulation. It is confirmed that the wall gives influence both on the particle arrangement and fluid velocity distribution close to the wall.
The lattice Boltzmann method for two-phase fluids is applied to penetration of liquid in gas through spherical bodies with various types of wettability. The previous wetting boundary condition on a solid wall is partially improved: the second derivative of the order parameter at a wall is approximated by the values and the first derivative at the wall and at the nearest node from the wall. In the preliminary calculations, the static contact angles obtained by the present approach are in good agreement with the theoretical predictions according to the Young's law between 24 and 156 degrees. Using this method, liquid penetration through the spherical bodies is simulated for various wetting conditions, several porosities of the porous structure, and several diameters of the spherical bodies.
This study developed an optic-axis controlling device using electrowetting. The device has three-layered liquid: two types of oil and water, in a glass cell with two electrodes. Voltage applied to the electrodes deformed the liquid-liquid interfaces and laser trajectory through liquid layers was refracted at the interfaces. We experimentally obtained about 3 degree of optic angle change at 80 V of applied voltage. We also numerically solved interface shapes between layers to reveal the relation between applied voltage and interface shapes.
Point energy release in water is a model of the early stage of laser breakdown in water. This initial model is indispensable in the simulation of the laser induced underwater phenomenon. In this study, we first constructed a series of equations of states for real water and steam, covering a wide range for temperature and pressure, required for a fully compressible flow simulation. Since the phase change is closely related to the formation of a bubble at the later stage, special care has been paid to maintain the high accuracy of the constructed formulas at saturation curves. We further developed a physical model for the setup of initial conditions for the point energy release using the constructed formulas. Using these models, the mechanism of a sudden energy release in water was investigated. The equations of states under consideration can be used to the simulation of both cavitation and boiling phenomenon.
Evaporation heat transfer from the hot water flow to the cold air flow in a horizontal duct was examined. Hot water was in the range of 35°C ~ 60°C. Cold water was approximately 25°C. The air velocity was varied from 0.0656 m/s ~ 1.41 m/s. The heat transfer rate from the water flow to the air flow became large with an increase in the air velocity. The higher the water temperature was, the larger the heat transfer rate was. When the total heat flux from water to the air flow is divided into two terms; the evaporation term and the forced flow convection term, the evaporation term dominate main part and that is about 90 ~ 80 % of the total heat flux. The measured values of the evaporation term and the forced flow convection term were larger than the predicted because of the effect of the diffusion of evaporated vapor. The correlation to predict the heat transfer from the hot water flow to the cold air flow with the evaporation was developed by modifying the laminar flow mass transfer correlation and the laminar forced convection heat transfer correlation. Good results were obtained.
A simple method for the rapid generation of superheated steam using a water-containing porous material was proposed in a previous paper (Mori and Okuyama, 2007). The start-up and cut-off responses are of the order of second, and the maximum energy utilization efficiency for input power is more than approximately 0.9. In the present paper, in order to clarify the process of the rapid generation of superheated steam, the steam temperature, the surface temperature of porous materials just below the wire heater, and the surface roughness of porous materials have been measured. As a result, it is found that narrow space between heater and porous material which is formed by surface roughness of porous material is one of important factors to generate superheated steam rapidly.
Container-less processing using levitation technique could prevent heterogeneous nucleation and contamination from container wall. Acoustic levitation is expected to be used for analytical chemistry and manufacturing new materials. Thus, it is important to gather the knowledge about acoustically levitated droplet. The purpose of this study is to investigate the heat transfer and flow characteristic of an acoustically levitated droplet. The following results were obtained from experiments. Evaporation process of levitated droplet was observed. Surface temperature lowers after levitation and volume decreases while levitation. External flow was observed and differs by water and ethanol. Toroidal vortex near the aqueous solution of ethanol droplet interface was observed. By measuring temperature, temperature distribution near the droplet surface was found. Finally, heat transfer coefficient was estimated and it was higher than the existing experimental formula.
Thermophysical property measurement of material without container is promising technique with high accuracy for high temperature molten state. Microgravity environment is useful to achieve containerless condition, and levitation technique also realizes the containerless processing even on the ground. The purpose of this study is to establish new measurement technique of viscosity for moderate viscous range, which has no conventional measurement methods. In this study, a droplet is levitated by electrostatic force, induced to rotate. The dynamic behavior of levitated droplet shape is observed, and viscosity measurement is discussed by using breakup of levitated droplet depending on viscosity.
In water reservoirs such as dam, the increment in sediments causes capacity reduction as well as water quality deterioration. In the present paper, the siphonic sediment removal system, invented by Sadatomi, is experimentally studied using different sized siphons and sands. From the present data and our previous data for spherical particles, a new correlation of particle volume flow rate fraction is obtained. By incorporating the new particle volume flow rate fraction correlation, the performance prediction model proposed in our previous study is revised and tested to know its applicability as a design tool of actual large scale systems. The validity of the revised model is confirmed by testing against the present data as well as previous data.
World has been faced with serious problems of water pollution. Recently, conventional chemical treatment has been replaced by a water treatment system using plasma. In this study, a method for decomposing organic compound by spraying solution directly as mist into reactive plasma was investigated using a newly developed mist-flow plasma reactor. The plasma reactor is a tube made of Teflon with a thickness of 0.5 mm and has an inner mesh electrode made of stainless and an outer grounded electrode made of copper. Non-thermal plasma is generated by dielectric barrier discharge (DBD) at the inner wall of a tube. An atomized solution including micro-sized droplets was introduced into the plasma reactor and was treated by ozone, free radicals and ultraviolet rays. Dissolved chemical species such as H2O2, reactive oxidation species and O3 are measured as liquid properties. The decomposition characteristics of this method were experimentally clarified through decolorization of methylene blue solution. The methylene blue solution is about 100 % decomposed by only one treatment under certain operating conditions. In addition, a water treatment method using aerosol plasma is compared with a activated air microbubble jet treatment.
A micro-bubble generator with a venturi tube generates a large number of micro-bubbles with a diameter of 10 μm ― 1 mm by bubble collapse. The bubble collapse is caused by pressure recovery in the diverging region of the venturi tube. The pressure recovery is expected to be a shock for supersonic flow in gas-liquid two-phase flow. However, a profile of Mach numbers to flow direction of the venturi tube has not been estimated experimentally. The present study reveals mechanisms of a bubble collapse. In order to achieve the objectives, we observe bubble behavior with the bubble collapse. In addition, we measure pressure and volumetric void fraction profiles in the flow direction. Pressure is measured by a differential pressure gauge. Void fraction is measured by a constant electric current method and Maxwell' s theory. From these measurements, gas-liquid mixture velocity, sonic speed and Mach number are estimated. In experimental results, bubble collapse is observed with high liquid inlet velocity. When bubble collapse is caused, bubbles expand once into a divergence region of the venturi tube. After that, they contracted rapidly and broken up into a great number of tiny bubbles. Pressure decreases sharply around the throat and increase at the bubble collapse point. On the other hand, the void fraction increase downstream from the throat and decrease around the bubble collapse point. From these results, it is confirmed that the flow is supersonic flow between the throat and the bubble collapse point, while it becomes subsonic flow downstream of the point. Therefore, it is proposed that a shock is present at the bubble collapse point.
The pressure wave propagation caused by the breakdown of a focused laser beam in a bubbly channel flow is investigated experimentally. Measurements were conducted in the downstream of a convergent-divergent channel with rectangular cross-section; microbubbles are generated downstream the constriction of the channel. Optical observation and pressure measurement were conducted using a high-speed video camera and a hydrophone, respectively. There are two major pressure waves: one which propagates with the speed of sound in water known as a precursor wave, another which propagates more slowly. The slower pressure wave is associated with the volume elasticity of gas-liquid mixture which depends on the local void fraction and bubble oscillations. The propagation speed of the slower wave is calculated from the image processing, and decreases down to 60 m/s. The influence of wave length, bubble size, and surface tension on the void fraction evaluated from the propagation speed of the slower wave is discussed. As a result, the void fraction is quantitatively estimated from luminance value and the propagation speed of slower wave in an isothermal homogeneous two-phase mixture. It was shown that with increasing the void fraction, the amplitude of the precursor wave decreases drastically while that of the slower wave decreases weakly.
Fluid slugs generated by a microchannel T-junction have various applications such as emulsion, micro reactor, microbubbles and so on. For these applications, it is required to elucidate formation mechanism of slug flow and control the size of two phase slugs. We study the influences of flow rate with slug length for the purpose of understanding the cause of fluctuation of slug length. We used the T-shaped microchannel with a width of 100 μm and a depth of 95 μm in the experiment. It is made of glass and silicon by anodic bonding, and it has hydrophilic surface in the micro channel. The slug flow was generated at T-junction of the microchannel by using pure water as continuous phase and silicone oil as dispersed phase which viscosity is 1 cs as the working fluid. We observed formation of slugs at T-junction in detail with measuring pressure by using a optical fiber micro pressure sensor. It was measured that pressure fluctuation which does not correspond to cycle of formation of slug and we observed length of two phase slugs fluctuated with opposite correlation. We studied correlation of length of two-phase slugs with the equation proposed by Garstecki.
The purpose of this study is to investigate the effects of a return bend and liquid properties on the pressure drop for gas-liquid two-phase flows in a horizontal rectangular microchannel having a hydraulic diameter of 0.25 mm. To know liquid properties effects, distilled water, pure ethanol, 49 wt% ethanol aqueous solution and HFE 7200 were used as the test liquid, while nitrogen gas as the test gas. To determine the effects of curvature radii of 0.875 mm and 0.500 mm in the return bend, pressure distribution upstream and downstream tangents of the bend was measured for single-phase and two-phase flows. From the pressure distribution data, the friction factor within the bends was determined. The bend friction factor for single-phase flow could be correlated with Reynolds number and the curvature ratio of the bend, while that for two-phase flows with the single-phase friction factor and a two-phase multiplier, being a function of gas volume flow fraction, Bond number and Lockhart-Martinelli parameter defined for the bend.