Numerical simulations of aerosol motion induced by sneezing and exhalation were carried out by CFD (Computational Fluid Dynamics) with modeled particles. For sneezing analysis, with and without covering by an elbow and wearing a face mask were modeled to clarify the effect of how to prevent scattering. Aerosol scattering from a talking passenger in a ventilated train was also analyzed to show a realistic situation that may cause infections of the virus. Those results have been published on the web through several SNSs to promote understanding the effect of infection prevention manner.
Inactivation of microorganism (bacteria, fungi, alga and plankton) using ultrasonic waves has attracted significant attention. However, the details of the inactivation mechanisms have not been cleared. This study aims to clarify the inactivation mechanisms of bacteria and fungi by acoustic cavitation. Escherichia coli (E. coli), Saccharomyces cerevisiae (S. cerevisiae) and Bacillus subtilis (B. subtilis) were used as the test microorganism. Inactivation was attempted by ultrasonic irradiation at frequencies of 20 kHz to 4.4 MHz and an acoustic power power of 10 W. Different frequency dependences of the inactivation were confirmed in E. coli, S. cerevisiae and B. subtilis. Based on these data, the mechanism of inactivation of microorganism is discussed with a focus on cell characteristics.
Extracorporeal membrane oxygenation (ECMO) is a respiratory or cardiac life support consisting of a vascular access cannula, a blood pump, and an artificial lung that removes carbon dioxide and adds oxygen. Venovenous ECMO is typically used for various forms of critical respiratory failure including severe pneumonia due to the new coronavirus, whereas venoarterial (VA) ECMO sees greater utilization in the patient with heart failure such as cardiac arrest or other clinical states in which cardiac dysfunction is a significant component of illness. The transference of gases occurs due to diffusion across the hollow fiber membranes packed in the module of the oxygenator. Emergence of microporous membrane tremendously enhanced gas diffusion, and currently microporous membrane with a dense layer on the blood-contacting surface is widely used for longer period of usage over weeks. As the usage period for respiratory failure is getting longer, development of more durable ECMO system is required in the future.
The growth of bubble nuclei in high intensity focused ultrasound (HIFU) is investigated. The imposed pressure on the nuclei is given by the experiments for the backscattering of HIFU from a laser-induced bubble interface. The lower limit of initial radius, R0min over which the bubble grows explosively, is calculated using the dynamical equation of motion for a spherical bubble in which the liquid compressibility and the thermal diffusion through the bubble interface are taken into consideration. The relationship between R0min and the absolute value of the minimum of imposed pressure |Pmin | is obtained for various temperature ranges. It is shown that temperature dependency of |Pmin | is in good agreement with that obtained directly from the corresponding experiments. It is also shown that appropriate choice of physical properties makes it possible to predict the relationship between R0min and |Pmin | in a temperature range from that in different temperature ranges even though the imposed pressure on the nuclei is different with each other. The results also show that temperature dependency of surface tension is a key factor to determine R0min.
The tiny bubbles with diameters less than 1 μm are called ultrafine bubbles (UFB). And the application of UFB in agriculture, fishery and environment is highly anticipated. In this study, the changes in the properties of oxygen, 5% hydrogen (95% nitrogen), and nitrogen UFBs were investigated with droplet formation. The UFB water was ejected through the orifice nozzle (diameter: 280μm) and formed tiny droplets. Particularly in the case of oxygen UFB, a change in the distribution of the bubble size was observed. We observed higher microbubble concentration and smaller bubble size below 50μm in the sprayed UFB water compared with in the control distilled water. The distribution of oxygen UFB itself changed before and after spraying, and its diameter tended to be larger. In addition, its water contained more MB, which resulted in a higher cleaning effect.
The pressure measurement using a fiber optic prove hydrophone (FOPH) and the observation with a high-speed video camera have been conducted simultaneously to investigate the collapse of a laser-induced bubble near a rigid wall and the corresponding shockwave generation due to the bubble collapse. Prior to the experiment of the bubble collapse near a rigid wall, the experiment of a laser-induced bubble in infinite water has been conducted to check the applicability of the FOPH to the pressure measurement. The directivity of FOPH is evaluated by controlling a directional angle to the spherical shockwave and a distance from a collapsing bubble. The peak pressure Pmax obtained at each θ was normalized by the peak pressure P0max obtained at θ = 0, and the calibration formula was obtained. Also, the peak pressure of the shockwave decays in proportion to the -1st power of the distance, which shows the characteristic of a spherical wave propagation. In the experiment of the collapse of a laser-induced bubble near a rigid wall, the bubbles collapse accompanying with the translation toward the wall. Then, the micro-jet penetrates the bubble surface toward the wall. By introducing a stand-off parameter γ, the peak pressure by the first collapse was evaluated. Due to the calibration of the directional angle and the distance, the peak value of the pressure of the shockwave takes a local minimum when γ is around 1.
Microbubbles have various properties and are expected to be used in a wide range of fields. In this research, we focus on one of microbubble generators with a converging- diverging nozzle, i.e. a Venturi tube. In order to control bubble distribution of generated bubble diameter, knowledge of bubble generation mechanism in the tube is required. The purpose of this research is to clarify the effect of complex flow in a Venturi tube on bubble collapse. In this research, we focus on the change of pressure loss coefficient calculated from the pressure distribution in the tube and the sound pressure generated at the time of bubble collapse. For results of the experiment, the pressure loss coefficient increased due to the cavitation in the liquid single-phase flow. The same tendency was further obtained in the bubbly flow. It was also confirmed that the collapse behavior of bubbles differed depending on the inlet conditions, and sound pressure increased under conditions where rapid bubble collapse occurred.
A simple one-way prediction method for wall thinning due to liquid droplet impingement (LDI) inside a valve using a steady flow simulation and a steady particle tracking has been proposed. The validity of the method was examined in a real globe valve configuration. We have confirmed that the position with high wall thinning rate obtained by the proposed method corresponded to the perforated position in a real valve. The present simple one-way method is expected to predict the position with high wall thinning. However, the present method overestimated the peak value of the wall thinning rate. The input droplet diameter greatly affected the value of wall thinning rate. Therefore, it is important to correctly model the droplet diameter for quantitative prediction of wall thinning.
Contactless liquid manipulation is important for applications in analytical chemistry, biotechnology and material science. The manipulation technology can prevent the wall effect, such as contamination, absorption, and heterogeneous nucleation for the wall. This study focuses on an ultrasonic phased array system based on the acoustic levitation method. Although the ultrasonic phased array system is promising for sample manipulation in mid-air, this method has not been fully investigated. This paper aims to demonstrate the contactless droplet levitation and three-dimensional (3D) transportation in mid-air via an ultrasonic phased array system. We have developed an opposing device with a face-to-face ultrasonic phased array system and demonstrated the transportation of the droplet. First, the droplet can be transported one-dimensionally (1D). Then, the followability of the droplet was shown with transport speed and droplet diameter. The sound field analysis showed the potential of the levitated point. Based on the experiments, it was found that oscillation amplitude of the droplet increased with the increase of the transport speed and the droplet diameter. For developing the contactless liquid manipulation, we demonstrated the 2D and 3D transportation. Our findings promote the feasibility of contactless droplet manipulation with a phased array for potential applications.
When a gas-liquid two-phase jet is discharged from a nozzle, its surface is fairly smooth for a certain distance from the nozzle exit. Then, deformation develops to cause jet breakup eventually. The deformation length that is defined as the length required for the jet deformation to develop is of industrial importance since significant splashing occurs when deformed jet impinges on a solid surface. The present work investigated the deformation length experimentally using air and water as the test fluids. The flow direction was vertical downward, and annular liquid film was formed at the nozzle exit. It was shown that the deformation develops faster with an increase in the gas flow rate. The measured deformation lengths were about 28.5 times the liquid film thickness estimated at the nozzle exit and no noticeable influence of the Weber number was found.
Experiments on air-water two-phase flows in a serpentine tube consisting of vertical U-bends and circular straight pipes were carried out to discuss the flow characteristics based on the high-speed video images of the flows at various gas and liquid superficial velocities. The pipe diameter, D, and the bend radius of curvature, R, were 20 mm, i.e. the bend ratio, R/D, was unity, and the length of the straight pipe was 1500 mm. The void fraction in each section was measured by using a quick-closing valve method. As a result, the following conclusions were obtained: (1) the top and bottom U-bends cause coalescence of large bubbles and liquid slug formation, respectively, which results in enhancement of flow pattern transition between the slug and annular flows, (2) the void fraction in the downward flow is larger than that in the upward flow due to acceleration of the liquid phase and the buoyancy acting on bubbles in the downward flow, and (3) void correlations based on the drift-flux model for fully-developed flows are applicable to the void fractions in the straight pipe sections under the present experimental conditions.
Currently, the MH21-S R&D consortium (MH21-S), supported by the Ministry of Economy, Trade, and Industry (METI), is developing the commercial production process of methane gas from methane hydrate. In order to develop a gas production method, the objective flow regime identification technique of gas-liquid two-phase flow under high-pressure conditions would be useful. Therefore, in the present research, a new method for identifying the flow regime by a convolutional neural network (CNN) using high-speed images obtained at a narrow view was proposed, and its performance was evaluated. Specifically, the upward gas-liquid two-phase flow images were recorded using a high-speed camera, and they were merged into a single image containing information from entire frames. The CNN was created and trained with these newly merged images, and it was utilized to identify flow regimes of unlabeled test images. As a result, the proposed method using the CNN model was capable of identifying the flow regime with high accuracy. Thus, the present study has shown that the CNN model with the proposed image conversion method can properly identify the upward gas-liquid two-phase flow regime. In addition, by utilizing the developed network, the bubbly-slug transition region, which has only been studied through visual observation or void fraction signals in the past, was successfully evaluated quantitatively.
Boiling heat transfer technology is considered as a promising way to remove the large amount of heat from electronic devices whose heat generation experiences a rapid increase recently. However, by employing the boiling heat transfer, there can be one concern that the pump efficiency may be reduced due to the vapor bubbles flowing into the coolant pump. Therefore, the occurrence mechanism of the pump performance degradation needs to be studied. In this study, a small-size centrifugal pump is selected and the pump performance degradation is investigated by measuring the liquid flow rate by changing the inlet gas bubble flow rate. In addition, the two-phase flow behavior in the pump is visualized and measured by using X-ray imaging. Numerical simulation is also performed to simulate the two-phase flow field around the pump impeller and the simulated results are compared with experiments. As a result, it is observed that the water flow rate decreases abruptly when the air flow rate exceeds a certain threshold value. In addition, both the X-ray imaging result and numerical simulation result shows that the accumulation of air bubbles occurs near the pump inlet, which may affect the water flow rate and in turn the pump efficiency.
Torque and flow pattern of gas-liquid two-phase flow driven by a rotating body are numerically studied. Two-dimensional direct numerical simulations are performed using a fixed mesh approach based on Volume Of Fluid (VOF) and Boundary Data Immersion (BDI) methods. We investigate the influence of rotational speed Ω, fluid properties, and rotating body shape. The interplay between the centrifugal and gravitational forces gives rise to transition of the flow pattern with Ω, that is qualitatively similar for all the body shapes chosen in the present study. The rotational speed of the transition to the annular flow in the non-axisymmetric system is found to be lower than that in the axisymmetric system. The flow structure is classified into four patterns with respect to the torque and the contact ratio of liquid phase at the rigid body interface. The torque and the contact ratio are found to be scaled by the Froude number.
Gravitational instability, which is a typical example of hydrodynamic instabilities, occurs due to density difference. When a heavy fluid lies over a light fluid in a constant gravitational field, fluctuations at the interface gradually increase and then macroscopic flows occur. The gravitational instability can be found not only in liquid-liquid interface but also in the gravitational settling of granular materials. However, the gravitational instability of liquid systems and granular systems have been discussed individually in most cases. We quantitatively find a close relationship of the gravitational instability between the physical gel and granular systems. We also find that those behaviors are determined by the thickness of the fluidization region.
We have simulated the motion of particle assemblage passing through fibrous media. On the assumption that the particle size is of the order of micrometers, the velocity of individual particles was calculated by use of the Stokesian dynamics that describes the Stokes flow in the presence of multiple particles. The results show that a part of particles is captured by fibers while others avoid them and permeate the media. From the velocity, traveling distance and direction of the particles which are not captured, the diffusion caused by the hydrodynamic interaction was observed. The results indicated that the motion of each particle greatly differs depending on the particle size particularly for the large fiber volume fraction, which would be closely related to the hydrodynamic diffusive behavior.
Cellulose Nano-fibrils (CNFs) are the fundamental building blocks extracted from wood fibers and have gained great attentions as a novel biological material. It is known that the outstanding mechanical properties of cellulose filament made of CNFs can be obtained with their orientation inside the filament, the orientation control of CNFs is essential in the material fabrication. In this study, the effect of channel configuration on the orientation of CNFs by elongational flow are clarified both by numerical simulation and experiment. The higher CNF orientation parameter can be obtained for the channel with the focusing angle of 45 degrees compared to that with 90 degrees and also by increasing the flow rate ratio of downstream sheath flow to the upstream one. The ultimate tensile strength and toughness of cellulose filament increases by 11 % and 59 %, respectively by increasing flow rate ratio from 1.0 to 3.0 with 45 degrees focusing angle as predicted from the numerical simulation results.
Thermohydraulic behavior in spent fuel pool is quite important in evaluating safety of a nuclear reactor under accidental conditions. When the liquid level of spent fuel pool decreases, fuel rods are exposed, heated up and finally failed. In order to avoid overheating of fuel rods, spray cooling is planned. Spay water is expected to enter rod bundle and cool down fuel rod. Therefore, quantity of spray water flowing into rod bundle has to be evaluated. A series of experiments were carried out for this phenomenon. The results showed that the water flowing into rod bundles is objected to the flooding and correlated by flooding correlations. However, the parameters of flooding correlations depended upon spray water flow rate and showed higher values compared with previous correlations.
This paper describes the design, fabrication, and the heat transfer performance of a miniature loop heat pipe (mLHP) aiming for cooling a small and high heat flux devices. For the design process, a one-dimensional numerical model was developed. Based on the numerical model, an mLHP was fabricated. The mLHP comprises an evaporator (15×16×t3 mm3), a condenser (12×12.5×t2 mm3) and transport lines (O.D.=1.6 mm, L=100 mm). The mLHP uses Shirasu porous glass (SPG) as a wick and is mainly made of stainless steel. The heat transport performance was investigated in horizontal attitude, anti-gravity (top heat mode) and gravity-assisted (bottom heat mode) with working fluid of water. When the condenser was dissipated by natural air convection, the mLHP performed 5-W (20-W/cm2) heat transport at 170-℃ heat source temperature in horizontal attitude. The thermal resistance was 7.9℃/W. In the bottom heat mode, the lowest thermal resistance of 2.6℃/W was obtained due to the gravity assist. Additionally, the mLHP was tested under the condition that the evaporator side was kept horizontal but the condenser side was bent upwards by 90 degree around the middle of transport lines. Consequently, in low heat load condition the bent mLHP showed the same performance with the bottom heat mode, but shifted to the horizontal mode as the heat load increased. Thus, the heat transport performance was comprehensively investigated for considering various uses.
Experiments on countercurrent flows of air and water in a 3×3 rod bundle were carried out, and countercurrent flow limitation (CCFL), pressure gradients dP/dz and void fractions α were measured under flooding conditions. Flooding with smooth and thin liquid film occurred at the top when the gas volume flux JG was small, whereas flooding with rough and thick liquid film took place at the bottom for medium and large JG. This behavior was similar to flooding in a vertical pipe. CCFL characteristics were expressed by Kutateladze parameters and agreed with those for the upper tie plates in BWR fuel bundles. The volume flux JL of falling water in the bundle was larger than JL in a vertical pipe, but it was smaller than JL through horizontal perforated plates without rods. α under flooding conditions were lower than α under stagnant water (JL = 0) conditions in the region of JG > 4 m/s. The wall friction and interfacial friction factors were obtained from the data and discussed.
Bubbles with a diameter of 1 to 100 μm are defined as microbubbles (hereinafter, this is called "MB"), and are used for floatation separation because of their adhesion to suspended substances. However, there are few research reports on floatation separation, and standardization for device design has not been performed, so improving the efficiency of floatation separation devices has become an issue. Therefore, in this study, the experiments were carried out to clarify the floatation separation characteristics by microbubbles as a basic research of floatation separation by microbubbles. In the experiment, the soft flour was used as a suspended substance, and as basic characteristics, the state of adhesion with microbubbles, the relationship with bubble diameter, and the rising velocity rate were measured. In addition, as the actual floating separation performance, the concentration time with each initial concentration was investigated. The conclusions are as follows: About 60% of the adhered form of the microbubbles and the soft flour was observed as a form in which a single microbubble adhered to the soft flour. The number of microbubbles with a bubble diameter of 80-90 μm was attached in the case of single microbubble attachment to flour is large. In the floating separation characteristics, floatation the soft flour concentration in the tank decreases sharply at the short time, and then gradually decreases. The higher the initial concentration, the longer the processing time required to reach the target concentration, but the tendency of the concentration decrease was the same. The final concentration remains around 20 ppm in all experimental conditions. These results are important for standardization of floatation separation characteristics in the future.
We develop a novel equipment that levitates giant drops, size of which is larger than the capillary length, over a moving wall. Generally, the levitation of giant drops is challenging since a drop larger than capillary length deforms and breaks up easily. We overcome this difficulty by equipping a special-shaped glass hollow cylinder. The novel equipment successfully levitates a drop of 5 ml in volume, which is more than 500 times larger than volumes of levitating drops reported in previous works. Remarkably, the giant drop levitates more than 24 hours for the special-shaped cylinder and a liquid film over the cylinder surface. This containerless manipulation of giant drops may open a new door for new method of material science and high-precision chemical analysis. In addition, we investigate the levitation mechanism of a giant drop over a moving wall. We adopt interferometry to observe the bottom shape of the drop which is complex because of the air flow between the drop and the moving wall. It is found that, when the drop size is large enough, the bottom shape of the drop shows surface waves, which has not been reported by any previous researches. Phase speed and wavelength of the surface wave vary with experimental parameters (i.e. wall velocity and drop diameter). Interestingly, both phase speed and wavelength have a linear relationship to the product of wall velocity and drop diameter.
Interlaboratory comparison can be effective to obtain consistent findings on ultrafine bubbles (UFBs). In this study, the stability of UFBs during international transportation was reported for the first time. We conducted stability tests in cooperation with German and Canadian institutions. UFBs were measured using two laser-based characterization techniques: particle tracking analysis (PTA) and dynamic light scattering (DLS). Acceleration and temperature during transportation were also measured. The results showed that regardless of transportation, the number concentration of UFBs decreased, while the size of UFBs increased with time. By filling the glass vial with UFB dispersion and minimizing the liquid flow in the vial, the effects of transportation were virtually eliminated. It was also suggested that the liquid flow caused by vibration during transportation may have enhanced the aggregation of UFBs. We report the results of an inter-instrumental comparison among PTA and DLS instruments using the transported UFB dispersions.
The external power supply of Fukushima nuclear power plant was lost by the Great East Japan earthquake, which might have resulted in shortage of water in the spent fuel pool. Many researchers are developing cooling system of spent fuel pool (SFP) by loop heat pipe without power supply. However, thermal stratification in SFP was not considered to predict the cooling performance. The purpose of this study is to develop the solving system for thermal stratification without power supply by injecting air to SFP. Simulated test using thermosensitive self-operated valve was done. The flow visualization test and the solving test for thermal stratification have been done by using three kinds of air injection structure. The following conclusions are obtained. ①Air injection method can solve the thermal stratification. ②Using the thermosensitive self-operated valve, the air can be injected without power supply. ③Elbow type was suitable structure as air injection structure.