An atomization process of a liquid sheet induced by co-current gas flows relevant to gas turbines and industrial applications is presented. We investigate the atomization mechanism, promoting comprehension of atomization. This paper explores the initial deformation and the breakup processes of the liquid sheet, influenced by the velocities and physical properties of gas and liquid and the atomizer’s geometries. A framework of a mechanistic model for predicting longitudinal and transversal wavelengths and the droplet diameter distribution is introduced. The model is examined and evaluated, demonstrating its effectiveness for optimizing spray systems. These findings offer significant potential for enhancing the understanding of multiphase flow dynamics.

Liquid atomization is a process of breaking down a mass of liquid into a group of many fine droplets, spray. The liquid atomization technology is used in a wide range of industrial fields, e.g. air-fuel mixture generation in internal-combustion engines, pesticide spraying, spray painting, powdered-food production, and solid-particle production from molten materials. As with other gas-liquid two-phase flows, the photographic observation is effective in the research on liquid atomization and in the development of related equipments, because a variety of information can be obtained from the images. In this article, the liquid atomization is outlined and the fundamentals of photographic observation of the atomization process and spray flow are presented. Several examples photographic observations are also presented.

There are various types of atomization measurements, mainly for liquids, and it is no exaggeration to say that it is the history of particle size measurement. When measuring the diameter of atomized droplets, it has long been a question of whether the measuring instruments are really showing the correct values. The Phase Doppler Interferometer (PDI), which appeared in this history, can simultaneously measure the particle size and flow velocity of droplets, and can also display the density and flux per unit volume (cc/cm2/s). When the measurement target is a spray, this means that larger particles have faster velocity and smaller particles have the opposite. In this article, I will look back on the history of measuring instruments related to atomization over the past 40 years, from the late 1980s when I first became involved in the world of atomization to the present day, focusing on the Phase Doppler Particle Analyzer.

From the Edo-period, a unique fragmentation cascade, the successive bursting of droplets, draws the ramifying sparks in a Japanese traditional sparkler, Senko-hanabi, depicting the fragile beauty. Involved potassium-salt droplets demonstrate nucleation without evaporation, while the thermal-decomposition supplies gas into a bubble following the thermal diffusion timescale. We specify the distinctions of Senko-hanabi from puffing/micro-explosion of multi-components volatile droplets and from bursting metal sparks rate-controlled by mass diffusion. But in the end, beyond the differences, we generally understand the underlying process of bursting droplet driven by a growth of internal bubble.

When a cone is immersed in liquid with its top upside down and rotated, a phenomenon is observed in which liquid is lifted up over the outer surface of the cone. The lifting-up phenomenon is related to Rayleigh-Taylor instability, and is caused mainly by the balance between centrifugal force and viscous resistance. When the viscosity of the liquid is smaller, the fluid is lifted up like a filmwise across the entire cone surface, because the viscous resistance is smaller. On the other hand, when a fluid with high viscosity is used as the working fluid, the viscous resistance is larger and the liquid is not lifted up in a filmwise shape. In such a case, the development of disturbance waves generated on the surface of the cone causes the flow to be lifted up in a filamentwise shape due to spinnability. In this paper, we introduce the phenomena by flow visualization on the transition of the flow patterns between filmwise and filamentwise lifting-up. Furthermore, we report the change in the lifting-up flow patterns with viscosity change and its relationship to starting rotation rates for the lifting-up. Finally, we perform a dimensionless rearrangement from the experimental results and show a dimensionless correlation equation.

In this study, we focus on developing a simple and flexible rheometer based on pressurized laminar flow. The developed device uses pressure- and flow-rate measurements to calculate the shear rates and shear stresses on the pipe walls, based on the Herschel－Bulkley model. The rotational viscometer, an existing measurement device, struggles with measuring systems that contain large solid particles because of its narrow-gap assumption. Moreover, conventional fluid-delivery measurement devices require large pumps and multiple sensors, necessitating large-scale and expensive setups. In contrast, our device, comprising only a flow meter and a pressure gauge, is simple and can measure various fluids by changing the connected pipe’s diameter. We employed glycerin as a Newtonian single-phase fluid, polymer aqueous solutions as non-Newtonian single-phase fluids, and multiphase food fluids (low-oil-type mayonnaise and ketchup) to compare the rheological analysis data obtained from both the rotational viscometer and our device. Both devices delivered consistent results for Newtonian fluids and non-Newtonian fluids exhibiting shear-thinning behavior. Upon comparing the actual measured shear stresses and shear rates for single-phase fluids, the discrepancy between the results from our device and those from the rotational viscometer was up to ~6.6%, whereas discrepancies of 12.1% and 17.6% were observed for mayonnaise and ketchup, respectively. For the single-phase fluids, the measurement results delivered by both devices were similar; however, discrepancies were observed for the multiphase fluids. These discrepancies are attributable to the accumulated shear stress applied to the fluid and slippage on the tube wall, suggesting that our device enables simple and flexible rheological measurements.

This study investigates the overall change in ultrafine bubble (UFB) size distributions in water under flowing conditions for Reynolds numbers ranging from ~300 to ~9500. The size distribution was segregated into 5nm bins, within which the change in UFB number concentration over a 120 minutes was found to exhibit 2^nd order rate dependency. A proposed correlation is able to predict the change in UFB size distributions in recirculating systems for diameters ranging from 100 to 400 nm. Notably, the loss of UFB and the shift in size distribution did not appear to be sensitive to the length of the recirculation loop within the experimental range of this study.

In recent years, there has been a need for cooling technology that can deal with the increasing heat generation by the miniaturized high output electronic devices. A finned heat sink with boiling cooling is considered to be a promising method that can cool those electronic devices efficiently. However, as the heat transfer increases, the void fraction between fins increases, leading to the increase in the pressure drop through a finned heat sink. The increasing pressure drop may result in the deterioration of the heat transfer efficiency. So, it is important to estimate the pressure drop through the finned heat sink. Though various researchers have already studied the pressure drop of the finned heat sink, however, the pressure drop evaluation model for the finned heat sink has not been established enough. In this study, first, the pressure drop through a finned heat sink with the single-/two-phase flow is measured, and then, the finned heat sink is regarded as a type of a packed bed system to obtain a universal evaluation model of pressure drop based on well-known Ergun/Lipinski models.

In recent years, many ultrafine bubble (UFB) water production methods have been developed. Recently, a batch-type UFB water production method by rapidly cooling and condensing a steam and non-condensable gas mixture has been invented. In this study, we improved the method above and developed a continuous-type UFB water production method. UFB water was produced by continuously cooling and condensing the steam and non-condensable gas mixture with a stainless-steel coaxial double-pipe condenser. By changing parameters such as cooling water flow rate, gas flow rate, and steam flow rate, we investigated the effect of parameter changes on the number concentration of UFB water produced. We found that the number concentration increased as the cooling water flow rate increased, decreased as the gas flow rate increased, and remained almost constant as the steam flow rate increased. These results were discussed from the perspective of heat transfer, and it was suggested that a correlation could be established between the amount of heat transferred and the number concentration.

Singular value decomposition (SVD), a data-driven fluid informatics method, was used to extract features of flow-induced single droplet breakup. The behavior of single droplet breakup induced by shockwave backflow was captured with high spatio-temporal resolution, and a large dataset was constructed by organizing the time and spatial luminance values according to the Weber number and its variations. SVD was performed on the dataset for space and variation, extracting spatial and variation features. Spatial modes indicating the upstream and downstream sides of the gas-liquid interface, as well as modes indicating planes perpendicular and parallel to the flow, were obtained. The sharp drop in the contribution rate of the second mode corresponds to the induction time of single droplet breakup. SVD was then applied to the dataset for space and time to extract spatial and temporal features. The first-order mode revealed features of the parent droplet and fragments with opposite signs, while higher-order modes captured detailed changes during the breakup process. The second mode, which shows the transition of the parent droplet, ligament, and fine fragment configurations, was emphasized. The moment when the time-mode feature becomes zero coincides with the induction time of droplet breakup. These results demonstrate that applying SVD to single droplet breakup provides a consistent and accurate determination of the breakup induction time.

Granular material is a closely packed collection of a large number of grains, often tens of microns to a few millimeters in size. The complex coupling between a wide range of parameters such as friction between grains, size dispersion, and particle shape makes even the simplest system of granular material challenging to be understood. Recently, it has been reported that when a solid sphere impacts on a deep and loose layer of granular medium, a broad splash of sand is generated, followed by a transient axisymmetric crater, and finally a granular jet is shot up vertically. Interestingly, such similar jets are also observed in ordinary liquid systems. Though such analogues are expected to provide key physical insights, the mechanism behind the phenomenon is not yet understood. Furthermore, it is difficult to investigate the details of the dynamic behavior of jet ejection using previous jet generation methods due to the effects of large deformation of the granular layer and particle scattering. Therefore, we performed an experimental study of jet generation and dynamics of granular material using the mechanism of liquid jet generation by a pressure impulse. We found quantitative similarities between jet phenomena in liquid systems and granular systems.

Though wall temperature can correlate with cavitation behavior, the influence of wall temperature on cavitation have received much less attention. In this paper, we present an exploratory experimental investigation of the transition from the attached cavitation to the nucleate boiling on a heated wall. A heated NACA0015 hydrofoil was used to observe cavitation on a heated wall. At a mainstream flow velocity of 3 m/s and a temperature difference of approximately 30 K between the hydrofoil and water, a sudden transition from attached sheet cavitation to nucleate boiling was observed at cavitation number σ = 1.8. Further decreasing the cavitation number only enhanced the nucleate boiling, even though local pressure conditions allowed for fully developed cavitation. The generation of vapor bubbles were observed at the detachment of the attached cavity sheet from the wall nucleation site. After being interfered by these isolated bubbles, the cavity sheet was swept downstream by the oncoming flow. It was suggested that the vapor bubbles of nucleate boiling at the detachment of the cavity sheet could suppress the formation of attached cavitation. Owing to these observation results, we conducted an analytical examination to compare the saturation pressure condition for both the inception of the cavitation and the onset of the nucleate boiling on a heated wall. The results suggest that, based on the mainstream flow conditions, there may be a specific range of wall temperatures that allow the transition from attached cavitation to nucleate boiling.