Akira Sou

Effects of Eötvös Number and Dimensionless Liquid Volumetric Flux on Lateral Motion of a Bubble in a Laminar Duct Flow

Experiments and numerical simulations on lateral migration of a single bubble in stagnant liquids and laminar flows were conducted in the present study to examine the effects of the Eotvos number Eo and dimensionless liquid volumetric flux on lateral forces. It was confirmed that (1) a lateral force due to the existence of the wall acts on a bubble and (2) a lift force due to the net circulation of liquid strongly depends on Eo. Empirical models for the wall and lift forces were also proposed and their validity was confirmed by comparing measured and calculated bubble trajectories.

Numerical analysis of bubble motion with the VOF method

Abstract Numerical analyses of a two-dimensional single bubble in a stagnant liquid and in a linear shear flow were conducted in the present study using the volume of fluid method, which is based on the local-instantaneous field equations. It was clarified that this method gives qualitatively appropriate predictions for the effects of the Morton number and the Eotvos number on fluctuating bubble motion in a stagnant liquid. Calculated velocity and pressure distributions indicated that the Karman vortex causes a sinuous movement of the bubble. As for the bubble motion in a linear shear flow, calculated bubbles migrated in a lateral direction. The direction of the lateral migration agreed to available experimental data. It was also confirmed that (i) the direction or the magnitude of the lateral migration is affected by the Eotvos and the Morton numbers, and (ii) the interaction among the internal flow of the bubble, the wake of the bubble and the external shear flow plays an essential role for the lateral migration.

Two-phase Flow Patterns in a Four by Four Rod Bundle

Air-water two-phase flow patterns in a four by four square lattice rod bundle consisting of an acrylic channel box of 68mm in width and transparent rods of 12mm in diameter were observed by utilizing a high speed video camera, FEP (fluorinated ethylene propylene) tubes for rods, and a fiberscope inserted in a rod. The FEP possesses the same refractive index as water, and thereby, whole flow patterns in the bundle and local flow patterns in subchannels were successfully visualized with little optical distortion. The ranges of gas and liquid volume fluxes, {JG} and {JL}, in the present experiments were 0:1 ≪ {JL} < 2:0 m/s and 0:06 < {JG} < 8:85 m/s, which covered typical two-phase flow patterns appearing in a fuel bundle of a boiling water nuclear reactor. As a result, the following conclusions were obtained: (1) the region of slug flow in the {JG}-{JL} flow pattern diagram is so narrow that it can be regarded as a boundary between bubbly and churn flows, (2) the boundary between bubbly and churn flows is close to the boundary between bubbly and slug flows of the Mishima and Ishiis flow pattern transition model, and (3) the boundary between churn and annular flows is close to the Mishima and Ishiis model.

Control of Ink Transportation in Electrostatic Inkjet Printer

In order to develop and realize a new electrostatic printer, the ways to reduce the switching voltage were examined by the numerical simulation in the present study. Furthermore, the possibility of the control of the volume of transported ink was numerically investigated. As a result, it was clarified that by charging about 1.0kV of bias voltage, by changing the shape of projection, and by decreasing the ink viscosity, the surface tension and the contact angle, switching voltage can be reduced. As for the volume of transported ink, linearity between pulse width and transported volume was observed, which show us a large possibility to realize a high quality inkjet printer.Copyright

Evaluation of cavitation in injector nozzle and correlation with liquid atomization

Present paper describes the correlation between occurrence of cavitation inside injector nozzle and progress of atomization in discharged liquid jet flow. A transparent two-dimensional shape nozzle was designed and it was used for flow visualization. The visualized images both of cavitation and atomization were obtained simultaneously. The cavitation was evaluated by applying image analysis. The quantitative analysis on length, thickness and their fluctuations of cavitation was made, and the correlation with atomization was examined. The authors proposed an evaluation method of atomization in the previous study, and it was combined with evaluation method of cavitation developed in this study. It is found that the change of cavitation and progress of atomization are synchronized, and that a strong correlation between them exists. The best performance of atomization may be obtained by controlling the cavitation inside nozzle, and obtained aspects with this study will be utilized for the design of fuel injection nozzle of internal combustion engines.Graphical abstract

Bubble Tracking Simulation of Cavitating Flow in an Atomization Nozzle

Numerical simulation of transient cavitating flow in a axisymmetric nozzle was conducted in order to investigate the detailed motion of cavitation bubble clouds which may be dominant to atomization of a liquid jet. Two-way coupled bubble tracking technique was assigned in the present study to predict the unsteady cloud cavitation phenomena. Large Eddy Simulation (LES) was used to predict turbulent flow. Calculated pressure distribution and injection pressure were compared with measured ones. Then, calculated motion of cavitation bubble clouds was carefully investigated to understand the cavitation phenomena in a nozzle. As a result, the following conclusions were obtained: (1) Calculated result of pressure distribution along the wall, the relation between injection pressure vs. flow rate, and bubble distribution agreed with existing experimental result. (2) Cavitation bubble clouds were periodically shed from the tail of vena contracta, which usually formed by the coalescence of a few small bubble clouds. (3) Collapse of cavitation bubbles due to the re-entrant jet was observed in the numerical simulation.Copyright

EVALUATION OF VOLUME TRACKING ALGORITHMS FOR GAS-LIQUID TWO-PHASE FLOWS

Qualitative and quantitative evaluations of volume tracking algorithms such as DA, FLAIR, MARS, CIP and VTEMC were conducted. Wide variety of two-dimensional test problems including (1) a circle transported in simple translation and rotational field, (2) a bubble rising in 45° slanted gravity field, (3) zigzag motion of a bubble in a vertical channel, and (4) a bubble rising in a stagnant liquid in axissymmetric cylindrical coordinate were chosen in the present study. As a result of these tests, the superiority of the cell-centered piecewise linear algorithm with the divergence treatment in operator split and especially with embedded micro cells was confirmed. Applicability of these algorithms to three-dimensional problems has to be examined in the future works.Copyright

A Hybrid Method for Simulating Flows Including Fluid Particles

A hybrid CMFD (computational multi-fluid dynamics) method is proposed for the prediction of multiphase flows including large-scale interface, poly-dispersed bubbles and/or drops. The method is the hybrid integration of an interface tracking method (ITM), three kinds of particle tracking methods (PTM) and an averaging method based on a multi-fluid model (MFM). The integration enables us (1) to cover a wide range of d* = d/Δx, where d is the particle diameter and Δx the grid size, and (2) to perform various kinds of multiphase CFD such as standard interface tracking, particle tracking and multi-fluid simulations, and hybrid simulations using an arbitrary combination of ITM, PTM and MFM. The field and constitutive equations of the proposed method are described in detail. A poly-dispersed air-water bubbly flow and several bubble plumes in a small open vessel are simulated using the proposed hybrid method to demonstrate its potential.Copyright

A Volume Tracking Method Based on Advanced Subgrid Counting Algorithm

A volume tracking scheme, ASCA (Advanced Subgrid Counting Algorithm), which is easily extendable to three-dimensions and possesses a good volume conservation property, is proposed. To examine the potential of ASCA, several two-phase flow simulations are carried out. As a result, the following conclusions are obtained: (1) Predicted shapes and breakup characteristics of single fluid particles in simple shear flows agree well with available data even with a low spatial resolution, (2) A water drop impinging on water surface and an air bubble rising through a stagnant water are successfully simulated with little errors in volume conservation, (3) Predicted shapes and terminal velocities of single drops in stagnant liquids under wide ranges of the viscosity ratio and Morton number agree well with measured one, and (4) In spite of a low spatial resolution, vapor bubbles flowing in a strong shear flow are simulated with good volume conservation. The interface sharpness is well preserved even after a large deformation of bubbles.Copyright

Effects of Adjacent Printer Heads in Electrostatic Inkjet Printer

In order to develop a high speed printing system, the system must be operated in high frequency, and must have multiple heads in rows. Through the present simulation, it was found that successive shots of high frequency (about 2kHz) were possible. By additional optimization of pulse shape and by decreasing the delay time for switching voltage to rise up, it will be possible to realize much higher speed printer. Then, the effects of adjacent printer heads in a row were investigated. It was found that a distance of 254µm (corresponding to 100dpi) was still not enough to achieve stable and well-controlled ink transportation if no electrostatic barriers in between were assigned in the present head design.Copyright