Naoya Ochiai

Computational study of the dynamics of two interacting bubbles in a megasonic field

Clarification of the mechanism of particle removal by megasonic cleaning and control of cavitation bubbles in the megasonic field are essential for cleaning of nanodevices without pattern damage. Multiple bubble interactions complicate the mechanism of particle removal. Therefore, it is important to understand multiple bubble dynamics to clarify the mechanism of particle removal by megasonic cleaning. In the present study, the dynamics of two bubbles in a megasonic field with several initial radii and initial separation distances were simulated by numerical analysis using a compressible locally homogeneous model of a gas-liquid two-phase medium. The present numerical method simulated the various complex behaviors of two bubbles, which are repulsive motion, coalescence, periodic and stable motion of the separation distance, and bubble breakup. The initial separation distance strongly affected the behavior of the two bubbles because the effect of the secondary pressure induced by the oscillation of one bubble on the other bubble depends on the separation distance. In particular, when the equilibrium radii are larger than the resonant radius and the radius of one or both bubbles is close to the resonant radius, the bubbles can show characteristic behaviors, such as periodic and stable motion of the separation distance.

Erratum: Numerical Analysis of Damping Effect of Liquid Film on Material in High Speed Liquid Droplet Impingement

By high speed Liquid Droplet Impingement (LDI) on material, fluid systems are seriously damaged, therefore, it is important for the solution of the erosion problem of fluid systems to consider the effect of material in LDI. In this study, by using an in-house fluid/material two-way coupled method which considers reflection and transmission of pressure, stress and velocity on the fluid/material interface, high-speed LDI on wet/dry material surface is simulated. As a result, in the case of LDI on wet surface, maximum equivalent stress are less than those of dry surface due to damping effect of liquid film. Empirical formula of the damping effect function is formulated with the fluid factors of LDI, which are impingement velocity, droplet diameter and thickness of liquid film on material surface.

Integrated Computational Study for Total Atomization Process of Primary Breakup to Spray Droplet Formation in Injector Nozzle

The advanced development and optimization of fuel atomization in port and direct injection systems for automobile engine is desired for the improvement of fuel combustion performance and thermal efficiency of the engine. In addition to the experimental procedure, Computational Fluid Dynamics (CFD) is useful for the prediction of computational prediction of spray flow from injector. The l iquid column of in jec ted fue l forms ligament and breaks up in to the liquid droplets. Furthermore, the droplets show the secondary breakup to further small droplets. In the past study , the primary breakup behavior of liquid jet is numerically analyzed by Volume of Fluid (VOF) and Level Set method. However, these Eulerian numerical methods have some numerical diffusivity and the computational prediction of the secondary breakup of the droplet and the droplet behavior are not applicable. Therefore, Lagrangian numerical method, Discrete Droplet Model (DDM) is used for the simulation of the spray behavior in the injection system after the primary breakup. DDM has the disadvantage that the method cannot consider the influence of injector nozzle inner flow to the spray behavior. Therefore, some procedures were proposed to use DDM for the simulation after the primary breakup. Okamoto et al. proposed a derivation method of the particle diameter and velocity after the primary breakup using the numerical analysis of nozzle internal flow and theory of liquid film breakup for the calculation of fan spray. Ishii et al. simulated liquid column breakup using hybrid model of Cubic Interpolated Propagation (CIP) method and Moving Particle Semi-implicit (MPS) method and used the results for the simulation of the secondary breakup. In this study, we have developed the integrated computational method of the total fuel atomization process of the injector nozzle. This new computational approach is taking into account the nozzle internal flow to form the primary breakup using Volume of Fluid (VOF) method in connection with the spray flow characteristics to the engine cylinder using Discrete Droplet Model (DDM). The flow field data of fuel velocity, turbulent energy and dissipation rate obtained by Eulerian approach (VOF) are transferred to Lagrangian process (DDM) as the initial numerical conditions of the droplet velocity, diameter and Integrated Computational Study for Total Atomization Process of Primary Breakup to Spray Droplet Formation in Injector Nozzle

Ultra-high heat flux cooling characteristics of cryogenic micro-solid nitrogen particles and its application to semiconductor wafer cleaning technology

The ultra-high heat flux cooling characteristics and impingement behavior of cryogenic micro-solid nitrogen (SN2) particles in relation to a heated wafer substrate were investigated for application to next generation semiconductor wafer cleaning technology. The fundamental characteristics of cooling heat transfer and photoresist removal-cleaning performance using micro-solid nitrogen particulate spray impinging on a heated substrate were numerically investigated and experimentally measured by a new type of integrated computational-experimental technique. This study contributes not only advanced cryogenic cooling technology for high thermal emission devices, but also to the field of nano device engineering including the semiconductor wafer cleaning technology.

A Detailed Observation of Hydrofoil Cavitation and a Proposal for Improving Cavitation Model

Cavitation of a hydrofoil is observed in detail by using a high speed video camera. A paint removal test is also carried out in order to evaluate cavitation aggressiveness for erosion. 2D hydrofoil profile is Clark Y 11.7% and its angle of attack is seven degrees. Cavitation number is σ = 1.08. The experimental results are compared with cavitation CFD. Numerous features of unsteady cavitation are observed such as cyclic fluctuation of the sheet cavity, existence of the glassy cones on a sheet cavity, generation of the cloud cavitation from the sheet cavity and the isolated bubbles traveling over the suction surface of the blade. The isolated traveling bubbles and their collapses are thought to be one of the main causes of the severe paint removals. The isolated traveling bubbles are derived from the flowing cavitation nucleus or from abrupt onset at the leading edge of the blade. For computing these complicated phenomena, combination of grid scale bubbles (GSB) and sub grid scale bubble model (SGSB) are proposed. GSB shall be computed by using the computational scheme for the free surface with phase change model. SGSB can be computed with conventional cavitation model. The breakup of GSB generates SGSB, and the coalescence of SGSB makes GSB. Upper limit of void fraction of SGSB is estimated in the range of five or ten percent from the simple speculation of the structure of packed spheres. The two types of cavitation bubble inception model are also discussed based on the generation of the isolated bubbles observed in the experiments. To verify the proposed concepts of cavitation model, a traveling air bubble over a hydrofoil is computed by using the free surface flow scheme of Volume of Fluid (VOF) approach. Cavitation on the hydrofoil is also computed by VOF approach with boiling model concerning the heat transfer. Both the computed results show qualitatively similar characteristics of the bubble dynamics to those in experimental results.Copyright