Zensaku Kawara

Bio: Zensaku Kawara is an academic researcher from Kyoto University. The author has contributed to research in topics: Two-phase flow & Subcooling. The author has an hindex of 10, co-authored 43 publications receiving 771 citations.

Pseudo-laminarization of micro-bubble containing milky bubbly flow in a pipe

Abstract: Micro bubble technology has become to attract people’s concerns due to its wide potential in practical applications to a variety of advanced and conventional science and technologies. However, our knowledge of micro bubbles containing bubbly two-phase flow is almost nothing. We developed a specially designed nozzle which generates high intensity micro air bubbles (mean diameter~ 40microns, number density more than 2×10/cc). Using this micro-bubble generator, we carried out, as a first step, a measurement of two-phase frictional pressure drop, cross-sectional average void fraction and liquid velocity profiles. The measurement of local void fraction is underway. The range of average void fraction was up to 0.6 % which is enough to realize milky bubbly flow. The most exciting result we found is that the two-phase flow becomes laminarized by injecting such ultra small bubbles into the water flow. For 0.3~0.5% void fraction, laminar-turbulent transition occurred at Re=10,000~20,000. The cross sectional averaged void fraction does not follow a homogeneous flow model, and radial liquid velocity profile was a little bit flattened. The mechanism of flow laminarization and flow structures are now being pursued.

Micropower generation using combustion: Issues and approaches

Abstract: The push toward the miniaturization of electro-mechanical devices and the resulting need for micro-power generation (milli-watts to watts) with low-weight, long-life devices has led to the recent development of the field of micro-scale combustion. The concept behind this new field is that since batteries have low specific energy, and liquid hydrocarbon fuels have a very high specific energy, a miniaturized power generating device, even with a relatively inefficient conversion of hydrocarbon fuels to power would result in increased lifetime and/or reduced weight of an electronic or mechanical system that currently requires batteries for power. In addition to the interest in miniaturization, the field is also driven by the potential fabrication of the devices using Micro Electro Mechanical Systems (MEMS) or rapid prototyping techniques, with their favorable characteristics for mass production and low cost. The micro-power generation field is very young, and still is in most cases in the feasibility stage. However, considering that it is a new frontier of technological development, and that only a few projects have been funded, it can be said that significant progress has been made to date. Currently there is consensus, at least among those working in the field, that combustion in the micro-scale is possible with proper thermal and chemical management. Several meso-scale and micro-scale combustors have been developed that appear to operate with good combustion efficiency. Some of these combustors have been applied to energize thermoelectric systems to produce electrical power, although with low overall efficiency. Several turbines/engines have also been, or are being, developed, some of them currently producing positive power, also with low efficiency to date. Micro-rockets using solid or liquid fuels have been built and shown to produce thrust. Hydrogen-based micro size fuel cells have been successfully developed, and there is a need to develop reliable reformers (or direct-conversion fuel cells) for liquid hydrocarbons so that the fuel cells become competitive with batteries. In this work, some of the technological issues related to meso and micro-scale combustion and the operation of thermochemical devices for power generation will be discussed. Some of the systems currently being developed will be presented and described.

A microfluidic device for conducting gas-liquid-solid hydrogenation reactions.

Abstract: We have developed an efficient system for triphase reactions using a microchannel reactor. Using this system, we conducted hydrogenation reactions that proceeded smoothly to afford the desired products quantitatively within 2 minutes for a variety of substrates. The system could also be applied to deprotection reactions. We could achieve an effective interaction between hydrogen, substrates, and a palladium catalyst using extremely large interfacial areas and the short path required for molecular diffusion in the very narrow channel space. This concept could be extended to other multiphase reactions that use gas-phase reagents such as oxygen and carbon dioxide.