Yasumichi Nomura

Fluctuation of Void Fraction in the Microbubble Generator With a Venturi Tube

Microbubbles are tiny bubbles with less than 1 mm diameter. These bubbles are utilized in various engineering fields, and it is very important to understand physics of flow with microbubbles. Especially, void fraction is one of the significant parameter for two-phase flow. Thus, developments of real-time measurement systems of void fraction are required. In the nuclear power engineering, electrical void fraction measurement methods have been proposed as one of the real-time measurement techniques. In the present study, we apply this method to a microbubble generator with a venturi tube and examine the performance of the generator. Constant electrical current method is adopted as electrical measurement method of void fraction. Microbubbles are generated with a bubble collapse phenomenon through a venturi tube. We can generate microbubbles in high void fraction. However, mechanism of bubble collapse in a ventrui tube is not made clear and void fraction distribution toward flow direction is less understood. The applicability of constant electrical current method in bubbly flow and the process of the bubble breakup in a venturi tube are discussed. In this experiment, a voltage between two electrodes in the generator is measured with various gas-liquid volume flows as inlet conditions. From results we succeeded to measure the void fraction profile in the venturi tube with constant electrical current method. The void fractions achieve a peak before the bubble collapse and it decreased drastically for 10 mm after collapse.Copyright

Study on Bubble Breakup Mechanism in a Venturi Tube

Microbubbles are expected to be applied in various subjects such as engineering and medical fields. Thus, on-demand microbubble generation techniques with high efficiently are required. In the present study, the microbubble generator using a venturi tube (converging-diverging nozzle) is focused. Although this technique realizes generation of many tiny bubbles with less than several-hundred-micrometer diameter, there are several unsolved parts of flow structure in a venturi tube on bubble breakup behavior. The purpose of this study is to clarify the bubble breakup mechanism in a venturi tube for practical use. In the present study, using a high speed camera for detailed observation of bubble behavior, the following features were obtained. In low velocity conditions, bubbles are divided in several pieces with a jet penetrating from the top (downstream) to the bottom (upstream) part of the bubble. In high velocity conditions, bubbles collapse in countless microbubbles with a drastic bubble expansion and shrinkage. Also, in order to clarify the flow structure in a venturi tube, pressure profile is measured in detail. Under chocking condition, the pressure profile shows the tendency of supersonic flow in a Laval nozzle and sudden pressure gradient appears in the diverging section. There are strong correlations between bubble fission points and pressure recovery points. It is suggested that bubble collapse is strongly influenced with pressure recovery in the diverging section.Copyright