Shin-ichiro Uesawa

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

Research and Development of Self-Priming Venturi Scrubber for Filter Venting: Preliminary Analysis and Observation of Hydraulic Behavior in Venturi Scrubber

As revealed by Fukushima Daiichi nuclear disaster, countermeasures against severe accident in nuclear power plants are an urgent need. In particular, from the viewpoint of protecting containment and suppressing the diffusion of radioactive materials, it is important to develop the device which allows filtered venting of contaminated high pressure gas. In the filtered venting system that used in European reactors, so called Venturi scrubbers are used to realize filtered venting without any power supply. The Venturi scrubber operates without a power supply of high pressure gas filled into the containment. In this apparatus, scrubbing of contaminated gas is promoted by both gas releases through a submerged Venturi tube which is one of the major components of Venturi scrubber and liquid splay flow formed by liquid suction through a hole for suction provided at the throat part of the Venturi tube. This type of Venturi scrubbers is called self-priming ones. However, the mechanism of a self-priming Venturi scrubber including effects of gas flow rate and shape of Venturi tube is understood insufficiently in the previous studies.In this study, to provide knowledge about the detailed mechanism of filtering and evaluation method for operating condition of the filtered venting system with Venturi scrubber, both experimental and numerical research works have been performed. In this paper, as a previous step of designing and making a self-priming Venturi tube, hydraulic behavior in a rectangular Venturi tube is taken by high speed camera, by the air-water experiment under atmospheric condition to check the operation in a self-priming mode and the validity of the experimental method and the visualization technique. And numerical analysis of a circular Venturi tube is conducted as a preliminary analysis, by TPFIT (Two-Phase Flow simulation code with Interface Tracking) to predict its operation. As results, the Venturi scrubbers used in experiment and simulated in numerical analysis were operated in a self-priming mode. By observed results of the hydraulic behaviors in the Venturi tube, the validity of the experimental method and the visualization technique were confirmed. And it is expected that the simulated circular Venturi scrubber in this numerical simulation was operated in a self-priming mode in a real situation.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

Measurement of void fraction in bubbly-slug flow with a constant electric current method

In several void fraction measurement methods, a constant electric current method which is one of conductance methods is focused in the present study. By using this method, void fraction can be measured with higher temporal resolution. However, it has been mainly applied to annular flow in previous studies. In the present study, Maxwells estimation, Bruggemanns estimation, low void fraction approximation and new estimations which consider the bubble shape are applied in order to measure more accurately void fraction of dispersed bubbly flow and slug flow. To understand the effect of bubble shapes and flow patterns, void fraction was measured by the constant electric current method for a rising single spherical bubble and a rising single slug bubble without a forced convection. In addition, void fraction was also measured in bubbly flow and bubbly-slug flow with a forced convection. Then, effects of flow patterns on the proposed estimations of void fraction and the accuracy of their estimations were discussed with the measurement results. From the result, the new estimations which consider a bubble shape are more accurate than the previous estimation in a slug bubble and bubbly-slug flow.

Estimation of Void Fraction in Dispersed Bubbly Flow With a Constant Electric Current Method

In several void fraction measurement methods, electric void sensors are online measurement method, simple construction and lower cost. In electric measurement, we research on a constant electric current method which is one of conductance methods. By using this method, we can measure volumetric void fraction with higher temporal resolution although the method cannot measure 2D and 3D distribution of void fractions. Besides, multiple measuring electrodes can be installed at a short distance. And then, flow is not obstructed by measuring electrodes. However, the constant electrical current method has been applied in annular flow in previous studies. Void fraction is estimated by cross-sectional ratio of gas and liquid phases in this method. For this reason, dispersed bubbly flow is not applied because cross-section ratio is not continuous in a flow direction. In the present study, Maxwell’s theory, Bruggemann’s treatment and polarization method are applied in order to measure void fraction of dispersed bubbly flow more accurately. Maxwell’s theory is an estimation of a resistance of a mixture with two difference resistivity by calculating electric potential in the mixture. Bruggemann’s treatment is based on Maxwell’s theory but it implies the assumption of a large size-range of particles in surrounding medium. In polarization method, bubbles are assumed to be dielectric bodies. Therefore if voltage is applied to gas-liquid two-phase flow, electrical charges in bubbles are polarized, and polarization electrical field generates. A difference of voltages in bubbly flow and liquid single phase flow assumes to be caused by polarization fields. Void fraction in vertical flow is measured experimentally by the previous method, Bruggemann’s treatment, Maxwell’s theory and polarization method in order to investigate the accuracy of these estimations. Working fluid is air and tap water. The accuracy is measured by comparing with a quick shut valve method and observations. Besides, we investigate effects of flow structure and bubble shape to measurement accuracy. Flow structure is changed by changing gas and liquid volume flow rate. In the experiment for bubble shape, a rising bubble by buoyancy is measured. The bubble shape observed by a high speed video camera is compared with the electrical signal measured by the constant electric current method. From experimental results, it is confirmed that void fraction in bubbly flow and froth flow is estimated more accurately by Maxwell’s theory, Bruggemann’s treatment and polarization method, and change of bubble shape correlates with fluctuation of void fraction measured by the constant electric current method.Copyright

Measurement of Void Fraction in Dispersed Bubbly Flow With Constant Electric Current Method

Void fraction is one of important physical values for gas-liquid two-phase flow in nuclear power plants, and it is an essential parameter for designs and performance evaluations of devices, including a core of BWR and so on. Therefore void fraction measurement with real time, high temporal resolution and high spatial resolution has been needed. In several void fraction measurement methods, electric measurement methods of void fraction can realize real-time measurement. In previous studies, conductance methods capacitance methods, wire mesh methods and tomography techniques have been studied.In the present study, we research on a constant electric current method. This method can measure void fraction with higher temporal resolution and simpler systems. In previous study, the constant electrical current method has been applied in annular flow mainly. However, the method cannot be applied to three dimensional dispersed bubbly flow. This is because void fraction is estimated by cross-section ratio of gas and liquid phases in this method. In the present study, Maxwell’s theory and polarization method are applied to calculation method of void fraction from voltage measured by constant electrical current method, and we try to measure void fraction in dispersed bubbly flow. Maxwell’s theory is a calculation of a mixture with two materials of different conductivity. The polarization method proposed in this study assumes bubbles to be dielectric bodies and void fraction is estimated by the polarization electric field of the bubbles.In the experimental results, the void fractions in the three dimensional dispersed bubbly flow can be estimated with Maxwell’s theory and the polarization method. The void fractions estimated with these methods are more accurately than the previous method. Furthermore, it is experimentally clarified that the present proposed method can follow highly temporal void fluctuations of bubbly and froth flows. In addition, effects of intervals between electrodes and structures of electrodes are experimentally investigated.Copyright