Since the existence of many bubbles is frequently encountered in the flow of liquid in hydraulic machineries such as water turbines, pumps, etc., research on the behavior of multiple bubbles has been recently paid much attention to problems related closely with the cavitation phenomena, or cavitation damage. In this paper, I will explain briefly the studies of multiple bubbles in our laboratory at the Institute of Fluid Science, Tohoku University.
In this investigation, an objective and quantitative method is developed for the identification and classification of flow regimes in vertical gas-liquid two-phase flow. It is clarified that the features of probability density function (PDF) and the parameters of the PDF expected according to the maximum entropy principle (MEP) can be used as objective flow regime indicators and quantitative flow regime discriminators, respectively. In this experiment, the void fraction fluctuations are measured by the constant current technique. The PDFs are classified into three major types and correspond to three dominant flow patterns: bubbly, intermittent and annular. That is, a uni-modal PDF with a low void fraction corresponds to a bubbly flow pattern; a bimodal PDF corresponds to an intermittent flow pattern; and a uni-modal PDF with a high void fraction corresponds to an annular flow pattern. Furthermore, the relationship between the statistical parameters of the void fraction fluctuations (the mean value, the standard deviation, and the coefficients of skewness and kurtosis) and the flow patterns is discussed. It is shown that classification of uni-modal and bimodal PDFs can be made according to the coefficients of skewness and kurtosis.
The three-fluid model is one of the most advanced models for phenomenologically analyzing annular mist flow. However, a numerical solution based on the three-fluid model often suffers from unclarified numerical instability. The numerical stability was therefore analyzed in the present study using the minimum relaxation distance of the three-fluid model for the annular mist flow in a vertical heated channel. This analysis made it clear that the instability is apt to occur at the onset of the annular flow and in the vicinity of the dryout location. It was also clarified that the virtual mass force has little effect on the improvement of the numerical stability. Then, a partially implicit method for momentum transfer terms was applied to the numerical integration of the three fluid model. It was confirmed that the numerical stability is improved by this method without increasing the difficulty of the programming.
In one proposal for an excellent, highly efficient collector, an injector is mounted at the collection mouth to fluidize the particle bed before particles are collected. In this paper, the inner diameter ratio of the injection mouth to the suction mouth is discussed, along with the ratio's effect on the concentration of particles and local efficiency at the collection section. Spherical glass beads are used as particles. This experiment clarifies that when the inner diameter ratio is too small, the velocity in the injection pipe is so high that the drop in pressure is very large whereas higher concentration is obtained. On the contrary, when the inner diameter ratio is too large, the flow area at the suction mouth becomes small and the beads are dispersed outside the collection region by the injection flow. As a result, an optimal value exists for the inner diameter ratio of a given inner diameter of the suction mouth and a given bead diameter. High local efficiency for the transportation of the beads is obtained at an inner diameter ratio equal to 0.28.