JAPANESE JOURNAL OF MULTIPHASE FLOW Vol. 31(2017) No. 1

We experimentally examined influences of microbubbles on generation of a localized turbulent puff in a circular pipe flow. Central velocity fluctuations in the pipe flow measured by LDV were utilized to distinguish status of flows: It enables to distinguish ‘decaying disturbances’ that emerges on intermediate state in between puffs and laminar flows. Forty trials to investigate status of flows were performed at different intensity of perturbation controlled to create turbulent puffs at fixed Reynolds number, Re = 1,900 for both of single phase and microbubbly liquid with volume fraction lower than 0.018%. The results indicated that the adding microbubble enhances creation of turbulent puffs and prevents decaying of disturbances. This means that dilute microbubble promotes flow transition. Finally, we quantify the results by using two methods which are based on a velocity dropping value and correlation coefficient. These methods make it possible to classify status of flows in a pipe.

The aim of this study is to investigate applicability of a numerical simulation method (ASG method) for coupling porous media system (saturation formulas) with an atmosphere (gas) － surface water two phase-flow system to practical problems for overtopping that surface water overflows a bank. In this study we conducted numerical simulations for overtopping a trapezoid bank with permeable surfaces, for overtopping trapezoid bank with a berm and permeable surfaces and for overtopping a trapezoid bank with a berm and impermeable surfaces. Moreover it was assumed that these banks were made of sand (the intrinsic permeability 1.737×10^-11 m²) and the impermeable surface had the intrinsic permeability 1.0×10^-18 m² and the thickness of 0.2 m. Consequently, it was found from the results of the numerical simulation with ASG method that there was a zone unsaturated with water (unsaturation zone) in the trapezoid bank with the permeable surfaces and two unsaturation zones in the bank with the berm at the elapsed time 9,000 sec after overtopping. Then the gas pressure in the unsaturation zone rose up to 12 kPa in these cases. Finally in these cases the gas in the unsaturation zone went out the bank. On the other hand the unsturation zone finally remained in two areas beneath the impermeable surfaces of the bank covered with the impermeable material as the results of ASG method. Then the gas pressure also rose up to 8 or 12 kPa in the unsaturation zone. Therefore it was recognized to be able to reproduce the overtopping the banks with various configurations, water contention curves and surfaces and that the gas in the unsaturated zone rose up to 8 － 12 kPa, by using ASG method. It is particularly significant that ASG method can represent an interaction between the gas and water in the porous medium and those in surface water considering balance of pressure.

One-region computations with the annular flow model were done for counter-current flow limitation (CCFL) at the sharp-edged lower end of vertical pipes to evaluate effects of diameters and fluid properties on CCFL characteristics. CCFL characteristics computed with several correlations for interfacial friction factors were compared with CCFL data and the correlation proposed by Bharathan et al. (which is a function of void fraction) was selected. The adjustment factors were obtained to give good agreement between CCFL characteristics computed with the correlation by Bharathan et al. and CCFL data and were correlated with the viscosity ratio of gas and liquid phases and the dimensionless diameter. By using the correlation for interfacial friction factors by Bharathan et al. modified with the viscosity ratio of gas and liquid phases and the dimensionless diameter, effects of diameters and fluid properties on CCFL characteristics were computed.

Production of the particles using hydrolysis reaction or oxidization in a flame is known as one of production processes of nanoparticles. This particle growth process is a complex phenomenon included reaction, fluid motion, particle transportation and particle growth by collision and sintering. To calculate this process, combining particle growth process with transport equation for each particle size and coupling transport equation for each particle size to CFD is general. But this method requires installing user routine to commercial CFD code and the work are not easy. In this work, simplified calculation method based on Lagrangian description was considered in order to calculate the phenomena using commercial CFD code. The method is calculating particle growth in the inspection volume moving along stream lines. This method has to satisfy some conditions such as practically conservation of the mass in the inspection volume. It was confirmed that this method satisfies these conditions for an actual laminar flame. And calculation results by the method consistent with characteristic of particles estimated from the flame property and with the trend of the measured particle diameter. From these results this calculation method can be used for flames which satisfy conditions.