The single-stage TMP is investigated by the direct simulation Monte Carlo (DSMC) method with a three dimensional analysis in a rotating reference frame. The generalized soft sphere (GSS) model and the modified Larsen-Borgnakke model are applied to simulate the intermolecular collisions. The pumping performances of TMP application with different gases on the same blade speed ratio are compared and analyzed in detail. The influence of clearance between the housing and rotor walls on them is predicted and analyzed. The performances and flow field characteristics of TMP pumping gas mixture are simulated, and the results for pumping performances are compared with the results by linear combination of each single gas. The present DSMC calculation results agree with the experimental data better than the former DSMC calculation with VSS model and general Larsen-Borgnakke model in the transitional flow.

View full abstract

Terminal velocities of single drops rising through infinite stagnant liquids under wide ranges of fluid properties were measured to examine the effects of initial shape deformation, surfactants and the viscosity ratio. As a result, the following conclusions were obtained: (1) the terminal velocity V_T of a drop is not affected by initial shape deformation due to strong viscous damping of shape oscillation, (2) the drop drag coefficient C_D is a function of the drop Reynolds number Re, the viscosity ratio κ and surfactant concentration, (3) surfactants increase C_D and the influence of surfactants on C_D becomes weaker as κ increases, which is consistent with the Levich's drag model, (4) the effect of κ on C_D can be evaluated by the factor in the Levich's model, [(2+3κ)/(1+κ)], even at intermediate drop Re numbers, and (5) the combination of the Levich's model and Schiller & Nauman's correlation gives good estimation of C_D of single drops in clean and fully contaminated systems under the conditions of -11.6 <log₁₀M < -0.9, 1.7×10^-1 < Re < 2×10², 1.7×10^-2 < Eo < 12.1 and 0.1 < κ < 100.

View full abstract

The mechanism of gas transfer, flow pattern and diffusion in respiratory air flow at the end zone of human lung, especially in bronchial and alveoli, has not been clarified in detail. Recently, it is known that high frequency oscillatory ventilation (HFOV) is an effective treatment for respiratory distress syndrome. However, the frequency effect on ventilation in relation to the gas transfer efficiency at the end zone of lungs has not been investigated. The velocity profile of oscillatory air flow in bronchial tube is one of the fundamental factors to consider the frequency effect. In this paper, velocity profiles of oscillatory flows in micro scale models of bronchial airway with single- and multi-bifurcation have been investigated for different frequencies corresponding to resting breathing and HFOV by using micro Particle Image Velocimetry (micro PIV). The temporal changes of velocity profiles were reconstructed by phase-averaged velocity maps obtained by micro PIV measurements, and the effect of frequency on the velocity profile in bronchial models has been discussed.

View full abstract

Coalescence of two planar air bubbles in stagnant water filled in two parallel plates was experimentally investigated. To examine the effects of bubble wake on coalescence, the bubble diameter, initial vertical and horizontal distances between two bubbles were chosen as experimental parameters. The vertical distance x from the bubble release point, at which coalescence took place, was measured. The number of coalesced bubble pairs was counted for about 100 bubble pairs, and the coalescence probability was calculated as the ratio of the number of the coalesced bubble pairs to that of the total bubble pairs. With decreasing the bubble diameter or with increasing the initial distance between two bubbles, the mean vertical distance x increased and the coalescence probability decreased. There was a strong correlation between the distribution of coalescence probability and that of bubble wake velocity, i.e. the wake structure does play a significant role in the bubble coalescence process and affects the coalescence probability.

View full abstract

The concentration field of a diffusing matter in the cross plane of a turbulent round jet injected into a counter-flowing uniform stream has been investigated using the PLIF technique. In order to capture images in good focus from a camera oriented obliquely to the object plane, the camera configuration has been designed according to the Scheimpflug condition and a water prism has been introduced to reduce the amount of radial distortions that are caused by capturing images through a channel wall. In addition to the mean and rms concentration fields in the cross-section, turbulent dispersion properties, such as the centroid and the absolute and relative spreads, have been examined by integrating the instantaneous concentration field over a cross-sectional area. It has been confirmed that a random fluctuation in the centroid of instantaneous concentration field exists in the cross plane and its amplitude increases downstream more rapidly as compared with the conventional jet in a quiescent ambient. These results could have a close relation with a radial oscillation of the jet core, which is expected to be responsible for its enhanced mixing efficiency.

View full abstract

Low Reynolds number (Re ∼ 0.001) flow and mixing in a Y-shaped micromixer with a four-paddle rotor placed in its junction were investigated using computational fluid dynamics (CFD). The rotation speed of the rotor ω ranges from 100 to 2000 RPM (revolutions per minute). The flow visualization shows the enhance mixing by the rotating paddle that are stretching the fluid particles, up and down flows in the junction, transverse velocity in a section of the mixing channel. Further, at a high value of ω, a circulatory flow formed in the mixing channel greatly enhances the mixing rate. The mixing potential of the mixer is measured by the strain rate. The dispersive mixing efficiency coefficient is considered as a criterion for evaluating the mixer. The other criterion is the distributive mixing efficiency, which is based on the homogeneity of the concentration distribution. Both the criteria indicate that the mixing significantly improves with increasing rotation speed. With regard to obtaining similar mixing efficiency, a mixer with a rotor reduces the required length of the mixing channel as compared with that of a mixer without a rotor. The reduced length is proportional to ω.

View full abstract

In earlier works by the present authors, it was shown that a large cross-flow excitation is induced to a circular cylinder by setting another cylinder downstream in a cruciform arrangement with a certain gap between them, caused by longitudinal vortices shedding periodically around the crossing. In this work, influence of the cross-sectional configuration of cylinders forming a cross on the longitudinal vortex excitation was investigated by wind tunnel experiments. Three systems, i.e. a two-circular-cylinder, a circular-cylinder/strip-plate and a square-cylinder/strip-plate system, with essentially equal structural parameters were compared. In the case of the circular-cylinder/strip-plate system, the trailing vortex excitation occurs over much wider velocity region, while the necklace vortex excitation is indefinite, as compared with the two-circular-cylinder system. In the case of the square-cylinder/strip-plate system, vortex excitation occurs in a certain velocity region of the galloping of single cylinder attributed to the necklace vortex.

View full abstract

Diesel engine combustion chamber which reduces exhaust emission has been designed using CFD analysis and optimization techniques. In order to save computational time for design, the Kriging model, one of the response surface models, is adopted here. For a robust exploration, both the estimated function value of the model and its uncertainty are considered at the same time. In the present problem, the k-means method is used to limit the number of additional sample points to a reasonable level. Among the additional sample points, two combustion chamber shapes dominate the baseline configuration in terms of all objective functions. Compared with the previous optimization with the evolutionary algorithm, its computational time for design was cut by 95%. The results indicate that the present method is a practical approach for real-world applications.

View full abstract

It is known that the pressure fluctuation in the runner will become large when a Francis turbine operates at the low flow rate and high head. One of the reasons is the occurrence of channel vortices, which are caused by the three-dimensional flow separating from the suction side of runner blades. In this study, two three-dimensional guide vanes (1# 3D GV, 2# 3D GV) are designed so as to depress the channel vortices and improve the operation performance of a Francis turbine. The flow rate equation for 3D GV is derived firstly in this paper. Then, in order to show the influence of 3D GV on the turbine characteristic, performance test and video-recording of the channel vortices are conducted. Numerical simulation for a part load operation is applied finally to the entire turbine flow passage (from the inlet of spiral case to the outlet of draft tube) using the RNG k-ε turbulence model to make clear the three-dimensional internal flow. From evaluation of both the experimental and calculation results, it is noted that the channel vortices from the blade suction side were suppressed effectively by 3D GV, and the turbine efficiency with 3D GV was 0.41 % higher than that with the conventional 2D GV.

View full abstract