[en] We have experimentally studied the influence of buoyancy force and inlet flow conditions on the laminar-turbulent transition process of fully-developed air flow in a heated horizontal tube with uniform wall heat flux at modified Rayleigh number Ra_q=3.1 x 10⁶. Eight time-series of the air temperature were simultaneously obtained using eight thermocouples positioned within the tube along a vertical line passing through the tube axis. We have determined the nature of the transition behavior in space and time by analyzing these instantaneous time-series. (author)

[en] In curved pipes, the high rate of heat transfer is obtained by the mixing action of the secondary flow induced in the cross sections, accordingly, they have been utilized to heat exchangers, and on the flow of air and water, many results of researches have been reported so far. On the other hand, on the flow of liquid metals in pipes subjected to the action of magnetic fields, most of the researches were related to straight pipes. In this study, considering the present status of the research on the flow in MHD curved pipes and that the liquid metals for cooling fusion reactors are subjected to the action of axial magnetic field after entering blankets, the motion of liquid metal flowing in a square curved channel subjected to the action of radial, vertical and axial magnetic fields was numerically analyzed. For the analysis, the channel walls were assumed to be electric insulator, and as the magnetic Prandtl number, the representative value of 10^-7 was used, thus the result for a square channel with radius ratio of 0.05 was obtained. The basic equations, the results of velocity distribution and current density distribution, and the coefficient of resistance are described. The magnetic field in every direction suppressed secondary flow, and had the effect to create the flow in a curved channel close to that in a straight chennel. (Kako, I.)

[en] The critical heat flux in the flow in a pipe in high dryness region is different in its property from that in low dryness region, and corresponds to the drying of tube wall when the cooling of the tube wall by liquid film or spray ceased, accordingly, it is called dry-out. Since this phenomenon occurs at relatively low thermal load, the wall temperature rise due to the deterioration of heat transfer is not directly connected to evaporating tube failure, but usually the violent fluctuation of wall temperature arises at the time of dry-out, and it has been pointed out that the life of evaporating tubes may be shortened remarkably due to the thermal fatigue. Especially in recent years, this has become a problem in view of the safety and reliability of the once-through steam generators for liquid metal-cooled FBRs. In this study, the characteristics of wall temperature fluctuation, and in particular, its relation to flow variation were examined. The experimental setup and experimental method, the general features of wall temperature fluctuation, the frequency components of wall temperature fluctuation and flow variation, and the form of a function of the power spectrum density of wall temperature fluctuation are described. The largest temperature fluctuation occurred near the middle of the variation zone. (Kako, I.)

[en] Although many fluid equipments using tube bundles such as steam generators are employed in various plants, the vibrational accidents of tube bundles are becoming frequent along with the increase of flow rate and flow velocity. The prevention of tube vibration is a very important problem because the tube breakage due to vibration may cause the critical accident leading to stop the functions of whole plants. One of the causes is hydrodynamic-elastic vibration. Connors confirmed experimentally that vibration amplitude rose abruptly at a certain flow velocity, using the vibration model for one row of cylinders, and proposed the equation between the critical flow velocity and the mass damping parameter. However, this equation is not sufficient, and the examples to which the equation cannot be applied have been reported. Since this vibration seemed to be affected largely by unsteady hydrodynamic force, the experiments were carried out. As a result, hydrodynamic-elastic vibration may be generated in the single row of cylinders. The critical flow velocity explains the fact better than the equation proposed by Connors if the wide range mass damping parameter is considered. The unsteady hydrodynamic force acting on the single row of cylinders differs much from the quasi-static fluid force, and its phase difference from that of the displacement of cylinders is large as well as it is dominated by the converted flow velocity. For this reason, the modeling to linearly add the plural hydrodynamic forces acting on respective cylinders may be possible. (Wakatsuki, Y.)

[en] A numerical analysis has been performed for three dimensional developing turbulent flow in a 180deg bent tube with roughened wall by algebraic Reynolds stress model which is adopted to solve anisotropic turbulent structure precisely. To our knowledge, very little has been reported about the numerical investigations of bent tube with roughened wall. In numerical analysis, an algebraic Reynolds stress model in conjunction with boundary-fitted coordinate system is applied to 180deg bent tube with roughened wall and it with smooth wall. As a result of this analysis, it is found that calculated results show comparatively a good agreement with the experimental data of time averaged velocity and secondary vectors. In bend tube with roughened wall, the location of maximum streamwise velocity moves more remarkably toward the outer wall than it with smooth wall and the intensity of secondary flow increases. These phenomena are realized by the present method. As for the comparison of turbulent energy, the present model has been found to simulate characteristic features of turbulent energy, but it has a tendency to overpredict its value. Moreover, it is elucidated by using calculated results that the decrease of averaged turbulent intensity over cross-section along streamwise is caused by the gradient of stream wise velocity with respect to bend angle. (author)

[en] Fragmentation of liquid metal takes place as basic processes of vapor explosions. This leads to rapid evaporation on the stretched interface. To date, a number of models explaining the fragmentation mechanisms have been proposed. However, few evidence has been obtained from the experiment because the phenomena are rapid. In Moving Particle Semi-implicit (MPS) method, grids are not necessary so that fluid fragmentation as well as multi-fluid thermal hydraulics can be analyzed. A numerical model of evaporation is developed for the present study. Impingement of water jets on a liquid metal pool is analyzed using the MPS method to investigate two typical models explaining the fragmentation mechanisms: Kim-Corradini and Ciccarelli-Frost models. Penetration of the water jet, which is assumed in Kim-Corradini model, is not observed in the calculation results. A filament of the liquid metal is observed between two water jets as assumed in Ciccarelli-Frost model. The filament appears when the jet density is smaller than the pool density, while the penetration appears when the jet density is hypothetically larger. The usual combinations of densities in vapor explosions are in the region of Ciccarelli-Frost model. (author)

[en] A non-intrusive technique for measurement of the velocity and pressure fields adjacent to a moving fluid surface is developed. The technique is based on the measurement of fluid surface profile. The velocity and pressure fields are derived with use of the boundary element method (BEM) by seeking for an incompressible flow field that satisfies the kinematic boundary condition imposed by the time-dependent fluid surface profile. The proposed technique is tested by deriving the velocity and pressure fields inversely from the fluid surface profiles obtained by a forward BEM calculation of fluid surface response to externally-imposed pressure. The inverse calculation results show good agreement with the imposed pressure distribution in the forward calculation. (author)

[en] If a flow obstruction such as a spacer is set in a boiling two-phase flow within an annular channel, the inner tube of which is used as a heater, the temperature on the surface of the heater tube is severely affected by the existence of the spacer. In some case the spacer has a cooling effect, and in the other case it causes the dryout of the cooling liquid film on the heating surface resulting in the burnout of the tube. But the burnout mechanism near the spacer is not still clear. In the present paper we discus the influence of the flow obstacle on the occurrence of burnout downstream of the flow obstacle in boiling two-phase upward flow within a vertical annular channel. (author)

[en] Linear stability analysis was applied to obtain the critical Rayleigh number Ra_c at the onset of natural convection for horizontal anisotropic porous fluid layers with vertical thin circular cylinders inserted by variational method with the consideration of non-Darcy viscous flow assumption. The permeability ratio of x-direction to z-direction is 0.5 and the effective thermal diffusivity ratio ξ of x-direction to z-direction for the anisotropic porous fluid layer is less than unity for any circular cylinders with high or low thermal conductivity. The analytical results show that Ra_c is the highest for the case of ξ=1 and decreases with the decrease in ξ. It decreases with the increase in Darcy number Da and asymptotically approaches to a constant value for Da→∞. The constant value of Ra_c for Da→∞ is the smallest to be 690. 8 for ξ→0. The analytical results were compared with experimental results performed with silicon oil and plastic resin as a working fluid and circular cylinders respectively for ξ=1. The experiment agreed well with the analysis based on non-Darcy viscous flow assumption. The analysis showed that Darcy flow assumption is applied for small Da dependent on ξ. (author)

[en] Since 1990s, an uprate of the thermal output has been carried out in USA using a method of measurement uncertainty recapture called as MUR. The principle of the MUR method is based on reduction of the measurement uncertainty of the feedwater flow rate using an ultrasonic flowmeter with high accuracy. Since the measurement uncertainty of thermal output can be reduced using that flowmeter, the thermal output can be increased within the reducing value of the uncertainty. The ultrasonic flowmeter using in a feedwater loop should be calibrated under the same condition of a nuclear power plant because of the correction factor of it is strongly affected by the condition of the flow field such as Reynolds number. To realize an actual flow calibration of feedwater flowmeter used at nuclear power stations at Reynolds numbers up to 18 million, a new test facility is constructed at National Metrology Institute of Japan (NMIJ). This very large Reynolds number is achieved at a flow rate of 12000 m³/h and water temperature of 70degC in a 600 mm pipe. Then the pipe prover system for 750 m³/h is also constructed to confirm of the temperature effect on the flowmeters. This paper describes overview and characteristic of the new facility. (author)