Nobuo Yamaoka

IFMIF/EVEDA lithium test loop: design and fabrication technology of target assembly as a key component

The engineering validation and engineering design activity (EVEDA) for the International Fusion Materials Irradiation Facility (IFMIF) is proceeding as one of the ITER broader approach activities. In the concept of the IFMIF, two 40 MeV deuteron beams are injected into a liquid Li stream (Li target) flowing at a velocity of 15 m s−1. The EVEDA Li test loop (ELTL) is aimed at validating the hydraulic stability of the Li target at a velocity up to 20 m s−1 under a vacuum condition of 10−3 Pa as the most important issue. Construction of the ELTL, which is the largest liquid metal loop possessing 5.0 m3 Li for the fusion research ever, was completed in the O-arai Research & Development Center in the Japan Atomic Energy Agency on 22 November 2010. This paper presents the design and fabrication technology of a target assembly called integrated target assembly, in which the Li target is produced by a contraction nozzle along a concave channel. There are two concepts regarding the target assembly: the integrated target assembly and the bayonet target assembly. Both target assemblies are outlined in this paper, and then the newly proposed design of the integrated target assembly for the ELTL and its fabrication technology are given. The integrated target assembly was processed by a five-axis milling machine and the processing accuracy was measured by 3D measurement tools. Finally, methods applied for the validation of the stability of the Li target are introduced in this paper.

Forced Convection Heat Transfer and Temperature Fluctuations of Lithium under Transverse Magnetic Fields

The MHD effects on a heat transfer of liquid lithium flowing in an annular channel were studied, using a newly constructed comparatively large-scale lithium loop. The data on the heat transfer characteristics were arranged in Nu-St diagrams where St=Ha2/Re. The diagrams indicate that Nu decreases as St increases as a whole trend. However, singular peaks of Nu were observed at about St=13, corresponding to the singular increments of temperature fluctuations, whereas the peaks occurred at St=1.5 in the previous experiment with a smaller channel. In considering a predominant MHD effect due to conductive wall duct, the current load factor KP for external circuit was taken into account. And Miyazaki number Mi=KPSt, which means the effective electromagnetic interaction, was adopted in substitution for St and the heater pin diameter was taken as the characteristic length in place of the conventional hydraulic equivalent diameter. Thus, an appreciably good agreement was obtained concerning the peaks at Mi=0.13 in the previous and at Mi=0.14 in the present experiments. Generation of vortex series in the singular phenomena was suggested from both the periodicity of temperature fluctuations in their PSD analysis and the proportionality between peak frequency and mean flow velocity, resulting in a 32 mm interval length of series.

Boiling two-phase flow under microgravity☆

To study the effects of reduced gravity on the flow regime and the heat transfer characteristics of a boiling two-phase horizontal flow, parabolic flight experiments were performed by using an aircraft. The gravity level during the parabolic flight and the duration time were about −0.01ge ∼ +0.01ge and 20 s, respectively. Under earth gravity, many small bubbles are detached very frequently from the heater rod surface, flowing into the upper stream due to the buoyancy and resulting in a stratified flow in the cases of lower inlet fluid velocity and higher heat flux. Under microgravity conditions, bubbles are hardly detached from the heater rod, growing and coalescing to become much larger along the heater rod, surrounding the heater rod in the downstream. This tendency was more noticeable in the cases of lower inlet fluid velocity, higher heat flux and lower inlet fluid subcooling. The local heat transfer coefficients at the bottom of the heater rod tend to decrease slightly under microgravity compared with those under earth gravity because of the reduction of the heat removal due to natural convection. On the other hand, the local heat transfer coefficients at the top of the heater rod tend to increase slightly under microgravity. However, the differences of the local heat transfer coefficients are very small in spite of large differences of the flow regimes under earth gravity and microgravity.

Magneto-hydro-dynamic pressure drop of lithium flow in rectangular ducts

The MHD pressure drop is measured by providing a lithium circulation loop of 40 lit/min and 0.3MPa heat with a square test section of 2a=15.7mm x 2b=15.7mm or a rectangular one of 2a=26.8mm x 2b=11.1mm inner cross-section made of t/sub w/=2.1mm thick 304-SS walls. The experiment covered ranges of B=0.2-1.5T (Ha=200-2100), U=0.2-4.Om/sec (Re=500-38000), and T/sub Li/=309-380/sup 0/C. Theoretical prediction is made on an assumption of a uniform electric current density, neglecting the friction with walls. The MHD pressure gradient -dP/dz is given by -dP/dz = K/sub rho/sigma/sub f/UB/sup 2/ where K/sub rho/= C/(1+a/3b+C) and C=sigma/sub w/t/sub w//sigma/sub f/a. The theory agreed well with the experimental data for both the square and rectangular test sections. Under the uniform magnetic field of the exit, the pressure drop data agreed with an approximated prediction of ..delta..P=..integral..K/sub rho/sigma/sub f/UB/sup 2/(z)dz.

MHD pressure drop of NaK flow in stainless steel pipe

An experiment on electric potential and pressure drop for NaK flow in uniform transverse magnetic fields was conducted. A test channel was constructed using 45.3 mm (or 28 mm) I.D. and 1.65 mm thick 304-SS circular pipe in the NaK-Blowdown MHD Experimental Facility of Osaka University. The experimental range covered had a driving gas pressure <8 bar, an applied magnetic flux density: B/sub 0/=0.3 about1.75 T, a mean flow velocity of NaK: U/sub 0/=2 about 15 m/sec, a Reynolds number Re = 8 X 10/sup 4/ about6.2 X 10/sup 5/ and a Hartmann number: Ha = 740 about4150. A theoretical analysis is given on the basis of a uniform-velocity thick-wall model. Good agreement between the theory and the experiment were obtained both for the potential and for the pressure drop, except a small deviation of the experimental pressure drop towards values lying above the theoretical ones in a weak B/sub 0/ and high U/sub 0/ region (Ha/sup 2//Re <15).

Heat transfer enhancement in lithium annular flow under transverse magnetic field

Magnetohydrodynamic (MHD) effect on heat transfer in liquid lithium annular flow was investigated with an emphasis on heat transfer enhancement and local turbulence. A test section was made of 10 mm heater pin and 2 in. B pipe of 304SS. The experiment covers ranges of U = 0.1-3 m/s (Re = 4 x 10 3 -1.4 x 10 5 ), B = 0-0.8 T (Ha = 0-3 x 10 3 ). The heat transfer was observed to increase over a region of St from unity to 100, and the peak value was 5-10% higher than that for B = 0 T. This was explained as a result of local turbulence enhancement in the vicinity of the heating wall. A new concept of heat transfer diagram was proposed with considering a current load factor of the test channel.

Flow and heat transfer characteristics in lithium loop under transverse magnetic field

A medium-scale lithium-loop with 40 l/min and 3bar ratings was constructed to gain basic information on MHD effects on the flow and heat transfer characteristics. The loop has two parallel test sections for pressure drop and heat transfer experiments, which were made of 15.75 mm I.D. and 19.05 mm O.D. 316-SS tubes and placed between magnet poles of 500 mm vertical length. The pressure drop test section was provided with two strain gage type pressure transducers and the heat transfer test section with a 300 mm long 7.6 mm O.D. high flux electric heater pin. The experiment covered the ranges of the magnetic flux density: 0-1.0 T, The Li flow velocity: 0.2 -5.0 m/sec, the heat flux: 0-120 W/cm/sup 2/ and the Li temperature: 350-400/sup 0/C. The experimental results of potential and pressure drop agreed well with the theoretical prediction based on the uniform-velocity thick wall model. The heat transfer coefficient, or Nusselt number, was decreased with increasing magnetic flux density, but not monotonically in a weak magnetic field region of 0.2-0.4 T, where a singular phenomenon, i.e. an elevation of Nusselt number was observed.

Lithium free surface flow experiment for IFMIF

Abstract High-speed lithium flow was demonstrated in free surface condition by using the lithium loop facility at Osaka University. The project aims at the verification of hydrodynamic stability of free surface lithium flow of up to 15 m/s. A horizontally aligned test channel was installed in the existing loop. Preliminary results show that the design of the nozzle and of the test channel is considered to be successful in order to realize a high-speed free surface flow of lithium in vacuum.

Experimental Study on Wave Propagation Behavior on Free Surface of Lithium Flow for IFMIF

Velocity measurement of surface waves on high-speed liquid lithium (Li) flow was conducted by using the Li circulation loop at Osaka University to support target monitor and diagnostics applications for the International Fusion Materials Irradiation Facility (IFMIF). Free surface shapes were recorded with a high-speed video camera, and surface waves were tracked with the statistical correlations of image intensity patterns over a velocity range of 5 to 15 m/s. As a result, the velocity distribution of surface waves was measured. The development of surface velocity beyond the nozzle edge was clearly measured. The velocity measurement by this method might suffer from some influence from stationary waves, which were generated by a damaged nozzle edge. The measured velocity in a region free from stationary waves was seen to exceed the mean flow velocity, probably due to two-dimensional regular waves. For this case, the measured velocity may consist of the advection velocity of the Li flow and the phase velocity of the wave. For an irregular wave region, the measured velocity was shown to be approximately the same as the mean flow velocity. The present flow velocity measurement can be used in the high-velocity range where random waves are generated.

MHD Pressure Drop of Liquid Metal Flow in Circular and Rectangular Ducts under Transverse Magnetic Field

Simple formulas were derived to estimate the liquid metal MHD pressure drop in a cooling system for fusion use and good agreements were obtained with NaK and Li experiments in use of various SS ducts.