Keiji Miyazaki

Present understanding of MHD and heat transfer phenomena for liquid metal blankets

Abstract A review of experimental work on magnetohydrodynamic (MHD) and heat transfer (HT) characteristics of liquid metal flows in fusion relevant conditions is presented. Experimental data on MHD flow pressure drop in straight channels of round and rectangular cross-section with electroconducting walls in a transverse magnetic field show good agreement with theoretical predictions, and simple engineering formulas are confirmed. Less data are available on velocity distribution and HT characteristics, and even less data are available for channels with electroinsulating walls or artificially made self-healing electroinsulating coatings. Some experiments show an interesting phenomena of HT increase in the presence of a transverse or axial magnetic field. For channels of complex geometry — expansions, contractions, bends, and manifolds — few experimental data are available. Future efforts should be directed toward investigation of MHD/HT in straight channels with perfect and nonperfect electroinsulated walls, including walls with controlled imperfections, and in channels of complex geometry. International cooperation in manufacturing and operating experimental facilities with magnetic fields at, or even higher than, 5–7 T with comparatively large volumes may be of great help.

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.

Ion-induced luminescence of silica glasses

Abstract We have performed in situ analysis of ion-induced luminescence of silica glasses in terms of incident ion energy and fluence, to investigate the origin of the luminescence and the influence of implanted deuterium and helium ions. The luminescence spectra of SiO 2 glasses induced by D + and He + irradiation showed a broad band centered at around 450 nm. By comparing with TRIM calculations, we can conclude that the luminescence is caused by electron excitation effects of implanted ions. In the early stage of the irradiation, most of the luminescence comes from intrinsic defect centers. With increasing ion fluence, the luminescence intensity increased, taking maximum and then decreased. This indicates that the first increase is attributed to the increase of newly produced defect centers by atomic displacements, while the decrease is probably owing to the growth and clustering of the defects and/or precipitation of Si clusters. When the fluence is translated to dpa, changes of the luminescence intensity between D + irradiation and He + are very similar. This result also supports that the additional luminescence centers are produced by atomic displacements. The luminescence centers are very likely oxygen vacancy or oxygen deficiency related ones but OH has some role.

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).

Energetic particle induced luminescence of Al2O3

Abstract In-situ observations of ion induced luminescence of sapphire (Al2O3) specimens are conducted with respect to incident energy, fluence and ion species. The luminescence spectra showed three band emissions centered at 335, 420 and 650 nm assigned to F1, F and 2nd order of F1 with some contribution of F2-type defects, respectively. Comparing with TRIM calculations it is clearly demonstrated that the ion induced luminescence of Al2O3 is simply caused by the electron excitation by the incident ion. The observed luminescence intensity changed with the ion fluence; F center monotonously decreased and mostly disappeared, whereas F+ first increased to a maximum and then gradually decreased to a nearly steady value after prolonged irradiation. Following the heavy irradiation the total luminescence intensity was reduced to less than 1/2 of the initial value owing to the clustering of the defects and possibly the precipitation of Al. When the fluence is translated to dpa, changes of the luminescence intensity with dpa for D+ and He+ irradiations are very similar, indicating that implanted D atom has little effect on the luminescence as assigned the luminescence centers of F and F+ to single oxygen vacancies.

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.

Condensing Heat Transfer in Steam-Water Condensing-injector

An experiment on the direct heat transfer process between supersonic steam and subcooled water jet was performed, using a steam-water condensing-injector. Photographic observation provided information on the state of flow, and establishment of a critical separate steam-water flow was confirmed. The temperature and pressure distributions along the flow were measured and the effective coefficients of condensing heat transfer were evaluated from the observed data, based on a model embodying an idealized interface between vapor and liquid. In the vicinity of the water nozzle exit, where the vapor-liquid interface was distinct, the heat transfer coefficients obtained were 14–28 (cal/°C.cm2.sec), and some correlation was observed among Nusselt, Reynolds and Jakob numbers, upon adopting the velocity and the physical properties of the steam phase. The relations Nu=6.0.Re 0.9(Pr=1.04–1.10), and Re=1.8×108.Ja 3.0, i.e., Nu=1.6×108.Ja 2.7 were derived as a rough estimation. No clear correlation could be discerned in...