Takehiro Ito

Development of a 22 kV/6.9 kV single-phase model for a 3 MVA HTS power transformer

We have developed a 22 kV/6.9 kV HTS single-phase transformer cooled by liquid nitrogen for field test, which is a practical model for the single-phase part of a 3 MVA HTS power transformer. First, we numerically simulated electromagnetic, mechanical and thermal conditions of the windings in accidental cases of short-circuit and lightning impulse, and considered the winding structure withstanding the severe loads. We constructed a small-sized model coil of Bi-2223 Ag/Mn-sheathed tapes and confirmed applicability of the design concept for the overcurrent and high-withstand-voltage tests. We designed and constructed a single-phase HTS transformer on the basis of the model-coil-test results. The primary and secondary windings are transposed parallel conductors of two and six Bi-2223 Ag/Mn tapes, respectively. The same tests for the HTS transformer as for usual oil-filled ones indicated the reliable operation and high performance. The field test in a distribution grid of Kyushu Electric Power Co. included in-rush-current test and long-term operation of the transformer cooled by a continuous supply system of subcooled liquid nitrogen with cryocoolers.

Investigation of variable thermophysical property problem concerning pool film boiling from vertical plate with prescribed uniform temperature

Abstract By the development of laminar two-phase boundary-layer analysis on pool film boiling heat transfer, system parameters and their fundamental effect on heat transfer have been disclosed. However theoretically obtained characteristics of heat transfer do not fairly compare with the experimental data, because the relevant thermophysical properties vary significantly across the vapor film, and the nature of flow process is often turbulent and of non-boundary layer. In this investigation an analysis was made on pool film boiling heat transfer from vertical plate of uniform temperature to saturated or subcooled liquid, taking into account temperature variation of all properties, and the significance of variable properties in pool film boiling was examined. By getting solutions for water at the pressure of 1,50,100 and 200 {bar}, effect of variable properties on heat transfer and distribution of velocity and temperature were clarified. The heat-transfer results were compared with the available data of pool film boiling from horizontal cylinder.

An analysis of free-convective heat transfer from an isothermal vertical plate to supercritical fluids

Abstract Theoretical studies have been made on the laminar free-convective heat transfer from an isothermal plate to fluids at supercritical pressures, taking into account of the temperature dependence of all the relevant physical properties. The heat-transfer characteristics of water at 230, 240 and 250 atm and carbon dioxide at 80 and 90 atm were clarified by integrating the similarity transformed differential equations numerically. Results were as follows: 1. (1) The heat-transfer characteristics are strongly dependent on the bulk fluid temperature and the wall temperature individually. 2. (2) For the larger temperature difference the conventional equation of laminar free-convective heat transfer fails to correlate the present analytical results. 3. (3) The analysis by Fritsch and Grosh for temperature-dependent density and specific heat compares favorably with the present one. Property-dependence of the heat-transfer characteristics of the supercritical fluids may be said to have been thoroughly examined.

Free-Convective Heat Transfer to a Supercritical Fluid

The influence of heater material on free-convective heat transfer from wires to carbon dioxide in the supercritical region was investigated. Three types of flow configuration were observed for nichrome and alumel wires: (a) conventional laminar-flow free convection, (b) an oscillating flow in which (a) and a bubble-like flow alternated, (c) a bubble-like flow in which masses of fluid similar to bubbles appeared successively near the heating surface. Neither (b) nor (c) was observed with a platinum wire. The heat-transfer rate increased suddenly when the flow configuration changed from (a) to (b). The transition in flow structure occurred when the temperature difference between the heater and fluid exceeded about 200 deg C.

Dynamic stability of superconductors cooled by pool boiling

Abstract The transient one-dimensional heat conduction equation in a NbTi composite superconductor with an initial localized heated zone and cooled by pool boiling of liquid helium ( 4 He) has been analysed numerically. The initial excess thermal energy associated with the localized heated zone was about the level of that associated with the minimum propagating zone (MPZ) equilibrium. The heat transfer characteristics of boiling helium, the variation with temperature of the heat capacity and thermal conductivity of the conductor and the temperature dependence of the Joule heat dissipation of the electric power were all stored in a computer to be referred to by a program. The stability of the conductor—coolant system was thus investigated. The conductor experienced quenching in many cases with the initial excess thermal energy less than the excess thermal energy stored at the MPZ equilibrium, and the recovery quenching was surprisingly dependent on the space-wise distribution of the initial excess thermal energy along the conductor. Thus, the MPZ criterion does not always guarantee safe operation of superconductors, though it might be an important quantity characteristic of a conductor—coolant system.

Numerical analysis of rapid solidification in a single roller process

The rapid solidification in a single roller process has been used to make amorphous ribbons. Because this process occurs rapidly, it is difficult to obtain useful experimental data. Therefore, a numerical analysis has been performed on the rapid solidification in a single roller cooling process. The VOF (volume of fluid) method was adopted as the numerical method used to simulate transient two-dimensional thermal and fluid flow with a liquid-solid phase change and free surfaces. We simulated the behavior of an aluminum alloy. The geometry of the amorphous ribbon, flow and temperature fields, temperature history of alloy particle whose initial location is at the center of the nozzle, and the cooling rate were obtained using as parameters the roll velocity, the nozzle slot breadth, and the gap between nozzle and roller.

Stability analysis of a forced-flow cooled superconductor

Abstract A numerical analysis of the thermal stability of a superconductor composite cooled by forced-flow supercritical helium has been performed. The analytical model consists of simultaneous two-dimensional coolant flow equations and a one-dimensional superconductor equation. The equations of conservation of mass, momentum and energy for coolant flow and the Fourier heat conduction equation for the superconductor were solved numerically. Current-sharing mode, temperature-dependent properties of the conductor, and pressure- and temperature-dependent coolant properties were fully taken into account. The transient behaviour of the conductor, coolant flow, the effects of disturbance-heated length of the conductor, the initial pressure and initial velocity of coolant on the thermal stability of superconductors are discussed.

Review of existing experimental findings on forced convection heat transfer to supercritical

Abstract A literature survey of existing experimental findings on steady state and transient forced convection heat transfer to supercritical helium is given, and the deterioration of heat transfer, the influences of buoyancy force and correlations of heat transfer are also discussed.

Numerical simulation of heat and fluid flow in basic pulse tube refrigerator

Purpose – To clarify the physical working principle of refrigeration in basic pulse tube refrigerators (BPTRs).Design/methodology/approach – A numerical simulation was performed. Transient compressible NS equation was solved utilizing the TVD scheme coupled with energy equation.Findings – The periodic flow and temperature field were obtained. The movement of the gas particles and heat transfer between the gas particles and wall were analyzed. These numerical results explained the mechanism of surface heat pumping (SHP) which is known as the working principle of refrigeration in BPTR.Research limitations/implications – Pulse tube refrigerator (PTR) is classified into the third generation. BPTR is the first generation. It is needed to clarify the working principle of refrigeration in the second and third generation by analyzing heat and fluid flow in the tube.Practical implications – A very useful source of information to understand the physical working principle of refrigeration in BPTR.Originality/value –...

Numerical Simulation of Heat and Fluid Flow in a Basic Pulse-Tube Refrigerator

The working principle of refrigeration in basic pulse-tube refrigerators (BPTR) has been explained by the mechanism called surface heat pumping (SHP) that heat is conveyed from the cold end to the hot end of the pulse tube by the successive heat exchange between the working gas and the wall. In this study, a numerical simulation has been performed to clarify the effect of the wall in BPTRs by comparing the numerical results in two physical models; one is the model considering the heat exchange between the working gas and the wall (HE model), and the other is the model ignoring that (AW model). As a result, the importance in the effect of the wall was shown clearly. In addition, the mechanism of refrigeration other than the SHP was made clear in the AW model.Copyright