V.V. Rao

Pressure Drop and Heat Transfer Analysis of Long Length Internally Cooled HTS Cables

High temperature superconducting (HTS) cables, internally cooled by liquid nitrogen (LN2), are being explored for efficient power transmission. The cooling requirements of a HTS cable depend on the geometry, length, and operating current. The HTS cable at the operating temperature receives heat flux from different sources such as the leads at the ends, ac losses through superconductor and the heat-in-leak through insulation. These losses need to be accommodated by a refrigeration system at one or more locations along the length of the HTS cable. In this context, it is important to estimate the pressure drop in long length HTS cables and associated pumping power required to pump the cryogenic fluid (LN2) internally through the central channel between two terminations. In the present work, long length HTS cables based on BSSCO and YBCO tapes are considered for analysis, where the thermophysical properties of LN2 are considered to be temperature dependent. Pressure drop and heat transfer analyses are carried out for different flow rates of LN2 to obtain the friction factor, pumping power and heat transfer rate. This analysis is useful in characterizing the operating flow regime and safe operating temperature range in HTS cables.

Low-temperature electrical transport in Heusler-type Fe2V (AlSi) alloys

The temperature variation of the electrical resistivity ρ and the Seebeck coefficient S of Heusler-type Fe2VAl1−xSix (0≤x≤1) alloys has been investigated. We have shown that the transport parameters are very sensitive to doping. For the x = 0 sample, high values of ρ and negative temperature coefficient of resistivity (TCR) have been observed. As the Si concentration increases, ρ decreases and the TCR changes its sign, while S shows significant changes in magnitude as well as sign when Al is replaced with Si. These changes appear to be reminiscent of a metal to semiconductor transition. It has been shown that the conventional transport theories proposed for intermetallic alloys or semiconductors cannot explain the transport behaviour in the whole temperature range of the present study. Low-temperature resistivity data of x = 0–0.02 samples could be described with a gapless semiconductor model. The strong composition dependence of S and ρ is attributed to the sharp variations in electronic density of states at the Fermi energy. It is also shown that by optimum doping one can achieve very large values of power factor (P). The estimated power factor at room temperature is observed to be highest (2.23 × 10−3 W mK−2) for x = 0.06 and comparable to that of conventional thermoelectric material. At lower temperatures P is found to be even higher than that of conventional thermoelectric material.

CFD Analysis of Cable-In-Conduit Conductors (CICC) for Fusion Grade Magnets

In the present work, dual channel Cable-in-Conduit Conductor (CICC) is considered for Computational Fluid Dynamics (CFD) analysis to understand the complex behavior of flow. A two dimensional (2D) axisymmetric computational model mimicking the CICC is generated and meshed in GAMBIT 2.0. The meshed model is exported to FLUENT 6.3, a commercial CFD code, for further analysis. Annular bundle channel of CICC is assumed to be porous medium with porosity of 0.37 and the central channel is assumed as clear region. The effects of variations in mass flow rate (6 g/s to 10 g/s) of Supercritical helium (SHe), which is used as coolant, through CICC on the pressure gradients and velocity gradients are studied. Reynolds Averaged Navier Stokes (RANS) model is considered for the analysis with standard k-epsilon (k - ε)turbulence, which is relevant to the separated flow models. Axial and Radial pressure gradients are calculated along the CICC axis and along the centre line of bundle channel. Friction factor is calculated using the shear stresses obtained from CFD analysis.

Correction to scaling critical exponents and amplitudes for amorphous Fe–Mn–Zr alloys

Abstract Asymptotic and leading correction to scaling critical exponents and amplitudes have been determined for quenched amorphous Fe 90− y Mn y Zr 10 ( y =0–8) ferromagnets through an elaborate analysis of temperature dependence of spontaneous magnetization, zero-field susceptibility and low-field AC susceptibility data obtained in the asymptotic critical region. From this analysis, it is found that the values of the critical exponents and amplitudes do not depend on the alloy composition and are in good agreement with the values predicted for three-dimensional Heisenberg ferromagnet system. The observed experimental results are consistent with the concept of scaling in that the exponent equalities β = γ ( δ −1) and α =2(1− β )− γ are obeyed to a high degree of accuracy. These results show that both amorphous and crystalline materials behave similarly in the critical region though amorphous alloys show a wide asymptotic critical region than the crystalline materials. The presence of disorder does not seem to have any influence on critical behavior of the system investigated in the present work.

Critical behavior of Mn-substituted a-FeZr alloys

Abstract The critical magnetic properties of the amorphous a-Fe 90− x Mn x Zr 10 ( x =0–16) alloy system have been studied systematically using AC susceptibility (ACS) technique. From our results, it has been observed that the Curie temperature ( T c ) decreases with increasing Mn concentration. Measurements have also been performed with a DC bias field. The critical exponent values γ , associated with the magnetic susceptibility, and δ , associated with the critical isotherm, have been determined from fits to the ACS data in the asymptotic critical region. Values of the effective exponents obtained from the fitting are consistent with values predicted for ordered three-dimensional Heisenberg system, indicating that the presence of disorder does not influence the critical behavior and these alloys exhibit a conventional phase transition to long-range ferromagnetic order at T c .

An ac Susceptibility Study of Mn Substituted Amorphous FeZr Alloys

The temperature dependence of the ac susceptibility of amorphous Fe 90-x Mn x Zr 10 (x = 0, 4, 8) has been measured at different ac fields in the frequency range of 80 Hz to 1 kHz. All the compositions of the present study exhibit a double magnetic transition below room temperature. It is found that the Curie temperature (T c ) decreases while the spin-glass transition temperature (T f ) increases as Mn concentration increases from 0 to 8 at%. The values of T c and critical exponent associated with the magnetic susceptibility were extracted from ac susceptibility measurements. The critical exponent is in close agreement with the value predicted by the three-dimensional Heisenberg model. The results obtained here can be explained on the basis of competing exchange interactions below the magnetic ordering temperature.

Dynamics of magnetic susceptibility in amorphous Fe80Mn10Zr10

Abstract Experiments on the field and frequency dependence of ac susceptibility (acs) for an amorphous Fe 80 Mn 10 Zr 10 alloy in the temperature range from 12 to 300 K are used to explore magnetic nature. There is a well-resolved frequency dependent maximum in the imaginary component of the acs at low temperature, which is the real significance of the reentrant spin-glass phase dynamics. The dynamic scaling analysis of the low field acs data indicates the presence of spin-glass transition T SG =40 K with a critical exponent Ψ =6.5 which seems to be a mixed state rather than a pure spin-glass state. The existence of mixed magnetic state could be understood based on exchange frustration model.

Current distribution mapping in insulated (Gd,Y)BCO based stabilizer-free coated conductor after AC over-current test for R-SFCL application

Uniformity of critical current (Ic ) over long lengths of (GdY)-Ba-Cu-O ((Gd,Y)BCO)-based high temperature superconducting (HTS) tapes after long periods of AC current excitation is an important criterion in their selection for resistive type superconducting fault current limiter (R-SFCL). The present work describes such critical current (Ic ) uniformity measurements performed over 1m long, stabilizer-free (SF), 12 mm wide, 2nd generation (2G) (Gd,Y)BCO based HTS tape. A non-destructive method using a static hall probe (Tapestar®) with moving HTS tape configuration was employed for estimation of Ic uniformity. Scanning Hall probe microscopy (SHPM) was then used to examine the weak superconducting regions (i.e. less Ic ) with a static HTS tape. Remanent field distribution on the HTS tape was measured to yield the critical current density distribution. Except for small degradation of Ic at some locations, these studies confirmed near-uniform critical current distribution over meter-long (Gd,Y)BCO tapes, both in virgin state and after exposure to AC over current.

Fault limitation characteristics of a lab-scale resistive type superconducting fault current limiter

With the recent development of advanced YBCO based 2nd generation (2G) coated conductors (CCs), many researchers started developing superconducting fault current limiters (SFCLs) based on these conductors. We have used an electrically insulated CC developed by Super Power to make a simplified laboratory scale resistive (i.e. non-inductive) type superconducting coil for SFCL technology in Indian power sector. This CC has unique thermal and electrical properties when compared to other commercially available HTS conductors. This YBCO based HTS tape has a silver over-layer and is made on Hastelloy®276 substrate which is highly resistive by nature. The conductor has polymide (kapton, 30% overlap) insulation for high voltage safety. Using this conductor, we made a small superconducting coil where adjacent turns carry equal and opposite currents to make it non-inductive. This SFCL coil is rated for 220 Vrms and 400 Arms (single phase) operation. The former of the coil is fabricated in-house using G-10 Glass fiber and fine holes are made on the former for both-side cooling of the CC. In this paper, a simplified design of a resistive SFCL without bypass path is carried out using 2G CCs. After that, the continuous current performance of the coil and voltage drop are measured when 425 Arms is applied. Over-current test on the coil is carried out to measure the effective fault limitation to verify the performance of the resistive SFCL without a bypass path.

Conceptual Design of a 440 V/800 A Resistive-Type Superconducting Fault Current Limiter Based on High

A conceptual design of a high-Tc-based resistive-type superconducting fault current limiter (SFCL) using second-generation coated conductors (CCs) has been carried out in this paper for possible power applications in India. In the design aspect, noninductive SFCL coils made with stabilizer-free CCs have been used for analytical calculations. In electrothermal analysis, a short-circuit characterization has been simulated to get the maximum resistance and temperature achieved during fault limitation in a 440 V/800 A single-phase SFCL. Furthermore, the recovery under no load is studied, and recovery time is calculated.