查看更多>>摘要：The pre-conceptual design phase of the EU DEMO magnet system relies on mechanical, electromagnetic and thermal-hydraulic analyses of different conductor designs for the Toroidal Field (TF) coils, the Poloidal Field (PF) coils and the Central Solenoid (CS) magnet. The cryo-magnetic system includes the superconducting magnets cooled by forced flow of supercritical helium at about 4.5 K, the cryo-distribution lines and valve boxes, and the cryogenic system with several cold boxes. The present analysis focuses on the cooling requirement of the TF coils with three winding pack options for the cable in conduit conductors based on 2015 DEMO baseline, featuring pancake or layer winding approaches. This analysis methodology would be further developed with the latest conductor designs and more complete heat load assumptions for the future conductor design studies and the specification of the cooling requirements. Parametric studies on the cold source temperature and on the supercritical helium mass flow rate have been performed on the three conductor designs in order to identify for each one the impact of the cooling conditions onto the temperature margin with respect to the current sharing temperature. In this study, the heat load contribution have been limited to the estimation of the neutron heating and some joint resistance heat loads when available. In addition, Simcryogenics, a dynamic modelling tool developed by CEA, is used to model supercritical helium loops for cooling different conductor designs. An algorithm has been developed to optimize both the cold source temperature and the supercritical helium mass flow, in order to minimize refrigeration power for each conductor design. Optimization studies are analyzed and compared in order to estimate for each TF winding pack design, the impact on refrigeration power. The interest of such quick cross-check analyses is to identify design improvements for the conductors and the cryodistribution, keeping acceptable temperature margins and minimizing the refrigeration power.
查看更多>>摘要：A launch vehicle that operates using cryogenic liquid propellants will require a pressurization process for a propellant tank. Usually, helium stored at cryogenic temperatures passes through the engine, is heated, and pressurizes the tank. Low-temperature helium is heated in a heat exchanger using high-temperature combustion gas from an engine. For the accurate performance prediction and weight optimization of the launch vehicle, the performance of the heat exchanger should be evaluated before launching. This study presents an evaluation method for predicting the performance of a heat exchanger in a launch vehicle. The heat exchanger performance was measured at a constant helium flow rate under actual engine operating conditions. A numerical analysis model for predicting the heat exchanger performance was developed and compared to the test results. The tests were conducted at different inlet temperatures. The test results and analysis confirmed that the effectiveness increased with the inlet temperature. Further analyses revealed that a change in the heat transfer coefficient, as well as the inlet temperature, changed the effectiveness by 6%. To accurately predict the performance of the launch vehicle, it was necessary to accurately predict the change in the heat transfer coefficient with temperature. A design method for the heat exchanger of a launch vehicle was also discussed. The heat-transfer area and weight should be optimized under the required conditions.
查看更多>>摘要：Under disturbances, superconductors may experience sudden, most undesirable phase transitions (quench) from superconducting to normal conducting state. Quench may lead to damage or even to catastrophic conductor failure. A superconductor is stable if it does not quench. In this paper, superconductor stability provides a methodical background for numerical investigations of the time dependence of phase transitions. Finite Element and Monte Carlo methods are applied to multi-filament, BSCCO 2223 and to thin film, coated YBaCuO 123 superconductors. Focus is on (i) transient temperature distributions under multi-component internal heat transfer (solid conduction and, in thin films, radiation), on (ii) a suggested operator method to solve the completeness problem of radiative transfer, and (iii) on time dependence of the order parameter obtained from a quantum-mechanical model. Explanation of the localizability of disturbances and their impacts in thin films are additional problems. These investigations shall contribute to improve understanding the physics behind the stability problem, in particular if under disturbances the material is already close to a phase transition. Traditional stability models cannot provide this information.
查看更多>>摘要：In the magnetic levitation system consisted of permanent magnets (PMs) and high-temperature superconductors (HTSs), the magnetization direction of the PM is often inconsistent with the c -axis of the HTS. Therefore, the interaction between the PM with an angle and the HTS should be studied to obtain the effects of the angle on the levitation properties due to the anisotropy property of the critical current density. In this paper, as the PM dipole oriented with the angle with respect to the c -axis of the HTS moves laterally above the semi-infinite HTS, the exact expressions for the lateral and vertical forces, the torque, and the total potential energy are obtained by using the advanced frozen-image model. The dependence of these physical quantities upon the angle and the initial cooling height have been systematically studied. Under the same conditions, the predicted force and torque calculations provided by the advanced frozen-image model qualitatively agree with the previous exper-imental and theoretical results. The study can provide some theoretical references for designing the HTS levi-tation system with the angle.
查看更多>>摘要：As a newly-developed microchannel heat exchanger, printed circuit heat exchanger (PCHE) is employed as the primary LNG vaporizer in floating storage and re-gasification unit for high efficiency and compactness. In this paper, thermal-hydraulic characteristics of trans-critical NG through the improved staggered S-shaped fin channels were numerically investigated. Variation tendencies of fin performance under different operating pressures and a wide range of Reynolds numbers were analyzed. Sensitivity analyses on the effect of various thermophysical properties on heat transfer around pseudo-critical temperature revealed that heat transfer capacity were deeply affected by specific heat, and effects of thermal conductivity and viscosity could be canceled each other. Additionally, to further estimate forced convective heat transfer mechanism, common features of HTC and f factor distributions were analyzed and some reasonable criteria were also discussed. The results showed that except for a few data points across critical temperature affected by entrance effect, the peak HTC always appeared in the vicinity of the pseudo-critical points. Furthermore, the Bo of 10-5 and q+ of 5 x 10 - 4 could be adopted to analyze buoyancy and flow acceleration influences qualitatively. The simulated results agreed with the predictions from Ngo Correlation within 16.32% and 12.36% errors respectively for HTC and f factors, which implied that though the drastic local changes of performance tendency could not be predicted well, suitable correlations were sufficiently accurate for engineering applications.
查看更多>>摘要：Under extreme conditions such as cryogenic temperatures, the application of optical fiber sensors inevitably encounters significant challenges, which include the calibrations associated with the thermal problems of sensor placement and heat sinking and the nonlinear dependence of cryogenic temperatures. Thus, fundamental investigations involving high-precision calibrations of the strain and temperature at cryogenic temperatures are essential. To this end, in this study, the thermo-optic coefficient, temperature and strain sensitivity coefficients for a Rayleigh-scattering-based distributed optical fiber sensor (DOFS) were calibrated. This experiment was implemented using a self-built low-temperature test device, and the calibration study was conducted over a large cryogenic temperature range, from 77 K to room temperature. The results indicated that the strain sensitivity coefficient of the Rayleigh-scattering-based DOFS is temperature-independent, whereas the temperature sensitivity coefficient has a significantly nonlinear temperature dependence and is associated with the coating polymer layers of the fiber. The coefficients and fitting functions were obtained over a large range of cryogenic temperatures, which indicated the invaluable properties of optical fibers in practice. Furthermore, the strain and temperature of an aluminum beam under thermal and bending loads were measured. The results indicated good agreement between the optical fibers and strain gauges, which verified the availability and reliability of Rayleigh-scattering-based distributed optical fibers in cryogenic environments.
Guner, Sait BarisSavaskan, BurcuKantar, Gunay Kaya
7页
查看更多>>摘要：The effect of bis oxalate (C22H22O6) addition on critical current density (J(c)), lattice parameters, microstructure, superconducting temperature (T-c), magnetic levitation force (F-z) and lateral force (F-x) properties of bulk MgB2 has been studied for the first time in this study. Bulk MgB2 samples containing x wt% C22H22O6;(x ranging from 0, 1.5, 3, 4.5, 6 and 9 wt%) were fabricated with by in-situ solid state sintering technique at 775 degrees C. F-z and F-x measurements at 25 K and 30 K were made under zero-field-cooled (ZFC) and field-cooled (FC) regimes. The analysis of XRD and SEM points out a drop in lattice parameters and grain size of bis oxalate added samples and endorses C replacement in B sites in MgB2 lattice. It was observed that the bis oxalate addition has a progressive effect on magnetic levitation performance of MgB2. Whereas the maximum F-z and F-x values of undoped sample show 16.91 N and 7.10 N at 25 K, the maximum force values of the 3 wt% (BO) added sample correspond to 18.62 N and 8.49 N at 25 K. Furthermore, the (BO) addition meaningfully rises the critical current density (J(c)) values of MgB2. The Jc values observed as 57 kA/cm(2) and 70 kA/cm(2) in self-field at 25 K for pure and 3 wt% (BO) doped MgB2 sample. The experimental results reveal that C22H22O6, as an example of carbohydrate, is an effective C-containing additive for the enhancement of the superconducting and magnetic levitation capacities of MgB2.
查看更多>>摘要：In the present study, corona inception voltage in liquid nitrogen under standard lightning impulse voltage (SLIV) and oscillatory impulse voltages were studied using fluorescent fiber sensor. A single discharge signal is observed near the peak of the applied SLIV. The amplitude of the fluorescent signal is high with positive polarity than under negative polarity. At higher voltages, the magnitude of the fluorescence signal increases and it shifts towards the rising edge of the applied voltage. Under unidirectional oscillatory impulse voltage (UOIV) and sinusoidal bidirectional oscillatory impulse voltage (SBOIV) conditions, Corona discharge occurs around the peak of the first cycle. When the applied voltage is increased, the discharges occur in the subsequent cycles. Under switching impulse voltage (SIV), corona discharge is noticed at the rising edge in positive polarity, whereas in negative polarity, it is observed at the falling edge. The corona discharge inception voltage (CDIV) is determined to be highest under Unidirectional Oscillating Lightning Impulse (UOLI) and lowest under SI. The negative polarity exhibits more temporal spread than the positive polarity under oscillating impulse voltages.
查看更多>>摘要：Hysteresis effect on thermal contact conductance due to load cycling is well documented for joints at ambient temperature. However very limited information is available at cryogenic temperature. The thermal cycling of joints to cryogenic temperature is also an unexplored area. In addition, the change in heat flow direction on thermal contact conductance across joints formed by dissimilar materials, which is referred as rectification, is an important parameter in maintaining desired thermal contact conductance across the joints. In the present study, these factors are experimentally evaluated for joints formed by stainless steel, aluminium alloy and titanium alloy over a temperature range of 150 K to 300 K at low interface loading. The present study reveals that the thermal contact conductance at cryogenic temperature is less affected by hysteresis by an average of 8% and 16% with thermal and mechanical loading cycles respectively. However, Rectification effect in thermal contact conductance is prominently observed in bi-metallic joints. More than 60% change in thermal contact conductance is observed with change in direction of heat flow. The study points out that rectification can be effectively utilised for controlling the thermal contact conductance at cryogenic temperature in joints subjected to low contact pressure, where the use of other controlling methods is constrained.
查看更多>>摘要：Collision of nitrogen droplets is a basic phenomenon in an array nitrogen spray cooling system, while the understanding on which is still lacking. We conduct a numerical simulation to investigate the collision dynamics of two nitrogen droplets in a low-temperature environment under various Weber numbers, Reynolds numbers and impact parameters, and simulation of collision of two water droplets is conducted for comparison. Incompressible Navier-Stokes equations are solved to simulate the colliding process, and Volume of Fluid (VOF) method and adaptive mesh refinement technique are used to capture gas-liquid interface. Three regimes of collision are found for nitrogen droplet collision, including coalescence, reflection separation and stretching separation. Upon collision, a lower surface tension and lower viscous dissipation consume less initial kinetic energy, which is conducive to the separation of droplets. Compared with water droplets, collision of nitrogen droplets easily enables breakage due to the lower surface tension and viscous dissipation. Separation of nitrogen droplets leads to the increase of secondary droplet number and enlarges heat transfer area of droplets and gas, which would enhance droplet vaporization and temperature descendence of the environment.
NSTL
Elsevier