Cryogenic design of liquid-nitrogen circulation system for long-length HTS cables with altitude variation

Liquid-nitrogen (LN₂) circulation systems with altitude variation are investigated and designed for application to practical long-length HTS cables. This study is motivated by the KEPCO’s immediate plan to install new HTS cables in existing utility tunnels including inclined and vertical sections. The distribution of pressure and temperature along the LN₂ circulation loop is examined for various geographic conditions, taking into account the gravitational effect of altitude variation. The cryogenic cooling requirements are defined in terms of the pressure and temperature of LN₂, and a design process is graphically demonstrated on phase diagram. It is concluded that the LN₂ flow rate along the cable with altitude variation should be carefully determined with the constraints on pressure and temperature, and the proposed graphical method is useful in the design.     

Experimental study of laminar natural convection heat transfer from a vertical cylinder to slush nitrogen under constant heat flux conditions

Natural convection heat transfer from a vertical cylinder immersed in slush and subcooled liquid nitrogen and subjected to constant heat fluxes was investigated in order to determine the relative merits of slush nitrogen (SlN₂) for immersion cooling. A glass dewar was used as a test vessel in which a cylindrical heater was mounted vertically, and heat transfer measurements were carried out for SlN₂ and subcooled liquid nitrogen (LN₂) in the laminar flow range. The results revealed advantages of SlN₂ over subcooled LN₂ in natural convection cooling. The local temperatures of the heated surface surrounded by solid nitrogen particles are measured to increase at much slower rates than in subcooled LN₂, which is due to the latent heat of fusion of solid nitrogen. Even after the solid nitrogen particles surrounding the heater are apparently depleted, the average heat transfer coefficients for SlN₂ are still found to be greater than those for LN₂ with the improvement in heat transfer being larger for lower Grashof number regime. Our analysis also indicates that solid nitrogen particles in close proximity to heated surface do not discourage local convection due to the porous nature of SlN₂, making the heat transfer in SlN₂ more effective than in the case of solid–liquid phase change of nitrogen involving melting and conduction processes.     

Numerical study of cryogenic micro-slush particle production using a two-fluid nozzle

The fundamental characteristics of the atomization behavior of micro-slush nitrogen (SN₂) jet flow through a two-fluid nozzle was numerically investigated and visualized by a new type of integrated simulation technique. Computational fluid dynamics (CFD) analysis is focused on the production mechanism of micro-slush nitrogen particles in a two-fluid nozzle and on the consecutive atomizing spray flow characteristics of the micro-slush jet. Based on the numerically predicted nozzle atomization performance, a new type of superadiabatic two-fluid ejector nozzle is developed. This nozzle is capable of generating and atomizing micro-slush nitrogen by means of liquid-gas impingement of a pressurized subcooled liquid nitrogen (LN₂) flow and a low-temperature, high-speed gaseous helium (GHe) flow. The application of micro-slush as a refrigerant for long-distance high-temperature superconducting cables (HTS) is anticipated, and its production technology is expected to result in an extensive improvement in the effective cooling performance of superconducting systems. Computation indicates that the cryogenic micro-slush atomization rate and the multiphase spraying flow characteristics are affected by rapid LN₂-GHe mixing and turbulence perturbation upstream of the two-fluid nozzle, hydrodynamic instabilities at the gas-liquid interface, and shear stress between the liquid core and periphery of the LN₂ jet. Calculation of the effect of micro-slush atomization on the jet thermal field revealed that high-speed mixing of LN₂-GHe swirling flow extensively enhances the heat transfer between the LN₂-phase and the GHe-phase. Furthermore, the performance of the micro-slush production nozzle was experimentally investigated by particle image velocimetry (PIV), which confirmed that the measurement results were in reasonable agreement with the numerical results.     

Influence of bubble size and flow velocity on AC electrical breakdown characteristics of LN2

To evaluate insulation design of high voltage for high T_c superconducting (HTS) power apparatus, the effect of bubble and electrode arrangements on the AC breakdown characteristics of LN₂ were investigated. Supposing that an outbreak of quench, the three electrode models were employed for breakdown voltage measurement in LN₂ with bubbles. Experimental results for various quench conditions revealed that the breakdown voltage of LN₂ with increasing the bubble size, flow velocity and electrode distance. Then, the relationship between the bubble conditions and the AC breakdown characteristics of LN₂ were clarified. Also, bubble movement phenomena were observed with an electrode model which consist of plane to cylindrical needle electrode immersed LN₂ for the simulation of the insulation environment in HTS pancake type coils and others of the quenching state.     

Image reconstruction algorithm based on the extended regularised total least squares method for electrical capacitance tomography

Electrical capacitance tomography (ECT) is a non-invasive imaging technology that aims at the visualisation of the cross-sectional permittivity distribution of a dielectric object based on the measured capacitance data. Successful applications of ECT depend greatly on the precision and speed of the image reconstruction algorithms. ECT image reconstruction is a typical ill-posed problem, and its solution is unstable, that is, the solution is sensitive to noises in the input data. Methods that ensure the stability of a solution while enhancing the quality of the reconstructed images should be used to obtain a meaningful reconstruction result. An image reconstruction algorithm based on the regularised total least squares (TLS) method that considers the errors in both the sensitivity field matrix and the capacitance data for ECT is presented. The regularised TLS method is extended using a combination robust estimation technique and an extended stabilising functional according to the ill-posed characteristics of ECT, which transforms the image reconstruction problem into an optimisation problem. In addition, the Newton algorithm is employed to solve the objective functional. Numerical simulations indicate that the algorithm is feasible and overcomes the numerical instability of ECT image reconstruction; for the cases of the reconstructed objects considered here, the spatial resolution of the reconstructed images obtained using the algorithm is enhanced; as a result, an efficient method for ECT image reconstruction is introduced.     

一种电容层析成像图像重建优化算法

提出一种电学层析成像(ECT)图像重建优化算法。通过将传统正则化算法转化为最小二乘问题进行求解,结合lp范数逼近正则化最小化问题,利用重新加权的方法进行迭代计算。以油-气两相流模型进行仿真及静态实验,将所提出的优化算法与常用的LBP、Landweber迭代及Tikhonov正则化算法进行对比。结果表明,与常用算法相比,采用该优化算法对管道中心物体及多物体分布流型进行图像重建,其图像相对误差均为最低,且重建图像的形状保真度明显提高。     

Optical properties of tetragonal germanium nanocrystals deposited by the cluster-beam evaporation technique: New light emitting material for future

The germanium (Ge) nanocrystals were deposited on substrates whose temperature was kept at room or liquid nitrogen (LN₂) temperature by the cluster-beam evaporation technique. The deposited films are found to consist of the tetragonal crystalline structure rather than the diamond structure of bulk Ge. Such a phase-transition has been theoretically predicted for sizes smaller than 4 nm, which agrees with the size measured by the transmission electron microscopy (TEM). The tetragonal Ge is expected to have a direct band gap of 1·47 eV. Furthermore, the Ge film deposited at LN₂ temperature exhibits unique properties, such as photo-oxidation and blue-light emission. The Ge-nanocrystal films deposited by the cluster-beam evaporation technique are attractive materials for application to light emitting devices in future.     

Cryogenic system with the sub-cooled liquid nitrogen for cooling HTS power cable

A 10 m long, three-phase AC high-temperature superconducting (HTS) power cable had been fabricated and tested in China August 2003. The sub-cooled liquid nitrogen (LN₂) was used to cool the HTS cable. The sub-cooled LN₂ circulation was built by means of a centrifugal pump through a heat exchanger in the sub-cooler, the three-phase HTS cable cryostats and a LN₂ gas-liquid separator. The LN₂ was cooled down to 65 K by means of decompressing, and the maximum cooling capacity was about 3.3 kW and the amount of consumed LN₂ was about 72 L/h at 1500 A. Cryogenic system design, test and some experimental results would be presented in this paper.     

Cryogenic Drilling of Ti–6Al–4V Alloy Under Liquid Nitrogen Cooling

This article is focused on experimental study of the effects of cryogenic liquid nitrogen (LN₂) coolant during drilling of Ti–6Al–4V alloy material with three different levels of cutting speed (V_c) and feed rate (f) at a constant depth. Cutting temperature (T), thrust force (F_z), torque (M_z), surface roughness (R_a), and hole quality are the output responses investigated by using cryogenic LN₂ coolant compared with a wet coolant. Tool wear and chip morphology were examined with the changes in cryogenic LN₂ coolant. It is found that cryogenic LN₂ coolant results in lowering cutting zone temperature which helps more removal of heat from the cutting zone. Lower thrust forces and surface roughness were observed due to less friction and better chip breaking in cryogenic LN₂ condition. Also better chipping results in improvement in hole quality, viz., circularity and cylindricity in cryogenic LN₂ condition. Less serration and uniform segmentation results in better chip morphology and no damage to the cutting inserts resulted in improved tool life in cryogenic LN₂ condition. The main application of cryogenic LN₂ coolant in the cutting zone provides better lubrication and is more effective than wet coolant. The effects of this investigation show that cryogenic LN₂ coolant is an alternative approach for a wet coolant in the drilling process.     

Experimental Investigations on Cryogenic Cooling in the Drilling of Titanium Alloy

In this study, a drilling experiment was conducted on titanium ASTM B265 Grade 2 material using PVD coated carbide inserts. Two types of coolants (Wet and LN₂) were used. The variables in the experiment were feed rate (f) and cutting speed (V_c). The depth of the drilling was constant. Cutting temperature (T), thrust force (F_t), surface roughness (R_a), and the hole quality (circularity, cylindricity, and perpendicularity) were analyzed. The tool wear and chip morphology were studied. The result of the experiment indicates that there is 6–59% reduction in cutting temperature when LN₂ is used, high thrust force values were recorded for LN₂ coolant condition, surface roughness (R_a) values were higher for LN₂ coolants. Hole quality is not favorable in LN₂ coolant supply.     

Nano/micro-structuring of oxide thin film under SHI irradiation

In the present work, we have studied the nano/micro-patterning of the surface of NiO thin films on different substrates (SiO₂, Si and Al) using 100 MeV Ag ions at LN₂ temperature and at an incidence angle of 75^° with the beam axis. The surface morphology of the irradiated surface is observed by Atomic force microscopy (AFM). AFM images of ion beam irradiated samples show the restructuring of initially flat and coherent NiO film into an almost periodic NiO lamellae structure. The quite regular lamellae with width, height and average distance of hundreds of nm are oriented perpendicular to the beam direction. Section analysis of the AFM images reveal that the width of the lamellae is less in case of NiO films deposited on SiO₂ substrate in comparison to Al substrate. The cracking and the development of lamellae structure is observed at higher fluence in the case of Al substrate in comparison to other substrates.     

Nano/micro-structuring of oxide thin film under SHI irradiation

In the present work, we have studied the nano/micro-patterning of the surface of NiO thin films on different substrates (SiO₂, Si and Al) using 100 MeV Ag ions at LN₂ temperature and at an incidence angle of 75^° with the beam axis. The surface morphology of the irradiated surface is observed by Atomic force microscopy (AFM). AFM images of ion beam irradiated samples show the restructuring of initially flat and coherent NiO film into an almost periodic NiO lamellae structure. The quite regular lamellae with width, height and average distance of hundreds of nm are oriented perpendicular to the beam direction. Section analysis of the AFM images reveal that the width of the lamellae is less in case of NiO films deposited on SiO₂ substrate in comparison to Al substrate. The cracking and the development of lamellae structure is observed at higher fluence in the case of Al substrate in comparison to other substrates.     

Cool-down studies on vacuum insulated cryogenic transfer lines

Analytical and experimental studies on the cool-down of vacuum insulated cryogenic transfer lines have been carried out. An expression for estimating the cool-down history is derived taking into account the gas conduction effects. Experiments were carried out for LN₂ flow in glass lines. The cool-down time decreases with increasing vacuum up to 10^?2 torr. Beyond this there is no significant change in cool-down time. The outer jacket size has no appreciable influence on the cool-down process. Higher mass flow rates result in faster cooling of the pipe line. The experimental and theoretical cool-down curves agree well, particularly at higher jacket vacuums and at higher mass flow rates.     

Design and development of a helium injection system to improve external leakage detection during liquid nitrogen immersion tests

The testing of assemblies for use in cryogenic systems commonly includes evaluation at or near operating (therefore cryogenic) temperature. Typical assemblies include valves and pumps for use in liquid oxygen-liquid hydrogen rocket engines. One frequently specified method of cryogenic external leakage testing requires the assembly, pressurized with gaseous helium (GHe), be immersed in a bath of liquid nitrogen (LN₂) and allowed to thermally stabilize. Component interfaces are then visually inspected for leakage (bubbles). Unfortunately the liquid nitrogen will be boiling under normal, bench-top, test conditions. This boiling tends to mask even significant leakage.One little known and perhaps under-utilized property of helium is the seemingly counter-intuitive thermodynamic property that when ambient temperature helium is bubbled through boiling LN₂ at a temperature of −195.8 °C, the temperature of the liquid nitrogen will reduce.This paper reports on the design and testing of a novel proof-of-concept helium injection control system confirming that it is possible to reduce the temperature of an LN₂ bath below boiling point through the controlled injection of ambient temperature gaseous helium and then to efficiently maintain a reduced helium flow rate to maintain a stabilized liquid temperature, enabling clear visual observation of components immersed within the LN₂. Helium saturation testing is performed and injection system sizing is discussed.     

GPU Computing based fast discrete wavelet transform for l1-regularized SPIRiT reconstruction

Parallel imaging is a technique to shorten the acquisition time by reducing the data size in phase encoding direction. Compressed Sensing is a technique to improve the performance of parallel imaging based reconstruction methods such as l₁-regularized SPIRiT by adding the regularization term, which leads to frequent calculations of Discrete Wavelet Transform (DWT) with high time cost. However, clinical practice of MRI scan requires fast or real-time reconstruction with high image quality. In this paper, by taking advantage of the properties of parallel imaging and GPU computing, we develop a fast three-dimensional DWT for parallel imaging based reconstruction methods such as l₁-regularized SPIRiT. Computational results show that fast DWT in l₁-regularized SPIRiT MRI reconstruction is approximately three times faster than the conventional DWT. Computational results also show that fast DWT for reconstructing an 80?×?150?×?32?×?80 Cardiac MRI dataset by l₁-regularized SPIRiT is approximately 20 per cent faster than l₁-regularized SPIRiT of the conventional DWT.     

Investigation of cryogenic level sensors for LN2 and LOX

Investigations of two different types of cryogenic level sensors (capacitance and High Temperature Superconductor (HTS) for level measurement of liquid nitrogen (LN₂) and liquid oxygen (LOX) are presented here. They were tested for an active length of 400 mm in LOX and LN₂. A discrete diode array level sensor was used as a primary standard for calibrating these sensors. Comparative studies on linearity, sensitivity and other parameters at the operating temperatures are presented.     

Effect of the connection gap on the heat-load characteristics of a liquid nitrogen bayonet coupling

A transfer system for liquid nitrogen (LN₂) installed at National Synchrotron Radiation Research Center (NSRRC) to provide LN₂ required for the superconducting equipment and experimental stations has a LN₂ transfer line of length 160 m and pipeline of inner diameter 25 mm, a phase separator (250 L) and an automatic filling station. The end uses include two cryogenic systems, one Superconducting Radio Frequency (SRF) cavity, five superconducting magnets, monochromators for the beam line and filling of mobile Dewars. The transfer line is segmented and connected with bayonet couplings. The aim of this work was to investigate, by numerical simulation, the effects on the heat load of the gap thickness of the bayonet assembly and the thickness of vacuum insulation. A numerical correlation was created that has become a basis to minimize the head load for future design of bayonet couplings.     

Investigation of cryogenic cooling effect in reaming Ti-6AL-4V alloy

This research article is based mainly on the investigation of the effect of cryogenic machining, while reaming Titanium grade 5 alloy (Ti-6Al-4V) material. Cutting speed (V_c) and feed rate (f) are two input parameters at three different levels for a constant depth of the hole. The output parameters considered by using a cryogenic LN₂ cooling compared to a conventional flood cooling are torque (M_t), thrust force (F_t), cutting temperature (T), quality of the hole (circularity and cylindricity), surface roughness (R_a) and chip morphology. The results show cryogenic liquid nitrogen (LN₂) cooling resulting in 15–31% reduction in the cutting temperature, 23–57% reduction in the thrust force and 14–65% reduction in torque. Higher surface roughness, circularity (Cir) and cylindricity (Cyl) were observed in the cryogenic LN₂ cooling condition. Furthermore, better chip breakability was observed in the cryogenic LN₂ cooling condition. No drastic change in the microstructure was observed in both flood and cryogenic LN₂ cooling. Increase in microhardness by 10–16% and 8–19% in cryogenic LN₂ cooling over flood cooling was observed.     

Installation and commissioning of a cryogen distribution system for the TPS project

A cryogen distribution system was installed and commissioned to transfer liquid nitrogen (LN₂) and liquid helium (LHe) from storage dewars to superconducting radio-frequency (SRF) cavities for the 3-GeV Taiwan Photon Source (TPS) project. The cryogen distribution system comprises one distribution valve box (DVB), four control valve boxes (CVB) and seven sections of multichannel transfer line (MCL). The DVB distributes the LHe and LN₂ to the CVB, and then to the SRF cavities through independent vacuum-jacketed transfer lines. The vaporized GHe and GN₂ from the cryomodules are collected via the MCL. The cryogen distribution system was installed and commissioned from October 2014 to the end of March 2015. This paper presents the installation, pre-commissioning and commissioning of the cryogen distribution system, and describes the heat load test. Thermal acoustic oscillation (TAO) was found in the GHe process line; this phenomenon and its solution are also presented and discussed.     

电容层析成像三维图像重建研究

进行了电容层析成像(ECT)三维图像重建的仿真研究。首先利用COMSOL软件对ECT传感器进行三维建模,基于其高精度的有限元求解,计算ECT的灵敏度矩阵;其次,基于Matlab软件实现了基于线性反投影(LBP)及Landweber迭代算法的ECT图像重建,并利用图像显示程序获得了三维重建图像;最后,对球体及圆柱体模型进行了三维ECT图像重建,获得了较好的重建结果。