查看更多>>摘要：Some cryogenic applications， such as for superconducting generators of wind power plants， require compact refrigerators。 Heat exchangers are the most bulky components within the cold box of a cryogenic refrigerator。 Thus， a reduction of their size would be advantageous for the development of a compact system。 Volume and performance of a heat exchanger depend strongly on the hydraulic diameter of the channels and of the efficiency of the heat transfer surface。 To improve the performance， a concept of aluminium 3D printed heat exchanger with optimised fin geometry has been developed。 A fin design with hydraulic diameters of 1。48 mm and 1。07 mm for the high and low pressure sides， respectively， has been realized。Two prototype aluminium heat exchangers with improved fins have been produced by 3D printing and studied within this project。 The accuracy of the geometry reproduction within the printing process has been investigated。 The heat exchangers have successfully withstood pressure tests。 The pressure drop has been measured at atmospheric and at higher pressure using helium and air as working fluids。 Furthermore， the heat transfer with a temperature difference of 40 K along the heat exchanger has been investigated using helium。 The Colburn and Darcy friction factors of designed and printed fin geometry have been calculated using Ansys CFX。 A full heat exchanger simulation model with consideration of longitudinal heat conduction through inner and outer walls has been developed in Python。 The results of the numerical simulations have been successfully validated against measurements。 Based on the obtained data， suggestions for an improved heat exchanger geometry for cryogenic applications have been worked out。
查看更多>>摘要：Feeding systems for railways include those which feed electric power via the overhead trolley line and those which supply power via the third rail installed on the ground。 Third rail systems require a large amount of electric current to operate electric railcars because the insulation distance is rather short and the voltage is limited to a low value。 To apply the superconducting cable to a railway with the third rail system， the superconducting cable was installed in an actual railway route to conduct the cooling test and current test。 It was confirmed that the system was capable of providing stable capacity for circulative cooling using the cryocooler and circulation pump。 In the cooling test for failure recovery， it was confirmed that the function of the system was restored when the power was turned on again after the power-off。 In the current test， it was confirmed that the superconducting cable with the two-layer structure was capable of carrying a current of 6 kA per one layer and had a sufficient capacity for operating trains。 In addition， the superconducting cable was installed along a railway route so that a part of a part of it crosses the track to determine that the cable is not affected by any external disturbance such as vibration and can be used for operation without problems。
查看更多>>摘要：We present a theoretical study， based on the mean-field theory， on magnetocaloric effect in both amorphous GdxCo1-x (0。16 <= x <= 0。25) and GdxFe1-x (x = 0。263 and 0。306) alloys in a temperature range up to Curie temperature and in magnetic fields up to 70 kOe。 Our calculations show a fair agreement with available experimental data which confirm ferrimagnetic order in these systems。 Both direct and inverse magnetocaloric effects are present。 For Gd0。16Co0。84， maxima of delta S-m are 1。32 and 1。62 J/kg K for magnetic field changes of 40 and 70 kOe respectively。 However， the peak values of delta S-m decrease as x increases。 Peak values of delta S-m are 0。64 and 0。82 J/K。kg for Gd(0。263)Fe(0。737 )and Gd0。306Fe0。694 respectively。 The Belov-Arrott plots， the universal curve， and temperature dependence of delta S-m for Gd-Co alloys， showed that the alloys exhibit a second order phase transition at their Curie temperatures。 The same conclusion is reached for Gd-Fe alloys。
查看更多>>摘要：In practical applications， the heat load changes caused by various factors require the pulsating heat pipe (PHP) to have good heat transfer performance and temperature response characteristics in different heating modes。 In order to understand the heat transfer performance of a hydrogen PHP in different heating modes， the temperature curves of the PHP in two heating modes (stepwise heating and sudden heating) are obtained experimentally。 The experimental results show that for a given heat load， the differences of the quasi-steady-state temperatures and the effective thermal conductivities between two heating modes are negligible。 It's concluded that the stable heat transfer performance of the PHP is reproducible in the two different heating modes。
查看更多>>摘要：Many optical devices can benefit from cryogenic cooling; however， the resolution of these devices suffers from microvibration from mechanical cryocoolers。 Unlike mechanical cryocoolers， sorption cryocoolers do not have any active moving parts and are essentially vibration-free。 Sorption cryocoolers are thermally driven and their performance is highly determined by the properties of the adsorbents employed in the compressors。 In this study， the thermodynamic and dynamic modelling of sorption compressors adopting Saran activated carbon， MOF-5 and HKUST-1 metal-organic frameworks as adsorbents are conducted to find suitable adsorbents to realize efficient compression。 It is found that a sorption compressor with MOF-5 as the adsorbent has the worst performance。 Saran carbon has the highest mass-flow rate。 And whether HKUST-1 or Saran carbon has higher efficiency depends on the operating conditions。 The effects of heat sink temperature and heater power on the performance of sorption compressors are also investigated。 The analysis method can be employed to choose suitable sorbent materials and operating conditions when designing sorption compressors。
查看更多>>摘要：High-temperature superconducting (HTS) pinning magnetic levitation (maglev) transit， with the advantages of passive stabilization， nearly resistance free and the ability to suspend in different directions， has attracted more and more attentions。 Though the USF (upper-suspension form) and SSF (side-suspension) form of HTS pinning maglev were widely studied and gradually applied in engineering， the weak loading capacity of the LSF (lower-suspension form) has been keeping it from application。 In this paper， a new field cooling method is proposed to increase the suspension force of the HTS pinning maglev system in LSF。 The vertical and lateral suspension force using the new proposed method was measured and analyzed by two rectangular three-seeded YBa2Cu3O7 bulks， as well as the LSF with the original field cooling method and the USF for comparison。 The result shows that， when the bulks move down in 5 mm， the vertical suspension force reaches to 207 N when the FCG (field cooling gap) is 5 mm， 70 N when the FCG is 15 mm。 The experimental data suggests that the new proposed field cooling method can increase the vertical suspension force by 32%， and also increase its stability。 With this new field cooling method， the loading capacity of the LSF can almost reach the USF， while the guiding capacity is far better than the USF。 This paper suggests that the LSF is a potential suspension form for the HTS pinning maglev with the new proposed field cooling method。 And this paper can provide insight and reference for different HTS pinning suspension form。
查看更多>>摘要：The clip-on extensometers are recommended by international standards (ASTM E 1450-16 or ISO 6892-4:2015) for tension testing of structural alloys at 4 K。 They are also frequently used to identify mechanical and electromechanical properties of superconducting wires or cables at cryogenic temperatures。 In this study， the time responses of clip-on extensometers during uniaxial tensile tests of the engineering materials at liquid helium temperature are analysed。 It turns out， that strong strain localization and self-excited vibrations of extensometers related to the plastic flow instability disturb the strain measurement。 These measurement artefacts may affect the identification of the mechanical parameters， such as the 0。2% proof stress or the total elongation。
查看更多>>摘要：In this part， a hybrid cryocooler composed of a four-stage Stirling-type pulse tube cryocooler (SPTC) and a Joule Thomson cryocooler (JTC) is worked out to verify the theoretical analyses carried out in Part A。 The heat loss on each precooling stage is calculated to evaluate the feasibility of thermal coupling between the SPTC and the JTC。 The JT compressors are tested and then the compression capacity is studied。 A bypass-accelerated-cooling approach is adopted to speed up the cool-down process。 The experimental results show that with He-4 in the SPTC and He-3 in the JTC， a no-load temperature of 1。36 K is reached when the three precooling temperatures are kept at 71。2 K， 40。5 K and 8。12 K， respectively。 The cooling capacity of 13。9 mW at 1。8 K is measured with the JT loop high pressure and the last stage precooling temperature being at 0。5 MPa and 8。1 K， respectively。 Satisfactory agreements are observed between theoretical and experimental studies。
查看更多>>摘要：Ships carrying liquefied natural gas (LNG) are generally propelled by dual-fuel engines that use boil-off gas (BOG) or diesel as fuel。 Fuelling the engine with pumped and vaporized LNG is operationally more convenient than compressing BOG。 BOG is usually reliquefied using reverse Brayton cycle (RBC) or Claude cycles。 In this paper， both these cycles are reconfigured to extract the cold of pumped LNG to reduce power consumption。 Steady-state simulations are done on process simulator Aspen HYSYS V8。6 (R)。 Capturing the cold from the LNG fuel improves the exergy efficiency of RBC and Claude reliquefiers from 37。8% to 57。5% and 48。5% to 75。6% respectively。 The surface area of heat exchangers in the Claude system is one-third that of RBC。 Mass flow through compressor and turbine in Claude system is one-fifth and one-ninth of RBC， while volume flow at turbine inlet is only one-fourteenth in the Claude system。 The evaluation shows that all equipment needs to be designed with sufficient operational flexibility to enable the system to extract cold effectively from the LNG fuel supply because it varies widely during the voyage。 Claude system with cold extraction from LNG fuel is highly compact and the most efficient reliquefier designed so far。
NSTL
Elsevier