查看更多>>摘要：The concept of a bending wavelength has been proposed in the description of strand deformation phenomena arising from electromagnetic loads， in cable-in-conduit conductor (CICC) with multistage twisted Nb3Sn cable， that result in the degradation of the CICC current sharing temperature (T-cs)。 A survey of CICC T-cs performance covering Nb3Sn CICC samples from the recent past two decades shows interesting results that can help relate bending wavelength to CICC parameters such as the void fraction (V-f) and the twist pitches of the CICC cable to which the latter has attributed to T-cs degradation according to prior research。 In this paper， a simple cable strand model is used to link CICC parameters especially the CICC V-f to the bending wavelength of the CICC strand。 Through this treatment， a range of bending wavelengths are identified that can be associated to characteristically different T-cs performance behaviour of the CICCs。 The bending wavelength values corresponding to transitions between these characteristically different CICC T-cs behaviours are identified as critical bending wavelengths。 Additionally， a picture of the internal mechanism resulting in T-cs change is presented along with comparisons of bending wavelength values derived from different theories and the value from destructive examination identified in the literature for Nb3Sn CICC samples。
查看更多>>摘要：At present， medium-scale helium cryogenic system is commonly based on modified Claude cycle with two expanders in series。 In this study， considering the minimum allowable heat exchanger temperature difference (MHTDamd)， the influence of key parameters (including compressor discharge pressure (Pcd) ， temperature at the inlet of expander1 (Tin-EXP1) and temperature at the inlet of Joule-Thomson valve (Tin-JT)) on the system performance (including refrigeration capacity (RC)/liquefaction rate (LR)， specific energy consumption (SEC) and total UA of the heat exchangers (UAtotal)) of the helium cryogenic system is investigated and analyzed in refrigeration and liquefaction modes， respectively。 The results show that in refrigeration and liquefaction modes， the optimal ranges of Pcd， Tin-EXP1， and Tin-JT are within 8 - 10 bar， 30 - 45 K and 5。2 K - 7。2 K， respectively。 Furthermore， the constraint relation between UAtotal and SEC is obtained。 By using genetic algorithm (GA)， it is found that keeping constraint SEC within 100%-115% of the chosen SEC from the optimal range can achieve a relatively economic trade-offs between SEC and UAtotal。 In the end， a comparison of T-S diagram obtained from design and experimental values in refrigeration and liquefaction modes is carried out， respectively。 The maximum temperature deviation is within 5%， which verifies the accuracy of the calculation models。 The optimized results in this study are also applicable to the same processes with mass flow rate of compressor lower than 100 g/s。
查看更多>>摘要：The 316LN stainless steel (SS) used for TF conduits in the ITER must undergo a series of pre-deformation during the fabrication。 In order to investigate the variation of mechanical properties of 316LN SS with different temperature and cryogenic pre-strain， the cryogenic pre-strained samples with 0%， 15%， 25%， and 35% pre-strain were prepared and stretched at room temperature (RT)， 77 K and 4。2 K， respectively。 The yield strength (YS)， ultimate tensile strength (UTS)， elongation and microhardness of the samples were measured。 Besides， the Scanning Electronic Microscopy (SEM) and Transmission Electronic Microscopy (TEM) were applied to observe the microstructure of 316LN SS。 The magnetic measurements for the samples were carried out to investigate the volume percent of phase transformation。 The results indicate that the YS， UTS and microhardness of 316LN SS are improved by means of cryogenic pre-strain and the decrease of elongation of pre-strained samples is different at different temperatures。 All 316LN samples show the ductile fracture。 The dislocation of samples promotes by cryogenic pre-strain and hinders by cryogenic temperatures。 The martensitic transformation occurs after all prestrained samples were stretched。 The volume percent of alpha' martensite is minimal at RT and increases dramatically at cryogenic temperatures (77 K and 4。2 K)。 The combination of dislocation density and martensitic transformation results in changes in the 316LN SS mechanical properties。
查看更多>>摘要：Spray cooling with liquid nitrogen has great advantages to achieve a cryogenic environment simulation system。 The nozzle number， the mass flow rate and the gas velocity have crucial impacts on the cooling efficiency and the high-precision temperature control in the system。 To gain insight into the multi-nozzle spray cooling perfor-mance， the small wind tunnel prototype with liquid nitrogen spray cooling was built， and the transient and steady-state cooling performance was experimentally studied。 The results show that during the transient cooling process， the temperature is lower when the region is closer to the spray field。 The temperature on the test plane near the spray has the biggest difference。 After flowing through two elbow pipes， the high uniformity of the temperature distribution is obtained。 The research of the steady state cooling shows that the average temperature gradually increases along the flow direction from the spray region， while the temperature uniformity is in the reverse order。 And the small deviation among the data on the later three planes means a good temperature uniformity within the wind tunnel except the spray region。 In addition， a lower gas velocity， a larger nozzle number and a higher injection pressure can enhance the cooling performance。 However， the temperature uni-formity becomes worse as the nozzle number increases。 The results could provide theoretical guidelines for the engineering application of the cryogenic spray cooling。
查看更多>>摘要：Subkelvin refrigeration refers to the refrigeration below 1 K and is widely used in the fields of condensed-matter physics， quantum computing， and astronomical observation。 Among the three conventional technologies， adsorption refrigeration， adiabatic demagnetization refrigeration， and dilution refrigeration， dilution refrigeration is the most popular with the advantages of lowest temperature down to several mK， continuous cooling， and relatively large cooling power。 The method was first proposed in the 1950s and the first complete system was built in 1960s， followed by a successful commercialization。 Since then， with the ever-increasing demands of low temperature physics research and due to its apparent advantages， it quickly becomes the most important technique to reach the millikelvin temperature range。 The lowest temperature achievable nowadays by a conventional dilution refrigeration is 1。75 mK， and largest cooling power at 100 mK is 2 mW。 In addition to the commercialization of dilution refrigeration， lots of other research， tailored to special applications， have also been done or going on， which include cold-cycle dilution refrigeration without room-temperature pumping system and zero-g dilution refrigeration for space applications， etc。 The cold-cycle dilution refrigerator on the ground， driven by two adsorption pumps， can reach a minimum temperature of 8 mK and the corresponding cooling power was 60 mu W at 110 mK。 For zero-g space applications， an open-cycle dilution refrigerator was successfully operated in space for 2。5 years with a cooling power of 0。1 mu W at 92。6 mK。 Closed-cycle dilution refrigerators for zero-g environment are still under investigation。 With a brief introduction on the thermodynamics of dilution refrigeration， this paper reviews the development of the conventional 3He-circulating dilution refrigerator， the 4Hecirculating dilution refrigerator， the cold-cycle dilution refrigerator on the ground， and dilution refrigerators for space。 Key issues related to non-conventional dilution refrigeration are pointed out with some discussions on their prospects。
查看更多>>摘要：Predicting the pressurant requirements is one of the key challenges for cryogenic propulsion systems。 In this context， a numerical model to simulate the tank pressurization that considers evaporation and condensation phenomena was developed and applied。 The novel solver combines a gradient-based phase change model with a weakly compressible multiphase solver of OpenFOAM based on the pressure implicit method with splitting of operator (PISO) algorithm。 To maintain a sharp interface the mass source terms are applied to the cells adjacent to the interface。 First， the model is validated against two analytical solutions: the one-dimensional phase change problem and secondly， the growth of a vapor bubble in a superheated liquid in the absence of gravity。 In a second step， the validated model was applied to a cryogenic pressurization experiment。 The measured pressure behavior could be confirmed with the numerical model being in a good approximation。 With the numerical model further insights into the physical behavior could be achieved。 The condensation and evaporation effects have a significant impact on the pressure development during and after the pressurization。 The mass flows due to phase change occurring at the vapor-liquid interface depend on interface location and time。 Directly at the wall， evaporation becomes dominant while condensation occurs at the center area of the liquid surface。
查看更多>>摘要：This article presents an investigation of the profitability of cryogenic power electronics at different cooling and ambient temperatures。 Thermodynamic fundamentals of low-temperature refrigeration processes are considered and the Carnot efficiencies of state-of-the-art refrigerators are evaluated in order to establish the necessary power loss reduction for energetic profitability of low-temperature to cryogenic power electronic systems down to 77 K。 In this context， special attention is paid to two loss contributions in a power electronic system which， based on investigations on active and passive components， show the greatest potential for loss reduction at low temperatures。 These are the on-state losses of Si and GaN transistors and the DC winding losses of inductors。 The analysis shows that over the entire low temperature range， the loss reduction in a cryogenic converter can hardly compensate for the electrical power required to provide the necessary cooling capacity when cooling against an ambient temperature of 300 K。
查看更多>>摘要：The aerodynamic sidewall heating is an inevitable phenomenon happening in a cryogenic propellant tank during a space mission。 The propellant tanks of the launch vehicle are designed as an integral part of the vehicle structure。 It is designed so that it has to withstand the internal fluid pressure and support the vehicle's static and dynamic launch loads。 The propellant state inside the propellant tank will be at their boiling temperature or subcooled condition so that chances of formation of stratification and self-pressurization are higher with minimal heat infiltration itself。 The propellant condition varies due to stratification， and it is essential to keep the state of propellant in a predefined state for the successful operation of the cryogenic engine。 This paper discusses about the mathematical modeling of the thermal stratification phenomenon in a cryogenic propellant tank。 A 1-D computer program is developed based on the available correlation of natural convection flow over a vertical flat plate to predict a double-walled cylindrical cryogenic storage tank's stratification parameters。 The model can predict stratification at various types of insulation such as foam， vacuum， MLI， and different cryogens; hydrogen， nitrogen， and oxygen。 The importance of gravity condition on the evolution of stratification is studied using the model。 Different gravity conditions ranging from 10-4 go to 10-1 go on stratification evolution are carried out。 Under 10-1 go， the time required for reaching the stratified layer to tank bottom in a cylindrical tank is obtained as 1347 s。 As the gravity value decreases further from 10-2 go to 10-4 go， the time required increases 1。59 times， 2。99 times， and 8。09 times。
查看更多>>摘要：Experimental setup for the thermal conductivity measurement of amorphous solids with the glass transition temperature below 250 K is presented。 This setup allows to heat the specimen to the melting point and then cool it fast to the glass transition temperature。 Thermal conductivity can be investigated in the temperature range from 5 K to 300 K。 Thermal conductivity of ethanol in amorphous and polycrystalline states was measured for the verification of the setup function。 The results of the measurements agree well with those presented in literature。
查看更多>>摘要：This paper investigates the effect of precooling stage application on steady-state performance of MR JT cryocooler。 Modelling is carried out using modified approach for nonprecooled MR JT cryocoler employing PengRobinson EoS。 This approach enables optimization of the MR composition for given precooling temperature。 The research shows that the use of precooling significantly improves the performance of MR JT systems。 In addition， it is investigated how precooling affects the optimum composition of MR。 It has been also shown that precooling temperature can be optimized in terms of systems COP。 This paper provides very clear insight on the effect and consequences of precooling application in MR JT systems。
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