Cool-down Rates Research Articles

A new experiment to enable rapid systematic investigations of flux trapping dynamics for superconducting radio-frequency cavity applications.

Many modern accelerators rely on superconducting radio-frequency (SRF) cavities to accelerate particles. When these cavities are cooled to the superconducting state, a fraction of the ambient magnetic field (e.g., Earth's magnetic field) may be trapped in the superconductor. This trapped flux can significantly increase the power dissipation of the SRF cavities. It is, therefore, crucial to understand the underlying mechanism of how magnetic flux is trapped and what treatments and operating conditions can reduce the flux-trapping efficiency. A new experiment was designed that enables a systemic investigation of flux trapping. It allows for independent control of cooldown conditions, which might have an influence on flux trapping: temperature gradient across the superconductor during cooldown, cooldown rate, and ambient magnetic field. For exhaustive studies, the setup was designed for quick thermal cycling, permitting up to 300 superconducting transitions in one day. In this paper, the setup and operation is described in detail and an estimation of the measurement errors is given. Exemplary data are presented to illustrate the efficacy of the system.

Experimental study and infrared detector application research of micro miniature refrigerators

The MMR（Micro Miniature Refrigerator）is a novel Joule-Thomson cryocooler manufactured by micro-machining technologies, its axial length is significantly shorter than traditional Joule-Thomson cryocoolers commonly employed in infrared detectors. MMRs can greatly reduce the size of infrared detectors upon successful implementation. However, contemporary MMR products encounter challenges such as relatively low cool-down rates, cooling power, and structural strength. To address these issues and enhance the cool-down performance of the MMR for effective application in infrared detectors, a calculation model describing flow and heat transfer besides working characteristics of the MMR is proposed and verified. MMR prototypes are fabricated and experimentally studied. Building upon theoretical analysis and experimental findings cooling performance enhancement methods including transitioning the MMR material from glass to metal and modifying the structure and channel patterns of the MMR are introduced, the cooling performance of the MMR is thus greatly improved. Furthermore, an integrated design incorporating an MMR into an infrared detector is proposed, the axial length of this infrared detector is reduced by 65.3 % compared to conventional infrared detectors. And the MMR is further enhanced to adapt the working conditions in infrared detectors, 60.9 MPa working pressure and 38 s cool-down time is achieved, the cool-down requirements of infrared detectors is satisfied. Notably, the proposed MMR and infrared detector design in this paper exhibit technical advantages over similar products reported in the literature.

An Algorithm for Modeling Thermoplastic Spherulite Growth Using Crystallization Kinetics.

Crystallization kinetics were used to develop a spherulite growth model, which can determine local crystalline distributions through an optimization algorithm. Kinetics were used to simulate spherulite homogeneous nucleation, growth, and heterogeneous nucleation in a domain discretized into voxels. From this, an overall crystallinity was found, and an algorithm was used to find crystallinities of individual spherulites based on volume. Then, local crystallinities within the spherulites were found based on distance relative to the nucleus. Results show validation of this model to differential scanning calorimetry data for polyether ether ketone at different cooldown rates, and to experimental microscopic images of spherulite morphologies. Application of this model to various cooldown rates and the effect on crystalline distributions are also shown. This model serves as a tool for predicting the resulting semi-crystalline microstructures of polymers for different manufacturing methods. These can then be directly converted into a multiscale thermomechanical model.

Investigation of Friction Stir Welding of Additively Manufactured Biocompatible Thermoplastics Using Stationary Shoulder and Assisted Heating.

Additive manufacturing (AM), also known as 3D printing, offers many advantages and, particularly in the medical field, it has stood out for its potential for the manufacture of patient-specific implantable devices. Thus, the unique properties of 3D-printed biocompatible polymers such as Polylactic Acid (PLA) and Polyetheretherketone (PEEK) have made these materials the focus of recent research where new post-processing and joining techniques need to be investigated. This study investigates the weldability of PLA and PEEK 3D-printed plates through stationary shoulder friction stir welding (SS-FSW) with assisted heating. An SS-FSW apparatus was developed to address the challenges of rotating shoulder FSW of thermoplastics, with assisted heating either through the shoulder or through the backing plate, thus minimizing material removal defects in the welds. Successful welds revealed that SS-FSW improves surface quality in both PLA and PEEK welds compared to rotating shoulder tools. Process parameters for PLA welds are investigated using the Taguchi method, emphasizing the importance of lower travel speeds to achieve higher joint efficiencies. In PEEK welds, the heated backing plate proved effective in increasing process heat input and reducing cooldown rates which were associated with higher crystallinity PEEK. Despite these findings, further research is needed to improve the weld strength of SS-FSW with these materials considering aspects like tool design, process stability, and 3D printing parameters. This investigation emphasizes the potential of SS-FSW in the assembly of thermoplastic materials, offering insights into the weldability of additively manufactured biocompatible polymers like PLA and PEEK.

First Results of Nb3Sn Coated Cavity by Vapor Diffusion Method at SARI

Nb3Sn is emerging as one of the focal points in superconducting radio frequency (SRF) research, owing to its excellent superconducting properties. These properties hold significant possibilities for cost reduction and the miniaturization of accelerators. In this paper, we report the recent efforts of the Shanghai Advanced Research Institute (SARI) in fabricating high-performance Nb3Sn superconducting cavities using the vapor diffusion method. This includes the construction of a Nb3Sn coating system with dual evaporators and the test results of 1.3 GHz single-cell coated cavities. The coated samples were characterized, and the growth state of the Nb3Sn films was analyzed. The first coated superconducting cavity was tested at both 4.4 K and 2 K, with different cooldown rates passing through the Nb3Sn critical temperatures. The causes of Sn droplet spot defect formation on the surface of the first cavity were analyzed, and such defects were eliminated in the coating of the second cavity by controlling the evaporation rate. This study provides a reference for the preparation of high-performance Nb3Sn-coated cavities using the vapor diffusion method, including the setup of the coating system, the comprehension of the vapor diffusion process, and the test conditions.

Magneto-thermal limitations in superconducting cavities at high radio-frequency fields

The performance of superconducting radio-frequency Nb cavities at high radio-frequency (rf) fields in the absence of field emission can be limited by either a sharp decrease of the quality factor Q0(Bp) above peak surface magnetic fields Bp ∼100 mT or by a quench. We have measured Q0(Bp) at 2 K of several 1.3 GHz single-cell Nb cavities with different grain sizes, and with different ambient magnetic fields and cooldown rates below the critical temperature. Temperature mapping and a novel magnetic field mapping systems were used to find the location of “hot-spots” and regions of trapped magnetic flux. The use of a variable input coupler allowed further exploring the dissipative state. The results showed a remarkable thermal stability in some cavities with up to 200 W of rf power dissipation at 2 K, whereas other cavities quenched at much lower rf power. We observed a narrow distributions of the onset fields of hot-spots which were not affected by thermal cycling or by conditions which favor the formation of Nb hydrides. Furthermore, a poor correlation was found between the location of hot-spots and trapped vortices. We suggest that the totality of our experimental data can be explained by a sharp increase of the residual surface resistance above 120–140 mT due to the field-induced breakdown of a proximity-coupled metallic suboxide layer at the surface.

RELAP5/mod3.3 Analysis of Natural Circulation Cooldown with One Inactive Loop for Nuclear Power Plant Krško (NEK)

The paper presents the RELAP5/mod3.3 analysis of natural circulation cooldown with one inactive loop for Nuclear Power Plant Krško (NEK). The aim of the analysis is to determine the limiting cooldown rates during operator recovery actions to minimize the effect of flow stagnation in inactive loop. Since this is typical asymmetrical transient, the RELAP5/mod3.3 NEK model with split reactor vessel model was developed (models of the reactor vessel and core were axially divided in two parts) and used for this analysis. The several transients of cooldown, with one inactive loop, for different time after shutdown (different decay heat) were performed. The extreme conservative assumptions were applied for the analyses, i.e. the complete loss of feedwater (FW) and auxiliary feedwater (AF), including turbine driven (TD) AF pump, and the cooldown has started after the SG is completely dry (inactive). The analyses show that the cooldown rate shall be significantly reduced, and, based on the results the procedure ES-0.2 “Natural Circulation Cooldown” was modified.

Methods of speeding up the cool-down of superconducting magnets that are cooled using Small coolers at temperatures below 30 K

The two primary ways of cooling superconducting magnets with small coolers are by conduction from the magnet to one or more the coolers cold heads through high thermal conductivity straps or by fluid flow through a thermal siphon cooling loop. Both methods have been used for cooling down magnets. The thermal siphon cooling loop can also be used to produce liquid helium or liquid hydrogen within the cooling loop. Over 85 percent of the cooldown time is used to cool a magnet coil from 300 K to 80 K. This is also true when a helium refrigerator is used to cool down a magnet. The primary reason for this is the magnet material enthalpy at 80 K is between 4 and 6 percent of the material enthalpy at 300 K. The cooler refrigeration increases with the cold head temperature, but the magnet material enthalpy increases more. If one wants to speed up the cool-down rate, one must remove the heat faster in the temperature range between 80 K and 300 K. Two primary methods for speeding up the cooldown of magnet that is kept cold using coolers will be presented.

Analytical approach to estimate the thermal stress distribution of reactor pressure vessel nozzle corners with a constant cooldown rate

Concerns about the integrity of the nozzle corners of reactor pressure vessels have been raised due to the long-term operation of nuclear power plants. Because ASME BPVC Section XI Appendix G (2013 edition) provided a stress intensity factor solution for nozzle corners, little progress has been made in relevant research. Using the Finite Element (FE) method for stress analyses appears to be successful; nevertheless, FE analyses of three-dimensional models are still computationally expensive. Therefore, this study develops a simplified approach to estimate the thermal stress distribution of nozzle corners. The thermal load is assumed to be caused by a change in the coolant temperature during normal cooldown operation. The cross-section temperature gradients on the nozzle corners are predicted based on the geometric shapes and through-wall temperature distributions of a cylinder and a semi-infinite plate. Thereafter, the cross-section thermal stress distributions are estimated based on the predicted temperature gradients; using the theory of circular plates and cylindrical shells. Three-dimensional FE analyses are carried out to validate the proposed approach. Six different nozzle geometries are considered based on pressurized water and boiling water reactor designs. The comparisons show that the estimates of the temperature and thermal stress distributions are in good agreement with the FE results. In addition, differences among the stress intensity factors calculated in accordance with ASME BPVC Section XI Appendix G are discussed.

Numerically investigated on the transient performance of fast cool-down J-T cryocoolers with non-isometric structure

To achieve fast cool-down is important for the miniature Joule-Thomson (J-T) cryocooler in infrared detectors and other applications. A novel non-isometric helically coiled heat exchanger, with a first-sparse-then-dense type structure, was adopted in this study to improve the cool-down performance of miniature J-T cryocoolers. A transient model that takes real gas characteristics, heat leakage and compressible flow into account, was established and verified. The transient thermodynamic behaviors (temperature, internal heat load, mass flow rate and cooling capacity) of J-T cryocoolers with the typical, first-dense-then-sparse and first-sparse-then-dense type structure were analyzed and compared to obtain the better performance. It can be concluded that the first-sparse-then-dense structure can increase the cool-down rate by 13.4% with a smaller structural mass. Besides, different operating conditions with charging pressure and cylinder capacity were also simulated and analyzed. The results show that the first-sparse-then-dense structure always has the fastest cool-down rate in any operating condition. The simulation results can be used for the accurate prediction of system performance and structural design.

Experimental investigation on the influence of ejector geometry on the pull-down performance of an ejector-enhanced auto-cascade low-temperature freezer

• Effects of ejector geometry on EARC based low-temperature freezer were explored. • D t and L m are the critical parameters influencing the pull-down performances. • Malfunction of pressure lift at start-up phase was prone to occur at small D m . • NXP seriously affects ejector performance instead of the pull-down rate Employing an ejector to recover the expansion work of the auto-cascade refrigeration cycle is a feasible method to improve the system performance. The system operation characteristics are closely relevant to the ejector geometry parameters. In order to obtain the critical structure parameters influencing the freezer's pull-down performance, experimental research was conducted on an ejector-enhanced auto-cascade refrigeration cycle applied in a low-temperature freezer. The impacts of the ejector nozzle throat, mixing chamber diameter and length, and the nozzle exit position on the system's pull-down and steady operation characteristics were explored. The experimental results illustrated that the cooling rate and the attainable freezing temperature were mainly influenced by the nozzle throat and mixing chamber length instead of the nozzle exit position and mixing chamber diameter. The nozzle throat diameter of 0.52 mm and the mixing chamber length of 25 mm was optimal concerning the fastest cool-down rate and lowest freezing temperature of -61.2 °C. At the early phase of the pull-down process, a small mixing chamber diameter would cause the ejector malfunction of the pressure lift. There was a worst nozzle exit position of slowing down the pull-down speed, rising the freezing temperature, and reducing the system COP and exergy efficiency at the given operations. The ejector yielded the maximum pressure lift ratio of 1.196 and the entrainment ratio of 0.523 at the optimal ejector geometries. This work would be helpful to guide the ejector structure optimization for the ejector-enhanced auto-cascade low-temperature freezers.

Modeling the warm-up process for hydraulic actuator components

This paper discusses warm-up processes for the hydraulic system components of road construction machinery using mathematical and physical simulations. The efficiency of these processes is governed by warm-up and cool-down rates of the hydraulic actuator components. It studies how various hydraulic system components interface as they warm up. The general task is to estimate the dynamic behavior of the components and loops comprising such components and the entire warm-upsystem. The models of the transient heat flow process of the warm-up system are based on the heat balance equation of the flowing medium (of the heat transfer fluid – a hydraulic fluid) within the control volume. For experimental verification, the hydraulic actuator components warm-up modes using the warmed-up hydraulic fluid from the hydraulic tank have been tested on the hydraulic test bench that we developed. The example of consecutive warm-up of the hydraulic system components is given. Based on this, after the experimental findings were processed, correlation coefficients for different warm-up modes and types of relationships between the hydraulic actuator component temperatures and their warm-up times were determined.

Experimental research on a Joule-Thomson refrigeration cycle with mixture R170/R290 for −60 °C low-temperature freezer

This paper presents an experimental investigation on a Joule-Thomson cycle based low-temperature freezer for −60 ℃ refrigeration. Natural binary mixture ethane/propane (R170/R290) adopted in the freezer was proved feasible to get the low temperature below −60 ℃ refrigeration in the experimental research. The influences of the mass fraction ratio and ambient temperature on the system’s dynamic operation characteristics in the cool-down and on–off cycle processes were analyzed in detail. The results indicated that the concentration of the mixture significantly impacted system performance. The lowest freezing temperature, fastest cool-down rate, and minimum daily energy consumption of 3.612 kW h were obtained at the optimal R170 concentration of 35%. The excessively high concentration of the low boiling temperature component R170 could contribute to a drastic increase in the discharge pressure and discharge temperature, which reduced the compressor operation reliability. The compressor worked with a high compression ratio ranging from 21.8 to 26.8 in the selected operating conditions. Hence, reducing the operating pressure and compression ratio should be the possible directions to improve the low-temperature Joule-Thomson refrigerator’s performance working with R170/R290.

Boil-off gas balanced method of cool down for liquefied natural gas tanks at sea

Cooling down the cargo tanks of liquefied natural gas carriers (LNGC) prior to the ships arrival at discharge and loading ports follow various widely adopted operating procedures. The tank cool-down procedures typically followed cannot though be considered as best practice, because they consume considerable boil-off gas while reducing tank temperatures. An alternative tested method, described here, consumes considerably less liquefied natural gas (LNG) during the tank cool-down process, which is beneficial, particularly during the ballast voyages. In certain circumstances, LNGC consume liquid marine fuels so that they are able to preserve enough LNG heel to complete tank cool down at sea to required low temperatures before reloading can be commenced. The novel method devised for cooling down LNGC, particularly those fitted with membrane cargo tanks, at sea prior to arrival at a loading terminal involves much lower LNG heel consumption than conventional methods. The boil-off gas (BOG)-balanced tank-cool-down method applies continuous spraying, at very low rates, of the tanks with LNG extracted from the heel. This procedure enables the ship’s engines to consume all excess BOG without the need to pass some of it as waste for combustion in the gas combustion unit or steam dump. It also ensures that the LNG cargo tanks are maintained at stable and constant pressure and reduces the coolant LNG quantity consumed. The BOG-balanced tank-cool-down is straightforward to implement and monitor, simplifying tank pressure control. Test results demonstrate that tank cool-down rates of 4 to 5 ◦C/per hour can be maintained such that tank temperatures can be reduced from +30 to -130 ◦C within 37 hours. The method could work on LNGC with Moss-type tanks but is likely to be less effective as they are typically fitted with fewer tank spraying nozzles. Cited as : Kulitsa, M., Wood, D.A. Boil-off gas balanced method of cool down for liquefied natural gas tanks at sea. Advances in Geo-Energy Research, 2020, 4(2): 199-206, doi: 10.26804/ager.2020.02.08

Examining the effects of microencapsulated phase change materials on early-age temperature evolutions in realistic pavement geometries

This study examines experimentally and numerically the effects of microencapsulated phase change material (PCM) additions on temperature rise and cool-down rate, and the associated impact on reducing the risk of thermal cracking in concrete pavements. Specimens representative of a realistic pavement geometry were exposed to diurnal temperature cycling at early ages while internal temperature evolutions were monitored. In spite of the fact that the lower thermal conductivity of the PCM inhibited heat dissipation, the results showed the presence of PCMs can reduce considerably the temperature rise and cool-down rate in the first 24 h following placement provided the PCM melting temperature is chosen suitably. A transient one-dimensional thermal model of a pavement section suitably captured experimental temperature evolutions and thereby offers the ability to inform the design of concrete pavements containing PCMs that feature early-age thermal cracking resistance.

A steam or gas pressurizer effect on the system pressure characteristics for an integral pressurized water reactor

A pressurizer is a major component used to control a system pressure in a primary coolant system of a pressurized water reactor. To identify the system pressure characteristics on nitrogen gas-water and steam-water pressurizers for an integral pressurizer water reactor, an experimental study is carried out on the integral test facility, VISTA-ITL, which is a scaled down facility of the SMART plant. According to the experimental results, the pressurization rate and peak pressure are affected by the pressurizer type. The pressurization and depressurization rate with the steam pressurizer is smaller than the gas pressurizer since water has its inherent characteristic having latent heat while a nitrogen has ideal gas properties. The safety valve in both the gas and steam pressurizers is not opened during the transient. It is one of the characteristics of the integral reactor SMART design with a large pressurizer volume. The cooldown rate of the primary coolant system during the later part transient depends on the decay heat level regardless of the pressurizer type.

Preparation of Monolithic Silica Aerogel for Fenestration Applications: Scaling up, Reducing Cycle Time, and Improving Performance

The ability of a rapid supercritical extraction (RSCE) process implemented in an industrial hot press to produce silica aerogel monoliths (AMs) with properties suitable for use in fenestration products was investigated. The effects of hot-press processing parameters such as press force and heat-up and cool-down rates on the size (area) and quality (presence of cracks and optical flaws) of the AMs produced, and on the manufacturing cycle times required to produce them, were studied. AMs of 14 × 14 × 1.27 cm were produced in a 6.5 h RSCE process using a laboratory scale (267 kN) hot press. Evacuated window prototypes were prepared by packaging aerogels in the interspaces of a double-glazing system under vacuum. The aerogels were found to have high translucency (>80% transmittance) in the red portion of the visible spectrum. The double-glazing evacuated aerogel windows were found to have thermal resistivity of 1.21–1.55 K·m2/W.

AB stacked few layer graphene growth by chemical vapor deposition on single crystal Rh(1 1 1) and electronic structure characterization

Graphene synthesis on single crystal Rh(111) catalytic substrates is performed by Chemical Vapor Deposition (CVD) at 1000°C and atmospheric pressure. Raman analysis shows full substrate coverage with few layer graphene. It is found that the cool-down rate strongly affects the graphene stacking order. When lowered, the percentage of AB (Bernal) -stacked regions increases, leading to an almost full AB stacking order. When increased, the percentage of AB-stacked graphene regions decreases to a point where almost a full non AB-stacked graphene is grown.For a slow cool-down rate, graphene with AB stacking order and good epitaxial orientation with the substrate is achieved. This is indicated mainly by Raman characterization and confirmed by Reflection high-energy electron diffraction (RHEED) imaging. Additional Scanning Tunneling Microscopy (STM) topography data confirm that the grown graphene is mainly an AB-stacked structure. The electronic structure of the graphene/Rh(111) system is examined by Angle resolved Photo-Emission Spectroscopy (ARPES), where σ and π bands of graphene, are observed. Graphene's ΓK direction is aligned with the ΓK direction of the substrate, indicating no significant contribution from rotated domains.

Reproducible operating margins on a 72 800-device digital superconducting chip

We report the design and test of reciprocal quantum logic shift-register yield vehicles consisting of up to 72 800 Josephson junction devices per die, the largest digital superconducting circuits ever reported. Multiple physical layout styles were matched to the MIT Lincoln Laboratory foundry, which supports processes with both four and eight metal layers and minimum feature size of 0.5 μm. The largest individual circuits with 40 400 junctions indicate large operating margins of ±20% on ac clock amplitude. In one case the data were reproducible to the accuracy of the measurement, ±1% across five thermal cycles using only the rudimentary precautions of passive mu-metal magnetic shielding and a controlled cool-down rate of 3 mK s−1 in the test fixture. We conclude that with proper mitigation techniques, flux-trapping is no longer a limiting consideration for very-large-scale-integration of superconductor digital logic.

Structural transformation and magnetic properties of Sm–Fe alloys with V doping

A systematical investigation was carried out on structure and magnetic properties in SmFe9−x V x (x = 0.4, 0.8, 1.2) compounds prepared by a single-roller quenching method. The high cool-down rate leads to metastable TbCu7 phase in the parent compound, which gradually transforms into equilibrium ThMn12 structure with V-doping content increasing. The Curie temperature increases from 470 to 590 K with V doping, which is consistent with the phase transformation. Surprisingly, simultaneous increase in both coercivity and remanence is resulted by V doping, reaching the highest value of 685 kA·m−1 and 44.8 × 10−3 A·m2·g−1 in x = 1.2 compound, respectively. This phenomenon can be explained by the combination of phase transformation and intergranular exchange coupling through δM-H plots.