Thermal Link Research Articles

Numerical Modeling of Thermal Link for Optical Cooler

Optical cooling is a novel research area for space applications, where materials are cooled through light interactions. This cooling is used to lower the temperature of a payload attached to a cooling element. To maximize cooling power, a thermal link is placed between the payload and cooling material, designed to conduct maximum heat while minimizing fluorescence transmission. This study investigates the performance of a tapered thermal link based on geometrical, operational, and material parameters, aiming to establish simplified mathematical correlations for optimizing the design. A validated numerical model was developed for analysis. Results show that the length and surrounding shell temperature significantly influence the thermal link’s performance, more so than the cross-sectional area, emissivity, and refractive index. Optical transmission decreased from 0.085% to 0.065% as length increased from 0.5 cm to 2.0 cm, stabilizing at 0.06%. Heat conduction dropped from 0.139 W to 0.132 W for lengths from 0.5 cm to 3.0 cm, while radiation load increased from 0.057 W to 0.085 W. A 240% rise in radiation load was observed with emissivity increases from 0.06 to 0.20.

Processing calibration data of low-temperature thermometer based on clustering algorithm.

The scarcity of cryogenic thermometers often stems from their high cost and lengthy lead times for calibration. Establishing an in-lab temperature calibration system is necessary to quickly make use of uncalibrated sensors or self-made sensors. This paper introduces a straightforward and high-accuracy thermometer calibration system. By employing copper screws as thermal links between the sensor platform and the cryogen-free refrigerator, temperature oscillation on the sensor platform is suppressed to a few millikelvins. In addition, this paper presents a data processing model based on clustering algorithms. These algorithms sort and group data based on distance, which is similar to human visual judgment of data. This paper discusses the parameter optimization process of the clustering algorithm to interpret the automated data process. The cryogenic temperature sensors calibrated by this system exhibited high accuracy, with relative errors of less than 1% compared to standard thermometers. Moreover, automatically processing calibration data from two uncalibrated thermometers takes just over 10min, highlighting the effectiveness of this calibration system.

Achieving high stability in Cryostat: A study on optimal thermal link parameters

Thermal link is an important carrier used to transfer the cooling capacity and suppress the temperature fluctuation in cryostat. To balance these two points, it is usually necessary to find the optimum thermal link parameters. This paper establishes a model for the cryocooler cold head-thermal link-second flange based on a cryostat. Utilizing the response surface method, response equations correlating thermal link parameters with the temperature and its fluctuations of the second stage flange are developed at the lowest temperature of cryocooler. Through dual-objective optimization of cooling capacity transfer and temperature fluctuations at the second flange, the optimal thermal link parameters are determined and experimentally validated based on predicted results. The experimental and predicted values show good agreement with an error of 2 %. The optimized thermal link led a minor temperature increase and a significant temperature fluctuation reduction, decreasing from 230mK at the cold head to 2.900mK at the second flange, achieving a 98.74 % reduction. Furthermore, compared with non-optimization structure, the optimization one has further lowered the temperature fluctuation at the second flange from 4.000mK to 2.900mK with 27.5 % improvement. These results show that the present methods are reliable and useful to help to realize highly stable low-temperature environment in cryostat.

Understanding thermal comfort expectations in older adults: The role of long-term thermal history

Understanding how long-term thermal history affects thermal comfort expectations in older adults (65+) has implications for designing energy-efficient spaces in a changing climate. A growing number of studies focus on thermal sensation/preference votes to represent the current thermal comfort expectations, often overlooking their limitations. This study, however, investigates how factors shaping long-term thermal history link to the current 65+ adults indoor thermal comfort expectations during exposure to heat, by focusing on the upper limit of thermally acceptable temperature range, represented by a self-reported temperature threshold at which 65+ adults believe to feel uncomfortable by indoor heat (Tit). To find Tit, we use answers to “Above what temperature do you start feeling too hot indoors?” by survey respondents in Warsaw (n = 678) and Madrid (n = 527), who lived in their apartment ≥5 years. Statistically, we find indoor factors affecting long-term thermal experiences more significant in explaining 65+ Tit, when compared to outdoor factors such as distance to water, vegetation, or surface thermal radiance. Better-insulated buildings were associated with a lower Tit (p < 0.05), suggesting a higher thermal sensitivity of such building residents to heat. Our key findings indicate for normal housing units’ area (∼80 m2), personal factors such as Basal Metabolic Rate (main effect 95% CI = +0.42 – +2.6 °C) and health status (+0.11 – +1.68 °C) were associated with a higher 65+ Tit and lower thermal sensitivities to heat (p < 0.05). Findings shed light on the complexity of factors shaping indoor thermal comfort expectations among 65+ adults.

Continuous Wave Mode Test of Conduction-Cooled Nb3Sn Radio Frequency Superconducting Cavities at Peking University

A liquid helium-free cryostat for radio frequency (RF) test of the superconducting cavity is designed and constructed. Gifford-Mcmahon (G-M) cryocoolers are used to provide cooling capacity, and the heat leakage at 4 K is less than 0.02 W. Vertical and horizontal tests of two Nb3Sn cavities are carried out in the cryostat with different surface treatments outside the cavities. Both of the cavities achieve stable continuous wave (CW) operation. A novel treatment, which cold-sprayed a 3.5 mm thick Cu layer onto the outside of the cavity, enables the maintenance of an average temperature of 5.5 K in the cavity at a RF loss of 10 W, implying that the thermal stability and uniformity of the cavity has been significantly improved. Through the synergistic control of four metal film resistors, a cooling rate of 0.06 K/min near 18 K is realized, and the cavity temperature gradient is reduced to 0.17 K/m, which effectively improves the RF performance of the cavity. The maximum Eacc of the cavity reaches 3.42 MV/m, and the Q0 is 1.1 × 109. An electromagnetic–thermal coupling simulation model for the superconducting cavity is established and is in good agreement with the experimental results. The simulation results show that the cavity with a Cu-spraying treatment and the thermal links of 5N Al can satisfy the Eacc of 10 MV/m under conduction cooling.

Enhancing Building Energy Efficiency: Leaf Transpiration Inspired Construction of Lignin-Based Wood Plastic Composites for Building Energy Conservation

Incorporating phase change materials (PCMs) into buildings represents a strategic method for reducing reliance on air conditioning systems and decreasing energy usage consequently. Inspired by leaf veins providing channels for transpiration, lignin porous carbon (LPC) was synthesized by chemical activation, showing a vein pore structure. The capillary network system of the LPC provided more channel to adsorb PCM through surface tension and capillary force, achieving an impressive loading capacity of LPC for polyethylene glycol (PEG) (75%), which demonstrated exceptional heat storage capability (133.21 J/g). The lignin-derived wood-plastic composite (L-WPC) was further prepared. With the gradual increase of LPC/PEG, the latent heat of L-WPC increased gradually. The ΔHm, ΔHc and energy storage efficiency of L-WPC-50 (incorporated 50 wt% LPC/PEG-75) reached 39.99 J/g, 42.65 J/g and 90.26%, respectively. The vein pore structure of LPC also provided a thermal conductivity link for L-WPC. L-WPC-50 demonstrated an increased thermal conductivity (0.66 W/(m·K)) and a reduced thermal diffusion coefficient (0.24 mm2/s), optimizing the effectiveness of the phase transition. Additionally, L-WPC-50 displayed satisfactory mechanical properties and dimensional stability, with tensile strength, bending modulus and water absorption thickness swelling rate measuring 14.40 MPa, 1043.72 MPa, and 1.61% respectively. It introduced a feasible approach for utilizing biomass in enhancing building energy efficiency, marking a significant stride towards reducing building energy consumption and advocating sustainable energy practices.

Neon pulsating heat pipe with innovative flexible evaporator to facilitate cryocooling of superconducting devices

Cryogenic pulsating heat pipes (PHP), also termed oscillating heat pipes, are passive thermal links that transfer heat by oscillatory motion of two-phase cryogen confined in serpentine-shaped capillary tubes. It is composed of three sections, namely, the condenser (cold sink), the evaporator (heat source) and the adiabatic part that can range from several centimetres to few metres. In spite of longer lengths, PHPs hold an advantage that their weight does not radically increase in comparison to counterparts like metallic thermal straps. The aim is to present cryogenic PHPs as one of the potential thermal links aiding in distant cooling of superconducting devices from active cryocoolers. A 0.4 m long neon PHP with 1 mm capillary tube diameter has been recently developed characterized by one of the highest thermal conductance reported till date both in vertical and horizontal orientation. An innovative modification in the construction of PHP evaporator is showcased in this article. This would considerably enhance the flexibility of PHPs in terms of their employment geometrically within the targeted application. Pilot experimental results for neon PHP coupled with the altered evaporator is presented for heat load up to 18 W.

Five year operation of a cooler Dewar assembly for infrared scanner on board GCOM-C

Global Change Observation Mission – Climate (GCOM-C) “Shikisai”, a satellite designed to observe global climate change, was launched from Tanegashima Space Center on December 23, 2017 by an H2A launch vehicle. The Second-generation GLobal Imager (SGLI) on GCOM-C is a multi-channel optical sensor for observing aerosols, vegetation, and temperatures. Through long-term monitoring, our understanding of climate change mechanisms will be improved. The infrared scanner (IRS) on SGLI has a Thermal InfraRed (TIR) detector requested to operate at 55 K. A Cooler Dewar Assembly (CDA) developed to keep the detector at 55 K is designed to minimize the heat load for the small cooler. The detector is supported on a thermal isolator made of Glass FRP and is thermally connected to the cooler by flexible thermal link. The Cooler Control Electronics (CCE) uses a heater to compensate heat load fluctuations, thereby maintaining temperature and stability. The heater power decreases gradually during five years, consequently decreasing the cooling power. Despite that cooler degradation, the detector temperature has been maintained at 55 ± 0.1 K for 5 years in orbit and has continued operating with 36 W power consumption. This paper describes the cooler Dewar Assembly and its five years of operation in orbit.

Performance of a foam copper-based chromium potassium alum salt pill in an adiabatic demagnetization refrigerator

Hydrated paramagnetic salt pills are important components in an adiabatic demagnetization refrigerator (ADR) for temperatures below 500 mK. To improve the heat transfer performance between salt crystals and external thermal links, thermal buses are commonly employed inside the salt pill. Common forms of thermal buses are either thin copper pillars made from electric discharge machining (EDM) or wire bundles, both of which generally involve considerable machining or labor. This paper introduces a novel design and fabrication of a simple foam copper-based paramagnetic salt pill. The foam copper has 30 pores per inch (PPI), a porosity of 98 %, 0.005 m2/g specific surface area, and an estimated pore diameter of 0.82 mm. The complete design, assembling process, and growth of the foam copper-based chromium potassium alum (CPA) salt pill (40 mm in length and 23 mm in diameter) are introduced. A single-stage ADR is built based on this salt pill to test its performance, while another salt pill with the wire thermal bus has also been tested for comparison. A typical cool-down from 4.3 K @ 4 T with a demagnetization rate of 0.0025 T/s led to a no-load temperature of 176.5 mK, slightly better than that using a wire thermal bus. In terms of cooling power per gram of salt, the cooling capacities at 600 mK of both salt pills have also been measured and both have shown similar performance. This preliminary study has shown the potential of the foam copper-based salt pill though more research is required to optimize its performance.

Verification of Wiedemann–Franz Law in Silver with Moderate Residual Resistivity Ratio

Electrical and thermal transport were studied in a vacuum-annealed polycrystalline silver wire with residual resistivity ratio 200 – 400, in the temperature range 0.1-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-$$\\end{document}1.2 K and in magnetic fields up to 5 T. Both at zero field and at 5 T the wire exhibits the Wiedemann–Franz law with the fundamental Lorenz number, contrary to an earlier report (Gloos et al., in Cryogenics 30:14–18, 1990). Our result demonstrates that silver is an excellent material for thermal links in ultra-low-temperature experiments operating at high magnetic fields.

A 220 GHz superconducting titanium transition edge sensor array developed for cosmic microwave background experiments

In this paper, we report on the design, fabrication, and characterization of a 220 GHz superconducting transition edge sensor (TES) array developed for ground-based cosmic microwave background (CMB) experiments. Unlike conventional TES arrays adopting thermometers made of bilayer superconducting films, the TES array has thermometers made of simpler single-layer titanium (Ti) film deposited on a suspended silicon nitride (SiN x ) membrane. The thermal weak link is realized by releasing the TES’s thermal island including the Ti thermometer by dry etching with xenon difluoride (XeF2), giving a typical thermal conductance of 25 pW K−1. Its thermal-conductance mechanism is further studied by fitting the measured TES’s complex impedance with a three-block thermal model. The dark and optical noise equivalent power (NEP) of a superconducting TES are both measured. Despite existing lens reflection and dielectric loss in the superconducting microstrip line, its typical optical NEP reaches 100 aW Hz−0.5, which meets the sensitivity requirement for ground-based CMB experiments.

Modeling of Sprinkler skipping for the suppression of large-scale rack-storage fires

A sprinkler skipping model is developed and implemented into FireFOAM. The model considers spray impingement and water accumulation on the sprinkler thermal link. A sub-grid spray impingement model is developed assuming a simplified link geometry, i.e., a sphere. Both the impinged cold water and the water evaporative cooling reduce the sprinkler thermal response. The model is calibrated with the plunge tunnel tests in the presence of water mist sprays and validated against a large-scale fire suppression test using Class 2 commodity. The skipping model improves the sprinkler activation prediction. With the skipping model, the second sprinkler activation is delayed, and the activation time is close to the test. The predicted number of activations and the activation pattern also show good agreement with the measurement, which improves the prediction of the fire suppression performance. The model is applied using different rack array heights (7.6, 10.7 and 13.7 m) under a 15.24 m high ceiling at two injection pressures. The model predicts that the highest array at the low injection pressure has several sprinklers skipped, which causes the failure of fire suppression.

Optimization of thermal link for a conduction-cooled superconducting magnet system

The thermal link between the second stage of cryocooler and the superconducting magnet has a significant influence on the initial cooldown time of conduction-cooled superconducting magnet. In this paper, a numerical investigation is carried out on the initial cooldown characteristics of a superconducting magnet system used flexible copper strap as the thermal link. The optimal shape ratio (cross-sectional area to length) of the copper strap is obtained to minimize the cooldown time. The results show that optimal shape ratio decreases with increasing the length. In addition, the effect of copper purity on the cooldown time is little at the optimal shape ratio or at RRR (Residual Resistive Ratio) > 100.

Cryogenic System Preliminary Design for a 0.5m-Long, Conduction-Cooled Nb3Sn Undulator Magnet Prototype

There are several NbTi superconducting undulator (SCU) magnets currently in operation at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). The development on Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn-based superconducting undulator magnets at APS is underway due to the potential to further enhance the performance of the SCUs. Superconducting undulator magnets need to keep temperature gradients minimized in order to retain thermal and operating current margin. We have designed the thermal links for efficient heat conduction using 3D finite element analysis (FEA) simulation in COMSOL Multiphysics software, which was later used for the calculation of the temperature distribution across a 0.5 m long, conduction-cooled, Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn undulator magnet prototype that includes both conductive heat transfer and radiative heating components. We have modelled the evolution of the thermal properties of the magnet winding as well as other cold parts during cool-down from ambient temperature with the operation of a SHI RDK-415D cryocooler to examine the estimated time that is needed for the cooldown as well as the baseline temperature we could achieve. A key result was that a maximum coil <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta $</tex-math></inline-formula> T = 0.11 K along the designed thermal links and a temperature range of 3.42 K–3.53 K for the winding were predicted at steady state. A pair of G10 support rods for carrying the magnet as well as a pair of current leads made of metals were also designed and optimized for the testing system, targeting for an operation temperature of 4 K and a coil current of 950 A DC.

High performance and working stability of an 18 W class neon pulsating heat pipe in vertical/horizontal orientation

This paper presents an experimental thermal and hydraulic characterization of a pulsating heat pipe (PHP) at neon temperature. In the search for efficient alternative cooling techniques, with a cryocooler as the cold source, this particular heat pipe is considered a potential candidate for passive thermal link to cool high-temperature superconducting devices. This wickless heat pipe is 0.4 m long with 0.1 m enclosed within the evaporator and condenser. It consists of 20 turns of 1.0 mm inner diameter stainless tube. A systematic study of the thermal performance and pressure of this PHP is presented as a function of thermal load and orientation covering the widest range of filling ratios (10 to 90 %). The cryocooler capacity limit is 18 W for which the PHP functions successfully. This cryogenic PHP has the best thermal performance ever measured with neon showcasing a thermal resistance as low as ∼0.17 K/W in vertical and ∼0.27 K/W in horizontal orientation. The limits of the operating conditions (start-up and dry-out) are also studied as well as the non-homogeneity in evaporator heat load. Finally, a 100 h stability test at constant operating conditions is also demonstrated.

Additional Possible Cause of the Erratic 2009 Monsoon Over South and East Asia: Large-Scale Change in the Upper Tropospheric Temperature

ABSTRACT It has been recognized by the scientific community for many years that erratic monsoon attribution should be prioritized in order to meet the growing demand for reliable monsoon forecasts. It is believed that the severe 2009 drought in the South and East Asian regions was caused by an erratic monsoon season, which has sparked enormous interest in its cause. In this study, we have identified a factor in the upper troposphere, i.e. the large-scale upper tropospheric temperature, which may have potentially functioned as a new cause. We have observed that a major change in the upper tropospheric temperature occurred, with one region over Central Asia and another over the eastern Tibetan Plateau; also observed is the interaction between these two local systems. The thermal wind link suggests that the 2009 circulations over the two different regions at 250 hPa – are characterized by a cyclone and an anti-cyclone over Central Asia and the eastern Tibetan Plateau, respectfully, and that this could be the dynamic cause of the 2009 monsoon prediction failure. Subtropical jets provide a means of connecting the dynamical systems in the two different regions, which have enhanced the convection over Central Asia and reduced the convection over South and East Asia during that period. Also revealed is the role of the thermodynamic processes, specifically the role of the vertical thermal contrast (VTC) over Central Asia and the eastern Tibetan Plateau, which is responsible for changing the convective patterns during the 2009 monsoon season. This is further substantiated by the inferred causal relationship between the upper VTC and longwave fluxes (LWFs) at the top of the atmosphere.

A design for optical refrigeration: The parallel configuration

For optical refrigeration to become a viable option for cryogenics, it is fundamental to improve the overall cooling power of an optical cryocooler. In this paper, we propose a design that might be able to cool down a thermal load to temperatures lower than ever before: the parallel configuration. We explore the possibility of attaching a thermal load to multiple Yb:YLF monocrystals, which would allow us to use more pump intensity without surpassing the saturation limit, and we show that by using two YLF:5% Yb crystals connected to an undoped YLF sample (that acts both as a thermal link and a thermal load) we obtained a ΔT increase in 40% compared to a setup with the load and a single crystal.

Thermal Preload for Predicting Performance Change Due to Pad Thermal Deformation of Tilting Pad Journal Bearing

Thermal deformation of journal bearings operating under high-temperature conditions can have a significant effect on changes in bearing performance. However, no attempt has been made to quantify this amount of thermal deformation and link it to the performance change of the bearing. The aim of this study is to investigate the quantitative performance change due to thermal deformation of the tilting pad journal bearing (TPJB) pad in terms of the change in preload amount. The variable viscosity Reynolds equation and the energy equation were coupled using the relationship between viscosity and temperature, and the solution was obtained using the finite element method. Heat transfer between the spinning journal, oil film and pads is considered, and a three-dimensional (3D) finite element (FE) model was used to calculate the thermal deformation of the bearing structure. The steady state of the rotor-bearing system was predicted using a bearing performance prediction algorithm with three closed loops. State variables for this steady-state prediction include the amount of thermal deformation of the structure. In order to investigate the amount of thermal deformation of the bearing pad in terms of bearing performance, the concepts of thermal offset preload and thermal performance preload were suggested and the change in thermal preload under various conditions was investigated.

Performance assessment of planar and non-planar cryogenic pulsating heat pipe with novel condenser

Pulsating heat pipe (PHP) is gaining popularity as a viable solution for thermal conductive links associated with a cryogenic cooling system. A competitive commercial environment requires PHP with a more compact configuration, high heat load removal capacity, and free of orientation dependency. Making orientation-free PHP, a planar PHP (2D layout), requires a large number of turns, increasing the overall size of the PHP device. A PHP with a non-planar (3D layout) layout is capable of accommodating a large number of turns in a short area to tackle both compactness and orientation-dependent issues. The present paper focuses on a comparative thermal performance assessment of planar and non-planar configurations of cryogenic PHP by developing a novel cylindrical shell-type condenser (CSTC). Liquid nitrogen is used as a working fluid. Two separate test setups are developed with 2D Four-Turn PHP (2D FT-PHP) and 3D Four-Turn PHP (3D FT-PHP) with a novel CSTC. The condenser, adiabatic, and evaporator lengths of PHP are kept as 60 mm, 120 mm, and 50 mm, respectively. The thermal performance is assessed by varying Filling Ratio (FR) (38−95 %) and heat load (1 W–130 W) with an evaporator at the bottom position. It is observed that, for a particular heat load, a 3D FT-PHP performed better than a 2D FT-PHP, irrespective of FR. The anti-dry-out ability of 3D FT-PHP is observed to be superior to 2D FT-PHP at a low FR. The frequency analysis results with Wavelet transform show that the working fluid oscillates with a more dominant frequency in 3D FT-PHP than in 2D FT-PHP configuration.

Optical refrigeration of payloads to T < 125 K.

A modified all-solid-state optical cryocooler prototype based on anti-Stokes fluorescence in a 10%-doped Yb:YLF crystal cooled a payload to temperatures below 125 K starting from room temperature. To achieve this record performance, the optical refrigerator employed a novel, to the best of our knowledge, textured-MgF2 thermal link to improve the thermal transport and fluorescence escape. Additionally, it used spectrally selective, high-reflection coatings in the pump circulator cavity to suppress parasitic lasing and amplified spontaneous emission.