Heat Leak Research Articles

Extendable origami multilayer insulation thermal characterization

Spacecraft commonly use multilayer insulation (MLI) for passive thermal control, however deployable structures such as telescoping booms impose the need to stow MLI preflight and extend with the structure for in flight operation. This paper investigates an octagonal origami folding pattern that allows predictable stowage and extension of multilayer insulation. Feasibility was demonstrated to fold, compress and extend a MLI blanket comprising 10 layers of aluminized Kapton, each separated by a layer of scrim cloth. Both folded and unfolded 10-layer MLI blankets were tested in thermal vacuum to characterize heat leak of deployed configurations. The 10-layer origami folded configuration increased thermal conductance and radiative heat transport relative to the unfolded configuration. Degradation factors for heat transport were empirically found to be Mk=2.5±0.3 and Me=1.3±0.2 for effective thermal conductance and emittance, respectively.

Effect of elastic deformation on the heat transfer characteristic of refrigerator gasket based on static analysis of structural assemblies

ABSTRACT Soft and hard contact deformation is most common issue of the refrigerator gasket and it has almost been ignored in previous studies. In this paper, the novelty is to propose a thermal-solid coupling simulation to study the effect of assembly states on the heat transfer of gaskets, improving the accuracy of heat transfer simulation analysis. The numerical results indicate that the calculated temperature based on physical model of assembly state is closer to measured temperature. The assembly distance affects the assembly state of the gasket by changing the contact positions and lengths of contact interfaces. As a result, the heat transfer path and condition at the gasket region also changes. As the deformation increases form −1.0 mm to 5.0 mm, the heat leakage load of gasket rises firstly and then falls, in the range of 1.61 ~ 3.33 W•m−1. The heat leakage at the transfer paths of internal cold air, outer shell and cold bridge accounts for over 20%. Thus, it is recommended to reduce the contact interface length of internal cold air, increase the thermal resistance of auxiliary air-bag, reduce the thermal conductivity and thickness of magnetic strip and increase the contact thermal resistance of cold bridge.

Design, Heat Leakage Analysis and Stirling Cryocooler Option of Stirling-type Lyophilizer

Background: Based on referring to the relevant patents, it was found that the traditional lyophilizer refrigeration system has the disadvantages of large volume and high power consumption. Meanwhile, considering the context of advocating low carbon emission and environmental protection, the use of energy-saving refrigeration technology and the use of non-fluorine and harmless refrigerants are the trends of future development. Objective: In order to solve the traditional lyophilizer problems and enhance the lyophilizer reliability, the paper designs a Stirling-type lyophilizer with a working temperature of -70°C. Methods: Stirling cooling has the advantages of ultra-low refrigeration temperature and high cooling speed. Based on the above advantages, combining the characteristics of the lyophilization box, a Stirling- type lyophilizer with a capacity of 0.120 m3 and a shelf area of 0.4 m2 is designed. The Stirlingtype lyophilizer refrigeration system structure is analyzed. The flat plate heat pipe to transfer the cold is used. The rigid polyurethane as the insulation material of the lyophilization box is studied. The heat leakage calculation method of the lyophilization box is given. Results: Stirling cryocooler is selected based on the lyophilization box heat leakage. The experimental test of the Stirling cryocooler is conducted. The feasibility of the design is verified. The Stirling cryocooler, with a charging pressure of 2.9 MPa and an operating frequency of 68 Hz, could achieve -87.057°C within 24 min. The lowest refrigeration temperature could be maintained at - 100.286°C after 97 min. The cooling capacity obtained is 40.0 W at an input power of 99.2 W, and the COP is 0.403. Conclusion: Stirling refrigeration technology is suitable for use in lyophilizer and Stirling refrigeration technology improves the robustness of the lyophilizer refrigeration system.

Numerical study of thermodynamic and hydrodynamics in a type B LNG cargo tank during sloshing excitation

The hydrodynamics and thermodynamics properties in a type B liquefied natural gas (LNG) cargo tank during sloshing excitation are investigated in this research using the numerical model that takes phase transition and exterior heat leaking into account. In order to forecast the movement of the vapor–liquid interface, the mesh motion treatment in conjunction with the volume of fluid (VOF) approach is used. The user-defined function (UDF) realizes the sloshing excitation. Its viability is confirmed by comparing the numerical model with related fluid sloshing experiments. The impacts of various sloshing periods, amplitudes, and filling rates on the hydrodynamics and thermodynamic properties were examined using numerical analysis of heat flux, the mass of boil off gas (BOG), internal energy, and pressure in the LNG cargo tank. Even with varying filling rates, the numerical findings demonstrate that resonance happens if the sloshing period corresponds to the natural period. As a result, this research serves as essential for gaining a thorough comprehension of thermodynamic, hydrodynamics and BOG mass properties during sloshing excitations in marine LNG transport.

Thermal conductivity enhancement and shape stability of composite phase change materials using MIL-101(Cr)-NH2/expanded graphite/multi-walled carbon nanotubes

Low heat conductivity and leakage much restrict the effective use of phase changing materials (PCMs) in heat energy management. Here, an expanded graphite (EG)/ multi-walled carbon nanotube (MWCNT) / metal organic framework (MOF: MIL-101(Cr)-NH2) composite was developed to solve these issues. The porous MOF can avoid the loss of liquid paraffin wax (PW). The heat conductivity of composite PCMs (CPCMs) was improved using EG/MWCNT with a mass ratio of 4:1. The thermophysical properties, heat stability and chemical compatibility of CPCMs were comprehensively investigated. Results demonstrate the successful preparation of CPCMs without any chemical reaction. The MOF (35 %)/EG(4 %)/MWCNT(1 %)/PW(60 %) CPCM exhibits superior thermophysical properties and its melting enthalpy (74.59 J/g) is near the theoretical level after 500 heating/cooling cycles. It exhibits a 792 % increment in heat conductivity from pure PW.

Preparation and crystallization behavior of sensitive thermochromic microencapsulated phase change materials

A novel sensitive thermo-chromic microcapsules with inorganic/organic nanoparticles-embedded composite shell was designed and synthesized, and the thermal properties, thermo-chromic performance, crystallization behavior of ethyl myristate (EM) in confined spaces were studied in this paper. As the reaction proceeded, the aminated silica (NH2-SiO2) nanoparticles modified outer shell in the aqueous phase and isophorone diisocyanate (IPDI) in the oil phase gradually self-assembled at the inner interface, leading to the formation of an inorganic/organic polymers composite shell. The effect of NH2-SiO2 nanoparticles content on the crystallization behavior, microstructure, morphology, and phase-change properties as well as the thermal stability of MPCMs were investigated in detail. Interestingly, the covalent attachment of NH2-SiO2 nanoparticles on the capsules surface offered long-term chemical stability compared to that of conventional physical adsorption. The NH2-SiO2 nanoparticles were found distributed uniformly on the surface of MPCMs, and the onset crystallization temperature of MPCMs increased by 7.3 °C with NH2-SiO2 nanoparticles addition of 15% content, which counteracts the super-cooling phenomenon. Microcapsule temperature change optical microscopy and non-isothermal crystallization kinetics revealed that the crystallization of RS-MPCMs were homogeneous as well. RS-MPCMs have significant potential in green energy applications due to their latent heat storage, thermal stability, thermal conductivity, leakage prevention, and mechanical properties.

Experimental research on the thermal performance of a flat evaporator loop heat pipe with a new composite wick

The heat leakage in evaporator is still a key challenge that impacts the heat transfer performance of the loop heat pipe (LHP). In this study, a new composite wick structure LHP, including sintered copper powder and sintered ceramic powder, was designed and fabricated to reduce the heat leakage into compensation chamber and enhance the heat transfer characteristics of LHP. The copper powder and ceramic powder were sintered as a whole by a low temperature-pressureless dry sintering technology. The heat transfer performance of water/air-cooled LHP with a new composite wick, including startup characteristics, operational characteristics and thermal resistance analysis, were investigated at the horizontal position. The experimental results indicated that the water/air-cooled LHP with a new composited wick can start up successfully when the heating power was not less than 30 W. And the maximum heating power of the new water/air-cooled LHP can reach 550 W and 500 W corresponding junction temperature of 83.42 °C and 94.22 °C, respectively. In addition, compared with the water/air-cooled LHP with a mono wick, the water/air-cooled LHP with a new composite wick has a lower startup temperature at the heating power of 20–50 W, and a lower steady-state junction temperature when the heating power was less than 500 W and 300 W, respectively.

Experimental investigation on a flat loop heat pipe coupled with thermoelectric cooler

The flat Loop Heat Pipe (LHP) has the advantages of small size, light weight, and direct contact with the heat source without additional thermal resistance, which has gradually drawn attention and importance in the aerospace field. However, for the vertically arranged flat LHP, there is a significant amount of reverse heat leakage from the evaporator to the compensation chamber, resulting in a low operating efficiency of the heat pipe. This paper proposes a thermal control structure coupling the flat LHP with thermoelectric cooler (TEC), which investigates the effects of TEC on the start-up speed, working temperature, system thermal resistance, temperature fluctuation, and reverse temperature control of the flat LHP through experimental studies. The results show that the TEC can help improve the start-up speed of the heat pipe by approximately 50 % under low heat load. The heat resistance of the LHP is greatly reduced to approximately 0.13–0.15 °C/W, and the working temperature curve changes from “V” type to a linear line with increasing heat load power. Moreover, by using the TEC to provide reverse power to the compensation chamber for temperature control, the temperature control efficiency is extremely high, and a temperature rise of approximately 10 °C can be achieved with only 0.1 W TEC power. The above research indicates that the TEC can significantly weaken the disadvantage of the flat LHP and improve its engineering practicality.

Thermal Conductivity of Bottle-Brush Polymers.

Using molecular dynamics (MD) simulations of a generic model, we investigated heat propagation in bottle-brush polymers (BBP). An architecture is referred to as a BBP when a linear (backbone) polymer is grafted with the side chains of different length Ns and grafting density ρg, which control the bending stiffness of a backbone. Investigating κ-behavior in BBP is of particular interest due to two competing mechanics: increased backbone stiffness, via Ns and ρg, increases the thermal transport coefficient κ, while the presence of side chains provides additional pathways for heat leakage. We show how a delicate competition between these two effects controls κ. These results reveal that going from a weakly grafting (ρg < 1) to a highly grafting (ρg ≥ 1) regime, κ changes non-monotonically that is independent of Ns. The effect of side chain mass on κ and heat flow in the BBP melts is also discussed.

Direct synthesis of porous aluminum nitride foams for enhancing heat transfer and anti-leakage performance of phase change materials

Organic phase change materials (PCMs) suffer from low thermal conductivity and easy leakage, which limit their application in heat storage and thermal management. In this work, porous AlN skeletons were synthesized to solve these problems of paraffin PCM. Using Al powders as raw material, porous AlN skeletons were prepared through the ice-templated method and the direct nitridation reaction. The skeletons were impregnated with paraffin to prepare phase change composites (PCCs). The thermal conductivity of the PCC with 55.6 wt% AlN reaches 9.1 W m−1 K−1 in the axial direction, which is 4550 % of paraffin. This PCC retains a high enthalpy of 105 J g−1 and has good thermal stability. Moreover, the porous AlN skeletons can effectively absorb paraffin so that all PCCs maintain a stable shape. This study provides a simple and efficient method to synthesize porous AlN and to solve the heat conduction and leakage issues of PCMs.

Study on thermodynamic response in liquefied natural gas storage tanks under static pressurization and sloshing conditions

AbstractLiquefied natural gas (LNG) is becoming a potential power fuel in ocean transport and will be widely utilized in the near future. However, severe thermodynamic imbalance issues, caused by environmental heat leakage and external sloshing disturbances, must to be efficiently addressed to improve the operation reliability and safety storage of LNG fuel tanks. In this paper, a comprehensive theoretical model is developed to investigate the thermal response in a type C LNG storage tank, with consideration of composition migration, heat penetration, liquid evaporation, fluid sloshing, and vapor pressure rise. The prediction accuracy of the theoretical model is validated by comparing with selected tank pressurization experiments, with deviation limited within 5.0%. Based on the theoretical model, the aging process of LNG is first involved. The variations of composition migration in vapor and liquid regions are specially considered and discussed. During static pressurization, the thermal physical performance, including tank pressure rise, vapor temperature change, and boil‐off gas (BOG) generation, is detailedly researched under heat penetration. Finally, the effect of external sloshing excitation on thermal behavior in LNG fuel tanks is explored. Compared to static pressurization, external sloshing excitation causes obvious influences on thermodynamic performance of LNG tanks, including promotion on tank pressure and enhancement of heat and mass transfer. With some valuable conclusions achieved, this work is significant to comprehensive understanding on the thermal response of LNG storage tanks under different operation conditions.

Non-degenerate-pump four-wave mixing kinetic inductance travelling-wave parametric amplifiers

Kinetic inductance travelling-wave parametric amplifiers (KITWPAs) have been demonstrated to achieve high gain over broad bandwidths whilst achieving near quantum-limited noise performance, properties which are extremely important for many ultra-sensitive experiments. In early KITWPA designs, the requirement for phase-matching lead to the creation of a large zero-gain gap in the centre of the gain profile where the peak gain is, which also slightly narrows down the operational bandwidth of the device. This has been mitigated in more recent designs by introducing a DC bias to the KITWPA device, which allows the gap to be tuned away from the amplification band. However, the added DC biasing requires a more complicated experimental setup and potentially leads to unwanted heat leak in the cryogenic environment. Additionally, operation with a DC bias also become challenging at higher frequencies beyond the microwave regime. In this paper, we present the concept of a KITWPA operating in a non-degenerate-pump four-wave mixing (NP-4WM) regime, whereby the injection of two pump tones along with a weak signal results in a broad, flat gain profile that removes the zero-gain gap as well as eliminates the need for a DC bias and the complexities associated with it. We demonstrate how a NP-4WM KITWPA is feasible to achieve broadband amplification at a range of frequencies, first in the microwave range where most KITWPAs reported to-date have been successfully experimentally characterised. We then extend the designs to several millimetre (mm) bands to illustrate how we can use this technique to design a broadband front-end pre-amplifier that covers several Atacama Large Millimetre/sub-millimetre Array (ALMA) Bands.

A study on the actual heat leak of a cryogenic vessel during storage

In the traditional method, as the loss product and gas are considered to be saturated, the heat leak of a cryogenic vessel is the latent heat of loss product. However, the temperature of loss product and gas exceeds the saturated temperature as measured in tests. Therefore, a portion of the heat leak is absorbed by the loss product when it diffuses into the gas phase of a cryogenic vessel in the storage process. In this study, the energy equations of the gas and liquid are established in cryogenic vessels during storage to investigate the actual heat leak of a cryogenic vessel in a storage process by considering the temperature distribution of the gas. Then, an experiment was conducted to demonstrate that the actual heat leak of a cryogenic vessel contains not only the latent heat but also the enthalpy change of the loss product, which is produced as it diffused into the gas phase. The results showed that the enthalpy change increased as the liquid level decreased and was critical for determining the actual heat leak in cryogenic vessels in the storage process, with a ratio of approximately 12% to the latent heat when the liquid level ratio is 85%.

Influence of Porous Inserts and Compact Resonators on Onset of Taconis Oscillations

Abstract Taconis oscillations represent excitation of acoustic modes due to large thermal gradients inside narrow tubes penetrating cryogenic vessels from a warm ambient environment. These oscillations are usually harmful, as they may drastically increase heat leakage into cryogenic vessels and result in strong vibrations of measuring instruments. Placing a porous material inside a tube with a goal to increase acoustic damping or attaching a small resonator to the main tube are some of the possible ways to suppress or mitigate Taconis effects. However, when the porous inserts are positioned in locations with large temperature gradients or the resonator parameters are selected incorrectly, these components may augment thermal-to-acoustic energy conversion and enhance Taconis oscillations. A low-amplitude thermoacoustic model has been extended and applied in this study to determine the effects of the insert location and pore radius, as well as the resonator dimensions, on the onset of Taconis phenomena in a hydrogen-filled tube of relevance to lines used in cryogenic hydrogen storage tanks. The presented findings can assist cryogenic specialists interested in suppressing or exciting Taconis oscillations.

Solar-driven gas turbine power plants

A general model for an irreversible solar-driven Brayton multi-step heat engine is presented. The model incorporates a regenerator, an arbitrary number of turbines (Nt) and compressors (Nc) and the corresponding reheating and intercooling pro cesses; thus the solar-driven Ericsson cycle is a particular case where NtNc . For the so lar collector we assume linear heat losses and for the Brayton multi-step cycle irreversibilities arising from the non-ideal behavior of turbines and com pressors, pressure drops in the heat input and heat release, heat leakage through the plant to the sur roundings, and non-ideal couplings of the working uid with the external heat reservoirs. We obtain the collector temperatures at which maximum over all e ciency max is reached as a function of the thermal plant pressure ratio and a detailed com parison for several plant con gurations is given.

Effect of cryogenic feed line thermal mass distribution and orientation on chill-down performance

Chilldown performance of cryogenic systems is highly influenced by parameters like feed line orientation, thermal mass of the feed system and its distribution, mass flux of the cryogen and heat in-leak. In this work, the impact of feed line orientation and mass flux of cryogen on chilldown is evaluated through a parameter defined as Chilldown Performance Index. A set of experimental data generated with Liquid Nitrogen employing a test section insulated with poly-isocynurate foam is used for this study. Chilldown Performance Index is found to be lower with downward inclination upto 60° downward orientation of test section as compared to that of horizontal orientation. Beyond this, at 90° downward orientation, it is seen to be better as compared to 60° orientation. Chilldown performance improves significantly with upward inclination of test section. Chilldown time is found to be highly sensitive to mass flux, whereas effect on Chilldown Performance Index is not so significant. Experiments also show that chilldown performance is significantly influenced by the distribution of feed line thermal mass. Chilldown is faster in experiments with higher thermal mass distributed near to the inlet to the test section as compared to the case wherein they are distributed near to the exit of the test section. Flow visualisation studies are also conducted employing a vacuum jacketed view glass made of borosilicate material to capture the flow structure prevailing in experiments with different upward and downward orientation and to assess its influence on chilldown. Flow structure is observed to be significantly different in experiments with upward inclination, compared to those with horizontal and downward orientation. Observations made with upward inclination, show the existence of plug or slug flow in the test section, leading to faster chilldown. Flow pattern observed in experiments with 30° and 45° downward orientation is also different compared to other orientations, as liquid flow is mostly confined to the core of the test section, due to gravity effects.

Minimum entropy generation paths for generalized radiative heat transfer processes with heat leakage

Heat transfer processes (HTPes) with generalized radiative heat transfer law (HTL) [q∝Δ(Tn)] and heat leakage (HL) are optimized. With net amount of heat transferred (NAHT) of working fluid at low-temperature side and given process period, optimal path, that is, optimal temperature configuration of reservoir (TCR), for entropy generation minimization (EGM) of HTP is derived. Universal result for optimal path is provided, and results for four special HTLs, that is, Newtonian, radiative, linear phenomenological, and mixed HTLs, are further provided. Optimal path with EGM is compared with traditional ones of constant heat flux rate (CHFR) and constant reservoir temperature (CRT). Effects of HL and NAHT on optimal results are analyzed. Results show that ratio of heat flux rates between reservoir and working fluid to (n1+1)/2 power of reservoir temperature for EGM of HTPes without HL is a constant, but the optimal relationship between temperatures of working fluid and reservoir with HL is significant different. The differences among TCRs with different heat transfer paths increase with increases of HL and NAHT. The path of CHFR is very close to that of EGM, and the two paths are superior to that of CRT.

Design and development of a cryogenic J-T valve for a 2 K cryostat with a single cryocooler

Abstract A cryogenic Joule-Thomson (J-T) valve for a 2 K cryostat cooled with a G-M cryocooler has been developed to continuously supply superfluid helium. Saturated liquid helium flowing through the valve transitions to superfluid helium below 2 K because of the J-T effect. The heat leak, sealing performance and the J-T effect of the valve were tested experimentally and analyzed. To reduce heat leak in the 2 K range, two thermal anchors were installed in the first and second cooling stages, respectively, resulting in 89.93% reduction of useful work loss. A unique sealing structure was developed for the J-T valve and was tested at both room temperature and liquid nitrogen temperature, yielding leak rates of 3 × 10-9 Pa· m3/s and 3 × 10-4 Pa· m3/s, respectively. The corresponding valve needle and seat structure were designed based on the flow resistance equation. The J-T effect of the valve in 2 K cryostat was tested at various valve openings. The cryostat can operate at 1.64 K with no load and provide 0.143 W cooling power at 2 K.

Влияние метеофакторов на среднесуточную интенсивность городского острова тепла Екатеринбурга

Urban heat island (UHI) intensity in the surface air temperature field is equal to the difference between temperatures (averaged for the period under consideration) in the city and in the immediate rural surroundings. Using multi-regression analysis, We investigated the meteorological factors determining the mean daily UHI intensity based on observational data at weather stations in Yekaterinburg and the village of Verkhneye Dubrovo in 2020–2022. The average value of an UHI intensity in May-September equals to 0.94 °C with a standard deviation of daily mean values of 0.71 °C. The multiple linear regression model includes the difference in the relative humidity of the atmosphere in the city and the village, the weather factor determined by wind speed and cloudiness, and atmospheric pressure. This “summer” model describes 60 % of the observed UHI variance. In December-March, the average UHI intensity equals to 0.81 °C with a standard deviation of mean daily values of 0.82 °C. The regression model includes only the weather factor explaining 27 % of the UHI variability. It is concluded that the UHI intensity in Yekaterinburg is primarily determined by solar radiation and various properties of urban and rural surfaces (albedo, emissivity, thermal properties of soils). They determine the maximal UHI intensity that is realized only in “ideal” weather (weather factor ~1). Deviations from “ideal” conditions described by meteorological factors only reduce the maximal UHI intensity. In winter, heat leaks from buildings and heating networks weakly dependent on the meteorological factors make an additional contribution to the UHI intensity.

Visualized investigation of a nitrogen pulsating heat pipe

This paper presents the results of a visualization study of a cryogenic pulsating heat pipe (PHP). The impact of radiation heat leakage was reduced by minimizing optical windows. Visual images of the adiabatic section of a 2-turn PHP with nitrogen as working fluid were obtained at a filling ratio of 50 % and an inclination angle of 90°. The lengths of the evaporator, adiabatic section, and condenser were 50 mm, 100 mm, and 50 mm, respectively. The adiabatic section was made of quartz, whereas the evaporator and condenser were made of stainless-steel capillaries soldered onto copper plates. The flow channel of PHP was 1 mm. The fluid characteristics of the PHP under quasi-steady state conditions at different heat inputs were captured using a high-speed camera. Based on the experimental results, the evolution of flow patterns and fluid motion characteristics, and the coupling relationship between flow and heat transfer were illustrated and discussed.