Multilayer Insulation Systems Research Articles

Optimized Fabrication Process and PD Characteristics of MVDC Multilayer Insulation Cable Systems for Next Generation Wide-Body All-Electric Aircraft

For wide-body all-electric aircraft (AEA), a high-power-delivery, low-system-mass electric power system (EPS) necessitates advanced cable technologies. Increasing voltage levels enhances power density yet poses challenges in aircraft cable design, including managing arc-related risks, partial discharges (PDs), and thermal management. Developing multilayer multifunctional electrical insulation (MMEI) systems for aircraft applications is a feasible option to tackle these challenges and reduce the size and mass of cable systems. This approach involves selecting layers of different materials to address specific challenges. Our prior research concentrated on the modeling and simulation-based design of MMEI systems for MVDC power cables. Experimental tests are essential for determining the behavior of PDs under varying pressure conditions. Also, the dielectric strength and time to failure of the designs need to be assessed. In this work, the fabrication process of a down-selected MMEI flat configuration is discussed and analyzed. This paper analyzes the fabrication process of power cables employing MMEI configurations and evaluates the PD characteristics of down-selected ARC-SC-T-MMEI cable samples. This study presents a detailed analysis of the characteristics of PD under atmospheric and low-pressure conditions, which will provide essential insights into the design of MVDC cables for future AEA applications.

Modeling the performance of multilayer insulation in cryogenic tanks undergoing external fire scenarios

Multilayer Insulation (MLI) is frequently used in vacuum conditions for the thermal insulation of cryogenic storage tanks. The severe consequences of the degradation of such materials in engulfing fire scenarios were recently evidenced by several large-scale experimental tests. In the present study, an innovative modelling approach was developed to assess the performance of heat transfer in polyester-based MLI materials for cryogenic applications under fire conditions. A specific layer-by-layer approach was integrated with an apparent kinetic thermal degradation model based on thermogravimetric analysis results. The modeling results provided a realistic simulation of the experimental data obtained by High-Temperature Thermal Vacuum Chamber tests reproducing fire exposure conditions. The model was then applied to assess the behavior of MLI systems for liquid hydrogen tanks in realistic fire scenarios. The results show that in intense fire scenarios degradation occurs rapidly, compromising the thermal insulation performances of the system within a few minutes.

Effects of hygrothermal and natural aging on the durability of multilayer insulation systems incorporating thermal mortars with EPS and aerogel

This study evaluated the durability of three innovative multilayer insulation systems incorporating thermal mortars with EPS aggregates and silica aerogel granules after hygrothermal accelerated aging and one year of natural aging at an urban site in Portugal. The loss of performance was assessed after the accelerated aging and every three months of natural aging using non-destructive testing. Chemical-morphological analyses were also carried out prior to and after accelerated and natural aging. Results obtained after accelerated and natural aging were compared, thus contributing towards a deeper understanding of possible synergistic effects of several degradation agents and mechanisms on the long-term durability of multilayer insulation systems. The Coffin-Manson equation showed that the accelerated aging procedure (∼13 days of heat/rain cycles and 5 days of heat/cold cycles) adopted herein corresponds to approximately 11 years of natural aging in typical urban conditions. The results show a significant increase in capillary water absorption and drying capacity after aging. Extensive surface microcracking was observed after accelerated aging and after 3 months of natural aging, especially in the systems facing North. Traces of biological growth were detected on both the artificially and naturally aged systems, whereas aesthetic alterations were more pronounced in North-oriented specimens after 3 months of exposure, with significantly lower surface gloss and a darker tone. On the other hand, color change cannot be detected in the artificially aged systems (ΔElab* < 2), due to the lack of degradation agents such as pollutants or UV radiation that were not included in the accelerated aging procedure.

Design and Optimization of the Insulation Performance of a 4000 m3 Liquid Hydrogen Spherical Tank

Efficient insulation technology is one of the key technologies for the development of large LH2 storage tanks. This paper aimed at a 4000 m3 LH2 spherical tank, many insulation schemes were designed, including multilayer insulation systems integrated with a vapor-cooled shield (VCS) and liquid-nitrogen-cooled shield (LN2CS). The heat transfer model was developed to predict the insulation performance of a LH2 spherical tank. The effect of the VCS position on insulation performance was studied, and the different configurations of double VCSs were compared and discussed. The results showed that the daily evaporation rate of MLI, hollow glass microspheres (HGMs) and vacuum was only 2.05 × 10−3%, 3.62 × 10−3% and 7.94 × 10−2% at 1.34 Pa, respectively. MLI was still the optimal insulation scheme for a 4000 m3 LH2 spherical tank. Meanwhile, it was found that when the single VCS was placed at the 10th layer, the heat leakage was reduced by approximately 40.5% compared with MLI. The heat leakage of parallel VCS(P-VCS) was 76.6% lower than that of MLI. The minimum heat leakage of series VCS(S-VCS) was 83.79%, 72.75% and 37.36% lower than that of MLI, single VCS and P-VCS, respectively. Additionally, the heat leakage of the LH2 tank could be reduced to less than 10 W when LN2CS was installed. These results provide a design reference for the highly efficient storage of large LH2 tanks.

Impact of natural and artificial ageing on the properties of multilayer external wall thermal insulation systems

Multilayer external wall thermal insulation systems are widely used in new constructions and for the retrofitting of building façades. Despite the increasing use as constructive solution, significant anomalies have been frequently identified on these systems only few years after their application, raising questions on their long-term durability. This paper focuses on the effects of one-year natural ageing and artificial ageing through hygrothermal cycles (heat/rain and heat/cold) on the water performance (capillary water absorption and drying kinetics), bio-susceptibility (mould development) and surface properties (colour, gloss, and roughness) of four multilayer external wall thermal insulation systems (three ETICS and a thermal rendering system). Results showed a significant loss of surface hydrophobicity after ageing. Considering the 24 h water absorption results, an increase up to 73% and 432% was obtained for the naturally and artificially aged systems, respectively, and when compared to the reference unaged systems. The drying kinetics was affected by ageing, with increases of the drying resistance ranging between 47% and 122% after artificial ageing. Traces of mould growth were observed on the artificially aged systems; however, no growth was detected on either the unaged or the one-year naturally aged systems. Substantial colour change for all systems after ageing was observed, confirming aesthetic alteration. Results contribute towards the development of implemented artificial ageing protocols, as well as for the delivery of multilayer thermal systems with enhanced performance and durability.

Thermal performance assessment of cold chain chamber with vacuum insulation panel envelope layer

As one of the super thermal adiabatic materials nowadays, Vacuum Insulation Panel has slim thermal conductivity, which can be no more than 0.0040 W/(m·K) at an internal pressure of less than 10Pa. the excellent thermal performance makes it prosperous in cold chin experiments. Actually, Vacuum Insulation Panel is normally installed with other insulation materials as multilayer envelope in practice. However, influenced by the fluctuation of environmental factors and the difference of thermal & physical properties of materials, the overall thermal performance of multilayer insulation systems with Vacuum Insulation Panel different arrangement sequences is significantly various. A reasonable Vacuum Insulation Panel location plays an important role on the thermal performance. In this paper, the thermal insulation performance of multilayer under unsteady state condition was studied. Three chambers, with the same exterior size, that is, 300 × 300 × 300 mm3, were employed for the innovation purpose. The multi-layer insulation construction involves a 10 mm layer of Vacuum Insulation Panels and a 10 mm layer of polyurethane. And three different envelope structures, namely, internal insulation, sandwich insulation and external insulation were built according to the Vacuum Insulation Panel's location. The numerical heat transfer model and physical model were established. The simulative and experimental results were comprised and analyzed. The results implied that, to maintain the constant temperature in cold\hot chamber, the vacuum insulation panels are located on the face of temperature fluctuation side, i.e. the ambient side, and have a better overall insulation performance.

Heat radiation reduction in cryostats with multilayer insulation technique

Multilayer insulation (MLI) is an important technique for the reduction of radiation heat load in cryostats. The present work is focused on investigation for the selection of suitable reflective layer and spacer material in MLI systems. In our analysis, we have selected perforated double-Aluminized Mylar (DAM) with Dacron, unperforated DAM with Silk-net and perforated DAM with Glass-tissue for their evaluation as the reflective layer as well as spacer materials in MLI technique. Current work would discuss the calculation of the effect of layer density and the number of layers on the heat load. Knowing the key parameters of MLI, we have compared the heat load generation in spherical as well as cylindrical cryostats and the different effect of shielding the cryostat inner or the outer surface on the heat load.

Performance of MLI Seams between 293 K and 20 K

The performance of multilayer insulation systems (MLI) are often determined not by the bulk acreage insulation but rather by the design details that allow the insulation to be attached to the hardware it is insulating. To help understand the impacts of multiple different types of seams on blanket performance, calorimeter testing was performed at Glenn Research Center. Seams investigated included interleaved seams, butt seams, and overlapped seams; the latter two types of seams with multiple sub-blanket offsets built into the seam. Results showed that the interleaved seam had the best performance, and butt seams outperformed overlapped seams. Furthermore, if overlap seams are used, the sub-blankets should be overlapped by less than two inches.

The temperature model of the thermal re-radiation model in multilayer insulation systems

Abstract In this manuscript, the temperature model of multilayer insulation (MLI) on a spacecraft solar panel in thermal radiation is considered. The thermal insulation performance of MLI and the stress of the spacecraft are also considered. The thermal equilibrium equation of the surface temperature of MLI spacecraft in the presence of current transfer, and the iterative temperature acquisition algorithm of the front and back surface of solar panel MLI are derived. The Gaussian elimination method is used to invert the high order sparse matrix composed of error coefficients, thus realizing the real-time calculation of thermal re-radiation(TRR). The thermodynamic model was applied to the motion simulation of GPS Block IIR satellites, and the theoretical model was verified based on the actual temperature measurement data of several GPS Block IIR satellites. The simulation results have certain reference value for studying the thermodynamic modeling of MLI spacecraft and TRR perturbation calculation.

Aerogel-Based Insulation Materials for Cryogenic Applications

Many different aerogel-based materials are now being used in thermal insulation systems for cryogenic applications. These materials include flexible composite blankets, bulk-fill particles, and polymer composites in both evacuated and non-evacuated environments. In ambient environments, aerogels provide superior thermal performance compared to conventional polymeric foam and cellular glass insulations while offering unique advantages in solving problems with weathering, moisture, and mechanical damage. Aerogels are also used as spacer materials in multilayer insulation systems. These layered systems provide combined structural-thermal capability for cryogenic systems in either vacuum-jacketed or externally-applied insulation designs. Test data (effective thermal conductivity) include a wide range of both commercial and experimental aerogel materials. Testing was performed using laboratory cryostats and standard methods for boundary temperatures 293 K and 78 K. Examples of aerogel-based insulation systems are given for both evacuated and non-evacuated applications.

Thermal conductivity survey of different manufactured insulation systems of rectangular copper wires

Especially in high power applications, thermal design of magnetic field coils is a critical partof efficient electromagnetic system design. Since thermal expansion of the coil effects magnetic fieldgeometry, temperature drop across the windings should be kept as low as possible. Here the insulationsystem between wires guides ohmic heat to the surface of the coil and influences the total thermalperformance. Because of very less information about the general thermal performance and quality ofmanufactured multilayer insulation systems, the present survey investigates several variants made ofenameled wires and Polyimide film wrapped wires. Hereby, different joining technologies like bondingor backfilling determine the thermal conductivity, which obviously differs from values of individual rawmaterials. Best performance could be gained with a Kapton®–CR film wrapped wire, backfilled withhigh thermal conductivity resin. Finally, the survey concludes that manufactured insulation systemsdrop approximately ten to twenty percent of the thermal conductivity, which could be theoreticallyachieved by an optimal layer composition of individual raw materials.

Heat Loads Due To Small Penetrations In Multilayer Insulation Blankets

The main penetrations (supports and piping) through multilayer insulation systems for cryogenic tanks have been previously addressed by heat flow measurements. Smaller penetrations due to fasteners and attachments are now experimentally investigated. The use of small pins or plastic garment tag fasteners to ease the handling and construction of multilayer insulation (MLI) blankets goes back many years. While it has long been understood that penetrations and other discontinuities degrade the performance of the MLI blanket, quantification of this degradation has generally been lumped into gross performance multipliers (often called degradation factors or scale factors). Small penetrations contribute both solid conduction and radiation heat transfer paths through the blanket. The conduction is down the stem of the structural element itself while the radiation is through the hole formed during installation of the pin or fastener. Analytical models were developed in conjunction with MLI perforation theory and Fourier’s Law. Results of the analytical models are compared to experimental testing performed on a 10 layer MLI blanket with approximately 50 small plastic pins penetrating the test specimen. The pins were installed at ∼76-mm spacing inches in both directions to minimize the compounding of thermal effects due to localized compression or lateral heat transfer. The testing was performed using a liquid nitrogen boil-off calorimeter (Cryostat-100) with the standard boundary temperatures of 293 K and 78 K. Results show that the added radiation through the holes is much more significant than the conduction down the fastener. The results are shown to be in agreement with radiation theory for perforated films.

Repeatability of Cryogenic Multilayer Insulation

Due to the variety of requirements across aerospace platforms, and one off projects, the repeatability of cryogenic multilayer insulation (MLI) has never been fully established. The objective of this test program is to provide a more basic understanding of the thermal performance repeatability of MLI systems that are applicable to large scale tanks. There are several different types of repeatability that can be accounted for: these include repeatability between identical blankets, repeatability of installation of the same blanket, and repeatability of a test apparatus. The focus of the work in this report is on the first two types of repeatability. Statistically, repeatability can mean many different things. In simplest form, it refers to the range of performance that a population exhibits and the average of the population. However, as more and more identical components are made (i.e. the population of concern grows), the simple range morphs into a standard deviation from an average performance. Initial repeatability testing on MLI blankets has been completed at Florida State University. Repeatability of five Glenn Research Center (GRC) provided coupons with 25 layers was shown to be +/- 8.4% whereas repeatability of repeatedly installing a single coupon was shown to be +/- 8.0%. A second group of 10 coupons has been fabricated by Yetispace and tested by Florida State University, the repeatability between coupons has been shown to be +/- 15-25%. Based on detailed statistical analysis, the data has been shown to be statistically significant.

Cylindrical cryogenic calorimeter testing of six types of multilayer insulation systems

Extensive cryogenic thermal testing of more than 100 different multilayer insulation (MLI) specimens was performed over the last 20 years for the research and development of evacuated reflective thermal insulation systems. From this data library, 26 MLI systems plus several vacuum-only systems are selected for analysis and comparison. The test apparatus, methods, and results enabled the adoption of two new technical consensus standards under ASTM International. Materials tested include reflectors of aluminum foil or double-aluminized Mylar and spacers of fiberglass paper, polyester netting, silk netting, polyester fabric, or discrete polymer standoffs. The six types of MLI systems tested are listed as follows: Mylar/Paper, Foil/Paper, Mylar/Net, Mylar/Blanket, Mylar/Fabric, Mylar/Discrete. Also tested are vacuum-only systems with different cold surface materials/finishes including stainless steel, black, copper, and aluminum. Testing was performed between the boundary temperatures of 78 K and 293 K (and up to 350 K) using a thermally guarded one-meter-long cylindrical calorimeter (Cryostat-100) for absolute heat flow measurement. Cold vacuum pressures include the full range from 1 × 10−6 torr to 760 torr with nitrogen as the residual gas. System variations include number of layers from one to 80 layers, layer densities from 0.5 to 5 layers per millimeter, and installation techniques such layer-by-layer, blankets (multi-layer assemblies), sub-blankets, seaming, butt-joining, spiral wrapping, and roll-wrapping. Experimental thermal performance data for the different MLI systems are presented in terms of heat flux and effective thermal conductivity. Benchmark cryogenic-vacuum thermal performance curves for MLI are given for comparison with different insulation approaches for storage and transfer equipment, cryostats, launch vehicles, spacecraft, or science instruments.

Transmissivity testing of multilayer insulation at cryogenic temperatures

The problem of degraded emissivity of thin films at low temperatures has been a long observed phenomena. Previous efforts at measuring properties have suggested that transmission of energy through the films may play a key role in the thermal performance of multilayer insulation systems at low temperatures. Similarly, recent testing on tank applied systems has suggested a radiative degradation at low temperatures. Two different approaches were used to attempt to measure the transmission of energy through MLI at low temperatures. A laser based measurement system was set up to directly measure transmittance and a calorimetric based measurement system was used to measure relative emittance of a single layer between aluminum foil and double aluminized Mylar. Minimal transmission at long wavelengths were observed through standard MLI blanket materials at deposition thicknesses of even 35nm. Where transmission was measured, it was too low to effect the performance of a multilayers system. Similarly, the calorimeter showed similar increases of emissivity for both standard blanket materials and aluminum foils. Multiple different methodologies of measurement have all yielded the same result: that there is no transmission through standard MLI blanket materials at wavelengths associated with temperatures as low as 2K.

Influence of extreme thermal cycling on metal-polymer interfaces

Thin metal films on polymer substrates are key components for a variety of terrestrial and space applications such as flexible microelectronics or multilayer insulation systems. Although these applications seem to be quite different, there is a similarity in terms of thermal stress. In both cases the material undergoes a considerable amount of thermal cycling of±hundreds of degree Celsius during its lifetime. Due to the composite nature of the material it is important to investigate interfacial properties as a function of this thermal cycling. This work investigates interface strength and electro-mechanical behaviour as a function of thermal cycling for vapour deposited Aluminum (Al) films (150nm) on 25μm Polyimide (PI). The material is subjected to various numbers of thermal cycles of ±150°C in a gaseous N2 atmosphere. Tensile induced delamination with in-situ resistance measurements is used to measure adhesion values as well as the electro-mechanical failure behaviour. It was found that although the adhesion of Al-PI is unaffected by the applied thermal load, a strong decrease in the electro-mechanical properties of the system was observed. Since good electro-mechanical properties are crucial for flexible electronic applications the resistance against thermal cycling needs to be taken into account when assessing the suitability of a material for flexible applications.

Demonstration of Hybrid Multilayer Insulation for Fixed Thickness Applications

Cryogenic multilayer insulation (MLI) systems provide both conductive and radiative thermal insulation performance. The use of radiation shields with low conductivity spacers in between are required. By varying the distance and types of the spacers between the radiation shields, the relative radiation and conduction heat transfers can be manipulated. However, in most systems, there is a fixed thickness or volume allocated to the insulation. To understand how various combinations of different multilayer insulation (MLI) systems work together and to further validate thermal models of hybrid MLI systems, test data are needed. The MLI systems include combinations of Load-Bearing MLI (LB-MLI) and traditional MLI (tMLI). To further simulate the space launch vehicle case wherein both ambient pressure and vacuum environments are addressed, different cold-side thermal insulation substrates were included for select tests.The basic hybrid construction consists of some number of layers of LB-MLI on the cold side of the insulation system followed by layers of tMLI on the warm side of the system. The advantages of LB-MLI on the cold side of the insulation blanket are that its low layer density (0.5 - 0.6 layer/mm) is better suited for lower temperature applications and is a structural component to support heat interception shields that may be placed within the blanket. The advantage of tMLI systems on the warm side is that radiation is more dominant than conduction at warmer temperatures, so that a higher layer density is desired (2 - 3 layer/mm) and less effort need be put into minimizing conduction heat transfer. Liquid nitrogen boiloff test data using a cylindrical calorimeter are presented along with analysis for spacecraft tank applications.

Standardization in Cryogenic Insulation Systems Testing and Performance Data

The close relationship between industrial energy use and cryogenics drives the need for optimized thermal insulation systems. Emerging cryofuels usage is enabled by adequate isolation of the liquid hydrogen or liquefied natural gas from the ambient environment. Thermal performance data for the total insulation system, as rendered, are essential for both engineering designs and cost-benefit decisions involving comparisons among alternatives. These data are obtained through rigorous testing with suitable apparatus and repeatable methods. Properly defined terminology, analysis, and reporting are also vital. Advances in cryogenic insulation test apparatus and methods have led to the recent addition of two new technical standards of ASTM International: C1774 - Standard Guide for Thermal Performance Testing of Cryogenic InsulationSystems and C740 - Standard Guide for Evacuated Reflective Cryogenic Insulation. Among the different techniques described in the new standards is the cylindrical boiloff calorimeter for absolute heat measurement over the full range of vacuum pressure conditions. The details of this apparatus, test method, and data analysis are given. Benchmark thermal performance data, including effective thermal conductivity (ke) and heat flux (q) for the boundary temperatures of 293K and 77K, are given for a number of different multilayer insulation (MLI) systems in comparison with data for other commonly-used insulation systems including perlite powder, fiberglass, polyurethane foam, and aerogels.

Multilayer insulation considerations for large propellant tanks

Multilayer insulation (MLI) systems for cryogenic instrument Dewars have demonstrated very high thermal performance in ground and orbit. Ground tests of insulation systems on propellant storage size tanks have shown performance and repeatability issues. This paper presents a summary of studies for National Aeronautics and Space Administration (NASA) focused on MLI systems on the larger Tankage. The sensitivity of boil off to MLI thermal conductivity is presented. The effect of compressions, assembly joints and MLI parameters are presented. A novel large tank simulator approach for MLI testing is presented along with recommendations for maturation of the MLI technology.

Studies on high-temperature multilayer thermal insulations

Experimental and theoretical studies are reported on the determination of the effective thermal conductivities of multilayer thermal insulation systems with specially designed shields for application in high temperature fuel cells. An experimental apparatus and instrumentation have been designed and built to measure heat transfer through such insulation systems at temperatures up to 1000 °C. The insulations consisted of both fibrous and microporous materials. A theoretical model based on combined conduction and radiation heat transfer through porous materials capable of absorbing, emitting and scattering has been used to predict the temperature distribution and heat transfer in insulations comprised of the materials separated by multiple screens (shields). The experimental results obtained for several multilayer thermal insulation systems are reported, and the experimental data are compared with theoretical predictions to assess the effectiveness of the screens in reducing the effective thermal conductivity of the insulations.