Vacuum Insulation Systems Research Articles

A novel approach to thermal insulation modelling in soft and medium vacuum insulation systems

The accuracy of vacuum-dependent models for predicting thermal performance of Multi-Layer Insulation (MLI) and other layered insulation systems is critical for the development of novel solutions in the aerospace and energy sectors, particularly long distance superconductors and cryogenic transfer lines. This paper presents a review of the current state of the art in cryogenic vacuum insulation systems and their associated modelling techniques and test methods. Current modelling techniques, namely the Lockheed and McIntosh MLI models, are compared to cryogenic, boil-off calorimeter test data for 3 types of MLI from the current literature. Both current models provide acceptable accuracy at high vacuum pressures but deviate from the test data when gas conduction becomes the dominant heat transfer mechanism (Kn≤1). Neither of the current models follow the characteristic S-curve observed by researchers during insulation tests. This paper presents the introduction of a novel modelling approach for layered insulation systems though changes to the current state of the art, specifically at soft (pvac≤7.5×105 mTorr) and medium vacuum (pvac≤7.5×102 mTorr) pressures, by substituting the gas conduction term in both equations with alternative terms based on the system Knudsen number (Kn) and molecule mean free path (l). This results in a stronger pressure dependence across the vacuum regime. Both modified models exhibited the characteristic S-curve with significantly reduced errors over the entire range.

Numerical approach to analyze fluid flow in a type C tank for liquefied hydrogen carrier (part 2: Thermal flow)

The present study aimed to analyze and compare vacuum insulation systems suitable for a type C liquid hydrogen storage tank. Multiphase thermal analysis was conducted via computational fluid dynamics, and experimental results for the boil-off rate (BOR) of a type C liquid nitrogen storage tank were obtained to verify the simulation technique. The simulation results showed a maximum error of 0.941 % compared to experimental results. Three insulation systems with superior thermal performance were selected and compared in a 3D system. The Vacuum+MLI Mylar-net insulation system showed the best thermal performance, with a result of 0.189 %/day. An analysis of the parts vulnerable to heat loss was conducted. These findings contribute to the development of high-performance insulation systems for the safe and economic transportation of liquid hydrogen. The study is an important resource for assessing insulation performance during the design phase and preventing local heat loss during operation.

Design and 3D printing of porous cavity insulation structure for ultra‐high electrical withstanding capability

AbstractVacuum‐dielectric interface is the most vulnerable part of vacuum insulation systems where surface electrical breakdown is prone to happen, hence severely restricts the development of advanced electro‐vacuum devices with large capacity and its miniaturisation. Generally, a direct and effective way to improve vacuum surface insulation is to alleviate the initiation and development of multipactor phenomena. Inspired by this approach, the authors report a 3D‐printed insulation structure designed with a millimetre‐scale surface cavity covered by periodic through‐pore array via stereolithography, exhibiting remarkable multipactor suppression and flashover threshold improvement, well outperforming the conventional flashover mitigation strategies. Experiments and simulations demonstrate that electrons in the multipactor region pass through‐pores and are unlikely to escape from the cavity, hence no longer participate in the above‐surface multipactor process, and eventually improve flashover threshold. The proposed approach provides new inspiration for the design of advanced insulators featuring ultra‐high electrical withstanding capability and brings up new insight into pertinent industrial applications.

Measurement of optical spectrum and mass spectrum in vacuum surface flashover for polymeric materials

The phenomenon of surface flashover in vacuum is a common problem in vacuum insulation systems. Although the theory of secondary electron emission avalanche is widely accepted, the developing process of vacuum surface flashover is not very clear and it needs to be further studied. The surface discharge process was diagnosed by spectral and mass spectrum analysis in this paper. The surface flashover was initiated by a microsecond pulse with a rise time of 500 ns and a full width at half maximum of 8 μs. Optical diagnostic methods were used to obtain the luminous properties of several polymeric materials. The time-domain optical signal was measured by a photomultiplier tube with wavelength range of 300-650 nm. The spectrograph was used to detect the optical emission from the surface discharge. The outgassing characteristics in the flashover were also measured by mass spectrometer connected to the vacuum experiment chamber. The results show that the optical signal appears before flashover and the light intensity reaches its maximum at about 80 ns after the flashover occurs. The optical emission spectrum is mainly in range of 400-800 nm, and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , H and Cu spectrum lines were detected in polymeric materials. The results of mass spectrum measurement show that the outgassing of polymeric materials in surface flashover includes the desorbed gases and decomposed gases of insulation materials. It is consistent between the results of emission spectrum and mass spectrum.

Formation of asperities on electrode surfaces of high-voltage vacuum insulation systems by striking microparticles

The authors analyzed the ability to produce asperities on the surface of discharge electrodes vacuum insulation systems by hitting the electrode surface small clumps of material − microparticles. Insulation systems with electrodes made of copper, aluminum and titanium were analyzed. The dependence on the distance between the electrodes and the minimum voltage value at the terminals of the insulation system was determined, under the influence of which accelerated spherical microparticles with different radii cause deformations of the electrode surface.

Application of mixed multiplicative statistical distributions in designing vacuum insulation systems

An algorithm suitable for transferring statistical behavior of results obtained on models to the final product is developed. Expressions obtained through statistical analysis were verified by processing a corresponding experiment conducted under well controlled laboratory conditions. Obtained results show that expressions derived on the basis of applying a mixed multiplicative distribution are satisfactory. The expressions proved to be exceptionally satisfactory in the case when one system of weak spots prevails at the surfaces of the electrode system. Because of the nature of vacuum electrical breakdown mechanisms, these expressions are very suitable and practical for application in designing vacuum insulation, but can also be applied to other (especially reversible) insulation media.

Modeling of Diffusion in Closed Cell Polymeric Foams

Closed-cell foams made of polymers have the lowest thermal conductivity of any currently available insulation material other than vacuum insulation systems. The increase of foam conductivity with age occurs as air diffuses into the foam while the blowing agent diffuses out, thus modifying the cell gas composition. Also, the change in cell gas composition influences the dimensional stability of the foams. To predict the long term aging behavior and dimensional stability of these foams, the diffusion characteristics of the different components need to be known. Several models exist in the literature which describe diffusion in foams. The most popular of these models are reviewed, and effective diffusivities predicted from one model are compared with experimental data. An unsteady state model is then proposed and solved numerically using a finite difference scheme. The numerical solution algorithm is developed to efficiently solve the large number of coupled equations resulting from this model. The uptake curves predicted from both the unsteady-state model and a discrete model (Bart and Du Cauze De Nazelle, 1993) are compared with available experimental data for the polystyrene-nitrogen system. From the analysis of uptake curves generated for different numbers of cells, the effective diffusivity of the PS/N2 system is predicted. Also, the effect of initial cell gas composition and cell size on both the long term aging profile and dimensional stability of polyurethane foam is considered. The proposed model can easily be extended to include the influence of blowing agent concentration on diffusivity in the polymer phase and the isotherm describing the distribution of blowing agent between the gas and polymer phases.

The influence of coatings and the surface state on the electric strength of vacuum insulated electrodes used with a 50 Hz supply

The electric strength of the vacuum insulation of plane electrodes not subject to surface conditioning was studied with a 50 Hz sinusoidal wave in the range up to 100 kV (peak). The electrodes with rounded-off edges of Rogowski’s profile were made of OFHC copper or aluminium and were of 50 mm diameter. The paper shows the influence of fundamental factors defining the conditions of a vacuum insulation system on its electric strength, such as: the presence of a cobalt–tungsten layer coating the surfaces of copper electrodes, the presence of aluminium oxide layer coating the surfaces of aluminium electrodes, the method of preparing the electrodes surfaces (mechanical polishing or electropolishing), the value of air pressure (within the range of 2 mPa–0.2 Pa) and at a constant value pressure of 0.2 mPa, the length of vacuum gap. Experiments showed an insignificant electric strength increase of unconditioned vacuum insulation systems could be obtained by means of electropolishing copper electrode surfaces instead of mechanical polishing. When copper electrode surfaces were coated with a layer of cobalt–tungsten or cobalt–tungsten–molybdenum deposited by electroplating, the investigated systems showed a 30% increase of electric strength at a pressure of 2 mPa. Aluminium electrodes coated with aluminium oxide showed a 100% increase of electric strength of unconditioned vacuum insulation systems at the same pressure.

Electric strength of vacuum systems with Co-Mo alloy coated Cu electrodes

The paper presents the results of investigations of the electric strength at 50 Hz AC voltage of vacuum insulation systems that were not subjected to conditioning. The experiments have shown that a Co-Mo alloy coating electrolytically deposited on the surface of copper electrodes causes, at the pressure 2 mPa, >25% increase of electric strength of unconditioned vacuum insulation systems. This breakdown strength decreases with increasing gap spacing between the electrodes, and amounts /spl sim/17 kV/mm for systems with Cu electrodes at the electrode gap of 1 mm, and /spl sim/9 kV/mm at a gap of 6 mm. Plane electrodes with rounded-off edges of Rogowski's profile were used in the investigations. The electrodes with 50 mm diameter, were made of OFHC Cu. The paper shows the influence of the value of pressure from /spl sim/1 mPa to 1 Pa and at constant pressure of /spl sim/1 mPa, the length of vacuum gap, and the presence of a Co-Mo alloy layer coating the electrodes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Outgassing of Multifoil Insulation Materials in Sealed Vacuum Systems

Multifoil vacuum insulation systems provide maximum thermal protection with a minimum of insulation. In order to obtain the maximum thermal performance in such systems, a good vacuum level must be maintained with the insulation system. In order to determine the amount of getter required to maintain the vacuum environment, the rate of offgassing of the insulation materials must be known. This paper presents the results of offgassing rate measurements on typical multifoil insulation materials in the temperature range of 100 ° to 1800 °F. Rates were determined for aluminum foil, tantalum foil, nickel foil, copper foil, glass-fiber paper, aluminum-opacified glass-fiber paper, quartz paper, copper-opacified quartz paper, and quartz cloth. These materials were subjected to periods of vacuum pumping at temperatures 100 °F in excess of the offgassing measurement temperatures. The composition of the offgas was also measured. The 100 °F over-temperature bakeout was generally effective in suppressing the offgassing rates to acceptably low levels. Because of the bakeout period, some materials became adsorbers when the temperature was reduced. All rates are presented as a function of the time of pumping at the over-temperature so that the length of the bakeout period can be determined for a given desired offgassing rate.