Fibrous Insulation Materials Research Articles

Cryogenic hydrogen fuel tanks for large hypersonic cruise vehicles

The design of light yet highly insulated cryogenic tanks is a key enabling technology for hydrogen-fuelled flights, especially when operation in the harsh thermal environment associated with hypersonic speeds is envisaged. This paper examines the use of different insulation systems for cryogenic hypersonic tanks. The investigation for a Mach 5 flight shows that a combination of foam and fully load-bearing aerogel blanket leads to the lightest tank, with a gravimetric efficiency of 73%. If the aerogel blanket cannot be strengthened sufficiently so that it can bear the full load, then a combination of foam and fibrous insulation materials gives the best solution with a gravimetric efficiency of 70.3%. This is around 10% better than (non-load-bearing) aerogel or multi-layer insulations. However, the performance of the foam-fibrous insulation is more sensitive to the external skin temperature than that of other insulation materials so that a foam-multilayer insulation system becomes more efficient at speeds beyond Mach 9. This demonstrates that it is critical to make the best use of each material and that proper combinations of materials can considerably improve the performance of the insulation system.

Synthesis and properties of lightweight fibrous ceramics with a 3D skeleton structure prepared by infiltration

In our report, a fibrous ceramics with the mullite fibers as the matrix and SiO2–AlPO4 sol as the high-temperature binder were fabricated by the infiltration method. The effects of sintering temperature on the properties, such as porosity, phase transition, microstructure, compressive strength and fracture mechanism, were investigated. Compared with the fibrous ceramics with the SiO2–B2O3 binder, the fibrous ceramics with the SiO2–AlPO4 binder exhibited lower densities (0.560–0.595g/cm3), lower thermal conductivities (0.157–0.165W/mK), high strain (7–8%), high rebound-resilience (77–90%) and the same level of compressive strengths (1.1–2.1MPa). During the cooling stage of the heat treatment process, α→β-cristobalite displacive phase transition in the SiO2–AlPO4 binder was successfully suppressed. The fracture behavior of the ceramics tends to be a layer-by-layer failure, which is quite different from that of the traditional ceramics. The non-brittle fracture mode and the excellent elasticity made this fibrous ceramics promising high-temperature elastic insulation materials.

Thermo-Insulation Properties Of Hemp-Based Products

Abstract As known, many multi-purpose plants can be used in different industries. This research is focused on the possibilities to utilize hemp as feedstock for thermal insulation products. The most advantageous features of hemp insulation are associated with health and environmental safety. The thermal conductivity of commercially available hemp insulation products is comparable with that of other fibrous insulation materials; however, it is possible to develop new products that could be more efficient in terms of cost and due to other important features.

Effect of parameters correlation on uncertainty and sensitivity in dynamic thermal analysis of thermal protection blanket in service

In this paper, an uncertainty propagation methodology considering correlations among the input variables for heat transfer analysis through fibrous composite insulation material subjected to severe aerodynamic conditions is presented, based on Monte Carlo (MC) simulation with Cholesky decomposition. The effects of correlation coefficient on the thermal evolution uncertainty, failure probability, and Sobol sensitivity indexes are discussed. The results show that the thermal response outputs become more scattered after taking into account the dependence of variables. The failure probability increase with the rise of the correlation coefficient, and nearly 15% increase is observed when the temperature limit is 470 K under the current uncertainty level. Sobol sensitivity analysis demonstrates that the correlated group variables of aerodynamic thermal and pressure load have the most significant first order and total effect on the thermal response simulations. However, the mean fiber diameter and the correlated group variables of density, specific heat and thermal conductivity of virgin material have weak contributions to the first order effect, but have relatively strong contribution to the interaction effect with other variables. The correlation effect of input parameters causes different influences on the sensitivity indexes of different variables. In all cases, the weakest correlation gives the highest total sensitivity index. The provided analyses demonstrate that incorporation of correlation information of stochastic variables is essential in random variable characterization, thermal evolution uncertainty level quantification, parameter importance identification and risk assessment, and it is especially imperative for thermal protection components with high reliability requirement to prevent the occurrence of catastrophic event.

Experimental study of head loss through an LOCA-generated fibrous debris bed deposited on a sump strainer for Generic Safety Issue 191

Fibrous porous media have been studied in diverse engineering fields. In nuclear power plants, fibrous debris produced during the blow-down phase of Loss of Coolant Accidents (LOCA) is transported into the sump and becomes an important cause of head loss through the sump strainer affecting the Emergency Core Cooling System (ECCS) performance. In this study, a vertically installed strainer plate in a horizontal pipe flow loop was designed to generate fibrous beds on the strainer and permit measurement of the head loss through the beds. NUKON fibrous insulation material was used as the fibrous debris prepared by shredder method explained in NUREG/CR-6224 report. The head loss data with the shredder method was mainly analyzed considering porosity and fibrous bed thickness. A modified head loss model was suggested and compared to the head loss models developed in previous studies.

The Thermal Insulation Capacity of Tree Bark

Abstract Nowadays increasing emphasis is placed on improving the quality of different insulation materials, and on developing such from materials of natural origin. The present research focuses on the thermal insulation capacity of the chipped bark of different broadleaved and coniferous wood species. We examined the bark of five tree species: black locust, a poplar clone, larch, spruce, and Scotch pine and compared their insulation characteristics to the traditionally used insulation materials. Results indicate that the thermal insulation capacity of chipped tree bark is comparable to that of generally used insulation materials, such as glass wool. Moisture content influences the thermal insulation capacity of chipped bark of the five examined species. Since energy requirement of producing chipped tree bark is very low, and it contributes also to storing carbon, therefore its CO2 balance is more advantageous compared to that of traditional fibrous or foamy insulation materials.

Modeling heat transfer with micro-scale natural convection in fibrous insulation

Fibrous thermal insulation materials are vastly employed in the thermal protection system of recoverable space vehicles. The accurate determination of their apparent thermal conductivity allows for an adequate sizing of the thermal protection system of space vehicles, with effects on the weight, structural strength, manufacture and launch costs. This property is strongly dependent on the pressure of the gas within the porous. The objective of this work was to improve the accuracy of a well-established model for estimation of the apparent thermal conductivity of fibrous insulation materials filled with gas at various pressures, which presents some increasing discrepancy as the pressure rises. After a sensitivity analysis in every term of the current model, some modifications are tested and proposed. A micro-scale convection term is modeled and included in the modified model. The results of both versions of the model are compared with experimental data, with the modified version presenting better agreement than the original model at higher pressures.

Evaluation of Thermal Properties of Fibre Insulation Materials for Inflatable Mini Cold Storage

Fibre materials, as thermal insulation materials, will be an optimum choice for inflatable mini cold storage. The evaluation index such as infrared spectrum, thermal stability and thermal conductivity is closely related to the selection and application of fibrous insulation materials. The results showed that the transmittance of bamboo fibre tested by Fourier Transform Infrared Spectroscopy (FTIR) measurements was about 35.3% with an advantage to the heat preservation, and radiation heat transfer was analyzed in the area of far-infrared. The initial decomposition temperature of 1#, 2# and 3# measured by TGA curve was respectively 383.9°C, 372.5°Cand 315.4°C, and the total thermal weight loss rate of 1#, 2# and 3# was 64.59%, 90.16% and 80.47%. The thermal conductivity of 1# at 0°C was 0.035 W·m-1·K-1, lower than 2# and 3#, also lower in the temperature range from 0°C to 8°C, measured by hypothermia thermal instrument.

The variation of thermal conductivity of fibrous insulation materials under different levels of moisture content

Thermal insulation plays an important role in determining the thermal and energy performance of a building. The effectiveness of thermal insulation is dependent on its thermal conductivity (k-value) and its ability to maintain its thermal characteristics over an extended period of time. However, the k-value can be greatly reduced by the presence of moisture within the insulation materials. For example, when circumstances are conducive, under hot–humid climatic conditions, condensation may occur within the insulation material, raising its moisture content well above the hygroscopic level. The objective of this paper is to investigate experimentally the impact of moisture content on the thermal conductivity of commonly used fibrous insulation materials. Three types of material with different densities are investigated with the emphasis on fiberglass. A comparison of the behavior and magnitude of k-value changes according to moisture content levels indicated appreciable variations. Higher thermal conductivity at a given operating temperature for the investigated densities is always associated with higher moisture content. The relationship between k-value and moisture content is found to be affected by the initial conditioning moisture content level. Materials having similar density but conditioned at different initial moisture content levels exhibit different relationships between k-value and moisture content. The rate of change in thermal conductivity with moisture content is higher at higher initial moisture content. The results should be of great importance to material manufacturers, building owners and designers when selecting suitable insulating materials and correctly predicting the thermal and energy performance of buildings and their energy-efficiency.

The impact of thermal conductivity change of moist fibrous insulation on energy performance of buildings under hot–humid conditions

The impact of the k-value change of fibrous insulation materials (i.e. fiberglass) in a typical wall–roof system due to moisture content levels on the thermal and energy performance of a typical residential building under hot–humid climatic conditions is investigated. Moisture performance is investigated utilizing theoretical long-term hygrothermal performance modeling and simulation techniques. Layer- and time-averaged levels of moisture content in the fibrous insulation are determined and the corresponding k-value change is evaluated from measured relationships. The impact of the k-value change due to moisture on the building thermal load and cooling energy performance of a residential building is then assessed utilizing detailed building energy simulation software. Limited change in the building cooling energy occurs as a result of the insulation k-value change due to moisture content occurring under moderate wetting conditions. When higher wetting conditions prevail the effect of k-value change on the building cooling energy continues to be limited although the impact of the roof on monthly cooling load is more pronounced especially during the summer months when as much as an 8% increase occurs. In general, the overall building energy performance is less affected by the k-value change of the moist fiberglass insulation under hot–humid conditions.

Analytical Monte Carlo Ray Tracing simulation of radiative heat transfer through bimodal fibrous insulations with translucent fibers

In this study, a Monte Carlo Ray Tracing (MCRT) simulation technique is developed to study steady-state radiative heat transfer through fibrous insulation materials. The simulations are conducted in 3-D disordered virtual fibrous media with unimodal and/or bimodal fiber diameter distributions consisting of fibers whose surfaces are specularly reflective, and are translucent to Infrared (IR) radiation. Scattering within the realm of geometric optics is incorporated into our MCRT simulations using Snell’s Law for ray refraction. Fibers’ optical properties are obtained from Fresnel’s law and Beer’s law based on the refractive index of the material. Two different treatments of “high” and “low” conductivities are considered for the fibers and their effects are discussed. Our results indicate that heat flux through a fibrous medium with translucent fibers decreases with increasing packing fraction of the fibers. It was observed that IR transmittance through the media increases with increasing through-plane orientation of the fibers, but is independent of their in-plane orientations. It was also found that fiber orientation has generally a negligible effect on the temperature profile across the media’s thickness. However, for the case of high-conductivity fibers, increasing fibers’ through-plane orientation tends to flatten the temperature profile. The results obtained from simulating bimodal fibrous structures indicate that increasing the fiber-diameter dissimilarity, or the mass fraction of the coarse fibers, slightly increases the radiation transmittance through the media, and accordingly reduces the temperature gradient across the thickness. Our simulation results are compared with those from the two-flux model and good agreement is observed.

A simple simulation method for designing fibrous insulation materials

Conductive heat in a fibrous material travels through both the air (interstitial fluid) and the fibers (solid phase). The numerical simulations reported in this paper are devised to study the effective thermal conductivity of fibrous media with different microstructural parameters. Simulations were conducted in 3-D fibrous geometries resembling the microstructure of a fibrous material. Assuming that the heat transfer through the interstitial fluid is independent of the geometrical parameters of the solid phase (for when the porosity is held constant), the energy equation was solved only for the solid structures, and the resulting values were used to predict the effective thermal conductivity of the whole media. This treatment allows us to drastically reduce the computational cost of such simulations. The results indicate that heat conduction through the solid fibrous structure increases by increasing the material’s solid volume fraction, fiber diameter, and fibers’ through-plane orientations. The in-plane orientation of the fibers, on the other hand, did not show any significant influence on the material’s conductivity. It was also shown that the microstructural parameters of fibrous insulations have negligible influence on the material’s performance if the conductivity of the solid phase is close to that of the interstitial fluid.

Core Materials of Vacuum Insulation Panels: A Review and Beyond

Vacuum insulation panels (VIPs) are regarded as one of the most promising high performance thermal insulation solutions on the market today. The insulation performance of VIPs mainly depends on the quality of core materials. This paper compared three types of core materials, namely foam insulation material, powder insulation material and fibrous insulation material. Novel structure of core materials which is fiber pore structures packed with different size powder particles is also put forward on this paper. The aim of this paper is to investigate and compare various properties, requirements and possibilities for traditional core materials and put forward possible future core materials of VIPs.

Determination of physical properties of fibrous thermal insulation

The objective of this study is to characterize both experimentally and theoretically, conductive and radiative heat transfer within polyester batting. This material is derived from recycled bottles (PET) with fibres of constant diameters. Two other mineral and plant fibrous insulation materials, (glass wool and hemp wool) are also characterized for comparative purposes. To determine the overall thermophysical properties of the tested materials, heat flux measurement are carried out using a device developed in house. The radiative properties of the material are determined by an inverse method based on measurements of transmittance and reflectance using a FTIR spectrometer and by solving the equation of radiative heat transfer. These measures are compared to results of numerical simulations.

A Monte Carlo simulation of radiative heat through fibrous media: Effects of boundary conditions and microstructural parameters

This work reports on a Monte Carlo Ray Tracing (MCRT) simulation technique devised to study steady-state radiative heat transfer in fibrous insulation materials. The media consist of specular opaque fibers having unimodal/bimodal fiber diameter distributions. The simulations are conducted in 2-D ordered geometries, and the role of lateral symmetric or periodic boundary conditions are discussed in detail. Our results indicate that with the symmetric or periodic boundary condition, view factor F i,i should be excluded from the calculations leading to temperature prediction. This is especially important when the media are made of fibers arranged in ordered configurations. In agreement with our previous 3-D MCRT simulations, the 2-D MCRT simulations presented here reveal that heat flux through a fibrous medium decreases by increasing packing fraction of the fibers, when fiber diameter is kept constant. Moreover, increasing fibers’ absorptivity was found to decrease the radiation transmittance through the media. In this work, we have also studied radiative heat transfer through bimodal fibrous media, and concluded that increasing fibers’ dissimilarity increases energy transmittance through the media, if porosity and number of fibers are kept constant. It was also found that temperature of the fibers is almost independent of the media’s porosity or diameter ratios.

Fast numerical upscaling of heat equation for fibrous materials

We are interested in numerical methods for computing the effective heat conductivities of fibrous insulation materials, such as glass or mineral wool, characterized by low solid volume fractions and high contrasts, i.e., high ratios between the thermal conductivities of the fibers and the surrounding air. We consider a fast numerical method for solving some auxiliary cell problems appearing in this upscaling procedure. The auxiliary problems are boundary value problems of the steady-state heat equation in a representative elementary volume occupied by fibers and air. We make a simplification by replacing these problems with appropriate boundary value problems in the domain occupied by the fibers only. Finally, the obtained problems are further simplified by taking advantage of the slender shape of the fibers and assuming that they form a network. A discretization on the graph defined by the fibers is presented and error estimates are provided. The resulting algorithm is discussed and the accuracy and the performance of the method are illusrated on a number of numerical experiments.

Modeling the role of microstructural parameters in radiative heat transfer through disordered fibrous media

Understanding the influence of microstructural parameters on the rate of heat transfer through a disordered fibrous medium is important for the design and development of heat insulation materials. In this work, by generating virtual 3-D geometries that resemble the internal microstructure of fibrous insulation materials, we simulated the influence of diameter, orientation, and emissivity of the fibers, as well as the media’s porosity and thickness on the radiative heat transmittance. Our simulations are based on a Monte Carlo ray tracing algorithm that we have developed for studying radiative heat flow in 3-D disordered media. The media were assumed to be made up of cylindrical opaque fibers with specular surface. The advantage of our modeling approach is that it does not require any empirical input values, and can directly be used to isolate and study the role of individual microstructural parameters of the media. The major limitation of the model is that it is accurate as long as the fibers can be considered large relative to the wavelength of the incoming rays. Our results indicate that heat flux through a fibrous medium decreases by increasing the packing fraction of the fibers when the thickness and fiber diameter are kept constant. Increasing the fibers’ absorptivity (or emissivity) was observed to decrease the radiation transmittance through the media. Our simulations also revealed that for constant porosity and thickness, the heat flux transmitted across the medium can be reduced by using finer fibers. The steady state temperature profiles across the thicknesses of media with different properties were obtained and found to be independent of the fibers’ emissivity.

A new procedure to measure water adsorption isotherms of porous fibrous materials

A typical relation between accumulated liquid, temperature, and vapour pressure characterizes any surface, in particular any porous structure. This relationship is usually described by means of “adsorption isotherms”, which are functions that correlate the amount of liquid in the porous material to the partial pressure of its vapour, for a given constant temperature. Speaking about fibrous insulation materials or building materials, the fluid of interest is simply water. The knowledge of water adsorption isotherms is of great importance in the formulation of heat and mass transfer models inside moist bodies. Typical experimental techniques for adsorption isotherm evaluation (salt solution methods, climatic chamber methods, etc.) require the conteporary control of both temperature and relative humidity of the environment surrounding the specimen. This often results in the need of dedicated and expensive experimental set-ups. In this work the possibility to use a simpler apparatus to obtain the sorption-desorption characteristics of a body is investigated from the theoretical point of view by means of simulated experiments. The proposed measurement procedure uses a closed, non permeable system in which only temperature is to be controlled, while the total water content, liquid plus vapor, remains constant. The new procedure offers an additional benefit, since both the constant water and the constant dry air mass constraints are used as further a␣priori information to improve the precision of the estimated results.

Radiative Heat Transfer Analysis of Fibrous Insulation Materials Using the Zonal-GEF Method

The zonal method, together with the concept of generic exchange factor, is shown to be an efficient and accurate approach to analyze radiative heat transfer in high porosity insulation materials which have a large scattering coefficient. Numerical predictions generated by the method correlate well with experimental data generated for LI900, a material used in the insulation tile for the space shuttle. Comparisons are presented for measurement in emitted surface intensity, effective thermal conductivity, and thermal response under radiant heating. The heat transfer characteristics for the material are also determined. For LI900 which is effectively a pure scattering material at the short wavelength region (λ < 5 μm), radiation from high temperature sources can be transmitted through the material even when the optical thickness is large. Radiative heat transfer in this class of material is nonlocalized in the optically thick limit. A nonlocalized radiation model such as the zonal-generic exchange factor method is needed to determine the thermal behavior of the material for design purposes.

Apparatus for in situ prediction of the thermal conductivity of fiberglass batts using acoustic propagation constant

This article describes a novel technique for the measurement of the thermal conductivity of low-density (12–18kg∕m3) fiberglass insulation and other related fibrous insulation materials using a noninvasive acoustic apparatus. The experimental method is an extension of earlier acoustic methods based upon the evaluation of the propagation constant from the acoustic pressure transfer function across the test material. To accomplish this, an analytical model is employed that describes the behavior of sound waves at the outlet of a baffled waveguide. The model accounts for the behavior of the mixed impedance interface introduced by the test material. Current results show that the technique is stable for a broad range of absorber thicknesses and densities. Experimental results obtained in the laboratory show excellent correlation between the thermal conductivity and both the real and imaginary components of the propagation constant. Correlation of calculated propagation constant magnitude versus measured thermal conductivity gave an R2 of 0.94 for the bulk density range (12-18kg∕m3) typical for manufactured fiberglass batt materials. As an improvement to earlier acoustic techniques, measurement is now possible in noisy manufacturing environments with a moving test material. Given the promise of such highly correlated measurements in a robust method, the acoustic technique is well suited to continuously measure the thermal conductivity of the material during its production, replacing current expensive off-line methods. Test cycle time is reduced from hours to seconds.