Radiation Effects in Heat Transfer Mechanisms by Dispersive Materials: Numerical Analysis in Diffusive Enclosure Model of Thermal Insulation

The article discusses experimental studies of the size and shape of structured particles of microsilica small angle x-ray scattering method and a photophonon theoretical description of the heat transfer process in complex heterogeneous structures to assessment of the structural characteristics of granular systems for the properties of thermal insulating materials. The mechanism of heat transfer in granular, porous systems is quite complex, since heat exchange occurs in a material consisting of two phases (solid and gas) and at the phase boundary. Heat transfer in liquid thermal insulation coatings can be carried out from one solid particle to another. In this case, the thermal conductivity will depend on: the chemical and elemental composition of the material; particle granulometry; surface topology - the presence of inhomogeneities, defects on the surface; the number of touches and the contact area between the particles. The heat transfer of gas in the pores is carried out when gas molecules collide. Thermal conductivity will be determined by the ratio of the free path of molecules and linear pore sizes, temperature and dynamic viscosity of the gas phase, the nature of the interaction of gas molecules with the solid phase. Heat transfer by radiation depends on the nature of the particles, the dielectric, magnetic permeability and the degree of blackness of the particle surface. Based on the analysis of possible mechanisms of heat transfer in granular systems, it can be argued that the effective thermal conductivity of the system depends, all other things being equal, on the structure of the pore space of granular materials, topology and the number of particle touches. Considering idealized models of the structure of granular materials in the form of ordered folds of perfectly smooth balls, we can obtain several variants of structures: with tetrahedral; hexagonal; cubic packing of balls.

This article emphasizes the significance of understanding the actual thermal properties of thermal insulation materials, which are crucial for avoiding errors in building design and estimating heat losses within the energy balance. The aim of this study was to analyse the thermal parameters of selected thermal insulation materials, particularly in the context of their stability after a period of storage under specific conditions. The materials chosen for this study include commonly used construction insulations such as polystyrene and mineral wool, as well as modern options like rigid foam composites. Experimental studies were conducted, including the determination of the thermal conductivity coefficient λ, as well as numerical analyses and analytical calculations of heat flow through a double-layer external wall with a window. The numerical analyses were performed using the TRISCO software version 12.0w, based on the finite element method (FEM). A macrostructural analysis of the investigated materials was also performed. The findings indicated that improper storage conditions adversely affect the thermal properties of insulation materials. Specifically, storing materials outdoors led to a deterioration in insulating properties, with an average reduction of about 4% for the standard materials and as much as 19% for the tested composite material. Insufficient understanding of the true thermal properties of insulation materials can result in incorrect insulation layer thickness, degrading the fundamental thermal parameters of external walls. This, in turn, increases heat loss through major building surfaces, raises heating costs, and indirectly contributes to greenhouse gas emissions.

RELEVANCE. Managing the surplus and deficit of electric power generation, which contributes to the stabilization of the energy system and enhances its reliability, is a pressing issue. One of the solutions is the development and implementation of thermal energy storage systems within distributed energy systems. An important task in their development is creating an effective insulation system. THE PURPOSE. To develop an algorithm for the effective design of insulation systems for thermal energy storages with high-temperature working bodies. METHODS. The research is carried out using theoretical methods, including thermal engineering calculation of thermal insulation layers and thermal conductivity analysis. Mathematical modeling methods were used to determine the thickness of the thermal insulation system of a thermal energy storage device. RESULTS. The design of a thermal energy storage device has been developed. Based on the developed algorithm, it was determined that the thickness of the thermal insulation system should be 151 mm (the thickness of the first thermal insulation circuit is 135 mm, the thickness of the second thermal insulation layer made of mineral wool is 16 mm), ensuring minimal heat loss at a temperature of the heat accumulator equal to 2000 °C. It was revealed that the radiant heat flux prevails in the layers closest to graphite, accounting for about 70% of the total flux. CONCLUSION. The study confirmed the effectiveness of the proposed multi-layer insulation system for thermal energy storage. The developed algorithm allows for the calculation of insulation systems of thermal energy storage, taking into account various parameters and operating conditions.

Thermal insulation materials are the key structures in many application areas for the conservation of energy and control of heat transfer. Phenolic foams (PFs) are known as rigid thermosetting polymeric foams with outstanding chemical resistance, outstanding thermal and acoustic insulation performances, excellent flame resistance, and low smoke production. Theoretical models and heat transfer mechanisms have been developed for the prediction of thermal conductivity by taking the cellular morphologies such as porosity, cell size, as well as cell structures and cell wall into consideration. Significant efforts have been devoted to decrease the thermal conductivity and improve the insulation properties of phenolic foams, which are expected to enable a broader range of applications. Various types of thermally insulating phenolic foams containing different matrix, micro- or nanofillers, and hollow particles, as well as the phenolic-based carbon foams are systematically summarized to understand the control and optimization of cellular morphology and thermal conductivity. Owing to the extremely low thermal conductivity combined with other satisfactory comprehensive properties, phenolic foams have attracted great attention for thermal insulation materials in applications such as aerospace, building, shipbuilding, petroleum, and chemical industry.

Pipe and equipment at refrigerated food processing and distribution facilities are thermally insulated to reduce energy use, control the refrigeration process, and reduce water condensation on the outside surfaces of insulated components. Ammonia is the typical refrigerant, with operating temperatures from a low of about –78°F (–61°C) to a high of about 110°F (43°C). The refrigerated pipe and equipment are typically located on rooftops and in unconditioned equipment rooms at industrial facilities. A major challenge for these insulation systems is water vapor intrusion, over time, into the insulation materials, with subsequent condensation to water or ice. This water vapor intrusion and condensation can significantly reduce the insulation's thermal performance while increasing its weight. Furthermore, severe corrosion of the steel surfaces beneath the insulation, known as corrosion under insulation (CUI), sometimes results, often requiring pipe replacement. In this research project, the author visited over 20 projects to inspect existing systems' thermal insulation, to witness the removal of old insulation systems on existing facilities, and to witness the installation of new insulation systems on both existing and new facilities. He also reviewed industry insulation installation practices and several of the insulation specifications. In all cases, new insulation systems comprised some different materials than those installed many years previously. The author notes that the design, specifications, and installation practices for these new systems better address the reduction in water vapor intrusion into the insulation materials, as well as minimization of CUI. However, he also notes that certain improvements in insulation system design, installation practices, and quality assurance or quality control practices, if consistently followed, could further improve insulation system performance and longevity.

Due to the extremely high porosity and extremely low density of nano-porous thermal insulation materials, the characteristic size of the pores inside the materials and the characteristic size of the solid skeleton structure are on the nanometer scale, which leads to the obvious nanoscale effect of the heat transfer law inside the aerogel materials. Therefore, the nanoscale heat transfer characteristics inside the aerogel materials and the existing mathematical models for calculating the thermal conductivity of various heat transfer modes at the nanoscale need to be summarized in detail. Moreover, in order to verify the accuracy of the thermal conductivity calculation model of aerogel nano-porous materials, correct experimental data are required to modify the model. Because the medium is involved in radiation heat transfer, the existing test methods have a large error, which brings great difficulties to the design of nano-porous materials. In this paper, the heat transfer mechanism, characterization methods, and test methods of thermal conductivity of nano-porous materials are summarized and discussed. The main contents of this review are as follows. The first part introduces the structural characteristics and specific application environment of aerogel. In the second part, the characteristics of nanoscale heat transfer of aerogel insulation materials are analyzed. In the third part, the characterization methods of thermal conductivity of aerogel insulation materials are summarized. In the fourth part, the test methods of thermal conductivity of aerogel insulation materials are summarized. The fifth part gives a brief conclusion and prospect.

Nano zinc oxide with a high refractive index has good thermal reflection performance, hollow glass microspheres have good thermal reflection and insulation performance, and sepiolite nanofibers with many nanostructural pores have good thermal insulation performance. The dispensability of nano zinc oxide in coating materials was improved by optimizing surface silane coupling agent modification process, leading to the good thermal reflection performance. The thermal insulation performance was improved by hollow glass microspheres and sepiolite nanofibers. On this basis, the thermal insulation coating materials were prepared by exploring the effect of amount, complex mode, and other factors of the above three kinds of functional fillers on the thermal reflection and insulation performance of coating materials. The results showed that the surface modification effect of nano zinc oxide was the best when the silane coupling agent addition was 6%. The reflection and insulation performance of the coatings were the best when the additions of modified nano zinc oxide, hollow glass microspheres, and sepiolite nanofibers were 3%, 4%, and 4%, respectively. Compared with the control coating materials, the thermal insulation effect was improved obviously, which was evaluated by the -13.5 degrees C increase of maximum temperature difference between the upper and the lower surfaces.

In this paper, the thermal insulation layer, thermal insulation support, and metal interface of the cryogenic tank are taken as the research objects, and the structural design, heat transfer mechanism analysis, heat transfer calculation model establishment, heat transfer calculation and result analysis, and temperature field numerical simulation of the cryogenic tank are studied. The heat transfer mechanism of Spray-On Foam Insulation (SOFI)/Varied Density Multi-Layer Insulation (VD-MLI) in the ground stage and space stage is analyzed. In the ground stage, SOFI heat conduction and radiation heat transfer between adjacent radiation layers in VD-MLI, gas heat conduction between adjacent radiation layers, and solid heat conduction between adjacent radiation layers were considered in terms of insulation layer design. In the space stage, SOFI heat conduction and radiation heat transfer between adjacent radiation layers in VD-MLI, residual gas heat conduction between adjacent radiation layers, and solid heat conduction between adjacent radiation layers were considered in terms of insulation layer design. Based on the heat transfer process of SOFI/VD-MLI in the ground stage and space stage, the heat transfer calculation model of SOFI/VD-MLI in the ground stage and space stage is established. According to the evaporation rate requirements of aerospace cryogenic propellant, the thermal insulation layer, thermal insulation support, and metal interface were optimized, which effectively controlled the heat flux density (HFD) and heat leakage (HL) of the cryogenic tank and ensured the evaporation rate index of cryogenic propellant and Zero Boil Off (ZBO) storage.

Triply periodic minimal surface (TPMS) structures with high surface area, high porosity, complex pore channels, and pore size distribution have great potential for application in thermal metamaterials and thermal engineering applications. To demonstrate the possibility of the use of TPMS structures as thermal metamaterials, the thermal insulation properties and heat transfer mechanisms of TPMS structures are investigated in detail. The results show that modulation of the volume fraction to within 15% by a rational geometric design indicates the possibility to obtain excellent lightweight properties. The effective thermal conductivity is within 0.25 W m−1 K−1, which is much lower than this component, indicating that the TPMS structure is designed to reduce the effective thermal conductivity and provide a lightweight design. However, in a high‐temperature environment, reasonable structural parameters can shield the cavity radiation in the TPMS structure and play an effective role to provide high‐temperature thermal insulation. Finally, based on the relationship between structural parameters and thermal insulation performance, a dynamic density TPMS‐graded structure is proposed, which exhibits a better thermal insulation performance than the conventional TPMS structure both at room temperature and at high temperature.

In the introduction of the work history was given, the importance of the application of thermal insulation and sound insulating materials in construction, the transfer of heat and sound, and the mechanisms of thermal and sound insulation. The reasons for the use of thermal and sound insulation and the consequences of insufficient heat and sound insulation are given. Heat and sound transfer and three modes of heat transfer and sound transmission in open and indoor spaces are described. In the continuation of the work, the division of thermal insulation materials was given based on formation: natural thermal insulation materials; synthetic thermal insulation materials; products of mineral origin; thermal insulation concretes and mortars and polyurethanes, and what they are made of, thereby distinguishing them and where they are used. The next chapter is the type of sound insulation material, where the characteristics, advantages, and disadvantages of the most common sound insulators are listed. The accent of the work is given to development trends and the most important modern materials for thermal insulation and sound insulation. Development trends state how thermal insulation materials are developed, according to what and what they are trying to improve. The most important modern materials for thermal insulation are singled out: aerogel, foam glass, Rockpanel plates, aerogel university of Singapore (NUS), and Bronya thermal insulation coatings with all their pros and cons. Similarly, innovative sound insulation products are listed below: sound insulation “Shumanet”; silka calcium silicate blocks; Green Glue sound insulation, and eco wool insulation. At the end of the paper, a conclusion was made as a reflection on the entire work.KeywordsHeat conductivityHeraclite platesExpanded polystyrene (EPS)Extruded polystyrene (XPS)Glass woolStone woolThermal insulation concretesThermal insulation mortarsPolyurethanesAerogelFoam glassRockpanel plates

Foams, flexible felt and thermal barrier coatings are widely used in the thermal insulation fields. This type of material has lower density and thermal conductivity, as well as higher specific surface area and porosity, and is generally referred to as the semi-transparent materials. The radiation heat transfer inside the semi-transparent materials belongs to medium radiation, so the radiation thermal conductivity is larger at high temperature. However, the metal radiation shields (MRS) play an important role in weakening radiation heat transfer. In order to study the effect of MRS on the thermal insulation performance of the semi-transparent materials, the numerical simulation of coupling heat transfer of heat conduction and heat radiation is carried out. The effective thermal conductivity of the semi-trans-parent materials with different extinction coefficient is calculated when the different layers of MRS are evenly inserted into the semi-transparent materials at the specific temperature, so that the influence of MRS on the thermal insulation performance of the semi-transparent materials can be obtained. The simulation results show that whether it is an optical thin medium or an optical thick medium, when seven layers of MRS are inserted into the semi-transparent materials, the thermal insulation performance is greatly improved. After that, the effective thermal conductivity changes little with the increase of the layer of MRS.

Electric power systems (EPS) for the future generation of electrified aircraft such as more electric aircraft (MEA) and all electric aircraft (AEA) are required to be high power delivery and low system mass. Due to the limited heat transfer by convection at the cruising altitude of a wide-body aircraft, designing cables for high power delivery and low system mass electric power systems (EPSs) based on typical standards e.g., IEC 60502 faces challenges such as thermal limits of the typical insulation systems. To design a low system mass cable system, aluminum should be used as the conductor and the overall diameter of the cable should be decreased. The former increases the joule losses of the cable, and the latter reduces heat transfer by radiation and convection, both resulting in exceeding the cable’s maximum permissible temperature. In this paper, a multi-layer insulation system for a ±5kV cable is designed for aircraft applications. The designed multi-layer insulation system experiences higher thermal conductivity and contains high-temperature materials such as AlN, PI, and PFA to compensate for the overall heat transfer reduction caused by decreasing the cable’s overall diameter. The designed multi-layer cable has a smaller thickness and a lower mass compared to insulation systems designed based on IEC 60502 standard. To determine the electric field and temperature field distributions across the designed insulation system a coupled study in COMSOL Multiphysics has been conducted. The main purpose of this study is to compare the designed multi-layer cable system’s overall diameter and mass to the cables designed based on IEC 60502 standard. Moreover, the obtained electric field distribution across the designed insulation shows that the designed insulation system is electrically safe.

Composite passive insulation technology has been proved to be an effective method to reduce heat leakage into the cryogenic storage tank. However, the current related research mainly focused on liquid hydrogen (LH2). The thermophysical properties of different cryogenic liquids and the thermal insulation materials at different temperatures are significantly different, so whether the results related to LH2 are applicable to other cryogenic liquids remains to be further determined. In fact, the insulation technology of LH2 itself also needs further study. In this paper, a thermodynamic calculation model of a composite insulation system including hollow glass microspheres (HGMs), multilayer insulation (MLI), and self-evaporating vapor cold shield (VCS) has been established. The accuracy of the calculation model was verified by the experimental results, and a comparative study on thermodynamic characteristics of the composite thermal insulation system with liquid methane, liquid oxygen (LO2), and LH2 was carried out. The results show that the heat leakage reduction of the proposed system for liquid methane, LO2, and LH2 is 25.6%, 29.7%, and 64.9%, respectively, compared with the traditional SOFI + MLI system (1 × 10−3 Pa). The type of liquid and the insulation system structure has a relatively large influence on the VCS optimal position. While for a specific insulation system structure, the insulation material thickness, storage pressure, and hot boundary temperature have a weak influence on the VCS optimal position.

The present study proposes a high-performance thermal insulator using a functional fibrous material of high reflectivity for thermal radiation, and the performance of thermal insulation was experimentally investigated for the glass fiber having aluminum powder on its surface as one of such functional fibrous materials. The heat transfer coefficient was measured in a rectangular cavity for vacancy, glass wool, glass wool with aluminum powder and glass wool with multilayer of aluminum foil. The experimental results showed that radiation heat transfer has serious effects on the performance of thermal insulator and that glass wool with aluminum powder is effective in thermal insulation.

ABSTRACTHigh‐performance thermal insulation is critically needed in applications where heat transfer must be substantially minimized. Traditional insulating materials, whether organic or inorganic, often suffer from thermal instability or mechanical fragility. Herein, we introduce a series of lightweight, highly porous polyimide/aluminum oxide cluster (PI/AlOC) composite aerogels that exhibit superior thermal insulation properties, achieved through freeze‐drying and thermal imidization processes. The aluminum oxide clusters serve as cross‐linking agents, enhancing the interaction between polyimide molecular chains and endowing the composite with improved structural integrity and mechanical robustness, as evidenced by a compression modulus of 8.7 MPa, six‐fold greater than that of pure PI aerogels. Moreover, the high porosity, reduced pore size, and three‐dimensional network structure of the PI/AlOC composite aerogel confer exceptional thermal insulation performance, particularly at elevated temperatures, surpassing that of commercial thermal insulation materials. Thus, the PI/AlOC composite aerogels, with their high mechanical strength and outstanding thermal insulation, are promising for practical applications in thermal insulation.