Thermal Conductivity Of Insulating Materials Research Articles

Ultra‐light fluorinated polyimide foams with excellent thermal insulation, low dielectric loss and high heat resistance

AbstractHigh efficiency thermal insulation materials are highly desirable in aerospace industry, whereas combining low density and low thermal conductivity of thermal insulation materials still remains a challenge. In this work, a series of fluorinated polyimide foams (PIFs) based on 4,4‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA) were successfully prepared by microwave‐assisted powder foaming. The modification of fluorination decreased the melt strength while simultaneously enhanced the thermal properties of fluorinated PIFs through the high bonding energy of CF bond. Thus, fluorinated PIFs exhibited one of the lowest densities of reported PIFs, among them, the density of PIF6FDA‐DMBZ was as low as 9.34 kg m−3. The glass transition temperature of all fluorinated PIFs was greater than 295°C. Microwave‐assisted foaming optimized the cell structure effectively, resulting in the thermal conductivity of PIF6FDA‐ODA as low as 0.029 W m−1 K−1. The incorporation of hexafluoroisopropylidene groups reduced the polarization and enhanced the free volume of fluorinated PIFs, leading to hydrophobic characteristics (CA >105°) and ultra‐low dielectric loss properties (tan δ <0.001 at 20 GHz). PIF6FDA‐ODA demonstrated the most excellent comprehensive performance. The excellent comprehensive performance of fluorinated PIFs indicated their potential application as thermal insulation and wave‐transparent materials in aerospace, railway, and shipbuilding industries.

RESEARCH OF ENERGY EFFICIENT SOLUTIONS FOR HEAT INSULATION OF CONSTRUCTIONS AND PIPELINES WITH AEROGEL

Ways of energy saving in buildings, namely compliance with the regulatory framework of European standards, possibilities to reduce heat losses during energy transportation through pipelines on the example of heating systems and reducing heat losses during the operation of buildings and structures are studied in the article. It was found that, despite the regular updates of the State Construction Norms of Ukraine, the coefficient of thermal conductivity of insulation materials is not brought into line with generally accepted European standards. The second part of the article presents the results of analytical studies of various thermal insulation materials that are widely used in the territory of Ukraine. After comparing the results, we concluded that the most common at this stage of development of insulation materials will not fully satisfy all the requirements, which must meet all the materials for insulation of structures. The use of some materials is impossible of thermal insulation of structures and pipelines with complex configuration. The solution of energy efficiency along with increasing soundproofing properties, reducing the fire hazard, reducing the load on the bearing structures of buildings and structures is possible through the use of aerogel as a thermal insulation material. The material is characterized by low dependence of the thermal conductivity coefficient on changes in ambient temperature, which makes it the best option for use in harsh operating conditions. After analytical studies of various insulating materials, it was found that aerogel in roll in addition to low coefficient of heat transfer is characterized by better operating properties, namely, non-combustibility of the material, environmental safety, light weight, which reduces the burden on the structure, ease of installation, the possibility of using the structures with high rate of sound insulation.

Thermal Conductivity of Insulation Material: Effect of Moisture Content and Wet-Drying Cycle

The insulation materials are widely used in petrochemical, power engineering and other industries. The thermal insulation materials play an important role in the energy saving of district heating systems and in the building sector. In this work, the thermal conductivity of rock wool with different levels of moisture content and density of the insulation material was investigated by an experimental method. Experimental studies were carried out on rock wool from three different manufacturers. The effect of the wetting and drying cycle on thermal conductivity and density of the insulating material is analyzed. The thermal conductivity of the insulating material was measured using the guarded hot plate method. It was found that the thermal conductivity of insulating materials significantly affected the moisture content. The increase in thermal conductivity was from 1.33 to 4.42 times, depending on the density and the manufacturer of the rock wool. The wetting and drying cycles increase the thermal conductivity and density of the insulating material up to 2 and 2.5 times, respectively.

INSTALLATION FOR DETERMINING THE THERMAL CONDUCTIVITY OF PLATES BY THE STATIONARY METHOD

When measuring the thermal conductivity of thermal insulation materials made by powder metallurgy and building porous materials, complications arise due to the fact that heat flows through the samples are commensurate with heat losses. The steady-state heat flow (SSHF) method is simple, does not require sophisticated equipment, and allows determining the thermal conductivity not in the near-surface layer but in the entire sample volume. Its disadvantage is low accuracy and the need to use reference samples. This work aims to develop an installation for measuring the thermal conductivity of largesized samples of low-conductivity material using the steady-state heat flow method with significantly higher accuracy than existing installations using this method. This is achieved by sandwiching the sample, which is a thin square plate of large dimensions, between the heater chamber and the refrigerator. The heating chamber is made of insulating material and its front wall, which is in contact with the sample, is made of the copper plate (which has good thermal conductivity). In the operating mode, the temperature in the heater chamber is maintained equal to the ambient temperature, which allows us to neglect heat losses and assume that in the steady-state mode, the heater power is equal to the heat flux through the sample. The thickness of the sample is much smaller than the size of its side (the sample should be square). This assumption is necessary to use the condition of isotropic temperature distribution in the cross-section of the sample. The refrigerator is filled with water and ice. The isotropic temperature distribution in the sample is ensured by its contact with the copper walls of the heater and refrigerator chamber. The temperature of the heated surface of the sample is measured using a thermocouple inserted through a hole in the front wall of the heater chamber. The proposed design of the installation and its operating conditions make it possible to significantly improve the accuracy of determining the thermal conductivity coefficient and make the error less than 2%.

Experimental Study on the Thermal Conductivity of Improved Graphite Composite Insulation Boards

The thermal conductivity of thermal insulation materials directly affects the building energy consumption. The types and constituents of thermal insulation materials in thermal insulation boards are the key to determining the insulation performance. By optimizing the material constituents and ratios, this paper proposes an improved graphite composite insulation board (GCIB), which has lower thermal conductivity and good fire resistance. Through theoretical derivation, it is found that the limit range of the thermal conductivity of the new GCIB is 0.042–0.064 W/(m · K). Combined with the results of theoretical value analysis, and according to the ratios of material components, the random distribution function of each material component is constructed, and the numerical model of GCIB is established. Through numerical analysis, the range of thermal conductivity of the new composite insulation board is 0.046–0.050 W/(m · K). Finally, we establish an experimental model of the new GCIB. Through the model test of six GCIBs, the thermal conductivity of the new GCIB is obtained as 0.046 W/(m · K), which is in good agreement with the results of theoretical analysis and numerical simulation. Through theoretical analysis, numerical simulation and a sample test, this paper verifies the better thermal insulation performance of the improved GCIB, providing theoretical and numerical simulation methods for the new GCIB, as well as a theoretical reference for the promotion and application of the GCIB.

A thermal conductivity model for insulation materials considering the effect of moisture in cold regions

Insulation is an important material for energy conservation and frost heave reduction in cold regions. The thermal conductivity of the insulation materials is usually considered as a constant. However, their thermal performance degenerates due to the absorption of water. To describe the variations of thermal conductivities with water content, a thermal conductivity model for three-phase insulation material was proposed by considering the insulation material's porous and moisture absorption characteristics. A tortuosity correction was introduced to reflect the heat conduction path. Then, the measured results and the existed models were used to evaluate availability of the proposed model. Finally, the quantitative influence of the porosity, water content, and thermal conductivity of solid phase on the thermal conductivity of wet insulation materials was analyzed. The results show that there is close agreement between the predicted results of the new model and the measured ones, with the maximum relative error <15%. The model with tortuosity correction can effectively calculate the thermal conductivities of the insulation materials in a full range of porosity (from 0 to 1). The thermal conductivity decreases with the increase of porosity, but increases with the increase of water content. The solid phase's thermal conductivity has little influence on the thermal conductivity of the insulation materials. The study can provide an effective method to predict the thermal performance variation of the insulation material due to the effect of moisture content in cold regions.

Mathematical modelling and model validation of the heat losses in district heating networks

Today the most popular system of district heating systems is based on pre-insulated pipes arranged in parallel or twin-pipe configuration. One of the greatest difficulties with heat distribution through pipelines is thermal loss from the distribution. The most efficient solution to that problem is optimising the insulation wall thickness layer according to the pipe diameter. Heat losses should be minimised at a relatively low investment cost to find the most suitable insulation thickness economically. Numerous studies focus on analytical (1D model) calculations and numerical simulations. However, there is a research gap related to laboratory devices that allow measuring the operation parameters (fluid flow, the temperature of the fluid in the supply pipe and the return pipe). This paper presents an analysis of the heat losses from pre-insulated pipes and twin pipes in the heating system network. This study compares the heat losses in the ground calculated by analytical solution (1D model) with the measurements on the dedicated experimental setup. The calculations have been made for several heating network pipe variants: twin pipes: DN40, DN50, DN65, and their counterparts in a single parallel pre-insulated system. The insulation thickness used in all cases is 30.85 mm for DN40 and 32.00 mm for DN50 and DN65. The insulation is made of rigid polyurethane foam that meets the requirements of the PN-EN 253 standard. During the investigation, the thermal conductivity of insulation material is examined. The obtained thermal conductivity results are used in the calculations. The results from laboratory devices and analytical models have been compared, demonstrating good agreement – with a low error level in the range of approximately 8%, depending on the type of district heating pipe. The validated mathematical model of the heating network is then used to calculate the heat losses in a heating network connecting an underground storage tank with a ground source heat pump. The economic analysis shows that after 5 y, a return on investment is expected when comparing twin-pipe systems and single-pipe pre-insulated heating networks.

Measuring the Specific Heat Capacity of Thermal Insulation Materials Used in Buildings by Means of a Guarded Hot Plate Apparatus

The specific heat capacity of building insulation materials is rather difficult to be determined using the conventional calorimetric methods. This is due to the small samples required for these methods which are not representative of the insulation material. Larger samples would not fulfill the requirements of the lumped system. Methods based on the transient heat transfer using heating wires or planes are commonly quick but less accurate measurements as they only consider the volume near the surface of the sample and need additional corrections and calibrations. The present investigation is based on a transient temperature control procedure using a common guarded hot plate device, which is normally used to determine the thermal conductivity of insulation materials. The procedure was performed on two different materials: one wood-based (wood fiber) and one mineral-based (expanded perlite). The results show a value of about 1200 J·kg−1·K−1 for the former and 600 J·Kg−1·K−1 for the latter. This method can also be extended to other thermal insulation materials for building application.

The impact of temperature and relative humidity dependent thermal conductivity of insulation materials on heat transfer through the building envelope

The accurate value of thermal conductivity of building insulation materials and the coupled heat and moisture calculation of the envelope structure are crucial for the analysis of energy consumption in building and the selection of air conditioning equipment. Currently, the thermal conductivity given in the relevant manuals is a fixed value at normal temperature and dry state. Studies have shown that temperature and moisture content are key factors influencing the thermal conductivity of insulation materials. The moisture transfer process in the building envelope has a significant impact on the heat transfer process, especially in extremely hot and humid areas. Therefore, this study experimentally determined the influence of temperature and humidity on the thermal conductivity of common building insulation materials. The influence of moisture transfer and thermal conductivity coupled with temperature and relative humidity on the sensible heat, latent heat and total heat passing through the building envelope has been analyzed through numerical simulations. The results show that the thermal conductivity of the most commonly used building insulation materials increases with increasing temperature and humidity, but the degree of change is different. Thermal conductivity varies depending on the temperature from about 8.8% to 21.4% (temperature is from 20 °C to 60 °C). Thermal conductivity varies depending on relative humidity from 14.8% to 186.7% (humidity is from 0% to 100%). In extremely hot and humid areas, when the thermal conductivity of five insulation materials changes with relative humidity, they have a greater impact on the heat transfer of the wall. In summer, the heat transfer through the wall when considering the transfer of moisture in the wall and the change of thermal conductivity with relative humidity is from 1.5 to 1.9 times, as much as the heat transfer through the wall when only considering the wall heat transfer and the thermal conductivity is a fixed value.

Numerical investigation on cooling effect in the circuit cabin of active cooling system of measurement-while-drilling instrument based on split-Stirling refrigerator

The thermal protection technology for high-temperature environment has made great progress in recent years. Restricted by the harsh environment of oil drilling, the design and development of measurement-while-drilling instrument (MWD) for high-temperature wells are still a worldwide issue. Although quite a few scholars have proposed a variety of thermal protection schemes, the feasibility or practical using effect has not been fully verified. Through comparative analysis of various cooling methods, it is considered that the active cooling scheme based on split-Stirling refrigerator is feasible, and a set of active cooling system is designed. The temperature distributions on the circuit cabin are presented. The influences of the structure parameters of circuit cabin, circuit board and refrigerator, the thermal conductivity of insulation material, the cooling power of refrigerator, the circuit board power, the annulus and water eye temperature on the cooling effect are discussed. The results show that the circuit board structure parameters, the insulation material thickness and thermal conductivity, the ratio between circuit board power and cooling power of split-Stirling refrigerator, the temperature difference between water eye and annulus are the key parameters affecting the cooling effect.

Measurement and identification of temperature-dependent thermal conductivity for thermal insulation materials under large temperature difference

Thermal insulation materials are widely used in the fields of thermal protection and the energy conservation at high temperature. Thermal conductivity is one of the most important parameters of characterizing the thermal insulation performance of thermal protection materials. Steady state method such as the guarded hot plate method is normally considered as the most accurate method to measure the thermal conductivity of thermal insulation materials, but the temperature range is limited to below 1000 K. Unsteady state method can be employed at higher temperature but the accuracy is lacking of validation. Consequently, thermal verification experiments are usually conducted by heating the front surface of thermal insulation materials to a very high temperature. However, it is difficult to obtain the temperature-dependent thermal conductivity by using the thermal insulation performance measured with a large temperature difference across the material. In this study, a sandwich structure experimental system is built to carry out the thermal insulation performance tests with a large temperature difference at both steady state and transient state. Both uniform temperature layer method and polynomial fitting method are adopted to extract the temperature-dependent thermal conductivity from the large temperature difference test at steady state. A new transient state identification method is proposed based on the niche genetic algorithm to identify the relation between thermal conductivity and temperature from the transient thermal response. The temperature-dependent thermal conductivity of four thermal protection materials are finally obtained. The identification results prove that the proposed transient identification method is reliable and robust.

Prospects for the Application of Wavelet Analysis to the Results of Thermal Conductivity Express Control of Thermal Insulation Materials

This article discusses an express control method that allows in situ measurements of the thermal conductivity of insulation materials. Three samples of the most common thermal insulation materials, such as polyurethane, extruded polystyrene, and expanded polystyrene, were studied. Additionally, optical and organic glasses were investigated as materials with a stable value of thermal conductivity. For the measurement of thermal conductivity, the express control device, which implements the differential method of local heat influence, was used. The case studies were focused on the reduction of fluctuations of the measured signals caused by different influencing factors using wavelet transform. The application of wavelet transform for data processing decreased the thermal conductivity measurement’s relative error for organic glass SOL and optical glasses TF-1 and LK-5. The application of wavelet transform thermal conductivity measurement data for polyurethane, extruded polystyrene, and expanded polystyrene allowed to reduce twice the duration of express control while maintaining the same level of measurement error. The results of the investigation could be used to increase the accuracy in express control of the thermal conductivity of insulation materials by improving the data processing. This approach could be implemented in software and does not require a change in the design of the measuring equipment or the use of additional tools.

Effect of environmental thermal disturbance on effective thermal conductivity of ground source heat pump system

Ground thermal conductivity, λs, is one of the important design factors for the ground source heat pump system. This study aimed to investigate the effect of environmental thermal disturbance on the λs estimation during the thermal response test (TRT), which is interpreted by the infinite line source model. For this purpose, a detailed numerical model considering the heat exchange between the circulating fluid inside the above-ground pipe of TRT rig and the outdoor environment in two representative seasons was developed. The numerical model was compared with the model with the adiabatic boundary assigned to the outer surface of above-ground pipe to evaluate the fluctuations of λs on account of the atmospheric disturbance. The parametric and sensitivity analyses were carried out to quantify the impacts of TRT seasons and above-ground pipe parameters including the surface absorptivity, thickness, and thermal conductivity of insulation material, and the length of the above-ground pipe on the λs and corresponding design borehole length. An approach of prolonging the TRT duration was proposed to impair the disturbance when the pipe is not insulated. Results show that λs fluctuates strongly and is largely misestimated considering the atmospheric disturbance, especially when TRT duration is limited. The λs varies widely as the length of above-ground pipe, insulation surface absorptivity, and insulation thermal conductivity increase, insulation thickness decreases, and TRT starts in summer. The pipe length is the most significant factor to the uncertainty of λs. The test time is recommended to last at least 3 days in summer and 3.6 days in winter when the pipe is not wrapped by a layer of insulation material. This study quantitatively assesses the impacts of environmental thermal disturbance during the TRT and gives guidance to carry out TRT properly.

Numerical investigation on heat transfer rate from the outside environment into the electronic compartment of the measurement‐while‐drilling tools

AbstractWith the increase of drilling depth, a large number of high‐temperature wells with bottom hole temperatures of more than 150°C appear. However, conventional measurement‐while‐drilling (MWD) instruments are no longer suitable. Therefore, it is necessary to design a new MWD instrument suitable for high‐temperature downhole for a long time. Compared with other active refrigeration methods, a split‐Stirling cryocooler is the best choice. In this paper, a new active cooling system is designed, based on the split‐Stirling cryocooler. The heat transfer rate entering the instrument cabin of MWD from the external environment is calculated, and the effects of the heat transfer device, insulation material, circuit board, split‐Stirling cryocooler, and working condition on heat transfer rate are discussed. The results show that the thermal conductivity of insulation material has a great influence on heat transfer rate. The thermal conductivity of the insulation material filled with the gap in the cabin must be small enough. Meanwhile, the influence of the length of the circuit board is greater than the height and width. If conditions permit, the double‐layer circuit can be used. And the thermal power of the circuit board has no effect on the heat transfer rate entering the instrument cabin from outside.

Design and construction of a portable apparatus to measure thermal conductivity

The characterization of the thermal insulation properties of construction materials represents a fundamental step on building insulation assessment. The present work aims to design and build a portable apparatus, namely, Portable LAMBDA UNI, capable of measuring the thermal conductivity of insulation materials. This portable apparatus is based on the standard ASTM C 518, which is a secondary method for measuring thermal conductivity. The apparatus also measures the effective thermal conductivity of square prism thermal insulation materials of 60 mm per side and a maximum thickness of 14 mm. The thermal conductivity of the drywall and adobe with Stipa ichu was measured with the Portable LAMBDA UNI, with the values being 0.265 W m−1 K−1and 0.357 W m−1 K−1, respectively.

Characteristics of heat fluxes of an oil pipeline armed with thermosyphons in permafrost regions

Abstract Permafrost degradation negatively affects the thermal stability of northern infrastructure. The quantification of heat loss through a buried oil pipeline is imperative to determine the thermal interaction between the pipe and underlying permafrost. However, the magnitudes and patterns of heat flux have not been studied methodically. This study quantifies how lateral and vertical heat fluxes from a buried warm oil pipeline increase the permafrost thawing rate and promote talik (i.e., bodies or layers of unfrozen ground in permafrost areas) development in subarctic regions by employing a three-dimensional conductive heat transfer model. In addition, the sensitivities of daily and annual heat fluxes to insulation thickness and its thermal conductivity were determined. Simulated results indicate that the heated oil pipeline acts as a heat source with the maximum daily heat flux of 6.7 W/m2 and maximum annual heat flux of 4.3 W/m2. The mean annual heat flux exponentially decreases with increasing insulation thickness, while has a logarithmical increase with the increase of the thermal conductivity of insulation material. For an insulated (0.04-m thickness) oil pipeline, a fast-growing talik initiates and enlarges laterally and vertically over time. By contrast, two-phase closed thermosyphons can be used as a remedying measure to mitigate the accelerated permafrost thaw around a buried oil pipeline. Thermosyphons increase the lateral daily heat flux by an average of 0.5 W/m2 and lower mean annual soil temperature at depth by an average of 2 °C. Within the context of climate warming, we expect that talik beneath the oil pipeline will enlarge faster than previously estimated in the coming decades due to the adverse effects of the subsurface water flow. The net founding of our results is applicable to improve the engineered design and assess the vulnerability of the oil pipeline in cold regions.

РАСЧЕТНО-ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ЭФФЕКТИВНОЙ ТЕПЛОПРОВОДНОСТИ ВОЛОКНИСТЫХ МАТЕРИАЛОВ

The article is devoted to the evaluation of the effect of porosity on the effective thermal conductivity of thermal insulation materials. The main factors influencing the thermal conductivity of the material, such as density, the type of porous structure of the material and humidity, are considered. The method of measuring the thermal conductivity by the stationary heat flow method and the hot zone method is described. A method for calculating the effective thermal conductivity of fibrous materials is presented. A computational and experimental study of the effective thermal conductivity is carried out and the results are analyzed.

Study on Water Absorption and Thermal Conductivity of Tunnel Insulation Materials in a Cold Region under Freeze-Thaw Conditions

A thermal insulation layer is often deposited on the lining structure of tunnels in cold regions to solve the problem of frost damage. When the air humidity in the tunnel becomes excessively high, the thermal insulation material tends to absorb water, leading to significant changes in thermal conductivity. Moreover, the temperature differences between the day and night cycles have been observed to be significant in portal sections of cold region tunnels, which facilitate the freeze-thaw cycle and, consequently, deteriorate the performance of the thermal insulation material. Therefore, the purpose of this study is to determine the changes in the water absorption, thermal conductivity, and microstructure of polyurethane and polyphenolic insulation boards under freeze-thaw conditions. To this end, an indoor water absorption test was conducted for both the insulation boards till they were saturated, which then underwent a freeze-thaw cycle test. It was determined that the water absorption and thermal conductivities of these boards increased linearly with the number of freeze-thaw cycles. In order to explore the change of thermal conductivity of thermal insulation materials after moisture absorption, this study provides insights into the relationship between the thermal conductivities and water contents of tunnel insulation materials under normal and freezing temperatures.

Thermal Conductivity Characteristics of Thermal Insulation Materials Immersed in Water for Cold-Region Tunnels

This study investigates the distribution of pore water on the basis of the measured mass moisture content after soaking the tunnel insulation material. This study also analyzes the influence of the distribution of pore water on the thermal conductivity of the material on the basis of this mass moisture content. Scanning images of phenolic and polyurethane insulation boards are obtained by computer tomography (CT). The gray volume moisture content (Gv) is deduced based on the CT scanning images, to determine the distribution of pore water (Gv is the ratio of the volume of the water sample (represented by the gray value) to the volume of the saturated water sample (represented by the gray value) which is the gray volume moisture content of the sample). The correlation between gray volume moisture content and mass moisture content is determined by comparing different algorithms of gray volume moisture content and volume moisture content. The relationship between mass moisture content and thermal conductivity can be determined using a self-made quasi-steady-state tester, whereas the relationship between gray volume moisture content and thermal conductivity can be derived indirectly. Related experimental research can predict the thermal conductivity of thermal insulation materials by using a new perspective and shows the influence of pore water distribution on the thermal conductivity of materials.

Optimization and Application Technology of Reflective Foils Using in Zirconia Fiber Insulation Materials

Graphite paper was used as reflective foils in the zirconia fiber insulation materials system, the effects of the number, spacing and emissivity of reflective foils on the thermal insulation properties of zirconia fiber insulation materials were studied. It was found that when the number of reflective foils is 10, interlayer spacing was 1 mm, the minimum thermal conductivity of the insulation materials was obtained. Zirconia superfine powder was used to reduce the emissivity of reflective foils, the modified reflective foils was used in the zirconia fiber insulation materials system, it was found that when the emissivity of the modified reflective foils was 0.37, the minimum thermal conductivity of insulation materials was obtained.