Thermal Resistance Values Research Articles

Thermal Performance of Foam Glass Aggregate at Different Compaction Ratios

The use of glass waste in the construction industry has a high potential of leading to a higher recycling percentage. Foam glass aggregate (FGA) is around 98% recycled glass waste of various origins and has good insulation properties with big grain size distributions ranging between 10 mm and 60 mm. FGA has a wide range of applicability in the construction industry, which significantly differs from each implementation in the case of built-in conditions of the material. Therefore, investigating the impact of different compaction ratios, temperature, and relative humidity conditions on the thermal performance of such material is very important. In the present work, the samples of foam glass aggregates have been prepared with four different compaction ratios (10%, 20%, 30%, and 40%) to measure their impact on the material’s mechanical and thermal insulation properties. The obtained results revealed that the dry density property of the material linearly increased with elevated compaction ratios. In contrast, the submergence density did not follow the same trend behaviour under the same circumstances. The vertical strain of the foam glass aggregates decreased with increased compaction ratios, and a significant correlation behaviour was observed between the vertical strain and increased compaction ratios at high compressional loads. The material’s thermal conductivity increased with increasing compaction ratios in both 50% relative humidity and 95% relative humidity, while for the submergence condition, a significant decrease in their values was observed after compacting the material by 40%. The thermal conductivity is tested at 10 °C and 30 °C using the TLS and GHP methods. The thermal resistance of foam glass aggregate layers was calculated based on the measured results, showing an approximately linear decreasing trend with increasing compaction ratios. While the submerged foam glass aggregate samples demonstrated stable thermal resistance values at 30% compaction, by raising the compaction ratio to 40%, the material’s thermal resistance increased once again. The experimental results also found the temperature conversion coefficients, which can be used to convert the compacted FGA materials’ thermal conductivity to the temperature experienced in a different built-in state than the laboratory measurements. Our study demonstrates the broad usability of foam glass aggregate as a compacted thermal insulating layer in the building industry.

A Similarity Model of Specific Heat Loss Determined by Dimensional Analysis

This article describes an innovative method for the determination of heat flow (specific heat loss; linear heat flow density) from a one-metre length of a twin pipe directly-buried heat network. Such heat losses are currently described by applying analytical procedures based on the heat transfer theory. It is rather complicated to accurately express the heat loss using such procedures, due to the necessity to determine the individual values of thermal resistance. A simpler method to express heat loss is the balance method, as it requires measuring a temperature gradient Δt between the starting point of the heat network and the end point of the heat collection. A suitable measuring device must provide high-accuracy measurements of the temperature. In the case of very well-insulated distribution pipelines and short pipes, the temperature measurements must be accurate to the hundredths of a degree Celsius. It is impossible to install such devices as fixed equipment on every heat distribution network, due to such networks measuring many kilometres and the cost of the appropriate measuring technology. For the aforesaid reasons, the authors created a mathematical model for specific heat losses based on dimensional analysis. This method facilitates the identification of dimensionless criteria based on the relevant dimensional quantities. Functional correlations between the identified criteria may be identified on the basis of the results of physical or numerical experiments. In this study, a database of the results obtained from physical experiments conducted on two heat networks was used. The output of the similarity model was a function describing the heat flow from a one-meter pipe length that was applicable to various alternatives in relation to the geometrical, physical and boundary conditions. The standard deviation of a difference in the heat losses identified by applying the balance method and using the proposed criterial equation for a twin pipe directly-buried pre-insulated heat network was 0.515 W·m−1.

A novel flat heat pipe for anti-gravity orientations: Leaf fractal evaporator and bi-directional transport capillary channel

Flat heat pipes (FHPs) could be an innovative solution for thermal management in aerospace applications; without the aid of gravity, FHPs could still transport high density energy fluxes with superior temperature control and almost zero energy consumption. Inspired by the powerful transpiration and liquid transport ability of leaf vein structure of plants, a novel FHP design with bionic grading evaporator structured surface was proposed. In addition, a bi-directional transport capillary structure was built inside this FHP, reducing the vapor–liquid flow path and enhancing condensed liquid return simultaneously, and thereafter promoting phase change intensity of FHPs. In the present research, heat transfer performance of FHP with self-wetting fluid as a coolant has been experimentally investigated under anti-gravity, concerning on heat inputs, filling ratios, diameter ratios, porosities of hybrid capillary wick, and two representative tree-like evaporator plate structures (H type and Y type). Our experimental results indicated that heat transfer capability of FHP showed a minimal temperature difference, which was suitable for multi-gravity working conditions. Interestingly, minimal thermal resistance value of 0.45 ℃/W and average enhancement ratio in heat transfer coefficient of 56.39% were achieved simultaneously at anti-gravity orientations for a novel FHP with the 70 PPI and αMF = 0.2 of hybrid capillary wick. Superior vapor flow and diffusion and permeability capability of FHP were further obtained as filling ratio of 30%. Moreover, due to the Marangoni effect, the addition of SRWFs could reduce the axial and radial thermal resistance of the FHP by 5–7 % under anti-gravity condition. Compared with traditional heat sink, the novelty of this passive heat transfer application was capable for realizing high heat transport performance for aerospace flights and applications.

Microscale mapping of thermal contact resistance using lock-in thermography

This paper proposes a new thermal contact resistance distribution measurement method using lock-in thermography. This method was used to evaluate a two-dimensional local thermal contact resistance distribution. To be able to measure the distribution, a uniform intensity laser heating system was developed. The developed system has a top-hat intensity distribution with a diameter of 30 mm. By combining this heat source and the lock-in thermography, a new measurement instrument was developed, which can evaluate local temperature behavior in the frequency domain, including the information of the contact interface in high spatial resolution of about 70 μm. Additionally, a new thermal contact resistance measurement principle was constructed based on a one-dimensional heat transfer equation that considers the reflected and transmitted temperature waves at the boundary and contact interface. The thermal contact resistance was acquired as a solution to the inverse problem of the temperature response done by fitting analyses. The validation of this method was performed quantitatively with two samples made of two bonded isotropic graphite plates. The one sample has an intentional defect area that has a slightly higher thermal resistance. The other sample has the known thermal contact resistance value measured with a validated method. Based on the results, the measurement value agreed with the referenced value. Also, the defect area was quantitatively detected clearly as a high thermal resistance region. Furthermore, as a practical example, the measurement method was applied for two different contact interface roughness samples consisting of aluminum alloys and thermal grease. Consequently, it was revealed and visualized that the contact interface with a rough surface has a high thermal resistance spot.

The effect of thermal insulation from autoclaved aerated concrete on the energy performance of a single-family house

The article examines the effect of thermal insulation from autoclaved aerated concrete on the energy characteristics of a single-family house. Analysis of mathematical models of energy characteristics of external enclosing structures of buildings depending on the thickness of AEROC autoclaved concrete products according to the criteria of heat loss shows that the thermal resistance value of 7.11 m2K/W and the heat transfer coefficient of 0.141 W/(m2K) are achieved using the wall block AEROC D 300 and AEROC Energy thermal insulation panel with a thickness of 200 mm. These indicators correspond to the passive house standard for thermal resistance (Ro ≥ 6.7 m2K/W) and heat transfer coefficient (Uo ≤ 0.15 W/m2K) of external walls, which confirms the feasibility of using AEROC Energy D 150 thermal insulation panels in combination with wall blocks AEROC D 300 for housing construction according to passive construction standards.

Modification and Validation of a Dynamic Thermal Resistance Model for Wet-State Fabrics

To investigate the dynamic thermal resistance of woven fabrics in different wetting states, ten commonly used clothing fabrics were selected and tested for fabric thermal resistance under different levels of water saturation in accordance with Chinese national standards. Based on Mangat’s eight thermal resistance prediction models, the study improved the models by replacing the original moisture content with water content saturation. The suitability of the eight models in predicting the thermal resistance of woven fabrics in wet states was compared using the sum of squared deviations (SSD), sum of absolute deviations (SAD), and correlation coefficient (R2). The results showed that during the process from initial wetting to complete immersion, the measured thermal resistance values of the ten fabric samples were consistent with the predicted values from Model 5 in the theoretical model of thermal resistance (R2 > 0.955). The characteristic of Model 5 is that the air thermal resistance and water thermal resistance are first connected in parallel and then connected in series with the fiber thermal resistance. The corrected predicted values from Model 5 were highly consistent with the experimental measurement values and can be used to approximate the thermal resistance of woven fabrics in wet states.

ВЛИЯНИЕ ТЕРМИЧЕСКОГО СОПРОТИВЛЕНИЯ ЗАГРЯЗНЕНИЙ ТЕПЛОПЕРЕДАЮЩЕЙ ПОВЕРХНОСТИ НА ТЕПЛООБМЕН

The influence of the thermal resistance of contamination of the heat transfer surface on the intensity of heat exchange is considered. The necessity of using exact values of thermal resistances of contamination from heat carriers in the design of heat exchangers is shown

Development of areca husk fiber/low melt PET needle punched nonwovens for building insulation applications

Thermal insulation plays an important role in contributing to energy savings in the building by heat gains and losses through the building envelope. The widely used insulation material in the construction industry is glass fiber based which is derived from silica sources and other synthetic fibers used are derived from petroleum-based resources. The development of insulation materials from natural and recycled resources will contribute to the cost-benefit as well as the green building initiative. In this research, the needle punched nonwovens were developed from 100% areca fibres and also blended with low melt polyester fibers in three different blend ratios (90% areca fiber/10% low melt PET, 80% areca fiber/20% low melt PET and 70% areca fiber /30% low melt PET). The developed nonwovens were characterized by thermal and sound insulation properties. The developed nonwovens had thermal resistance values in the range of 0.062 m2K/W to 0.100 m2K/W. The developed areca fibers/ low melt PET nonwovens had a sound absorption coefficient of 0.35 dB at a high frequency (6300 Hz) range. Since the developed areca fiber/ low melt PET nonwovens have good sound insulation along with thermal insulation, they can be used in building insulation applications. The bio-degradation behavior of the developed nonwovens was also studied and the results showed that developed nonwovens have better degradation properties.

Исследование связи тепловых и энергетических характеристик монокристаллических солнечных элементов

The paper shows the possibility and results of measuring the thermal resistance junction-to-board of monocrystalline solar cells using the modulation method. Shown the dependance between the solar cell efficiency on temperature and its “junction-to-board” thermal resistance value. Approved the possibility of express solar cells diagnostics based on the thermal resistance measuring results at the input control in the production of solar batteries.

Effects of weaving parameters on acoustic and thermal insulation properties of handmade carpets

Carpeting is one of the most efficient approaches to provide both the acoustic and thermal insulation, and improve the comfort of building inhabitants. In the current research work, the acoustic and thermal properties of handmade carpets as one of the commonly used textile products in several countries, especially in Middle-East countries, have been investigated. Aiming to find the optimum weaving condition to maximize the insulation performance of the handmade wool carpets, the weaving variables of knot type, knot density, and pile height were changed. By performing a set of experiments according to the Response Surface Methodology (RSM), the sound absorption coefficient and thermal resistance values were measured for each sample. The statistical analysis showed that the sound absorption and thermal properties of carpets were appropriately described with the linear model. The pile height was the most effective factor for the both sound absorption and thermal resistance responses. Knot density was marginally significant, while the knot type had a negligible contribution. Increasing the height of the pile and knot density led to an increase in sound absorption and thermal resistance. The optimization procedure suggested that within the experimental range, the best acoustic and thermal insulation, can be obtained at knot density of 35 (per 7 cm), and the pile height of 3 cm.

Optimization of interfacial mixing for thermal transport along Si/Ge heterostructures: A molecular dynamics study

Interfaces can play critical roles in the dissipation of heat, generated during the electronic nanodevices operation. Therefore, engineering of the interfacial thermal resistance is a key factor in the thermal management design of nanoelectronics. In this research, the engineering of the interfacial thermal resistance of silicon/germanium heterostructure is explored, by varying the content of silicon or germanium atoms at the interphase (mixing region). For this goal, we conducted systematic non-equilibrium molecular dynamics simulations. For a specific interphase length, it is noticeably predicted that the interfacial thermal resistance versus the percentage of germanium atoms shows an optimal minimum value. By substituting germanium atoms with silicon atoms, but with preserving their atomic mass, it is interestingly shown that the interfacial thermal resistance increases significantly, although the optimal value for the interfacial thermal resistance is still observable. By altering the length of the interphase region and the samples’ temperature (300 and 600 K), the optimal interfacial thermal conductance in terms of the percentage of germanium atoms in the mixing region was also detectable. The findings of this work can be useful in the thermal management engineering of nanodevices, especially in nanoelectronics.

Thermal performance analysis of heat pipe using response surface methdologyUdayvir

Heat pipes are the specific class of heat exchangers. They are used in thermal management of electronic components. Research community is continuously working to obtain the optimum heat transfer performance. In present work, parametric study of heat pipe using nano- fluid has been carried out. The operating parameters of heat pipe like power supply, orientation (gravity assisted angle), filling- ratio, and nano-fluids concentration are being investigated to find the optimum thermal performance of heat pipe. Response surface method (RSM) is used to analyze the effect of operating parameters on thermal performance. The optimum value of thermal resistance and thermal efficiency are 0.3994 °C/Watt and 68.44% respectively. Most suitable power supply, inclination angle, filling ratio and nanofluid concentration are 185.85 W, 60.09°, 50.7% and 1.05 % respectively. The experimental results confirm and validate the RSM predicted results.

Experimental Study of Thermal Resistance Values of Natural Fiber Insulating Materials under Different Mean Temperatures

The purpose of this paper is to experimentally study the thermal resistance (RSI value) of building insulation materials made mainly from natural fiber. Natural fibrous materials or renewable resources and their reinforcement composites are currently being used in building and construction as a potential solution to significantly reduce thermal load and energy consumption. The RSI value is used in describing the thermal efficiency of insulating material and in an analysis of heat transfer through the structural components of a building (such as walls, roofs, and windows) under steady-state conditions. In this study, the thermal resistance values of several samples made from coir fiber, rice straw fiber, energy reed fiber, and coconut wood were calculated from the thermal conductivity which was measured at mean temperature of 20°C, using the heat flow apparatus. The lowest RSI value was recorded in the phenol-formaldehyde polymer composites reinforced by rice straw fiber (0.115 m2·K·W-1) and coir fiber (0.128 m2·K·W-1) due to the relative thinness of the tested samples (8 and 12 mm). However, these samples can be used as an additional layer in multi-layered assemblies because of their low thermal conductivity value. The highest RSI value was reported on the binderless coir fiber panel (0.909 m2·K·W-1) at the thickness of 50 mm. Another investigation examined the relationship between RSI value and mean temperature to observe the influence of variations of ambient temperature on the heat resistivity of building insulation materials. Practical data showed the decreased linear proportion between thermal resistance and specific mean temperatures increased from 0 to 40°C. It is apparent that an increase in the interior and exterior temperature of a building significantly influences the thermal resistance of its insulation materials. Based on the experimental study, once the thermal conductivity coefficient of each sample was determined, the calculated RSI value was a valuable parameter to evaluate the thermal resistant effectiveness of a multi-layered installation, which allows us to investigate practically the effect of the thickness of additional layers from different insulating materials used in building envelopes.

Combined heat conductive boards with polyimide dielectrics

Heat-conductive properties of thin heat-conductive polyimide dielectrics have been studied and their thermal resistances have been calculated. Possibility of creating combined printed circuit boards on heat-conductive bases with reduced thermal resistances of polyimide dielectrics from ~ 0.2 to ~ 0.04 °С /W is confirmed.
 Design parameters and thermal properties of the combined boards with thin polyimide (PI) dielectrics for receivers of concentrated solar radiation are studied. Possibility of providing thermal resistances of PI dielectrics not exceeding 0.43 °С/W has been confirmed.
 Technical solutions of volumetric light-emitting diode (LED) modules on combined heat-conductive boards, which are 3D-holders-heat sinks, made in the form of single heat-conductive light-reflecting mirrored element, are studied. High thermal characteristics of the modules were achieved due to increase in the area of heat sink holders by more than 2.5 – 3 times compared to flat-type LED modules.
 Scientific and technical sources were analyzed for selection of modern polyimide materials intended for development and manufacture of combined boards on heat-conductive bases with dielectrics made of polyimide films with increased thermal conductivity up to 0.36 – 0.75 W/(m•K).
 Potential possibility of creating effective combined printed circuit boards on heat-conductive basis, including those that can be bent, is confirmed using modern industrially manufactured thin heat-conductive PI films with heat-sealable thermoplastic coatings that provide the value of total thermal resistance of boards from 1.5 up to 2.8 °C•cm2/W.

Effect of bonding techniques on thermal insulation and biodegradation of banana fiber/recycled polyester nonwovens

In this research, the needle punched and chemical bonded nonwovens were developed by blending banana fiber and recycled polyester (r-PET) in three blend ratios (90% banana/10% r-PET, 70% banana/30% r-PET, 50% banana/50% r-PET). The developed nonwovens were characterized by thermal insulation and biodegradation properties. Results showed that all developed nonwovens have an excellent insulation performance; the thermal resistance value is in the range of 0.299 to 0.248 m2K/W. It was observed that nonwovens made from 90% banana/10% r-PET have better thermal insulation compared to other blend ratios in both techniques. It was also observed that the chemical bonded nonwovens had a 6.69% higher thermal resistance value compared to the needle punched nonwovens. A bi-layer hybrid structure made with chemical bonded nonwoven and needle punched nonwoven provided a thermal resistance value of 0.43 m2K/W. The chemical bonded nonwovens showed lower biodegradation compared to needle punched nonwovens.

Analysis of thermal comfort and threshold range of airflow supply parameters for different types of work in humid-heat coal mines

For the purpose of investigating the effects of various factors as to the thermal comfort of miners, obtaining the most comfortable threshold range of air supply parameters. Five types of jobs with moderate and light labor intensity were selected for the study. The effects of labor intensity, environmental parameters and thermal resistance of clothing on the thermal comfort of miners were analyzed using PMV-PPD index, air motion and temperature difference. Under the same environment, moderate manual workers showed more satisfactory thermal comfort than light manual workers. In a humid and hot mine, the degree of influence of environmental parameters on thermal comfort of workers is ranked as: temperature > humidity > wind speed. Considering the influence of thermal environment and dust on people, the temperature of 27–30 °C, humidity of 40–50% and wind speed of 1.5–2.5 m/s are an optimal combination of ventilation parameters that can simultaneously satisfy human comfort and low energy consumption. The worker can compensate the temperature by adjusting the heat resistance value of the garment. 0.38–2.12clo clothing thermal resistance value, can compensate the temperature range of 27–30 °C. These findings can provide a reference for reducing energy consumption and improving human thermal comfort.

Effects of fiber cross-section shape on thermal comfort properties of polyester interlock fabrics

In this study, some dimensional and thermal comfort properties, i.e. thermal resistance, water vapor permeability, and air permeability properties of interlock fabrics knitted from different cross-section shaped polyester fibers were investigated. Four polyester yarns of different fiber cross-section shapes (hollow-round, round, triangular, and W-shape) were used to produce the samples. From the results, it was found that the triangular and W-type fibers had higher fabric density values due to the denser and closer fiber settlement. The highest bursting strength value belonged to the sample knitted from hollow-round yarn, which had also the highest yarn-breaking load value. The samples knitted from both round cross-section polyester yarns had significantly higher air permeability and thermal resistance values than the other samples due to the lower fabric density values of these fabrics. On the other hand, the water vapor permeability values of the fabrics from triangular and W-type fibers were slightly higher than the others because of their lower thicknesses.

Optimization of Knitted Fabrics for better Thermo-Physiological Comfort by using Taguchi-based Principal Component Analysis

The water, air permeability and thermal resistance of fabrics are important attributes that have a significant impact on the thermal comfort properties of sportswear fabrics in different environmental conditions. In this work, terry and fleece fabrics were developed by varying the fibre content and mass per unit area of fabrics. Moreover, the thermo-physical properties of the developed fabrics, including air permeability, water vapor permeability and thermal resistance, were analysed before and after washing. The multi-response optimization of the thermal comfort properties of knitted fabrics was performed using principal component analysis (PCA) and the Taguchi signal-to-noise ratio (PCA-S/N ratio) to achieve optimal properties. It was determined that the selected parameters (fabric type, finishing, fibre content and fabric mass per unit area) had a significant effect on the thermal comfort properties of knitted fabrics. The PCA analysis showed that 100% cotton terry fabric before washing with an aerial weight of 220 g/m2 had higher air and water vapor permeability value, but a lower thermal resistance value.

Natural sub-bituminous coal as filler enhances mechanical, insulation and flame retardant properties of coir-polypropylene bio-composites.

Additives provide substantial improvement in the properties of composites. Although bio-based composites are preferred over synthetic polymer and metal-based composites, they do not have the requisite properties to meet specific needs. Hence, organic, inorganic and metallic additives are included to improve the properties of bio-based composites. Coal is a readily available resource with high thermal insulation, flame resistance and other properties. This work demonstrates the addition of 20-30% natural sub-bituminous coal as filler for coir-reinforced polypropylene (PP) composites and exhibits an increased tensile strength by 66% and flexural strength by 55% compared to the composites without any filler. Such composites are intended for insulation applications and as a replacement for gypsum-based false ceiling tiles. Various ratios of coal samples were included in the composites and their effect on mechanical, acoustic, thermal insulation, flame and water resistance have been determined. A substantial improvement in both flexural and tensile properties has been observed due to the addition of coal. However, a marginal improvement has been observed in both thermal conductivity (0.65W/mK) and flame resistance values due to the presence of coal. Adding coal increases the intensity of noise absorption, particularly at a higher frequency, whereas water sorption of the composites tends to decrease with an increase in the coal content. The addition of coal improves and adds unique properties to composites, allowing coir-coal-PP composites to outperform commercially available gypsum-based insulation panels.

Wavelength- and Time-Division Multiplexing via Pump Current Variation of a Pulsed Semiconductor Laser—A Method of Synchronization for Quantum Key Distribution

The dependence of the semiconductor laser wavelength on the pump current is a well-known phenomenon, which is generally attributed to a change in the refractive index of the active layer due to carrier injection. This effect is usually considered to be a drawback as it causes frequency chirping of the pulses produced via direct current modulation. Here, we show that at high values of thermal resistance of a laser diode, the lasing wavelength is red-shifted due to the change of the refractive index caused by a thermal effect and this shift may significantly exceed (in absolute value) the blue shift related to a refractive index change induced by the carrier injection. We propose to benefit from the ability to tune wavelength by the pump current variation and use the same laser to generate qubits and synchronization optical pulses for quantum key distribution at different wavelengths. To demonstrate the proposed method of synchronization, we perform numerical simulations as well as proof-of-principle experiments.