Search Results (6)

This paper concerns measurements and CFD simulations of vacuum-insulated glazing (VIG), which consists of two glass panes separated by a narrow gap from which air has been removed. Distancers, e.g., in the form of small balls, are inserted into this gap every few centimeters to prevent the glass from deflecting. In the first part, simulations of two-pane VIG thermal transmittance with the Ansys Fluent program are described, resulting in thermal transmittance of VIG without the network of distancers equal to 2.18 W/(m²K) and with the distancers equal to 2.29 W/(m²K). The influence of the supports on the thermal transmittance of VIG is also determined. The CFD results show that the supporting balls increase the two-pane VIG thermal transmittance by about 0.15% with respect to the glazing without the distancers. Then, VIG is analyzed both numerically and tested in two measurement stands. Firstly, the tests are performed in a guarded hot-plate apparatus, according to the EN ISO 8302 standard. The two-pane glazing with one low-emissivity coating has a measured thermal transmittance equal to 1.75 W/(m²K). Other measurements were undertaken in the calorimetric chamber equipped with the hot-box apparatus. The results of the numerical assessment are then compared to the measurements of the existing three-pane vacuum-insulated glazing with two low-emissivity coatings, the same as simulated. The procedure follows the EN ISO 8990 standard. Measurement results of 1.10 W/(m²K) are compared to the simulation results of VIG thermal transmittance equal to 1.09 W/(m²K). A satisfactory agreement is reached. Additionally, this paper considers a new correction coefficient to thermal transmittance according to standard EN 673 in order to achieve a proper calculation of vacuum-insulated glazing in the center-of-glass region. The authors propose to use an adjustment coefficient of 1.05 when calculating the thermal transmittance of vacuum-insulated glazing without taking into account convection in the vacuum space and the thermal influence of distancers. Full article

The paper presents investigations devoted to the analysis of the thermal contact conduction in a bundle of round steel bars. The phenomenon can be expressed quantitatively with the use of thermal contact conductance (h_ct). The starting points for the presented analysis were the results of the experimental measurements of the effective thermal conductivity. The measurements were performed for samples of a medium in the form of flat packed beds of bars with three different arrangements: staggered, in-line, and crossed and four bar diameters: 10, 20, 30, and 40 mm. Next, a mathematical model was developed, thanks to which the values of the h_ct coefficient were calculated for the analyzed cases. This approach consists in analyzing thermal resistances in the medium model, which is defined with an elementary cell. It was established that the value of the h_ct coefficient in the temperature range of 50–600 °C changes within the range of 50–175 W/(m²·K), and it decreases with an increase in the bar diameter. The final effect of the present study was to develop generalized approximation equations describing changes in thermal contact conductance in the heated bar bundle simultaneously in the temperature and bar diameter function. Full article

In many cases of heat treatment of steel products, the heated charge has a porous structure. The examples of such charges include bundles of long steel components e.g., bars. The basic thermal property of the charge in this form is effective thermal conductivity k_ef. This paper presents the results of experimental examinations of effective thermal conductivity of the porous charge, which is composed from various types of steel long components. Due to the specific nature of the samples, a special measurement stand was constructed based on the design of a guarded hot plate apparatus. The measurements were performed for sixteen different samples across a temperature range of 70–640 °C. The porosity of the samples, depending on the type of components used, ranged from 0.03 to 0.85. Depending on these factors, the effective thermal conductivity ranged from 1.75 to 8.19 W·m⁻¹·K⁻¹. This accounts for 0.03 to 0.25 of the value of thermal conductivity of the solid phase of the charge, which in the described cases was low-carbon steel. It was found that the effective thermal conductivity rises linearly with temperature. The intensity of this increase and the value of coefficient k_ef depend on the transverse dimension of the components that form the charge. The results may represent the basis for the validation of various models of effective thermal conductivity with respect to the evaluation of thermal properties of the porous charge. Full article

Insulation performance in older buildings is usually poor, so retrofitting the insulation in these buildings would reduce the energy required for heating, resulting in cost and energy savings. Windows account for a significant amount of the heat loss, therefore, we have developed vacuum layer type vacuum insulation panels (VIPs) with a frame structure that is also slim and lightweight. The developed VIPs are inexpensive and easy to install, as well as being slim and translucent, so retrofitting the window insulation of existing buildings can be easily performed. In this paper, we propose a frame covering with a low emissivity film and a gas barrier envelope coating, with a focus on a reasonable design method. Firstly, a structural model was created to evaluate the safety and specifications of the frame using element mechanical analysis. Next, a finite element model (FEM) was created to predict the insulation performance. Subsequently, experimental validation was completed and the insulation performance was evaluated with the measured thermal conductivity by a guarded hot plate (GHP) apparatus. Finally, case studies were used to evaluate the insulation performance under different conditions. The optimum design included a reasonable frame-structure to hold the vacuum layer with a high insulation thermal conductivity performance of approximately 0.0049 W/(m·K). Full article

The authors develop slim and light-weight vacuum insulation panels (VIPs) by producing vacuum layers with spacers and plastic plates. The developed VIPs have the advantages of a low cost and easy installation, thus facilitating retrofitting insulation of existing buildings. In addition, one of the developed VIPs is slim and translucent so it can be easily used for windows in an internal installation. In this paper, the authors first propose a vacuum layer type slim translucent VIP and focus on a reasonable design method. Next, the authors introduce the design process in which the structural design is obtained with element mechanical analysis and a three-dimensional analysis is conducted for the VIP element. In the study, a heat transfer model is used to predict the insulation performance through finite element analysis (FEA). Subsequently, the authors perform an experiment to measure the thermal conductivity in a guarded hot plate apparatus to validate the performance prediction. Finally, case studies are performed to confirm how the different design conditions affect the insulation performance. The optimum design of the vacuum layer type slim and translucent VIP will have a sufficient structural strength to hold and maintain the vacuum layer. The thermal conductivity is approximately 0.007 $\mathrm{W}/\left(\mathrm{m}·\mathrm{K}\right)$ , which can effectively improve the insulation performance in applications. Full article

The thermal conductivity of straw bales is an intensively discussed topic in the international straw bale community. Straw bales are, by nature, highly heterogeneous and porous. They can have a relatively large range of density and the baling process can influence the way the fibres are organised within the bale. In addition, straw bales have a larger thickness than most of the insulating materials that can be found in the building industry. Measurement apparatus is usually not designed for such thicknesses, and most of the thermal conductivity values that can be found in the literature are defined based on samples in which the straw bales are resized. During this operation, the orientation of the fibres and the density may not be preserved. This paper starts with a literature review of straw bale thermal conductivity measurements and presents a measuring campaign performed with a specific Guarded Hot Plate, designed to measure samples up to 50 cm thick. The influence of the density is discussed thoroughly. Representative values are proposed for a large range of straw bales to support straw-bale development in the building industry. Full article