A numerical implementation of the Dynamic Thermal Network method for long time series simulation of conduction in multi-dimensional non-homogeneous solids

The Dynamic Thermal Network (DTN) approach to the modelling of transient conduction was conceived by Claesson (1999,2002,2003) as an extension of the network representation of steady-state conduction processes. The method is well suited to the simulation of building fabric components such as framed walls and thermally massive structures such as basements but can also be applied to the long timescale simulation of other conduction problems. The theoretical basis of the method and its discretized form is outlined in this paper and a new numerical procedure for the calculation of the necessary weighting factor data is presented. Such data has previously been generated for three-dimensional bodies by a heuristic process of blending analytical solutions and numerical data. The numerical approach reported here has the advantage of accommodating parametric representations of multi-dimensional geometries and allows the data to be produced in an automated fashion and so more easily incorporated into simulation tools. Enhancements to the data reduction procedure and a generalised approach to representing complex boundary conditions are also presented. The numerical procedure has been validated by a series of comparisons with analytical conduction heat transfer solutions and discretization errors were found to be acceptably small. Compared to numerical methods, calculations using the DTN method were found to be up to four orders of magnitude quicker but with comparable accuracy

Dynamic test method of PCM content in fiberous insulation

Przedstawiono koncepcję metodyki prowadzenia testowych pomiarów zawartości materiału fazowo-zmiennego, w dużych i w założeniu jednorodnych próbkach izolacji z włókna szklanego lub celulozy, z użyciem aparatu płytowego. Podano sposób szacowania stałej czasowej próbki w procesie ustalania się strumienia ciepła po skoku temperatury.Dynamic test method of PCM content in fiberous insulation is considered. Heat balance of a sample of material with PCM is analyzed. Time constants for 7.6 cm thick samples of PCM-Enhanced Cellulose Insulation are estimated

Effect of moisture on the accurancy of steady state thermal conductivity test for a cellular concrete

W pracy przedstawiono wyniki symulacji numerycznych i badań doświadczalnych dotyczące badań współczynnika przewodzenia ciepła metodą stacjonarną dla betonów komórkowych o różnej początkowej wilgotności względnej. Numerycznie przeanalizowano wpływ gęstości materiału, grubości próbki, różnicy temperatur i zawartości wilgoci na dokładność pomiaru, a także przebieg zjawisk fizycznych podczas badań. Omówiono wyniki badań doświadczalnych dla trzech typów betonu komórkowego o różnej wilgotności.In this paper som e results of numerical simulations and experimental investigations conceming steady state tests ofthermal conductivity for cellular concretes with various initial relative humidity are presented. Effect of the material density , specimen thickness, temperature difference and moisture content on the tests accuracy, as wen as physical phenomena during the test have been numerically analysed Results of the experimental measurements for three types of cenular concrete of various moisture content have been discussed

Freeze-drying method as a new approach to the synthesis of polyurea aerogels from isocyanate and water

Polyurea (PUA) aerogels were prepared as the polycondensation product of triisocyanate and water. PUA aerogels were formed through supercritical drying (SC) and, for comparison, by freeze-drying (FD). The effect of isocyanate concentration, catalyst concentration, and drying method on properties like density, specific surface area, pore volume, compressive strength, and morphology was investigated. The properties of aerogels strongly depend on the concentration of the isocyanate in the parent solution and, to a lesser extent, on catalyst concentration. For example, aerogels with higher isocyanate concentration exhibited a higher surface area and modulus. Depending on the formulation, the materials had a density between 0.128 and 0.220 g/cm3, surface area between 140 and 210 m2/g, pore volume between 0.593 and 1.500 cm3/g, compressive modulus between 6.5 and 25 MPa, and thermal conductivity between about 0.028 and 0.033 W/m K. Most importantly, the drying method did not strongly affect the properties of the materials. These were within ~15% for SC and FD. The results indicate that freeze-drying can be successfully used to fabricate aerogels in a cost-effective way. Open image

Density and shrinkage as guiding criteria for the optimization of the thermal conductivity of poly(urethane)-class aerogels

We investigated the effect of gelation solvent, monomer type, and monomer concentration on the physical properties of freeze-dried poly(urethane)-poly(isocyanurate) (PUR-PIR) aerogels, with particular emphasis on their thermal conductivity. It was found that the gelation solvent considerably affects aerogel morphology and physical properties. Aerogels with the lowest thermal conductivity were obtained using a mixture of tetrahydrofuran (THF) and acetonitrile, in a 50% volume ratio. The influence on thermal conductivity of polyol and isocyanate structure and of their concentration was also investigated. Rigid precursors, phloroglucinol (POL), and an aromatic polyisocyanate based on toluene diisocyanate (Desmodur RC) yielded the lowest thermal conductivity. Our results were compared with recent work reporting on parameters that could be used as predictors of thermal conductivity and other physical properties of organic aerogels. None of these parameters were found to be satisfactory predictors of aerogel properties. For example, no systematic correlation between solvent solubility parameters and aerogel properties was observed. We also examined the role of the K-index. This index, defined as the ratio between porosity and contact angle, was shown recently to be a good predictor of the properties of polyurea aerogels. While the thermal conductivity scaled with the K-index, the scaling was different for each of the isocyanate monomers considered in our experiments. Thermal conductivity, instead, scaled well with the product of density and shrinkage of aerogels, independent of monomer type. The reasons of this dependence on shrinkage and density are discussed, and the use of these parameters to guide experimentation on other systems is discussed. Physical properties such as static and dynamic compression modulus and thermal stability of the most promising formulations were also examined

Linseed oil as a natural modifier of rigid polyurethane foams

Fast growing awareness of environmental and economic problems associated with global climate warming and anticipated future depletion of petroleum resources is driving the fast growing demand for new sustainable biomaterials. A scale of new developments of versatile materials made from abundant and inexpensive natural sources is growing worldwide. Following a wide volume of research studies focused on application of bio-based components for production of plastic foams, composites, coatings, sealants or adhesives, this paper reports the development of a novel type of rigid polyurethane foams (RPUFs), containing linseed oil (LO) as a natural modifier of the rigid polyurethane foams. The use of natural bio-oils for polyurethane composites, broadens the range of functional properties and reduces the costs of production. The bio-composites are more ecofriendly and creates the opportunity to utilize linseed oil, a raw material available in many countries in large quantities. Our research shows that polyurethane foams containing linseed oil demonstrate variety of favorable properties, including the improvement of mechanical strength characteristics. The aim of this work was to determine the influence of the linseed oil on the foam morphology and their physical properties. Our major goal was to optimize the foam formulation, including the content of linseed oil in the reaction mixture

Optimization of Preparation Method, Nucleating Agent, and Stabilizers for Synthesizing Calcium Chloride Hexahydrate (CaCl2.6H2O) Phase Change Material

This study investigates improvements in low-cost latent heat storage material calcium chloride hexahydrate (CaCl2.6H2O). Its melting point is between 25 and 28 °C, with relatively high enthalpy (170–190 J/g); however, this phase change material (PCM) shows supercooling and phase separation. In CaCl2.6H2O incongruent melting causes lower hydrates of CaCl2 to form, which affects the overall energy storage capacity and long-term durability. In this work, PCM performance enhancement was achieved by adding SrCl2.6H2O as a nucleating agent and NaCl/KCl as a stabilizer to prevent supercooling and phase separation, respectively. We investigated the PCM preparation method and optimized the proportions of SrCl2.6H2O and NaCl/KCl. Thermal testing for 25 cycles combined with DSC and T-history testing was performed to observe changes in enthalpy, phase transitions and supercooling over the extended period of usage. X-ray diffraction was used to verify crystalline structure in the compounds. It was found that the addition of 2 wt.% of SrCl2.6H2O reduced supercooling from 12 °C to 0 °C compared to unmodified CaCl2.6H2O. The addition of 5 wt.% NaCl or KCl proved to effectively suppress separation and the melting enthalpy achieved was 169 J/g–178 J/g with congruent melting over 25 cycles, with no supercooling and almost no reduction in the latent heat

Optimization of Preparation Method, Nucleating Agent, and Stabilizers for Synthesizing Calcium Chloride Hexahydrate (CaCl₂.6H₂O) Phase Change Material

This study investigates improvements in low-cost latent heat storage material calcium chloride hexahydrate (CaCl2.6H2O). Its melting point is between 25 and 28 °C, with relatively high enthalpy (170–190 J/g); however, this phase change material (PCM) shows supercooling and phase separation. In CaCl2.6H2O incongruent melting causes lower hydrates of CaCl2 to form, which affects the overall energy storage capacity and long-term durability. In this work, PCM performance enhancement was achieved by adding SrCl2.6H2O as a nucleating agent and NaCl/KCl as a stabilizer to prevent supercooling and phase separation, respectively. We investigated the PCM preparation method and optimized the proportions of SrCl2.6H2O and NaCl/KCl. Thermal testing for 25 cycles combined with DSC and T-history testing was performed to observe changes in enthalpy, phase transitions and supercooling over the extended period of usage. X-ray diffraction was used to verify crystalline structure in the compounds. It was found that the addition of 2 wt.% of SrCl2.6H2O reduced supercooling from 12 °C to 0 °C compared to unmodified CaCl2.6H2O. The addition of 5 wt.% NaCl or KCl proved to effectively suppress separation and the melting enthalpy achieved was 169 J/g–178 J/g with congruent melting over 25 cycles, with no supercooling and almost no reduction in the latent heat