Driving Potentials of Heat and Mass Transport in Porous Building Materials: A Comparison Between General Linear, Thermodynamic and Micromechanical Derivation Schemes

In systems of coupled transport processes the question of the appropriate driving potentials is a point of discussion. In this article, three different approaches to derive models for transport currents are systematically compared. According to a general linear approach, an arbitrary full set of independent state variables and material properties is sufficient to describe any transport current. This approach is derived here from a symmetry principle. Thermodynamic and micromechanical approaches are more complex and even less general, but they allow additional statements about the transport coefficients and they reduce the number of transport processes. In the thermodynamic approach the additional information stems from the calculation of the entropy production rate; the micromechanical approach involves a microphysical model of the considered porous system. As a practical example, the three derivation schemes are applied to the often-encountered case of non-hysteretic heat and moisture transport in homogeneous building materials. It is shown, how the general state variables of a porous system are reduced to only two. Then from the general linear approach it can be seen, that all equations for the moisture transport current using a main driving potential (e.g. moisture content, vapour pressure, chemical potential) and an independent secondary driving potential (e.g. temperature, liquid pressure) are equivalent, without recurrence to the thermodynamic or micromechanical approach. However, the transport coefficients are arbitrary phenomenological functions depending on the two state variables. Based on a literature survey it is shown, which additional statements can be made in the thermodynamic and in the micromechanical approach. The latter yields the pressure-driven model (vapour and liquid pressure as the two driving potentials). Finally it is shown, what is to be expected, if in more complex systems the number of state variables increase

Slim building envelopes using vacuum glazing and high-performance insulation

To get a slim building envelope with low thermal transmittance, and hence a low energy consumption, vacuum glazing and high-performance insulation material have been combined. Preliminary investigations both experimental and numerical have been carried out to determine thermal, hygric and mechanical properties of these building components according to either existing standards or to best knowledge procedures. This includes measurement of the center-of-glass U-value, the simulation of the influence of support pillars on the glass surface temperature and comparison of measured and calculated results. Several specimens were measured under different temperature gradients to get more information about accuracy and deviation in the production of the vacuum glazing elements. Further, mechanical stability of larger specimens was tested by the standard pendulum tests for flat glass and compared to identical tests on conventional double-glazing elements of the same size. Finally, a onedimensional hygro-thermic analysis has been conducted to determine the temperature and moisture distribution within a thin wall construction containing high-performance insulation when submitted to climatic conditions in the Swiss Midlands

Brandverhalten verschiedener Pflanzen und deren Eignung für begrünte Fassaden

Nicht nur in Südostasien, sondern immer mehr auch in kälteren Regionen wie West- und Mitteleuropa breiten sich vertikale Begrünungen bzw. grüne Wände rasant aus. Das gleichzeitige Wachstum und die Verdichtung größerer Städte und deren Bedarf an Grünflächen haben diesen Trend in den letzten Jahren beschleunigt. Die Begrünung von Gebäudefassaden und vertikalen Flächen im urbanen, gewerblichen und privaten Bereich bietet gestalterische und klimatisch relevante Vorteile für Bestands- und Neubauten

Correlation between charring rate and oxygen permeability for 12 different wood species

The charring rates of 12 different wood species originating from Europe and the tropics with densities ranging from 350 to 750kg/m3 were investigated to obtain clues on their fire resistance behavior. This was done by measuring the thickness of the charred layer after a 30-min exposure to the standard fire ISO 834-1. No correlation was observed between charring rate and density. In search of another physical property that could be used as an indicator of fire resistance, the oxygen permeabilities of the selected wood types were measured. A strong correlation between oxygen permeability perpendicular to the wood fiber direction and charring rate was found, which is quite straightforward given that oxygen is the necessary component to enable smoldering and ignition, both affecting the charring rate. It seems that oxygen permeability is potentially more suitable as a parameter to evaluate the fire resistance of char-sensitive wooden constructions, rather than density. No general preference of the tree ring orientation from 0° (tangential) to 45° and 90° (radial) was found for these measurement

Vacuum insulation panels (VIP) in refrigerator room, freezing room & fridge

With respect to the continuous trend of reduction in energy consumption vacuum insulation panels (VIP) find an increasing application in freezing and cold storage rooms of supermarkets as well as in refrigerators. This implies the investigation of VIP behaviour subject to conditions different from those encountered in building application.The present contribution deals with the evolution of the internal pressure of VIPs subjected to low and freezing temperature for a period of 5 years confirming the prediction made earlier on its linear increase 0.6 mbar/year in the refrigerator room and 0.10 to 0.26 mbar/year in freezer room depending on the VIP type.This is a clear indication that the aging process decelerates compared to the standard test condition of VIP due to the low temperature condition.Further, a refrigerator with different VIP configuration was analysed with respect to their energy performance by means of thermal bridge analysis and possibilities to reduce them

Heat and moisture balance simulation of a building with vapor-open envelope system for subtropical regions

Global warming and the resource depletion induced discussions on sustainable developments within the construction sector. Also the rapid urbanization in subtropical regions is becoming one of the most important global issues. Appropriate measures must be taken in such developments to avoid further damage to the environment. In this study, the heat and moisture balance simulation of building with a sustainable building envelope system for subtropical climate was proposed. In the moisture balance simulation the moisture buffering by the interior materials was taken into account. The prediction of moisture buffer value (MBV) of the interior finishing materials was attempted and validated by measurements. Subsequently, the whole building calculation was carried out and the contribution of the moisture buffering to the indoor comfort and energy consumption was investigated. The MBVs of the mineral-based materials were predicted with high accuracy. However, that of wood-based composite was much higher than the experimental value. In order to create a more accurate model, nonlinear moisture conductance should be accounted when modeling wood-based materials. The heating and cooling demand of a test house was 9.4 kWh/m2 and 14.5 kWh/m2, respectively. It was concluded that the utilization of the building envelope system has a high potential to provide sustainable houses in subtropical regions. In order to enhance both energy efficiency and indoor comfort of buildings in subtropical regions, there still is a strong need to develop a holistic method to find the optimum building design considering not only moisture buffering but also all the relevant factors. The presented model will be validated by in-situ measurements in the near futur

Dynamics of microcrack propagation in hardwood during heat treatment investigated by synchrotron-based X-ray tomographic microscopy

The process of crack propagation in wood during pyrolysis is strongly linked to heterogeneities in its hierarchical porous structure. Fundamental understanding of this process is necessary for the analysis of the behavior of wood structural elements during fire exposure. Synchrotron-based X-ray tomographic microscopy combined with a recently developed laser-based furnace at the TOMCAT beamline of the Swiss Light Source provides a unique opportunity to study the heat-induced propagation of microcracks in hardwood in situ with high spatial and temporal resolutions. In this study, attention was focused on the 3D microstructure of beech and the interconnectivity between morphology and cracking patterns. It is shown that thermal cracks initiate mainly along the ray cells in hardwood and in the junction of seasonal growth layers. There is a clear indication of increased total porosity of the wood due to charrin

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Dynamics of microcrack propagation in hardwood during heat treatment, investigated by synchrotron-based X-ray tomographic microscopy

The process of crack propagation in wood during pyrolysis is strongly linked to heterogeneities in its hierarchical porous structure. Fundamental understanding of this process is necessary for the analysis of the behavior of wood structural elements during fire exposure. Synchrotron-based X-ray tomographic microscopy combined with a recently developed laser-based furnace at the TOMCAT beamline of the Swiss Light Source provides a unique opportunity to study the heat-induced propagation of microcracks in hardwood in situ with high spatial and temporal resolutions. In this study, attention was focused on the 3D microstructure of beech and the interconnectivity between morphology and cracking patterns. It is shown that thermal cracks initiate mainly along the ray cells in hardwood and in the junction of seasonal growth layers. There is a clear indication of increased total porosity of the wood due to charring