The Leidenfrost effect, namely the levitation of drops on hot solids
, is known to deteriorate heat transfer at high temperature
. The Leidenfrost point can be elevated by texturing materials to ...favour the solid-liquid contact
and by arranging channels at the surface to decouple the wetting phenomena from the vapour dynamics
. However, maximizing both the Leidenfrost point and thermal cooling across a wide range of temperatures can be mutually exclusive
. Here we report a rational design of structured thermal armours that inhibit the Leidenfrost effect up to 1,150 °C, that is, 600 °C more than previously attained, yet preserving heat transfer. Our design consists of steel pillars serving as thermal bridges, an embedded insulating membrane that wicks and spreads the liquid and U-shaped channels for vapour evacuation. The coexistence of materials with contrasting thermal and geometrical properties cooperatively transforms normally uniform temperatures into non-uniform ones, generates lateral wicking at all temperatures and enhances thermal cooling. Structured thermal armours are limited only by their melting point, rather than by a failure in the design. The material can be made flexible, and thus attached to substrates otherwise challenging to structure. Our strategy holds the potential to enable the implementation of efficient water cooling at ultra-high solid temperatures, which is, to date, an uncharted property.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Interfacial thermal resistance (ITR) is the main obstacle for heat flows from one material to another. Understanding ITR becomes essential for the removal of redundant heat from fast and powerful ...electronic and photonic devices, batteries, etc. In this review, a comprehensive examination of ITR is conducted. Particular focus is placed on the theoretical, computational, and experimental developments in the 30 years after the last review given by Swartz and Pohl in 1989. To be self-consistent, the fundamental theories, such as the acoustic mismatch model, the diffuse mismatch model, and the two-temperature model, are reviewed. The most popular computational methods, including lattice dynamics, molecular dynamics, the Green's function method, and the Boltzmann transport equation method, are discussed in detail. Various experimental tools in probing ITR, such as the time-domain thermoreflectance, the thermal bridge method, the 3ω method, and the electron-beam self-heating method, are illustrated. This review covers ITR (also known as the thermal boundary resistance or Kapitza resistance) of solid-solid, solid-liquid, and solid-gas interfaces. Such fundamental challenges as how to define the interface, temperature, etc. when the materials scale down to the nanoscale or atomic scale and the opportunities for future studies are also pointed out.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
•FEM simulations of different construction joints for the wall-foundations-ground node.•Regression equations for all the configurations to evaluate linear transmittance.•Correlations to evaluate the ...ground-contact thermal bridges in existing buildings.•Tests for more than 1000 practical cases confirmed accuracy of proposed correlations.
The issue of transmittance of thermal bridges in existing buildings has been addressed aiming to achieve correlations able to evaluate the ground-contact thermal bridges. A bi-dimensional steady state FEM model has been implemented to simulate wall-to-floor structural node and then validated in accordance with the EN ISO 10211:2017 standard. Different construction joints have been then simulated for the structural node foundations-vertical elements up to a total of 19 configurations, from which 1700 cases have been derived varying walls and floor stratigraphies and ground properties. Correlations for the linear thermal bridge transmittance have been calculated through regression technique for all the configurations, together with their validity ranges expressed in terms of 95% confidence interval values. Tests performed for more than 1000 practical cases confirmed the accuracy of the proposed correlations. Through those correlations ground contact thermal bridges in existing buildings can be hence analyzed in a simple and operative way, offering technicians in the sector a tool that covers most of the possible situations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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•This study presents a state-of-the-art on thermal bridges focusing on slab on grade TB, including calculation methods, norms and software summary.•A numerical study of possible ...technical solutions to decrease slab on grade TB’s effect is presented.•Different external insulation solutions to renovate existing building were compared.•The study highlights the effect of soil thermal properties on slab on grade TB calculation.•Improving insulation thickness and depth does not necessarily reduce the thermal bridges effect.
The building industry is responsible for considerable energy consumption and the production of large amounts of greenhouse gas emissions worldwide. Hence, reducing the building heat losses that occur through the building envelope, especially through the floor foundations, is important.
This paper provides an up-to-date review on thermal bridges (TBs), mainly with respect to slab-on-grade TBs. First, different thermal bridges types are defined; then the standards used to evaluate TBs are presented. Common approaches to evaluate TBs are discussed, such as the U-value method, the equivalent method approach, and the three-dimensional dynamic method. This latter focuses on heat loss through the foundation and soil in existing buildings. Soil’s effect on thermal bridges is described and our case studies showed that thermal bridges heat losses are inversely proportional to soil thermal conductivity. This analysis is involved in the present paper. Finally, exterior insulation solutions at ground level are presented among those the trapezoidal double insulation type can reduce TBs up to 65% compared to no insulation case. As conclusions, directions for further works are suggested to fill current literature gaps.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Dynamic effect of thermal bridges on energy performance of a residential building.•Three methods modeling thermal bridges in building energy performance investigated.•Equivalent U & equivalent wall ...method underestimates annual heating load.•Cooling load underestimated by equivalent U & equivalent wall method for hot climate.
The existence of thermal bridges in building envelopes affects the energy performance of buildings, their durability and occupants’ thermal comfort. Typically the effect of thermal bridges on the energy performance is taken into account by implementing an equivalent U-value in 1D whole building energy simulation program. This treatment accounts for the effect of thermal bridges on the overall thermal transmittance, while their thermal inertia effect is ignored. The presence of thermal bridges not only reduces the overall thermal resistance but also changes the dynamic thermal characteristics of the envelope.
This paper investigates the dynamic effect of thermal bridges on the energy performance of residential buildings through simulations. A two-story residential building is used as a case study. Three methods, namely equivalent U-value method, equivalent wall method, and direct 2D/3D modeling method, are implemented in WUFI Plus, a whole building Heat, Air and Moisture (HAM) modeling program. Simulations are carried out for two climates with different insulation levels. Simulation results show that for the cold climate the annual heating load of this building with the inclusion of thermal bridges modeled using 3D dynamic method is 8–13% higher than that modeled using the equivalent U-value method, and 4–9% higher than that modeled using the equivalent wall method. With the increase of the insulation level, the percentage effect of thermal bridges on the heating load increases, while the difference among the three methods decreases. For the hot climate, simulation results show that the presence of thermal bridges increases the annual cooling load by 20%. Compared to the 3D dynamic method, the annual cooling load is underestimated by 17% using the equivalent U-value method and by 14% using the equivalent wall method, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Abstract
In response to increasingly stringent energy performance requirements for buildings, the need for energy-efficient construction becomes ever more important. Although new construction ...technologies have emerged, thermal bridges are still common in new and especially in to-be renovated buildings. To facilitate analysing the effect of thermal bridges this paper investigates the development of a method to automatically integrate their effect into energy simulations. The developed workflow starts with integrating thermal bridges into Building Information Modeling (BIM) software. Subsequently, the thermal bridges are exported to the open source format Industry Foundation Class (IFC). Finally, a conversion of the IFC file to a readable file for the energy simulation software Modelica, using the ifc2modelica tool is shown. Furthermore, it investigates the impact of thermal bridges on different key performance indicators (KPIs) for both renovation and new build cases. This research aims at efficiently integrating thermal bridges in energy simulations during early design stages. The foundation for a BIM-library for standard thermal bridges has been developed, allowing an easy integration in existing BIM models with good simulation results.
•Automation of thermographic building inspections and categorization of pathologies.•Automatic detection of thermal bridges through geometric and thermal characteristics.•Quantitative thermography ...for thermal characterization of the thermal bridges.•Automatic computation of linear thermal transmittance of thermal bridges.•Thermal image rectification to improve geometric accuracy.
There are several techniques for the performance of energy studies in buildings, where infrared thermography is widely used for the study of the composition of the envelopes, to find faults in the building materials and in the composition of the envelopes with influence on their thermal behaviour, as well as to detect areas with humidity. Common thermographic building interpretations are performed by a human operator, which involves a high level of subjectivity and mainly relies on the expertise of the operator.
With the aim at minimizing this subjectivity and maximizing the accuracy of the inspections, this paper presents a procedure for the automation of thermographic building inspections mainly focused on thermal bridges. The procedure, in addition to detecting the thermal bridges by their geometric characteristics and their temperature differences with the surroundings, includes the computation of the thermophysical property of linear thermal transmittance of each candidate to thermal bridge, thus implying their characterization in addition to their detection. With this addition, together with a previous process of rectification of the thermal images analysed, the accuracy of the detection of thermal bridges regarding existing methodologies is improved in 15% considering the false positives and negatives obtained in each methodology.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Dynamic effect of balcony slab on energy performance of a high-rise residential building.•Two modeling methods, i.e. dynamic 3D modeling vs equivalent U-value, investigated.•Equivalent U-value ...method underestimates annual heating load by up to 15%.•Implementation of thermal breaks in balcony reduces the dynamic effect.
Typically the effect of thermal bridges on the energy performance of buildings is taken into account by implementing an average U-value in 1D whole building energy simulation program, which is referred to as the equivalent U-value method. This treatment accounts for the effect of thermal bridges on the overall thermal transmittance of envelope details, while their thermal inertia effect is ignored. In this paper, the dynamic effect of balcony slab as thermal bridges on the energy performance of a high-rise residential building is investigated by comparing the annual space conditioning loads obtained by the equivalent U-value method to that obtained from 3D dynamic modeling of the balcony junctions implemented in WUFI Plus program.
The simulation results show that the equivalent U-value method underestimates the annual heating loads by 2.8–4.4% for the building as-designed. With the increase of balcony portion and better insulated walls, the difference in annual space heating loads modeled using these two methods is increased to 8.6–15.2%. With the implementation of balcony thermal break the annual heating loads are reduced by 8.8–25.7% and the difference between these two modeling methods is also reduced to 1.9–11.5% for the cases investigated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Steady-state heat transfer of built-in composite insulation walls close to column had been investigated in this paper by using finite element analysis (FEA). Firstly, the thicknesses of the wall and ...the thermal insulation were determined according to energy saving requirement of 75%. The heat transfer of the wall with the commonly used details was studied and it was found although adding local insulation on the outside of the column can improve the heat flow concentration, there is still a thermal bridge at the junction of the wall and the column. Then the influence zone of the thermal bridge was investigated and a construction measure of extending the local insulation width was proposed based on the analysis results. Lastly, the partial thermal insulation extended widths for different column sections and thermal insulation were suggested to break thermal bridge completely, which can meet the energy saving requirement of Beijing government.
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