Advanced Modeling of Enclosed Airspaces to Determine Thermal Resistance for Building Applications

Enclosed airspaces to reduce heat flow have been recognized for well over 100 years. Airspaces with one or more reflective surfaces define reflective insulation (RI) assemblies, a product type used in walls, roofs, windows with multiple panes, curtain walls and skylights. The thermal resistance (R value) of airspaces depends on the emittance of all surfaces, airspace dimensions and orientation, heat flow direction and surfaces temperatures. The modeling of RI now includes CFD coupled with radiation to quantify the total heat transfer. This study compares a validated model for airspace R values with existing methods such as ISO 6946 and hot-box results that provide the R values in the ASHRAE Handbook of Fundamentals. The existing methods do not include an airspace aspect ratio. This study showed that the aspect ratio can impact the R value by a factor of two. The impact of aspect ratio was calculated for double airspaces variation such as that for single airspaces. The present calculations are two-dimensional and also consider all the bounding airspace surfaces, while previous methods are one-dimensional and do not include surface temperature variations or detailed radiative transport.

Evaluation and use of airspaces for thermal resistance in buildings

The thermal resistance (R-value) of airspaces depends on the emittance of surfaces around the airspace, dimensions, heat-flow direction, and the temperatures of bounding surfaces. Assessing the energy performance of building envelope components and fenestration systems requires accurate results for the R-values of any enclosed spaces. The evaluation of reflective insulation R-values has evolved to include use of computational fluid dynamics and surface-to-surface radiation to quantify convective and radiation contributions to the heat transfer across airspaces of all types. This paper compares an advanced and validated computational tool for calculating enclosed airspace R-values with the widely-used ISO 6946 and airspace R-values in the ASHRAE Handbook-Fundamentals. The tool evaluates construction defects, air-infiltration impact, and dimensional aspect ratios that 1-D methods do not address. The differences between the methods that are currently being used to evaluate the R-value and the advantages of the advanced method for evaluation of reflective insulation applications are discussed.

Thermal protection by integration of vacuum insulation panel in liquid-cooled active thermal management for electronics package exposed to thermal radiation

Thermal Management Systems (TMSs) working for electronics packages under harsh environments like intense thermal radiation is challenging due to external thermal interactions. Thermal insulation protection for TMS is very critical in these harsh conditions. An experimental set-up was developed to analyse the effect of insulation protection against thermal irradiation over a pumped liquid-cooling Active Thermal Management System (ATMS) with varying heat dissipation rate (0 - 4.2 kW/m2), thermal irradiation (0.85-3.80 kW/m2) and coolant temperature (15-25 C). Three configurations of ATMS are considered in the experimental study- ATMS without thermal insulation protection, ATMSs integrated with Cellulose Fibre Insulation (CFI) and Vacuum Insulation Panel (VIP). The effect of insulation on each parameter in all three ATMS configurations over the temperature of the electronics component, cooling load, and nature of heat flow in ATMS was analysed. VIP outperformed CFI on achieving a significant reduction in temperature of electronics systems (35.67 %) and cooling load (45.64 %) experienced by the ATMS. VIP effectively reduced the impact of temperature and cooling load change in ATMS against change in thermal irradiation. The study concluded that thermal insulation protection was most effective at high thermal irradiation and low heat dissipation rate. Heat Flow Direction Index (HFDI) concept was developed to find the nature of heat transfer direction in ATMS without temperature distribution trend. Heat generation rate and irradiation possess significant influence over the nature of heat flow direction.

Defects in Graphene/h-BN Planar Heterostructures: Insights into the Interfacial Thermal Transport Properties

In this work, the defects (local stress generated) induce the formation of graphene/h-BN planar heterostructure (Gr-hBN-PH) to form "unsteady structure". Then, the coupling effects of external field (heat flow direction, strain and temperature field) and internal field (defect number, geometry shape and interfacial configuration) on the interface thermal conductivity (ITC) of Gr-hBN-PH were studied. The results show phonon transmission is less affected by compression deformation under the action of force-heat-defect coupling, while phonon transmission of heterostructure is more affected by tensile deformation. The non-harmonic interaction of the atoms in the composite system is strengthened, causing the softening of high-frequency phonons. The greater reduction of thermal transport at the interface of heterostructures will be. The interface bonding morphology plays a significant role on the ITC of the Gr-hBN-PH. The relationship between structure and properties in the low dimension is analyzed from the perspective of defect energy. It is helpful for us to understand the physical mechanism of low-dimensional structure, realize multiple structural forms, and even explore new uses.

Microstructure and Microtexture Development in Grain Oriented Electrical Steel

The first Si-Fe electrical steel was produced in 1905, and the grain-oriented steel was discovered in 1930 after Goss demonstrated how optimal combinations of heat treatment and cold rolling could produce a texture giving Si-Fe strip good magnetic properties when magnetised along its rolling direction. This technology has reduced the power loss in transformers greatly and remains the basis of the manufacturing process today. Since then, many postulations reported on the mechanism on abnormal grain growth (AGG) which is the key for Si-Fe superior magnetic properties. However, none have provided a concrete understanding of this phenomenon. Identifying and classifying the driving force behind Goss abnormal grain growth is of industrial and academic importance to further optimise the manufacturing process and reduce losses. In the current investigation, the deviation from easy magnetisation direction <001> was studied to find a correlation between crystallographic orientation and magnetic domain structure. Both deviation angles α: the angle between <001> and in-plane rolling direction (RD), and β: the angle between <001> and out-plane rolling direction were calculated using electron backscatter diffraction (EBSD) raw data. Further, EBSD combined with forescatter detector (FSD) is used to reveal the magnetic domain configuration within individual oriented grains. The magnetic domain patterns were directly imaged and correlated to the crystal orientation and α and β deviation angles. It was demonstrated that the size of the deviated orientation grains from ideal (110) <001> Goss orientation is a critical microtexture parameter for the optimisation of magnetic property. It is concluded that the magnetic domain patterns and α and β angle of deviations are strongly correlated to the magnetic losses in GOES (grain oriented electrical steel).Furthermore, the effect of grain boundaries, grain size, heating rate and dislocation density on Goss abnormal grain growth was investigated using EBSD. It was found that in the early stages of secondary recrystallisation random grains grow and abnormal growth of Goss achieved in low heating rate. The advantage of Goss abnormal grain growth in secondary recrystallisation is lost while annealing at a high heating rate, and random orientation can grow abnormally. Also, statistical analysis of grain boundaries, including CSL (coincident site lattice), shows no distinct behaviour and high angle grain boundaries and CSL are not exclusive to Goss oriented grains. In addition, GND (geometrically necessary dislocation) and Taylor Factor showed to be randomly distributed around Goss grains, and the hypothesis of Goss grains grow by consuming high GND and Taylor Factor grains cannot be the reason for Goss abnormal grain growth. Neutron diffraction experiment was conducted at Rutherford Appleton Laboratory, ISIS facility at Oxford, UK using GEM beamline. It was demonstrated that Si atom positions in the solid solution disorder α-Fe cubic unit cell that cause lattice distortions and BCC symmetry reduction is the most influential factor in early stages of Goss AGG than what was previously thought to be dislocation related stored energy, grain boundary characteristics and grain size/orientation advantages. Finally, heat flux, heat flow direction, and strain effect on Goss abnormal grain growth investigated. It was found that heat flow direction greatly impacts the rate of abnormal grain growth of Goss. Also, strain areas can disrupt Goss AGG and promotes randomly oriented grains to grow abnormally.

Orientation and Microstructure Evolution of Al-Al2Cu Regular Eutectic Lamellar Bifurcating in an Abruptly Changing Velocity under Directional Solidification

In an abruptly changing velocity under directional solidification, microstructures and the growth orientation of Al-Al2Cu eutectic lamellar were characterized. The change in solidification rate led to an interfacial instability, which results in a bifurcation of the eutectic lamella into new, refined lamellae. The growth orientation of the eutectic Al2Cu phase was also only in its (001) direction and more strongly oriented to the heat flow direction. The results suggest that the eutectic lamellar Al-Al2Cu bifurcation and the spacing adjustment may be caused by the rate determining lateral diffusion of the solutes after interfacial instability.

Building–Soil Thermal Interaction: A Case Study

This paper presents an analysis of thermal interaction between a building and surrounding soil. The examined building was located in southern Poland. Measurements of selected indoor and outdoor air temperature parameters were made in order to determine the boundary conditions. The soil temperature measurements were conducted at 42 points. The analysis of results is divided into four periods: summer, autumn, winter, and spring. The analysis show that weather conditions significantly affect the temperature in soil, but the range of residential building impact decreases with distance, and it varies depending on the season. The residential building impact on the soil temperature is in the range of 1.2–3.3 m. This paper also includes a study of the heat flow direction in soil and a quantitative estimate of heat exchange between a building and the soil. The greatest energy losses 2082 kWh (21.24 kWh/m2) from the building to the soil were recorded in winter. In spring, the energy losses were reduced by about 38% as compared with the energy losses in winter, and the energy losses in spring were comparable to autumn.

Characterization of the Pore Structure in Ice from Water Added at Different Temperatures

The present study describes ice characterization technique by utilizing light microscopy. Two different optical setups were used and compared. For ice structure characterization, ice was formed by adding water to an ice base layer: pouring hot water (+20 °C), pouring cold water (+3 °C) and spraying of hot water (+20 °C) water. Ice substrate specimens were made (20 x 30 x 20 mm prepared in a polypropylene mould) at -20 °C for 48 h. The heat flow direction governed the orientation of the air bubbles, and set the inner structure of ice. Pouring hot water onto ice slowly solidified the top layer and created channel-like pores perpendicular to the top surface, but cold water quickly solidified the top layer and created chaotically oriented air bubbles.

Composites with Excellent Insulation and High Adaptability for Lightweight Envelopes

Lightweight insulation materials are widely used in lightweight buildings, cold-chain vehicles and containers. A kind of insulation composite, which can combine the super insulation of state-of-the-art insulation materials or structures and the machinability or adaptability of traditional insulation materials, was proposed. The composite consists of two components, i.e., polyurethane (PU) foam as the base material and vacuum insulation panel (VIP) or silica aerogel as the core material. The core material is in plate shape and covered with the base material on all sides. The thermal conductivity of the core material is nearly one order lower than that of the base material. The effective thermal conductivity of the insulation composite was explored by simulation. Simulation results show that the effective thermal conductivity of the composite increases with the increase of the thermal conductivity of the core material. The effective thermal conductivities of the composites decrease with the increase of the cross-section area of the core material perpendicular to heat flow direction and the thicknesses of the core material parallel with heat flow direction. These rules can be elucidated by a series-parallel mode thermal resistance network method, which was verified by the measured results. For composite with a VIP as the core material, when the cross-section area and thickness of the VIP are respectively larger than 60% and 21% of the composite, the composite’s effective thermal conductivity can be 50% or less than that of the base material. Simulated heat loss of the envelope adopting the insulation composites with VIP as the core material is nearly a half of that of the envelope adopting traditional insulation materials.

Influence of heat flow direction on solder ball interfacial layer

This paper deals with the research of an intermetallic layer of SAC305 solder balls soldered from three directions of the heat flow in the ball-attach process for BGA package. From the point of view of the heat flow direction, the samples were soldered by infrared heating. The heat sources were placed on the top, bottom and both lateral sides of the BGA package. After the solder balls-attach process, a metallographic cross-section was performed, followed by selective etching to visualize the relief of the intermetallic layer. Images of the interfacial between the solder and solder pad were taken from the created samples, followed by measurement of the average thickness and root mean square roughness of the intermetallic layer. The results showed changes in the intermetallic layer. The largest thickness of the intermetallic layer was observed on samples soldered from the top and both sides. Soldering with the bottom infrared heater resulted to the smallest thickness of the intermetallic layer. The same trend was in the roughness of the IMC layer. The greatest roughness was found for samples soldered by the top and both side heaters. The top soldered samples exhibit the smallest roughness.