Thermal radiation and radiant barriers
Thermal radiation is composed of all wavelengths of light, however most of the energy of the thermal radiation of objects at room temperature is in the infrared part of the spectrum according to Wien's displacement law. As with all electromagnetic radiation, it requires no medium in which to travel. The amount of energy radiated by an object is proportional to its surface temperature and its emissivity. Any object above Absolute Zero radiates thermal radiation. As all objects radiate energy towards one another, the important consideration is the net direction of energy flow.
Thermal radiant barriers possess the characteristics of low emissivity, low absorptivity and high reflectivity in the infra-red spectrum. They may also exhibit this for other wavelengths including visible light but this is not necessary to function as thermal barrier. Only a small fraction of radiant energy is absorbed by such a material (most being reflected back away) and therefore only a small fraction is re-emitted. Highly polished metals are one example. Conversely, dark materials with low reflectivity will absorb a large fraction of energy, and similarly emit a large fraction. (see Black Body, Grey Body)
Thermal conduction and conductive barriers
Conduction occurs when heat travels through a medium. The rate at which this occurs is proportional to the thickness of the material, the cross-sectional area over which it travels, the temperature gradients between its surfaces and its thermal conductivity.
Most gases including air are poor conductors, good insulators. Conductive barriers often incorporate a layer or pockets of air to reduce heat transfer. Examples include styrofoam and double glazed windows. Conductive heat transfer is largely reduced by the presence of the air-filled spaces (which has low thermal conductivity) rather than by the material itself. Metals exhibit high thermal conductivity and allow heat conduction to occur readily.
The effectiveness of an aluminum foil radiant barrier in preventing conduction is negated if it abuts any material with high thermal conductivity. Reflective foil needs an adequate air gap to function adequately as a conduction insulation material. A radiant barrier system is defined as a reflective material facing an air space. When the radiant barrier faces an enclosed air space it becomes a reflective insulation with a measurable R-value. Reflective insulation traps air within layers of foam, or plastic bubbles.
没有评论:
发表评论