| Author | Message | ||
     John Clements |
On high temperature industrial furnaces something in the order of 90% of heat released by the gas is in the below 6 micron region of the infrared spectrum. I have heard it said that the refractory walls - often and alumina/silica material - are partially transparent to the shorter wavelenths (<6 microns). Has anyone seen anything published on this phenomena? | ||
| hlmark |
John - well, both alumina and silica are pretty nearly transparent in the NIR (and extending into the mid-IR, as per your wavelength specification) except for absorbance due to absorbed and chemically-bound water: -OH will absorb strongly at ca 3 microns (3,000 nm). So that will depend to some extent on the exact nature of the material, and how much -OH is chemically bound (which can be appreciable). But unless it's in a crystalline form, such as sapphire, optical scatter effects due to the particulate nature of the material will cause much of the impinging radiation to be reflected back on the source side. In fact, sintered high-purity alumina disks are often used as reference reflectors in the NIR region, since their reflectivity is close to 100% due to this scattering phenomenon. You have to consider another effect, too: since the walls of the furnace will also be heated, they will emit radiation of their own. That probably will NOT be close to a blackbody distribution, but outside the furnace the self-emitted radiation will be nearly indistinguishable from whatever radiation is scattered through the walls of the furnace. So you wind up with the worst possible combination of effects, I think. Howard |