Microwave absorption

[Tunalover]

Incropera and DeWitt's book has an example problem involving a terrestrial solar collector panel. In the problem they consider microwave absorption. Does anyone have any data on this phenomenon? Why is it seemingly ignored in all other thermal radiation problems?


Tunalover

 

[m777182]

I think it is not ignored. At least microwaves are used on purpose in some heating applications(domestic oven), we are aware of the potential danger of even such small microwaves sources as a cellular telephone. Maybe it is as for now an expencive alternative for heating that can be done on more "archaic" way and a less controlable source of radiation- you do not see it or feel it but when you do it is too late.
m777182

 

[25362]

They say it heats but doesn't ionize...

 

[Tunalover]

I worked through the example problem. It acccounted for 25% of the radiated heat! If it makes such a big difference, why do all the terrestrial radiation heat examples (put forth by the PhDs in textbooks and so forth) always ignore the phenomenon? Are there an thermal radiation gurus out there?!



Tunalover

 

[MintJulep]

Tuna,

What edition do you have? I couldn't find a solar collection example with microwaves in my second ed.

The delineation between thermal radiation and microwave radiation at at 10^2 micrometers seems rather arbitrary to me. I find it hard to believe that radiation at 10^1.999 micrometers transfers heat, but at 10^2.00001 doesn't.

 

[hacksaw]

semantics only, reference is being made to the wave length of the IR.

 

[chicopee]

Probably because there is not enough intensity to heat objects w/ solar microwave at least on earth surface. In contrast TV transmitting antennae having power outputs of 25kw to at least 100kw can cause serious eye injuries such as cataract from the emitted microwavesand consequently any antennae technician will make repairs when the transmitters are shut down.This microwave in essence can "cook" the inside of the eyeballs.

 

[IRstuff]

I guess I'm not convinced that the critism is true.

Almost all of the heat transfer texts I own include microwave energy implicitly in the use of the Stefan-Boltzman sigma*T⁴ relationship, which includes microwave radiation. For the simple body to body radiative transfer, microwaves are included.

However, we should note that most of the heliostats were developed using mirror arrays with limited apertures. These limited apertures will not properly reflect nor focus wavelengths comparable to the aperture dimension efficiently. Thus, it might well be that the issue is an optical and not thermal radiation problem.

TTFN

 

[chicopee]

The sun emits over 90% of its total radiation between vawlengths of 0.1 and 3 micron. Probably the highest wavelength of the sun's radiations is 3.5 microns. Microwave wavelength range between 10E-3 and 10+2 meters which is well outside the sun's radiation wavelength.

 

[hacksaw]

microwave(rf) is generally considered a coherent source and not black body radiation

that said thermally emitting objects contain all way bands if they are black body radiators....

 

[IRstuff]

Microwave is coherent ONLY when generated by an antenna driven electronically.

Passively generated microwave is incoherent, since it's from the same quantum process that generates the infrared.

TTFN

 

[hacksaw]

but having said that you are describing thermally generated radiation and we agree on that.

the problem statement in Incropera and DeWitt is unfortunate, they should have indicated that longer wavelength IR or "black body" radiation was to be considered. to me a matter of semantics...

 

[IRstuff]

Solar collectors are different. There is an issue with diffraction-limited optics.

It's obvious that stellar microwave radiation CAN be collected, but NOT with a solar collector. Collection of stellar microwave emissions is done every day with radio telescopes. Their receiving aperture range from the small one at over 40 ft across to VLBI arrays that are hundreds of km across.

TTFN

 

[Tunalover]

Folks-
FYI, this radiation term is proportional to the sky temperature (in absolute units) to the fourth. Does that help any?

I'm just trying to understand WHY this term is not shown in every other terrestrial thermal radiation problems.


Tunalover