Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Radiative Heat Transfer: Simple Overview
#1
The Inconvenient Skeptic

November 2,2010

EXCERPT:

The linchpin of the global warming debate centers on what the impact is of increased levels of CO2 in the atmosphere. If there is little to no impact, then there is not a problem. If it does make a big difference, then there is reason to worry. That question is answered through the analysis of radiative heat transfer. This is the most misunderstood science in the entire debate. This is meant to be a simple overview of radiative heat transfer (RHT).

The first step is to understand that all objects that have a temperature are radiating energy of some type. How much energy it radiates is primarily determined by its temperature. A hot object gives off a lot more energy than one that isn’t hot. Temperature is everything in RHT.

LINK

It is our attitude toward free thought and free expression that will determine our fate. There must be no limit on the range of temperate discussion, no limits on thought. No subject must be taboo. No censor must preside at our assemblies.

–William O. Douglas, U.S. Supreme Court Justice, 1952
Reply
#2
I've been hitting that link at least three times a day since it appeared.
Waiting for his promised detailed post. Cool

Environmentalism is based on lies and the lies reflect an agenda that regards humanity as the enemy of the Earth. - Alan Caruba
Reply
#3
He said the magical words "net energy". He also commented:

Quote:The net effect of a cooler body on a warmer body is more complex and that will be discussed in a later post.

Lets keep the heat transfer down in the comments at this time. Glenn clearly understands that the hand is not warming the fire, but the hand does reduce the rate of heat loss in the view factor that the hand intersects the heat leaving the fire.

I recalled that second thermodynamics law have several enunciates and didn´t sound familiar any of them talking of photons or radiation. Searching for it I came across this very interesting link of the Institute of Human Thermodynamics:

Variations of the 2nd Law of Thermodynamics


Scrolling down the more than a hundred "enunciates" you can find a funny poll to vote for your favorite one and following it some notes and resources. Note 2:

Quote:A reason why there are so many varieties of the second law is that heat, or energy in transit, and the varieties of work, technically weight lifted through a height, produced from such heat movements are by no means limited to mechanical engines. As it is currently understood there are four principle varieties of energy: gluons, photons, bosons, and gravitons. Thus, the concept that "the flow of energy moves from hot to cold" is a very general statement applicable to virtually all types of dynamics. For example, heat from the sun, i.e. photons, works to make sunflowers by lifting the weight of atoms and molecules, from the soil of which they originate, though a vertical height and into their structural configuration. Hence, a corollary of the second law of thermodynamics governs the ontogeny or lifecycle of organisms as well as the phylogeny or evolution of species of organisms. According to this reasoning, by logical extrapolation, there exist numerous 2nd Law variations, each applicable to its own relative system. By reduction, however, all such varieties have simply to do with the natural direction in which heat produces work or energy instills dynamics.

The Kelvin-Planck and Clausius statements are most frequently used in engineering thermodynamic courses. The Hatsopoulos-Keenan statement is most frequently used in the environmental sciences. That is, this statement postulates the existence of (stable) equilibrium natural states reached by systems, living or otherwise, subjected to specified constraints as internal partitions, external conservative force fields, rigid impermeable walls, etc. This statement implies that one particular equilibrium state will be reached. Thus, the approach to equilibrium is quite directional in nature.
(bold mine)

Maybe someone else would appreciate this link to quickly refresh like me long forgotten basics: http://www.peoplephysics.com/physics-laws6.html#20
Ni cien conejos hacen un caballo, ni cien conjeturas una evidencia (F. Dostoyevski)
Reply
#4
The first step is to understand that all objects that have a temperature are radiating energy of some type. How much energy it radiates is primarily determined by its temperature. A hot object gives off a lot more energy than one that isn’t hot. Temperature is everything in RHT.

I think I've got it. Here is my dumb bunny layman understanding of RHT.

When a solid body is radiating from a surface, the material under the surface is warmer. Heat is migrating from the interior to the surface as the body loses heat through radiation (and conduction to the atmosphere).

Dependant on the temperature of that body it will be radiating at a finite range of IR frequencies. Photons being radiated will each occupy a very narrow and specific energy band in that range of emitted IR.

According to the physics, whatever a body can radiate, it can also absorb. Thus any photons arriving at the surface of that body which are in the SAME energy band
being radiated will indeed be absorbed. BY THE COOLER SURFACE MOLECULES ONLY! THAT ABSORBED ENERGY CANNOT PASS INTO THE INTERIOR OF THAT BODY! LOW ENERGY DOES NOT MOVE INTO AREAS OF HIGH ENERGY.

Therefore photons in the local IR temperature range are absorbed and almost immediately radiated away. This does reduce the rate of cooling. If the rate of cooling becomes zero this is a state of equilibrium. Incoming energy is exactly balancing outgoing energy. The body is not being warmed.

If the incoming photons are of a higher energy band, higher frequency, the absorbed energy can then move into the interior of the body and warm it. That energy is STORED.

Downwelling radiation from the atmosphere AT NIGHT is generally cooler than upwelling surface radiation and CANNOT warm the surface.
Environmentalism is based on lies and the lies reflect an agenda that regards humanity as the enemy of the Earth. - Alan Caruba
Reply
#5
(05-24-2011, 12:24 AM)Richard111 Wrote: -snip-
Therefore photons in the local IR temperature range are absorbed and almost immediately radiated away. This does reduce the rate of cooling. If the rate of cooling becomes zero this is a state of equilibrium. Incoming energy is exactly balancing outgoing energy. The body is not being warmed.

If the incoming photons are of a higher energy band, higher frequency, the absorbed energy can then move into the interior of the body and warm it. That energy is STORED.

Downwelling radiation from the atmosphere AT NIGHT is generally cooler than upwelling surface radiation and CANNOT warm the surface.

This is partly correct but only applies to heat loss by radiation, there is also convection, conduction and latent heat of evaporation!

I don't think that back IR can be absorbed onto a surface that is already emitting IR at that wavelength because the atoms and molecules of that surface are already excited to sufficient energy level to be emitting at wavelength = x, so that wavenumber X of radiation cannot excite the surface further. You need more energetic "hotter" shorter wavelength radiation (visible light, UV, gamma rays, etc etc) to heat the surface. So rather than being absorbed and restricting cooling, the IR is immediately absorbed and re-emitted without any net heat being transferred back to the surface so back IR cannot heat the earths surface, or significantly impede cooling.

Reply
#6
(05-24-2011, 12:48 AM)Climate Realist Wrote: This is partly correct but only applies to heat loss by radiation, there is also convection, conduction and latent heat of evaporation!

I don't think that back IR can be absorbed onto a surface that is already emitting IR at that wavelength because the atoms and molecules of that surface are already excited to sufficient energy level to be emitting at wavelength = x, so that wavenumber X of radiation cannot excite the surface further. You need more energetic "hotter" shorter wavelength radiation (visible light, UV, gamma rays, etc etc) to heat the surface. So rather than being absorbed and restricting cooling, the IR is immediately absorbed and re-emitted without any net heat being transferred back to the surface so back IR cannot heat the earths surface, or significantly impede cooling.

Fair enough, but I did mention conduction in brackets above. If we can limit this discussion to a solid surface, then I claim that heat from that surface by radiation AND conduction will exceed heat loss by radiation alone or conduction alone. Therefore I assume there will be a surplus of surface molecules in the required condition to accept incoming radiation of the same frequency they recently emitted.

We seem to agree this absorbed radiation will not warm up the surface but I have reservations on "significantly impede cooling". I am basing my thoughts on personal oberservation of local temperature change at night between clear sky and cloudy sky cooling rates.

Thinking about this reminds me to try an experiment. I will attach a simple digital thermocouple to a sheet of steel and leave it outside on different nights and record the rate of temperature drop. I will lay the sheet of steel on a concrete surface to reduce air flow under the sheet so hopefully I will record mostly radiation induced temperature change.
Environmentalism is based on lies and the lies reflect an agenda that regards humanity as the enemy of the Earth. - Alan Caruba
Reply
#7
Cloudy skies impede convection due to the weather conditions at the time. The best experiment to try is Wood's experiment to disprove the greenhouse effect.
Reply
#8
Regarding Wood's experiment; I don't think I can afford a half square metre sheet of rock salt! Big Grin

Made myself a solar cooker couple of years ago. On a clear windless day it cooks like a dream but I live under the Green One air lane and a stationary contrail in just the right place takes cooker temperature down to useless and the pot has to be taken to the kitchen stove much to wife's annoyance.

Had a quick trial of my experiment yesterday and last night. In sunlight plate temperature more than double air ambient but after sunset temperature dropped rapidly and then tracked ambient. Will try to build an open top polystyrene box to keep moving air away from the plate. Will use very thin clear plastic sheet for the top. Sort or stuff garden centres provide. If that blocks IR then my night time temps should stay above ambient. I am hoping it will go below, especially on a CLEAR night. Last night clouded over again and minimum temp was 8.7C on digital thermometer and about 9C on standard spirit thermometer.
Environmentalism is based on lies and the lies reflect an agenda that regards humanity as the enemy of the Earth. - Alan Caruba
Reply




Users browsing this thread: 2 Guest(s)