Conceptual Model of Heat Transfer by Radiation

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As a conceptual model of the mysterious phenomenon of heat transfer by radiation one may think of heat transfer by conduction and as a model of heat transfer by conduction one may think of the transfer of water in a system of lakes at different altitudes,  with altitude corresponding to temperature.

In the lake system there is one-way flow of water from higher to lower altitude driven by hydrostatic pressure. Conduction transfers heat energy from higher to lower temperature. Radiation transfers heat energy from higher to lower temperature.

The flow of water, or heat energy, can be seen to be governed by a 2nd law stating that Nature always seeks to decrease difference in water level, or temperature.

Why does Nature have this tendency to decrease difference, which by the way Nature shares with social-democrats in Sweden and elsewhere?  Why is Nature not seeking instead to increase difference, like a good old capitalistic system is supposed to do?

Because, in a certain sense Nature is simple-minded and it is easy for a simple mind to simply decrease difference by the simple process of mixing, which requires no distinction. Just stir and mix.

On the other hand, the reverse process of increasing difference by unmixing is difficult because it requires distinction.

In short: decreasing difference is simple and stable and can be easily be accomplished by a natural simple process, while increasing difference is difficult and potentially unstable and thus requires some form of intelligence, or at least some process for separation or selection.

The above model can be read as a conceptual version of my analysis of radiative heat transfer in Mathematical Physics of Blackbody Radiation and of the 2nd law of thermodynamics in Computational Thermodynamics.

7 Comments

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  1. steveta_uk

    The flowing water analogy for radiative heat trasnfer is very appropriate.

    In a river, the bulk flow of water is clearly downhill. However, at the microscopic level individual water molecules are moving all over the place, and it would be difficult to determine the bulk flow if you examine a tiny volume within the river.

    Equally, while the bulk flow of radiation will be from the hotter to the cooler object, at the photon level there is an enormous flow of radiative enegy in all directions; certainly not just from hot to cold!

  2. claesjohnson

    You are speculating about IR photons. Remember that there are no IR-photons. It is
    fantasy, not physics. This is what Einstein said.

  3. Doug Cotton

     
    Of course radiation can go in all directions. It does so in a microwave oven, but not everything gets warmed in the oven, now does it? Radiated energy is not thermal energy. It has to be converted to thermal energy, and that only happens if the source of the radiation was warmer than the target. Some or all of the radiation can and will resonate with the target. When this happens it supplies energy to the target, yes, but the energy is used by the target to do some of its own radiating. It can use it because it is identical in frequency to what it can emit itself. And this happens because the Planck curve for a cooler body is always fully contained within that for a warmer body. So the two-way radiation which corresponds to the area under the Planck curve for the cooler body just resonates in each body and gives it energy that can only be used for new radiation. Because the new radiation is identical, it looks as if the original radiation has been scattered. Hence the term “pseudo scattering” or, as in my paper, I use “resonant scattering.”

    So the process is not in any way violating the Second Law of Thermodynamics (SLoT) because the radiated energy is never converted to thermal energy. However, there is additional radiation in the warmer body which corresponds to the area between the Planck curves. This is the energy which does get converted to thermal energy in a cooler target. And, since S-B calculations are based on the integrals of Planck functions, the normal calculations (subtracting the two S-B values which represent areas under the large and small curve) still apply, because the difference is the area between the curves.

    So the analogy of the lakes is totally appropriate for radiation, because you may think of radiation being scattered each time it strikes a target, but only “dropping off” some of its thermal energy when it meets a cooler target. The new radiation then continues just as if emitted by that target, so more thermal energy is only dropped off if the next target is cooler than the last one.

    Because the incident radiation supplied radiated energy to the cooler target, that target does not have to convert some of its own thermal energy in order to radiate what it is “allowed” to radiate as per the Planck function. Hence the target’s rate of radiative cooling will be slower, as we observe. So, yes, a wooden table in your back yard may stop dew forming on the ground below it because the table is warmer than the atmosphere and thus slows the cooling more. This is because gases in the atmosphere are cooler, and also because ones like carbon dioxide don’t radiate with a full Planck spectrum. Instead they just have a few spectral lines of radiation which can resonate with the surface. So carbon dioxide is like a picket fence with most of its pickets missing, standing up against a full flood of radiation from the surface. Even water vapour molecules can do much better when it comes to slowing radiative cooling of the surface.

    Whilst the calculations are the same, there are huge differences in the physical consequences. These are explained in the FAQ’s in Appendix Q.7 of my paper which is linked from my site. Because there is no conversion to thermal energy, there can be no subsequent heat transfer to other bodies instead of radiation. Hence, in the case of radiation from a cooler atmosphere, there can be no effect upon the rates of cooling by evaporation, conduction and other sensible heat transfer mechanisms. Nor is there any slowing of the radiation that gets through the atmospheric window to space. So only about a quarter of all the surface cooling is affected, and only by a minuscule amount by carbon dioxide with its limited range of frequencies.

    But wait, there’s more. The temperature of the surface is stabilised by both the underground temperatures and, more importantly, the amount of solar radiation reaching the surface. The temperature gradient in the atmosphere is governed by the adiabatic lapse rate, and that in turn is controlled by the force of gravity. So, if that gradient is represented by a simple linear equation y = mx + b then m is fixed by gravity and b is controlled by solar insolation which only varies a little beyond our control. Thus surface climate is beyond our control and any slowing of the radiative cooling is simply compensated by an increase in sensible heat transfer.

    So there is no overall slowing of the rate of surface cooling, no transfer of thermal energy from the atmosphere back to the surface, and so no greenhouse effect.
     

    • steveta_uk

      If some person were interested in your theories, Claus, but every time they tried to engage with you some Australian fanatic always intervened and destroyed any attempt by you to present the theories in a sensible manner, would you not start eventually to get annoyed at said fanatic and eventually ask him to STFU!

      I know I would!

      • Doug Cotton

        Do you ever discuss physics, steveta? Or do you find explanations by Claes (not Claus) and myself difficult to understand, or just difficult to believe? I suggest you have no more empirical evidence for your believe system than we have for ours: probably far less.

  4. claesjohnson

    No, the road to insight is not crowded and Doug is free to do what he wants a to do. You are also welcome.

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