Never forget heat CONVECTION (especially you who proselytise radiation models of the Shroud of Turin)

antibes-300x199I’m rather fortunate to be just a few hundred yards from the place in the picture. Yes, I’m on the Med, and we’ve been blessed with clear blue skies and sunshine the last few days, but the air is cool (presently 7 degrees C). That’s hardly surprising, given the proximity of those snow-capped peaks in the background.

Yet in two hours time, folk will be congregating at their favourite outdoor restaurants, and the top coats will in all probability start coming off.

There are few well-situated places that rarely have a vacant table and it’s not just because of good food and service  No prizes for knowing the answer. They are part natural, part man-made SUNTRAPS

What factors make one place a suntrap, and another a cold corner (they can be a few yards apart)?

The first factor is a sheltered position that is out of the wind (obviously). The second is to have nothing nearby that blocks the sun – trees, buildings etc.

There are less obvious factors, like the furniture: are the tables shiny and white, or matt and a dark shade of brown or green. Is the frontage of the restaurant white stucco or dark stone or brick? And finally, what are the diners wearing – light or dark clothing?

Get all the factors right, then you can find yourself in a balmy bubble of warm air at a comfortable 15 degrees of more, and be amazed at  the temperature of the arm of your coat that receives direct sunshine?

How come? The answers are not difficult to find if you recall what you learned at school about the 3 methods of heat gain and heat loss (heat transfer).

From the internet:

“Sunlight’s composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light and 32 watts of  ultraviolet radiation.

Where intercepting and trapping warmth outdoors is concerned, leaving aside sun tans, we need to focus on the heat rays (infrared) and the visible light rays.

Both of them tend to be reflected off, or scattered, by light shiny surfaces. Their ability to heat the latter kind of surface will be small. Conversely, both tend to be absorbed by matt (non-shiny) dark surfaces. In that case, something interesting happens. The temperatures of those absorbing surfaces – clothing, furniture, walls, paving stones begins to rise. That because the energy in the radiation has been converted to atomic and molecular motion (vibration). As the temperature rises those atoms and molecules radiate back more and more infrared radiation. They have to shed extra energy in order to maintain heat balance.  Some of that back-radiation off walls etc comes back to the customers at their tables, instead of being immediately lost to deep space, so there’s what might be termed a  ‘secondary radiation’ that helps to maintain the sun trap (handy for temperature buffering, say when the sun goes behind a cloud). In time there is a new steady state in which the surfaces at our outdoor restaurant are now emitting the same amount of total radiation as they receive, but are at a HIGHER TEMPERATURE than the surrounding air.

But they lose or shed some of the extra energy not just by back-radiation, but by  two additional mechanisms: conduction and convection.

Those heated paving stones will lose heat into the subsoil by conduction. Conduction is heat loss by direct atom-to-atom contact, with no air gaps.  When the sleeve of one’s coat feels warm, or the back of one’s chair, that is also due to heat conduction into the skin and its temperature receptors.

It’s the third form of heat loss that tends to be overlooked – not least of all in the Shroud literature – and by people who in some cases ought to know better, namely CONVECTION. When the cool air comes in contact with those irradiated surfaces, it acquires heat by conduction initially (atom-to-atom contact between solid and air). But having acquired additional kinetic energy, the air expands and becomes less dense. There is then a buoyancy effect, and the warm air rises upwards, with cool air taking its place. One has set up a cooling convection current. It’s usually invisible, unless there are folk smoking, and one can see the smoke rising on convection currents (miniature thermals – the larger sort being the kind that keep gliders aloft)

As I said, the infrared and visible light both behave in essentially the same fashion through both being absorbed preferentially by dark matt surfaces.

Let’s now imagine we go indoors into a dark room, with little or no natural light, and do a thought experiment with an electric iron as a source of heat (infrared) with no emitted light. I say a thought experiment – the one I’m about to describe would not work terribly well with an ordinary iron with a shiny stainless steel sole. It would need to have a sole that was matt black to make it a good emitter. (Yes, paradoxical though it may seem, matt black surfaces are not only the best absorbers of infrared radiation, but the best emitters i.e. radiators too).

Imagine you plug in your iron, then press it down onto linen. Little happens initially, though there may be a hint of steam (from moisture being driven off). You then become aware of a burning smell, and find you have scorched the linen.

This scorchophobic Shroudologist believes she is deflecting cosmic rays onto linen (to model a highly superficial Shroud image).

Oops. But radiant energy from an electric iron, or any surface at a comparable temperature,  is unlikely to have ever scorched fabric., certainly not white linen.  The latter needs direct contact, or, just possibly, especially with sensitive fabrics, CONVECTED  hot air.

That is clearly the result of heat CONDUCTION (direct atom-to-atom contact between steel and linen) with NO AIR GAP.

Now imagine you stand the iron on end, and hold samples of fabric, some light, some dark, vertically and close to the sole of the iron. Wait long enough and you will feel those samples become warm, but probably not hot, and certainly not scorched. The smallest air gap between hot metal and linen prevents scorching. There is infrared radiation crossing that air gap, to be sure, but it will NOT scorch the fabric. Why not? Because at the temperature of an electric iron, say 150 -200 degrees C, there is not sufficient absorption of that radiation to elevate the temperature of the cloth to that which produces scorching. It would of course be a different matter if one’s cloth was held close the incandescent red hot coil of an electric fire, say, at 800 degrees C or more. Even if a mere 5% of the radiant energy (mainly infrared with some visible orange/red light) were to be absorbed, that is then sufficient to quickly drive moisture out of the fabric, which then proceeds to become scorched through pyrolysis of carbohydrates to coloured products.

Now here’s a third experiment that can be done with our electric iron that can give a result that is in between conduction (scorching) and radiation (gentle warming) and which risks producing some light scorching. One gets someone to hold the iron horizontally, sole up, and then stretches some fabric close to the sole, but not touching. What happens now is that the fabric begins to get much warmer than the previous experiment with the sole upright/ Why? It’s our old, all-too-often forgotten friend, namely heat convection. Hot air rising off the sole of the iron quickly drives moisture out of the fabric, and then there is the possibility, at least in principle, of scorching, quite unlike the situation where there was radiated heat, but with a geometry that protected from convected heat. Do not mistake any scorching for radiated heat!!!!!  Do not start talking about radiation and cloth-object distance in the same breath.

Take away message – needed to be borne in mind when reading the next posting – NEVER FORGET CONVECTED HEAT (that’s assuming the researchers have not done their experiments in vacuum chambers!).

I shall post now.  Later in the day – after lunch in one of those suntraps – I shall start to post instalments (my preferred method for maintaining a work/life balance) about the correct – and incorrect- ways of interpreting the results of those attempts to model the Shroud image, especially, as I say, where a cloth is draped over the TOP of a bas relief or other heated 3D object. Shroud Center of Colorado kindly take note.


About Colin Berry

Retired science bod, previous research interests: phototherapy of neonatal jaundice, membrane influences on microsomal UDP-glucuronyltransferase, defective bilirubin and xenobiotic conjugation and hepatic excretion, dietary fibre and resistant starch.
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