07 October 2008

Radiative Heating

Short example of radiative heating from today's lunch. We were at a Japanese restaurant and they did the usual bit of cleaning the surface and then spraying it down with something flammable and igniting it. We noticed that we felt the heat even from the burners 20-30 feet away as the big whoosh of flame went up.

I've mentioned before that there are three methods of moving heat around -- conduction (molecules bouncing in to each other; very slow), convection (carrying the hot air from one place to another, and radiation. Now hot air rises, and we were not sitting above the burners! So what is left is radiation. The short-lived, but hot, sheet of flame radiated some heat over to us 20-30 feet away.

Some fireplaces take advantage of the principle by using a 'firebrick' which absorbs heat and then efficiently radiates it back in to the room, rather than letting it go with the air up the chimney..

Anyone else have a daily experience with (non-solar) heat transfer by radiation?

7 comments:

silence said...

Everybody who has a vacuum flask can see that the contents do eventually reach the temperature of the environment the flask is in (though it takes a day or two)

kcsphil said...

Hum, non-solar examples . . . well why not solar examples with a twist? Like a lot of houses, ours has some ceramic tile floor in downstairs rooms. That tile soaks up sun in the winter, and then radiates it back once the sun goes down. I know this is happening because the programmable thermostat hooked to my boiler (feeding my cast iron radiators) takes far longer to reach its "kick in" point on sunny days then cloudy days, or days I forget to open the windows. Such solar mass passive temperature control is also becoming a "standard" tool in the design box of many a sustainable architect.

drj11 said...

Making cheese on toast under a grill. The cheese is underneath the gas burners of the grill, so I assume it isn't convection that makes it melt?

RonSpross said...

Microwave ovens.

Penguindreams said...

Silence and kcsphil: How can we test that it's radiation rather than conduction?

The passive solar design kcsphil mentions is an oldie. I remember it from the 1970s, and even have (later acquisition) a Rodale book from then that gives designs.

Ronspross, drj11: Interesting how food and cooking show up in our examples. Earlier in talking about greenhouse effect, a commentator mentioned cooking under salt -- as a method that lets the infrared radiation hit the food.

drj11 said...

Bonfires. In our country we ceremoniously burn a catholic on top of a large pyre every November (no, really!). At a suitable distance you can feel the radiate heat from the fire. You can tell it's radiation because if you turn away, your face getting cold is a clear indication that the air surrounding you is cold.

Penguindreams said...

drj -- maybe you could elaborate on exactly what is burned, where?!

Bonfires themselves are a good example. Thanks.