Imagine your BBQ’ing over some very hot coals. You slowly run your hand say 6" over the coals and naturally, you burn your hand.
My assumption is that what is burning my hand is, for the most part, the air heated by the coals through conduction and convection. If I was suddenly standing in a vacuum, would the infrared radiation from red hot coals be enough to burn my hand say 6 inches from them? If not, how much hotter would the coals need to be in order for the radiation to be sufficient to cause thermal burns similar to the ones that would be caused in our atmosphere (through the heated air)?
Place the hot coals on a grate, lift them up, and place your hand 6" under them. That should remove the convection component of heat transfer, giving you an idea of the amount of radiant heat transfer.
Most of the heat coming off of hot coals is going to be in the form of direct (thermal) radiation. Although there is some convective heat occurring as well, there really isn’t enough air flow through a charcoal grill to provide sufficient convective load to cook food. (Convection or commercial impingement ovens designed to augment radiative heating or cook primarily by convection used forced air to increase convective load.) You can test this by taking a piece of aluminum foil, punching a bunch of small holes in it (to permit relatively unrestricted air flow) and putting your hand over it. While it’ll still be hot, the foil will reflect enough of the radiation to make a significant reduction in heating until the foil heats up and comes to thermal equilibrium itself.
There is, of course, no convection in a vacuum, and only objects in physical contact can transfer heat energy by conduction. Radiation will actually be slightly more effective in vacuum than in atmosphere, although at a 12cm distance I don’t think air is going to absorb that much unreflected heat. Even if we assume that a significant amount of heat is transferred by convection in terrestrial conditions, the intensity of radiative energy flux is proportional to the temperature of the source to the fourth order (W ∝ T[sup]4[/sup]), so even just a small increase in temperature gives a large increase in power output.
I’m pretty sure Stranger is right. I do a lot of work in heat transfer and have wondered about this and decided the effect was mostly radiant unless you are pretty close and directly overhead. Being pretty close and yet laterally outboard of the edge of the coals - that is, not directly above them - still gives you quite a good deal of heat.
I think radiant heating of the skin at a flux of about 1000 W/m^2 is very noticeable and, if your surroundings are not cool, objectionable when delivered over your entire body for more than a minute or so.