I was just listening to a new Home Depot ad on the radio, with two guys that are kind of a cross between “Dumb and Dumber” and “Mr. Rogers’ Neighborhood.” They are talking about grills. Then the voiceover comes in talking about a grill that cooks with “one hundred percent infra-red heat.”
Well, what other kind of heat is there?
I am thinking they mean “radiation” to distinguish it from “convection” but still, every gas grill I’ve seen works with radiation anyway, which is radiation of infra-red.
No. People who call infrared “heat” are ignorant. Infrared is given off by hot objects, but any EM radiation can heat things up. Look at a microwave oven for an everyday example.
Gas grills don’t work with just radiation. There’s a huge convective element to it. Unless you’re just wasting gas to heat up a hot dog or something. Anytime the lid is closed or you’re using indirect heat (90% of my grilling and bbq’ing), it’s convection.
Oh, there are also some gas grills that don’t product a flame. Either their primary source of heat (not my grill) or a secondary unit (my grill’s rotisserie) consumes gas but only makes the burner element red hot, i.e., infrared cooking not unlike using your electric broiler with the door open (of course leaving the door closed brings convection back into the picture). I think my convection oven sucks out the hot air in broiling mode, because the instructions indicate to keep the door closed which is everything against I’ve ever learnt about broiling!
OK, let me elaborate by stating my meager knowledge.
Any object with a temperature of greater than absolute zero emits infrared radiation. When another object absorbs this radiation and becomes warmer as a result, this is heating by radiation. This is how a heating coil in an electric broiler or a toaster work. I guess technically microwave ovens also work by radiation, although it’s in a different frequency band than infra-red (but my understanding is that microwaves don’t heat just anything, but only certain things, most notably water).
When an object touches a hotter object, some heat is transferred and the cooler object gets hotter. This is heating by conduction. This is what happens when you put an egg in the frying pan.
When a hot object heats the surrounding air, and that air in turn heats another object. This is heating by convection.
In a gas grill, there is a combination of cooking by radiation plus convection. In this case the radiation is infrared.
So what possible meaning does it have to advertise this way (see OP)?
Most of the energy dissipated by an incandescent bulb is in the infrared. Depending on factors such as wattage, the visible light output only accounts for about 20-25% of the total energy.
Oh, yeah… forgot that part of the applicability to grilling. You still need conduction to get the nice char marks on your food.
Without hearing the ad the OP refers to, though, I still think he heard an advertisement for the newish infrared grills. The distinction I imagine is that they give off much, much more infrared than a gas flame. My previous grill’s gas rotisserie burner was weak compared to the infrared rotisserie that my current grill has. A gas flame doesn’t have near the energy (in infrared) that these infrared burners have. Let’s see what Wikipedia says… nothing. Only a stub with zero content. But here’s a Detroit News article (with photos) about them.
I heard a story abputh this a while ago. Apparently someone patented a gas-powered infrared burner a while ago. He licensed it out a a hefty rate so only commercial kitchens could afford them. The patent recently ran out and now it is available in commercial versions.
I am intrigued and plan to look at them for my next barbeque.
Slight nitpick: Technically, there’s a bit of conduction, too. Air is made of matter, after-all, and any matter to matter contact with heat transfer has conduction. Oh, and it’s not just air that can convect, it’s any fluid. Though you may only have been referring to the specific act of cooking on a gas grill, in which case yes, there would only be convection from air.
there are space heaters that are billed as infra red because they use quartz tubes to surround a heating element. they project heat differently than just the heating element alone. There are grills that use ceramic tiles arranged to project the heat in a similar manner.
In my heat transfer classes we were told that the micro-scale conduction that goes on between a fluid and a solid doesn’t count as anything other than convection.
…which seems to be your point here, as well. Did I miss something? I thought that the three forms were defined as:
Conduction - Heat transfer between two solid masses
Convection - Heat transfer into or out of a mass of liquid or gas, regardless of the phase of the other mass
Radiation - Heat transfer between two masses that aren’t in contact
Does the convective fluid need to be in motion for it to count as convection, or does Brownian motion count?
Brownian motion counts, but you’ve got the definition wrong. Convection isn’t heat transfer into or out of a mass of liquid or gas. It’s heat transfer within a mass of liquid or gas.
Convection is characterized by movement of the fluid, so you heat it at one location, it moves (either because you push it or because its buoyancy changes relative to other fluid due to thermal expansion), and in its new location it heats something else.
If you have a hot solid object above a cold solid object, both objects having an emissivity of zero so they don’t emit or absorb radiation, with air or water or other fluid between them, and you don’t stir the fluid, that fluid will be stagnant with the hottest and least dense fluid at the top and the coolest and most dense fluid at the bottom. It will still transfer heat by conduction. The thermal conductivity of air, for instance, is about 0.04 watts per meter kelvin. If you stir the fluid, or if you put the cold object above the warm one so that the fluid’s buoyancy stirs it, the heat transfer will increase, by a factor known as the Nusselt number.
Close. The radiation doesn’t have to be infrared: The dominant wavelength of the radiation will be determined by the temperature of the object. For most things in our everyday experience that we think of as “warm” or “hot”, the dominant wavelengths are in the infrared, but cooler objects will produce microwaves and radio waves, and hotter objects will produce visible or above, in exactly the same manner. There’s nothing special about infrared that makes it and it alone “heat”.
