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Cooking Up Some Heat Transfer

As many of you know, I enjoy cooking.

Need an excuse to grill? Fire up the BBQ, get out the imager and call it a lesson in Thinking Thermally!

Not only because I like the results—good food—but also because it’s a fun place to practice Thinking Thermally™. With the grilling season fast upon us, there is no better way to learn about radiational heat transfer than to fire up the old grill! Most of us understand we can “measure” temperatures on the grill by using our built-in infrared sensors—our hands!

Measuring Temperature
If the heat is low (225-250°F/107-121°C) you should be able to hold your hand about 4” away for 11-14 seconds; for medium heat (325-350°F/163-177°C) you’ll be uncomfortable in about 6-8 seconds, and you will only be able to stand high heat (450-650°F/232-343°C) for 2-3 seconds. Of course a number of BBQ tools now include some sort of a built-in temperature-measuring device, many of which are either contact probes or radiometers.

Seeing Heat Transfer
Another interesting way to see heat transfer is by cooking potatoes. Select three whole ones all about the same size. Toss one into boiling water, cut one in half and put it in a hot frying pan, and put the last in the microwave. After they’ve all cooked for a while, cross-section them and, voila, you can see the path that heat flow took as it moved into the potatoes.

A thermal imaging system is an important tool in any well-stocked kitchen. Here we can see the effects of convective, conductive and radiational heat transfer by observing potatoes that have been boiled (left), fried (center) and microwaved (right).

Whether in a bottle or a glass, it is important, when conducting your “research,” to include a cold beverage and study heat transfer in both directions.

Boiling water (convection) provides a fairly uniform distribution of heat through the surface while the frying pan (conduction) is non-uniform—heat moves mainly from one side. The microwave (radiation) doesn’t care so much about the surface of the potato because it is transparent to microwaves, a form of electromagnetic radiation in the radiowave portion of the spectrum. Microwaves pass right through the surface and are absorbed fairly uniformly by the water found throughout the volume of the potato. The thermal images reveal a great deal about heat transfer that it might be hard to learn from formulas!

I don’t ever need an excuse to cook, eat, or Think Thermally, but why not combine all three and have some fun doing so?. By the way, don’t forget to image some cold things too as heat can flow in either direction!

Thinking Thermally,
John Snell—The Snell Group, a
Fluke Thermal Imaging Blog content partner

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