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Radiational Heat Transfer: Moving Heat Magically

My recent trip to the beach was a pleasant reminder of the “magic” of radiational heat transfer. How else can we describe warmth delivered from 94 million miles away? We feel similar radiant transfer when we BBQ, sit in front of a fire, or feel a hot brick wall on a cool evening. The truth is: all objects radiate a wide spectrum of energy, called electromagnetic energy, including the infrared energy our imaging systems see. When things get hot enough, about 1000oF (538oC), they also radiate the form of energy-light-our eyes detect.

Radiation is important to thermographers for two reasons:

• It is a key mode of moving heat around and,

• It is the way our imagers see the world.

As an object heats up it gives off more energy. Put your hand close to a motor that is operating and you’ll feel more energy being radiated from the motor than when it is not running. A thermal imager gives us more refined and useful information, but it, too, simply “sees” the radiant energy our hand feels.

As anyone who’s used a thermal imaging system knows, the world of radiant heat transfer is remarkable! Not only are we warmed on the beach, but the beach at night is cooled as it re-radiates heat to the sky. A light bulb radiates heat to the nearby wall and a masonry building “glows” long into the night after a sunny day. The examples are endless because everything, everywhere is radiating all the time.

Mostly, this new thermal world is easy to understand—but we need to go slowly as we learn to Think Thermally, because some of the rules are less familiar. For example, bright metal surfaces are inefficient at emitting radiant energy. These low-emissivity surfaces can be hot (or cold) but not look that way to our imagers. I like to say they only “whisper the truth” about their temperature.

To make matters worse, low-emissivity surfaces are—not unlike a mirror is to visible light—highly reflective of their thermal surroundings. As a result, we see very confusing images—a little bit of thermal truth and a lot of thermal lies!

The good news is that most surfaces, except bright metals, are very efficient at radiating energy, so what we see about their temperature in our thermal imager is what is true. For any surface we can make corrections for emissivity and reflected background temperature that help us refine the radiometric temperature reading. Unfortunately, for bright metals, the margin of error is still unacceptably high in most cases. Carefully used, however, we can measure temperatures differences on many surfaces with a very high degree of precision, typically as small as ±3.6ºF (2ºC).

To more fully understand radiant energy, try this simple experiment: heat a non-stick, aluminum pan on the stove until it is very hot (WARNING: Please be careful!). Hold your hand near the aluminum side. You’ll need to get quite close before you feel its radiant warmth! Flip the pan over and do the same to the non-stick side. There, you’ll have a much better sense of how hot it is because the non-stick side emits efficiently. Now take infrared images of both sides and you’ll see what your hand sensed—the differences are startling.

The aluminum side of the hot pan appears cool (left) because it doesn’t emit efficiently and reflects the cooler surroundings. Flip the pan over and the non-stick side (right), being a much more efficient emitter, reveals how hot the pan really is. Have fun with this, but be careful!

Next week we’ll talk about what happens when you touch the pan: conduction!

Thinking Thermally,

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

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