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Convection

Last week we discussed conductive heat transfer in solids. That is, how we feel warmth from a mug of coffee. But why is the handle on the mug so much cooler? One reason, of course, is the greater conductive pathway—or the distance to the heat source—but a more significant reason is probably convection. Heat transfers from the handle to the surrounding air leaving the surface cooler. The warm air rises and is replaced by cool air to continue the process. This process is called “natural convection.” You can get a good sense of how much heat is transferred by convection simply by putting a towel over the whole mug for a few minutes, thereby reducing transfer, and feeling hot how the handle gets!

When heat is transferred by moving fluids, big thermal changes can happen quickly. If our coffee is too hot, without even thinking, don’t we blow air over it to cool it off? Along with the wind, fans and pumps, blowing on your coffee is an example of “forced convection,” typically 3-5 times more powerful than natural convection.

Convection, regardless of the circumstances, is a powerful mode of transfer that affects nearly everything we look at with our imaging systems. Typically, the fluid we are most concerned about is air. In the case of heat exchangers, boilers and cooling ponds, or even swimming in water, it is a liquid.

Regardless of the type of fluid, several factors affect the rate of convective transfer. The temperature difference between the surface and the fluid is a significant driver. A greater difference means more transfer. Think of how you feel in a light wind at the beach where there is only a small temperature difference, compared to that same “light breeze” while on the ski slope in the winter!

An oil-filled circuit breaker (left) as seen on a day when winds were 15mph. The next day—with similar loading and air temperature but winds of only 2mph—the problems have increased in temperature and become more obvious. Wind can be a powerful convective influence!

A number of other factors—velocity, direction of flow, geometry, etc.—are grouped together by engineers and are termed the “coefficient of convective transfer.” If that value is greater, then transfer is greater. Clearly, velocity is often the most significant. Even a slight increase in wind over a hot connector in an outdoor substation nearly always results in significant cooling. Some of the hotspots may be cooled to the point where they are not easy to detect! Thermographers must always use care when viewing surfaces in the wind, especially if that wind may diminish at a later time.

In complex machinery and in buildings, in particular, we also see the influence of direction of flow and geometry. “Sticky” layers of fluid, or boundary lays, build up along the surfaces reducing convection. So parts of a machine will be less effectively cooled and will, as a result, be warmer, often to the point of failure. In buildings, the corners between the walls and the ceilings will often be at a different temperature for the same reason.

While we can simplify convection, it is, in fact, very complex. Again, you can probably better appreciate that fact if you sit down with that cup of coffee, your imaging system and your favorite engineer!  Thermographers should always be aware of convection and try to determine what effect it is having on the surfaces we are viewing. Next week, we’ll bring together many of the aspects of heat transfer we’ve been discussing by looking at a case study example.

Thinking Thermally,

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

2 comments to Convection

  • It is quite refreshing to read an article associated with convection and its effect on thermography. I fear it is an area which is often not given the condideration it deserves particularly when relating to building thermography. Thank You.

  • Fluke Thermography

    Thank you for your input, and we’re glad you could find this post helpful! Keep contributing to our blog and let us know what other topics you’d like to see more of.

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