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“h” or the Convective Heat Transfer Coefficient

When heat transfer occurs in fluids—defined simply as non-solids—the rate and total transfer are governed by several factors, two of which are easily known: temperature difference and area. More challenging to define precisely is “h” or the Convective Heat Transfer Coefficient. This all-important variable is the amalgamation of a number of influences on heat transfer in fluids. Let’s look at the most important of them and how that might affect what we see with our imaging systems:

• Velocity of the fluid

• Orientation to the flow

• Geometric shape

• Surface condition

• Viscosity

All of these factors are important to consider, whether you are a thermographer working on buildings, industrial equipment or both. For most of our work, the fluid in question is the air surrounding us and the surfaces we are inspecting. The transfer of heat between the air and the surfaces can be considerable and can also result in patterns that can be confusing if we are not paying close attention to convection.

This remarkable image, taken by one of my students, show changes in wall temperature caused by air currents moving around and through fiberglass batt insulation in the walls of a home. Interestingly, the blower door was not being used to depressurize the home at the time!

Of the five major influences, velocity typically has the greatest affect. Whether we work out-of-doors, where we must pay attention to the wind, or indoors where air currents can have a dramatic influence, the velocity of the fluid has a powerful affect on “h” and, as a result, on total convective heat transfer. The relationship is direct: increase velocity and you increase convective heat transfer.

A simple experiment demonstrates this. Put your hand in the air and notice what it feels like. Now wave it back and forth several times. Notice you can literally feel the difference in heat transfer—cooling in this case—caused by the increased velocity of the air movement. You have, in essence, increased “h” simply by waving your hand.

If you look at the upwind and downwind sides of any building or any piece of industrial equipment influenced by air currents and you’ll see differences in surface temperature—both warmer and cooler—related to velocity.

I have found it important to measure the velocity of air movements so that I better understand their influence. This can be done very easily using any of the simple “personal weather stations” now readily available in the marketplace. Kestrel in particular, makes several fine, reasonably priced products that work well.

A personal weather station, like this Kestrel brand model, provides a simple means of quantifying both wind and air currents.

Convection is both a large and an important topic so next week we’ll continue the discussion about “h” and other issues related to convection. In the meantime, please keep your thermal eyes open and watch for thermal images showing the influences of this powerful mode of heat transfer!

Thinking Thermally,

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

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