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Convective Heat Transfer

As thermography professionals, we must be well grounded in the basics of heat transfer. If not, we’ll make mistakes in understanding, interpreting, and presenting our data. If you don’t feel 100% confident in your understanding, I urge you to move in that direction and will offer these posts as simple starting points.

Convection happens in fluids, like this cup of tea. With a carefully adjusted image you can actually see the currents of warm and cool fluid moving!

“Simple” is an important part of the process for me. Anyone doing research on heat transfer can quickly run smack into “COMPLEX!” While that approach is often necessary, let’s begin by simplifying—recognizing that the world is not always so—in order to start the foundation on which we can learn more and more “complex” later on.

Convection happens in fluids. Let’s again go back to our mug of hot coffee. When you hold your hands above the mug, heat is transferred by convection.  Your hands gain energy as air, having been warmed by the mug, moves over them. How much heat is transferred is determined in large part by various circumstances,,all lumped together and called the Convective Heat Transfer Coefficient (or h). These can include velocity and direction of flow among others. Like conduction, transfer also depends on the difference in temperature between the fluid and the surfaces with which heat is being transferred (in either direction), and, if we are concerned about total energy transfer, the area over which transfer is happening.

Newton’s Law of cooling succinctly describes conductive heat transfer:

Q = h • ΔT • A

In which:

Q = total conductive heat transfer

h = Convective Heat Transfer Coefficient

ΔT = temperature difference between the fluid and the surfaces involved in the transfer

A = Area of surfaces over which transfer is taking place

As was the case with conductive transfer, remember the net transfer can occur in either direction. Stick your finger in the hot coffee and the transfer is into your body. Once the coffee has cooled to room temperature (70°F/21°C), heat is transferred from your warmer body into the coffee.  There may be instances where the coffee and your finger are exactly the same temperature, in which case ΔT equals zero and no heat transfer happens!

We are all prone to simplifying how we speak about convection by saying such things as “warm air rises.” While this seems to describe reality, what is really true is that the less dense warmer air is displaced by the more dense, cooler air surrounding it. Think of a cork in water. Literally the cork is pushed upward as gravity pulls the water downward. “I thought we were going to keep this simple,” you say! Yes, but we also need to be accurate. You can read more (some of it just plain “hot air”) about the movement of fluids in a great online discussion on Home Energy Pros, a great website for building scientists.

Here I’m holding a piece of paper just above the neck of a thermos bottle filled with hot coffee. The heat transfer from the more buoyant warm air can be heating the paper. Is the warm air rising or being “pushed” by the cold air around it? That is an interesting question to explore!

Between now and next week, give some further thought to the issues related to convective heat transfer and take time to look at examples of convection with your imaging systems. We’ll come back to that mysterious “h” or Convective Heat Transfer Coefficient next week.

I also wanted to take a moment to wish all our Chinese friends—especially the Fluke China team and our mutual customers there—a Happy New Year of the Dragon.

1 comment to Convective Heat Transfer

  • John Nicholas

    I like the analogy of ‘a cork in the water’ – I will use that when I break out the Ti 32 with the Bloor Door going.

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