As we continue the discussion about convective heat transfer, it is useful to define two types of convection, natural and forced. When quantities of fluids are moved in either way, we also use the term “bulk energy transfer” because it is really the movement of the fluid itself, and the energy inherent in it, that results in the transfer.
Natural convection is powered by the difference in temperature, and thus the density, of the fluids involved. As fluids are warmed, they become less dense, and, at the same time, are also displaced by more dense, cooler fluids. As a result, warmer fluids are pushed upward as the cooler fluids sink. The greater the temperature difference, the greater the convective movement, and the more energy transferred. A summer thunderstorm is a classic example of natural convection.
Forced convection involves a pump or fan or some other means of artificially moving the fluid. Forcing convection in this way typically results in a great deal of energy being transferred. Think of how quickly the air temperature can change when a sudden windstorm springs up.
As thermographers we have to watch for the effects of both natural and forced convection. Inside a building, as anfor example, you may see a difference in temperature between the floor and the ceiling of 3-5°F due to natural circulation patterns. Outside the wind can quickly change the temperature, typically by cooling, of the side of a building or a connector in a substation. If you fail to take these changes into account, you will not be able to accurately and fully understand the nature of what you are seeing.
Other variables (aside from velocity, which we talked about last week) must be considered as well. Geometric shape and orientation to the fluid flow can both have a large influence in the patterns we see.
I often notice this around windows. This is because windows are typically either a good deal warmer or cooler than the walls, and because they also project in or out geometrically. In the winter, I can watch cool air falling across the inside of a window to chill the floor below and in the summer the effect of air being warmed by the window can be seen on the ceiling above.
Of course architects recognize this fact and often put HVAC ducts near windows to mask over these effects of convection. Look at any active heating or cooling system and you will see the affects of disruption to the moving air by the surrounding surfaces.
A blower door forces convective airflow through small cracks and holes in the envelope and their shape and location determine what the resulting thermal patterns will look like. You’ll see similar patterns around active machinery where airflow from both from the machine itself as well as the HVAC system, what is termed “spurious convection,” can result in interesting thermal signatures.
The consequences of not understanding convection or not paying attention to its effects can mean making mistakes in your understanding of what you are seeing. Take the time to learn the basics and then be careful as you apply that knowledge in the field. Most of what we need to know is firmly rooted in a common sense understanding so we can all be successful if we just apply ourselves.
Next week, we’ll talk about heat transfer by electromagnetic radiation. This is important to us both because of the effect on temperatures around us, but also because our imagers detect this kind of radiation.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner