This time we’re finishing up our discussion of thermal diffusivity before moving on to further discussion of thermodynamics. Hopefully taking these topics a step at a time is doing its job and will allow you to absorb the knowledge more easily, and hold onto it longer.
In The Snell Group’s Level I course, we see several mathematical equations related to heat transfer. We encourage you in class to not concern yourself with being able to calculate anything with them, that’s for Stefan, Boltzmann, Fourier and those guys. As Level I thermographers, we need only to understand the relationships between the variables in these equations so we can better interpret what we see through our imagers. Let’s look at this pesky formula again that we touched on in part one of this blog series: http://thermal-imaging-blog.com/?p=3602 :
k = Thermal conductivity
ρ = Density
Cp = Specific heat
This is a linear equation, so whatever happens on one side of the equal sign, also happens on the other side of the equation, and to the same degree. If conductivity increases, so does diffusivity. If either density or specific heat increases, diffusivity decreases. Materials with higher conductivity will tend to be more diffusive than materials with lower conductivity. Materials that are high density will tend to be less diffusive than materials that are less dense (with similar conductivity values, that is).
How does the concept of thermal diffusivity impact thermographers in the field? Most often, the heat we’re interested in is developed internal to the object we’re inspecting. How we “see” the heat is dependent upon how the heat gets from inside (where we can’t see it) to the outside surface (where we can). The thermal pattern detected from the surface on of an object is going to be influenced by ambient conditions as well as variances in the operating parameters of whatever system the object is part of. Likewise, the thermal characteristics of the material through which the heat flows will impact the pattern we detect. Often, we will be more concerned with small areas of highly concentrated heat than we are with larger areas with an even heating pattern. Consider though how an object would appear thermally if the object is very diffusive. A “spread out” or diffuse pattern may be difficult to observe thermally but it doesn’t always mean there isn’t trouble.
Another common infrared application involves nondestructive testing, NDT, where the thermographer intentionally disturbs thermal equilibrium by adding or removing heat from a material and observing, thermally, subsurface anomalies. Data collected during testing like this can identify purity of material, porosity and thickness.
Thermal diffusivity occurs in all materials whenever there is a transient event or thermal equilibrium is disturbed. Knowing its effects takes a careful eye and some basic understanding. Hopefully you’ve gained some of that basic knowledge from this blog post. Check back for our fourth and final installment of Thermodynamics 101 and Think Thermally®!
Think Thermally,www.thesnellgroup.com The Snell Group, a Fluke Thermal Imaging Blog content partner