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Thermodynamics 101: Thermal Diffusivity Part I

If you’ve taken a basic Level I thermography course, you’ve already received a brief introduction to thermodynamics.  In Level II thermography courses, students get to dig a little deeper into “Thermo” (as we like to call it) in an effort to help advanced thermographers better understand why objects appear thermally they way that they do.  If you’re an engineer, some of this information is a refresher, but for other folks it’s brand new, and maybe it doesn’t all stick the first go around.   We’re going to explore some principles of thermodynamics in three, smaller blog installments because, like the old riddle suggests, you eat an elephant one bite at a time.

Diffusivity Part 1

Many of you have at least heard the term “thermal diffusivity” used, and if you have been to Level I or II course, you probably have even seen it demonstrated.  For those of you who haven’t been to a Level I or II infrared course, I need to issue a spoiler alert.  The Miracle Thaw demonstration illustrates how a highly thermally diffusive material will allow heat transfer to occur more rapidly than a material with lower diffusivity, because the heat transfer is occurring both linearly and non-linearly.  We give a brief description of the concept in our Level I course, but we largely leave it at that.

For those engineers out there, you may have seen the formula below before Thermal diffusivity, α, is used to describe or define heat transfer situations that are not at steady state, or said another way,  when temperature distribution changes with time.  It is related to the steady state variable thermal conductivity as shown in the equation below where k is the thermal conductivity.  Cis the specific heat and ρ is the density.  The product of the specific heat and the density of a material it’s thermal or heat capacitance and is based on the volume of a material.

 

Formula

 

 

 

The SI unit of measurement for thermal diffusivity is m2/s, and essentially it’s a measurement of thermal inertia, or more simply put, how quickly material moves heat throughout its volume.

When discussing thermal conductivity, most of us consider heat energy moving linearly through a material, from the side where heat is applied to the other side.  Many of the demonstrations we use in class reinforce this concept.  When heat is applied evenly across a surface, the transfer will tend to be more linear than if the heat is applied in a more concentrated, non-linear fashion. Another important point to keep in mind involves the direction of heat flow.  Heat energy moving through the volume of a material can be triggered by adding or removing heat energy. In transient heat transfer situations, the kind we might experience in the field as thermographers, diffusivity of a material plays a part in how the heat will move through the objects we’re inspecting.

 

This important concept, once mastered, will help you as a thermographer in the field. Check us out next time for another installment on Thermodynamics 101 and Think Thermally®!

 

Think Thermally,
www.thesnellgroup.com
The Snell Group, a Fluke Thermal Imaging Blog content partner
 
 

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