Thermal conductivity of Dry Wood

Thermal conductivity of Dry Wood

This is a follow up post on a study we preferred in December 2018. At that time, we looked freshly cut birch wood and its thermal conductivity. You can find that post here: https://thermtest.se/thermal-conductivity-of-green-wood

For the study we are using two methods, first the Transient Line Source method utilizing Thermtest’s TLS-100 and then the Transient Plane Source method utilizing a TPS 2500 S. The TLS-100 was originally designed to test soil but is also very useful when analyzing larger samples of construction materials, like rock, concrete, polymers and wood. The TPS 2500 S is a versatile instrument for testing most solids, powders and pastes.

Now, we have gone back to the very same samples as tested before, to see how their properties have changed over the past 16 months. The main difference is that the wood is now dry, as it has been kept in the laboratory the entire time. Just as last time, there are two different sample geometries, a larger piece with a 10 cm deep hole along its central axis for testing with the TLS-100 and two smaller discs for testing with the TPS method, all cut from the same original tree trunk. The samples were still in good shape since last time and no further sample preparation was needed.

Thermal Conductivity Testing of Dry Wood using Thermtest TLS-100
Figure 1. TLS-100, samples for TPS testing and sample for TLS testing. The bark on the large sample was partly removed to allow it to dry faster.

As noted in the earlier blog post, wood is an anisotropic material and we must consider in which direction we test the thermal conductivity. You can read all the details in the older post, but in short, the TLS will test the sample in the radial direction while the TPS will provide a geometrical mean value. With this knowledge we can quantify the anisotropy of the material by combining the methods.

Results

The TLS-100 was set to automatically perform five measurements. The TPS 2500 S was programed to also perform 5 tests, using a Hot Disk sensor with radius 6.4 mm. The average results from these tests are presented in the below table.

InstrumentThermal conductivity of green birchThermal conductivity of dry birch – New test
TLS-1000.275 W/m·K0.117 W/m·K
TPS 2500 S0.361 W/m·K0.215 W/m·K

To investigate the anisotropy, we again utilize the following relationship, using the green wood as the example:

thermal conductivity of dry wood formula

The corresponding value for the dry wood is 0.395 W/m·K.

From this we can conclude that the anisotropy is 1.72 to 1 for the green wood and 3.38 to 1 for the dry wood (axial to radial). Seemingly, the drying process affects the cross-fiber direction more than the direction along the fibers.

Conclusion

From the above data we can conclude that the drying process results in lower thermal conductivity, as expected. Clearly, conductivity is reduced more in the radial direction, likely explained but reduced fiber to fiber contact as the moisture leaves the material. Again, we have successfully combined two transient methods to generate additional information about a sample.