Why is the thermal conductivity of wood important?
Wood is one of the oldest construction materials and has also been used a fuel by humans for thousands of years. Today, it is important to understand the thermal properties of wood for energy design and evaluation in buildings constructed with timber. Timber is an excellent building material. The low thermal conductivity of wood means that it can act as an effective insulator to protect the inhabitants of the building from extreme environmental temperatures. Wood is also a popular fuel source as it burns slowly and predictably. However, extended periods of temperature variation can have adverse effects on the strength of wood and its physical properties. Knowledge of the factors that affect the thermal conductivity of wood can help engineers and designers in decisions about which materials to use during construction, and also contribute to our knowledge of how wood acts as a fuel. Suleiman et al. (1999) used the Hot Disk TPS thermal conductivity system to collect data on how temperature, density, porosity, and anisotropy affect the thermal conductivity of wood (hardwood) in Sweden.
Figure 1. Building construction1 and fuel2 are two of the major uses of wood today.
Suleiman et al. (1999) used six samples of birch wood from Sweden. These samples were placed in an oven at 120°C until their mass was constant, indicating that all free and bound water had been removed. Measurements were performed in both the longitudinal and transverse directions. Some samples were cut parallel to the grain, while other were cut perpendicular to the grain. This enabled the researchers to test the anisotropic properties in a unidirectional manner, as it limited the amount of heat flow that would travel in the opposite direction of measurement. In order to understand the effect of temperature, thermal conductivity testing was performed at room temperature (20°C) and at 100°C.
The Hot Disk sensor composed of nickel foil surrounded by Kapton was firmly clamped between the two sample halves, and thermal conductivity and thermal diffusivity were measured (Figure 2). The Hot Disk TPS thermal conductivity measurement system is ideal for this type of work as it has a specific anisotropic testing module that enables researchers to determine if heat is conducted at different rates in different directions. As wood is an extremely anisotropic material due to the arrangement of cellulose microfibrils, the TPS facilitates accurate directional results. A one sided sensor, the TPS-S, is also available for thermal conductivity testing. This sensor can perform all of the same measurements as the two sided sensor, but only requires one piece of a sample (Figure 2).
Figure 2. An illustration depicting the set up used to measure thermal conductivity with a two sided Hot Disk sensor like the one used by Suleiman et al. (1999) is on the left, on the right is the TPS-S, which can measure thermal conductivity with only one sample piece.
Data from the Hot Disk TPS thermal conductivity measurement system indicated that the thermal conductivity of the samples increased with temperature. This trend was seen in both the longitudinal and the transverse direction. Porosity was observed to have a substantial effect on the thermal conductivity of the samples, and Suleiman et al. (1999) theorized this was due to the majority of heat is transferred through the voids themselves. A low thermal conductivity makes wood a popular building material. Investigations into the effect of anisotropy on thermal conductivity measurement gave results that aligned with the scientific literature, thermal conductivity is higher longitudinally (parallel) than it is in a transverse direction (Tables 1 and 2).
This study is an excellent example of how the Hot Disk TPS system can adapt to produce results for a variety of different samples. Due to its ability to measure anisotropic properties, a clear distinction in how the wood conducted heat in different directions was revealed. The ability of the system to perform tests at both room temperature and at higher temperatures enabled researchers to analyze how temperature change affects the thermal conductivity of wood, which is an important factor in understanding how heat flow in buildings can change as the materials in the structure age. This research has given us a better understanding of how the thermal properties of hardwood change with temperature, and how factors intrinsic to the wood affect the thermal conductivity.
Note: For comprehensive results and an in depth discussion, please follow the link to the scientific paper in the reference section.