WPS 264 Thermal properties

Heat affects

• Hygroscopicity

• Shrinkage & swelling

• Mechanical properties

• Acoustical properties

• Electrical properties

Dimensional changes

• Changes very small compared to shrinkage & swelling due to MC changes

• No practical importance because temp changes normally within narrow limits & MC produces higher change.

Thermal expansion coefficient

• Calculated for ovendry wood

• d = (l₂ - l₁) / l₁

• Expansion coefficient a = d / l₁*ΔT (given)

• Elongation with 1C increase

• smallest in axial & largest in tangential direction

Other dimensional

• In plywood thermal expansion is different for each layer in same direction; washboard effect

• a is largest with change in density in tangential direction. Radial lower. Axial very low with small change.

Thermal conductivity κ

• Measure for amount of heat (kcal) flowing during time t (h) through a body with length t (m) & surface area A (m²) at temp T (K).

• kcal*m/(h*m*K) → W/(m*K)

• Inverse to thermal insulation capacity

• Good insulation quality because of porous structure

Thermal conductivity dependencies

• Wood structure

• Density

• MC

• Temp

• Extractives

• Defects

κ with structure

• κ higher in axial direction (about twice)

• κ hardly varies between radial & tangential directions

  • axial: 0.191-0.284, radial: 0.104 - 0.151, tangential: 0.090 - 0.14

• Higher axial value due to fibrous morphology & axial orientation of wood cells

• Deviations of fibrils in vertical orientation reduce axial conductivity

• Radial conductivity about 5-10% higher than tangential because of wood rays

• Higher tangential conductivity in species with pronounced density difference between early/latewood

• Ratio of axial:tangential conductivity depends on angle of microfibrils in S2 wall

  • large angle decreases ratio

Thermal conductivity linearity

• TC varies linearly with density, MC, temp.

Density and MC & TC formula

• κ = 0.177R₁₂ + 0.0205

• R₁₂ is density at 12% MC

• κ = 0.01864 + r₀(0.1941 + 0.004604*MC)

Extractives & TC

• High extractive content means higher conductivity

Insulation properties

• Wood is good thermal insulator

• Light & dry wood better

• Low temp better

Specific heat

• Amount of energy needed to increase temp of a mass by 1C

• J/ g*C

• High specific heat with low thermal conductivity makes wood suitable for handles, matches, ect.

• Not affected by species, density, but increases with temp & MC

Combustion

• Wood burns at high temp; suitable for energy/heat production, limitation as building material

• Result is chemical decomposition

• Flammable gases produced

• Process:

  • Moisture evaporates (up to 100C)

  • Volatile substances evaporate (95 – 150^oC)

  • Carbonisation of the surface and slow exit of flammable gases (150 - 200^oC)

  • Increased exit of flammable gases, ignition and glow (200 - 370^oC)

  • Ignition of flammable gases, formation of glowing charcoal (370 - 500^oC)

Flammability dependencies & stages

• Depends on species, MC, temp, dimensions, type of structure.

• Species;

  • extractives (mainly resin) increases flam

  • long, open passages (vessels) more flam

  • SW tracheids decrease flam, because short & closed ends

• MC retards ignition & decreases flam

• Temp increase increases flam

• Stages

  • 200 - 275^oC: a external flame (spark) is needed to ignite the wood and maintain the flame.

  • 260 - 290^oC: Wood needs to be ignited external but the flame remains.

  • 330 - 520^oC: Spontaneous ignition.

Flammability & char

• Small pieces burn easier & ignite at lower temp

• In large structures only outside layer is burnt & core maintains strength

• Carbonised layer has lower thermal conductivity & high specific heat. Acts as thermal insulator. Burning of core delayed.

• Flammability reduced by impregnation

Heating value H

• Amount of energy produced from burning 1kg (1g) of wood

• kcal/kg or cal/g OR MJ/kg or J/g

• Average max heating value of ovendry wood: 4500 cal/g

• 1kcal = 4.1868 kJ

MC & heating value

• H of air dry wood about 15% lower than of ovendry wood

• H of moist wood estimated by; H = H₀*(1-u) - 2.45*u, u is MC in %

Extractives & heating value

• SW generally have higher H

Chemical composition & heating value

• Lignin has higher H value than cellulose

• Lignin: 6100 kcal/kg

• Cellulose: 4200 kcal/kg

Density and particle size & heating value

• Increases with density

• Decreases with particle size, but huge jump from smallest particle size to next range