Thermal behaviour of Norway spruce and European beech in and between the principal anatomical directions

Thermal conductivity (ThCond), thermal diffusivity and heat capacity of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) have been determined for all principal directions - radial (R), tangential (T) and longitudinal (L) - depending on the moisture content (MC) and ThCond was additionally measured in 15 degrees steps between these directions. The ThCond was determined in a guarded hot plate apparatus. For determining thermal diffusivity and heat capacity, the same apparatus was supplemented with thermocouples and the temperature evolution was evaluated numerically by a partial differential equation. The results show expectedly that ThCond increases with increasing MC, whereby the highest increment was observed in T and the lowest in L direction. ThCond is higher for beech than for spruce in all anatomical directions and the conductivity for both species is more than twice as high in L direction than perpendicular to grain. The highest ThCond is found for beech at a grain angle of approximately 15 degrees. The lowest ThCond shows spruce at an angle of approximately 60 degrees between T and R direction. Thermal diffusivity is similar for both species and decreases with increasing MC. Its differences with regard to the anatomical directions correlate with those of the ThCond values. Heat capacity is lower for beech than for spruce and shows a clear increase with increasing MC.