HEAT ABSORPTION AND TRANSFER IN SOFTWOODS AND THEIR KNOT SURFACES

This paper deals with the effects of density and fiber orientation of wood on the thermal conductivity and the heat absorption of softwood l umber surfaces. This is to deal with the problem of detecting knots in lumber by thermography based on the differences temperature increasin g rates between knots and surrounding wood. The thermal conductivity o f densified spruce (Picea sp.) wood specimens with various densities a nd grain angles were measured by a laser-flash method. It was found th at thermal conductivity can be expressed by a combined equation of a l inear relationship with the density and tensors given together with th e principal components of longitudinal and transverse conductivities. The longitudinal thermal conductivities of knots and branches of hinok i (Chamaecyparis obtusa Endl.) and sugi (Cryptomeria japonica D. Don) were separated from the relationship to clear tissue, but were similar to transverse conductivity. This behavior resembles that of compressi on wood of branches. From measurements of temperature increases in lum ber surfaces irradiated with incandescent lamps, the heat absorbance o f knots was about 1.6 times that of the surrounding wood.