Transfer processes in animal coats. II. Conduction and convection

Abstract

According to reliable literature, the thermal conductivities of animal coats range from about 40 to 150 mW m -1 K -1 compared with 25 mW m -1 K -1 for still air at 20 °C. Greater rates of heat transfer in coats can be accounted for by ( a ) radiative transfer between hairs; ( b ) free convection induced by temperature gradients. A simple theoretical analysis of radiative transfer for the special case of a linear temperature gradient showed that in the region where boundary effects are negligible, a radiative conductivity can be estimated from 4 b /3 p where b is the increase of black-body radiant flux per degree Kelvin and p is the interception function defined by Cena & Monteith (1975 a ). Taking account of radiation, the combined molecular and radiative conductivity of coats is expected to fall between 30 and 45 mW m -1 K -1 . Higher values, e.g. for fleece, can be accounted for by free convection. The importance of free convection in a sample of fleece with a diameter of 20 cm was demonstrated by showing that the thermal conductivity was independent of windspeed but increased with the temperature difference between the skin and the air or ( T s — T a ). When the sample was held vertically, the Nusselt number for free convection was given by 1.66 ( T s — T a ) 0.7 . The Nusselt number for a 2 cm fleece was very close to the value expected for a flat plate with the same diameter.