Air movement and heat loss from sheep. III. Components of insulation in a controlled environment

Abstract

The rate of sensible heat loss from a Clun Forest ewe was studied at several fleece depths in a temperature-controlled chamber. A simple resistance analogue was used to describe the heat flow from different body regions. Heat loss from the trunk depends largely on the mean fleece depth î . The fleece resistance was about 1.5 s cm -1 per centimetre depth. Heat transfer through the fleece was accounted for by molecular conduction, thermal radiation and free convection. The fleece conductivity k̄ b attributed to free convection depends on the mean temperature dif­ference ( T ¯ st — T ¯ ct ) across the fleece according to the relation k̄ b = 8.9 ( T ¯ st — T ¯ ct ) 0.53 . Estimates of the sensible heat flux from the trunk at environmental temperatures, T a , between 0 and 30°C range from about 8 W ( î = 7.0 cm, T a = 30°C) to about 160 W ( î = 0.1 cm, T a = 0°C). In contrast, the sensible heat loss from the legs depends mainly on the local tissue resistance. For environmental temperatures between 0 and 30°C, the calculated tissue resistance for this region of the body varied from about 8 to 1 s cm -1 . The corresponding heat loss from the legs was between 10 and 20 W, compared with between 3 and 7 W from the head. The fastest heat loss from the legs occurred at an environmental tempera­ture of about 12°C. Although the proportion of the heat loss from the extremities depends on environmental temperature, the total heat loss (sensible or latent) was closely related to the mean skin temperature of the trunk.