Chapter 10 - Heat Transfer

Abstract

This chapter deals with the mechanisms of heat transfer between organisms and the atmosphere by convection and conduction. The principles of convective heat transfer are introduced using non-dimensional groups from engineering theory. The Nusselt number Nu provides the basis for comparing rates of convective heat transfer from similar-shaped bodies of different scales. Leaves and animals can often be approximated by flat plates, cylinders, and spheres, and this allows heat transfer to be estimated from theoretical and empirical relations. Methods of estimating Nu in forced and free convection and in laminar and turbulent flow are explained, and the concept of resistance to convective heat transfer is introduced. Measurements of convective heat loss from leaves and animals are then compared with estimates from engineering theory, demonstrating reasonable agreement. Conduction is discussed primarily in relation to heat transfer through animal tissue and coats and human clothing. Thermal resistances to heat transfer through peripheral tissue and animal coats are reviewed, and it is shown that heat transfer through animal coats usually involves multiple mechanisms: molecular conduction, radiation, and free and forced convection.