Heat and Mass
Transport
6
Life depends on heat and mass transfer between organisms and their
surroundings. Such processes as carbon dioxide exchange between leaves
and the atmosphere, oxygen uptake by micro-organisms, oxygen and
carbon dioxide exchange in the lungs of animals, or convective heat loss
from the surfaces of animal coats are fundamental to the existence of
life. A thorough understanding of these exchange processes is therefore a necessary part of the study of biophysical ecology.
Now that we have the ability to describe the state of the organism
environment (temperature, gas concentrations, wind, etc.) and the temperature or concentration at the organism surface, we are ready to show
how to compute fluxes of heat and mass between organisms and their
surroundings. In this section we briefly review the transport laws, and
then show how to integrate the laws of heat and mass transfer to obtain
forms of them that are easily used in environmental biophysics. We then
show how to apply these laws to compute fluxes. In Ch. 7 we use principles from engineering and micrometeorology to compute conductances
for organisms and their surroundings, and then, in Chs. 8 and 9 we briefly
present the theory for heat and water flow in soil.
The rate of transport of mass or energy is usually expressed as the
product of a proportionality factor and a driving force. The most familiar
transport laws are the following:
Newton's law of viscosity for momentum transport:
Fick's law for diffusive transport of material:
Fourier ' s law for heat transport: