Environmental Engineering Reference
In-Depth Information
Chapter 3
Transport
3.1 The Conservation Principle
Transport is a general term, denoting processes which determine the distribution of
biogeochemical species or heat in an environmental compartment. In this chapter
transport is understood in a narrower sense as interaction of physical processes with
an effect on species or components, or on heat. Other processes, which also may be
relevant for the environment, like sorption, degradation, decay and reactions of
various types, are not conceived as pure transport processes and are treated in
chapters below.
These transport processes are relevant in almost all environmental systems. The
term is not restricted to a specific compartment of the environment. Heat and mass
transport are a common phenomenon which can be found almost everywhere, in the
hydrosphere, in the pedosphere as well as in the atmosphere, in surface water bodies -
rivers, lakes and oceans, in sediments, in groundwater, in the soil, in multi-phase
systems as well as in single phases.
There are two different types of transport processes in the narrower sense:
advection and diffusion/dispersion. Advection denotes transport in the narrowest
sense: a particle is purely shifted from one place to another by the flow field.
Diffusion and dispersion are processes which originate from concentration
differences. Within all systems there is a tendency to equalize concentration
gradients. If the species are mobile, e.g. if they have the possibility to move from
one place to the other, there will be net diffusive or dispersive flux from those
locations with high concentrations to locations with low concentrations.
Transport can be described by differential equations as will be shown below. In
fact it is one differential equation for each species or component. The differential
equation, the so called transport equation can be derived from the principle of mass
conservation and Fick's Law.
Concerning heat transport a differential equation for temperature
T
as dependent
variable results. The equation is derived from the principle of energy conservation
and from Fourier's Law. From the mathematical point of view it is the same
