Geology Reference
In-Depth Information
fluid flow
Buoyant
fluid
Heat
transport
Thermal boundary layer
Figure 5.1. Essential elements of convection in a fluid layer.
The other fundamental thing to say about convection is that it is driven by
buoyancy. Buoyancy arises from the action of gravity on density differences. The
density differences can be due to differences in composition or differences in
temperature - this gives us the distinction between compositional convection and
thermal convection. Differences in temperature cause differences in density because
of the phenomenon of thermal expansion. Thus warmer fluid expands, its density
is lower, its weight (per unit volume) is lower, so it rises relative to other fluid.
Conversely, cooler fluid contracts, its density is higher, it becomes heavier and
sinks. Thus we get to the familiar phenomenon that hot air rises.
If we put these two things together, the fluid in a thermal boundary layer is
buoyant (positively or negatively), its buoyancy can cause it to rise (or sink), and
the flow driven by that rising or sinking fluid is convection. A rising column of
buoyant fluid is depicted in Figure 5.1, and the flow it induces is shown by the thin
arrows. Because some fluid is rising, other fluid must sink to compensate. However,
this return flow does not have to occur in simple closed circulation patterns, if the
flow is time-dependent or three-dimensional. Because warmer, rising fluid carries
extra heat within it, it transports that heat upwards through the fluid. Thus thermal
convection is also a heat transport mechanism. That is the essence of thermal
convection. This overview can be a guide as we go through the specifics of each
part of the process, and then put them together into a description of thermal
convection.
There are several material properties that are important in convection. We have
just encountered thermal expansion, which depends on the particular material
involved, which is why it is called a material property. We have also encountered
density, another material property. Viscosity was covered in the previous chapter.
It is a measure of how much the fluid resists deformation, and we need that to
figure out how fast the fluid will flow under the action of a given buoyancy. Later
we will need the thermal conductivity, which controls the thickness of thermal
boundary layers. Specific heat will also crop up, but we'll worry about that when we
get to it.
