heat transfer is the transport of thermal energy by a moving fluid. The
transfer is controlled by the motion of the fluid, turbulent fluctuations
within the fluid and heat conduction within the fluid. Convection occurs in two
types; forced convection where the fluid motion is externally imposed and is
driven by external energy such as a fan or pump. Or free convection in which
fluid motion is driven by bodily
interactions and density gradients due to temperature differences. 1.
Convection occurs in two forms; Laminar flow where each particle of the fluid follows a smooth path at
constant velocity which never interfere with one another. 2. Or Turbulent
flow which is characterized by irregular fluctuations in which the velocity
changes drastically and mixing can occur. Since the Reynolds equation for turbulent heat
transfer is being studied, Turbulent flow will only be discussed further.
thermal boundary layer develops when a fluid at specific temperature flows over
a surface which is at a different temperature. In Turbulent heat convection,
the wall heat flux is controlled by the thickness of the viscous sublayer. The
flow type can be defined by the Reynolds number which depends on free stream
turbulence and surface roughness. The Reynolds number represents the ratio of
the inertia to the viscous forces. When the Reynold’s number is above 6000, the
flow is fully turbulent. 3. In internal pipe flow, the addition of heat or
removal at the walls changes the energy content of the flowing stream, and
therefore the bulk fluid temperature changes along the flow direction. This is
known as the no free stream condition. The advantages of turbulent flow are
that ”it has a greater heat transfer capability than laminar flow” and ”it
can extend the useful life of the process tooling by slowing the build-up of
precipitates on the heat transfer surface.” 4.
There are many benefits
to using convection heating in turbulent pipe flow; first of all ”it allows for
tighter temperature uniformity of heat-treated parts, particularly those with
specific load geometries, alloys that do not transfer heat well and/or many expensive
parts that require a tight temperature uniformity range.” 5. Cycle times can
also be reduced by using convection heating especially for large and dense
loads. 5. Some examples of the applications are blade cooling in turbines and
the transfer of heat in internal combustion engines.