In engineering, the term convective heat transfer is used to describe the combined effects of conduction and fluid flow. At this point, we have to add a new mechanism, which is known as advection (the transport of a substance by bulk motion). The properties that are carried with the advected substance are conserved properties such as energy, which is a commonly advected quantity. Advection is sometimes confused with the more encompassing process of convection, but convection includes energy transfer by both the bulk fluid motion (advection) and the random motion of fluid molecules (conduction or diffusion).
From the thermodynamic point of view, heat flows into a fluid by diffusion to increase its energy, the fluid then transfers (advects) this increased internal energy (not heat) from one location to another, and this is then followed by a second thermal interaction which transfers heat to a second body or system, again by diffusion. Advection requires currents in the fluid (as in a river or pipeline), and so cannot happen in rigid solids. It does not include transport of substances by molecular diffusion.
The advection equation for a conserved quantity described by a scalar field ρ is expressed mathematically by a continuity equation:
∂⍴⁄∂t + ∇ . (⍴ ͞v) = σ
- ∇ . is divergence,
- ρ is the density of quantity q,
- ⍴ ͞v is the flux of quantity q,
- σ is the generation of q per unit volume per unit time. Terms that generate (σ > 0) or remove (σ < 0) q are referred to as a “sources” and “sinks” respectively. If q is a conserved quantity (such as energy), σ is equal to 0.