In industrial heat exchangers, heat transfer is often neither purely forced nor purely natural. The interaction of both mechanisms is called **mixed convection**.
Regime Classification
The parameter determining the importance of natural convection relative to forced convection is the Richardson number:
\[Ri = \frac{Gr}{Re^2}\]
Where \(Gr = \frac{g \beta \Delta T D^3}{\nu^2}\) is the Grashof number and \(Re = \frac{u_m D}{\nu}\) is the Reynolds number.
- \(Ri < 0.1\): Forced convection dominates (neglect buoyancy).
- \(Ri > 10\): Natural convection dominates (neglect axial pump flow).
- \(0.1 < Ri < 10\): Mixed convection regime. Secondary buoyancy currents rotate perpendicular to flow axis, enhancing heat transfer up to 400% compared to pure forced laminar limits.
References
- Metais, B., & Eckert, E. R. G. (1964). Regimes of free, forced, and mixed convection for flow through horizontal tubes. J. Heat Transfer, 86(2), 295-296.
