Steady, Single-Phase Heat Convection in Micro Ducts

Abstract

Flows completely bounded by solid surfaces are called internal flows, and they include flows through ducts, pipes, nozzles, diffusers, etc. External flows are flows over bodies in an unbounded fluid. Flows over a plate, a cylinder or a sphere are examples of external flows, and they are not within the scope of this chapter. Only internal flows, in either liquid or gas phase, in micro ducts will be discussed emphasizing size effects, which may potentially lead to a different behaviour in comparison with similar flows in macro ducts. For the practical application of single-phase heat convection in micro-ducts, e.g. heat exchangers, the friction factor and the heat transfer coefficient are the parameters of prime interest. As long as material properties, such as viscosity, are nearly constant over the temperature range of operation, the friction factor can be predicted analytically without invoking the energy equation. The extensive experimental and theoretical research has matured to a point where the size effects are well understood. Unfortunately, this is not the case with respect to the heat transfer coefficient, mainly due to lack of reliable experimental data needed to confirm or refute the numerous theoretical analyses and numerical simulations.