In the development of modern industrial world, tremendous
works on heat transfer enhancement have been conducted and a
large number of techniques for convective heat transfer enhancement
have been developed since 1970s. Most of these techniques
are targeted at the tube fluid flow. The flow in a pipe can be divided
into two parts [1]: boundary flow, which represents boundary layer
in the entrance length or the fluid layer near wall in the fully developed
length, and core flow. To raise the heat transfer efficiency of
heat exchangers, most attentions have been paid to the surface of
tube [2]. The commonly used methods are: (a) disrupting the fluid
boundary layer near the wall; (b) extending the solid surface to
transfer more heat flux; (c) changing the physical and chemical
natures of the surface, etc. This kind of methods is focused on the
heat transfer boundary and thus it may be called heat transfer
enhancement in the boundary flow. However, when these methods
effectively increase the amount of heat transferred, they also
increase the flow resistance inevitably: as the velocity gradient, viscous
diffusion as well as the momentum loss of the fluid near the
boundary increases, the shearing strength and friction between
the fluid and boundary may have different extents of growth.
Different from traditional manner of heat transfer enhancement,
Liu et al. [3] developed the new concept of enhanced heat
transfer in the core flow along a tube. He believes that the core
flow of tubes is worthy to be well used for heat transfer augmentation.
The most direct way is to make temperature as uniform