Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
From previous studies, it is known that the use of wall
deformations in alternate directions while keeping a quasiconstant
cross-section is an efficient way to enhance the heat
transfer in a laminar flow regime inside a tube. In the present
study, the tube cross-section shape gradually changes along the
tube length while keeping the same cross-sectional area to
prevent flow separation areas thereby limiting pressure drops.
These wall deformations create vortical macrostructures inside
the flow that significantly modify the transfer properties. Two
geometrical parameters characterize the tube wall shape: the
radial deformation amplitude and its streamwise wavelength.
Through a numerical study, the effects of the variation of these
two parameters on the flow and on the heat transfer have been
studied. An important finding is that the ratio between the
wavelength and the amplitude has a significant impact on the
observed results: both the friction and the heat transfer
increases as this deformation ratio decreases. At the same time,
a local analysis of the flow mechanisms has been performed to
outline the modifications that occur in the flow pattern when
the wall deformations are increased. Flow in the entrance
region has also been specifically considered: it has been found
that geometrical parameters do have an influence on the length
needed for the flow to get fully hydrodynamically and
thermally established. Finally a performance analysis has been
conducted to assess, for a given performance criterion, the
deformation parameters that give optimal results. Through this
parametric study, for the given Reynolds and Prandtl numbers,
an alternate wall deformed tube geometry that maximizes the
heat transfer without significantly increasing the pressure drops
can thus be defined.