NUMERICAL ANALYSIS OF HEAT TRANSFER INCREASE IN A TUBE WITH ALTERNATE SUCCESSIVE GRADUAL WALL DEFORMATIONS
Résumé
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 section area to
prevent flow separation areas thereby limiting pressure drops.
These wall deformations create vortical macro-structures 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 holds a significant impact on the
observed results: both the friction and the heat transfer increase
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 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