Numerical analysis of heat transfer increase in a tube with alternate successive gradual wall deformations

Abstract

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.