Efficient heat transfer in a regime of Elastic Turbulence – HotEt

The improvement of the heat transfer and heat-exchanger performance is the subject of much intensive research work.
Improved heat-exchanger designs must satisfy several criteria: they should preferably be compact, should have high
thermal efficiency with low pressure losses, and, for the chemical and food industries, must be easy to clean and must not
alter the quality of the product by overheating at the wall or high shear stresses. The usual technique for heating a fluid is
to heat a wall by some means, and heat-transfer performance is generally enhanced in two ways. Passive techniques
either employ complicated three-dimensional designs of the boundary conditions or the use fluid additives for
enhancement, while active techniques require supplementary external power. Chaotic advection, which is the production
of chaotic particle paths in the laminar regime, is a novel passive technique for increasing heat transfer. The increase in
mixing and heat transfer in the chaotic advection regime compared to the regular flow has already been established.
Chaotic advection can be generated in two-dimensional unsteady or three-dimensional steady flows. Generally, the
improvement of the overall heat-transfer coefficient reaches a maximum at a optimal laminar Reynolds number, before
decreasing with Reynolds number. The improvement tends to be reduced consequently for small Reynolds number (<30).
However, progress over the last decade in miniaturization of various technological devices has led to the new discipline of
microfluidics, which regroups various scientific domains concerning flows in structures or devices of length scales from
one to a few hundred microns. Many studies have recently been dedicated to characterizing the heat transfer in
microchannels in components of electronic cooling applications. In this case, Reynolds numbers reach the smallest values
and the use of “classical” chaotic advection regime requires a special design of fully tri-dimensional micro-channels which
may become unpractical. An alternative way of generating a chaotic flow (laminar or turbulent like) is to introduce a
source of non-linearity in the hydrodynamic equations, other than the inertial one. This can be realized by using complex
fluids, made of high molecular structure building blocks that can interact in a nonlinear fashion with the flow field. An
example of such chaotic flow is the Elastic Turbulence that will be briefly introduced in the next section. The global
objective of the project is to understand how the random fluid motion in a regime of Elastic Turbulence could enhance the
transport of heat in a viscous fluid. This global aim would be tackled at two levels. First, a fundamental physical
understanding of the coupling between a smooth elastic turbulent flow field and a temperature field will be studied in a
macroscopic swirling flow system. Second, the fundamental understanding of the coupled space-time dynamics of
temperature and flow fields will be employed in designing microscopic an open-flow heat exchanger powered by the
phenomenon of Elastic Turbulence.