Thermal electro-hydrodynamic heat transfer augmentation in vertical annuli by the use of dielectrophoretic forces through a.c. electric field

We consider a fluid-filled cylindrical enclosure with an inner heated cylinder of radius ri maintained at temperature Ti and an outer cooled cylinder of radius ro maintained at temperature To. This vertical annulus is of height H with adiabatic top and bottom boundaries. The temperature difference ΔT=To-Ti induces natural convection in the gap width d=ro-ri at small values of ΔT. This base flow experiences transition to instabilities depending on the radius ratio η=ri/ro, aspect ratio Γ=H/(ro-ri), physical properties of the fluids (weighted with the Prandtl number) and on the driving force (weighted with the Rayleigh number). In this work, we study experimentally the heat transfer enhancement by the dielectrophoretic force induced by the application of an alternating (a.c.) electric field superposing natural convection in the vertical annulus. This procedure allows to introduce an electric Rayleigh number for the system. We will show a clear distortion of the base flow via flow visualization, and we quantify heat transfer by measurements of the Nusselt number for the inner cylinder. For low values of the electric Rayleigh number the superposed electric field damps the heat transfer, whereas for large values we observe a clear increase via an increase of the Nusselt number.