Fluid Mechanics and Acoustics Laboratory - UMR 5509

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Article in Int. J. Heat Mass Transf. (2021)

Acoustic streaming enhanced mass transfer at a wall

Nouhayla El Ghani, Sophie Miralles, Valéry Botton, Daniel Henry, Hamda Ben Hadid, Benoît Ter-Ovanessian & Sabrina Marcelin

Acoustic streaming enhanced mass transfer at a wall

The influence of an impinging acoustic streaming jet on wall mass transfer is studied both experimentally and numerically. The idea is to show that acoustically-driven jets generated by ultrasounds can be used to enhance transfer phenomena at a distance, by creating localized friction zones. An experimental setup has been developed consisting in a cavity containing an electrolytic solution of $[Fe(CN)_6]^{4-}/[Fe(CN)_6]^{3-}$. A jet forced by an ultrasound beam impinges on the upper wall instrumented with electrodes, at which the mass transfer influenced by the streaming is measured by electrochemical technics. Numerical simulations of the flow and mass transfer in the same configuration are also performed. A significant enhancement of the mass transfer at the electrodes (represented by the Sherwood number $Sh$) with the injected acoustic power (quantified by the acoustic Grashof number $Gr_{ac}$) is observed. An order of magnitude of the expected Sherwood number and friction coefficient is proposed on the basis of the Leveque law and momentum budget considerations. Scaling laws involving both experimental and numerical mass transfer at the electrodes ($Sh$), numerical wall shear stress and injected power ($Gr_{ac}$) are finally derived.
(a) 2D schematic side view of the experimental setup with a 2 MHz acoustic source put in water and oriented with an angle of θ = 27.5° to shoot on the instrumented plate at the top of the investigated volume. The tank is filled with $[Fe(CN)_6]^{4-}/[Fe(CN)_6]^{3-}$ electrolytic solution. The 3 mm diameter platinum working electrodes are connected one at a time to a potentiostat. The counter electrode at the bottom of the cavity and the reference electrode are used to close the circuit and control the electric potential. The solid red line departing from the transducer represents the acoustic beam axis. The super-imposed colormap is the computed velocity field for an acoustic power Pac = 1.099 W. Details are given in Section 5. (b) Geometry of the instrumented top-plate supporting the electrodes. A colormap of the normalized acoustic force Fac/Fac,max has been superimposed on the sketch. The white circles represent the positions of the electrodes.

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