Natural convection in shear-thinning fluids: velocity and temperature measurements by MRI

Abstract : An experimental investigation of the Rayleigh–Bénard convection in shear-thinning fluids is presented. The experimental setup consists on a cylindrical cavity defined by a finite aspect ratio A = D/d = 6. Both velocity and temperature fields are obtained by using MRI, in particular, we use an innovative technique to measure temperature called the Magnetic Resonance Thermometry (MRT). In our case, the temperature fields are obtained by measuring the transverse relaxation time T2, we have validated this technique with the Glycerol and we obtain a temperature resolution close to 1°C. Concerning the Rayleigh-Bénard configuration, flow structure is determined at different Ra number from the velocity and temperature mappings for a Newtonian fluid, Glycerol, and for shear-thinning fluids, Xanthan gum aqueous solutions with weight concentrations ranging from 0.1% to 0.2%. In the case of the Glycerol and the Xanthan solution at 0.1%, one recovers similar experimental results in terms of criticality with Rac close to 1800 and patterns since the convection is characterized by rolls. When the Xanthan concentration is increased, the critical Rayleigh number is not modified, however the onset occurs with hexagonal pattern. Because the critical temperature differences increase with the concentrations due to an increase in viscosity, one can think that hexagonal patterns are due to variations of physical properties with temperature (non Oberbeck–Boussinesq effects). Similarities with some results obtained in the Newtonian case are highlighted. We have observed a transition from hexagonal patterns to rolls by increasing the Rayleigh number. This pattern transition is characterized by a discrepancy in the maximal velocity values. By using shear-thinning fluids, results show an increase in the intensity of convection compared with the Newtonian case.
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https://hal.univ-lorraine.fr/hal-01857713
Contributor : Sebastien Leclerc <>
Submitted on : Friday, August 17, 2018 - 9:30:45 AM
Last modification on : Thursday, March 28, 2019 - 9:18:08 AM

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  • HAL Id : hal-01857713, version 1

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Christel Metivier, Sébastien Leclerc, Mohamed Darbouli, Dynmuhambet Baymbetov. Natural convection in shear-thinning fluids: velocity and temperature measurements by MRI. Annual European Rheology Conference 2018, Apr 2018, Sorrento, Italy. ⟨hal-01857713⟩

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