Comportement hydrodynamique et thermique d'un fluide non newtorien pseudolplastique dans une geométrée complexe

Abstract

In this work, numerical simulations were performed using CFD code in order to study the flow characteristics of non-Newtonian power-law fluid in two geometries, C-shaped and straight channels. First, the kinematic behavior of the fluid flow was characterized in terms of the Vorticity rate, Deformation rate and Rotation rate. As known, the enhancement of these parameters in the fluid flow maximizes the mixing performances of the fluid. The results illustrate that the Newtonian fluid is more favorable than the non-Newtonian cases to increase the considered parameters. Furthermore, at high values of generalized Reynolds number, the rotation and the deformation rates are more important in Newtonian case compared to the other fluid model. Then, hydrodynamic and thermal performances are characterized by calculate the Poiseuille (Po) number and the Nusselt number (Nu). Case validation of the non-Newtonian power-law model was performed for straight channel under the condition of wall heat flux. The chaotic configuration displays a heat transfer enhancement in terms of the mean Nusselt number compared to the straight channel, however the pressure drop in this geometry increases (high Poiseuille number) for all examined generalized Reynolds number. Although this, the ratio of the Nusselt number to the Poiseuille number (Numean/Pomean) is higher in the C-shaped geometry, showing that the heat transfer enhancement is important pressure loss increase. After that, thermal mixing was characterized for both straight and the C-shaped channels by calculate the degree of mixing (Dm) of two fluid with different temperatures and for various values of generalized Reynolds numbers (Reg = 50-200). The results have shown that the C-shaped geometry has more effective improvement in the mixing efficiency than the straight channel due to the existence of the recirculation zones. In addition, the thermal mixing performance can be improved by increasing the generalized Reynolds number (Reg). Furthermore, the power-law index (n) has a significant effect on the thermal mixing performance. Also, we analyzed the second law of thermodynamics for the considered geometries to show the thermodynamic performances. The results showed that the increase in generalized Reynolds number causes the reduction in heat transfer contribution and the increase in the pressure drop one, that is mean the entropy generation due to heat transfer decreases while the entropy generation due to friction factor augments. As the power-law index decreases, the global entropy generation is dominated by heat transfer irreversibility for larger external heat flux. Moreover, for given generalized Reynolds number, the entropy generation due to heat transfer rises with increasing of external heat flux and decreases as the value power-law index increases. Therefore, the reduction in heat transfer entropy generation of non-Newtonian fluids becomes higher than that of the Newtonian one. Finally, this work allowed presenting a chaotic geometry of a highly efficient of thermal mixing system and better performances in thermodynamics, for both Newtonian and non-Newtonian fluids. As related to the present work, the following suggestions are recommended for future developments:  Study the problem experimentally.  Take in consideration various type of fluids (Bingham, dilatant fluids…etc)