Heat transfer during agitation of Bingham viscoplastic fluid is studied in this paper. The fluid is agitated with an anchor impeller and the heating is made by a jacketed wall of the stirred vessel. Transfers in the agitated vessel, translating hydrodynamic and thermal phenomena, are numerically predicted by means of Computational Fluid Dynamics (CFD) in transient regime. The purpose of this numerical study is to identify the rigid zones and to optimize mixing and heating performances. The Navier-Stokes and energy equations are discretized using finite volume method, and a two-dimensional analysis of the hydrodynamic and transient thermal behaviours generated in the agitated vessel are performed. Fluid rheology is modeled by the Bingham approximation and Papanastasiou's regularization model. Results show the presence of recirculation zones and permit to explain the unpredicted Nusselt number increasing when Oldroyd number increases. This study shows also the importance of the anchor position on the size and the shape of the rigid zones and on the heating performances.
Ceramic plasma-sprayed coatings are successfully used for prolonging the service life of industrial components where high wear and corrosion resistance are required. In this work, various types of coatings based on alumina were deposited by atmospheric plasma spraying on 304 L austenitic stainless steel substrate. These coatings were used in further tribological studies. For the atmospheric plasma spraying applications, spraying parameter choices such as carrier gases, plasma enthalpy, current intensity, spraying distance, and particles granulometry are the key issues. Two kinds of Al2O3 particle sizes 2–12 µm (Amperit) and 15–45 µm (Norton) were used to prepare pure and composite coatings. The spraying distance was varied from 90 mm to 120 mm. Microstructure characterization performed by scanning electron microscopy showed that the Amperit powders projected at a distance of 90 mm provided better cohesion and a more dense microstructure. However, the choice of the spraying distance of 120 mm was defined to manufacture composite structures, which allowed more stability in the plasma jet and enabled large metal particles to be mixed with ceramics. To develop hard and wear-resistant coatings, alumina and 316SS were mixed with different addition rates by volume (Al2O3–5 vol.%316SS and Al2O3–25 vol.%316SS) and fed through a single injection port. Composite coatings include porosity and unmolten particles in a lamellar microstructure. The addition of 316SS powder led to the formation of typical layered structure due to the effect of viscosity and different densities of the two particles. These coatings were investigated by different tests to evaluate microhardness, cohesion, and fracture values of such materials. Despite the significant decrease in the microhardness values, the 316SS particle addition demonstrates an improvement in the toughness and crack resistance.
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