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Numerical prediction of heat transfer by natural convection of a Herschel-Bulkley non-Newtonian fluid inside a square cavity has been computationally analyzed. Unsteady 2D fluid mechanics and heat transfer were described in terms of the non-linear coupled continuity, momentum and heat equations. These equations were solved by the control volume finite element method (CVFEM) with Gauss-Seidel/System Over-Relaxation coupling algorithm. The effect of the Ra, Pr, Bn and the rheological behavior index (n) on the non-Newtonian fluid thermal and momentum behavior were studied. The non-Newtonian fluid flow was described by the rheological model of Herschel-Bulkley. Results for the streamlines and isotherms along the enclosure walls are presented. It was found that the effect of the Pr and Bn is more important when the Ra is lower (103). In addition, the behavior index had a significant effect on the CPU time for the different studied cases. [ABSTRACT FROM AUTHOR]

The main results of an experimental study of the effect of temperature and nanoparticle concentration on thermal conductivity and viscosity of a nanofluid are shown. The nanofluid was prepared with Octadecane, an alkane hydrocarbon with the chemical formula CH 3 (CH 2 ) 16 CH 3 , as a base fluid and 75-nm CuO spherical nanoparticles. Since the base fluid is a phase change material (PCM) to be used in thermal storage applications, the engineered nanofluid is referred to as Nano-PCM. Three Nano-PCMs were prepared by the two-step method (2.5% w/v, 5.0% w/v, and 10.0% w/v). In order to increase the stability of the Nano-PCM, the surface of the CuO nanoparticles were modified with Sodium oleate, and it was verified by IR analysis. The modified CuO nanoparticles were dispersed with an ultrasonic horn. The thermal conductivity was measured with a thermal properties analyzer in the temperature range of 30–40 °C. The viscosity was measured in the temperature range of 30–55 °C. The results for the Nano-PCM showed that thermal conductivity is almost constant in the analyzed temperature range, and the viscosity decreases non-linearly with temperature. With respect to the effect of nanoparticle concentration, both thermal conductivity and viscosity increased with nanoparticle concentration. Thermal conductivity increased up to 9% with respect to the base fluid, and viscosity increased up to 60%, in both cases with increasing concentration. Finally, the viscosity measurements for different deformation rates (30–80 RPM) showed that the addition of nanoparticles modifies the rheological behavior of the base fluid, from a Newtonian to a shear thinning (power-law) non-Newtonian behavior. [ABSTRACT FROM AUTHOR]

This paper presents the results of a numerical study on the unsteady three-dimensional natural convection of a power-law fluid in a thick walled vessel being driven by natural heat convection in the surrounding air inside a cubical cavity. Continuity, linear momentum and energy equations for the internal Otswald the Waele shear thinning fluid, with a power index n = 0.4, are used to describe fluid mechanics and natural heat transfer along the conjugate continuity, Navier–Stokes and energy equations for the external Newtonian air flow and the transient heat diffusion equation in the walls of the inner fluid container. Unsteady 3D thermal convection of the inner shear-thinning fluid, with n = 0.4, is analyzed in comparison with the fluid mechanics and heat transfer results for the Newtonian fluid, with n = 1, for two values of the Rayleigh number: Ra = 2.09 × 10 4 and 2.09 × 10 6 . Thermo-physical properties of the inner fluid are allowed to change with temperature for the two values of the Rayleigh number and for both the Newtonian and the power law non-Newtonian inner fluids. Numerical simulations were carried out by an -in house- Fortran 90 Finite Volume Method parallelized code. Results of the evolution of the fluid mechanics are described in terms of the streamlines and those related to the unsteady convective heat transfer in terms of the isotherms. [ABSTRACT FROM AUTHOR]

Abstract: Transient phase change of a power law non-Newtonian fluid inside an inner thin walled container caused by external mixed convection in a square cavity has been analyzed numerically. Air was chosen as external cooling fluid and modified non-Newtonian water as the phase change fluid. Fluid mechanics and conjugate convective heat transfer, described in terms of continuity, linear momentum and energy equations, were predicted by using the finite volume method. Solidification was treated in terms of a phase change function varying linearly with temperature. The effect of the external Reynolds number, for Re=200 and 1000 on solidification was studied along the influence of the non-Newtonian power law index (n =0.5, n =1.0). Results for the time evolution of streamlines, isotherms and freezing curves are analyzed. The effect of the Reynolds number on streamlines of the external fluid is remarkable, principally near the region close to the internal water filled container. Differences between cooling and freezing times are found for Newtonian (n =1.0) and non-Newtonian modified (n =0.5) water. [Copyright &y& Elsevier]