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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]

Nanofluids are a type of composite material with a demonstrated potential for improving heat transfer processes present in industries such as computers, electronics, and automobile. However, they have a limitation, which is that the suspended nanoparticles tend to agglomerate and in that way decrease their thermophysical properties. The present work studies experimentally the stability of a nanofluid synthesized with TiO 2 nanoparticles (6 nm) dispersed by continuous ultrasonication in water, determining the effect that exposure of the nanofluid to an atmosphere pressurized with N 2 at 1000 kPa has on its stability. A method is proposed for the quantitative measurement of the stability of a nanofluid based on the transient study of its thermal conductivity and the implementation of a model that describes such behavior. The results allow inferring statistically that the pressure treatment improves the stability of the nanofluid due to a presumed decrease of the average diameter of the agglomerations of the suspended nanoparticles. However, this improvement depends on the temperature. [ABSTRACT FROM AUTHOR]

The present work was originally carried out as a project by HVAC-R engineering students of a heat transfer undergraduate course. The main goal was to introduce to the students to the implementation of a commercial simulation tool, ANSYS/Transient thermal, to analyze a transient heat transfer process of their professional interest. Behind this goal there were a set of activities that lead students to learn very important lessons that deal with computational simulation: physical and mathematical modeling, validation, measurement of thermal properties and grid size analysis. Specifically, a computational simulation of the transient cooling process of two composed building walls initially at 20 °C was performed. Boundary conditions of third kind were imposed at the outer surfaces of the walls. The composed walls are made of light materials: oriented strand board and plasterboard, which are porous building materials and whose thermal properties can be improved by imbibe them with phase change materials (PCM). The convective heat transfer coefficients were taken from the Chilean normative (Nch853-Of91) and some of the thermophysical properties were experimentally obtained. [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]

Numerical simulations of a geothermal reservoir often assume that the primary heat source is a magmatic system; however, heat from the host rock and the coupled complex transport phenomena between magma chamber and host rock also need to be considered. This research numerically simulated the cooling history of an enclosed magma chamber and the thermal effects on the host rock around it from which geothermal energy could be extracted. Modeling of the magma body included natural convection, the effect of latent heat of phase change when the crystals are being formed between the liquidus temperature and the solidus temperature, and heat conduction when the temperature is below the solidus temperature. This study takes as an example a geothermal reservoir in southern Peru (Western Cordillera) whose heat source is a rhyolitic magma chamber like those that gave rise to the intrusive rocks of the Peru coastal cordillera. This analysis varies the chamber shape and uses three solidification temperature ranges for convection and conduction models above the rock formation temperature (solidus) to study the implications for heating of the host rock. This is the first study of its kind in this area. The center of an average-sized magma chambers takes approximately 500 ka to cool from 800 °C to 300 °C. Simulated cooling times between intrusion and solidus temperatures decreased 3 ka when convection was modeled along with conduction cooling. Cooling times decreased by up to 6 ka when the solidification temperature range was increased. The host rock temperature pattern depends strongly on the stage at which the magma chamber is modeled to begin cooling. The temperatures results near the surface of the host rock obtained in this work match well with measurements at hot springs founded in several places in the Western Cordillera. Application of the methodology proposed in this study can reduce uncertainties in planning geothermal energy extraction wells. The accuracy of the numerical model described here could be improved by including more ground data from exploration wells, e.g., soil stratigraphy and temperatures variation with depth. [ABSTRACT FROM AUTHOR]