Elisa Ricci

Thermophoresis-induced oscillatory natural convection flows of water-based nanofluids in tilted cavities

ABSTRACT A two-phase model based on the double-diffusive approach is used to perform a numerical study on the natural convection of water-based nanofluids in differentially heated square cavities, inclined with respect to gravity so that the heated wall is positioned below the cooled wall, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum, and energy for the nanofluid, and continuity for the nanoparticles, is solved through a computational code that incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on several sets of literature experimental data. Pressure–velocity coupling is handled by the way of the SIMPLE-C algorithm. Numerical simulations are executed for tilting angles in the range 50−70 deg, such that the nonuniform distribution of the suspended solid phase gives rise to a nonnegligible solutal buoyancy force, whose effects are investigated using the nanoparticle diameter and average volume fraction, the cavity width, the nanofluid average temperature, and the temperature difference imposed across the cavity, as independent variables. It is found that the competition between the solutal and thermal buoyancy forces results in an oscillatory flow, with an oscillation amplitude that increases on increasing the cavity size and the imposed temperature difference. Moreover, the impact of the nanoparticle dispersion into the base liquid is found to be higher at higher average temperatures, whereas, by contrast, the other variables have moderate or negligible effects.

Optimal Inclination for Maximum Convection Heat Transfer in Differentially-Heated Enclosures Filled with Water Near 4°C

ABSTRACT Natural convection in water-filled square cavities inclined with respect to gravity, having one wall cooled at 0°C and the opposite wall heated at a temperature ranging between 4°C and 30°C, is studied numerically for cavity widths spanning from 0.02 m to 0.1 m in the hypothesis of temperature-dependent physical properties, with the main aim to determine the optimal tilting angle for maximum heat transfer. A computational code based on the SIMPLE-C algorithm is used to solve the system of the mass, momentum and energy transfer governing equations. Once the vertical configuration, in which the cavity is differentially heated at sides, is identified by the zero tilting angle, and positive angles denote configurations with the heated wall facing upwards, it is found that the optimal tilting angle is positive if the heating temperature is equal or higher than 8°C, whereas it is negative whenever the heating temperature is lower than 8°C. Moreover, the optimal tilting angle is found to increase as the cavity width is decreased and the temperature of the heated wall is either decreased or increased, according as it is higher or lower than 8°C. Sets of dimensionless correlating equations are developed for the prediction of both the optimal tilting angle and the heat transfer rate across the enclosure.

Buoyancy-Induced Convection of Alumina-Water Nanofluids in Laterally Heated Vertical Slender Cavities

ABSTRACT A two-phase model based on the double-diffusive approach is used to perform a numerical study of natural convection of alumina-water nanofluids in differentially heated vertical slender cavities. In the mathematical formulation, Brownian diffusion and thermophoresis are assumed to be the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles is solved through a computational code relying on the SIMPLE-C algorithm for the pressure-velocity coupling. The effective thermal conductivity and dynamic viscosity of the nanofluid, and the coefficient of thermophoretic diffusion of the suspended solid phase, are evaluated using three empirical correlations based on a high number of experimental data available from diverse sources, and validated by way of literature data different from those used in generating them. Numerical simulations are executed for different height-to-width aspect ratios of the enclosure, as well as different average temperatures of the nanofluid. The heat transfer performance of the nanoparticle suspension relative to that of the base fluid is found to increase as the nanofluid average temperature is increased and, at low to moderate temperatures, the aspect ratio of the enclosure is decreased. Moreover, at temperatures higher than room temperature, a peak at an optimal particle loading is found to exist for any investigated configuration.

Use of nanofluids as coolants in buoyancy-driven thermal management of embedded heating components of small-scale devices

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in square cavities partially heated at the bottom wall and cooled at both sides, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. Numerical simulations are basically executed for Al2O3 + H2O, using the diameter and the average volume fraction of the suspended nanoparticles, the cavity width, the heated fraction of the bottom wall, the average temperature and the temperature difference imposed across the cavity, as independent variables. Additional simulations are also performed using CuO or TiO2 nanoparticles. It is found that the cooperation between the solutal and thermal buoyancy forces results in a significant enhancement of the heat transfer performance of the nanofluid compared with the pure base liquid.

A Demonstrative Study on the Two-phase vs. Single-phase Modeling of Buoyancy-driven Flows of Enclosed Nanofluids

ABSTRACT A demonstrative numerical study on natural convection of water-based nanofluids in square enclosures with different boundary conditions imposed at the walls, and different orientations with respect to the gravity vector, is performed using both the single-phase and the two-phase approaches, with the main scope to evaluate in what measure the single-phase approach fails in describing the basic heat and fluid flow features, as well as in determining the thermal performance of nanofluids. The system of the mass, momentum and energy transfer governing equations is solved by way of a computational code based on the SIMPLE-C algorithm. Empirical correlations are used for the calculation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient. The following configurations are investigated: a tilted cavity differentially-heated at two opposite walls; a vertical cavity partially-heated at the bottom wall and cooled at both sides; and a vertical cavity differentially-heated at the vertical and horizontal walls. It is found that the non-uniform distribution of the suspended solid phase throughout the enclosure gives rise to a solutal buoyancy force, whose competition with the thermal buoyancy force results in a periodic flow detectable only if the two-phase approach is applied. Moreover, the impact of the dispersion of the nanoparticles into the base liquid, which turns out to be notably higher at higher average temperatures, is found to be systematically underestimated by the single-phase approach.