EXPERIMENTAL STUDY ON CAVITY FLOW NATURAL CONVECTION IN A POROUS MEDIUM, SATURATED WITH AN Al2O3 60%EG-40%WATER NANOFLUID

Natural convection is convection where the fluid motion is driven by buoyancy forces. Porous media and nanofluids have an impact on the heat transfer capabilities of thermal systems. The present experimental study is part of ongoing research and lies at the intersection of buoyancy driven flow in a cavity, porous mediums and nanofluids. The nanofluid consists of Al2O3 nanoparticles in the base fluid of 60% ethylene glycol (EG) and 40% water. A Rayleigh number range of 6 × 10 < Ra∗ < 1.6 × 10, for a volume fraction of 0.2% nanoparticles. The porous medium used is glass spheres of 16mm. In this research the effective viscosity of the nanofluid was determined experimentally while the effective thermal conductivity was available in the literature. The results showed that heat transfer is affected by both the porous medium and the nanofluid. The results show that the heat transfer in the case of porous media with nanofluid is more than the case of pure base fluid. However, more experimentation for a wider range of Rayleigh numbers will be a part of future works of this ongoing research. INTRODUCTION Convective heat transfer in fluid-saturated porous medium has received much attention in the last few decades and is relevant to a wide range of applications such as underground heat pump systems, solar engineering, packed sphere beds, chemical catalytic converters, heat exchangers and geothermal applications. Extensive reviews of the subject are given in various textbooks for instance, Nield and Bejan [1], Vafai [2], Pop and Ingham [3], Ingham [4] and Bejan et al. [5]. Natural convection inside a porous filled cavity has attracted the attention of many researchers and is very applicable to solar receiver technology. Although more numerical works have been published, a few examples of experimental papers could be found. Prasad et al. [6, 7] presented experimental results for natural convective heat transfer in an annulus. Bories and Combarnous [8] presented numerical and experimental results in a sloping porous layer. The porous medium used was spherical glass beads. Seki et al. [9] presented an experimental study with a rectangular cavity. The fluids included water, transformer oil and ethyl alcohol. A recent and emerging field, first coined by Choi [10], is the field of nanofluids. The field is based on the premise that suspending solid nanometer sized particles, with higher thermal conductivity, in a conventional heat transfer fluid, would augment the heat transfer capabilities of that fluid. Because nanoparticles could potentially be added to any liquid heat exchangers to augment heat transfer, the potential applications are endless. However, the addition of the nanoparticles to a NOMENCLATURE Da [-] Darcy Number g [m/s2] Gravity Constant H [m] Cavity Height h [W/(m2 ·K)] Convective Heat Transfer Coefficient k [W/(kg ·K)] Conduction Heat Transfer Coefficient K [m2] Permeability L [m] Cavity Length Nu [-] Nusselt Number Pr [-] Prandtl Number Ra [-] Rayleigh Number T [K or ·C] Temperature Special Characters β [1/K] Volumetric Expansion Coefficient μ [Pa · s] Dynamic Viscosity ν [m2/s] Kinematic Viscosity ρ [kg.m2] Density φ [-] Nanofluid volume fraction or Porosity depending on the subscript Subscripts bf Base Fluid f Fluid H Height L Length nf Nanofluid p Nanoparticle pm Porous Medium s Solid Matrix fluid effects more than its thermal conductivity and different models to predict fluid properties and behaviors have been proposed. Buongiorno [11], Tiwari and Das [12] and Aybar et al. [13]. Free convection of nanofluids in clear cavities with differentially heated walls has been studied both numerically and experimentally. Hwang et al. [14] performed a numerical investigation on a rectangular cavity, following the experimental results presented by Putra et al. [15]. A previous 2D numerical work on Al2O3 is also presented by Khanafer et al. [16] Putra et al. [15] performed an experimental investigation of free convection of nanofluids in a horizontal cylinder that is heated from the one side and cooled from the other side. They reported paradoxical results; when the volume fraction was increased, the heat transfer rate was decreased. The work of Putra was extended by an experimental study performed by Nnanna et al. [17] of Al2O3 nanoparticles in water. Their focus was to estimate the range of volume fractions for which heat transfer is enhanced. They also wanted to determine the impact of volume fraction on the Nusselt number. They found that for small volume fractions 0.2% < φnf < 2% heat transfer is augmented, but for higher volume fractions, the heat transfer coefficient is reduced due to the reduction of 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics