Experimental study of nanoparticles distribution in natural convection of Al2O3-water nanofluid in a square cavity

The objective of the present study is to investigate experimentally the non-homogenous distribution of nanoparticles in the nanofluid by natural convection heat transfer inside a square enclosure. There are some phenomenon such as fluid motion, Brownian effect, thermophoresis effect and gravity which cause nanoparticles movements throughout the nanofluid. An experimental set-up was built to carefully take micro-litter samples of nanofluid from a 8 x 8 x 18 (cm(3)) square cavity. The nanofluid was prepared using 20 nm-gamma type Al2O3 nanoparticles dispersed in deionized distilled water. The experiments were done for three Rayleigh numbers 0.992 x 10(7), 0.51 x 10(8) and 1.53 x 10(8) and the particle loading of 1% was constant during the experiments. The contours of nanoparticle fraction shows that the assumption of non-homogeneity is correct and the regions of high and low nanoparticle volume concentration are distinguishable. The fluid motion is the most important factor that may come in to play especially at higher Rayleigh number and causes the maximum differences between measured nano particle concentration. The result showed that the differences between maximum and minimum volume fraction in the whole cavity increases from 18% at Ra = 0.992 x 10(7) to 28.74% at Ra = 1.51 x 10(8). It was observed that the average nanoparticle volume fraction along cold wall is 3.10% greater than that along hot wall for minimum. Ra number however at higher Ra numbers this difference would weaken. To check precision of replicates and ensure accuracy, the data were analyzed using analysis of variance method (ANOVA method). The present work reveals that there are still some issues for flow and thermal mechanisms in nano fluids, particularly regarding the nanoparticles distribution for future works. (C) 2016 Elsevier Masson SAS. All rights reserved.