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The present work investigates the natural convection of nanofluids in Rayleigh-Benard problem. New models, particularly some enhancement factors, are presented in this research to calculate the heat transfer characteristics of nanofluids around critical Rayleigh numbers, including the criteria in which the creation of Rayleigh-Benard and transition to turbulent flow occurs. The present models are compared with available models in the literature to reveal discrepancies of theoretical models in prediction of nanofluids heat transfer characteristics. This research is conducted for SiO2, Cu,CuO and Al2O3 as nanoparticle and water as the base fluid and discussed the SiO2-water nanofluid in more detail. Results indicate that the Nusselt number and convective heat transfer ratio are reduced for SiO2-water nanofluid for almost all volume fractions and as a result using SiO2 as nanoparticle into water (the base fluid) is not desired for a natural convection in Rayleigh-Benard problem. Moreover, the onset of natural convection is delayed due to the reduction in enhancement factor. [ABSTRACT FROM AUTHOR]

The transformation group theoretic approach is applied to perform an analysis of unsteady free convection flow over a vertical flat plate immersed in a power law fluid. The thermal boundary layer induced within a vertical semi-infinite layer of Boussinseq fluid. The system of governing partial differential equations with boundary conditions reduces to a system of ordinary differential equations with appropriate boundary conditions via two-parameter group theory. The obtained ordinary differential equations are solved numerically for velocity and temperature using the fourth order Runge-Kutta and shooting method. The effect of Prandtl number and viscosity index (n) on the thermal boundary-layer, velocity boundary-layer, local Nusselt number, and local skin-friction were studied. [ABSTRACT FROM AUTHOR]

This paper applies the particle image velocimetry (PIV) to measure the laminar boundary layer thickness of TiO 2 –water nanofluid free convection over the vertical and horizontal flat plates with constant heat flux. The results are presented for two values of heat flux strength equal to 3000 and 7000 w/m 2 . The effects of nanoparticles and heat flux strength on the boundary layer thickness and velocity profiles are investigated in details. Finally, a comparison is performed between the experimental, theoretical and numerical results for different conditions. The obtained results revealed that the velocity boundary layer decreases by adding the nanoparticles to the base fluid. These reductions, at ϕ = 0.1%, are in the vicinity of 7.5% and 5.2% for q ″ = 3000 and 7000 wm - 2 , respectively. Also, the maximum reductions in the vertical velocity component by adding the nanoparticles at ϕ = 0.1% are in the vicinity of 4% and 3.3% for q ″ = 3000 and 7000 wm - 2 , respectively. [ABSTRACT FROM AUTHOR]