Free convection enhancement in an annulus between horizontal confocal elliptical cylinders using hybrid nanofluids

In the present paper, natural convection in an annulus between two confocal elliptic cylinders filled with a Cu-Al2O3/water hybrid nanofluid is investigated numerically. The inner cylinder is heated at a constant surface temperature while the outer wall is isothermally cooled. The basic equations are formulated in elliptic coordinates and developed in terms of the vorticity-stream function formulation using the dimensionless form for 2D, laminar and incompressible flow under steady-state condition. The governing equations are discretized using the finite volume method and solved by an in-house FORTRAN code. Numerical simulations are performed for various volume fractions of nanoparticles (0 <= phi <= 0.12) and Rayleigh numbers (10(3) <= Ra <= 3 x 10(5)). The eccentricity of the inner and outer ellipses and the angle of orientation are fixed at e(1) = 0.9, e(2) = 0.6 and gamma = 0 degrees respectively. It is found that employing a Cu-Al2O3/water hybrid nanofluid is more efficient in heat transfer rate compared to the similar Al2O3/water nanofluid.