Numerical analysis of unsteady natural convection from two heated cylinders inside a rhombus enclosure filled with Cu-water nanofluid

In this paper, natural convection due to two heated cylinders inside a rhombus enclosure with lower temperature with respect to the cylinders is investigated. Here, Cu-water is the working nanofluid. The numerical algorithm is based on an iterative direct immersed boundary method (IBM). The focus of this article is on stability, instability, symmetry as well as asymmetry of nanofluid flow pattern in three different Rayleigh (Ra) numbers, i.e. 10(4), 10(5) and 10(6). The effects of important parameters such as cylinder diameters, nanoparticle volume fraction and distance between two cylinders are analyzed. The results of this study disclose that at both Ra = 10(4) and 10(5), nanofluid flow is symmetric and stable, while at Ra = 10(6) four different flow regimes including steady-symmetric, steady-asymmetric, unsteady-asymmetric with periodic oscillations and unsteady-asymmetric with irregular oscillations are observed. In addition to geometrical parameters, nanoparticle volume fraction has a considerable impact on the flow regime. Furthermore, the possibility of unsteady-asymmetric flow increases considerably by increasing the distance between two cylinders.