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Convective heat transfer is a broad field of science that has extensive engineering applications. Natural convection is the motion fluid resulting due to the interplay of an external body force and density difference. This external force is usually gravity. In the present work we have performed modeling and simulations of the experimental assembly of Arshad et al. (2011). The thermal hydraulic characteristics of convective heat transfer in an array of heated cylinders can be studied using computational fluid dynamics (CFD). One set of simulations were performed by including individual rods which we call full structural details (FSD) model. Another set of simulations was performed using the porous media modeling (PMM) approach. The assembly consisted of 3x3 square array of rods completely enclosed in a tank of water (Arshad et al., 2011). CFD was used to model an enclosed assembly of vertically heated rods at different heat fluxes. Grid independence was ensured for both approaches before detailed investigations of hydrodynamic parameters. The averaged out flow and temperature profiles were determined with porous media approach and the FSD simulation will provide the feature of the flow near the cylinder surface not captured by PM model. The computational requirement in PMM reduced dramatically due the porous media approximation. However, the porous media model (PMM) gives the overall picture of flow and temperature but the local information like the surface temperature of rod is lost in this model. In order to get the local information around the heated cylinders, the FSD model is required. In this model the whole geometry is model as it is physical present without any approximation. The simulation results for FSD were validated against experimental data of Arshad et al. (2011).