Researchers have always been interested in identifying new materials with suitable properties and using them to improve performance in various applications. One of these new materials are open cell metal foams. Nowadays, due to the advancement of 3D printing technology and the possibility of making materials with desired structures, metal foams with regular structure have received much attention. Computational fluid dynamics is a method with high accuracy and relatively low cost to study the behavior of these materials under different conditions. In the present research, fluid flow and conduction, convection and radiation heat transfer have been studied for three-dimensional periodic cells with lattice structure for a wide range of porosity (0.7-0.99) and Reynolds numbers (1-150) by using the Fluent software. In this study, a steady, incompressible flow of air with constant thermophysical properties inside an aluminum cell is simulated numerically. First, the hydrodynamic behavior of the flow in the cell without thermal gradient was investigated. It was observed that by decreasing the porosity, parameters like specific surface area, conductive thermal conductivity, pressure drop, inertia coefficients, Darcy and non-Darcy coefficients and friction factor are increased, but the equivalent pore diameter and permeability are reduced. Then in absence of radiation, conduction and convection heat transfer was investigated under two different boundary conditio constant temperature at solid surfaces and constant heat release inside the solid. The results show that by decreasing porosity at the same Reynolds number, temperature in the whole cell increases for the case of constant temperature boundary condition due to the increase of solid surfaces with high temperature, but for the case of constant heat transfer boundary condition, temperature decreases due to the reduction of heat flux on solid surfaces. In both cases, at a constant porosity, the temperature in the cell decreases with increasing Reynolds number, because of increasing the inlet flow velocity and its ability to cool the cell. Moreover, by reducing the porosity at a constant Reynolds number, the velocity in the cell increases due to the reduction of the flow cross section in the cell. By increasing the porosity to more than 0.95, heat transfer coefficient increases but it doesn’t change much for less porosities. But the Nusselt number always increases with the porosity due to the dependence on both heat transfer coefficient and permeability. Further in this research, radiation was added to the problem at hand, and it was observed that radiative-equivalent thermal conductivity increases with increasing porosity, which is greater at higher average cell temperatures. The radiative-equivalent thermal conductivity increases slightly with increasing solid radiative emission coefficient and the applied temperature difference, but increases linearly with increasing the cell size. It was also observed that at low porosities, the conduction heat transfer, and at higher porosities, the radiative heat transfer play major roles in the effective thermal conductivity. At the average cell temperature of 1800K, the ratio of radiative-equivalent thermal conductivity to the effective thermal conductivity, is calculated 95% for the porosity of 0.99, and 12% for the porosity of 0.7. After examining all three modes of heat transfer at the constant heat rate boundary condition, it was observed that with decreasing Reynolds number, temperature of solid surfaces increases so the ratio of radiation heat transfer to the total heat transfer increases exponentially (for the porosity of 0.99, this ratio increases 6 times as the Reynolds number decreases from 150 to 1.5625), and the temperature in the whole cell reduces accordingly. Keywords: Heat transfer, Open cell metal foam, Lattice materials, CFD