The optimization of heat transfer in turbulent upward pipe flows is a very common problem in engineering, due to the wide range of applications. However, this set-up is usually categorised as mixed convection flow, which means that gravity plays an important role. Sometimes, it might lead to heat transfer deterioration. This research will simulated three different physical flow conditions gained by Shehata et Al. [3]: turbulent, sub-turbulent and laminarizing. The model will be constructed with Open FOAM using two different approaches: RANS and LES. The former is applied to a two-dimensional geometry, whereas the latter is employed for a 3D cylinder mesh. For both of them air physical properties are implemented as function of temperature and the Boussinesq approximation is selected to forecast density variations. Results is an assessment of RANS and LES differences and which one is more accurate, comparing normalized temperatures at axial locations to real experimental data. In the end, the role of gravity is studied, showing laminarization and using the sub-turbulent case to demonstrate how the presence or the absence of buoyancy forces influences heat transfer.