My main research during my 1st Master’s program was to investigate the feasibility of using nanofluids in heaters of gas pressure reduction stations to enhance heat transfer rate in indirect fired water bath heaters. The investigation was performed numerically using open source CFD software package, ”OpenFOAM”, using high-performance computational cluster of University of Tabriz . In this study Different nanoparticles were studied and the best nanofluid with best thermal performance and stability was introduced. Unsteady and three dimensional turbulent momentum, energy and concentration equations of free convection heat transfer were solved using shear stress transport turbulence model with a new developed heat transfer solver,“MassBuoyantBoussinesqPimpleFoam”, and turbulence model, ”rhokOmegaSST”. In addition, sedimentation of nanoparticles, was examined and the best nanofluid from an economic perspective was recommended. The new solver was developed implementing a two-“fluid” (nanoparticles+base fluid), four-equation (2 mass+1 momentum+1 energy) model by adding the concentration equation to the basic solver BuoyantBoussinesqPimpleFoam, which is a transient solver for simulating incompressible buoyant laminar and turbulent flows. The main feature of the new solver is that all nanofluid properties are variable and are considered as a function of nanoparticle concentration, which is obtained from the nanoparticle transport equation. Moreover, density has been considered inside the relative derivative terms because of its variable nature in this study. Therefore, the governing equations constitute a set of fully coupled equations, which all properties are updated in each iteration. On the other hand for turbulent flow of nanofluid a new turbulent model was developed which is the extended form of kOmegaSST model for incompressible fluids, where density is a function of concentration and is inside the relative derivatives.