Thermal and vegetable oil-based nanofluids are being explored for heat transfer in solar thermal systems, especially for temperature range 373-573 K. The need for a systematic assessment on the heat transfer-related gain is realised. Therefore, in this paper, a steady heat transfer analysis, for the turbulent flow of Therminol55/diathermic oil and their nanooils, is performed with a 3.5 m long, straight, 2D axissymmetric model of a circular tube, with uniform heat fluxes 100 kW/m² and 300 kW/m², and temperature-dependent thermo-physical properties. A single-phase approach is selected for this simulation, and realizable κ-ε