The Natural Circulation Loop (NCL) has many applications in nuclear and renewable energy systems including solar heaters and geothermal power derivation. Applying nanofluids in NCL has distinctive advantageous, including higher stability and effectiveness of NCL. The present study is a numerical investigation of a rectangular single-phase natural circulation loop utilizing Water, water-based Al2O3, and TiO2 nanofluids as working fluids. Both steady-state and transient simulations are in good agreement with experimental data. The effects of heating power and nanoparticle concentration on the stability and flow transitions were investigated in this study. Flow rate and heat transfer capacity enhancement were significantly observed by applying nanofluids in NCL compared to Water. The comparison of effectiveness factor, temperature difference over heater, and mass flow rate of NCL utilize different working fluids, revealed thermal performance improvement by particle concentration increment, and this is more considerable for the nanofluids containing bigger particle size.
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