OICC Press International Journal of Nano Dimension 2228-5059 12 3 2024 02 03 Thermal performance of natural circulation loop filled with Al2O3/Water nanofluid 10.22034/ijnd.2021.681099 EN Praveena Devi Nagireddy Mechanical Engineering Department, SR University, Warangal, Telangana-506371, India. 10.22034/ijnd.2021.681099 Srinivasa Rao Chalamalasetti Mechanical Engineering Department, Andhra University, Visakhapatnam, Andhra Pradesh, India. Kiran Kumar Kupireddi Mechanical Engineering Department, National Institute of Technology Warangal, Telangana, India. Journal Article 2024 02 03 Natural Circulation Loop (NCL) which is also called as a thermosyphon system is the heat transfer loop which uses no pump or external device to drive the loop fluid. In the present paper, a comparative study on thermal characteristics of two loop fluids viz. water and Al2O3/water nanofluid is made. Experiments are conducted on in-house designed test rig. Thermo-hydraulic behaviour of loop fluid is presented. Two parameters such as heat input, nanofluid concentration are varied in order to study their individual and combined effects. It is concluded that Al2O3/water nanofluid as loop fluid results in higher mass flow rates as compared to the water. Different derived quantities such as Nusselt number and Grashof number are calculated. Quantitative comparison is made between water and Al2O3/water nanofluid. Time to reach steady state is reduced by 22 % using Al2O3/water nanofluid as loop fluid when compared with water. Mass flow rate and Grashof number of the Al2O3/water nanofluid based NCL are enhanced by 6.75% and 26% respectively, when compared with water-based NCL at 1000W heat input. At the heater, the temperature gradient is reduced by 30.2% due to the improved thermal and transport properties of Al2O3/water nanofluid when compared with water at 1000 W heat input. As particle concentration increases from 1% to 5%, Nusselt number increases from 10.1 to 20.1, for the heat input of 1000W.