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Thermal/vegetable oil-based mono/hybrid nanofluids (Nano-oils) may be potential working fluids in single-phase natural circulation loops for medium temperature applications due to their favorable thermophysical properties (high thermal expansion coefficient and low viscosity) and high Prandtl number; however, not studied yet. Hence, the present study investigates the different nano-oils in a natural circulation loop. Therminol-VP1 and soybean-based mono/hybrid nano-oils with different nanoparticles (Al 2 O 3 , CuO, SiC, and MWCNT) are used at a volume fraction of 0.1%. The influence of power input and loop inclination on the energy-exergy performance parameters (Effectiveness, Entropy generation rate and Induced mass flow rate) are presented. The use of nano-oil shows enhanced heat exchanger effectiveness and lower entropy generation rate, whereas the induced mass flow rate may increase or decrease depending on nanoparticle shape and power input. Al 2 O 3 +MWCNT nano-oil exhibits superior performance, with a maximum mass flow rate reduction of 8%, effectiveness enhancement of 36%, and entropy generation reduction of 20% as compared to base oil. Therminol-VP1 shows quick flow establishment than soyabean oil. The usage of nano-oil is more beneficial at the higher power input. Loop inclination reduces the mass flow rate while increasing the effectiveness and entropy generation; however, mass flow reduction is much lower for counter-clockwise inclination. • Hybrid nano-oils having dissimilar shaped particles are used in natural circulation loop. • Mass flow rate, heat exchanger effectiveness and entropy generation rate are evaluated. • Effect of power input and loop inclination are investigated for a medium temperature range. • Therminol-VP1 shows quick flow establishment and better performance than soyabean oil. • Counter-clockwise inclination shows higher mass flow rate than the clockwise inclination. [ABSTRACT FROM AUTHOR]

Nanoparticle shape strongly affects the thermal-hydraulic behavior of nanofluid-enhanced single-phase natural circulation loop (SPNCL) but is yet to be explored experimentally. Hence, various mono/hybrid nanofluids having dissimilar shaped nanoparticles (Al 2 O 3 +Water, Al 2 O 3 +CuO + Water, Al 2 O 3 +SiC + Water, Al 2 O 3 +MWCNT + Water) with a 0.1% volume concentration, are used in SPNCL to investigate mass flow rate (MFR), heat exchanger effectiveness (HEE) and total entropy generation rate (TEGR). The effect of power input, coolant inlet temperature and loop inclination (both counter-clockwise and clockwise) on transient and steady-state performances are presented. The result reveals that mono/hybrid nanofluid shows enhanced HEE and lower TEGR as compared to base fluids, whereas the MFR may increase or decrease since the performance of SPNCL is significantly impacted by the nanoparticle's shape. Spherical-shaped nanoparticle-dispersed nanofluids show early flow initiation and higher MFR than cylindrical-shaped nanoparticle-dispersed nanofluids. MFR and TEGR rise with higher power input, whereas HEE declines for all working fluids. MFR and TEGR increase as the coolant inlet temperature rises, whereas HEE decreases for all working fluids. The loop inclination decreases the MFR, while it increases the HEE and TEGR. The Counter-clockwise inclination of the loop shows a higher MFR than the clockwise inclination at the same angle of inclination. • Hybrid nanofluids having dissimilar shaped particles are used in natural circulation loop. • Mass flow rate, heat exchanger effectiveness and entropy generation rate are evaluated. • Effect of power input, coolant inlet temperature and loop inclination are investigated. • Spherical-shaped shows early flow initiation and higher flow rate than cylindrical-shaped. • Counter-clockwise inclination shows higher mass flow rate than the clockwise inclination. [ABSTRACT FROM AUTHOR]

Transient analysis of vertical heating and horizontal cooling rectangular single-phase natural circulation loop with various water-based hybrid nanofluids (Al 2 O 3 + Ag, Al 2 O 3 + Cu, Al 2 O 3 + TiO 2 , Al 2 O 3 + CNT, Al 2 O 3 + Graphene) with 1% volumetric concentration is studied numerically. Temporal fluctuation and time required to attain the steady-state, transient mass flow rate and energy-exergy performance parameters (effectiveness and total entropy generation) using hybrid nanofluids are compared with water. The effect of power input and geometry parameter (diameter and height) of the loop on transient performances is studied as well. The result reveals that the fluctuation of mass flow rate and time required to attain the steady-state are less for hybrid nanofluids as compared to water. However, platelet and cylindrical shaped nanoparticles yield a lower stability as compared to spherical shaped. The mass flow rate is enhanced with hybrid nanofluids, except Al 2 O 3 + CNT and Al 2 O 3 + Graphene, as compared to water. The energy-exergy performance of hybrid nanofluids is higher than water. The maximum increment in mass flow rate is shown by Al 2 O 3 + Ag hybrid nanofluid (3%), whereas Al 2 O 3 + Graphene shows the highest increment in effectiveness (25.4%) and highest decrement in total entropy generation (14.3%) as compared to water. Smaller diameter and lower height of loop are found preferable for the flow stability. [ABSTRACT FROM AUTHOR]