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Transient and steady‐state characteristics of the natural circulation loop are studied using various water‐based tri‐hybrid (different nature and shaped nanoparticles) nanofluids. Effects of input power, loop inclination and loop aspect ratio on the mass flow rate, effectiveness, and entropy generation rate are studied. Results disclose that tri‐hybrid nanofluids reduce the oscillation and attain steady state faster than water. Effectiveness increases and entropy generation rate decreases by using tri‐hybrid nanofluids. Nanoparticle shape has found a significant impact. Al2O3 + Cu + CNT/water shows the best performance. The mass flow rate increases with input power, loop aspect ratio, whereas, reduces with loop inclination. Effectiveness decreases and then increases with input power, whereas increases with loop inclination and decreases with loop aspect ratio. Entropy generation upsurges with input power and loop inclination, whereas decreases with loop aspect ratio. Loop aspect ratio of 1.5 to 3 is found suitable for better performance. [ABSTRACT FROM AUTHOR]

The turbulent flow of binary hybrid nano-oils is investigated and Nusselt number correlation is developed for futuristic concentrated solar thermal application. Available Nusselt number correlations for water-based hybrid nanofluids are nanoparticle-specific property correlations dependent and substantially over-predict the values for hybrid nano-oils. Therefore, a generalized Nusselt number correlation for turbulent flow of water and oil-based binary hybrid nanofluids is deduced using the separation approach. Dissimilar to the available correlations, the developed correlation needs only the thermophysical properties of base fluid and nanoparticles. It is valid for the Reynolds number range 10,000-30,000 and the Prandtl number range 0.5-2000. It is found that the proposed correlation predicts the published experimental values for different hybrid nanofluids mostly within ±10-20%. Computational fluid dynamics simulation is also performed for turbulent flow of different hybrid nano-oils to assess the developed correlation. The comparative assessment also confirms that the developed correlation predicts the numerical values for hybrid nano-oils within ±10-20%. The deduced Nusselt number correlation will be useful for a realistic heat transfer analysis with different water or oil-based hybrid nanofluids. The need for experiments with different hybrid nano-oils is realized for further improvement. [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]

It is necessary to investigate the relative effect of various phenomena before neglecting the same for simulating the single-phase natural circulation loop to obtain an acceptable prediction. Hence, the present study focuses on the validation and effect of different assumptions related to Boussinesq approximation, property variation, bend effect, heat loss, axial fluid and wall conduction on the transient and steady-state characteristics. Effect of assumptions for different working fluids (water, brines and hybrid nanofluids), and assess the effect of different heat flux distributions, like uniform, linear, non-linear, sinusoidal and Gaussian, applied to heater are also explored. Mass flow rate (predicts heat transport capability), effectiveness (predicts heat transfer ability) and entropy generation rate (predicts irreversibility) are evaluated. The results disclose that the Boussinesq approximation with constant other properties under-predicts the steady-state mass flow rate and effectiveness and over-predicts the entropy generation rate. This assumption cannot be applied for all working fluids. The fluid and solid conduction is the crucial parameter for simulating transient characteristics. Gaussian heat flux distribution shows the highest mass flow rate and non-linear increasing heat flux shows the highest effectiveness and entropy generation rate. This study will help to consider reasonable assumptions for the numerical simulation. [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]