*5.7. Effect of Cold Fluid Temperature on First and Second Law Characteristics*

The variation in first and second law characteristics of hybrid nanofluid with OSshaped nanoparticles for various volume fractions and cold fluid temperatures is depicted in Figure 16. The cold fluid temperature is varied as 10 ◦C, 20 ◦C and 30 ◦C. The cold fluid at the lower temperature absorbs more heat and presents the higher temperature gradient and heat transfer rate. The pressure drop is not significantly affected by change in the cold fluid temperature. Therefore, the performance index increases as the cold fluid temperature decreases. With the increase in volume fraction, the dominance of the increase in heat transfer is superior to the increase in the pressure drop; therefore, the performance index increases with the increase in the volume fraction. The performance index of the Al2O3/Cu nanofluid with OS-shaped nanoparticles decreases by 12.5% and 24.99% when

the cold fluid temperature increases from 10 ◦C to 20 ◦C and 10 ◦C to 30 ◦C, respectively, for all volume fractions. With an increase in the volume fraction from 0.5% to 2.0%, the performance index increases by 9.43% for all cold fluid temperatures. The thermal entropy generation increases with the increase in volume fraction and decrease in the cold fluid temperature because the heat transfer rate increases with the increase in volume fraction and decrease in the cold fluid temperature. The friction entropy generation rate decreases with the increase in volume fraction despite an increase in pressure drop, because the increase in the average temperature with an increase in the volume fraction is dominant compared to an increase in pressure drop. Similar to the hot fluid temperature, the lower cold fluid temperature presents higher values of the friction entropy generation rate, and vice versa. The higher Bejan number is obtained at the lower cold fluid temperature because the dominance of the thermal entropy generation increase is higher than the friction entropy generation increase with a decrease in the cold fluid temperature. The thermal entropy generation increases and the friction entropy generation decreases with an increase in the volume fraction, which results in an increase in the Bejan number. The Bejan number of the Al2O3/Cu nanofluid with OS-shaped nanoparticles decreases by 1.16%, 1.12% and 1.07% as the cold fluid temperature increases from 10 ◦C to 20 ◦C, and that decreases by 2.66%, 2.59% and 2.45% as the cold fluid temperature increases from 10 ◦C to 30 ◦C for volume fractions of 0.5%, 1.0% and 2.0%, respectively. With the increase in volume fraction from 0.5% to 2.0%, the Bejan number of Al2O3/Cu nanofluid with OS-shaped nanoparticles increases by 0.63%, 0.73% and 0.85% for cold fluid temperatures of 10 ◦C, 20 ◦C and 30 ◦C, respectively. Garud et al. have proved that the performance of the heat exchanger is optimum for the lower mass flow rate of cold fluid [66]. The contributions of thermal and friction entropy generation rates for Al2O3/Cu with OS-shaped nanoparticles at various cold fluid temperatures are depicted in Figure 17. trend becomes steeper with the volume fraction at the higher mass flow rates. On the other side, with the increase in the hot fluid mass flow rate, the increase in the pressure drop is dominating compared to the increase in average temperature; therefore, the friction entropy generation rate is high at a higher mass flow rate and vice versa for all volume fractions. Based on the trends for the thermal and friction entropy generation rates with volume fractions and hot fluid mass flow rate, the Bejan number is evaluated. The Bejan number increases with the increase in volume fraction and decrease in the hot fluid mass flow rate. The thermal entropy generation increases, and the friction entropy generation decreases with the increase in volume fraction, which results in an increase in the Bejan number with an increase in the volume fraction. The thermal and friction entropy generation rates have both increased with the increase in the hot fluid mass flow rate but the increasing rate of the friction entropy generation rate is significantly higher than the thermal entropy generation rate. Therefore, the Bejan number decreases with the increase in the hot fluid mass flow rate for each volume fraction. The Bejan number of the Al2O3/Cu nanofluid with OS-shaped nanoparticles increases by 0.21%, 0.73% and 1.74% for hot fluid mass flow rates of 10 kg/h, 20 kg/h and 30 kg/h, respectively, as the volume fraction increases from 0.5% to 2.0%. The Bejan number of Al2O3/Cu with OS-shaped nanoparticles decreases by 3.06%, 2.89% and 2.57% as the hot fluid mass flow rate increases from 10 kg/h to 20 kg/h, and that decreases by 10.72%, 10.24% and 9.37% as the hot fluid mass flow rate increases from 10 kg/h to 30 kg/h for volume fractions of 0.5%, 1.0% and 2.0%, respectively. Soroush and Chamkha have also shown that the first and second law characteristics of single-particle nanofluids enhances as the volume fraction increases for all nanoparticle shapes [30]. The contributions of thermal and friction entropy generation rates for Al2O3/Cu with OS-shaped nanoparticles at various hot fluid mass flow rates are depicted in Figure 15.

dominant than the increase in pressure drop, hence the friction entropy generation rate decreases as the volume fraction increases for all hot fluid mass flow rates. This decreasing

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**Figure 14.** Effect of hot fluid mass flow rate and volume fraction on first and second law characteristics of hybrid nanofluid with OS-shaped nanoparticles. **Figure 14.** Effect of hot fluid mass flow rate and volume fraction on first and second law characteristics of hybrid nanofluid with OS-shaped nanoparticles.
