## 4 Results and Discussion
### Effect of Reynolds Number on Cutting Tool Heat Transfer Characteristics  
In the current research study, the effect of the velocity of the nanofluid from the duplex jets on the chip-tool interface is investigated during the drilling of Ti-6 Al-4V. As stated before, the jets have impinged concurrently with the heat sources. The tool life deteriorates drastically because of the tremendous heat generation caused by the adhesion between the cutting tool and chips. Nanofluid will act as lubrication and cooling at the chip-tool-workpiece interfaces, maintaining the optimum temperature as the high-speed drilling process will impact tool life and the workpiece surface by giving rise to microcracks tensile residual stresses and thermal damage.  
Figure 8: Variation of Nusselt Number with respect to volume fractions.  
Figure 7: Variation of the convective heat transfer coefficient of the drill bit with different volume fractions of dispersed nanoparticles.  
The conventional coolants failed to significantly decrease heat dissipation in the cutting zone region, raising the environmental concern related to toxic and unsustainable conventional coolants. A hybrid nanofluid with an increased concentration is used to overcome this issue. In this study, the volume fraction of hybrid nanoparticles ranged from 2% to 8%, and the best volume fraction was used to determine the effective velocity. It is intriguing to note that the average temperature decreased with an increase in volume fraction.  
The cutting fluid is varied in the range of 10 m/s to 25 m/s with different combinations of Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), Cu, SWCNT, and MWCNT hybrid nanoparticles. Figure 9 shows the maximum temperature in Kelvin in terms of Reynolds number in different hybrid nanofluids at the same 8% volume concentration of the hybrid nanoparticles with cutting fluid velocities varied in the range of 10 m/s to 25 m/s.  
This numerical study observed that with an increase in Reynolds number, there is a linear temperature drop in the curve in the range between 800 and 1000 Reynolds numbers. Beyond this range, the temperature increases with an increase in Reynolds number because of the increase in kinetic energy. The hybrid nanofluid with Cu-Al\({}_{2}\)O\({}_{3}\) nanoparticles showed the highest drop in cutting temperature reduction, followed byTiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\), SWCNT-Al\({}_{2}\)O\({}_{3}\), and MWCNT-Al\({}_{2}\)O\({}_{3}\). Thus, we can conclude there is a reduction in cutting temperature with an increase in hybrid nanocolant velocity and Reynolds number.  
The Nusselt number is also an essential criterion for investigating the thermal performance of a particular hybrid nanofluid. It is the proportion of convective and conductive heat transfer at a boundary in the fluid. Figure 10 illustrates the Nusselt number for hybrid suspension, and Nu increases when the graph is plotted against Re. It is apparent from the graph that the Nusselt number for any given hybrid nanofluid increases as the Reynolds number gains because of the rise in thermal conductivity. Furthermore, the Nusselt number increases at a higher rate when the Reynolds number is more significant than 950. The Cu-Al\({}_{2}\)O\({}_{3}\) dispersed nanofluids exhibit higher thermal performance than the TiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\), followed by SWCNT-Al\({}_{2}\)O\({}_{3}\) and MWCNT-Al\({}_{2}\)O\({}_{3}\) dispersed nanofluids.  
Figure 9: Variation of maximum drill bit temperature with respect to Reynolds number.