## 5 Conclusions  
In this research, a numerical investigation is performed using computational fluid dynamics (CFD) to compare the temperature distribution on the surface of the drill bit and workpiece during drilling operation using different hybrid nanocoolants with duplex jets. The computational domain consists of a heated workpiece and tool, and hybrid nanocoolants are sprayed using a dual nozzle located above the machining zone. The hybrid nanoparticles of Cu-Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\), SWCNT-Al\({}_{2}\)O\({}_{3}\), and MWCNT-Al\({}_{2}\)O\({}_{3}\) are dispersed in mineral oil for various volume fractions. The heat transfer performances of different hybrid nanocoolants are compared by varying the hybrid nanoparticle volume fraction and coolant velocity. The parameters such as the maximum tool temperature, Nusselt number, and convective heat transfer coefficient are investigated for different volume fractions and Reynolds numbers. The results indicated that a hybrid nanofluid with higher thermal conductivity and lower specific heat capacity would exhibit better thermal performance. The conclusions from this investigation are summarized below:  
The duplex jets of hybrid nanocoolant are competent in producing adequate cooling. The applied liquid jets' most predominant heat dissipation methodology was forced convection. The convective heat transfer has the potential to be improved by the existing coolant employment system. The maximum cutting tool temperature reduces linearly with an increased volume fraction of dispersed nanoparticles. This is attributed to the fact that a higher volume fraction results in a greater thermal conductivity which leads to enhanced heat transfer. Metal-metal-oxide hybrid nanoparticles are more efficient in heat dissipation of the drilling system compared with metal-oxide-metal-oxide and CNT-metal oxide. The convective heat transfer coefficient improves significantly with an addition in the volume fraction of nanoparticles. At the same time, SWCNT-Al\({}_{2}\)O\({}_{3}\) and MWCNT-Al\({}_{2}\)O\({}_{3}\) nanocoolants have the highest heat transfer coefficient amongst the other nanocoolants. The SWCNT-Al\({}_{2}\)O\({}_{3}\) nanocoolant shows the highest temperature drop per unit length with an increase in Reynolds number. The convective heat transfer coefficient rate proliferates with an increase in Reynolds Number. Moreover, the Nusselt number rises with an increase in Reynolds numbers. This can be attributed to the conventional fact that increasing the Reynolds number enhances the rate of forced convection. The results of the numerical studies show that the overall temperature of the components, i.e., workpiece and drill bit, decreases with an addition in nanoparticle volume fraction. This is because of the enhanced thermal conductivity of the mixture. It was observed that Cu-Al\({}_{2}\)O\({}_{3}\)-doped nanocoolants showed the lowest temperature among all other variants. Moreover, the  
Figure 10: Variation of Nusselt number with respect to Reynolds number. system reaches a steady state after approximately 50 s of simulation time. The temperature distribution was found to be in accordance with previously published literature. Furthermore, an increase in the nanoparticle volume fraction aids the enhancement of the convective heat transfer coefficient. In contrast, it was noticed that the Nusselt number declines with an increase in nanoparticle volume fraction. Cu-Al\({}_{2}\)O\({}_{3}\)-based nanocoolants showed the highest values of the Nusselt number. Hence it can be concluded that Cu-Al\({}_{2}\)O\({}_{3}\) nanocoolants showed enhanced heat transfer because of the convection compared with TiO\({}_{2}\)-, SWCNT-, and MWCNT-doped nanoparticles with Al\({}_{2}\)O\({}_{3}\) and mineral oil. Conceptualization, R.M., S.S., V.J. and R.H.; Data curation, R.M. and V.J. ; Formal analysis, R.M., S.S., V.J. and R.H.; Investigation, S.S. and R.H.; Methodology, R.H.; Supervision, R.H. All authors have read and agreed to the published version of the manuscript.