# Numerical Study of Coolant Flow Phenomena and Heat Transfer at the Cutting-Edge of Twist Drill
## 4 Results and Discussion
It is also observed that the heat dissipation at the cutting-edge area deteriorates as the diameter of the coolant channel increases, particularly in the case of the profile models. Furthermore, introducing the notch at the exit of the channels did not yield any improvements in heat dissipation. Additional investigations on these three modified drill models are carried out whereby the inlet is adjusted with a constant pressure instead of a constant fluid flow rate, with pressures of 40 and 60 bar applied at the inlet while conducting the numerical simulations. This extended investigation aims to gain further insights into the performance of the modified drill models under varied operating conditions. Figures 6 and 7 illustrate the differences in coolant flow velocity at the flank face and cutting-edge area for the three modified drill models under constant pressures of 40 bar and 60 bar at the inlet, respectively. From the velocity contour plots of the resulting streamlines and velocity near the surface, it is evident that the fluid flow rate has increased significantly for all models compared to that of the constant inlet flow rate (5.5 lt/min) (Figure 3). Especially at 60-bar inlet pressure, a very high coolant velocity at the cutting-edge area is observed for the standard-with-notch model drill, in particular (Figure 7). However, some parts of the cutting-edge, and most importantly the outer corner, remain blue in colour, indicative of a dead zone with 0 m/s coolant velocity, for all models, even when the inlet pressure is 60 bar (Figure 7). While using the profile models allows for higher coolant flow velocities at the flank face and borehole ground, these modified models are not entirely effective in eliminating dead zones from the cutting-edge area. Among all three modified models, the standard-with-notch model performs the best concerning the accessibility of the coolant to the cutting-edge area. Using the standard-with-notch model, particularly at high pressure, dead zones at the cutting-edge can be reduced significantly. It is extremely challenging to eliminate such dead zones by increasing the coolant flow rate, either by adjusting pressure, fluid flow rate, or changing the coolant channel diameter. This limitation definitely impacts the heat transfer at the outer edge. Figure 6: Velocity profile for standard-with-notch drill model and profile model drills (with and without notch) for constant pressure of 40 bar at inlet. Note: the colour legend is in m/s. Figure 7: Velocity profile for standard-with-notch drill model and profile model drills (with and without notch) for constant pressure 60 bar at inlet. Note: the colour legend is in m/s. In comparison to pressure inlet and constant flow rate inlet conditions, the fluid pressure varies for each drill model (Figure 4) when the inlet condition is a constant flow rate, impacting the coolant flow at the cutting-edge area. With an increase in the diameter of the coolant channel (profile models) or the extension of the exit area due to the implementation of the notch, fluid flow is provided with an additional passage. Consequently, fluid flow does not acquire as much pressure as needed to reach the complex cutting-edge region under constant flow rate conditions. However, applying a higher pressure at the inlet changes the dynamics of the fluid flow at the bore hole and cutting-edge area. The temperature distributions of the fluid along the cutting-edge and outer corner of all modified models under both 40 and 60 bar pressure conditions are shown in Figure 8. The primary objective of coolant flow during drilling operations is to mitigate thermal load. From Figure 8, it is evident that the entire cutting-edge is subjected to a significantly high temperature, with the very edge of the outer corner experiencing the highest temperature. Nevertheless, heat dissipation shows a notable improvement when utilizing higher pressure at the inlet. Among the three given models, the standard-with-notch model demonstrated the most effective reduction in thermal load.