# Numerical Study of Coolant Flow Phenomena and Heat Transfer at the Cutting-Edge of Twist Drill
## 4 Results and Discussion  
The primary objective of this study is to explore the influence of coolant channel arrangements and inlet pressure differences on the coolant flow pattern at the cutting-edge area, with a concurrent focus on reducing thermal load. Numerous numerical investigations are conducted under various operating conditions to comprehensively address this objective. As depicted earlier, four types of twist drills are employed for this investigation (Figure 1), with the standard drill (without a notch) serving as the reference for performance comparison. Initially, investigations are conducted using a coolant flow rate of 5.5 litres per minute (lt/min) for each drill model. Subsequently, the study is expanded by replacing the constant flow rate with a constant pressure at the inlet. Figure 3 presents the velocity profiles at the flank face and cutting-edge area for all drill models at the constant flow rate of 5.5 lt/min at the inlet. To assess the performance of the coolant channel arrangements with respect to coolant flow behaviour, the resulting streamlines and velocity profiles near the drill's surface are examined. The velocity profiles reveal distinct characteristics among the drill models. The standard model drill exhibits the highest coolant exit velocity of approximately 90 m/s. In contrast, the flow velocity for the standard model drill with a notch is comparatively lower, ranging between 75 and 80 m/s. The profile model drill and the profile-with-notch model drill exhibit significantly lower coolant flow velocities, ranging from 50 to 65 m/s, compared to the standard models. However, the profile model drills showcase a relatively even coolant flow distribution across the borehole area. Despite this, all drill models exhibit a significant drop in coolant flow velocity at the cutting-edge area, reaching very low velocities, almost 0 m/s in some parts, referred to as a dead zone, indicating an 80-100% drop in coolant flow velocity from the exit bore area to the cutting-edge. This suggests insufficient cooling agent supply to these areas. Remarkably, the dead zone area is notably higher in both profile models compared to the standard model, despite having a relatively uniform flow velocity at the flank face and borehole ground, as observed from the CFD results. Under the same initial condition of 5.5 lt/min, the pressure differences between the coolant channel inlet and outlet were analysed and are presented in Figure 4, which illustrates that the pressure difference is lower in the profile-shaped drill models compared \begin{table}
\begin{tabular}{c c c} \hline \multicolumn{2}{c}{**Boundary Conditions**} \\ \hline \multicolumn{2}{c}{**Fluid**} & \multicolumn{2}{c}{**Water**} \\ \hline \multicolumn{2}{c}{Inlet flow rate} & \multicolumn{2}{c}{5.5 lt/min} \\ \hline \multirow{2}{*}{Coolant channel diameter} & Standard & 0.8 mm \\ \cline{2-3}  & Profile & 1.0 mm (approx.) \\ \hline \multicolumn{2}{c}{Rotation} & \multicolumn{2}{c}{2000 rpm} \\ \hline Turbulence model & k-\(\epsilon\) \\ \hline \multicolumn{2}{c}{Run mode} & steady \\ \hline \multicolumn{2}{c}{Solver duration} & 3000 s \\ \hline \end{tabular}
\end{table} 
Table 1: Boundary conditions. \begin{table}
\begin{tabular}{c c} \hline \multicolumn{2}{c}{**Drill Models**} & \multicolumn{2}{c}{**Number of Cells**} \\ \hline Standard model & 962,696 \\ \hline Standard model with notch & 962,277 \\ \hline Profile model & 988,536 \\ \hline Profile model with notch & 969,643 \\ \hline \end{tabular}
\end{table} 
Table 2: Number of mesh elements. to the standard drill models. In other words, under the same operating conditions, the pressure of the coolant upon exiting the channel decreases as the diameter of the coolant channel increases. Differences are also evident when a notch is employed at the exit of the coolant channels, as the pressure decreases for both the standard and profile models. This observation emphasizes the influence of the coolant channel design, with implications for the pressure characteristics within the system.