## 4 Results and Discussion  
A modular drill bit was designed for drilling boreholes with a diameter of 216 mm, and flow field characteristics were analyzed with an inlet flow rate of 300 L/min and outlet pressure at ambient pressure based on the COMSOL software. In this simulation, the diameter of the internal coring bit was 87 mm, and 5 nozzles were opened on the internal non-coring bit. Figure 3 is a velocity nephogram of the modular drill bit; according to Figure 3a, the drilling fluid in the modular bit for coring drilling flows into the clearance between the external bit and internal bit, and then most of it flows to the borehole bottom through the isolating casing, while a small part of it flows to the bottom of the borehole through the chamber between the isolating casing and internal bit and the passageways between the torque ring and internal bit. After the above two streams of drilling fluid have converged at the borehole bottom, it flows upward through the annulus between the external bit and borehole wall. At the outlet of the isolating casing radial holes, a maximum velocity of 5.07 m/s is generated after drilling fluid collides with the inside wall of the external bit. Then, the velocity decreases when it flows from the chamber between the external bit and the torque ring to the bottom nozzles of the external bit. In the bottom nozzles of the external bit, the velocity increases again and reaches to 4.58 m/s. At the outlet of the nozzles, a vortex is formed, and rocks at the borehole bottom are eroded by the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameters** & \(\rho\) & \(\mu\) & \(\sigma_{k}\) & \(\sigma_{\varepsilon}\) & \(C_{\varepsilon 2}\) \\ \hline Values & 1.0 \(\times\) 10\({}^{3}\) & 1.01 \(\times\) 10\({}^{-3}\) & 1.0 & 1.2 & 1.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main parameters of the modular bit model. drilling fluid. Finally, the drilling fluid flows upward at an average velocity of 1.07 m/s. Furthermore, it is observed that the flow velocity near the core is relatively low, signifying effective preservation of the core. For the non-coring bit, shown in Figure 3b, similar laws apply when the drilling fluid flows to the borehole bottom through the isolating casing and nozzles on the external bit. Meanwhile, most of the drilling fluid flows in the center passageway of the internal non-coring bit, and the maximum velocity is generated in the nozzles on the internal non-coring bit with a value of 3.6 m/s; the velocity flowing upward in the annulus of the external bit and borehole can reach to 0.84 m/s. Figure 4 illustrates the pressure inside the drill bit. According to this nephogram, pressure decreases gradually as drilling fluid flows through the modular bit. The pressure in the modular bit for coring drilling, shown in Figure 4a, is up to its maximum value of 21,766 Pa at the radial holes of the isolating casing, and the pressure at the core is around 2800 Pa. For the non-coring bit, as shown in Figure 4b, there are different pressure variation laws when drilling fluid is flowing through the external bit and internal bit. The maximum pressure of the external bit occurs at the inlet of the bottom nozzles with a value of 11,249 Pa, and then the pressure decreases rapidly. Meanwhile, the pressure increases sharply from the inlet of the internal bit to its bottom, and the highest pressure occurs at the nozzles of the internal non-coring bit with a value of 12,741 Pa. As the fluid jets out from the bottom nozzles of the internal bit, the pressure gradually decreases, but it experiences a rise at the central region of the borehole bottom, reaching a value of 11,309 Pa.  
Figure 3: Velocity nephogram of the large diameter modular bit: (**a**) velocity nephogram of the coring drill bit; (**b**) velocity nephogram of the non-coring drill bit.