## 4 Results and Discussion
1—clearance between the external bit and internal bit; 2—annulus between the external bit and the borehole; 3—chamber between the external bit and torque ring; 4—nozzles on the external bit; 5—water gaps of the external bit 6—water gaps of the internal bit; 7—center channel of the internal non-coring bit; 8—nozzles of the internal non-coring bit. Figure 5 is a velocity and the pressure nephogram of the bottom section perpendicular to the bit axis when the modular bit drill is in a borehole. As illustrated in Figure 5, once the drilling fluid flows out from the bit, it forms a complete overflow zone in the area of the borehole bottom surface, which is profitable for rock cuttings removal at the borehole bottom and for drill bit cooling. For the coring bit, as in Figure 5a, a dense flow line and high flow velocity are produced in the outlet area of the nozzles on the external drill bit. After flowing out of the nozzles and the clearances between the external bit and internal bit, drilling fluid is mixed and eventually returns upward. In Figure 5c, at the bottom of the borehole, the jet out of the external bit nozzles causes uneven pressure distribution, resulting in a lower pressure zone at every nozzle outlet and two higher pressure zones distributed radially along the drill bit on both sides of the nozzle. As Figure 5b shows, for the non-coring bit, there are two drilling fluid high velocity zones, which are at the outlet of the external drill bit nozzles and the outlet of the internal drill bit nozzles. Around the internal drill bit nozzles, a high pressure zone is generated, while at the nozzles on the external bit low pressure zones are generated, as shown in Figure 5d. With the high pressure at the internal drill bit nozzles, a hydraulic jet will be generated, which helps to improve the capacity of rock cuttings removal and increases rock-free faces so as to raise penetration rates. In Figure 5a,b, the lines AB and CD are drawn on the coring bit and non-coring bit, respectively. The gradient of velocity on these two lines is illustrated in Figure 6. According to Figure 6a, for the coring bit, the gradient of velocity is symmetrically distributed on line AB relative to the center of the coring bit. There is a trend of initially decreasing, then increasing, and finally decreasing again from the borehole wall to the center of the bit. Figure 6b illustrates that the gradient of velocity at the bottom of the non-coring bit fluctuates, and it is difficult to describe the changing patterns with mathematical expressions. It is worth mentioning that a larger gradient of velocity means a higher  
Figure 4: Pressure nephogram of the modular bit: (**a**) pressure nephogram of the coring drill bit; (**b**) pressure nephogram of the non-coring drill bit. drilling cuttings carrying capacity. In other words, when the drilling fluid flows through the bottom of the coring bit or non-coring bit, rock cuttings can be cleared effectively. Figure 5: Flow velocity and pressure nephogram of the bottom section of the large diameter modular bit: (**a**) velocity nephogram of the coring drill bit; (**b**) velocity nephogram of the non-coring drill bit; (**c**) pressure nephogram of the coring drill bit (**d**) pressure nephogram of the non-coring drill bit. Flow Characteristics with Different Clearances between the External Bit and Internal Bit for Coring Drilling  
The clearance between the external bit and the internal bit is one of the most important structural parameters for the coring bit; it affects the sizes of the internal bit, bit cooling, and rock cuttings removal. When the size of the external drill bit is fixed, increasing the clearance will cause a reduction in the size of the internal bit, and thus small diameter core will be obtained, which may lead to the core-taking volume hardly meeting drilling engineering requirements. Meanwhile, decreasing the clearance will decrease the drilling fluid flow rate and increase pressure loss, which will reduce the bit cooling effect and rock cuttings removal.