# Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control
## 5 Results and Discussion
### Influence of the Dominant Effective Parameters
Figure 9b shows a much better mixing uniformity of the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together. As shown in Figure 9c, the non-uniformity zone was also presented at the upper outlet boundary of the center passage and the zone near the wall. One possible explanation is that the flow too close to the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage diameter is larger. Yet, with the increase of the spray angle \(\theta\), unnecessary energy consumption was increased due to the increase of multiple fluid collisions in the radial direction. Figure 8: Comparison of the predicted results and experimental data for \(\omega\) with different \(\theta\). Figure 9: _Cont._  
Figures 10 and 11 present the impacts of flushing nozzle diameter and center-passage diameter on the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center-passage diameter, and then the curve tended to be smoother. It can be inferred that the interaction of nozzles was obvious and the back pressure at the supersonic nozzle outlet would be increased when the expansion process was restricted in a smaller space. The pressure difference between the center-passage and the air in the annular space was likely to decrease, which resulted in poor suction performance. However, the effect gradually decreased when the gas expansion was accomplished and insufficiently supplemented the flow. The flushing nozzle diameter, by contrast, presented a negative relation with the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to suspend the particles in the flow for better suction. However, the backflow caused by the flushing nozzle disturbed the suction process of the annular air, which can be seen in Figure 5.0 Moreover, it is likely that the total inlet flow was shunted by the flushing nozzles, which caused the flow rate decrease of the supersonic nozzle. Thus, the velocity at the nozzle outlet decreased and poor suction performance was achieved. Figure 5.1 Comparison of predicted results and experimental data for \(\omega\) with different \(D_{\mathrm{c}}\). Figure 5.2 Schematic view within the reverse-circulation bit based on the contours of the Mach number: (a) \(\theta=24^{\circ}\); (b) \(\theta=32^{\circ}\); and (c) \(\theta=48^{\circ}\). According to the analysis above, the CFD method is realiable to estimate the overall suction performance and flow field charactristic. The deviations of calculated results were no more than 17%. The result indicates that the standard \(k\)-e model was not sufficient to accurately present the whole details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic, and it is reliable to calculate \(\omega\). It was also found that the calculated values were generally less than the experimental values. Undiscovered leakage may be a reason for the difference.