## 4 Results
### Effect of Nozzle Configuration Under Different Inlet Pressures  
In this group of experiments, the dimensionless standoff distance, \(S\), was normalized by the nozzle diameter, and increased from 1 to 10. The ambient pressure was set constant as 10 MPa, and the fluid temperature was set at 330 K. The inlet pressures were set at 20, 30, 40 and 50 MPa, respectively.  
Figure 6 illustrates the effects of nozzle configuration at various standoff distances on the rock erosion ability of the SC-CO\({}_{2}\) jet under four inlet pressures. It is shown in Figure 6 that, nozzle configuration greatly affects the rock erosion ability of the SC-CO\({}_{2}\) jet and the effects largely depend on standoff distance and inlet pressure, which is claimed by evaluating the erosion volume of the cavity on each specimen. Overall, with increasing inlet pressure, the rock erosion ability of the three test nozzle increases. This is because the higher inlet pressure allows more pressure energy to be converted into the kinetic energy of the jet, thereby enhancing the impingement of the jet. Moreover, different nozzle configurations, the convergent nozzle, or the Laval nozzles, influence the rock erosion performance differently. In more specific terms, compared with the commonly used convergent nozzle, the Laval-1 nozzle can maximally enhance the erosion volume by about 10%, 21.2% and 30.3%, respectively, corresponding to inlet pressure of 30, 40 and 50 MPa (Figure 6b-d); while the Laval-2 nozzle has the capability of maximally enhancing the erosion volume by about 32.5%, 49.2% and 60% at inlet pressure of 30, 40 and 50 MPa (Figure 6b-d), respectively. However, at inlet pressure of 20 MPa, both the two Laval nozzles turn to play an opposite role of reducing the intensity of rock erosion with the maximum erosion volume being only 71% and 88% of that of the convergent nozzle (Figure 6a). In other words, at relatively higher inlet pressure, the Laval nozzles can generally increase the rock erosion ability of the SC-CO\({}_{2}\) jet; whilst at relatively lower inlet pressures, they affect the erosion behavior in an opposite way of reduction. In addition, as the inlet pressure increases, the Laval nozzles can increase the erosion capability of the SC-CO\({}_{2}\) jet more significantly.  
Figure 5: Photos of rock specimens after erosion: (**a**) Specimens eroded by the convergent nozzle; (**b**) Specimens eroded by the Laval-1 nozzle; (**c**) Specimens eroded by the Laval-2 nozzle.  
It can also be seen from Figure 6 that each jet has an optimum standoff distance at which position the erosion volume reaches a maximum, which means the existence of an optimum standoff distance could be a typical feature of the SC-CO\({}_{2}\) jets for erosion. This is because the undeveloped jet and backflow resistance at small standoff distance lead to large energy loss and then the poor erosion intensity. A standoff distance larger than the optimal value results in significant entrainment of the jet flow and then large energy dissipation [12]. The curves suggest that the optimal standoff distance is about 4 times the nozzle diameter for the SC-CO\({}_{2}\) jet discharging from the Laval nozzles, while it reduces from 4 to 2 with the increasing inlet pressure for the convergent nozzle. It is inferred that the jet issuing from the convergent nozzle has a smaller standoff distance range to effectively erode the specimens at relatively high inlet pressure.