# Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control
## 1 Introduction  
Air drilling technology is a basic and important technique which is widely used in mineral production and foundation construction. The heavy dust due to drilling processes has always been a key issue. The adverse effects of fugitive dust emissions to the local environment and human health, including workers and the general public, has attracted attention from researchers [1; 2; 3]. Hence, we proposed a reverse-circulation drilling method to solve this problem. This method utilizes the nozzle suction effect to draw in and pump a significant amount of air with the cuttings and dust to the ground for collection. It is a highly efficient and cleaner drilling technique aimed to protect the environment and human health [4]. A high penetration rate and good quality of borehole are also important advantages to shorten time and save cost. Satisfying dust control performance was achieved after drilling to a certain depth, however, heavy dust can still be observed in the initial drilling phase because of lower annular space pressure and deficient suction capability. Additionally, leakage loss of air at fractured formations is another problem due to deficient suction performance. The combination of a supersonic nozzle and reverse-circulation drilling method is likely to be a potential solution to this issue. A supersonic nozzle is a minor and simple mechanical component utilizing the conversion of pressure energy and kinetic energy of the motive or primary stream to entrain and pump flows. It is widely used in aerospace [5], fuel cell [6], refrigeration [7], waste heat recovery [8], and solar power technologies [9]. Considering its simplicity in construction and high efficiency, the supersonic nozzle seems to be available to the reverse-circulation drilling technique for better dust control. How to make the best of the supersonic nozzle to maximize the entrainment ratio has attracted significant attention from researchers. Some of them focused on the design of the nozzlestructure. A lobed supersonic nozzle was proposed by [PERSON] et al. [10] to improve entrainment and mixing within a rectangular ejector. [PERSON] et al. [11] compared the performance of the Chevron nozzle and a conventional nozzle, and stated that the Chevron nozzle has a positive effect on the supersonic ejector performance. [PERSON] et al. [12] developed two supersonic nozzles named the Tip Ring nozzle and Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle, and the results show that both nozzles achieved a 30% increase in entrainment of the secondary flow. Nevertheless, the relative complexity of these nozzle geometries is such that it is not easy to fabricate them. A group of studies focused on geometry parameter matching. [PERSON] et al. [13] investigated the optimum area ratios of the nozzle/mixing chamber and operating conditions with an experimental setup. [PERSON] et al. [14] confirmed that the nozzle exit position (NXP) and primary nozzle geometry had a strong influence on the entrainment performance. [PERSON] et al. [15] and [PERSON] et al. [16] also observed that the position of the nozzle was an important design parameter, and the nozzle exit position affected the critical back pressure considerably. For the reverse-circulation method, the effect of area ratio can be regarded as the relationship between the nozzle and the center passage area. The difference is that the supersonic nozzle is not coaxial with the \"mixing chamber\" (center passage). The supersonic nozzle was designed inclined with an unequal-length divergent part which was different from the axial symmetry nozzle in previous studies. Furthermore, multiple supersonic nozzles were distributed in the radial direction instead of a single nozzle. The flows from nozzles in adjacent or opposite positions may interact and, hence, change the back pressure in the center passage. Therefore, it is necessary to thoroughly understand the flow characteristic and mixture mechanism for suction performance improvement.