Pre-swirl nozzle is regarded as an important part of preswirl system. Theoretical analysis shows that the temperature drop of pre-swirl system directly relates to the pre-swirl effectiveness of nozzle. When the inlet and outlet pressure are given, there are three ways to improve the nozzle pre-swirl effectiveness. Firstly, increase the discharge coefficient by optimizing the geometry of nozzle; secondly, decrease the preswirl angle; thirdly, minimize the deviation angle of outlet flow to decrease the actual throat area and then increase the tangential velocity.
A new kind of pre-swirl nozzle called vane shaped hole (VSH) nozzle was presented and designed in this paper. The vane height/ pitch ratio of VSH nozzle could be adjusted to an appropriate value, which made its performance of acceleration and deflection better than that of cascade vane nozzle. With the same throat area, radial location and pre-swirl angle (15°), the performance of VSH nozzle was numerically analyzed and compared with three other traditional ones at pressure ratio range 1.1∼1.9, and Reynolds number range 1×105∼5×105. The three traditional nozzles were the simple drilled nozzle, aerodynamic nozzle and cascade vane nozzle. To consider the mixing and rotating influence in the pre-swirl cavity downstream of nozzle, the rotating pre-swirl cavity and receiver hole were included in the computational models. Entropy generation analysis was also made for each nozzle to assess the source of irreversibility.
Numerical results show that the discharge coefficient of aerodynamic nozzle increase 13.7% compared with that of simple drilled nozzle owing to the smaller pressure gradient at nozzle inlet. For cascade vane nozzle, pressure gradient generated in the course of acceleration and deflection would decrease due to the well-designed vane profile. The discharge coefficient of cascade vane nozzle can be as high as 0.97, whereas the preswirl effectiveness is only 0.86 because of a large deviation angle ( 2.42°). For VSH nozzle, higher vane height/pitch ratio and close to zero trailing edge radius lead to a further decrease of endwall secondary flow loss and trailing edge loss, which results in a very small deviation angle ( 0.56°). Consequently, the pre-swirl effectiveness of VSH nozzle is about 8% higher than that of cascade vane nozzle, though the discharge coefficient is about 5% lower. Since the volume of solid block in VSH is bigger than the volume of vane in cascade vane nozzle, which could reduce the difficulty in manufacturing but increase the total weight. Thus, the final nozzle design should comprehensively consider the aerodynamic performance, cost, manufacturing, and stress constraints.
