Aerodynamic analysis of a 1.5 kW two-stage counter-rotating partial-admission impulse turbine for small-scale power system with a high expansion pressure ratio

Partial-admission impulse turbine is suitable for small-scale power cycles with a high expansion pressure ratio (EPR) and a low mass flow rate. However, the exit velocity of a single-stage turbine is very high, leading to a large exit loss. A new two-stage counter-rotating impulse turbine has been developed to recover the exit loss, which has a novel structure without stator between the two rotors. However, the internal flow characteristic of such a turbine is still unclear. In this study, the aerodynamic performance of the two-stage counter-rotating partial-admission impulse turbine is analyzed. The internal aerodynamic characteristics are estimated via CFD simulation. First, the geometric parameters are determined based on the mean-line model. Then, the characteristic line method is adopted to design the blade profile of the first supersonic stage based on the vortex flow theory. Subsequently, the performances are estimated under the design and off-design conditions via 3D CFD simulation. The results indicate that the modified convergent-divergent supersonic nozzle exhibits a better adaption with the impellers. The load is principally imposed on the upper zone of the blades. The power output of the turbine reaches 1501.3 W with an EPR of 52 and an efficiency of 45.99 % at the design point and the power of the first impeller accounts for 76.5 %. The mass flow rate increases almost linearly with the augmentation of the turbine inlet pressure. The power output proportion of the second impeller may arrive at 27.63 % under off-design conditions, showing the addition of the second stage can improve the turbine performance evidently.