Experimental Investigation on Off-Design Performance of a Small-Scale Two-Stage Counter-Rotating Impulse Turbine

The high-efficiency turbine design is of great importance for small-scale power systems with a high expansion pressure ratio (EPR). A two-stage counter-rotating impulse turbine is optional, with the advantages of a high efficiency and a compact size. However, few experimental investigations were reported. The performance characteristics incorporating power output and turbine efficiency against operation parameters need to be revealed. In this study, the off-design performance of a two-stage counter-rotating impulse turbine with a rated power of 1.5 kW is investigated experimentally. The internal flow is analyzed based on a 3D numerical simulation. First, a test rig is set up for the small-scale Rankine cycle. The performances are estimated under various rotational speeds and mass flow rates. Then, a 3D CFD simulation is employed to investigate the internal flow inside the supersonic nozzle and the flow passages of the two rotors. The effect of the second counter-rotating impeller is analyzed. The results indicate that the mass flow rate increases almost linearly from 7.2 to 18.5 kg/hr as the inlet pressure rises from 200 to 540 kPa. The isentropic efficiency of the turbine increases as the EPR descends, and the superheated degree ascends and is correlated as a linear function of the reciprocal of EPR and superheated degree. The measured maximum efficiency is 0.574. When the turbine speed ascends, the power proportion of the first impeller increases while the second impeller declines. Because the increment of the impeller 1 is more prominent, the overall power output is enhanced. Meanwhile, the second impeller can improve the power output apparently under the off-design conditions with a high EPR and a low rotational speed. The power proportion of the second stage reaches 32.59% when the EPR is 52.34 and the speed is 17,201 r/min. In this novel two-stage counter-rotating impulse turbine, the exit energy of the first rotor can be recovered effectively by the second rotor thanks to a compact structure because of no stator between the two rotors.