Power density of a super-critical carbon dioxide cycle is very high due to its fluid-like density. For this reason, generally size of turbines are very compact compared to that of the air Brayton cycle. However, such an advantage sometimes becomes a challenge for aerodynamic design, because low volume flow rate of the turbine requires design point at a very low specific speed. One of the solution for the challenge is to design a turbine stage as a partial admission stage in which flow enters the turbine nozzle over only a portion of its annulus. Then it secures a sufficient turbine inlet area, even though performance degradation should be taken in to account.
In this study, aerodynamic design of an axial turbine has been carried out and its performance has been assessed with numerical simulations. One of design requirements for the axial turbine was to minimize rotor inlet and outlet pressure difference to avoid potential axial thrust. In spite of a small amount of expansion ratio in the turbine stage, the absolute pressure difference could cause severe damage to rotor dynamic system and require complicated bearing system. For this reason, in this study, the turbine was designed as impulse type axial turbine with partial admission.
Required rotating speed and resultant low volume flow rate restricted mean diameter and blade height at the stage inlet. The final design has a very low aspect ratio, less than unity. The number of nozzle and rotor are 12 and 34, respectively. The rotating speed of the rotor is 45,000 rpm. The ratio of nozzle arc to blade pitch is approximately 3, which determines efficiency deterioration due to the partial admission.
During the numerical simulations, to implement real gas property, Redlich-Kwong-Aungier cubic equation was used. As the turbine operating point is far from its critical point, the Redlich-Kwong-Aungier cubic equation showed a good agreement with real supercritical gas property.
To assess full and partial admission turbine performance, steady state numerical simulations have been performed. The full annulus CFD domain was constructed for the partial admission stage. At the design condition, there was 15% isentropic efficiency drop in case of the partial admission stage relative to the full admission stage. Also similar amount of power output penalty was investigated from the partial admission case. As the nozzle was choked at the design condition, the mass flow rate was conserved regardless of the admission type. Then in the flowing region, design velocity triangle in front of the rotor well established, while additional loss was generated along the circumferential direction over non flowing region.