Performance analysis of low speed axial impulse turbine using two type nozzles for small-scale organic Rankine cycle

Expander is one of the core components in organic Rankine cycle (ORC). High rotating speed and limited working range are the key limitations on using turbine expander in small-scale ORC system. In this work, an axial turbine, consisting of 6 nozzles and 36 blades with mean diameter of 160 mm, is designed with partially admitted method to evaluate the feasibility of reducing rotating speed for small mass flow conditions and extending working range with R245fa used as working fluid. The results from 3D simulation show reasonable performance in design and off-design conditions. In design condition, the mass flow rate, turbine power output and turbine efficiency using Laval nozzle are 1.035 kg/s, 17.0 kW and 54.1%, higher than using convergent nozzle. However, the performance using convergent nozzle is better than using Laval nozzle when total mass flow is lower than 0.6 kg/s. Furthermore, reducing the number of nozzles can effectively increase power output of turbine and thermal efficiency of ORC for small mass flow conditions. With the designed turbine using 6 Laval nozzles and rotating at 10,000 RPM, the maximum net power output and thermal efficiency of bottoming ORC are 18.3 kW and 7.8%, respectively.