FAST DESIGN METHODOLOGY FOR SUPERSONIC ROTOR BLADES WITH DENSE GAS EFFECTS

asme-orc2015 Tracking Number 34

In Organic Rankine Cycle turbines where a large work output is required from a turbine in a single stage, it is necessary to use high pressure ratios across the nozzle blades, thus producing supersonic velocities at the rotor inlet unless very high rotational speeds are used. Supersonic flow in the rotor can lead to significant losses unless a careful design of the rotor blades is adopted to avoid focusing of the characteristic lines. Moreover, the design of Organic Rankine Cycle (ORC) expanders requires numerical models taking into account the dense gas effects the working fluid undergoes in given ranges of operating conditions. This work describes a fast 2-D design methodology based on the method of characteristics (MOC) for rotor blade vanes of supersonic axial ORC impulse expanders. The MOC is generalized to gases governed by complex equations of state to fully take into account dense gas behavior. The fluid thermodynamic behavior is described by highly accurate multiparameter equations of state based on Helmholtz free energy. Several working fluids are considered, including R245fa, Novec649, R449, RE347mcc. The designs generated by the generalized MOC are compared with those obtained under the classical perfect gas model. Finally, CFD simulations of both isolated rotor blades and a full turbine stage are carried out to assess the performance of the designs using the ANSYS CFX solver. The nozzle blades are also designed by means of an extended method of characteristics previously developed by some of the present authors.