Innovative small axial multistage turbine with partial admission for bottoming ORC

Abstract

The expander is the key element of an organic Rankine cycle (ORC) and has a significant influence on the cycle efficiency. Therefore, in this study, an innovative small axial multistage turbine with partial admission for a bottoming ORC (ORBC) is investigated. The ORBC was used for the waste heat utilization of exhaust gases and jacket cooling water from a 537 kW biogas internal combustion engine (ICE) .The initial design of the turbine flow part was developed using a mean diameter two-dimensional (2D) preliminary design (PD) method. The high rotational speed of the axial turbine, which is its most critical characteristic, was overcome by employing partial admission. To achieve high isentropic efficiency, new (nonstandard) profiles of the nozzle and moving blades were designed using an analytical method. The initial full flow part of the turbine was further investigated by three-dimensional (3D) computational fluid dynamics (CFD) simulations, with the aim of improving its design. The simulations also provided a better insight into the physics of flow than the mean diameter 2D method. Both the 2D PD calculations and 3D CFD simulations were performed using real gas properties.The innovative axial action–reaction turbine with partial admission has 8 stages, a mean diameter of 140 mm, and a rotational speed of 9000 rpm. According to the CFD simulation results, a mass flow of isopentane as the working fluid of 1.171 kg/s achieves an effective power of 60.35 kW and isentropic efficiency of 74.8%. These represent an 11.2% increase in the overall power and a 4.5% increase in the overall thermodynamic efficiency by employing an ORBC on a 537 kW biogas ICE.Therefore, the new turbine is competitive with other expanders in this power range.