In this study, blade tip leakages were calculated for a Radial Outflow Turbine (ROT) designed for an Organic Rankine Cycle (ORC) at a 150kW power output. Since the turbine blade sizes are relatively very small for low-capacity systems, the leakages through the blade tip clearances considerably affect the turbine isentropic efficiency. Therefore, labyrinth seals were applied at the blade tips and the ROT’s performance degradation due to blade tip leakages was investigated.
In order to determine the preliminary ROT sizes, an in-house developed 1-D code was utilized. The blade profiles were optimized with CFD analyses to reach high power output and isentropic efficiency. The designed ROT has 8 stages. Toluene is used as the cycle fluid at inlet conditions of 24bar of total pressure, 310°C and outlet conditions of 0.25bar of static pressure. These conditions are chosen for exhaust conditions of a common biogas engine. Thus, the ORC is supposed to operate at a heat source temperature of 460°C and a heat sink temperature of 35°C. The turbine speed of 14000 rpm is determined.
The CFD model for the entire 3-D turbine geometry is built in the FlowVision software. The real gas equation is employed for the compressible flow. The SST turbulent flow model is employed. The CFD model uses transient state and rotating frame approaches.
Four blade tip configurations were analyzed. The CFD results reveal the followings. The turbine isentropic efficiency is calculated to be 87.62% for the unshrouded geometry with no clearance, which is an ideal case. For a manufacturable and manageable blade tip clearance of 0.2 mm, the turbine isentropic efficiency is calculated to be 71.03% for the unshrouded geometry. The shrouded geometry with the same clearance increases the efficiency to 74.03%. When a labyrinth seal is applied to the shrouded geometry, the efficiency reaches to 77.03%. The best practice in terms of turbine power output and efficiency is the shrouded geometry with labyrinth seal applications.