NUMERICAL STUDY OF ORGANIC RANKINE CYCLE RADIAL-INFLOW TURBINES FOR HEAVY-DUTY DIESEL ENGINE COOLANT HEAT RECOVERY

asme-orc2015 Tracking Number 84

Low rotating speed radial-inflow turbines are promising for the organic Rankine cycle systems in the small power size applications such as heavy-duty trucks and passenger cars waste heat recovery. Small mass flow rate and low rotating speed will lead to the turbine design specific speed lower than the optimal range of 0.34 to 0.72. Until now, few literatures reported the performance characteristics and loss mechanisms of low specific speed ORC radial-inflow turbines. The present study makes a RANS simulation of the low specific speed of 0.28 radial-inflow turbine using R245fa as working fluid to evaluate its performance characteristics and investigate the loss mechanisms. An optimal specific speed of 0.47 radial-inflow turbine in the same nominal operating condition and working fluid is also simulated for comparative analysis. The CFD results indicate that the low specific speed turbine nominal efficiency decreases 1.7% compared with the optimal one. And furthermore the off-design efficiency decreases are in the range of 2.6% to 0.6% from low to high pressure ratio conditions. The effect of specific speed on the efficiency characteristics mainly lies on rotor tip clearance and passage losses. The low specific speed turbine shows larger tip clearance losses but smaller passage losses compared with the optimal specific speed turbine. Flow field analysis of rotor entropy generation indicates that for the low specific speed turbine, the entropy generation caused by the tip clearance flow is much larger, and the tip clearance loss is mainly located on the suction surface in the inducer to midchord region, but on the pressure surface in the exducer region.