Design of Counter-Rotating Turbine to Improve the Off-Design Performance of Turbo-Compounding Systems

Abstract

Engine turbo-compounding (TC) is an effective technology to improve the engine efficiency and cut down CO2 emission. A power turbine downstream of the conventional turbocharger turbine is used to recover the exhaust energy and transfer it into mechanical work. However, the performance of turbo-compound system deteriorates seriously at engine off-design points. It is mainly caused by the fact that the designs of turbocharger turbine and power turbine are independent from each other, failing to consider the effects of upstream swirls on the downstream power turbine performance. Analysis showed that the off-design conditions, off-design incidence loss, transition duct loss and non-uniformity inlet loss contributed to the deterioration performance of power turbine at off-design conditions. Among these reasons, the incidence loss played significant effect on power turbine’s performance. The current research aims to improve the turbo-compounding performance at engine off-design conditions by counter-rotating turbine (CRT) configuration. CRT consists of a radial-flow turbocharger turbine and an axial-flow power turbine with counter-rotating direction. The main purpose is to decrease the incidence loss and turning loss in the power turbine stator by CRT. Performance comparisons between the counter- and co-rotating turbines have also been conducted. First, meanline analysis was carried out to investigate the influences of design parameters on the velocity triangles and turbine performance. Analysis shows that counter-rotating turbine requires the design of high reaction radial turbine. Then, the 1D preliminary design starts and it is followed by 3D detail design. Finally, the computational fluids dynamic (CFD) method was used to evaluate the counter-rotating turbines performance. Results show that the designed counter-rotating turbine improves the off-design performance effectively, with 3.8% increase of power turbine efficiency points at 1200rpm condition. Further analysis on the flow field was conducted and it was found that the flow angle distribution at upstream turbine exit was highly non-uniformity along the span. To weaken the non-uniformity may be a potential way to further improve the performance of turbo-compounding systems.