Aerodynamic interaction of turbines in a regulated two-stage turbocharging system

Two-stage turbocharging becomes prevailing in internal combustion engines due to its advantage of flexibility of boosting for the variation of operational conditions. Two turbochargers are closely coupled by engine manifolds in the system especially under the requirement of compactness. This paper studies the influence of the interaction of two turbines in a two-stage turbocharging system on the performance. Results show that the performance of low-pressure turbine is highly sensitive to the stage interaction. Specifically, compared with the cases without interaction, the efficiency of low-pressure turbine increases maximumly by 2.8% when the bypass valve is closed, but reduces drastically by 7.5% when the valve is open. Detailed flow analysis shows that the combined results of swirling flow from the high-pressure turbine and the Dean vortex caused by the manifold elbow result in the alleviation of entropy generation in the turbine rotor. However, when the bypass valve is open, interaction of the swirling flow with the injected bypass flow results in strong secondary flow in the volute and distorted inlet flow condition for the rotor, leading to the enhancement of entropy generation in low-pressure turbine. The study provides valuable insights into turbine performance in a two-stage turbocharging system, which can be used for the modeling and optimization of multi-stage turbocharging systems.