Numerical and experimental investigation of irreversible energy losses in a two-stage radial turbine under asymmetric inflow conditions

In a two-stage turbocharger, the coupling between stages and the structure of the inter-stage duct may lead to severe flow distortions, which have a destructive impact on turbine performance. In this paper, the aerodynamics and energy loss mechanisms of two-stage turbines matched with a V-6 diesel engine are experimentally and numerically investigated. To reveal the formation mechanism of the asymmetric inter-stage flow, the vortex transportation in the high-pressure turbine is first analyzed. Entropy generation analysis is applied to quantify the irreversible losses while considering the coupling influence of the asymmetric inflow conditions, and two types of curved inter-stage ducts are studied. The sources of the dominant irreversible losses are revealed and origin of the corresponding energy loss is localized. The results show that the entropy generation rates associated with viscous dissipation, turbulent dissipation, and heat conduction account for about 30% each in the low-pressure turbine. Compared to uniform inflow conditions, the entropy generation rates increase by an average of 43.8%, 196.9%, and 19.2%, respectively under asymmetric inflow conditions. The pivotal turbulent dissipation is primarily concentrated in the inter-stage duct, the impeller inlet, and the volute tongue. The peak value for the difference in the cumulative entropy generation rates from turbulent dissipation is mainly caused by the mixing of the leakage vortex and the separation flow. The novelty of the present study lies in revealing the source of irreversible losses in two-stage turbines, which may provide a fundamental basis for the optimal design of a two-stage turbocharger.