Performance Characterization of Twin Scroll Turbine Stage for Vehicular Turbocharger Under Unsteady Pulsating Flow Environment

Abstract

Unsteady three-dimensional computations have been implemented on a turbocharger twin-scroll turbine system (volute-turbine wheel-diffuser). The flow unsteadiness in a turbocharger turbine system is essentially driven by a highly pulsating flow from the upstream combustor which causes a pulsating stagnation pressure boundary condition at the inlet to the turbine system. Computed results have been post-processed and interrogated in depth in order to infer the significance of the induced flow unsteadiness on performance. The induced flow unsteadiness could be deemed important since the reduced frequency of the turbine system (based on the time scale of the inlet flow fluctuation and the flow through time) is higher than unity. Thus, the computed time-accurate pressure field and the loss generation process have been assessed to establish the causal link to the induced flow unsteadiness in the turbine system. To do this consistently both for the individual subcomponents and the system, a framework of characterizing the operation of the turbine system linked to the fluctuating inlet stagnation pressure is proposed. The framework effectively categorizes the operation of the unsteady turbine system in both spatial and temporal dimension; such a framework would facilitate determining whether the loss generation process in a subcomponent can be approximated as unsteady (e.g. volute) or as locally quasi-steady (e.g. turbine wheel) in response to the unsteady inlet pulsation in the inlet-to-outlet stagnation pressure ratios of the two inlets. The notion that a specific subcomponent can be approximated as locally quasi-steady while the entire turbine system in itself is unsteady is of interest as it suggests a strategy for an appropriate flow modeling and scaling as well as for the turbine system performance improvement. Also, computed results are used to determine situations where the flow effects in a specific subcomponent can be approximated as quasi-one-dimensional; thus for instance the flow mechanisms in the volute can reasonably be approximated on an unsteady one-dimensional basis. For a turbine stage with sudden-expansion type diffuser, the framework for integrating sub-component models into a turbine system is formulated. The effectiveness and generality of the proposed framework is demonstrated by applying it to three distinctly different turbocharger operating conditions. The estimated power from the integrated turbine system model is in good agreement with the full unsteady CFD results for all three situations. The formulated framework will be generally applicable for assessing the new design configurations as long as the corresponding high fidelity steady CFD results are utilized to determine the quasi-steady (or acoustically compact) behavior of each new sub-component.