For modern gasoline engines the beneficial miller cycle strategy is used to improve engine efficiency. The resultant demand of increased boost pressure at similar exhaust gas enthalpy level requires enhanced turbocharger efficiencies both for low flow as well as for high flow operating conditions. Variable nozzle turbines are the key technology to achieve such a wide efficient operating range. Due to the large number of relevant and strongly correlated design parameters, the optimization of a variable nozzle turbine stage by means of 3D computational fluid dynamics is challenging and time-consuming. To reduce the order of complexity a surrogate model based on superordinate and non-dimensional parameters was established for optimization. Those functional parameters were derived from the most sensitive geometric properties. Thus, the geometric properties of a turbine stage are retroactively predetermined by a certain setting of the functional parameters. Finally, the turbine characteristics are determined by those functional parameters, which allow preliminary design considerations based on a small number of parameters.

With the establishment of a surrogate model based on named functional parameters, a multi-point, multi-disciplinary optimization can be proceeded to achieve an optimal solution according to the layout target. Not only the aerodynamic performance can be discussed. But also, the structural integrity of the rotor as well as cost sensitive parameters can be considered. Furthermore, different turbine stages with different absolute geometric values can be evaluated on basis of the functional parameters corresponding to similarity coming from their non-dimensional character. Hence, comparative investigations based on named functional parameters can be performed for the evaluation of competitors.

In the following paper functional parameters are derived from turbomachinery analytics and rated on corresponding values of existing turbine stages. The influence of different parameters on aerodynamic performance is discussed based on CFD results and arranged according to their sensitivity for different engine relevant operating conditions. A surrogate model for the preliminary layout of variable nozzle turbine stages, which was derived from a DoE (design of experiment) based on named CFD results, is evaluated regarding its predictive quality on several exemplary turbine stages. Both, CFD- and experimental results are therefore used while the experimental results are made up of hot gas stand measurements such as measurements on engine test bench. Thus, not only the influence of functional parameters can be verified on measured turbine efficiency characteristics. But beyond that, also the predictive quality of the matching tool and the relevance of given target values can be assessed.


Preliminary Design of Variable Nozzle Turbines Based on Sensitive Parameters

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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