Vaneless turbocharger turbines are commonly used for automotive engines due to its low cost and better off-design performance. It consists of a vaneless volute and a radial or mixed flow rotor, where both components are important to the overall device performance. The volute has the function of increasing the kinetic energy of exhaust gas and directing the flow towards the rotor inlet at a favorable flow angle. The rotor can convert the flow energy to the mechanical energy, which can be used for either driving a compressor or power generation. With the pulsating nature of the exhaust flows, most energy is contained at the peak of the pulse. Therefore, during one engine cycle, optimizing the turbine performance for the peak pulse region is more straightforward to improve the cycle-averaged shaft power generation.

 

This study sought to optimize both the volute and rotor simultaneously for the peak point of the pressure pulse (2.4 bar). Thirteen design parameters in total are considered during the optimization process. Six volute design parameters were used to control the aspect ratio, intake area, exit area, and the circumferential distribution of the cross-sectional area. Seven rotor parameters were utilized to modify the cone angle, blade axial location, and the camber-line angle distribution. To avoid the optimized results trapped in the local minima, the optimization was conducted twice with two different algorithms used, namely the genetic algorithm and the kriging surrogate model. Commercial turbulent viscous CFD solver ANSYS-CFX was used to predict the turbine performance. Full-stage turbine including ten blade passages is explicitly modeled for better accuracy. In order to ensure the matching between turbocharger and engine maintained same as the original turbine, special attention was paid to constraint the swallowing capacity characteristic of the optimized turbine to be similar as the baseline turbine, with maximum 2.5% difference at the design point. Compared with the baseline turbine, the turbine efficiency was improved by 4.1 percentage point with the using the genetic algorithm, and a 4.8 percentage point was achieved by using kriging surrogate model. Although the optimized turbine has a lower peak efficiency, the optimal velocity ratio of optimized design shifted from 0.71 (baseline value) to 0.61, implying a better performance will be achieved under high loading conditions. Quasi-steady method was used to assess the cycle-averaged performance, it is found that the optimized design has a higher energy output than the baseline as soon as the peak pressure greater than 2.6 bar


Optimization of Vaneless Volute and Mixed Flow Impeller for Pulsating Flow

Division / Sponsor:
International Gas Turbine Institute (IGTI)

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

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