Investigating the Suitability of Multi-Scroll Volutes for Improving Spanwise Incidence of Mixed Flow Turbine Rotors With Varying Blade Cone Angles in Automotive Turbocharging Applications

Abstract

Targets to reduce fuel consumption and reduce CO2 emissions have been met using engine downsizing and turbocharging. In automotive applications, it is important that the turbocharger responds well to transient events and operates efficiently at both the design and off-design conditions. A mixed flow turbine (MFT) is not constrained to a radial inlet blade angle, allowing the peak efficiency to be shifted to a lower U/C_is, providing additional freedom to the designer. As the MFT leading edge varies in radius, the spanwise incidence angle also varies, leading to additional separation on the suction surface (SS) of the blade near the hub because of increasingly positive incidence, which is most noticeable at off-design conditions. A multi-scroll volute was previously paired with an MFT with a 45-deg blade cone angle (Λ), which generated a non-uniform spanwise flow that improved efficiency at off-design at the cost of peak efficiency. The current study identified the range of blade cone angles that benefitted from a multi-scroll volute to reduce incidence at the hub region. A numerical investigation was conducted, which determined the influence a multi-scroll volute can have on MFTs with varying levels of blade cone angle. When the MFT with a large blade cone angle (Λ = 60 deg) was paired with a multi-scroll volute, the efficiency improved by 2.2%pts at design and 0.5%pts at off-design conditions. The incidence improved, and the mass flowrate increased at the hub region. The MFT with a smaller blade cone angle (Λ = 30 deg) had performance losses at both operating conditions because the multi-scroll volute increased incidence within the hub region, which reduced the peak efficiency by 1.3%pts. The off-design condition had an excessively positive incidence angle, which was further increased at the hub region by the multi-scroll volute. This resulted in a 0.8%pts reduction in off-design efficiency. The multi-scroll volute concept was shown to offer efficiency improvements for MFTs with larger blade cone angles through better management of the non-uniform spanwise velocity distribution at the rotor inlet.