An Investigation into the impact of the spanwise flow field at rotor inlet on the performance of mixed flow turbines

Abstract

Current driving trends have caused automotive engines to undergo a period of downsizing. Downsizing refers to a reduction in engine swept volume, which corresponds to a lower power output. To counteract this, downsized engines are increasingly equipped with turbochargers to increase the power output. This has placed a greater emphasis on turbocharger performance across the full operating range. Therefore, turbocharger turbines must be capable of operating with high efficiency at off-design conditions, as well as possessing a low rotational inertia. One potential method to achieve this is the use of mixed flow turbines. However, compared to conventional radial flow turbines, mixed flow turbines experience increased flow separation from the hub side of the blade suction surface. This has a detrimental impact on turbine efficiency. The separation is the result of an increase in incidence angle at the hub, due to the reduction in leading edge tip radius.

This study examined the effectiveness of modifying the spanwise distribution of flow angle at the rotor inlet, with the intent of counteracting the increase in incidence angle at the hub. Two different approaches were considered. The first was leaned stator vanes, which were investigated using numerical and experimental techniques. The second approach was multi-scroll volutes, which had asymmetric limbs on the hub and shroud sides of the passage. These were examined with a numerical analysis. Both approaches provided an effective means to create a favourable spanwise flow distribution at the rotor inlet. This resulted in an improvement in turbine efficiency, when compared to baseline cases. Both approaches provided a more substantial benefit at off-design conditions compared to at the design point. The leaned stator vanes achieved the largest increase in efficiency and provided an improvement at all operating conditions. The benefits of the multi-scroll volutes were primarily limited to low speed, off-design conditions. Two key factors were identified that contributed to the improved turbine efficiency. A reduction in flow angle towards the hub side of the passage counteracted the positive shift in incidence angle, which reduced flow separation from the blade suction surface. This was most significant at the off-design operating points. The second factor was a spanwise variation in mass flow rate. Decreasing the mass flow rate towards the shroud reduced the tip leakage loss. This was the primary reason for improved efficiency at the design point.

In addition to examining the impact of different spanwise flow distributions, a method of producing variable geometry operation with the multi-scroll volutes was also investigated. This method consisted of permitting flow through only one volute limb at off-design conditions. This resulted in an increased turbine power output, at the expense of lower stage efficiency and higher engine back pressure.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	IHI Charging Systems International GmbH
Supervisor	Geoffrey McCullough (Supervisor) & Sung Kim (Supervisor)