Abstract
Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. The purpose of turbocharging is to increase the engine inlet air density by utilising, the otherwise wasted energy in the exhaust gas. This energy extraction is commonly accomplished through the use of a radial turbine. Although less commonly used, mixed flow turbines can offer aerodynamic advantages due to the manipulation of blade leading (LE) angles, improving performance at low velocity ratios. The current paper investigates the performance of a mixed flow turbine with four volute designs, two radial and two tilted volutes each with one variant with an aspect ratio (AR)=0.5 and one with AR = 2. To ensure constant mass flow parameter (MFP) for aerodynamic similarity, volute area to radius ratio (A/r) was manipulated between the design variants. The maximum variation of cycle averaged normalized efficiency measured between the designs was 2.87%. Purely in the rotor region, the variation in normalized cycle averaged efficiency was 3%. The smallest volute AR designs showed substantial secondary flow development. The introduction of volute tilt further complicated the secondary flow development with the introduction of asymmetry to the flows. It was established that both AR and tilt have a notable effect on secondary flows, rotor inlet conditions and over all mixed flow turbine performance.
Highlights
Due to environmental concerns, engine downsizing has become increasingly important in the automotive industry to reduce engine emissions
To overcome the limited mixed flow effect achieved with the employment of standard radial volute designs, Lee et al.[9] investigated the impact of volute tilted on the performance of a mixed flow turbine and found an efficiency benefit of up to 1.64% mainly due to a reduction in shroud surface separation
To achieve equal mass flow parameter (MFP) between the different aspect ratio (AR) volute designs, the AR 1⁄4 0.5 volute required an area to radius ratio (A/r) 1⁄4 23.0 while the AR 1⁄4 2 volute required an A/r 1⁄4 24.2. This effect is the result of the larger AR designs achieving a lower MFP and a larger A/r being necessary to compensate for this, as discussed by Lee et al.[14]
Summary
Engine downsizing has become increasingly important in the automotive industry to reduce engine emissions. To overcome the limited mixed flow effect achieved with the employment of standard radial volute designs, Lee et al.[9] investigated the impact of volute tilted on the performance of a mixed flow turbine and found an efficiency benefit of up to 1.64% mainly due to a reduction in shroud surface separation. It was observed that increasing aspect ratio reduced volute secondary flows and a small reduction in rotor LE span-wise variation was achieved.
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