Analysis of leading edge flow characteristics in a mixed flow turbine under pulsating flows

Abstract

Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. A turbocharger increases charge air density using a turbine to extract waste energy from the exhaust gas to drive a compressor. Most turbocharger applications employ a radial inflow turbine. However, to ensure radial stacking of the blade fibers and avoid excessive blade stresses, the inlet blade angle must remain at zero degrees, creating large incidence angles. Alternately, mixed flow turbines can offer non-zero blade angles while maintaining radial stacking of the blade fibers and reducing leading edge separation at low velocity ratios. Furthermore, the physical blade cone angle introduced reduces the blade mass at the rotor outer diameter reducing rotor inertia and improving turbine transient response. The current paper investigates the performance of a mixed flow turbine under a range of pulsating inlet flow conditions. A significant variation in incidence across the LE span was observed within the pulse, where the distribution of incidence over the LE span was also found to change over the duration of the pulse. Analysis of the secondary flow structures developing within the volute shows the non-uniform flow distribution at the volute outlet is the result of the Dean effect in the housing passage. In-depth analysis of the mixed flow effect is also included, showing that poor axial flow turning ahead of the rotor was evident, particularly at the hub, resulting in modest blade angles. This work shows that the complex secondary flow structures that develop in the turbine volute are heavily influenced by the inlet pulsating flow. In turn, this significantly impacts the rotor inlet conditions and rotor losses.

The impact of volute aspect ratio and tilt on the performance of a mixed flow turbine

“…The impact of rotor inlet flow condition were studied by Lee et al 8 over a wide range of pulsating flow conditions. The authors showed that the LE span-wise flow distribution varied over the pulse period and was dependent on the pulse characteristics.…”

Analysis of the Three-Dimensional Accelerating Flow in A Mixed Turbine Rotor

“…Abidat et al [13] studied the clearance effect between the rotor blades and the casing on the mixed turbine performance. In two cases, papers [14,15] studied the effects of the cone and the inlet blade angles on mixed inflow turbine performance. The first is for a fixed outlet volute section and the second for a variable outlet volute section.…”

Investigation on the Flow Angle of a Mixed Flow Turbocharger Turbine Under Steady State Operating Conditions Authors

“…Lee et al, [15] stated the presence of axial flow element at the inlet rotor was discovered to obtain swallowing capacity than the radial flow element as it is mainly due to the variation of distribution of an incidence at the respective location. Lee et al, [16] further stated that the efficiency at low velocity ratio and loss correlated to the flow angle could be improve with the use of a mixed flow turbine. Leonard et al, [17] found a significant presence of secondary flow and loss at highly incidence angle as the flow enter the rotor.…”