No abstract

In order to reduce the aerodynamic efficiency loss of deeply scalloped radial turbines, asymmetrically scalloping of turbines’ backdisc has been invented for a number of years. However, the mechanism how this works and its mechanical implication have not been reported in the open literature. In this paper, taking the symmetrically scalloped turbine of a turbocharger for marine generator application as the baseline, two asymmetrically scalloped turbines were developed from it: one with scalloping bias to the pressure side, and the other to the suction side of the blades. Mechanic and aerodynamic analyses of these three turbines were carried out using ANSYS Workbench. The results indicate that the asymmetrically scalloped turbine with scalloping bias to the pressure side can reduce the backdisc-heat shield cavity leakage flow going into the blade suction surface through the scalloped backdisc, due to the influence of the high pressure near the pressure surface of the blade, thus decrease the size of vortices the leakage flow generated near the suction surface inside the blade passages and associated mixing loss. Compared to the symmetrically scalloped turbine, it increases the maximum principal stress by 0.39% and 5.2% at blade fillet and backdisc respectively, but the aerodynamic efficiency of the turbine is increased by 0.78% point at a turbine U/C value of 0.43, and the smaller the value of the U/C is, the greater the efficiency advantage will be. By contrast, the turbine with suction side bias scalloping shows the poorest aerodynamic performance, due to an increased leakage flow through its scalloped backdisc.

Alignment of a turbocharger turbine rotor hub with its housing can affect aerodynamic performance of the turbine, and studies may have been done in industry to quantify this effect. However, there is no related publication in open literature and so the effect and the underline mechanism remain trade secrets. A numerical investigation into the effect and the mechanism was carried by current authors using two turbine wheels with identical flow passage but different backdisc configurations: one with a full backdisc and the other a deeply scalloped backdisc. For each configuration, five alignments of the wheel hub with housing were studied, these include a full alignment of the rotor hub and the housing and four misalignments of the two turbine components. Effects of the alignment and turbine operating condition on aerodynamic performance of the turbines were studied. The flow field of the turbines was interrogated, and the underline mechanism that produced the performance changes was investigated. The study yields some interesting results: For the full backdisc configuration, the exposure of the backdisc and the wheel recess from housing both decrease turbine efficiency. It has been found that the exposure of the backdisc in the housing outflow creates a stagnation point at the tip of the backdisc, generating some loss. The stagnation then forces the incoming flow to move away from the hub and toward the shroud side of the wheel, producing further losses within the wheel passage. The high pressure at the tip of the backdisc also reduces the flow entering the backdisc cavity between the wheel and heat shield, thus decreases the loss in the cavity. However, this loss is relatively small compared with the loss in the main flow passage. It has also found that the misalignment has greater negative effects on aerodynamic performance of the turbine with deeply scalloped backdisc, and a stronger leakage flow from the backdisc cavity into rotor passage occurs due to the opening in the backdisc by the scalloping. Turbine operating condition has an effect too: lower U/C condition shows higher impacts of misalignment than higher U/C condition. These results are reported and analysed.