The turbocharger, with its advantages in increasing fuel economy and reducing emission, has been widely used in internal combustion engines. Aiming to achieve both fast transient response and high boost at low engine speeds of the diesel engines, variable nozzle turbine (VNT) turbochargers are necessary to match well with the engines in various operating conditions. At different engine speeds, both the mass flow rate and expansion ratio of the exhaust gas could be controlled by changing the opening of the guide vanes to adjust the nozzle throat area. At low engine speeds, the small opening of the VNT nozzle can accelerate the exhaust gas, resulting in the increase of the compressor boost pressure. The large opening of the VNT nozzle is capable of avoiding the excessive boost pressure at high engine speeds.

The high exhaust gas pressure accompanying the small opening of the guide vane at the engine braking condition leads to shock waves. Loading fluctuation on the surface of the rotor blade, in response to the shock wave at the variable guide vanes, could increase the high cycle fatigue (HCF) risk of the rotor blade significantly. In this study, the generation and weakening mechanism of the shock wave in the VNT have been investigated by adopting three guiding vanes with different span chord ratios. Numerical simulation indicates that two shock waves are produced in the geometry throat and the trailing edge of the guide vane, named throat shock wave and trailing edge shock wave, respectively.

The trailing edge shock wave at mid-span area of the rotor-stator interface has the largest intensity, and it is gradually weakened towards both the rotor blade leading edge and the guide vane trailing edge. As the distance between the guiding vane and rotor blade changes, the trailing edge shock wave also varies in intensity, shape, and relative position to the blade, while the throat shock wave has no variations. Further analysis shows that the generation of the shock wave is mainly due to the excessive acceleration of the exhaust gas. Reduction of the gas expansion ratio across the variable guide vane and the channel shrinkage degree between the nozzle vane trailing edge and the rotor blade can eliminate the throat shock wave and weakens the trailing edge shock wave. As the trailing edge shock wave weakens, pressure fluctuation inside the rotor blade can be reduced, which will significantly reduce the turbine blades forced response and enhance the reliability of the VNT turbine.

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