Study On Unsteady Flow Caused By Non-axisymmetric Tip-clearance In Turbine

With the widely use of large rotating machines in industry, steam excitations have been becoming more widespread. It is clear that steam excitations are caused by unsteady flow, which can generate force acting on rotor of the machine. However, there is more than one reason inducing unsteady flow, this study is on steam excitations caused by non-axisymmetric rotor-tip clearance.4 kinds of models with different tip clearance and shroud ring are constructed for this study. With the help of High Performance Computing and Communications (HPCC), unsteady flow fields in 4 different 3D models are successfully simulated. Subsequently, coordinate transformation and contrastive analysis have been used to study its (tip clearance) affect on unsteady flow field and Alford force. Additionally, Alford force can induce rotor-whirl, which may cause instability for the rotor system.This study indicates results as the following:(1) In the small tip clearance, pressure distribution in the flow field is higher than the one in the large tip clearance. And when eccentricity becomes greater, the pressure will have more differences between small tip clearance and large tip clearance.(2) The results indicate that flow field is of the aggregation of sinusoidal distribution, especially on the face of Rotor-top. Besides, it can also be of the aggregation of conic curve on face of Rotor-high near the outlet location.(3) The Alford force consists of two parts: the pressure force part and viscous force part. In this study the pressure force part is much higher than the viscous force part, and both of them will induce rotor forward whirl. After resolving Alford force into two components in Y axis and Z axis, it shows force in Z axis will induce forward whirl and force in Y axis have the ability to make the eccentric rotor back to the concentric position.(4) The Alford force increases linearly with increased eccentricity.