Session: 01-04-01 Fluid Machinery Symposium

Paper Number: 86773

86773 - Effects of Tip Clearance on Internal Flow and Loss Generation Mechanism in an Axial Compressor at Windmilling Conditions 

The internal flow structure and loss generation mechanism of a single-stage compressor at windmilling operation were investigated by experimental and computational analyses. The windmilling state occurs when air flowing through an unlit engine drives the compressor rotor blades, similar to a turbine. This phenomenon applies mostly to aircraft engines, where it is caused by ram pressure. This research focused on two windmilling operations: free windmilling (FW) and highly loaded (HL) windmilling. In the case of FW, the net work performed by the rotor blades to the fluid is canceled out, and the rotor is in an idle state. In the HL windmilling condition, the work performed to the rotor blades by the fluid increases, the compressor acts as a turbine, and power is generated. In the windmilling state, the large total pressure loss was generated. The main factor of the total pressure loss under windmilling operations were influenced by the tip leakage flow and the separation due to the highly negative incidence.

The study focused on the effect of tip clearance size on the internal flow at windmilling operations. Therefore, the detached eddy simulations were conducted at three tip clearance conditions, which were the without clearance (nonCL), the design clearance (DC) and the wide clearance (WC). As a result, the total pressure loss was increased when the size of tip clearance was increased at design point. On the other hand, the total pressure loss was decreased when the size of tip clearance was expanded at HL. In the HL windmilling condition, the separation on the pressure side due to the highly negative incidence interfered in the leading-edge vortex, then it was the main reason of the total pressure loss in the mid span. When the size of the clearance was increased, the size of separation was decreased because of the tip leakage flow. Thus, the total pressure loss was decreased. According to the detailed numerical results in the windmilling state, the tip leakage flow affected the internal flow field not only near the tip area, but also whole span. In addition, the area of the highly total pressure loss in the mid span shifted to the hub side because of a blockage due to the tip leakage flow.

Presenting Author: Tomohiro Inada Waseda University