Session: 31-06 Tandem Design

Paper Number: 129574

129574 - Investigations on Unsteady Flow Structure Formation in Tandem Bladed Axial Flow Compressor Stage 

The axial compressors suffer from the risk of flow separation upon increasing the loading beyond a certain limit due to increased boundary layer thickness on blade surface. The aerodynamic loading of axial compressors can be enhanced by implementing tandem configuration on rotor. The tandem configuration re-energizes the stalling boundary layer over blade suction surface as well as increases the overall loading of the blade. This increase in loading comes at the cost of increased blade row interaction between rotor and stator which impacts stage stability and losses. This study explores the unsteady rotor-stator interaction at design and peak pressure rise operating point of an experimental tandem bladed axial flow compressor. The axial compressor stage consists of highly loaded tandem bladed rotor and single bladed stator fine-tuned to match the dual wake rotor exit flow. The stage has a high mean flow coefficient of 1.05 and design pressure rise of 1400 Pa. An experimental study for performance map along with detailed sector- annular measurements has been conducted before the unsteady simulations. The unsteady simulations have been fine-tuned with actual boundary conditions observed during the experiments. The tandem configuration results in impingement of dual wakes emanating from the rotor on the downstream stator resulting in unconventional flow structure on the stator entry. The steady state simulations predict the overall flow field but important rotor-stator aerodynamic interaction is undermined due to numerical averaging at the interface plane. The experimentation analysis though real, is limited to finite number of measuring stations for detailed analysis. Important flow interactions within the rotor and stator passage is difficult to access in experimental program due to limitations of probe placement and associated technicalities. Hence a detailed unsteady numerical investigation has been carried out to explore the flow structure at design and peak pressure mass flow rate. The unsteady computations are performed by blade row transformation methods using ANSYS CFX. The flow structure exhibits remarkably different physics during design point and peak pressure rise operating conditions. The rotor is designed strategically with varying loading from hub to tip. The effect of radially varying loading is pronounced on the unsteady aerodynamics of stage. The study is carried out at 60% axial distance between rotor and stator. The tandem configuration leads to formation of dual wakes from the rotor. The rotor wakes are observed to mix out midway distance before impinging on the stator at design point in the lower to mid-span region. The mid to higher span regions are observed to have distinguishable dual wake structure till LE of the stator. This radial variation in diffusion of wakes leads to highly three dimensional flow structure at the stator entry. The flow through the tandem nozzle affects the development of boundary layer on rotor surface which further complicates the rotor passage flow field. The high aerodynamic loading on rotor result in strong tip leakage flow and its associated interaction with endwall. The cumulative effect of semi-diffused rotor wakes and tip leakage complexities result in highly non-uniform flow entry to the stator. The aforementioned flow interactions are observed to have qualitative and quantitative variations for design and peak pressure rise operating conditions.  Additionally, the peak pressure rise operating point depicts strong back pressure oscillations due to increased loading. The spectral analysis of unsteady data gives detailed insights into pressure oscillations. The study describes the flow physics behind this unique stage configuration using computational and experimental evidences as a whole.

Presenting Author: Ajey Singh Indian Instiute of Technology Khargapur

Presenting Author Biography: Ajey Singh is a PhD Research Scholar at Department of Aerospace Engineering, IIT Kharagpur.

Authors:

Ajey Singh Indian Instiute of Technology Khargapur
Chetankumar Mistry Indian Institute of Technology Kharagpur