# Design of a Compressor Test Rig for Immobilization of the Stall Cell
## 1 Introduction  
The design and stable operation of axial compressors at a given shaft speed are limited by stall cells, which have been extensively studied as the predominant flow instability phenomenon in compressor systems over the past four decades. Notably, [PERSON] et al. [1] were among the first to propose a propagation mechanism for stall cells. These cells typically create blockages that result in increased incidence ahead of the cell and decreased incidence behind it, thereby facilitating stall recovery in adjacent passages. Furthermore, subsequent researchers have conducted non-linear analyses of stall cell propagation [2, 3]. Since then, most studies have focused on understanding and mitigating stall inception due to its practical relevance for industrial applications. The stall cell was initially considered to be a stagnant region with a wake downstream. [PERSON] and [PERSON] [4] conducted the first comprehensive velocity measurement of rotating stall cells in axial compressors, revealing their highly active and three-dimensional nature with significant circumferential velocity. In their subsequent publication, [PERSON] and [PERSON] [5] developed a model to correlate the propagation speed of the stall cell with pressure changes at its trailing edge. [PERSON] and [PERSON] [6] reported measurements from a three-stage axial flow compressor, while [PERSON] and [PERSON] [7] presented 3D velocity vectors of a rotating stall cell for the first time. [PERSON] et al. [8] performed a 2D CFD simulation on post-stall behavior in compressors. [PERSON] et al. [9] presented experimental results from a two-stage axial flow compressor, indicating that the axial dimension of the stall cell is significantly larger than its circumferential dimension, suggesting that measurements or computations should cover both far upstream and downstream regions for post-stall flow investigations. [PERSON] et al. [10] presented results using both experimental and computational methods for an axial flow transonic fan. One useful piece of information from these results concerns the dimension of the cell. The cell extended upstream of the rotor by 4-5 axial chords based on the production of entropy. [PERSON] et al. [11], through numerical simulations on an entire annulus of an axial flow compressor stage,demonstrated that while CFD tools are effective in predicting stall occurrence, they may not accurately capture detailed flow characteristics. Therefore, elaborate measurements are still necessary to fully understand the complex flow structures within a stall cell. [PERSON] et al. [12], using PIV techniques on an annular cascade configuration, detected stall-like flow structures propagating from the pressure side to the suction side of one blade; this finding further supports [PERSON] et al. 's explanation regarding stall cell propagation. [PERSON] et al. [13] proposed a computational method for rotating stalls and surge transients in an axial compressor. [PERSON] et al. [14] researched the physics of pre-stall propagating disturbances in axial compressors. The measurement of flow within a rotating stall cell poses significant challenges due to its highly three-dimensional and unsteady nature. The primary source of unsteadiness in the cell is its rotation, while the secondary source is the passage of blades through the cell. Additional sources of unsteadiness may include large vortices and turbulent eddies, among others. [PERSON] et al. [15] employed the large eddy simulation (LES) to computationally investigate rotating stalls in a pump, revealing that blade-passing-induced fluctuations are considerably lower in the stalled region compared to the un-stalled region. This observation suggests that blades are not crucial within a stall cell. Several test rigs have been documented in the literature for investigating the structure of a rotating stall flow [16; 17; 18; 19; 20; 21].