Analysis of Unsteadiness on Casing Treatment Mechanisms in an Axial Compressor

Abstract

Control devices based on casing treatments have already shown their capability to improve the flow stability in compressors. However their optimization remains complex due to a partial understanding of the related physical mechanisms. The present paper proposes to use a budget analysis of the Navier Stokes equations to support the understanding of such flow phenomena. Based on the original work of Shabbir and Adamczyk (2005), the strength of the present contribution is to generalize the flow analysis method to all Navier-Stokes equations, including unsteady terms. A high-pressure multistage compressor equipped with circumferential casing grooves is chosen to demonstrate the potential of this approach. Steady and unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are solved with a structured multi-blocks solver. Results are then briefly compared to experimental data to validate the numerical method. The analysis of the unsteady axial momentum equation for configurations with and without casing treatment points out some of the mechanisms responsible for the stability improvement. The analysis also indicates that the flow unsteadiness generated by upstream stator wakes (stator/rotor interaction) reduces viscous efforts and increases convective forces, significantly modifying the compressor stability. Finally, the proposed post processing method shows very interesting results for the understanding of circumferential grooves and it should be also used for non-axisymmetric casing treatment configurations.

A Novel Approach to Evaluate the Benefits of Casing Treatment in Axial Compressors

“…Moreover, Legras et al [37] present local flow comparisons between CFD results and LDA measurements done in the rotor tip clearance and inside the CT of CREATE compressor. They proved that the CFD model correctly predicts the main characteristics of the flow in this region and particularly the rotor tip vortex, the flow inside the CT, and the flow interactions between the CT and the throughflow (i.e., radial flow exchanges).…”

“…Due to the radial flow exchanges between the grooves and the throughflow (see [9,37,44] for images of the flow exchanges), CT is expected to contribute to balance the axial momentum equation with additional terms. As previously assumed in the SW analysis, the radial flow exchanges might influence the axial momentum equation through the force Δ(ρW z W r A r ).…”

“…Numerical validation and other modelling details are given, respectively, for (i) the NASA Rotor 37 in Legras et al [8,9], (ii) the CREATE case in Legras et al [37], (iii) the CBUAA in Legras et al [44], Castillon and Legras [42].…”

“…However, for the purpose of the current study, the CT flow analysis is only focused on the grooves implemented in the R2 rotor row. Furthermore, notice that the numerical results are based on Legras et al [37]. Performances are obtained on the overall configuration R1-S3, whereas the CTs implemented in R2 are investigated using modelisation of the S1-R2 configuration.…”

Investigation of groove casing treatment in a transonic compressor at different speeds with control volume method

“…The results showed that the radial transport of axial momentum across the groove and main flow interface is the flow mechanisms for stall margin improvement. On the basis of Shabbir and Adamczyk's research, Legras et al [20][21][22] proposed an extended control volume method, which conducted budget analysis of the Navier-Stokes set of equations. It was concluded that the change of radial velocity induced by the casing grooves was important for the flow stability near the blade tip.…”