Effects of partial foam metal casing treatment on fan stability

The application of casing treatments is a widely recognized method for improving the stall margin of fans and compressors. However, dimensional constraints in modern aero-engines, particularly in designs with S-duct or boundary layer ingesting inlets, limit the use of treatments requiring bulky auxiliary structures. This paper investigates the effectiveness of a circumferentially partial foam metal casing treatment (PFMCT) combined with an adjacent circumferential cavity. The study is conducted experimentally on a low-speed, single-stage axial flow fan under both uniform and distorted inlet conditions. The effects of varying the circumferential coverage of the foam metal and the influence of the relative angular position between the PFMCT and a 180-degree inlet distortion are systematically examined. Results indicate that PFMCT provides a significant stall margin improvement, which is further enhanced by the presence of the circumferential cavity, especially under uniform inflow. It is found that unlike full-annulus casing treatments, especially the ones with annulus back-chamber, the partial configurations do not alter the inherent spike-type stall inception of the fan. The stabilization mechanism is attributed to two distinct functions: the foam metal section is primarily responsible for dissipating unsteady disturbance energy and reducing the blade tip's loading, while the circumferential cavity contributes by achieving a more uniform steady-state circumferential load distribution. Furthermore, stability enhancement is shown to be highly sensitive to the relative circumferential position of the casing treatment and the inlet distortion zone.