An interpretation of unsteady lift mechanism in highly loaded axial compressors

Focusing on rotor-stator interaction in compressor performance, this paper addresses the question posed by Furber and Williams: “Is the Weis-Fogh principle exploitable in turbomachinery?”-a question for which no clear conclusions or applications have yet been established. Specifically, it investigates the feasibility and potential costs of applying the unsteady lift mechanism, with an emphasis on total pressure ratio (PR) and isentropic efficiency (EF), respectively. Using unsteady numerical simulations, the study analyzes the performance of highly loaded axial compressor stage elements with a contracted flow path, examining the effects of rotor-stator spacing and operating conditions. The results indicate that at reduced rotor-stator spacing, pressure-potential-induced vortex shedding occurs, manifested as the accumulation of negative vorticity from the rotor wake within the stator passage. Under such conditions, a notable increase in PR is observed. Similar to the vortex lift described in the Weis-Fogh mechanism, the strength of this accumulated negative vortex leads to a PR benefit, where a linear positive correlation is observed at small spacing. However, EF does not follow the same trend as PR. Unlike the ideal wake recovery mechanism, EF losses occur at small spacing, particularly near choke, due to the accumulation of another negative vorticity on the stator pressure surface, resulting from a negative jet effect. Under idealized conditions, excluding three-dimensional(3D) effects and geometric constraints, the unsteady lift mechanism can provide a sufficiently obvious PR benefit, e.g., 1.04 for a 10-stage compressor with a design target of 22.6, despite the potential for a decrease in EF.