Abstract
To address the situations where the casing treatment needs to be used to stabilize axial compressors through strong recirculation, this paper initiated a CFD study to investigate how the flow could be suitably controlled in the casing treatment to minimize the efficiency penalty and increase the flow range. A counter-swirl self-recirculation casing treatment was first designed on a low speed axial fan rotor as a baseline case. Then three different slot positions and the influence of including the noise baffle were numerically studied. Based on the understanding of their coeffects, the shorter noise baffle was considered and it was found that the highest efficiency was achieved in the case of the upstream slot when the length of baffle was suitably adjusted to balance the incoming flow and recirculation. The largest flow range was achieved by locating the slot at the most downstream position and using a 50% length baffle since it suitably controlled the recirculating flow and relieved the separation at the low-span region. An optimization study showed that the optimum length of the baffle for efficiency was always larger than for the flow range. Both of the two optimum values reduce as the slot moves downstream.
Highlights
In aeroengine and other applications where a wider flow range is required, casing treatments are commonly employed on compressors for their cost-effectiveness in improving the system stability
A low speed axial fan-rotor was chosen for this study, which follows the geometry used in reference [14]
Since the stall flow was judged by a strict criterion which took into account the outlet static pressure rise and high entropy is associated with lower pressure, the reason why CTD,baffled resulted in improved Δm compared to CTD should lie in its suppression of the upstream extent of the recirculation flow and the corresponding reduced loss near the stall point
Summary
In aeroengine and other applications where a wider flow range is required, casing treatments are commonly employed on compressors for their cost-effectiveness in improving the system stability. The concept of casing treatment is not new and the earliest experiments date back to the 1960s when the NASA Lewis Research center conducted tests of bleeding and blowing through the casing for their influences on the stall range of a high-speed single stage axial-flow compressor [1]. The recent research of Guinet et al [9] employed a 1.5stage transonic test rig and a carefully designed recirculation duct to study the influence of the tip gap on the stall margin improvement and efficiency penalty of the casing treatment.
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