Abstract
Flow mechanisms suppressing the flow separation in two diffusers, a low-solidity cascade diffuser and a vaned diffuser with additional small vanes near the inlet, were compared mainly by numerical simulation. As the superiority of the low-solidity cascade diffuser was expected, a series of experiments was conducted using a transonic centrifugal compressor with a maximum pressure ratio of 7. The performance of the compressor with the vaned diffuser was comparable to that of the low-solidity cascade diffuser only between the surge point and the design flowrate at a pressure ratio of 3.5. The maximum flowrate of the vaned diffuser was lower than that of the low-solidity cascade diffuser. At higher rotational speeds, the pressure ratio at the surge point, the efficiency, and the flow range of the low-solidity cascade diffuser exceded those of a vaned diffuser at a pressure ratio of 3.5.
Highlights
Flow mechanisms suppressing the flow separation in two diffusers, a low-solidity cascade diffuser and a vaned diffuser with additional small vanes near the inlet, were compared mainly by numerical simulation
When the damper opening was 40%, the surge flow rate decreased by 35% and the maximum head rise increased by 15%
A rotating stall occurred just near the surge flow rate at both damper openings. These results suggested that the added small vanes did not fully suppress a flow separation in the diffuser channel
Summary
As a flow distortion is stronger on the shroud side than on the hub side, small vanes were added near the diffuser’s vane inlet on the shroud side This diffuser had a remarkable effect in a highsubsonic, centrifugal compressor with a pressure ratio of 2, with a suction damper (Nakagawa et al, 1995). A rotating stall occurred just near the surge flow rate at both damper openings These results suggested that the added small vanes did not fully suppress a flow separation in the diffuser channel. In the first half of this article, the flow mechanisms suppressing the flow separation of two diffusers, a low-solidity cascade diffuser (called an SVD in this paper, for simplicity) and a vaned diffuser with small vane added near the vane inlet (called a VDA), were compared in experiments using a high subsonic. To investigate the flow distribution at the surge limit, static pressures were measured on the shroud sides of both diffusers
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