Abstract
The flow field at the rotor exit of a low aspect ratio axial flow fan for different tip geometries and for different flow coefficients is measured in the present study. The following configurations are tested: (1) rotor without partial shroud, designated as rotor (wos), (2) rotor with partial shroud, designated as rotor (ws), and (3) rotor with perforated (perforations in the shape of discrete circular holes) partial shroud, designated as rotor (wps). From steady state measurements, the performance of rotor (wps) is found to be the best. Both the rotors with partial shrouds have stalled at a higher flow coefficient compared to that of rotor (wos). From periodic flow measurements, it is concluded that the low velocity region near the tip section is considerably reduced with the use of partial shrouds with perforations. The extent of this low velocity region for both rotor (wos) and rotor (wps) increases with decreasing flow coefficient due to increased stage loading. This core of low momentum fluid has moved inwards of the annulus and towards the pressure side as the flow coefficient decreases. The extent of the low momentum fluid is smaller for rotor (wps) than that of rotor (wos) at all flow coefficients.
Highlights
Leakage of flow around the tip of unshrouded rotor blades is inevitable in turbomachines
The performance of a fan or compressor is sensitive to the tip region geometry and flow in this region, as it affects the flow over much of the blade span
A significant fraction of the end wall losses is attributed to the tip leakage flow and its interaction with the secondary flow, blade boundary layer, annulus wall boundary layer, and the scraping vortex
Summary
Leakage of flow around the tip of unshrouded rotor blades is inevitable in turbomachines. It is well known fact that the end wall losses comprise a substantial proportion of the total losses, in low-aspect-ratio turbomachines. A significant fraction of the end wall losses is attributed to the tip leakage flow and its interaction with the secondary flow, blade boundary layer, annulus wall boundary layer, and the scraping vortex. The dominant source of leakage loss is due to the mixing of the leakage vortex with the primary flow, which has passed through the blades. Most of the earlier research was confined to understand this flow field in large-aspect-ratio rotors (Hunter and Cumpsty [1], Lakshminarayana et al [2], Inoue and Kuroumaru [3], and Lakshminarayana et al [4]). A comprehensive review of the investigations on the turbomachinery tip clearance effects is given by Peacock [5]
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