Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator
Axial compressors in aero engines are prone to suffering a breakdown of orderly flow when operating at the peak of the pressure rise characteristic. The damaging potential of separated flows is why a safe distance has to be left between every possible operating point and an operating point at which stall occurs. During earlier investigations of stall inception mechanisms, a new type of prestall instability has been found. In this study, it could be demonstrated that the prestall instability characterised by discrete flow disturbances can be clearly assigned to the subject of “Rotating Instabilities”. Propagating disturbances are responsible for the rise in blade passing irregularity. If the mass flow is reduced successively, the level of irregularity increases until the prestall condition devolves into rotating stall. The primary objective of the current work is to highlight the basic physics behind these prestall disturbances by complementary experimental and numerical investigations. Before reaching the peak of the pressure rise characteristic flow, disturbances appear as small vortex tubes with one end attached to the casing and the other attached to the suction surface of the rotor blade. These vortex structures arise when the entire tip region is affected by blockage and at the same time the critical rotor incidence is not exceeded in this flow regime. Furthermore, a new stall indicator was developed by applying statistical methods to the unsteady pressure signal measured over the rotor blade tips, thus granting a better control of the safety margin.
Evolution of the flow instabilities in an axial compressor rotor with large tip clearance: An experimental and URANS study
Abstract With modern engine designs trending toward smaller cores to increase propulsive efficiency, the gap-to-span ratio in compressors is expected to increase, especially for the rear stages. Detailed knowledge of stronger tip clearance flow and its impact on compressor instability under large clearance condition become increasingly important. The present study is aiming for illuminating the instability evolution and the underlying flow physics in a large-tip-gap compressor, with the use of casing-mounted pressure transducers to acquire the unsteady nature of tip flow, and full-annulus URANS to obtain the three-dimensional flow details related to instability generation. Results show that flow instability evolution of the compressor along speed-line experiences three stages: stable state, rotating instability, and rotating stall. Two critical behaviors of tip leakage vortex (TLV) are found to relate to the transition of instability pattern. As TLV moves to the adjacent blade trailing edge, it starts to oscillate under the interaction with the adjacent blade, leading to a short-length rotating disturbance, i.e. rotating instability. At near-stall condition, as the interface of TLV and main-flow exceeds the passage inlet plane, forward spillage occurs along with radial vortexes periodically shed from blade leading edge. The periodically generation, movement and decay of leading edge vortexes (LEVs) constitute an orderly propagating disturbance rather than evolve into a stall cell. However, as the compressor is further throttled, the orderly propagation of LEVs collapses due to their scattering, resulting in the generation of stall cells with local stronger blockage and higher entropy.
Venus: Evolution of the Upper Atmospheric Clouds
Abstract The large number of UV photographs taken by different observatories throughout the world are grouped for analysis at the IAU Planetary Photograph Center of Meudon Observatory. Three horizontal V, Y, or psi-shaped dark-hued cloud features are usually aligned along the equator and move 110 m s−1 westward in a planetary-wide rotation. The more intense and distinctly V-Shaped features last several weeks. The smaller scale cloud structures usually show significant changes after each successive rotation in 4 days around the planet. The average UV contrast is 32% but can fluctuate from disappearance to 37%. For periods of several years, the polar areas are sometimes covered by a white cloud. According to a set of photographs taken at Pic-du-Midi, this never occurred between 1962 and 1966 but has happened during 25% of the time for at least one pole since 1967, and only when the planet was in a given half of its orbit. These polar clouds are ephemeral and usually last several weeks or months; they evolve...