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.
Influence of Mach Number and Aerodynamic Loading on Rotating Instability in an Annular Compressor Cascade
An experimental study was conducted to investigate the influence of Mach number and aerodynamic loading on Rotating Instability (RI). Investigations were performed on an annular compressor cascade with radial clearance at the hub. In addition to steady flow field measurements, time resolved measurements of static wall-pressure at the hub, close to the tip clearance, were performed. For a specific range of incidence angles, RI could be detected at all flow speeds (Ma = 0.2–0.7). Shortly before the inception of RI, a sudden increase in pressure loss coefficient and outflow angle was detected by steady flow field measurements. The time resolved measurements showed an abruptly broadband increase of amplitude in the frequency range, in which RI occurs at higher loadings. RI propagates with 25–33% of the circumferential component of the upstream flow speed. Furthermore it is possible to scale the power spectral density function of the unsteady data with Strouhal number.Copyright © 2012 by ASME
Laminar heat or mass transfer in rectangular channels and in cylindrical tubes for fully developed flow: comparison of solutions obtained for various boundary conditions
Abstract A comparison is made of results obtained with various boundary conditions at the wall of the channels. By comparing blood Sherwood numbers for finite wall resistance with those for constant concentration boundary condition only a small variation is found for various points along the blood channel.