چكيده به لاتين
The predictions of the onset of rotating stall and surge are very important in the preliminary design stage of a compressor. Rotating stall and surge are complex instabilities that cause efficiency loss and reduced pressure rise, and, therefore, compressor designers attempt to avoid them in the design stage. There are many criteria for predicting stability limits, including empirical, theoretical, and numerical investigations in the literature. However, these investigations have important limitations.
In the first part of the thesis, the present investigation establishes a new method in which the stall and post-stall behavior of a compressor is estimated by an equivalent reconstructed compressor using special combinations of single passage flow behavior in different mass flow rates. The combinations are generated such as pre-stall, in-stall, and surge flow regimes are reproduced in the full annulus compressor. The numbers of one to eight stall cells are reproduced in the full-annulus compressor. The method requires the least computational requirements and is time efficient. In the second part of thesis, the effects of two modifications to a vaned recessed casing treatment is investigated numerically. First, the shape of a circular curve was used in the top of the treated casing. Second, a fully curved guide vane was also applied. The goals of the modifications are to enhance flow recirculation as well as to relieve the low-speed flow, which is normally accumulated within the corners of the vaned recessed casing treatment. The solid casing in addition to two vaned recessed configurations with 23.2% and 53.5% rotor blade tip axial chord exposure have been studied numerically. In the third part of the thesis, the unsteady characteristics of the modified vaned-recessed casing treatment with 23.2% rotor blade tip axial chord exposure were studied numerically.
The solid casing and casing treatment configurations were simulated using the Unsteady Reynolds-Averaged Navier–Stokes equations (URANS), and the results were validated by experimental results. The time-averaged results are composed of velocity components analysis and velocity triangles which are presented first, and the unsteady results are presented afterwards. The standard deviation and frequency analysis were performed to find the sources of unsteadiness. Furthermore, the velocity components analysis, including velocity triangles, was presented instantaneously to clarify their effects on rotor tip flow fields as well as stall margin improvement. Moreover, the unsteady interactions between the rotor and casing treatment flow fields, including flow structure and pressure distributions, were discussed. In the end, flow streamline patterns, in addition to the physical mechanism of the vaned-recessed casing treatment, were also discussed.
The results indicate that secondary flow total energy and spectral entropy are indeed correlated with compressor operating conditions. The predictions of the onset of stall and surge for the investigated compressor show good agreement with the experimental data. The results also indicated that two mechanisms are involved in the stall margin enhancement. First, the circumferential pressure gradient is reduced for both configurations. The reduction in pressure gradient largely reduces the development of tip leakage vortex and, thus, the generation of low-speed fluid is diminished. Second, the main flow/tip leakage interface moves toward downstream and the movement of interface toward the leading edge is delayed. The second configuration with a greater rotor blade tip exposure enables extra flow recirculation due to decreasing surface area and, therefore, could be superior to the application of the first casing treatment configuration. The unsteady results also showed that unsteadiness plays an important role in the flow mechanism and cannot be ignored. The unsteadiness increases as the mass flow is reduced toward the stall/surge condition. Moreover, the analysis of velocity components demonstrated that the casing treatment has distinct behavior at the last operating points before the onset of the stall for solid casing and casing treatment configurations in terms of axial velocity change.
