| As an effective method to extend the stability range of axial compressor, circumferential groove (CG) casing treatment has always been the focus of researches. The axial location of CG is the key factor for enhancing the stability range, as it is directly related with the tip region flow. The researches on single groove in recent years demonstrate the two peaks of stall margin improvement (SMI) as the groove’s axial location is changed, which is difficult to be captured by numerical simulations. With this regard, the single circumferential groove with various axial locations is mainly studied in experimental method in this dissertation. A new stall inception with relatively long-scales disturbance is revealed for the groove which locates between the two peaks of SMI but shows almost no SMI. Then the tip region flow with groove treatment is further studied. In order to study the influence mechanism of the groove, a special method is designed to measure the flow velocity on the opening surface (OS) of the groove, which is seldom reported in public literatures. The different influence mechanism on tip region flow for the grooves locating at different axial locations is then discussed. At last the double grooves are also discussed on basis of single groove. This dissertation consists of four main parts:1. Change of stall inception when treated by single grooveThe results of dynamic measurements on the casing indicate it is the typical spike-type stall inception for both smooth casing (SC) and the groove CG7 which provides high SMI, while it is relatively long-scaled stall inception for CG5 which shows almost no SMI. The difference of stall inception between CG5 and CG7 implies the tip region flow may be affected by CG5 in another method which is different from CG7.2. The influence of single groove on tip region flow and the flow features on its opening surface (OS)The dynamic measurements on casing and at rotor outlet are both perfomed to study the flow features in tip region with single groove treatment. The different influences between the two grooves on tip region flow are demonstrated:the tip leakage flow (TLF) is strengthened by CG5 and moved from the casing into the tip region, while it is suppressed by CG7 and still stays close to the casing at near stall condition.In order to discuss the groove’s influence mechanism, the flow features on the opeing surface (OS) between the groove and blade passage are discussed. A special measuring method by hot-wire probes is applied in this dissertation. It is shown that more intense groove-passage interaction is found on the OS of CG7 than that of CG5, and the measuring inside CG7 shows the attenuation trend of this interaction. These experimental researches on OS are seldom seen in public literatures, and to some extent the relationship between the groove-passage interaction and SMI is verified by these results.3. The influence mechanism of the single groovesThe influence mechanism of CG5 and CG7 are further analyzed. Results by phase locking averaging (PLA) demonstrate a peak of flow velocity with high fluctuation exists on the OS of CG5 when compared to that of CG7, implying the TLF enters CG5 and is strengthened. Then the long-scaled stall inception occurs due to the flow blockage caused by this phenomenon. Then the difference of the influence mechanism between the two grooves are discussed:for CG5 with no SMI, the strengthening of TLF results from the variation of blade loading and the relative position between CG5 and the trajectory of TLF; for CG7 with high SMI, the TLF reaches the pressure side of the neighboring blade before entering the groove, and it is then suppressed by this groove.4. Further discussions on double grooves treatmentBased on CG5, the double grooves treatment of CG59 (combination of CG5 and CG9) is discussed. The SMI of CG59 is a little lower than that of CG9, and it is still long-scaled disturbance when stall occurs. The flow phenomenon with high fluctuation similar to the results of single groove CG5 is also found on the OS of CG5 for CG59, but it is weakened by CG9 and the TLF at rotor exit is also suppressed. Therefore the stall inception is still changed by CG5 while it is postponed by CG9.The main innovations of this dissertation are:1) the new stall inception which is different from the usual spike and modal type is experimentally demonstrated, and this is the further understanding about the stall process with CG treatment; 2) the flow features on OS is revealed and discussed in a special measuring method in this dissertation, which is seldom seen in public literatures; 3) on basis of these measurements the different influence mechanism of the single groove on tip region flow when its axial location is changed is studied, especially for the groove which locates between the two peaks of SMI but shows almost no SMI. The work in this dissertation is helpful to both the understanding of the influence mechanism of CG treatment and the verification of numerical simulations. |
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