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1、COMPARISON OF TWO METHODS TO INCREASE TIP CLEARANCE AND ITS EFFECT ON PERFORMANCE OF TURBOCHARGER CENTRIFUGAL COMPRESSOR STAGEAbstract: Tip clearance between th; ble tip and casing of a centrifugal compressor can be varied through two methods: by changing the hijde height (MP or by cnanging the casi
2、ng diameter (M2). Numerical simulations are carried out to cv.p&ie Uiese two methods and their effect on the stage and impeller performance. The i.T,peiier and diffuser are connected through rotor stator boundary using frozen rotor approach. Overall stage performance and the flow configuration have
3、been investigated for nine tip clearance levels from no gap to 1 mm. Impeller and difruser performances are also presented separately. It has been found that the overall and impeller performance are comparatively better for Ml below tip clearance of 0.5 mm whereas M2 is found advantageous above 0.5
4、mm of tip clearance. Both Ml and M2 show performance degradation with the increase in tip clearance. Two models have been proposed for the stage total pressure ratio and efficiency, which are found to be in agreement with experimental results. The impeller efficiency and the pressure ratio are found
5、 to be maximum at tip clearance of 0.1 mm for both the cases however minimum diffuser effectiveness is also observed at the same clearance level. Difruser effectiveness is found to be maximum at zero gap for both cases. As it is practically impossible to have zero gap for unshrouded impellers so it
6、is concluded that the optimum thickness is 0.5 mm onwards for Ml and 0.5 mm for M2 in terms of difruser effectiveness. Mass averaged flow parameters, entropy, blade loading diagram and relative pressure fields are presented, showing the loss production within the impeller passage with tip clearance.
7、 Key words: Centrifugal impeller Diffuser Stage Tip clearance Numerical simulation Entropy0 INTRODUCTIONThe flow structures within the centrifugal compressors are considered amongst the most complicated and convoluted in all turbomachinery. In recent past paramount advancement in the performance of
8、the centrifugal compressor has been made primarily because of the computer aided design and analysis techniques that are cautious combination of empirical correlations and extensive modeling of the flow physics.Unshrouded centrifugal compressors are mostly favored compared to shrouded compressors in
9、 order to avoid high stresses involved with the increased weight. As a result the leakage flow through the tip clearance between the blades and casing is inevitable that further complicates the flow and may depreciate the overall performance of the centrifugal compressor. There are two unique and ev
10、enly significant aspects of the tip clearance flows as suggested by DENTON, et alm. First, the reduction in the blade force and the second foremost aspect is the mixing of flow through the tip clearance gap with the flow between blades. The interaction of tip clearance flow field, blade vortex flow
11、and leakage vortex flow generates an extremely complex flow structure.In recent past, a number of numerical and experimental investigations have been conducted26 to investigate the effect of tip clearance in unshrouded compressors. DANISH, et al2), numerically investigated the effect of tip clearanc
12、e on the performance and flow characteristics of a centrifugal impeller. Entropy fields and the secondary flow development were presented showing the loss production within the impeller passage. No optimum clearance was found for all simulated results except no gap level. USHA, et alt3, numerically
13、predicted that the performance was degraded with the increase in tip clearance. GAO, et al41, used heir own computation fluid dynamics (CFD) code to investigate the effect of tip clearance on 3D viscous flow field and performance of NASA LSCC impeller with a vaneless diffuser. The study indicated th
14、at the location of the throughflow wake was influenced by the tip clearance and there probably exist an optimal clearance at which flow loss was minimum. Their simulations indicated that the optimum clearance was about 0.9 percent of the blade height. EUM, et al5, numerically studied six clearance l
15、evels. The effect was decomposed into inviscid and viscous components using one-dimensional model expressed in terms of the specific work reduction and the additional entropy generation. Both inviscid and viscous effects affected performance to similar extent, while efficiency drop was mainly influe
16、nced by viscous loss of the tip leakage flow. Performance reduction and efficiency drop due to tip clearance was proportional to the ratio of tip clearance to blade height. The proposed 1D model was found to be in close agreement with the experimental results. HONG, et al61, experimentally measured the discharge flow of a centrifugal compressor at six levels of tip clearance. The study found an optimum tip clearance ratio of 0.12 in terms of surge margin,
