Entropy Generation Research Of Flow Loss In Tip Region On Near Zero Clearance Compressor And Its Casing Treatment

The existence of tip clearance causes leakage loss at compressor blade tip. And reducing tip clearance can improve the operation efficiency of compressors. In recent years, the development of the material and craft makes it possible for the clearance between blade tip and casing to reach zero, which can improve the aerodynamic performance of compressors.It is difficult to achieve a tip clearance less than 0.3 mm for traditional materials and craft. In this paper, this hard-to-achieve clearance is called near zero clearance. When the clearance is near zero, the high frequency friction and extrusion effect between rotor and abradable casing will cause wear and tear of the blade and deformation. This article take the ingenious idea of using of circumferential grooves as a major means of casing treatment at near zero clearance to guarantee that the aerodynamic performance is not affected and the wearing area is reduced at the same time.Under the concept of near zero clearance, tip flow stability and flow loss mechanism will face a new challenge, so we need to explore the internal flow loss at near zero clearance. Casing treatments can alter the blade tip load and the distribution of flow loss, so it is necessary to study the casing treatments’internal flow loss distribution at near zero clearance.This thesis was aiming at the flow loss of compressors and their casing treatments at near zero clearance. Other than previous researches using time and space average parameters, this paper used the entropy generation as the parameter to quantify the flow loss, and entropy generation control volume method was put forward to measure the internal irreversible flow loss. The specific internal flow losses were not strictly distinguished and clarified, but the local irreversible flow losses were quantified in different areas of the internal flow field in the compressor, in order to provide initial guidance to the design of casing treatment. The entropy generation research of flow loss in tip region at near zero clearance and casing treatment were carried out on low-speed compressor IET-LAC and typical high-speed compressor rotor J69 were studied the respectively. The main research content could be summarized as follows:1, Using the design clearance of IET-LAC as the research sample, the establishment of the entropy generation analysis method and analysis method were introduced. First IET-LAC was calculated and the model was verified, and the expression of local entropy generation of internal flow was derived. After acquiring numerical solution of compressor internal flow field, the entropy generation analysis method was used to study the corresponding relationship between the efficiency and the entropy generation with the change of clearance. The local entropy generation distribution of the normal clearance (0.6 design clearance, design clearance,1.3 design clearance) were analyzed. Through the detailed division of compressor flow region of space and combining with the flow field structure, the characteristics of flow loss under normal clearance were clarified.2, The local entropy generation of near zero clearances and their casing treatments were studied using the entropy generation control volume method on IET-LAC near zero clearance(no space,0.1 design clearance). Using the design clearance for comparison, the main source of the flow loss of near zero clearance was located with the method of entropy generation control volume method, and the entropy generation was lower at 20%～80% axial chord in blade tip region of near zero clearance. Combining with the flow structure analysis of near zero clearance, it was found that the unique form of leakage flow caused higher efficiency and smaller flow loss at near zero clearance. Four different circumferential grooves of radial dimensions were adopted at near zero clearance, and efficiency could maintain at a high level with circumferential grooves, and the entropy generation decreased at opening surfaces of circumferential grooves. Flow field analysis found that the leakage flow at opening surfaces was the reason that entropy generation dropped. Finally, using the bell-curve control volume method to estimate that the circumferential grooves did not damage the stability of the compressor, but even could improve the stall margin.3, The entropy generation control volume method was applied to the analysis of subsonic rotor J69 of near zero clearances (0.06 design clearance,0.2 design clearance) and their flow loss casing treatment. Using the entropy generation analysis method to study the effect of efficiency and flow loss with the change of clearance on J69, the relation between efficiency and clearance was found to be linear. The entropy generation space distribution of near zero clearance was analyzed with design clearance as comparison, and it was found that the flow losses were concentrated in the leading edge and the central chord of the blade. When clearance reduced, local entropy generation near the leading edge dramatically increased, and entropy generation of middle blade chord decreased. And when tip clearance gradually increased, the two parts of the entropy generation took on the opposite change. Further, the flow field structure analysis showed that the shock wave caused the entropy change at leading edge, and the tip leakage vortex was the reason of flow loss at middle blade chord. Based on the above understandings, circumferential grooves were adopted at leading edge at near zero clearance in order to bring down shock wave loss by increasing local clearance near the leading edge, and by using the entropy generation method to verify the effects of circumferential grooves.Finally, the research content was summarized and the future research work was discussed.