Influence Of The Tip Clearance On The Matching Of A Multistage Axial Compressor

The matching of a multistage axial compressor is essential for the efficiency and the stability of a compressor.In this study the matching of a multistage compressor is tackled with the means of a tip gap variation.The tip clearance is not only an important parameter during the blade design process,but it is also subject to changes in the transient operation of the compressor.This research features a numerical investigation of a 5-stage axial compressor using the software ANSYS CFX.Six tip gaps ranging from 0% to 3% relative tip gap are chosen to evaluate the influence the gap size has on the multistage compressor and the matching.The efficiency decrease with more than 0.2% decrease in isentropic efficiency per 0.1% tip gap increase is found to be larger than reported in literature,which can be attributed to the investigation of a whole compressor in contrast to the investigation of a single stage.The front stages are progressively moved closer to the stall region with increasing tip gap,stage 1 heavily contributing to the efficiency decrease with losing 10% polytropic stage efficiency from 0% to 3% tip gap.A quantitative method for evaluating the matching from the efficiency point of view is developed.Application of this quantitative measure shows that the matching of the compressor under investigation is best for medium tip gaps.This quantitative measure discloses the potential of a better matching,from 0.31% isentropic efficiency at 0% tip gap to only 0.10% at 1% tip gap.Without tapping the full potential of improving the matching by modifying blade stagger,0.3% compressor efficiency increase with a simultaneous increase in stall margin are achieved in a few simulation attempts.With increasing tip gap the compressor characteristic is moved to lower mass flow values.Eliminating this mass flow dependency from the results for a single stage,it is found that the stage efficiency decrease for increased tip gaps reaches almost the same values irrespective of the mass flow.Simultaneously the split between rotor and stator influence with increasing tip gap is found to have a ratio of 1:1 for a rear stage.A relation of the change of incidence in the tip region can only be established with the mass flow.An attempt at redesigning the leading edge towards improved incidence values confirms that the incidence in the tip region bears no strong link to the stage efficiency.