## 1 Introduction
In recent years, more and more studies have shown that seal cavity leakage flow will obviously deteriorate the performance of axial compressor and turbine in aeroengines, and such influence cannot be ignored. Usually, for every 1% increase in seal-tooth clearance-to-span ratio, the decrease in pressure rise is 3%, and the reduction in efficiency is 1% [14; 15]. As a reference, a usual relationship between efficiency deterioration and blade tip clearance in an axial compressor is that the compressor efficiency decreases by 1.5% when the rotor tip clearance increases by 1% with respect to the span [15; 16; 17; 18]. For a cantilevered stator, the decrease ranges from 1.0% to 2.0% [15; 17; 19]. [PERSON] and [PERSON] [20] investigated the effect of seal cavity leakage flow on 1.5 stages of a 10 stage axial flow compressor. Numerical results indicate 0.86% efficiency reduction for a seal clearance of 1.3% span. [PERSON] et al. [21] have performed a full three-dimensional unsteady calculation for a high-speed six-stage advanced axial-flow compressor. The calculation results with a sealed cavity under the stator blade reduce the efficiency by 1.7% compared with the results without a cavity. Similar to compressors, losses caused by leakage through the shrouded cavity contribute significantly to the overall losses of turbines [22]. [PERSON] et al. [23] compared experimental data and numerical simulations in a three-stage turbine and pointed out that calculation of the leakage flows and cavities is necessary in order to get good agreement between calculation and measurement. The flow in a two-stage LP-turbine at relatively low Reynolds numbers is simulated by [PERSON] et al. [24] with an in-house 3D code. The overall isentropic efficiency derived from computations with cavities lies within 0.2% of the experimental result, and that is 1.3% closer than computations without cavities. Considering the advantages of the two-dimensional method in the simulation of full-aeroengine and the non-negligible influence of the seal cavity leakage flow on the performance of turbomachinery, we have reason to believe that a two-dimensional throughput program considering seal cavity leakage flow can undoubtedly assist in the performance prediction of an aeroengine. Adopting the model of [PERSON] [25] and [PERSON] et al. [26], [PERSON] and [PERSON] [27] introduced the mixing loss, the secondary flow loss and the change of the flow angle caused by seal cavity leakage flow into their throughput program based on a stream function approach. [PERSON] et al. [28] introduced the effect of seal cavity leakage flow into their flow through program based on axisymmetric Euler equations in the form of source terms. Their program can reflect the impact of seal cavity leakage flow on the mainstream, but cannot directly reflect the flow field inside the labyrinth sealed cavity. Our goal is to develop a two-dimensional steady-state simulation software for aeroengine design and quick analysis using a circumferentially averaged method. While the coupled simulation of the main flow and the leakage flow can be conducted for both compressor and turbine stages, this paper focuses on the compressor side. In our previous work [29], the IGV and the first stage of a multistage axial-flow compressor is the research object, on which the grid independence has been verified and the influence of the seal cavity leakage flow on the main flow and its mechanism has been investigated. In this paper, the research object is the IGV plus the first three stages of the same multistage axial-flow. The work of this article can be divided in three parts. First, to investigate the influence of seal cavity leakage flow on the multistage compressor performance. Second, to investigate the influence of seal cavity leakage flow on a certain stage or a certain blade row. Third, to investigate the propagation of this influence in multistage compressor.