CFD modelling of leakage flows in low-pressure compressors

Abstract

With the customer’s increasing requirements on engine performance, the optimisation of every engine component is critical to maximize the performance. Therefore, the secondary effects have to be taken into account, to understand where the sources of losses come from and how reduce them. Among the many secondary effects that exist in compressors, leakage flows between stationary and rotating parts are one of the main sources of losses in turbomachinery. The standard method to take into account leakage flows relies on chimera grids of rectangular cavities at the bottom of which a mass flow and a flow direction are imposed. The evolution of the design methods gives rise to boosters geometries with ascending and descending flow path for which the method was not calibrated. Thus, a new advanced modelling of higher fidelity is needed to evaluate leakage flows and to analyse their behaviour. The complete modelling of seal cavities has highlighted that the leakage flow rate was twice overestimated by the standard modelling. Moreover, the advanced modelling increases the compressor performance thanks to its higher fidelity. Near the stall conditions, the higher-fidelity results of the advanced modelling show that the flow behaviour at the stator foot is better than predicted by the standard modelling. However, this result does not allow to conclude if the stator is weaker on the foot or on the head. However, simulations based on the advanced modelling are costly compared to the standard modelling. Therefore, an analytical model based on the Stodola formula has been developed in order to approximate the leakage-flow rate and the windage heating. The calibration of the formula for two engines has shown that the discharge coefficient CD only depends on the geometric shape of the fin head. Finally, a trade-off between the computation cost and the fidelity of the simulations is developed through a simplified modelling of cavities. This modelling was built for further optimisation of the corner-radius shape of the upstream cavity opening. The geometry of the upstream cavity seal is reproduced since it has a critical effect on the compressor performance. This simplified modelling gives faithful results compared to the advanced modelling on the compressor performance.