Diffuser is a key component affecting the aerodynamic performance of an axial turbine significantly. However, there is little research on the matching parameters between a rotor and a diffuser. In present study, effect of matching parameters which are normalized axial clearance (ACLR‘) and normalized radial size deviation (RDev‘) between rotor and diffuser on the aerodynamic performance and flow loss characteristic of a typical diffuser is revealed. A validated Computational Fluid Dynamic (CFD) model which couples the cavity between the rotor and diffuser is also proposed. The results illustrate that the isentropic efficiency is decreased with the increase of ACLR‘ and RDev‘. The RDev‘ presents a larger influence on isentropic efficiency. When ACLR‘ is increased from 0.21 to 0.43, the isentropic efficiency is only decreased by 0.3%, while the isentropic efficiency reduction of 0.65% is achieved when RDev‘ is increased from 0.091 to 0.192. A “leakage flow”, which is a result of the egress and ingress of the fluid near diffuser-cavity interface, is formed. It presents limited effect on the main flow in the diffuser when ACLR’ is only included and the flow loss near the guide cone surface is increased slightly. However, obvious back flow caused by the “leakage flow” is observed near the diffuser-cavity interface, when RDev’ is included. A flow separation is thus formed downstream and the flow loss in the diffuser is thus increased. The efficiency reduction caused by actual matching parameters (ACLR‘ with 0.21 and RDev‘ with 0.091) is increased with the decrease of expansion ratio. It can be more than 1.26% when the expansion ratio is less than 2.0. An isentropic efficiency reduction of 0.65% is found when different tip clearance are further adopted, and the mixture loss between tip leakage flow and back cavity flow can be neglected. In summary, matching parameters, especially for radial deviation, should be minimized reasonably in further optimization.

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