Diffuser is a key component affecting the aerodynamic performance of an axial turbine significantly. It reduces leaving loss of rotor, decreases the pressure downstream of the turbine blades and increases the power output of the turbine. Existing literatures for the diffuser mainly focus on the influence of upstream flow field parameters, such as tip clearance leakage flow of blade and outlet flow angle of rotor. The installing angle of strut and profile optimization of the diffuser are also conducted in previous researches. However, there is little research on the matching parameters for the assembly of a rotor and a diffuser, and the corresponding effect of cavity between rotor disk and diffuser is also neglected in most literatures.

In present study, a typical diffuser adopted in axial turbine is selected as the research object. The effect of matching parameters which are axial clearance (*A*~clr~) and radial size deviation (*R*~Dev~) between rotor and diffuser on the aerodynamic performance and internal flow characteristic is revealed. In order to conduct the investigation, a validated Computational Fluid Dynamic (CFD) model, which includes stator, rotor and diffuser are proposed by coupling the cavity between the rotor and diffuser. The model is solved by using commercial software ANSYS CFX which based on the three-dimensional, steady the Reynolds-Averaged Navier-Stokes(RANS) equations.

The results illustrate that the isentropic efficiency is decreased with the increase of axial clearance and radial size deviation. When axial clearance is increased from 0 mm to 4 mm, the isentropic efficiency is decreased by 0.3%, while the isentropic efficiency is only decreased by 0.1% when axial clearance is increased from 4 mm to 8 mm. At the same time, the isentropic efficiency is decreased by 0.66% when radial size deviation is increased from 0 mm to 1.7 mm, while the isentropic efficiency is only decreased by 0.2% when the radial deviation increases from 1.7 mm to 3.4 mm. In general, the radial size deviation presents a greater influence on isentropic efficiency compared to axial clearance.

The effect of axial clearance and radial size deviation on the internal flow field of diffuser is also analyzed in present study. Different leakage flow structure are found for axial clearance and radial size deviation respectively. The leakage fluid from cavity between rotor and diffuser is observed near the deflection cone surface when axial clearance is only included. However, more leakage fluid can be found in the main flow region when radial size deviation is included. The influence range of leakage fluid is increased with the increase of the axial clearance and radial deviation. What’s more, due to the strong adverse pressure gradient, inverse flow is also observed at outlet of the cavity, upstream the leading edge of the strut.

Effects of actual matching parameters on isentropic efficiency at off-design condition are also investigated. The efficiency reduction increased with the decrease of total pressure ratio when actual matching parameters (*A*~clr~ =4mm, *R*~Dev~ =3.6 mm) are employed. Isentropic efficiency can be reduced by 0.8%, 1.26%, and 10.4% when total pressure ratio is 2.67, 2.01 and 1.34 respectively.

The present study can provide an effective reference for the further design optimization of the axial turbine.


Effect of Matching Parameters Between Rotor and Diffuser on the Flow Loss Characteristic in an Axial Turbine

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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