Abstract
This thesis presents CFD assessments of axial turbine chute rim sealing flows. Firstly, a systematic study of the performance of an axial turbine chute rim seal using unsteady Reynolds averaged Navier Stokes (URANS) methods is reported. This extends previous studies from a configuration without external flow in the main annulus to cases with a circumferentially uniform axial flow and vane generated swirling annulus flow (but without rotor blades). This includes variation of the mean seal to rotor velocity ratio, main annulus to rotor velocity ratio, and seal clearance. The effects on the unsteady flow structures and the degree of main annulus flow ingestion into the rim seal cavity are examined. Sealing effectiveness is quantified by modeling a passive scalar, and the timescales for the convergence of this solution are considered. It is found that intrinsic flow unsteadiness occurs in most cases, with the presence of vanes and external flow modifying the associated flow structures and frequencies. Some sensitivities to the annulus flow conditions are identified. The circumferential pressure asymmetry generated by the vanes has a clear influence on the flow structure but does not lead to higher ingestion rates than the other conditions studied at high rotational Reynolds numbers.
Next, the wall modelled large eddy simulation (WMLES) methodology is considered since it requires considerably less computational resources compared to wall resolved LES, and has the potential for extension to representative engine conditions. Therefore, this methodology is implemented, validated and adapted for the assessment of the sealing effectiveness at representative experimental test conditions. The WMLES approach is shown to be effective, giving significant improvements over an eddy viscosity turbulence model in prediction of rim seal effectiveness compared to research rig measurements. When the cavity is not fully sealed, all the WMLES solutions show rotating inertial waves in the chute seal. Good agreement between WMLES and measurements for sealing effectiveness in the configuration without vanes is found at relatively low and medium sealing flow rates. For cases with vanes fitted, the WMLES simulation predicted a lower ingress compared to the measurements in the inner part of cavity and possible reasons of these mismatches can be explained by the use of a limited sector size and from the failure of WMLES to capture the flow separation in the mid part of the cavity.
Finally, for the vaned configuration, a CFD assessment of the purge flow distribution over the main annulus is reported and compared against experimental measurements at a representative turbine rotor blade leading edge axial position. WMLES has proven to be effective in the resolution of the turbulence associated with the interaction between the main annulus and cavity flow as opposed to URANS in which the sealing flow is mostly entrained on the rotor hub boundary layer.