The Effect of Annulus Performance Parameters on Rotor-Stator Cavity Sealing Flow

Abstract

The amount of cooling air assigned to seal high pressure turbine rim cavities is critical for performance as well as component life. Insufficient air leads to excessive hot annulus gas ingestion and its penetration deep into the cavity compromising disk or cover plate life. Excessive purge air, on the other hand, adversely affects performance. This paper is Part-2 of the authors’ work on ingestion reported last year [1]. Whereas, the main focus of that investigation was to qualitatively describe ingestion driven by annulus circumferential pressure asymmetry under constant annulus conditions and rotational speed, in this paper, the research team investigated the variation of annulus circumferential pressure fluctuation and rotational speed on the double overlap platform rim seal cavity reported in Part-1, and mapped out the resulting non-dimensional minimum sealing flow (minimum value of Cw or Cw,min) as it relates to entrained ingestion in the absence of cavity cooling flow (Cw,ent). As was done in Part-1, the runs were made with 3D CFD in setup/run mode option using Fine/Turbo. At two rotational speeds, annulus conditions were varied by reducing turbine inlet pressure (i.e. mass flow) from the baseline operating condition, and the resulting pressure fluctuation was quantified. In addition, a preliminary investigation to assess the aft-located mixing plane steady state solution for this study was performed. The results yielded the linear decrease in Cw,ent at fixed rotational Reynolds number as annulus Reynolds number was decreased. Moreover, the rate of change in entrained flow sharply increases with increase in rotational Reynolds number. As annulus mass flow is reduced to a critical value defined by annulus-to-rotational Reynolds number ratio, the CFD prediction for Cw,ent converges to the turbulent boundary layer entrainment solution for the rotor, and Cw,min reverts to the rotational Reynolds number dominating region. The results from this study were compared to what has been observed by a previous study for a single overlap platform geometry. The resulting design curve allows insight in relating cavity purge flow requirements versus turbine cycle parameters which could lead to better efficiency.

A New Interpretation of Hot Gas Ingress Through Turbine Rim Seals Influenced by Mainstream Annulus Swirl

“…Previous studies (e.g. [3][4][5], [13][14] and others) have attributed the circumferential pressure asymmetries in the annulus as the driving mechanism for ingress. It was shown that as the annulus flow coefficient CF increased, the annulus pressure asymmetries (quantified by the peak-to-trough non-dimensional pressure variation, ΔCp,a) increased, and the ingress was generally enhanced.…”

Computational extrapolation of turbine sealing effectiveness from test rig to engine conditions

“…Zhou et al 3 reviewed previous computational studies related to ingress, including those by Hills et al, 4 Laskowski et al, 5 Rabs et al, 6 Johnson et al, 7 Wang et al, 8 Zhou et al 9 and Mirzamoghadam et al 10,11 A number of authors have noted that the statistical convergence for pressure variations in the annulus is more quickly and more easily obtained than the cooling effectiveness field in the wheel-space.…”

Unsteady 360 Computational Fluid Dynamics Validation of a Turbine Stage Mainstream/Disk Cavity Interaction

“…In addressing both (a) and partly (b) objectives, the survey by Mirzamoghadam et al [8] in 2009 looked at the influence of turbine mainstream boundary conditions on the HP turbine disk cavity minimum sealing flow using 3D CFD. The periodic sector model runs were made using the steady state model with the mixing plane located aft of the cavity.…”