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

Abstract

The amount of cooling air assigned to seal high pressure (HP) 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 a continuation of the authors' work on ingestion reported by Mirzamoghadam et al. (2008) (“3D CFD Ingestion Evaluation of a High Pressure Turbine Rim Seal Disk Cavity,” ASME Paper No. GT2008-50531), where 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. The outcome from this study was to map out the resulting nondimensional 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 computational fluid dynamics (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, an investigation to assess the selected aft-located mixing plane steady state solution for this study as compared to the forward-located steady run was performed using unsteady (nonlinear Harmonics) CFD as the referee. 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.