DOI 10.1515/tjj-2012-0038 Int. J. Turbo Jet-Engines 2013; 30(1): 15 – 31 Dongil Chang and Stavros Tavoularis* Effect of the Axial Spacing between Vanes and Blades on a Transonic Gas Turbine Performance and Blade Loading Abstract: Unsteady numerical simulations have been con- ducted to investigate the effect of axial spacing between the stator vanes and the rotor blades on the performance of a transonic, single-stage, high-pressure, axial turbine. Three cases were considered, the normal case, which is based on the geometry of a commercial jet engine and has an axial spacing at 50% blade span equal to 42% of the vane axial chord, as well as two other cases with axial spacings equal to 31 and 52% vane axial chords, respec- tively. Present interest has focused on the effect of axial gap size on the instantaneous and time-averaged flows as well as on the blade loading and the turbine perfor- mance. Decreasing the gap size reduced the pressure and increased the Mach number in the core flows in the gap region. However, the flows near the two endwalls did not follow monotonic trends with the gap size change; instead, the Mach numbers for both the small gap and the large gap cases were lower than that for the normal case. This Mach number decrease was attributed to increased turbu- lence due to the increased wake strength for the small gap case and an increased wake width for the large gap case. In all considered cases, large pressure fluctuations were observed in the front region of the blade suction side. These pressure fluctuations were strongest for the smaller spacing. The turbine efficiencies of the cases with the larger and smaller spacings were essentially the same, but both were lower than that of the normal case. The stator loss for the smaller spacing case was lower than the one for the larger spacing case, whereas the opposite was true for the rotor loss. Keywords: high-pressure turbine, axial spacing, CFD PACS ® (2010). 47.11.Df, 47.40.Hg, 47.85.Gj. *Corresponding author: Stavros Tavoularis: Department of Mechanical Engineering, University of Ottawa, Ottawa, Ontario, K1N 6N5 Canada. E-mail: stavros.tavoularis@uottawa.ca Dongil Chang: Department of Mechanical Engineering, University of Ottawa, Ottawa, Ontario, K1N 6N5 Canada 1 Introduction In gas turbines, an axial gap necessarily exists between blade rows of compressors and turbines. The determina- tion of an optimum gap size is one of the crucial design considerations for aircraft engines, because this dimen- sion not only affects the size and weight of the engine, but also its performance and the blade life. In this paper, we focus on high-pressure turbines. At first glance, it seems obvious that the axial gap size must be kept as small as possible for the engine to be rela- tively small and light. However, reducing drastically the gap size would produce strong unsteady pressure fluctua- tions near the leading edges of blades and the trailing edges of the vanes. In transonic high-pressure turbines (HPT), these fluctuations are mostly the result of potential interactions (shock waves and pressure wave reflections) between the rotor and the stator and may introduce exces- sive vibrational stresses on the blades, thus causing mate- rial fatigue and shortening the blade life. When a small turbine size is not an essential requirement, as in the case of turbines operating on ground and sea, a moderate in- crease of gap size for better turbine performance would be acceptable. As the axial gap size is increased, the flow uni- formity at the rotor inlet would be improved, because a larger spacing would allow a better mixing of the three- dimensional (3-D) flows induced by the radial pressure gradient and periodic stator-rotor interactions due to vane wakes and/or shocks. Nevertheless, an increased unifor- mity level at the rotor inlet is not necessarily beneficial for the turbine performance (Gaetani et al. [1]). Dring et al. [2] were the first to examine experimen- tally the influence of axial gap size on the surface pressure and the heat transfer on the blade of a subsonic HPT. The turbine consisted of a stator with 22 vanes and a rotor with 28 blades. They performed their experiment in a large test rig, which allowed them to scale their turbine model to five times an actual turbine size. The turbine rotational speed was 410 rpm and the aspect ratios (i.e., height-to- axial chord length ratios) of the vanes and blades were 1.3 and 0.96, respectively. The compressibility effect was negligible as the turbine exit Mach number was 0.2. They Brought to you by | Yale University Library New Haven Authenticated | 134.99.128.41 Download Date | 3/13/13 9:48 PM