AEROTHERMAL PERFORMANCE OF PARTIAL AND CAVITY SQUEALER TIP IN A LINEAR TURBINE CASCADE

Three-dimensional highly complex flow structure in tip gap between blade tip and casing leads to inefficient turbine performance due to aerothermal loss. Interaction between leakage vortex and secondary flow structures is the substantial source of that loss. So as to improve the turbine performance, different kinds of squealer tip geometries have been tried in the literature. The current research deals with comparison of partial and cavity squealer tip concepts for higher aerothermal performance. Effects of squealer tip have been examined comprehensively for an unshrouded HP turbine blade tip geometry in a linear cascade. In the present paper, flow structure through the tip gap have been comprehensively investigated by Computational Fluid Dynamic (CFD) methods. Numerical calculations have been obtained by solving threedimensional, incompressible, steady and turbulent form of the Reynolds-Averaged Navier-Stokes (RANS) equations using ANSYS CFX. Two equation turbulence model, Shear Stress Transport (SST) has been used. The axial turbine blade tip profile belongs to the Pennsylvania State University Axial Flow Turbine Research Facility (AFTRF). The tip profile of the AFTRF that was used to create an extruded solid model of the axial turbine. For identifying dimensions of squealer rim in terms of squealer height and squealer width, our previous studies about aerothermal investigation of cavity squealer tip have been examined. In order to generate the mesh, an effective parametric grid generation has been performed using a multizone structured mesh. Numerical calculations indicate that partial and cavity squealer designs can be effective to reduce the aerodynamic loss and heat transfer to the blade tip. Future efforts will include novel squealer shapes for higher aerothermal performance. INTRODUCTION In order to allow the relative motion of blades and to prevent the blade tip surface from rubbing, clearance gap between blade and casing is required in turbomachinery. The overall performance of the turbomachines is strongly related to the flow within tip gap. The flow in this gap is 3D and highly complex. The pressure difference across the pressure and suction side of the blade forms a leakage flow passed over the blade tip surface. The pressure driven flow throughout the gap results in approximately one-third of the aerodynamic loss in the rotor of an axial gas turbine [1]. The flow structure in the tip gap is a significant source of inefficiency in terms of aerodynamic loss and heat transfer to the blade tip and casing. The leakage flow passes over the blade tip without being turned as the passage flow, thus a reduction in work extracted from the turbine is observed [1-4]. The leakage flow is also a significant source of higher thermal loads on the blade tip platform which is exposed to the hot gas stream [4,5]. There are many studies in order to clarify the structure of the tip leakage flow and to reduce its adverse effects. In an early investigation Moore and Tilton investigated the leakage flow both analytically and experimentally considering the flow through the tip gap as a flow through an orifice [6]. Bindon and Aerothermal Performance Of Partial And Cavity Squealer Tip In A Linear Turbine Cascade 2 FIGURE 1: CONSEPTUAL VIEW OF BLADE TIP DESIGNS Morphis revealed the effects of tip geometries and radius at the pressure side corner on aerodynamic loss [7]. Yaras and Slojander investigated the effect of relative motion between blades and casing and experiments revealed that the leakage flow rate was decreased by the effect of rotation [8]. Passive control methods such as squealer, partial squealer and winglet blade tip designs are widely used in order to reduce the effects of leakage flow. Heyes at al. figured out that the implementation of squealer tip geometries could be effective in reducing the leakage flow since a separation bubble blocks the flow [2]. Ameri et al. numerically investigated the effect of a squealer geometry on heat transfer and efficiency and obtained that the leakage flow rate was reduced whereas the heat transfer to the blade tip was increased [9]. Azad et al. investigated different types of squealer geometries in a linear turbine cascade experimentally and indicated that a suction side squealer had better results with respect to a pressure side squealer [4]. Krishnababu et al. [3] studied numerically different types of geometries including both cavity tip squealer geometry and suction side squealer geometry. Numerical results showed that a cavity squealer reduced aerodynamic loss and heat transfer to the blade tip. In this study, it was stated that width of the squealer could be effective in determining the performance of the squealer. Lee and Kim investigated the flow structure over a cavity squealer tip experimentally for a linear turbine cascade [10]. Zhou numerically studied the thermal performance of the baseline cavity tip with wider and higher cavity tip. In this research, thin and low rim yielded the lowest aerodynamic loss [11]. Wei et al. simulated the aerodynamic performance of five passive control methods: pressure side winglet, suction side winglet, cavity squealer, inclined pressure side squealer and partial suction side squealer tip [12]. They revealed that inclined pressure side squealer had the best turbine efficiency. Camci et al. carried out an experimental and numerical study in a rotational test rig (AFTRF) for aerodynamic performance of full and partial-length squealer rims [13-15]. Schabowski and Hodson investigated both numerically and experimentally the aerodynamic performance of various blade tip designs including squealer and winglet geometries [16]. In this paper, the effect of a squealer tip section has been examined comprehensively for an unshrouded HP turbine blade in a linear cascade. A special emphasis is placed on obtaining three dimensional and complex grid systems in a parametric effort. The parametric approach in tip leakage mitigation studies provides significant time savings for the construction of the solid model and its associated flow grid. Conceptual view of flat, partial squealer and cavity squealer tip designs has been given in Fig. 1. METHOD A numerical study has been conducted in present paper. Computational Fluid Dynamics (CFD) is an important tool to make a better understanding flow in turbomachines whereas experimental measurements may become difficult, expensive and time consuming. Three different tip designs have been studied: flat tip (FLAT), squealer (SqBASE), suction side squealer (SSSqBASE) in Fig.1 Fig. 1: Flat tip and squealer tip geometries Axial blade geometry belongs to Pennsylvania State University Axial Flow Turbine Research Facility (AFTRF). Tip profile of the turbine in AFTRF was used to form solid model of axial turbine. Some design features of turbine blade is given in Table 1. Aerothermal Performance Of Partial And Cavity Squealer Tip In A Linear Turbine Cascade 3 Table 1 Blade specifications Blade Height [mm] 123.615 Tip Gap Height [mm] 0.615 Axial Chord [mm] 85.04 Flow Angle (o) (inlet) 71.3