Numerical Investigation of Aerothermal Characteristics of the Blade Tip and Near-Tip Regions of a Transonic Turbine Blade

In modern gas turbine engines, the blade tips and near-tip regions are exposed to high thermal loads caused by the tip leakage flow. The rotor blades are therefore carefully designed to achieve optimum work extraction at engine design conditions without failure. However, very often gas turbine engines operate outside these design conditions which might result in sudden rotor blade failure. Therefore, it is critical that the effect of such off-design turbine blade operation be understood to minimize the risk of failure and optimize rotor blade tip performance. In this study, the effect of varying the exit Mach number on the tip and near-tip heat transfer characteristics was numerically studied by solving the steady Reynolds Averaged Navier Stokes (RANS) equation. The study was carried out on a highly loaded flat tip rotor blade with 1% tip gap and at exit Mach numbers of Mexit = 0.85 (Reexit = 9.75 x 10 5 ) and Mexit = 1.0 (Reexit = 1.15 x 10 6 ) with high freestream turbulence (Tu = 12%). The exit Reynolds number was based on the rotor axial chord. The numerical results provided detailed insight into the flow structure and heat transfer distribution on the tip and near-tip surfaces. On the tip surface, the heat transfer was found to generally increase with exit Mach number due to high turbulence generation in the tip gap and flow reattachment. While increase in exit Mach number generally raises he heat transfer over the whole blade surface, the increase is significantly higher on the neartip surfaces affected by leakage vortex. Increase in exit Mach number was found to also induce strong flow relaminarisation on the pressure side near-tip. On the other hand, the size of the suction surface near-tip region affected by leakage vortex was insensitive to changes in exit Mach number but significant increase in local heat transfer was noted in this region. NOMENCLATURE

[1]  Ali Ameri,et al.  Effects of Tip Clearance and Casing Recess on Heat Transfer and Stage Efficiency in Axial Turbines , 1998 .

[2]  John Moore,et al.  Tip leakage flow in a linear turbine cascade , 1987 .

[3]  L. El-Gabry.  Numerical Modeling of Heat Transfer and Pressure Losses for an Uncooled Gas Turbine Blade Tip: Effect of Tip Clearance and Tip Geometry , 2009 .

[4]  Jiang Luo,et al.  Prediction of Heat Transfer and Flow Transition on Transonic Turbine Airfoils Under High Freestream Turbulence , 2008 .

[5]  Karen A. Thole,et al.  Effects of Large Scale High Freestream Turbulence and Exit Reynolds Number on Turbine Vane Heat Transfer in a Transonic Cascade , 2009 .

[6]  Richard J Goldstein,et al.  Local Mass/Heat Transfer on Turbine Blade Near-Tip Surfaces , 2003 .

[7]  Chao Zhang,et al.  Flowfield and heat transfer past an unshrouded gas turbine blade tip with different shapes , 2013 .

[8]  Je-Chin Han,et al.  Heat Transfer and Pressure Distributions on a Gas Turbine Blade Tip , 2000 .

[9]  Srinath V. Ekkad,et al.  Effect of Tip Gap and Squealer Geometry on Detailed Heat Transfer Measurements Over a High Pressure Turbine Rotor Blade Tip , 2004 .

[10]  Ali Ameri,et al.  Effects of Tip Clearance and Casing Recess on Heat Transfer and Stage Efficiency in Axial Turbines , 1998 .

[11]  Li He,et al.  Turbine Blade Tip Heat Transfer in Low Speed and High Speed Flows , 2011 .

[12]  A. A. Ameri,et al.  Heat Transfer and Flow on the First-Stage Blade Tip of a Power Generation Gas Turbine: Part 2—Simulation Results , 2000 .

[13]  Budugur Lakshminarayana,et al.  Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part I—Effect of Tip Clearance Height , 2001 .

[14]  Ali Ameri,et al.  Heat Transfer and Flow on the First-Stage Blade Tip of a Power Generation Gas Turbine: Part 1—Experimental Results , 2000 .

[15]  Je-Chin Han,et al.  Heat-Transfer Coefficients of a Turbine Blade-Tip and Near-Tip Regions , 2002 .

[16]  Li He,et al.  Overtip Shock Wave Structure and Its Impact on Turbine Blade Tip Heat Transfer , 2011 .

[17]  A. J. Rawlinson,et al.  Effects of Inlet Turbulence and End-Wall Boundary Layer on Aerothermal Performance of a Transonic Turbine Blade Tip , 2013 .

[18]  Ronald S. Bunker,et al.  Axial Turbine Blade Tips: Function, Design, and Durability , 2006 .

[19]  D. E. Metzger,et al.  HEAT TRANSFER AT THE TIP OF AN UNSHROUDED TURBINE BLADE , 1982 .

[20]  D. E. Metzger,et al.  The Influence of Turbine Clearance Gap Leakage on Passage Velocity and Heat Transfer Near Blade Tips: Part II—Source Flow Effects on Blade Suction Sides , 1989 .

[21]  Zhenping Feng,et al.  Numerical investigations on the steady and unsteady leakage flow and heat transfer characteristics of rotor blade squealer tip , 2011 .

[22]  F. Menter.  Two-equation eddy-viscosity turbulence models for engineering applications , 1994 .

[23]  R S Bunker,et al.  A Review of Turbine Blade Tip Heat Transfer , 2001, Annals of the New York Academy of Sciences.

[24]  Li He,et al.  Transonic Turbine Blade Tip Aerothermal Performance With Different Tip Gaps—Part I: Tip Heat Transfer , 2011 .

[25]  Je-Chin Han,et al.  Heat Transfer Coefficients on the Squealer Tip and Near-Tip Regions of a Gas Turbine Blade With Single or Double Squealer , 2003 .

[26]  Wing Ng,et al.  Effects of Tip Clearance Gap and Exit Mach Number on Turbine Blade Tip and Near-Tip Heat Transfer , 2013 .

[27]  Nicole L. Key,et al.  Comparison of Turbine Tip Leakage Flow for Flat Tip and Squealer Tip Geometries at High-Speed Conditions , 2006 .

[28]  Richard D. Sandberg,et al.  DIRECT NUMERICAL SIMULATIONS OF A TRANSONIC TIP FLOW WITH FREE-STREAM DISTURBANCES , 2013 .

[29]  Nicholas R. Atkins,et al.  Unsteady Effects on Transonic Turbine Blade-Tip Heat Transfer , 2012 .