Date of Award
Master of Engineering (Research)
Department of Mechanical and Manufacturing Engineering
Dr. Ken Grove
Following a recent series of failures of overhauled third stage turbine blades in the JT8D- 15A gas turbine engine, new inspection criteria have been introduced by the engine manufacturer Pratt & Whitney. One such inspection checks for airfoil bowing and lean. A blade will bow or lean towards the suction face of the airfoil as a result of creep during engine operation. Third stage turbine blades from the JT8D low pressure turbine have been observed with as much as two millimetres of deflection measured at mid-span. In a geometrically correct T3 blade, the tensile centrifugal load due to engine rotation will act in a near radial line. In a bowed blade, the centroids of the blade cross-sections at different radii are displaced tangentially away from their design positions. This displacement introduces a centrifugal bending stress which may result in part failure. Blade lean, where the shroud displaces relative to the blade root, has a similar effect on blade loading. Bowing and lean can result in up to 30% of blades being considered unacceptable for overhaul, a problem of critical importance to the industrial partner, SIFCO Ireland. This project is thus concerned with determining whether airfoil bowing, or airfoil lean, will raise the stresses in the blade by an order of magnitude to cause the part to fail at maximum engine power setting.
An analytical method for the calculation of the blade-to-blade flow field around the airfoil of an axial flow turbine rotor is presented. This analysis tool is utilised to predict the pressure distributions on the rotor airfoil using an airfoil stacking technique. The method treats the flow as a sequence of radially stacked two-dimensional calculations. Radial variation of flow properties and angles is included. The analysis considers the time dependent Navier-Stokes equations in conservation-law form for inviscid, thermal, compressible flow. These laws are expressed in terms of partial differential equations, which are discretised through finite element based techniques.
During operation, gas turbine blades are subject to both structural and thermal stresses. The geometry of the third stage rotor blade is modelled using p-type brick elements. The structural and thermal loads are derived from the flow models described. The stress distribution in the rotor blade material is computed for variously leaned and bowed models within limits specified by the industrial partner. Extensive finite element model results are presented, analysing the maximum compressive and tensile principal stresses at arbitrary radial positions along the span of these airfoil models. The Von Mises theory of elastic failure is utilised to contrast the intensity of stress in these models to the yield strength of the Inconel 713C Nickel-base superalloy.
The finite element analyses undertaken indicate that bowing of the airfoil portion of JT8D-15A third stage turbine blades is a much more significant factor in raising significant stress levels than comparable leaning of a blade. 11
O'Donoghue, Kevin, "An Investigation using the Finite Element Method into Bowing & Lean of the 3rd Stage Turbine Blade of the Pratt & Whitney JT8D Gas Turbine Engine" (1998). Theses [online].
Available at: https://sword.cit.ie/allthe/356