Boundary layer development in multi-stage low pressure turbines

Abstract

<p>Comprehensive experiments were conducted to study the development of boundary layers along airfoil surfaces of multi-stage low pressure turbines. The test vehicle consisted of a large scale, low speed turbine containing two stages of blading. Tests were carried out over a wide range of Reynolds numbers and loadings relevant to low pressure turbines of modern commercial engines;Unsteady measurements of the boundary layer were acquired using arrays of densely-packed, surface hot-film sensors mounted to the airfoil suction and pressure surfaces. Airfoils from the second stage nozzle and rotor bladerows were instrumented. Measurements of the time unsteady velocity and turbulence were obtained at the inlet and exit of each turbine bladerow. For the first time, quantitative measurements of turbulence length scale are reported for a multi-stage low pressure turbine;For all test conditions, the boundary layers along the blade suction surfaces of the embedded second stage were predominantly laminar and transitional. In general, the boundary layers developed along two distinct but coupled paths. The first developed approximately beneath the wake that convects through the bladerow. The second occurred between the wakes. Along both paths, regions of laminar, transitional, and turbulent flow were identified. The two paths were coupled by regions of calmed flow that developed behind turbulent spots. The calmed regions, characterized by elevated levels of non-turbulent shear stress, effectively suppressed boundary layer separation in areas of adverse pressure gradient;Along the suction surfaces at high Reynolds numbers, equivalent to aircraft takeoff conditions, some transitional flow persisted to the trailing edge. At low, cruise Reynolds numbers, the boundary layer was laminar along more than seventy percent surface distance;Both the airfoil boundary layers and bladerow aerodynamic loss were influenced significantly by clocking of upstream bladerows;Along the pressure surface, boundary layer development for all test conditions was influenced strongly by the adverse pressure gradient at the leading edge. At decreased loading, transition occurred abruptly due to the presence of a laminar separation bubble. At increased loading where the incidence angle was lower, attached-flow transition was observed. In all cases, little periodic unsteadiness persisted downstream of transition.</p>