Characterization of Leading-edge Laminar Separation Bubble for Various Angles of Incidence and Reynolds Numbers

Low-pressure turbine is a crucial component of the gas turbine engine. The overall performance of the engine strongly depends on the efficiency of the LP turbine. The aerodynamics within the blade passage of an LP turbine is influenced by the presence of high freestream turbulence intensity, off-design angle of incidence, endwall contouring and unsteady upstream wake, which in turn lead to early flow transition and formation of laminar separation bubble on the blade’s suction surface. Notably, LP turbine operates at large positive angle of incidence due to a decrease in Reynolds number during cruise. In such situation, large laminar separation bubble forms near the leading edge on the suction surface of LP turbine blade. The presence of high turbulence intensity and large angle of incidence brings distinct characteristics to the separation bubble and makes it potentially different from the laminar separation bubble that is generally formed on the suction surface of a wing at moderate angle of attack and low turbulent intensity. The extent, height and location of the separation bubble greatly affect the overall drag and lift characteristics of the turbine aerofoil. It is essential to perform detailed characterization of leading-edge laminar separation bubble to understand its effect on the efficiency degradation of LP turbine during cruise. The paper discusses the effect of Reynolds number and angle of incidence on the behaviour of such leading-edge laminar separation bubble, experimentally studied over a flat test plate with elliptical (4:1) leading edge. The test plate is equipped with a flap at the rear to simulate the angle of incidence change on leading edge of LP turbine during cruise. The flow physics concerning the formation process of leading-edge LSB along with its evolution with change in AOI and Re have been investigated using surface pressure measurement and planar PIV. The bubble length and the height increase with the increment in the angle of incidence. At higher Reynolds number, bubble formation is delayed to higher angle of incidences. The aspect ratio of the bubble also changes with the change in the Reynolds number. Although the separation point moves upstream with an increase in the angle of incidence, the movement rate is significantly lower than the downstream movement of the reattachment point. The obtained results with respect to the bubble formation process and bubble properties agree well with the literature and are discussed in detail in this paper.