A numerical study of active flow control for low pressure turbine blades

Abstract

Expansion of the slow, recirculating separated region of flow inside a separation bubble on the suction surface of Low Pressure Turbine (LPT) blades is the main source of loss generation as the Reynolds number is reduced. At axial chord Reynolds numbers as low as 30,000, typical of small or high altitude jet engines, the separation bubble expands all the way to the trailing edge of the blade, with a large increase in losses caused by the fully separated flow field. Active Flow Control (AFC) is a promising tool for reducing these losses without altering blade geometry, in order to make smaller, higher flying engines more cost-effective. Alternatively, AFC allows an increase in blade loading such that the number of turbine blades required can be reduced for a given operating point. Jet Flap (JF) is one such control device, and consists of a small opening on the pressure surface of the LPT blade near its trailing edge. By injecting fluid into the free stream normal to the pressure surface, the passage between blades is effectively contracted causing increased acceleration through the throat formed by adjacent blades. This induces a more favourable pressure gradient over the aft portion of the blade below, potentially suppressing the separation bubble altogether, while increasing the suction peak and moving it rearwards. In the current study the Pak-B LPT blade was used to evaluate the suitability of the JF concept as an AFC device for LPT blades. A numerical model was developed in STAR-CCM+ and validated against experimental data