Secondary Flow Response to Endwall Jets in a Low Pressure Turbine

Abstract

Due to the significant losses contributed by the secondary flow features, an active flow control system was implemented in a low-pressure turbine linear cascade which consisted of localized endwall jets with small mass ratios to perturb the dominant passage vortex. Benefits included significant area-averaged total pressure loss reduction and improved exit angle deviations which help to open the design envelope to application of high-lift front-loaded blades. This report looks to reveal the impact of steady and pulsed endwall blowing on the secondary flow dynamics. High-speed stereoscopic particle image velocimetry for an in-passage measurement plane was utilized to investigate the time-dependent behavior of key flow features such as the passage vortex. At baseline conditions, the passage vortex is characterized by time-varying oscillatory motion in the pitchwise direction, streamwise undulation, bursting, and fluctuating strength. Upon actuation of endwall jets, some of these defining dynamics of key flow features were greatly affected. A complementary investigation of the endwall jets mounted outside of a turbine environment in order to study the emitted structures at varying conditions was used to explain the observations found in the turbine passage. Insights into the secondary flow responsiveness demonstrated that loss reduction was realized by inducing reduced coherence of the passage vortex. Despite pulsed blowing at discrete frequencies associated with the passage vortex, there was no indication that instability excitation was exploited. Rather, the endwall jets acted as a periodic shape-change to the endwall which weakened the passage vortex and forced it closer to the suction-surface.

Comparison of Endwall Loss Reduction Techniques in a High-Lift Turbine Passage

“…9). This second structure is believed to be the result of the jet oriented in cross-flow which is well known to produce streamwise vortices [25]. A weak signature of the pressure side vortex was also visible in the steady jet oil flow visualization.…”

“…Further detailed analysis using the highspeed SPIV data set to describe the response of the passage secondary flow to the localized endwall jets was included in Ref. [25].…”

“…The cyclic response of the passage vortex at various pulsing conditions is discussed in detail in Refs. [20,25]. The strength of the passage vortex responded in a cyclic manner to pulsing.…”

“…Further analysis of the dynamic response of the secondary flow to jet actuation using high-speed stereo particle image velocimetry (SPIV) can be found in Ref. [25].…”

“…Portions of data collected and reported separately in Refs. [21][22][23][24][25] are combined with new data and analysis to provide insight into the flow physics associated with each of the three different secondary flow loss techniques.…”