Transport of swirling entropy waves through an axial turbine stator

Authors

Andrea Notaristefano - Politecnico di Milano, Dipartimento di Energia
Paolo Gaetani - Politecnico di Milano, Dipartimento di energia

Abstract

The combustion flow exiting modern combustion chambers results highly unsteady, the transport of entropy wave and their impact on the stage aerodynamic are still open points. This paper shows the results of a research work on the evolution of swirling entropy wave in a stator of a high pressure axial turbine stage: the issue is addressed by a wide experimental campaign where different injection parameters and positions are studied. The experimental data allows to quantify the aerodynamic impact of the swirling entropy waves coming from combustors on the first stage of gas turbines and characterize their transport through the stator. The prescribed swirling entropy waves are axially directed and they are injected at 70% of the blade height. The experiments are carried out employing a novel Entropy Wave Generator which ensures a wide set of different injection parameters. First the frequency is varied in a range from 10 – 110 Hz; for each frequency the best injection pressure is applied, chosen to get the best entropy wave uniformity at the injector discharge. Thus, depending on the frequency and on the injection pressure, different temperature perturbations are released by the injectors and enter in the stator. Four different clocking positions are studied: at mid-pitch, at the leading edge and, with respect to the leading edge, at 1/3 of the pitch on the pressure side and suction side, as well. In the experimental campaign different measuring techniques are used both upstream and downstream of the stator: a 5-hole probe to catch the steady aerodynamic field and a fast thermocouple to measure the fluctuating temperature pattern. Upstream of the stator, a fast pneumatic total pressure probe is also applied to estimate the turbulence level. To characterise the residual fluctuating patter, a Fast Response Aerodynamic Pressure Probe is applied downstream of the stator. Measurements showed a severe temperature attenuation of the swirling entropy wave downstream of the stator. The temperature pattern resulted modified accordingly to the injection position as a consequence of its interaction with the stator secondary flows systems. The attenuation rate is in fact independent of the frequency in the range studied. As for the aerodynamic field, it is perturbed by the swirling motion of the injected entropy wave; it also shows a dependence on the injection position.