Design and Commissioning of a Combustor Simulator Combining Swirl and Entropy Wave Generation

Abstract

Modern aero-engine combustion chambers burn a lean and premixed mixture, generating a turbulent flame which involves large heat-release fluctuations, thus producing unsteady temperature phenomena commonly referred to as entropy waves (EWs). Furthermore, to enhance the fuel air mixing, combustion air is swirled, leading to vorticity disturbances. These instabilities represent one of the biggest challenges in gas turbine design. In this paper, the design and testing of a novel entropy wave generator (EWG) equipped with a swirler generator (SG) are described. The novel EWG will be used in future works on the high-speed test rig at Politecnico di Milano to study the combustor–turbine interaction. The paper shows the process of the EWG geometry and layout. The EWG is able to produce an engine-representative EW, the extreme condition is at the maximum frequency of 110 Hz, a peak-to-valley temperature value of 20 °C and swirling angles of ±25° are measured. By virtue of these results, the proposed system outperforms other EWG devices documented in the literature. Furthermore, the addition of a swirling generator makes this device one of a kind.

Transport of Swirling Entropy Waves through an Axial Turbine Stator

“…Finally, an unavoidable drawback of the employed injector is the significant wake released by the stem. However, previous experimental campaigns in a wind tunnel have shown that the wake is completely reabsorbed at stator leading edge [20]. As such, the aforementioned flow features of the generated swirl make the generated vortex comparable to what documented in literature [16,17,25,26].…”

“…These authors agree that the higher the frequency, the higher is the EW dissipation caused by diffusion and dispersion inside the duct from the combustor to the turbine inlet. Therefore, an EW is expected to preserve a significant magnitude at turbine inlet only at low frequencies: 10 and 110 Hz seem to match the previous statement, as also showed in Notaristefano and Gaetani [20].…”

“…Despite the EW is injected at mid-span, the swirled motion transports radially upward the temperature disturbance whose peak is found at 65% blade span, 8 mm over the mid span. If the frequency does not influence significantly the mean temperature, it dramatically changes the peak-to-trough temperature difference (frames C and D) due to the severe mixing process taking place at high frequency, as discussed in [20]. Overall, the local peak-to-trough temperatures are 70 K at 10 Hz and 25 K at 110 Hz.…”

“…The two ducts are then coupled in the injector head which injects the EW in the turbine. This novel EWG, extensively described in [20], allows to tune different parameters: the valves frequency that is the corresponding EW fluctuation frequency, the valves duty cycle, the feeding pressure and the heater power. For each frequency, the aforementioned parameters are set to guarantee the best EW penetration in the mainstream and the highest peak-totrough temperature value.…”

“…These are the lowest and highest, respectively, among the five frequencies tested and are chosen as representative. The complete assessment of the flow field downstream the EWG can be found in [20]. For each frequency, the best combination of feeding pressure, valves duty cycle and heater power can be set to maximize the EW penetration and the peak-to-trough temperature difference while keeping the same averaged momentum.…”