Background-oriented schlieren of entropy waves advected in straight duct

Entropy waves generate indirect combustion noise when accelerated in the turbine stages of aeroengines. This indirect combustion noise constitutes a significant part of the overall combustion noise. In addition, the indirect combustion noise from entropy waves can constructively interact with flame and induce high amplitude self-sustained oscillations. A deep understanding of the advection and dispersion of entropy waves is therefore important for the prediction of noise emissions and thermoacoustic instabilities. This experimental study contributes to that goal: a highly turbulent airflow in a rectangular duct at ambient temperature is subject to the forcing of a siren supplied with hot air. This pulsed hot-jet-in-cold-crossflow yields coherent entropy waves, which are advected through the channel. The line-of-sight integrated temperature amplitude and phase are measured using high-speed Background-oriented Schlieren (BOS), thermocouples and simultaneous particle image velocimetry (PIV). This non-invasive method facilitates the observation of low amplitude temperature fluctuations at high frequencies, with a simple and inexpensive setup.