On the dispersion of entropy waves in turbulent flows

Predicting and controlling entropy-wave-driven combustion instabilities is challenging, because the production, advection and dispersion of entropy waves in practical systems is difficult to model. The present paper aims to shed new light on this problem by considering a highly turbulent configuration with experiments and large eddy simulations. In this configuration, the decay of entropy waves is not only governed by the shear dispersion of an idealized turbulent pipe flow, as assumed in the recent studies on the topic, but also enhanced by the highly three-dimensional dispersion due to large-scale coherent structures. A novel post-processing approach for background-oriented schlieren (BOS) thermometry is proposed and enables the measurement of entropy waves with high spatio-temporal resolution for wide ranges of entropy wave amplitudes and frequencies. Instantaneous BOS snapshots are arranged in a panoramic coordinate frame using velocity data. This work, therefore, contributes to filling the knowledge gap in experimental data on entropy waves. The new dataset is accompanied with large eddy simulations to further elucidate the mechanisms dominating the amplitude decay of the entropy waves. It is shown that shear dispersion models that are only based on mean profiles of the axial velocity significantly underestimate the decay in the present configuration, where the entropy waves are formed by periodically injecting hot air pockets in the main stream. It is shown that the turbulent nature of the coherent hot pockets plays a key role in the dispersion enhancement.