The ECOFlex‑turbo project, carried out from 1 July 2019 to 30 September 2023, investigates how an evaporating liquid spray interacts with acoustic fields in a cross‑flow environment. The research was led by Prof. Dr. Ing. Christian Oliver Paschereit at the Technical University of Berlin and was funded by the German Federal Ministry of Economics and Energy (BMWi) under grant number 0324295E. The industrial partner was GE Power, and the work was part of the larger “Last‑ and Fuel‑Flexible Combustion” (LaBreVer) consortium.

A key deliverable of the project was a generic test rig that provides optical access to a spray undergoing evaporation and acoustic excitation. The rig allows simultaneous measurement of droplet velocity and diameter distributions using phase‑resolved laser Doppler anemometry (PDA). Droplet size data were collected at acoustic frequencies of 49 Hz, 98 Hz, 69 Hz, and 180 Hz with excitation amplitudes ranging from 0.2 µ to 0.4 µ. The initial droplet diameter was 25 µm, and the cross‑flow velocity was 15 m s⁻¹. Results showed that acoustic forcing induces significant droplet breakup, leading to a broader size distribution and a shift toward smaller droplets at higher frequencies. The phase‑resolved PDA data revealed that droplet trajectories oscillate in sync with the acoustic field, producing a pronounced vertical displacement of the spray plume. This vertical motion causes periodic concentration fluctuations in the evaporated vapor.

To quantify these concentration variations, a tunable diode laser absorption spectroscopy (TDLAS) system was developed. The TDLAS probe measured water vapor concentration (used as a surrogate fuel) with spatial and temporal resolution across the channel cross‑section. The system captured phase‑resolved concentration fields in both horizontal and vertical directions. At 69 Hz excitation with an amplitude of 0.2 µ, the concentration in the upper part of the channel increased during the quarter‑phase of the acoustic cycle, while at 180 Hz the effect was spread over a larger phase range. These observations were corroborated by the PDA measurements, which showed simultaneous spray expansion and increased vapor concentration during the upward motion of the droplets.

Numerical simulations of droplet evaporation under acoustic excitation were performed to validate the experimental findings. The simulations incorporated the measured velocity and droplet size distributions and predicted droplet lifetime and vapor concentration profiles. Comparison with the experimental droplet breakup index (DBI) demonstrated good agreement when acoustic forcing was included, whereas simulations without acoustic forcing underestimated droplet breakup and vapor concentration fluctuations. The study thus confirms that acoustic fields can significantly alter spray atomisation and evaporation, leading to measurable changes in fuel concentration that are critical for thermo‑acoustic stability in combustion systems.

While the mid‑pressure test rig planned for the later phase of the project was delayed due to COVID‑19 related supply chain disruptions and design revisions, the atmospheric‑pressure rig achieved the primary objectives. The results provide a foundation for developing predictive models of spray‑acoustic interaction, which will aid the design of future fuel‑flexible gas turbines with improved combustion stability and reduced emissions.