Influence of acoustic excitation upon the entrainment phenomenon in combustion/propulsion applications

This paper presents the results of an experimental investigation performed about the entrainment phenomenon induced by an axisymmetric jet, which can be used for premixed combustion appliances or in free diffusion flames (for instance in propulsion applications). Particularly, the main goal of the research activity was a systematic analysis of the influence of an acoustic excitation upon the jet development and entrainment of the surrounding stagnant air. The analised jet presents a quite low Reynolds number (ReD@3600), constituting a test case non yet thoroughly studied in literature, but peculiar of some technical appliances, for instance in the field of premixed gas burners. At first, the flow field generated by the stationary free jet has been characterised both through laser Doppler velocimetry, to estimate the global and local entrainment coefficient, and hot wire anemometry, to attain the natural frequency (Strouhal number) of the jet. Subsequently, the jet has been acoustically excited through an active loudspeaker placed in a stagnation chamber upstream the jet outflow, operating at a frequency corresponding to the natural one of the stationary jet (210 Hz). The flow field induced by the excited jet has been analised through laser Doppler velocimetry, comparing the jet development (mean axial velocity and turbulence intensity profile) and the entrainment phenomenon with respect to the stationary (i.e.: not-excited) jet. The results put into evidence that the excited jet presents, especially in the initial region, higher turbulence levels and a larger radial expansion, contributing to a noticeable reduction of the potential core length (backwarding of the jet virtual origin). Moreover, this induces an increase of the entrainment phenomenon with respect to the stationary jet (up to 25% of the entrained flow rate from the surrounding stagnant air).