The impact of heating the breakdown bubble on the global mode of a swirling jet: Experiments and linear stability analysis

TL;DR

This paper explores how heating the recirculation bubble in swirling jets affects their dominant helical structures, using experiments and linear stability analysis to identify conditions that suppress or enhance these coherent modes.

Contribution

It demonstrates experimentally and analytically that heating can suppress or amplify the global helical mode depending on the density ratio and heating intensity.

Findings

Mild heating increases the energy of the dominant helical mode.

Strong heating reduces the coherent structure energy by 30%.

A density ratio of ~0.8 correlates with suppression of the global mode.

Abstract

This study investigates the dynamics of non-isothermal swirling jets undergoing vortex breakdown, with an emphasis on helical coherent structures. It is proposed that the dominant helical coherent structure can be suppressed by heating the recirculation bubble. This proposition is assessed with Stereo Particle Image Velocimetry (PIV) measurements of the breakdown region of isothermal and heated swirling jets. The coherent kinetic energy of the dominant helical structure was derived from PIV snapshots via Proper Orthogonal Decomposition. For one set of experimental parameters, mild heating is found to increase the energy content of the dominant helical mode. Strong heating leads to a reduction by 30\% of the coherent structures energy. For a second set of experimental parameters, no alteration of the dominant coherent structure is detectable. Local linear stability analysis of the time-averaged velocity fields shows that the key difference between the two configurations is the density ratio at the respective wavemaker location. A density ratio of approximately 0.8 is found to correlate to a suppression of the global mode in the experiments. A parametric study with model density and velocity profiles indicates the most important parameters that govern the local absolute growth rate: The density ratio and the relative position of the density profiles and the inner shear layer.