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This study investigates the dynamics of non-isothermal swirling jetsundergoing vortex breakdown, with an emphasis on helical coherent structures.It is proposed that the dominant helical coherent structure can be suppressedby heating the recirculation bubble. This proposition is assessed with StereoParticle Image Velocimetry (PIV) measurements of the breakdown region ofisothermal and heated swirling jets. The coherent kinetic energy of thedominant helical structure was derived from PIV snapshots via Proper OrthogonalDecomposition. For one set of experimental parameters, mild heating is found toincrease the energy content of the dominant helical mode. Strong heating leadsto a reduction by 30\% of the coherent structures energy. For a second set ofexperimental parameters, no alteration of the dominant coherent structure isdetectable. Local linear stability analysis of the time-averaged velocityfields shows that the key difference between the two configurations is thedensity ratio at the respective wavemaker location. A density ratio ofapproximately 0.8 is found to correlate to a suppression of the global mode inthe experiments. A parametric study with model density and velocity profilesindicates the most important parameters that govern the local absolute growthrate: The density ratio and the relative position of the density profiles andthe inner shear layer.

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