Fig. 22. Summary of all confined cases. CRZ1(purple), CRZ2 (yellow) and PVC (blue) are the coherent structures observed. (A) Open Square Confinement, 4.A; (B) Pyramidal Confinement, 4.B; (C) Sudden Square Confinement, 4.C. The remaining CRZ2 is encircled; (D) Open Circular Confinement, 4.D; (E) Conical Confinement, 4.E; and (F) Sudden

Circular Confinement, 4.F. No CRZ2 observed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Essentially most of the flame was of an annular shape and moved back to being located near the burner exit when the pres-

sure wave retarded flow at the inlets, then moving downstream when flow accelerated through the system. Thus the mechanism appears to be as follows.

An initial stable burning swirling flow forms with the flame located in the shear flow leaving the burner via the action of the CRZ. Natural fluctuations in the shape and location of the CRZ due to the PVC cause variations in heat release (both extent and position) which can then couple with an axial acoustic (i.e. Helmholtz) oscillation. The CRZ (1 and 2) as well as the PVC respond to this coupling, the change in swirl number not only alters the size and shape of the CRZs but also that of the PVC, accentuating the effect further. The PVC may well disappear if the swirl numbers drops to 0.2 as indicated by Schildmacher [34].

5. Conclusions

This study has provided a better understanding of the complex three-dimentional time dependent behaviour of swirl burner flows. Main findings are:

 The PVC–CRZ system co-exist.

 The CRZ is highly dependent on Sg and Re, being compacted at higher Re and thinned at high Sg.

 Low Sg demonstrated the wobbling behaviour of the vortex breakdown, appearing as a lifted CRZ structure not always attached to the burner.

 The PVC is a semi-helical structure, rapidly straightening towards the end of the CRZ.

 CRZ and PVC were demonstrated to be very coherent structures. Standard deviation velocity analysis of the boundary of the CRZ showed highest levels in and around the CRZ base and at the end of the CRZ.

 The CRZ appears as a highly three-dimentional time dependent structure. When confined, the zone is highly asymmetric and rotates with the same frequency as the high momentum crescent shaped velocity region.

 CRZ2 was observed under confined and rapid pressure decay conditions. The system showed strong second and third harmonics. Overall CRZ1 and CRZ2 increased the extent of the recirculation regions.

 Higher axial pressure gradients destroy CRZ2, creating a more compacted recirculation region. Vortex core precession almost disappears as well.

 The PVC length is suppressed by 30% under rapid pressure decay confined condition, (with large exhaust nozzles to the confinement).

Acknowledgements

The authors gratefully acknowledges the receipt of a scholarship from the Mexican Government (CONACYT) and for the assistance of Paul Malpas and Mario Alonso during the setup of the experiments.

References

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