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The PVC has been known to interact with the CRZ, for many years as has been the presence of multiple harmonics [13,27–31]. The analysis was focused on the interaction, occurrence and characterisation of the higher harmonics.
Fig. 7. FFT analysis, showing the presence of strong harmonics at 55.0 and 110.0 Hz.
The power spectrum from the HWA was thus examined and comparison made between power in the second and third harmonics to the first was performed. Fig. 7 shows some FFT data showing the first and second harmonics at 55 and 110 Hz.
Configuration IC-25_25 an axi-symmetric case, showed conditions where the PVC’s second harmonic is as strong as the first one. The data are in agreement with those results encountered by Claypole and Syred [30], who noted the presence of two PVCs in the system, as did Syred [13]. At this condition for Re 26,700 traces of strong second harmonics are indicated, matching the results of Fick [25] which are linked to the existence of a second PVC.
Other swirl numbers ranges gave indications of a third strong harmonic. Examples occur with one inlet no inserts, IC-0, (more limited occurrence of the phenomenon), also for the IC-50 case
[29].
The configurations that showed traces of strong secondary harmonics were analysed using High Speed Photography and although the PVC was clearly visualised, little evidence of the second harmonic could be found.
3.2. Phase locked PIV. Unconfined cases
Fick [25] gathered some initial results using Phase Locked PIV techniques, showing that the PVC can be identified by these means.
The aim of the experiments was to quantify and visualise the interrelation between the PVC and the CRZ which have never been experimentally quantified. Some numerical models have indicated the presence of strong helical structures around the recirculation zone, although with no experimental validation. Hence another aim of this work was to examine the level of twist in the PVC.
Table 1 shows the analysed cases, (UNC is an abbreviation for ‘‘Unconfined”). UNC_1 and UNC_2 related to configurations where a second strong harmonic was evident. UNC_3 is the case where non uniform inlet areas were used, but the swirl number is very close to UNC_1. Finally, UNC_4 shows the effects of low swirl near the vortex breakdown point. The most stable case UNC_1 was also recognised by Fick [25] and Syred [13] as the most coherent among their experiments.
By careful analysis of the data, a quantitative boundary for the CRZ was defined via the zero velocity region. The CRZ can be seen to be elliptical in shape, and under certain conditions extends back into the burner. The length each CRZ extended past the burner exhaust being 0.96 D (UNC_1) and 1.18 D (UNC_2). This is a function of Re, since the first harmonic varies from 55 Hz to 32 Hz for UNC_1 and UNC_2, respectively. Case UNC_3 developed an enlarged PVC with minimal twist, whilst UNC_4 showed a lifted unattached behaviour, with a very unstable PVC wobbling with periods of expansion and contraction. Since this is the configuration where the vortex breakdown has just occurred, this behaviour is attributed to the unstable nature of this phenomenon with substantive periods of intermittency.
Case UNC_1 was studied in more detail due to its higher stability. Fig. 8 shows the radial section at the exit nozzle. The PVC is clearly visualized, accompanied by a High Momentum Region. Although the signal analysis identified the first and second PVC harmonics, these were difficult to identify during the PIV studies.
Table 1 Analysed unconfined cases.
Case Inserts used
(%) Flowrate (l/ min) Sg
number Re number First harmonic
(Hz)
UNC_1