Pressure decay plays the most important role as a consequence of geometry change in the development of coherent structures. These are fundamental in the flow dynamics involved in the cold flow patterns observed in many swirl burners. Moreover, it has been shown that well controlled expansions can suppress the formation of CRZ2, whilst a strong anchored central vortex can interact with the PVC reducing its precession considerably. In this sense, the vortex can be used to improve mixing, as observed by the multitude of eddies formed in the vicinity of the vortex [27].
Fig. 22 summarises the coherent structures found in all the confined cases, showing the high dependence on the geometry used, and confirming that secondary structures develop when the sudden expansion into which the swirl burner fires has been changed. 4.2. Combustion effects
One of the most important effects arising with premixed and partially premixed combustion is the appearance of the PVC. This can have significant effects as follows:
(1) The presence of large coherent structures, especially the three-dimentional CRZ and region of high momentum will considerably affect the Damkohler, Karlovitz and turbulent Reynolds numbers, hence turbulent flame speed in regions where flame stabilisation occurs, as well as initial flame location relative to the burner exit. The CRZ boundary is characterised also by large velocity fluctuations, especially towards its base, doubtless it is here where highest levels of heat and mass transfer are occurring. The end of the CRZ also appears to be irregular and suffer intermittent changes in shape. The CRZ/PVC structures appear to wobble together.
(2) The propensity to excite combustion driven oscillations via the Rayleigh criterion. These flows are well known to be capable of exciting combustion driven oscillations with appropriate acoustic stimulation. Examination of the signals from the HWA is useful in elucidating the mechanism, This signal is caused not by the PVC but by the associated high momentum crescent shaped region of flow in the shear flow leaving the swirl burner, as highlighted in Fig. 8.
This region lies next to the PVC and arises from the PVC squeezing the flow leaving the swirl burner exit. This must also affect the size, shape and the length of the CRZ and hence the PVC. Thus the modulation of the HWA signal also must reflect significant variation in the CRZ size and length. The PVC has a well known susceptibility to small perturbations [10] which can grow into much larger effects. This is possibly the initial mechanism of excitation and arises from natural PVC fluctuations which couples through into the CRZ. The position of flame stabilisation, dependent on the CRZ, will then naturally follow this movement. When the oscillation of the CRZ occurs, any natural frequency of the system can then act to further excite this process and lead to increased oscillatory behaviour. Dawson et al. [27] showed that for a geometrically similar swirl burner with attached furnace (similar to Fig. 4.E) a 250 Hz oscillation could be generated with partially premixed natural gas combustion. Phase locked Laser Doppler Anemometry measurements showed that the oscillation was causing a sinusoidal variation in flow into the burner, thus giving a large variation in swirl number, correspondingly the strength of the CRZ varied considerably over the limit cycle, reflecting the variation in swirl number. Schildmacher et al. [34] derived experimental variations in swirl number for a gas turbine combustor with a 250 Hz oscillatory flow field, finding maximum variation between 0.2 and 0.8.
Froud et al. [11] showed phase locked fluctuating velocity and temperatures over a limit cycle oscillation in a similar swirl burner/furnace to that of Dawson, but at much lower frequency, typically 41 Hz. The mechanism was much more apparent here with complex three-dimentional CRZs appearing at various points in the cycle (probably CRZ1 and CRZ2), the CRZs acting as a flame reservoir and pilot flame. 燃气涡轮机英文文献和中文翻译(11):http://www.751com.cn/fanyi/lunwen_51784.html