Oscillations input

The second device comprised a set of three circumferentially located pintle-type injectors Keihin, 10450-PG7-0031) to inject fuel radially into the main duct of the first flow arrangement as near-rectangular pulses. The frequency and duration of fuel injection were software controlled, and the fuel flow from each injector was delivered close to the outer edge of the annular ring flame holder by a cross-jet of air (1.2 x 5 mm), directed along the duct axis with exit velocity up to 100 m/s. The amplitude of the oscillated input was limited by the volume injection rate of the injectors. Propane, rather than methane, provided up to 3.5 kW of the total heat release of around 100 kW. With fluid dynamic damping, the RMS of the oscillated fuel flow corresponded to a heat release of around 1.8 kW. 

The results of Fig. 19.8 for a swirl number of 1.35 show that the attenuation increased to 10 dB with the velocity of the axial jet up to 42 m/s, and further increase to 47 m/s caused the amplitude to fall from around 6 kPa to less than 1.5 kPa and the attenuation to decrease from 10 dB to almost zero. Similar results were observed with the swirl number of 0.6 the attenuation improved with axial jet velocity up to 60 m/s, after which the amplitude and attenuation decreased. The decline in the amplitude of oscillation and its attenuation by active control was due to the interaction between the axial jet with a large velocity and the central recirculation zone, which caused the flame to move further downstream of the swirler and heat release to occur further from the pressure antinode. The consequent increase in the distance between the point of entry of the oscillated fuel and the active burning zone reduced the effectiveness of the oscillated input due to increased fluid dynamic damping and development of a large difference in phase between different parts of the oscillated flow, especially with swirl surrounding the oscillated axial jet. 

Figure 19.10a quantifies control performance with the oscillation of 5% of the total fuel in the axial jet for a swirl number of 1.35. It was increasingly effective with values of overall equivalence ratio less than 0.8 and the decline in amplitude with equivalence ratio greater than around 0.7. As explained earlier in connection with the results of Fig. 19.7, this was due to the downstream movement of the flame and the decline in effectiveness of the oscillated input. It should be noted that control was also hampered by the effect of the pressure oscillations at the pressure antinode on the coherence of the oscillated input. 

With flow rates of fuel and air larger by a factor of two, Fig. 19.106 shows that the amplitude of pressure oscillations increased by around 40% so that the power associated with the pressure oscillations increased by a factor of two, the factor by which the heat release and the oscillated input were increased. The attenuation of pressure oscillations, although around 2 dB less than in the flows of Fig. 19.10a, is important and, again, control was not effective for overall equivalence ratio greater than 0.78. The... 

Figure 19.11 shows that, for an overall equivalence ratio of 0.73 and a swirl number of 0.6, the amplitude of oscillation increased with the proportion of oscillated fuel due to the unpremixedness caused by the higher value of mean fuel concentration in the oscillated flow. The attenuation also increased with the fuel flow to around 5.5 dB with oscillation of around 10% of the fuel, compared with around 7 dB by the oscillation of 7% of the fuel in the axial jet and the same overall equivalence ratio and swirl number. As expected, control was less effective with higher swirl numbers due to the greater dissipation of the oscillated input. 

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

Control was more effective with the oscillation of fuel through a central jet than in the main flow of the swirl-stabilized arrangement with attenuations between 8 and 10 dB for equivalence ratios up to 0.75. With equivalence ratios greater than 0.78, the position of the flame moved from the upstream to the downstream end of the expansion. The attenuation was limited by the decay of the effectiveness of the oscillated input with distance from the point of injection, and the effect of the pressure oscillations at the pressure antinode on the coherence of the oscillated input. 

Figure 1. The output of a pH meter immersed in a CSTR with the Landolt oscillator. Input concentrations [SO3 ] = 0.075 M, [BrOs l - 0.065 Af, [Fe(CN)/[ = 0.02 M, [H2SO4] = 0.01 M.Residence time = lOOOs, at room temperature. (Reproduced from reference 7 permission of Hie Royal Society of Chemistry bn behalf of the PCCP Owner Societies)...

If the phase angle between the oscillation (input) and the response (output) is S, the torque on the plate or the inner cylinder can be written as... 

In the actual experiment, the amplitudes of the oscillation input (y, ) and output (CTq), and the phase angle (p) are measured. Therefore, each oscillatory shear flow... 

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