Swirl number

A two-color pyrometer has been used along with the phase-Doppler anemometer to simultaneously measure the local velocity and size of kerosene droplets and the temperature of burning soot mantle in a swirl burner.[648] The measurements were conducted within the flame brush that develops in the shear layer of a swirl-stabilized, gas-supported kerosene flame with a swirl number of about 0.19 and potential heat releases of 10.6 and 15.5 kW, respectively. The results showed that the maximum burning fraction of the droplets occurs adjacent to the region denoted as gas flame but the value ranges from 20 5 to 40 5% depending on the axial station, and decreases sharply across the shear layer. The flame mantle temperature was found to be independent of droplet diameter, which agrees with previous results in the literature. 

Figure 17.5 Percent increase in flame speed versus the inverse of a squared average chamber (global) swirl number...

The mass flow rates of air and fuel were measured with a set of float-type flow meters to a precision of 1%, and the overall equivalence ratio was determined with an uncertainty of less than 2%. The swirl number of the second-flow arrangement was defined at the exit of the swirler as... 

The flammability and stability limits of Fig. 19.7 were obtained using fuel-air mixtures with the same equivalence ratio in the radial and tangential inlets, and without an axial jet. The lean flammability limit decreased from 0.57 to 0.4 as the swirl number was increased from 0.6 to 3.75, and the region of high-heat release moved closer to the swirler which represented an acoustic pressure antinode for the naturally occurring oscillations associated with a quarter wave in the entire duct, with frequency close to 200 Hz. Thus, swirl led to an increase in the amplitude of oscillations and to an earlier transition from smooth to rough combustion with antinodal RMS pressures up to 10 kPa, and initiated at an equivalence ratio of 0.5 for a swirl number of 3.75... 

Figure 19.7 also shows that the amplitude of oscillations decreased with equivalence ratios greater than around 0.8 for swirl numbers up to 1.35, and at smaller values of equivalence ratio for larger swirl numbers. This is in contrast with results for ducted flames behind steps and bluff bodies, where the amplitude is nearly always a maximum near stoichiometry. This appears to be due to a shift in the location of flame stabilization by up to 50 mm, from close to the exit of the swirler to the end of the expansion section, since the amplitude of oscillations depends strongly on the intensity of heat release near the acoustic pressure antinode. This shift in flame location may have been related to the movement of the flame attachment with pressure oscillations. 

The needle valve oscillated up to 12% of the total fuel, and flow rates larger than around 3% of the total implied that the bulk mean velocity in the axial jet was greater than the bulk flow in the swirler and the mean equivalence ratio was greater than unity. It was found that the amplitude of oscillations was unaffected by values of bulk mean velocity of the axial jet greater than 2.5 times that in the swirler for a main flow swirl number of 1.35, and 4 times that value for a swirl number of 0.6. Larger values of axial jet velocity led to a decrease in amplitude due to the penetration of the swirl-induced recirculation region by the jet and the consequences for the distribution of heat release. 

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. 

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. 

Pressure oscillations in the first arrangement depended on the equivalence ratio of the flow in the annulus and decreased with velocities in the pilot stream greater than that in the main flow due to decrease in size of the recirculation zone behind the annular ring and its deflection towards the wall. Increase in swirl number of the second arrangement caused the lean flammability limit to decrease, and the pressure oscillations to increase at smaller values of equivalence ratio. Unpremixedness associated with large fuel concentrations at the centre of the duct increased the pressure oscillations. Pressure oscillations caused the position of flame attachment to move downstream in both flows with a decrease in amplitude of oscillations. 

Axial and swirling air streams in the combustor issued from a circular chamber through a conical nozzle. The chamber was utilized both as an acoustic resonator and a settling chamber. It contained a honeycomb to straighten the how and two acoustic drivers to apply acoustic excitation to the jet. The nozzle exit diameter was 3.8 cm and the maximum Reynolds number based on this diameter and the exit velocity with and without air forcing was 4800 and 1400, respectively. The tests were performed with total air how rate of 85 1/min, and fuel how rate of 0.063 1/min. The swirl was applied with tangential air injection and the maximum swirl number tested was Ns = 0.30. 

As noted earlier, in this configuration extremely high swirl numbers (greater than 3) can be generated. In presenting the temperature and velocity data for... 

The swirl number, a dimensionless ratio of the angular momentum to the product of the axial momentum and the radium of the burner, can be varied through separate control of the two secondary air streams in order to study various burner designs. The air flows were measured using sharp edged orifices. Control of the air flows and calibration of the coal feeder made it possible to duplicate combustion conditions as determined both by exhaust gas analysis (CO, CO2, O2, NO, NOx) and aerosol characteristics. 

Figure 3. Composition distribution of the aerosol produced by near stoichiometric combustion ((f> = 0.96) and swirl number of 4.3. Oxides were determined as elements using FIXE carbon was inferred from absorption measurements.

Figure 4. Composition distribution determined by FIXE analysis of the aerosol produced by fuel lean combustion ( — 0.57) and swirl number of 0.14.

The conical coal injector was replaced with a blunt cyclin-der with a single axial jet so the flame could be stabilized at lower swirl numbers, thereby reducing the centrifugal deposition on the furnace walls. The radiation shield between the combustor and heat exchanger was removed to reduce particle losses further. The increased radiative transfer decreased the wall temperature substantially. The later experiments were also carried out at lower fuel-air equivalence ratios, i.e., (J> = 0.57. The combination of increased heat losses and increased dilution with excess air reduced the maximum wall temperature to 990°C for the experiments reported below. 

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