RANS Study of Hydrogen-Air Turbulent Non-Premixed Flames

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Processes and Technologies for a Sustainable Energy duced to generate buoyancy-driven structures external to the flame. Once a vortex is developed it rolls along the flame surface while it is convected downstream. During this process, the vortex interacts strongly with the flame, making the flame surface bulge and squeeze. This motion is simulated by the time-dependent calculations. Fig. 5 Comparison of computed hydrogen/air flames for fuel-jet velocity of 106 m/s: (a) instantaneous temperature contours of computed flame; (b) iso-temperature contours obtained from time-averaged flame; (c) iso-temperature contours of steady-state flame. Since the mean temperature reflects the time the flame spends at a given location, the presence of the first bulge indicates that the flame spends considerable time in the bulged position at an axial location between 110 and 140 mm. The isotherms in the interior of the jet (r < 5 mm) are only moderately affected by the dynamic motion of the outer structures, as evidenced by the similarity of the instantaneous (Fig. 5a) and averaged (Fig. 5b) isotherms. To illustrate the importance of simulating the dynamic flames using unsteady CFD codes, calculations were also performed for the same flame using the steady-state option of FLUENT. Solution for this case converged to a flame having perfectly smooth surface. The iso-temperature visualization of the resulted flame is shown in Fig. 5c, which does not resemble either the instantaneous flame (Fig. 5a) or time-averaged flame (Fig. 5b). Fig. 6 Impact of outer and inner vortices on temperature for the flame shown in Fig. 5. Blowups of the two different vortex-flame interactions are shown insets. Vortex-flame interactions result in the presence of two types of vortices: one located on the fuel side of the flame and the other on the air side. Both types of vortices create positive (stretch) and negative (compression) stretch regions when interacting with the flame, as 4
Ischia, June, 27-30 - 2010 shown in Fig. 6. The outer vortex-flame interaction is shown in the left insert, and the fuel side vortex-flame interaction is shown on the right. Vortices are more evident in the isoradial-velocities contours of Fig. 7: red and blue colors represent positive (outward) and negative (inward) radial velocities, respectively. Fig. 7 Instantaneous radial velocity fields in the first part of the combustor for : on the left is depicted the case with Vortices motion are time dependent. Frequency spectra from the temperature fluctuations (measured on a single point in the domain, x=10cm; y=0.95cm) are obtained using FFT method. Figure 8 shows frequency spectra obtained from temperature data collected at a radial location of 9.5 mm within the shear-layer at a distance of 100 mm from the nozzle exit. The data, stored from 60000 time-steps, covered a real time of 0.6 s. We can distinguish two peaks over a frequency range between 0 to 1000 Hz. The one at 273.9 Hz corresponds to the fundamental frequency, and the subharmonic of this (547.8 Hz) appears as the second peak. The frequency of the highest peak can be compared with a dimensional analysis that defines a frequency as the ratio of mean velocity and characteristic length of the vortices: (1) where L v is the distance between two consecutive vortices. Vortex frequency value is very close to the peak obtained from the FFT and displayed on Fig. 10. Physically this phenomenon can be explained considering turbulent fluctuations as different spatial and temporal scales vortical structure overlap. When these move inwards the fluid they induce a perturbation on the characteristic scalars. 5
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RANS Study of Hydrogen-Air Turbulent Non-Premixed Flames

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