RANS Study of Hydrogen-Air Turbulent Non-Premixed Flames

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
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