Postgraduate Research Conference 2009
The Prize Winners
PRESENTATION: Methane Diffusion Flame Dynamics at Elevated Pressures
student: Hamidreza Gohari Darabkhani
supervisor: Dr Yang Zhang
Abstract
The understanding of combustion has been enhanced considerably by studies of laminar flames
at atmospheric conditions. However, many practical combustion devices operate at high
pressures to increase thermodynamic efficiency and decrease their physical size [1]. The current
understanding of the influence of pressure on thermo-physical properties of sooty flames is still
weak [2].
In this study the coupling of chamber pressure and fuel flow rate effects on the
flickering (oscillation) behaviour of methane-air diffusion flames was studied over the pressure
range of 1 to 14 bar. The flickering of a laminar diffusion flame is known to be caused by the
interaction of the flame and the vortices both inside and surrounding the flame jet [3]. It was
speculated that the vortical structures outside the luminous flame are due to buoyancy-driven
instability and are also responsible for the low frequency flame oscillation. Therefore, the
frequency of the outer vortices correlated with the flame oscillation frequency. Laminar
diffusion flames, at atmospheric pressure, are known to oscillate at a low frequency, typically
ranging from 10 to 20Hz, depending upon the operating conditions [3].
In this study, chemiluminescence and high speed photography along with image processing
techniques have been used to study the change in global flame shape as well as the frequency
and magnitude of the flame oscillation. The high pressure burner used in this study has an
internal diameter of 120 mm with a fuel nozzle exit diameter of 4.57 mm and produces a classic
over ventilated Burke–Schumann laminar co-flow diffusion flame. The optical system used for
the real-time measurement of flame light emissions is shown schematically in Fig 1. The
summation of the soot light and chemiluminescence of OH* and CH* radicals at 308±2.5 nm The evolution of the structure of the flame was captured using a digital monochrome high speed camera (FASTCAM-Ultima APX-RS) which can record up to 250,000 fps (frames per second). The camera uses a mega pixel resolution CMOS sensor and provides full resolution images (1024 x 1024) at frame rates up to 3,000 fps. It has been found that a framing rate of 3,000 fps with a camera shutter speed of 1/3000 s is optimum to capture the full details of the flame flickering and to avoid image saturation. |
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The experimental results clearly demonstrate that both pressure and fuel flow rate have a strong effect on the diffusion flame dynamics. The pressure is observed to change the flame shape, structure, flickering magnitude and frequency due to enhancement in formation and growth of outer toroidal vortices. The mass flow rate effects on oscillation magnitude and structure of vortices emerge from subtracted high speed images. It has been confirmed that flow rate does not have a significant effect on flickering frequency unlike the pressure.
High-speed images has shown that the periodic break-up of the methane flame at higher flow
rates (0.2 and 0.25 slpm) and elevated pressures is almost symmetric, with a pair of equal size
pockets of flame highlighting the structure of the outer toroidal vortices with further separation
at the tip of the breakaway flame part and it then splits into at least two wrinkled coherent flame
structures. However, the methane flames at lower flow rates (0.1 and 0.15 slpm) oscillate in a
more waving manner due to the alternating lateral nature of the outer vortices and the flame tip
is burnt out consisting of one portion of flame tongue. A steeper change was also observed on
the flame structure, temperature field and dynamics of methane diffusion flame at early stages
of the increase in chamber pressure.
The average flame luminosity was observed to increase with pressure up to 6 bar then it starts to
decrease with the further increase of pressure. The flame oscillation magnitude (Lf , the distance
between the flame lowest and highest heights) and oscillation wavelength (l, the length of the
separated part of the flame at the moment of separation) were obtained from the high speed
imaging database. It has been observed that the trends of these parameters correlate well with
the standard deviation (s) of mean pixel intensity (MPI), measured from the flame high speed
images. The increase in fuel flow rate increases the magnitude of oscillation, the flickering
frequency, however, remains almost constant at each pressure (see Fig 2). The peak flickering
frequency of a methane diffusion flame generally varies with the chamber pressure as a function
of Pn (f=15.7P0.17).
Fig 2: Peak oscillation frequencies of the methane air diffusion flame and the best curve fit
References
[1]. McCrain, L. L. and Roberts, W. L. Measurements of the soot volume field in laminar
diffusion flames at elevated pressures. J of Combust. Flame, 2005, Vol. 140, No. 1-2, pp.
60-69.
[2]. Gohari Darabkhani, H., Bassi, J., Huang, H. W., and Zhang, Y. Fuel effects on diffusion
flames at elevated pressures. J of Fuel, 2009, Vol. 88, No. 2, pp. 264-271.
[3]. Chen, L. D., Seaba, J. P., Roquemore, W. M., and Goss, L. P. Buoyant diffusion flames.
Proc. Combust. Inst., 1989, Vol. 22, No. 1, pp. 677-684.