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METHANE DIFFUSION FLAME DYNAMICSAT ELEVATED PRESSURES

Hamidreza Gohari Darabkhani1 and Yang Zhang21School of Mechanical, Aerospace and Civil Engineering, The University ofManchester, Manchester, UK2Department of Mechanical Engineering, The University of Sheffield,Sheffield, UK

The chamber pressure and fuel flow rate effects on the flickering behavior of methane-air

diffusion flames was studied over the pressure range of 1 to 10 bar. Photomultipliers and

high-speed imaging techniques have been used to study the frequency and magnitude of

the flame oscillation and the change in global flame shape. The instability behavior of the

flame was observed to be sensitive to both the fuel flow rate and pressure. Particularly, it

has been observed that the flame responds to the change of pressure more when the pressure

is relatively low. High-speed imaging has shown that the periodical break-up of the methane

flame at higher flow rates is almost symmetric. However, the methane flames at lower flow

rates oscillate in a more waving manner due to the alternating lateral nature of the outer

vortices. The average flame luminosity was observed to increase with pressure up to 6 bar

and then starts to decrease with the further increase of pressure. The flame oscillation mag-

nitude (Lf) and oscillation wavelength (k) were obtained from the high-speed imaging data-

base. It has been observed that the trends of these parameters correlate well with the

standard deviation (r) of mean pixel intensity (MPI), measured from the flame high-speed

images. The increase in fuel flow rate was observed to increase the magnitude of oscillation.

The dominant flickering frequency of a methane diffusion flame varies with the chamber

pressure as a function of Pn(f¼ 15.7P0.17).

Keywords: Diffusion flames; Flame dynamics; Flickering frequency; High-pressure

INTRODUCTION

Detailed knowledge on the simultaneous pressure and flow rate dependence ofthermo-physical properties of laminar diffusion flames are of significant relevance tothe understanding of combustion process and instability. Many systems operate athigh pressures, such as diesel engines and gas turbine combustors. Research on lami-nar diffusion flames at high pressures has been limited to a number of studies focusedon the soot and temperature profiles within a stable laminar flame (Bento et al.,2006; Flower & Bowman, 1986; Joo & Gulder, 2009; Lee & Na, 2000; Liu et al.,

Received 22 May 2009; revised 4 October 2009; accepted 15 October 2009.

Address correspondence to Hamidreza Gohari Darabkhani, School of Mechanical, Aerospace, and

Civil Engineering, The University of Manchester, Manchester, M13 9PL, UK; Tel.: þ44 161 3063731;

E-mail: [email protected], [email protected]

Combust. Sci. and Tech., 182: 231–251, 2010

Copyright # Taylor & Francis Group, LLC

ISSN: 0010-2202 print=1563-521X online

DOI: 10.1080/00102200903418252

231

2006; McCrain & Roberts, 2005; Schalla & McDonald, 1955; Thomson et al., 2005).However, only a few experimental studies have been carried out on diffusion flamedynamics at sub-atmospheric (Albers & Agrawal, 1999; Durox et al., 1997) and elev-ated (Gohari Darabkhani et al., 2009) pressures. The current understanding of theinfluence of pressure on thermo-physical properties of sooty flames is still weak(Bento et al., 2006; Thomson et al., 2005). It is well known that increased pressureplays a significant role in increasing soot concentration, leading to an overalldecreasing soot temperature in diffusion flames (Bento et al., 2006; Flower, 1989;Thomson et al., 2005). Pressure also has significant effects on parameters such asflow velocity, flame structure, and thermal diffusivity (Glassman, 1998; GohariDarabkhani et al., 2009).

The phenomenon of flame flickering in diffusion flames is well known andhas been extensively studied at atmospheric pressure (Albers & Agrawal, 1999;Chamberlin & Rose, 1948; Chen et al., 1989; Grant & Jones, 1975; Katta et al., 2009;Kimura, 1965; Mishra & Kiran, 2009; Toong et al., 1965; Yilmaz et al., 2008) anddifferent gravity levels (Arai et al., 1999; Bahadori et al., 2001; Durox et al., 1995;Sato et al., 2000). However, the exact mechanism of this buoyancy-induced instabilityhas not been fully understood. It is speculated that the mechanisms responsible forflame oscillations are highly sensitive to various burner configurations, flame flowconditions, and gravitational effects due to buoyancy (Yilmaz et al., 2008). It wasnoted (Arai et al., 1999; Bahadori et al., 2001) that flicker frequency increases withincreasing gravitational level, and that flicker was dominated by buoyancy.Low-gravity experiments by Bahadori et al. (2001) show that the flame flickerfrequency is proportional to the square root of the gravitational acceleration.

The fuel flow rate effects on flickering behavior and oscillation of diffusionflames at atmospheric pressure have also been studied extensively (Albers &Agrawal, 1999; Chamberlin & Rose, 1948; Chen et al., 1989; Durao & Whitelaw,1974; Kimura, 1965; Toong et al., 1965; Yilmaz et al., 2008). According to theliterature, laminar diffusion flames are known to oscillate or flicker at a low fre-quency, typically ranging from 10 to 20Hz, depending upon the operating con-ditions. Chamberlin and Rose (1948) reported that the flicker frequency of about10Hz for diffusion free flame was not greatly affected by either the fuel type andflow rate or the burner size tested. The existence of discrete frequencies in the vicinityof 11Hz in a range of methane fuel exit velocities (0.9 to 2.6m=s) were measured byDurao and Whitelaw (1974). Toong et al. (1965) studied both free and enclosedflames, and the frequency of oscillation of the enclosed flames was found to be astrong function of their surroundings. In their study, the free-flames again repro-duced the 10Hz behavior found by Chamberlin and Rose (1948).

Hamins et al. (1992) analyzed experimental data over a wide range of operat-ing conditions and found that the flicker frequency correlated with burner size andfuel jet exit velocity. Kimura (1965) revealed that a regular flame oscillationappears when the fuel-flow rate is increased above a critical value, and it is axiallysymmetric. The oscillation was found to have no connection with the size of thecombustion chamber and the length of the fuel pipe. The frequency of the oscil-lation for propane diffusion flames was found to be between 10 to15Hz for therelatively wide change in the fuel flow rate and in the diameter of the port(Kimura, 1965).

232 H. GOHARI DARABKHANI AND Y. ZHANG

The flickering of a laminar diffusion flame is known to be caused by theinteraction of the flame and the vortices both inside and surrounding the flamejet. An elementary flame-vortex interaction is typically periodic, regular, and repro-ducible in nature, and can therefore be easily characterized (Albers & Agrawal, 1999;Chamberlin & Rose, 1948; Grant & Jones, 1975). Chamberlin and Rose (1948)ascribed the periodic nature of the phenomenon to alternate increases and decreasesin the rate of diffusion of oxygen into the flame. That is to say, when the fuelflow exceeds some critical value, an envelope of combustion products is created thattemporarily separates the reactants (quenching of the flame front). There is thus atime interval before the slower moving air can diffuse through this envelope andagain create a flame sheet (Grant & Jones, 1975). It can be considered that the flameoscillations occur due to quenching of the flame front in one specific location orregion (Yilmaz et al., 2008).

Generally two distinct vortices have been identified in the jet diffusion flamesstudies (Chen et al., 1989, 1991; Davis et al., 1994). They are characterized by largetoroidal vortices outside the luminous flame and small roll-up vortices inside. Largetoroidal vortices in the oxidizer side of the luminous flame convect downstream andinteract with the flame to create wrinkles on the flame surface (Albers & Agrawal,1999). It was speculated that the vortical structures outside the luminous flame aredue to buoyancy-driven instability and are also responsible for the low-frequencyflame oscillation (Chen et al., 1989; Davis et al., 1994). Therefore, the frequencyof the outer vortices correlated with the flame oscillation frequency (Albers &Agrawal, 1999). The outer vortices develop because of the Kelvin-Helmholtz typeinstability in the buoyancy-induced shear layer surrounding the flame surface(Buckmaster & Peters, 1988; Davis et al., 1994; Katta & Roquemore, 1993). Thebuoyancy effect on the shape and flickering frequency of a diffusion flame has beenstudied by various researchers in the literature (Agrawal et al., 1998; Albers &Agrawal, 1999; Davis et al., 1990; Durox et al., 1995; Sunderland et al., 1998; Yuan& Durox, 1993). Buoyancy is directly related to the Froude number, which is inver-sely proportional to the product of gravitational acceleration and square of theambient pressure (Agrawal et al., 1998; Davis et al., 1990). According to the numeri-cal results of Ghoniem et al. (1996), in an axisymmetric vortex sheet of pool fires, theKelvin-Helmholtz instability is augmented by Rayleigh-Taylor type instability as theroll-up pushes the hot gases outward and the cold air inward with respect to theinterface. Williams et al. (2007) also noted the conclusion of several recent studiesthat buoyant vorticity generation (created by a misalignment of the density gradientand the gravitational vectors) and baroclinic vorticity generation (resulting from amisalignment of the density gradient and the pressure field vectors) are the primarycauses of the large-scale toroidal vortices at the periphery of the plumes, which arereferred to as ‘‘puffing’’ instability (Cetegen & Ahmed, 1993; Hamins et al., 1992).

It was reported by Miller and Maahs (1977) that a very marked changeoccurred in the shape, structure, and the region of stable combustion of the meth-ane–air diffusion flame as pressure was increased. It is found (McCrain & Roberts,2005; Thomson et al., 2005) that buoyancy-induced instabilities will lead to flameflicker at slightly elevated pressures because the buoyancy forces scale with pressure.Furthermore, experiments by Yuan and Durox (1993) reveal that the flame flicker issuppressed at low buoyancy, achieved by reducing the ambient pressure. Still, there

METHANE DIFFUSION FLAME DYNAMICS AT ELEVATED PRESSURES 233

is a lack of understanding on the influence of elevated pressure on the flame dynam-ics of a laminar=transitional diffusion flame (Gohari Darabkhani et al., 2009).

Experiments were performed by Albers and Agrawal (1999) at sub-atmosphericpressures to scale the effects of buoyancy on flame flicker. The flame flicker fre-quency was found from Fourier analysis of angular deflection data to vary linearlywith operating pressure. In their study, different buoyancy levels were achieved byvarying the jet exit Froude number, which is inversely proportional to the gravi-tational acceleration (g) or square of the ambient pressure (p2) (Agrawal et al.,1998; Davis et al., 1990). Flame flickering of jet diffusion flames has been character-ized experimentally in terms of pressure and gravitational force by Durox et al.(1997). The measurements show that the relationship between the frequency andthe reduced gravity at varying sub-atmospheric pressures is very close tof/ (Pg2)1=3 , when the burners have a sufficiently small outlet diameter. Otherwise,the linear relationship between pressure and frequency holds. Most of the currentinvestigations have been conducted at pressures below atmospheric pressure toreduce the effects of buoyancy. At reduced gravity or ambient pressure, a diffusionflame is wider and longer than their normal gravity flame counterparts (Sunderlandet al., 1998; Yuan & Durox, 1993).

In our recent study (Gohari Darabkhani et al., 2009), we investigated the influ-ence of elevated pressures up to 16 bar on the flame structure of laminar diffusionflames, whereas particular attention has been paid to the effect of fuel variability.It has been observed that the flame properties are very sensitive to the fuel typeat elevated pressures. The geometry of the stable flame and the instability regionof the flame were observed to change dramatically with pressure according to thefuel type. In this study, the coupling of fuel flow rate and chamber pressure onthe flame dynamics (flickering behavior) of methane-air diffusion flames was studiedover the pressure range of 1 to 10 bar. Optical diagnostics (chemiluminescence andhigh-speed imaging) have been used to study the change in flame geometry and fre-quency and the magnitude of flame oscillations. A wide range of fuel flow rates andpressure were studied in order to gain a better understanding of the influence ofthese two parameters.

EXPERIMENTAL SETUP

The high-pressure burner used in this study is similar to the burner applied inreference (Miller & Maahs, 1977). The chamber, which has an internal diameter of120mm, is shown schematically in Figure 1. A classic over-ventilated Burke–Schu-mann (Burke & Schumann, 1948) laminar diffusion flame is produced. The flameis stabilized on a fuel nozzle with an exit diameter of df¼ 4.5mm. Gaseous methane(CH4) fuel was supplied from a compressed gas cylinder regulated by a needle valveand measured by a calibrated mass flow meter with 1% full scale accuracy. The cali-bration of mass flow meters are performed by manufacturer at the standard con-ditions for temperature and pressure (STP), which are 70�F (21.1�C) and 14.7 psia(1.01 bar) respectively, using dry nitrogen gas. The calibration can then be correctedto the appropriate gas desired based on relative correction gas factors. The flowranges of methane and air mass flow meters are 0–0.6 slpm and 0–50 slpm respect-ively, with the maximum working pressure of 500 psia (34.47 bar). During each set


of the experiments, the methane mass flow rates of 0.1, 0.15, 0.2, and 0.25 slpm (stan-dard litres per minute) were kept constant at all pressures. Co-flow air is suppliedfrom a compressed air bottle into the burner and is diffused using a layer of glassbeads, after which a honeycomb structure with 1.5mm diameter holes is used tostraighten the flow. Co-flow air was controlled by a needle valve to produce a con-stant mass flow rate of 15 slpm through a coaxial air nozzle with an equivalent exitdiameter, da¼ 37.2mm, for all the diffusion flames. Table 1 shows the physical para-meters of fuel and air streams for experiments. The fuel nozzle jet exit Reynoldsnumbers (Re) of the methane flow rates of 0.1, 0.15, 0.2, and 0.25 slpm were 31,46, 61, and 76, respectively. The Reynolds number of the co-flow air through theair exit nozzle was 607. The Reynolds number was calculated under the assumptionthat the flow had properties at atmospheric temperature and pressure. Increasing thepressure for a constant mass flow rate (m) does not significantly change the Reynoldsnumber (Re¼ 4m=pld) because viscosity (l) only weakly depends on pressure(Higgins et al., 2001). One may then conclude that all flows were laminar.

Table 1 Fuel and air parameters

Gas type

Volume flow rateMass flow rate

[mg=s]

Velocity

[m=s] Re number Fr numberSlpm [L=min] [m3=s]

Methane (CH4) 0.1 1.67E-06 1.13 0.115 30.51 0.312

0.15 2.50E-06 1.7 0.172 45.76 0.703

0.2 3.33E-06 2.27 0.230 61.02 1.249

0.25 4.17E-06 2.83 0.287 76.27 1.952

Air 15 2.50E-04 301 0.230 606.79 0.145

Figure 1 Cross-section of the high-pressure burner.


To pressurize the chamber, nitrogen flow is introduced through the base of theburner using a ring of 1.5mm-diameter holes. The nitrogen flow also keeps thechamber walls cool and the window ports free of condensation and soot. Increasingthe pressure within the vessel is achieved by increasing the flow rate of nitrogen (inthe range of 0–15 slpm) and simultaneously decreasing the flow rate of the exhaust byadjusting the back-pressure regulating valve. This maintains the chamber pressurebetween 1 and 20 bar. Optical access to the burner is provided by four windows witha diameter of 50mm and a thickness of 20mm. Two are made from fused quartz andtwo from high-resistivity float-zone silicon (HRFZ-Si). The fused-quartz windowsprovide optical access in the visible band for current study while the HRFZ-Siwindows provide access in the far infrared up to 1000 mm, which were used for adifferent study based on terahertz-time domain spectroscopy (Bassi et al., 2005).

The optical system used for the real-time measurement of flame light emissionsis shown schematically in Figure 2. The global flame light emission is focused onto a5mm-diameter bundle of fine fiber optical cable using a spherical lens. The bundle offine fibers is bifurcated randomly into two equal subdivisions to produce two chan-nels of the same signal intensity from the same imaged volume. Each channel is thenguided to a photomultiplier tube (ORIEL model 70704). At the end of each channel,OH� and CH� interference filters at wavelengths of 308� 2.5 nm and 430� 5 nm,

Figure 2 Schematic of the experimental setup.


respectively, are used. The summation of the soot light and chemiluminescence ofOH� and CH� at the two chosen wavelengths are measured. The availability oftwo wavelengths may provide qualitative information on the flame chemistry. Theintensity of the filtered light is converted into voltage signals, which are capturedby an analogue to digital data acquisition card (National Instruments PCI-MIO-16E-1) at 5000 samples per second averaged over a duration of 4 s. Real-time signalprocessing was performed by using a LabVIEW 8.5 virtual instrument (VI) to obtainthe flame flickering frequency spectrum of the flame light emission. Optical accessgained through the second fused quartz window is used to capture the evolutionof the structure of the flame using a digital monochrome high-speed camera. Thecamera uses a mega pixel resolution complimentary metal-oxide semiconductor(CMOS) sensor and provides full resolution images (1024� 1024) at frame ratesup to 3,000 frames per second (fps). It has been found that a framing rate of3,000 fps with a camera shutter speed of 1=3000 s is optimum to capture the fulldetails of the flame flickering and to avoid image saturation.

RESULTS AND DISCUSSION

The stable operating range for the burner is dependent on the fuel type, flowrate, and chamber pressure (Gohari Darabkhani et al., 2009). The effect of elevatedpressure on the flickering phenomena of diffusion flames can be clearly observed inthe unstable regime of methane flames for the chosen flow rates of 0.1, 0.15, 0.2,and 0.25 slpm. At atmospheric pressure, the flame shape is convex, and its maximumwidth is wider than the burner nozzle exit diameter. The soot formation is dominantat the tip of the flame (Bento et al., 2006). At higher pressures, the soot formation dra-matically increases because pressure affects the mechanism of soot formation. Theflame also narrows with a concave shape due to increase in fuel and air densities, whilemaintaining a constant mass flow rate, according to the law of conservation of mass(Gohari Darabkhani et al., 2009; Liu et al., 2006). Pressure enhances the outer vorti-ces to induce stronger flame oscillations. At atmospheric pressures, the flames appearstable at all test fuel flow rates. However, by increasing the pressure, a very markedchange occurred in the shape, structure, and region of stable combustion. It has beenobserved that stronger changes of the flame geometry, temperature distribution, andflickering behaviors occur at the early stages of the pressure increase.

The oscillation (flickering) of a laminar diffusion flame is known to be causedby the interaction of the flame and the vortices both inside and surrounding theflame jet. Change in both flow rate and pressure can cause vital changes in the inter-action of the flame and the outer flame vortices. It is well known that the outer vor-tex is due to buoyancy-driven Kelvin-Helmholtz type instability, and the frequenciesof these vortices are well correlated with the flame bulge and the flame oscillationfrequency (Buckmaster & Peters, 1988; Chen et al., 1989; Davis et al., 1994; Katta& Roquemore, 1993). Although a prediction was made and later confirmed byexperiments, the influence of fuel flow rate on the flickering frequency is not signifi-cant over the range of flow rates used, but it has a significant effect on the magnitudeof oscillation at different tested pressures. The magnitude of oscillation (Lf) isdefined by the distance between the flame’s lowest and highest heights. The oscil-lation wavelength (k) is defined by the length of the separated part of the flame at


the moment of separation. The definitions of flame scale parameters and the outervortices position are presented in Figure 3.

It is observed that a regular flame oscillation appears when the fuel flow rate isincreased above a critical value. At a higher fuel mass flow rate, the threshold pres-sure for a flame to become unstable is lower. At a flow rate of 0.15 slpm, the onset ofobservable flame flickering occurs in the flame tip at a chamber pressure of 2 bar,and a small flame is periodically detached from the main flame at 4 bar. However,for a lower fuel flow rate of 0.1 slpm, the burner appears to produce a stable flameuntil 6 bar. During the entire operating regime of the burner, it is shown via subtrac-tion of two consecutive high-speed images that even a stable flame always has a verysmall flickering at the tip. At larger flow rates (0.2 and 0.25 slpm) and at atmosphericpressure, flames are visually stable. However, above a chamber pressure of 2 bar,very large oscillations were recorded. At a flow rate of 0.2 slpm, a large flame sep-aration was found at 4 bar. In contrast, flame separation started at 2 bar for a flowrate of 0.25 slpm.

The size of the oscillation wavelength has been observed to increase initiallywith both fuel flow rate and pressure and then decreases with a further increase ofpressure. For methane flame at lower flow rates (0.1 and 0.15 slpm), the maximumoscillation wavelength has been found to occur at 8 bar. However at higher flow rates(0.2 and 0.25 slpm), this length was a maximum at 4 bar (see Figure 4). The oscil-lation magnitude generally was increased with fuel flow rate. However, themaximum oscillation magnitude has been observed to occur again at 4 bar and8 bar for higher and lower methane flow rates, respectively. The trends are consistent

Figure 3 Definitions of flame scale parameters in (a) laminar and (b) flickering flames, and also the outer

vortices location (Arai et al., 1999; Katta et al., 2009).


with the trends of oscillation wavelength for both the higher and lower methane flowrates (see Figure 4). The initial increase in the flame oscillation magnitude can beexplained by the dominant effect of pressure on enhancement of Kelvin–Helmholtztype of instabilities, after which, at a certain pressure, soot generation effects becomemore dominant. It is found by Katta et al. (2009) that in a jet diffusion flame, themagnitude of flame oscillations (flicker) decreases with the amount of soot generatedin the flame; however, the frequency of the oscillations does not change. Themaximum oscillation magnitude of methane at flow rates of 0.2 and 0.25 slpm wasmeasured to be 24 and 26mm, respectively. This length was 15.7 and 18mm at itsmaximum for 0.1 and 0.15 slpm methane flow rates, respectively.

A sequence of high-speed images of methane-air diffusion flames at 0.15 and0.25 slpm fuel flow rates and at three different pressures of 2, 6, and 10 bar (labeleda, b, and c) are shown in Figures 5 and 6, respectively. The time interval between twoconsecutive images is 3.3ms. The lower part of each row (a, b, and c) contains theresultant images obtained by subtraction of two consecutive images of the flame witha 1.6ms time interval. The high-speed images illustrate the structure of the outertoroidal vortices outside the luminance flame, and the subtraction of images showhow the luminance boundaries of the flame is growing in the base and tip of theflame. Therefore, in a sequence of the flame oscillation, the image subtraction tech-nique is able to highlight the flame surface change at certain time interval. For amethane flow rate of 0.15 slpm and at 2 bar, the flame appears stable to the nakedeye (see Figure 5a); however, through the subtraction results of the images (seeimages below row (a) in Figure 5), the oscillating nature of the flame is visualizedfrom the flame moving boundaries. The rate of the flame’s boundary changesincreases and then decreases in a complete cycle of flame oscillation. At higher pres-sures, the flame luminosity dramatically rises due to the increased formation of soot.The flame narrows and changes to concave shape, leading to flame bulge, necking,and separation. The flame bulge is believed to be formed due to the rotational flow

Figure 4 Oscillation wavelength (k) for the methane flame at different flow rates at elevated pressures.


inside the outer vortices. The mechanism is speculated to be that the toroidal vortexbelow the flame bulge moves the flame surface radially outward, while the one abovethe bulge drags the flame surface inward (Chen et al., 1989). The outer vorticesenhance the fuel-air mixing at some instant; as a result, the local burning rateincreases, leading to necking and quenching of a portion of the flame tip. By increas-ing the chamber pressure, the formation and growth of outer toroidal vortices

Figure 5 Methane (0.15 slpm)-air (15 slpm) diffusion flame pictures, taken by a digital high-speed camera

(FASTCAM-Ultima APX) at shutter speed of 1=3000 s at pressures (a) 2 bar, (b) 6 bar, and (c) 10 bar. The

framing rate is 3000 fps and the time interval between two consecutive images is 3.3 ms. The lower part of

each row (a, b, and c) belongs to the subtraction of two consecutive images of the flame at that pressure

with 1.6ms time interval.


become more evident, and regular flame necking is observable (see Figure 5b). It isalso interesting to note that this flame changes more dramatically at the flame tip andthe lower part, while the necking part has a slower pace of change. From Figure 5b,it appears that the flame oscillation is not symmetric at this flow rate, and theouter vortices make the flame oscillate side by side. This phenomenon becomes more

Figure 6 Methane (0.25 slpm)-air (15 slpm) diffusion flame pictures, taken by a digital high-speed camera

(FASTCAM-Ultima APX) at shutter speed of 1=3000 s at pressures (a) 2 bar, (b) 6 bar, and (c) 10 bar. The

framing rate is 3000 fps and the time interval between two consecutive images is 3.3ms. The lower part of

each row belongs to the subtraction of two consecutive images of the flame at that pressure with 1.6ms

time interval.


dominant at higher chamber pressures. The flame images at 10 bar (see Figure 5c)and the resultant images of consecutive image subtraction clearly show the wavingstructure of oscillation.

The periodic nature of the flame necking and separation is due to alternatevortex shedding. The flame is then seen in its original state, as there is a time intervalbefore the slower moving air can diffuse through this envelope and again form anenvelope of flame that will extinguish later (Chamberlin & Rose, 1948). The flicker-ing behavior of methane diffusion flames at lower flow rates (0.12 and 0.17 slpm withthe fuel jet exit mean velocity of Uf¼ 0.1 and 0.15m=s, respectively) appears to beconsistent with the planar visualized methane diffusion flame of Chen et al. (1989)with Re¼ 110 (Uf¼ 0.19m=s). They did not mention the waving oscillation behaviorof low jet exit velocity flames; however, it is evident from the images they have pre-sented (Figure 3b of Chen et al., 1989). It is worth noting that in their study, theinner roll up vortices did not appear at the lower flow rates (Re¼ 110 and 410),but the well-defined roll-up vortices inside the luminance flame appeared at a flameheight of 20mm at Re¼ 2390, where the size of the vortices grew rapidly upstream.

For higher flow rates, obvious flame bulge and necking starts to occur at lowerpressures. A higher fuel flow rate means that the fuel jet is able to induce a strongervortex, which would then produce better fuel and air mixing. As a result, the fuelcould be burnt faster near the vortex, which causes the necking and separation tooccur faster. The effect of increased buoyancy at higher flow rates is clearly high-lighted in the high-speed images shown in Figure 6 for a methane flame with a flowrate of 0.25 slpm. The flame appears to be almost stable at 1 bar, but strong oscil-lation at a low-frequency which results in flame necking and separation was observedat 2 bar. The images of this flame at 2 bar have been presented in Figure 6a. Theseimages highlight the presence of large outer toroidal vortices outside the luminanceflame. The subtracted images of Figure 6a (below the original images) show how theluminance boundaries of the flame is growing in the base and tip of the flame duringflame oscillation. The increase in oscillation magnitude and wavelength due toformation of the bigger outer vortices are clear from comparison of these images(see Figure 6) with methane flame images at the same pressure but lower flowrate (see Figure 5). According to the subtraction of images, presented in Figures 5and 6, at lower pressures and flow rates, oscillations mainly occur at the flame tip.However, change of luminance flame boundaries at higher pressures and flow ratesshow that oscillation is dominant at both flame tip and base. It has been observedthat the luminance boundaries of flame tip are growing almost twice as fast as theflame base boundaries.

At higher pressures, a larger chunk of the flame tip is periodically detachedand burned out (see Figure 6b and 6c at 6 and 10 bar, respectively). This may beexplained by the scaling of buoyancy with pressure that will increase the convectivevelocity at the flame surface. This causes an increase in the acceleration of toroidalvortices, which correlates with the flame surface oscillations at the same frequency(Chen et al., 1989). The vortices bring in more fresh air to the flame surface; there-fore, a stronger local fuel-air mixing occurs, resulting in local necking of the flameand larger break-up of the flame tip at elevated pressures. Images in Figure 6band 6c show that the breakup of the methane flame at higher flow rates is almostsymmetric with a pair of equal size pockets of flame, highlighting the structure of


the outer toroidal vortex at the base of flame bulge with further splitting of thebreakaway flame. Conversely, the methane flames at lower flow rates oscillate in amore waving manner with a lower oscillation magnitude and wavelength and smallerdetached flame tip (see Figure 5b and 5c).

Figure 7 shows the typical flame=vortex interaction in a jet co-flow diffusionflame at pressures of 6 and 10 bar at methane flow rates of 0.15 and 0.25 slpm (allat the same instant sequence of 1.65ms in a complete flickering cycle). This is accord-ing to the typical flame=vortex interaction presented by Renard et al. (2000) andKatta et al. (2009). The side-by-side formation and growth of outer vortices at lowermethane flow rates is evident in Figure 7a and 7c; however, these vortices are almostsymmetric at higher flow rates of methane (see Figure 7b and 7d). The first identifi-able outer vortex, at a flow rate of 0.25 slpm and pressure of 6 bar (at a particularinstant presented in Figure 7b) is located at 7 to 24mm above the burner nozzle.An obvious flame bulge is seen after the ‘‘dislocated saddle’’ point A, which islocated at a height of 25mm above the burner nozzle (see Figure 7b). The differentpositions of the outer vortices at 0.15 and 0.25 slpm methane flow rates are clearer athigher pressures (see Figure 7c and 7d, respectively). The identifiable outer vorticesat a flow rate of 0.15 slpm and the pressure of 10 bar is located at approximately 3 to13mm followed by a second vortex at 14 to 24mm at one side above the burner.However, at the other side, the first vortex is located at 6 to 18mm, followed by avortex located at above 19mm (see Figure 7c). The first outer vortices of flame with

Figure 7 Typical flame=vortex interaction in a jet co-flow diffusion flame at pressures 6 and 10 bar

for methane flow rates of 0.15 and 0.25 slpm (all at the same instant sequence of 1.65ms in a complete

flickering cycle).


0.25 slpm methane at 10 bar pressure are located symmetrically at 5 to 21mm eitherside of the flame at this sequence (see Figure 7d). As the evolution of a large-scalestructure is governed by Kelvin-Helmholtz instabilities, the frequency, mutual inter-action, and energy distribution among various length-scales are controlled by theinitial conditions of the flow. Combustion processes strongly modify the instabilitymechanisms, but coherent structures similar to those observed in non-reacting flowsmay be generated in an annular jet diffusion flame by forcing it at a preferred modefrequency (Renard et al., 2000).

The graphs of mean pixel intensity (MPI) of the flame high-speed images versustime, at methane flow rate of 0.25 slpm at 6 bar, and the Fast Fourier Transform(FFT) analysis of the data for peak frequency measurement have been shown inFigure 8a and 8b, respectively. The MPI as an arbitrary unit (a.u.) was measuredby image processing using MATLAB1. The maximum points of the cyclic MPIgraph correspond to the maximum emission of the flame; similarly, the minima referto the minimum flame emission after the burning out of the detached part period-ically. The average of MPI values for a full data-range of high-speed images at

Figure 8 Methane (0.25 slpm)-air (15 slpm) diffusion flame at 6 bar. (a) Mean pixel intensity (MPI) of

flame high speed images; (b) FFT analysis of MPI for peak frequency measurement.


different pressures have been presented in Figure 9. The average flame luminosity at0.25 slpm is almost 1.6 times that of the flame at 0.15 slpm. These intensities havebeen observed to increase until a certain pressure, then decrease with the furtherpressure increase. According to these results, the maximum intensities occur at6 bar for all flow rates tested (see Figure 9). The trend of this result is consistent withthe previous results (Gohari Darabkhani et al., 2009) in which the maximum heightof a stable laminar methane diffusion flame occurred at 6 bar. In addition, the stan-dard deviation (r) of MPI, measured from the flame high-speed images, showsthat the higher flow rate generally results in more flame fluctuation and intensityvariation in comparison with lower fuel flow rates (see Figure 10). The maximum

Figure 10 Standard deviation (r) of mean pixel intensity (MPI) from high-speed images of the methane

diffusion flames at different fuel flow rates.

Figure 9 Average of mean pixel intensity (MPI) values for the methane flames at different fuel flow rates

and pressures.


standard deviation of MPI for 0.2 and 0.25 slpm of methane flow rates were observedat 4 bar. This maximum occurred at 8 bar for a methane flow rate of 0.1 and0.15 slpm. The higher value of this intensity fluctuation refers to giving off a largerportion of the detached flame (indicated by oscillation wavelength) during the flameoscillation. These results are also consistent with the trend of maximum oscillationmagnitude observed for these flames (see Figure 4). It appears that the standarddeviation of MPI can be a general indicator of the flame oscillation wavelengthand magnitude trends.

The power spectra of the flame emissions, by collecting the radiation spectrumat OH� and CH� emission bands using the interference filter, are shown in Figure 11.The frequency spectra of methane flame at 0.15 slpm (see Figure 11a) shows that atatmospheric pressure, the flame has a dominant (peak) frequency of 16Hz and threeother noticeable peak frequencies at 5.75, 11.75, and 12.75Hz, each with lower

Figure 11 Frequency spectra for methane diffusion flames at elevated pressures. (a–c) Methane (0.15 slpm)

at 1 bar, 6 bar, and 10 bar, respectively; (d–f) Methane (0.25 slpm) at 1 bar, 6 bar, and 10 bar, respectively.


amplitude than the dominant frequency. Under elevated pressure, the methane flameat 0.15 slpm clearly exhibits frequency spectra with the peak frequencies of 21.25Hzand 23.25Hz at 6 and 10 bar, respectively, each with as many as six harmonics of thepeak flickering frequency (see Figure 11b and 11c). These observed harmonics indi-cate that the flame has a clearly defined coherent structure, which can be confirmedfrom the flame images in Figure 5.

The influence of pressure on the flickering frequency of methane at a higherflow rate (0.25 slpm) is shown from comparison of power spectrums at 1 bar (seeFigure 11d) with 6 and 10 bar (see Figure 11e and 11f, respectively). At atmosphericpressure, methane at 0.25 slpm flickers with the dominant frequency of 16.5Hz andthree other noticeable peak frequencies of 4.75, 11.75, and 14.5Hz. This flame underelevated pressure oscillates with the peak frequencies of 21Hz and 23.25Hz at 6 and10 bar, respectively. The corresponding harmonics are also observable at these pres-sures. For both fuel flow rates, the dominant flickering frequencies are very close toeach other at a certain pressure.

The flickering frequency results of methane flame at different flow rates (0.1,0.15, 0.2, and 0.25 slpm) and elevated pressures up to 14 bar are presented inFigure 12. It has been observed that the influence of fuel flow rate on the flickeringfrequency is not significant over the range of flow rates used, although it has a quitesignificant effect on the flame oscillation magnitude and wavelength. This confirmsthat the weak influence of fuel flow rate (jet momentum) on dominant frequencyis valid not only at atmospheric pressure but also at elevated pressures. However,pressure has a significant effect on the dominant frequency enhancement. This con-firms the hypothesis previously made on the weak influence of the jet momentumcompared to the buoyancy effects (Durox et al., 1997). It has been observed thatmethane flames at different fuel flow rates flicker with one dominant frequencyand as many as six harmonic modes at elevated pressures. The dominant flickeringfrequency of a methane diffusion flame varies locally in a linear manner withthe chamber pressure. However, globally, the trends suggest a curve fit of f/Pn.

Figure 12 Peak oscillation frequencies of the methane air diffusion flame at different fuel flow rates at

elevated pressures and the curve of the best fit.


The pressure exponent (n) was found to be about 0.17 at different flow rates. Thepower-law fashion indicates a decreasing dependence of flickering frequency onpressure above 10 bar. Figure 12 shows the dominant flickering frequency of a meth-ane diffusion flame at different fuel flow rates with the curve of best fit on the trendof dominant frequency change by pressure (f¼ 15.7P0.17).

It is worth highlighting that diffusion flames at elevated pressures are verysooty, the power spectra collected by the photomultipliers representing the sum-mation of soot particles emission and the OH� and CH� spectra at wavelengths of308� 2.5 nm and 430� 5 nm, respectively. It has also been observed that in all thecases of flame, chemiluminescence at elevated pressures (unstable flame) power spec-tra at CH� and OH� emission band is superimposed, but the CH� band radiation(power spectrum) is higher than the OH� band radiation at the dominant frequency.However, at higher frequency harmonics, the relative position of each peak swapposition (see Figure 11b, 11c, 11e, and 11f). At atmospheric pressure (stable flame),both spectra are completely superimposed with the same magnitude of power spectra(see Figure 11a and 11c). At lower pressures, the blue zone at the center of the flameenvelope and next to the nozzle exit is quite clear (this part of the flame mostlycontains CH� radicals). The images by Clemens et al. (1997) reveal OH� zones thatappear as thin filaments connected by a diffuse region; also, Leo et al. (2007) sug-gested that the OH� concentration strongly increases with the oxygen content inthe oxidizer stream. At higher pressures, the yellow part of flame will be more domi-nant, and the stronger outer vortices bring more oxygen to the flame diffusion bor-der. Thus, the emission of OH� radicals will be more dominant at a flickering flametip and detached part of the flame tongue.

A steeper change in the thermo-physical properties of methane diffusion flamewas observed at the early stage of the chamber pressure increase. The average inten-sity of the flame high-speed images was observed to increase by an order of about 3.5times at fuel flow rates of 0.15 slpm and 0.25 slpm from 1bar to 2 bar. However, themaximum change in the average pixels intensity at higher pressures was of the orderof 1.3 for both previously mentioned fuel flow rates (see Figure 9). The change offlame flickering frequencies from 1 to 2 bar is also very pronounced. At 1 bar, onlythe dominant frequency was clearly observable (see Figure 11a and 11d), while therewere as many as six corresponding harmonics at 2 bar for both flow rates. Withfurther increase in chamber pressure, the change of peak frequency follows the trendpresented in Figure 12.

The main reason for these steeper changes of flame properties at the initialsteps of pressure increase can be attributed to the quick change in the flame shapefrom convex to concave through the pressure dependence of the mixture density(q/P) and the binary diffusion coefficient (Dij/P�1), as described in detail in theprevious study (Gohari Darabkhani et al., 2009). At elevated pressures, due to theincreased presence of soot (which acts as a reaction intermediary), the reaction pro-cesses are different from those acting at atmospheric pressure. Temperature willdecrease due to the formation and elimination of more soot particles. Flower andBowman (1987) indicated a decreasing dependence of soot yield on pressure above10 bar. Also the results of the present study show a decreasing dependence of flick-ering frequency on pressure above 10 bar. It seems after a certain pressure the gradi-ent of changes in flame properties will decrease.


CONCLUSION

Experiments were conducted in a high-pressure burner to investigate the effectsof fuel flow rate and chamber pressure on the flickering behavior of methane-air dif-fusion flames. Global flame emission dynamics and high-speed imaging have beenapplied to study the methane diffusion flame dynamics at different flow rates acrossa pressure range of 1 to 10 bar.

The experimental results clearly demonstrate that both pressure and fuel flowrate have a strong effect on the diffusion flame dynamics. The pressure is observed tochange the flame shape, structure, flickering magnitude, and frequency due toenhancement in the formation and growth of outer toroidal vortices. The mass flowrate effects on oscillation magnitude and structure of vortices emerge fromhigh-speed images. The flickering frequency, however, remains almost constant ateach pressure despite an increase in fuel flow rate.

High-speed images have shown that the periodic breakup of the methane flameat 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 tor-oidal vortices with further separation at the tip of the breakaway flame part before itsplits into at least two wrinkled coherent flame structures. However, the methaneflames at lower flow rates (0.1 and 0.15 slpm) oscillate in a more waving manner,and the flame tip is burnt out consisting of one portion of flame tongue. A steeperchange was also observed on the flame structure, temperature field, and dynamicsof methane diffusion flame at early stages of the increase in chamber pressure.

The average of mean pixel intensity (MPI) values in a full sequence of flameimages (proportional to the flame emission intensity) at all methane flow rates hasbeen found to increase with pressure up to 6 bar, and then it starts to decrease withthe further increase of pressure. For methane flame at lower flow rates, the maximumoscillation wavelength (k) and maximum standard deviation (r) of MPI have beenobserved to occur at 8 bar. However, at higher flow rates, these parameters wereat maximum at 4 bar. The increase in fuel flow rate increases the magnitude of oscil-lation (Lf). The flickering frequency, however, remains almost constant at each press-ure. It has been observed that methane flames flicker with one dominant frequencyand as many as six harmonic modes at elevated pressures. The peak flickering fre-quency of a methane diffusion flame generally varies with the chamber pressure asa function of Pn (f¼ 15.7P0.17).

ACKNOWLEDGMENTS

The first author wants to gratefully acknowledge the School of MACE, TheUniversity of Manchester for the Ph.D. research scholarship. The first author alsoappreciates the general comments of Dr. Mark Jabbal and Mr. Robert Ward regard-ing this paper. The high-pressure burner was developed under the EPSRC BasicTechnology Program.

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