Plasma-assisted combustion of methaneusing a femtosecond laser

Xin Yu,1,2,* Jiangbo Peng,1,2 Rui Sun,3 Yachao Yi,1,2 Peng Yang,3 Xiaochuan Yang,3

Chunhong Wang,1,2 Yongpeng Zhao,1,2 and Deying Chen1,2

1National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China2Institute of Opto-electronics, Harbin Institute of Technology, Harbin 150080, China

3Institute of Combustion Engineering, Harbin Institute of Technology, Harbin 150001, China*Corresponding author: pengjiangbo_2004@126.com

Received February 28, 2011; revised April 18, 2011; accepted April 19, 2011;posted April 20, 2011 (Doc. ID 143344); published May 13, 2011

We first research the effects of femtosecond-laser-induced plasma (FLIP) on a laminar premixed methane/oxygen/nitrogen flame speed with a wide range of equivalence ratios (0:8–2:0) at atmospheric pressure. It is experimentallyfound that the flame speed increases by 30.8% at equivalence ratio 1.33, and the effects of the FLIP on the flamespeed are more remarkable when the methane is rich. The self-emission spectra from the flame and the plasma arestudied, and the presence of the oxygen atom is likely to be a key factor in enhancing flame speed. © 2011 OpticalSociety of AmericaOCIS codes: 350.5400, 350.3450, 300.6365, 140.3440.

Plasma-assisted combustion is lately receiving increasingattention, driven by the need for increasing the efficiencyof fuel combustion and decreasing pollutant emissions inaircraft engines, gas turbines, internal combustionengines, and so on. Plasmas can significantly enhancecombustion and reduce pollutant emissions throughthe formation of “active” species (for example free radi-cals or excited-state molecules and atoms) or the disso-ciation of fuel molecules into smaller and more easilycombusted fragments. Various experimental works havereported that atmospheric pressure plasma discharges,i.e., single-electrode corona, dielectric-barrier discharge,and repetitive ultrashort-pulsed discharge, can enhanceflame stability, flame speed, and combustion chemistry[1–5]. However, the plasma discharge electrodes areinstalled on the inner wall of the combustor and cannotbe changed or transferred after being installed. Theproblems listed above could be solved by the use oflaser-induced plasma because it has many advantages.The major advantages are as follows: controllability ofinput energy and its duration and nonintrusive nature.Moreover, with the absence of electrodes, heterogeneouseffects and wall heat loss can be eliminated and highlyreactive plasmas can be induced wherever and wheneverneeded, because the laser beam can be focused at anythree-dimensional spatial points wherever accessible.Recently, laser-induced spark ignition has become an

active research topic [6–8]. However, there are manyfewer studies to date on laser-induced plasma to enhanceflame speed and improve flame stabilization, which de-pend on the energy deposition and the lateral growthof the deposited volume. That makes a single laser pulsedeposition for this application highly unlikely, becausethe lateral expansion of a laser spark might not be strongenough to span the flow field. Periodic energy depositionmight be of practical importance, because the depositedpulse energy needs to be sufficient only to initiate a self-sustaining outward flame propagation, which eventuallyinteracts with the flames from the preceding and subse-quent pulses. Thus, in the repetitive case, a discretestream of propagating spherical flame is generated.

Finally, the burned gas region with its flame front ex-pands laterally [6]. Therefore, enhancing the flame speedand stabilizing the flame requires laser pulses with a highrepetition rate and great irradiance (≥1010 W=cm2)[7,9,10]. The femtosecond (fs) laser can generate a trainof pulses with a maximum irradiance of about1014 W=cm2 at a repetition rate of up to 1 kHz in ourstudy. In this Letter, we report on the influence offemtosecond-laser-induced plasma (FLIP) on the burningrate and structure of a laminar premixed methane/oxygen/nitrogen flame in a wide range of equivalenceratios (0:8–2:0) at atmospheric pressure by examiningflame appearance and flame speed. Furthermore, weanalyze the reasons for the effects of plasma-assistedcombustion by testing radical species of flame withand without plasma enhancement. For these purposes,we employed two diagnostic tools. two-dimensionalflame imaging and time-averaged emission spectroscopy.

Figure 1 is a schematic of the experimental scheme inour experiment. For generating plasma, fs-laser pulses of2mJ during 40 fs with a 800 nm wavelength at a repetitionrate of 1kHz from a regenerative amplifier were used.The energy of plasma generated by the fs-laser pulse

Fig. 1. (Color online) Schematic of the experiment setup(MFM, mass flow meter).

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0146-9592/11/101930-03$15.00/0 © 2011 Optical Society of America

is about 0:38mJ. The exciting laser beam was focusedwith lens L1 (f ¼ 25 cm) into a Bunsen burner alongthe burner axis. The focal spot size was estimated tobe about 44 μm in radius, giving a maximum peak inten-sity of about 1014 W=cm2. The focal spot was at the exit ofthe burner. A Bunsen burner that is a straight quartz tubewith an inner diameter (D) of 7:6mm and length of450mm is employed. The burner diameter and lengthare chosen to ensure that the flow remains laminar(Reynolds number, ReD < 2300) and that the flameprevents flashback and blowoff. In order to allow simplecontrol over the equivalence ratio (Φ) and the averagevelocity through the burner while maintaining the de-sired methane/oxygen/nitrogen composition, the desiredmixture is first prepared using a bank of calibrated massflow meters, one for each gas. After mixing thoroughly,the mixture goes to the burner. Digital images of theflame emission are captured with a 12 bit scientificCCD camera (1024 × 1032 pixels) and a telescope system.The camera system is of high sensitivity in the UV andvisible regions (∼300–700 nm) and has large dynamicrange and large well capacity. For performing time-averaged emission spectroscopy, the self-emission fromthe flame and the plasma was collected and imagedonto an ANDOR SR-750-B1 spectrometer (focal length750mm) fitted with a 1600 × 200 pixel EMCCD camera(ANDOR DU420A-BU2). The resolution (FWHM) of thespectrometer is 0:026 nm in the range 200–1100 nm.In this Letter, the equivalence ratio (Φ) is defined as

Φ ¼ CH4=O2

ðCH4=O2Þst; ð1Þ

where ðCH4=O2Þst refers to the stoichiometric value of(CH4=O2). Some typical images of the flame radiationare given in Fig. 2. As shown in Fig. 2, the FLIP changesthe flame shape and markedly reduces the flame height.When the volumetric flow rate of the unburned mixtureis constant and the flame height is decreased, theflame speed must be increased. Figure 2 also shows thatthe change of the flame shape at Φ ¼ 1:33 is mostobvious.One of the prime objectives of the present study is

to measure the flame speed of laminar premixedCH4=O2=N2 flame with and without FLIP. We utilized

the oldest known Bunsen flame approach to measurethe flame speed [11,12]. The flame speed can beshown as

Su ¼_m

ρuAb¼

_Q

Ab; ð2Þ

where ρu is the density of the unburned mixture; Su is theflame speed of the laminar premixed CH4=O2=N2 flame;Ab is the surface area at the end of the heat release zone(the luminous cone surface area shown in Fig. 2) and _Q isthe volumetric flow rate of the unburned mixture. Sincechemiluminescence is primarily produced in the thinheat release zone of the flame, the surface area measuredfrom a chemiluminescence image can approximate Ab.According to [11], the same method was utilized to deter-mine Ab from the cheminluminescence images. The volu-metric flow rates of CH4, O2, and N2 are recordedseparately by thermal mass flow meters.

Figure 3 shows the measured flame speeds (Su) of la-minar premixed CH4=O2=N2 flame with and withoutFLIP; the trend of flame speeds versus equivalence ratiois the same. The relevant uncertainties of the measuredflame speeds of laminar premixed flame with and with-out FLIP are less than 2.5% and 2.0%. The FLIP markedlyenhances the flame speed, which is increased by 5.3%–30.8% as the equivalence ratio varies from 0.8 to 2.0.The increases of the flame speeds are slightly less thanreal values, because the laser-induced blast wave pushesthe flame front outward and makes the inner coneslightly wider. When the equivalence ratio is under 0.9or above 1.6, the FLIP has less effect on the laminar flamespeeds and the increase of flame speed is under 10%. Thisis mainly because the CH4 or O2 is poor and the probabil-ity that the active atoms and molecules generated by theFLIP are reactive with CH4 or O2 is small.

From Fig. 3, atΦ ¼ 1:33, the increase of flame speed isup to 30.8%. This figure is the highest when the flame canstabilize on the exit of the burner. AtΦ ¼ 1:11, the flamecould not stabilize on the exit of the burner and the flash-back phenomenon appeared with FLIP. This shows thatthe effect of the FLIP on the laminar flame speed may bebiggest and the increase of flame speed may be above

Fig. 2. (Color online) Images of CH4=O2=N2 laminar premixedflame for various equivalence ratios (Φ).

Fig. 3. (Color online) Flame speeds of laminar premixedCH4=O2=N2 flame (38∶62 O2∶N2) with and without FLIP at p ¼1 atm and Tu ≈ 300K.

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30.8% at Φ ¼ 1:11. Moreover, we can find out, when CH4is rich (at Φ > 1:0), the effects of the FLIP on the flamespeeds are more remarkable. At Φ > 1:0, the flamespeeds are up by 10% to 30.8%. However, at Φ < 1:0,the flame speeds increase by 5.3 to 17.4.The foregoing results demonstrate that a low power

FLIP represents an effective and energy-efficient methodto enhance flame speed. However the physical–chemicalprocesses leading to the observed combustion enhance-ment are not yet understood. We recorded spectra ofFLIP in N2=O2 current and laminar premixed CH4=O2=N2 flame to measure the species generated just afterthe laser pulse. As shown in Fig. 4, the oxygen atomat 777.2, 777.5, 777.6 and 844:8 nm appears in the spectraof FLIP in the N2=O2 current, and the absence of theseemission features in the flame indicates that the oxygenatoms are consumed or quenched. The oxygen atom isthe key species for the initiation of combustion as it isthe main specie responsible for breaking C─H bondsin hydrocarbon fuels [5,13]. It also accelerates thechain-branching reactions of methane combustion.The molecular band of the second positive system of

N2 (the transition C3Πu–B3Πg) in the range of300–480 nm is well pronounced and the (0, 0), (0, 1),(0, 2), (1, 2), (1, 3), and (2, 3) bands of the first negativesystem of the N2þ ion at 391.4, 427.8, 470.9, 423.7, 465.2,and 419:9 nm, respectively, appear in the spectra of FLIPin the N2=O2 current and at a reduced level in the spectraof the plasma in the laminar premixed CH4=O2=N2 flame(Fig. 4). These excited N2 and N2

þ ions appear to beconsumed or quenched in the flame. Previous studieshave shown that electronically excited nitrogen mole-cules are quenched by an oxygen molecule to producean oxygen atom [3,14], which mainly proceeds throughthe following reactions:

N2ðA3ΣuþÞ þ O2 → N2ðX1ΣgþÞ þ Oþ O; ð3Þ

N2ðB3ΠgÞ þ O2 → N2ðX1ΣgþÞ þ Oþ O; ð4Þ

N2ðC3ΠuÞ þ O2 → N2ðX1ΣgþÞ þ Oþ O: ð5Þ

Thus it can be seen the oxygen atom is likely to be thekey “active” species to enhance the laminar premixedCH4=O2=N2 flame speed. These results also help usunderstand why at Φ > 1:0 the effects of the FLIP onthe flame speeds are more remarkable. At Φ > 1:0, theCH4 is rich; the prime reason for slowing the flame speeddown is that the oxidizer is not enough. The FLIP canafford a lot of “active” oxygen atoms. Therefore, the ef-fects of the FLIP on the flame speeds are very sharp.However, at Φ < 1:0, the CH4 is poor; the key reasonfor slowing the flame speed down is that the fuel isnot enough. Thus, the effects of the FLIP on the flamespeeds are less.

In conclusion, we demonstrate the possibility of en-hancing the combustion efficiency of a laminar premixedflame at lower power through the use of fs-induced plas-ma located in the exit of the burner. The plasma mark-edly enhances the laminar flame speed (up to 30.8%).Moreover, we discovered that the effects of the FLIPon the flame speeds are more remarkable at Φ > 1:0.Through spectroscopic measurements, we believe thatthe oxygen atom may be the key species in drivingfavorable chemistry for combustion enhancement.

This work is supported by the National NaturalScience Foundation of China (NSFC) grant 60978016and the Natural Science Foundation of HeilongjiangProvince of China (grant F2007-05).

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Fig. 4. (Color online) Emission spectra: (a) FLIP in N2=O2current, (b) FLIP in laminar premixed CH4=O2=N2 flame.

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