Experimental investigation of bio-butanol laminar non-premixed flamelets

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Experimental investigation of bio-butanol laminar non-premixed flamelets

Maria S. Agathou, Dimitrios C. Kyritsis ⇑University of Illinois at Urbana–Champaign, Department of Mechanical Science and Engineering, Urbana, IL 61801, United States

a r t i c l e i n f o

Article history:Received 21 September 2011Received in revised form 11 December 2011Accepted 17 December 2011Available online 10 January 2012

Keywords:Strained flamesBio-butanolNon-premixed flamesExperimentsExtinction strain ratesScalar dissipation rates

a b s t r a c t

Pure butanol as well as methane-assisted butanol non-premixed flames were studied in a counter-flowburner configuration in view of the emergence of butanol production from agricultural sources. Majorcombustion species were measured across the flame using line Raman imaging and temperature mea-surements were obtained with K-type thermocouples. Of particular importance was the comparison offlames of the same heating value and equivalence ratio, as well as flames of the same overall equivalenceratio but varying reactant flow rates. Extinction strain rates were measured for a wide range of condi-tions. It was shown that there was a decrease of extinction strain rate with increasing overall equivalenceratio for both constant fuel flow rates and constant heat release of combustion and that butanol flameswere more vulnerable to extinction compared to methane. Species concentration measurements indi-cated that N2 concentrations anti-correlate with temperature across the non-premixed flame, in agree-ment with non-premixed flame theory. Finally, the possibility of estimating the scalar dissipation rateat the stoichiometric surface vstoich was investigated through a measurement of the mixing layer thick-ness d, performed with an intensified CCD camera and a modest-power laser.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The possibility of producing butanol efficiently from agricul-tural sources, with the use of clostridia through fermentation pro-cesses, has recently been demonstrated [1–4], thus highlightingthe potential use of butanol as an alternative fuel. Bio-butanol fuelcan be competitive to the widely used bio-ethanol, primarily dueto its higher energy density. Specifically, the energy densities ofbutanol and ethanol are 36.4 and 24.8 MJ/kg respectively, whereasthe energy content of gasoline is 44.9 MJ/kg. The reason for thehigher energy content of butanol lies in the smaller oxygen contentof the butanol molecule (22%) compared to the oxygen content ofethanol (35%).

Relatively few fundamental studies of butanol combustion havebeen reported, with their majority appearing within the last3 years. In our previous work [5,6] we presented experimental re-sults on non-premixed bio-butanol flames and performed exten-sive comparisons of butanol combustion with the correspondingflames of ethanol and methane [5,6]. In addition, butanol kineticmodeling was studied in [7]. Sarathy et al. [8] presented numericaland experimental results using a jet-stirred reactor, whereasDagaut et al. [9–11] proposed modeling of butanol combustionkinetics based on results from a jet-stirred reactor. Premixedflames of all butanol isomers were studied in low pressure

environments by Yang et al. [12]. In the area of internal combus-tion engines Rakopoulos et al. [13–17] have studied the effect ofbio-butanol usage on engine performance and emissions [13–17].Liu et al. investigated the effect of ethanol and butanol as additivesin soybean biodiesel combustion [18], whereas Komninos and Rak-opoulos [19] performed a numerical analysis on emissions of bio-alcohols in HCCI engine environment. Older studies involve fueldecomposition for kinetic modeling purposes in mixtures of CH4

with the four butanol isomers [20], as well as introductory internalcombustion engine work in the field [21,22].

The current paper focuses on the non-premixed counter-flowlaminar flamelet, which constitutes the fundamental element ofturbulent flames [23–26]. Butanol flame structure and flamedynamics was probed with laser-based diagnostics. Specifically, acounter-flow burner was used that has been developed in previouswork [27,28], and non-premixed flames of both pure butanol aswell as of butanol–methane mixtures were established. The flameswere studied for varying overall stoichiometry and total heat re-lease. To our knowledge, we present the first spatially resolvedtemperature and species measurements in butanol methane-assisted laminar counter-flow flamelets, as well as measurementsof extinction strain rates for flames burning on both pure butanoland butanol-containing blends. Scalar dissipation rate approxima-tions were performed based on the measured temperature distri-butions and were compared to the respective calculations basedon the gradient of the mixture fraction at the stoichiometricsurface.

0306-2619/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.apenergy.2011.12.060

⇑ Corresponding author. Tel.: +1 217 333 7794.E-mail address: [email protected] (D.C. Kyritsis).

Applied Energy 93 (2012) 296–304

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Applied Energy

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2. Experimental set-up

2.1. The counter-flow burner

An axisymmetric, laminar, methane assisted, butanol-oxygenflame was established in a counter-flow burner configuration,shown schematically in Fig. 1. The oxygen stream was nitrogen-di-luted and nitrogen was used as a shroud flowing from the outerannulus of the lower nozzle in order to isolate the flame from diffu-sion from ambient air as well as drafts in the laboratory. Fine gridsand glass beads were used to achieve uniformity of the velocity pro-files of the two reactants. The diameter of each of the nozzles was12.5 mm, and the gap between them was 15 mm. Butanol flow ratewas controlled by a syringe pump that fed a 4.5 mm diameter, 1 mlong, copper tube surrounded by heating tape. The tape was heatedup to 170 �C to assure butanol vaporization. Both methane andbutanol streams were nitrogen-diluted before mixing at the inletof the counter-flow burner. The outside surface of the burner lowersection was also wrapped with heating tape and its temperaturereached 220 �C, so as to avoid butanol condensation.

2.2. Species concentration measurements

Major combustion species (C–H bond, O2, N2, CO2 and H2O)were measured using line Raman imaging. The experimentalset-up is shown schematically in Fig. 2. A Quanta-Ray Pro-250Nd-YAG laser delivering 300 mJ/pulse at 532 nm was used as anexcitation source and the Stokes-shifted Raman signal was col-lected at the wavelengths shown in Table 1. It is noticeable that,for an incident laser beam wavelength of 532 nm, all three pure

fuels have a detection wavelength around 629 nm, thus this isthe wavelength selected in order to detect the C–H bond in thefuel-mixture cases (Row 4 of Table 1). Separation of the severalC–H lines corresponding to different organic molecules was notpossible within the resolution of the spectrograph/camera couplethat was used. The laser beam was focused to an approximately500 lm thick and 60 mm long horizontal line with a 1 m plano-

Nomenclature

EnglishA nozzle cross-sectional area (m2)A regression constantB regression constantC regression constantC concentration of the measured species (kmol/m3)D diffusivity of the involved species (m2/s)d distance between the nozzles (m)E energy of light (J)K strain rate (s�1)l sampling length (m)_m mass flow rate (kg/s)

MW molecular weightn number density of scatterers along the beam (kmol/m3)P pressure (Pa)Q heat released by fuel combustion (W)R ideal gas constant (8314 J/kmol K)R regression coefficientT measured temperature (K)U speed of the counter-flowing streams (m/s)_V volumetric flow rate (m3/s)x distance from the fuel nozzle (m)Y mass fraction of involved speciesZ mixture fraction

Greekd mixing layer thickness (mm)e collection efficiencyq density of a mixture (kg/m3)r standard deviationu overall equivalence ratio

v scalar dissipation rate (s�1)X solid angle

Subscripts and superscriptsatm atmosphericext extinction conditionsfuel fuel streami fuel or oxidizer streamI reference (known) location between the burner nozzlesj species mix butanol–methane mixtureN2 nitrogenox oxidizer streamstoich stoichiometric conditionstot totals signals arbitrary location between the burner nozzleso incident light

AbbreviationsC–H bond of C and H atoms, considered as fuel in the concen-

tration measurementsNd-YAG neodymium-doped yttrium aluminum garnet, Y3Al5O12

laser usedF/A fuel-to-air ratioLHV lower heating values of the involved fuels (kJ/kg)@r@X differential Raman cross-sectionS/N signal to noise ratioM1–M6 flame cases of butanol-methane mixturesB1–B6 flame cases of pure butanolFWHM full width at half maximum

Fig. 1. Cross-sectional area of the counter-flow burner.

M.S. Agathou, D.C. Kyritsis / Applied Energy 93 (2012) 296–304 297


convex lens. The selection of a relatively long lens that will gener-ate an inevitably not-so-tight focus was dictated by the need toavoid sparking that would happen during a tighter focus. The effecton spatial resolution is not dramatic because this is essentiallydetermined by the limited resolution of the intensifier. An ActonResearch 300 mm imaging spectrograph was used for the disper-sion of Raman signals. Dispersion was achieved with a 68 mm �68 mm ruled grating with 1200 grooves/mm and a 500 nm blazewavelength.

The spectral images were captured using an Andor I-star ICCDwhich was coupled to the spectrograph. The horizontal imagesfrom the flame were rotated by 90� with a dove prism and thenfocused onto the inlet mirror using a Nikon 50 mm, f # 1.8 lens.An OG550, 3 mm-thick glass filter was used to reject stray reflec-tions from the burner hardware. The burner was mounted on a ver-tical translation stage so that the flame was scanned in the verticaldirection. For each measurement, three sets of 300 Raman linesthat were integrated on the chip were averaged, in order to achievean acceptable signal-to-noise ratio (S/N > 10). It should be notedthat although the Raman technique is operated in a ‘‘line imaging’’configuration, it was effectively utilized in a ‘‘single-point’’ fashion,because the signal was integrated in the spatial sense. This wasnecessary because tightly focusing the laser beam to a point causedintense sparking and limited the energy of the excitation pulse tolevels that were not adequate for the acquisition of signals withhigh signal-to-noise ratio.

2.3. Temperature and extinction measurements

Temperature measurements were obtained across the flamewith an increment of 1 mm using a K-type thermocouple with abead diameter of 0.50 mm. The thermocouple was positionedperpendicularly to the burner axis with its bead at the center ofthe inner annulus of the burner. The thermocouple was rated fora maximum temperature of 1473 K and the maximum flame tem-

perature observed was 1382 K. Flame extinction was achieved byincreasing the diluent mass flow rate while the ones of fuel andoxidizer were kept constant. No radiation correction was appliedto the temperature measurements.

3. Results and discussion

3.1. Calculation of flame parameters

Six butanol–methane mixtures were considered. The measuredmass flow rates of fuel, oxidizer and diluent in each of the flames in(g/min), are presented in Table 2. The mixture of gases at the exit ofthe burner was assumed ideal and the density q was calculatedthrough the equation:

qi ¼_mtot

_Vtot

¼_PatmMWmixi

RTi

: ð1Þ

The subscript i denotes either the fuel or oxidizer nozzle loca-tion, _mtot and _Vtot are the total mass and volumetric flow ratesreaching the nozzle respectively, R is the ideal gas constant(8314 J/kmol K), T is the measured temperature and Patm is theatmospheric pressure. MWmix is the molecular weight of themixture:

MWmix ¼1Pj

1yj

MWj

; ð2Þ

where yj and MWj are the mass fraction and molecular weight ofspecies j. The speeds Ui were calculated through:

Ui ¼_mtot;i

qiA; ð3Þ

where A is the nozzle cross-sectional area. The strain rate at thestoichiometric surface was then estimated, as suggested in [29]:

K ¼ 2Uox

d1þ Ufuel

Uox

qfuel

qox

� �12

" #; ð4Þ

where d is the distance between the nozzles. The calculated densi-ties and speeds for the six mixture flame configurations are pre-sented in Table 3, both for the oxidizer and for the fuel nozzleexit location. In addition, strain rates at the stoichiometric surfaceare presented in the last column of Table 3, in (s�1).

The fuel-to-air ratio (F/A), the stoichiometric fuel to air ratio (F/A)stoich, as well as the overall equivalence ratio u, are presented inTable 4. In order to calculate the overall equivalence ratio u for thebutanol–methane mixtures, both combustion reactions were takenunder consideration:

C4H9OHþ 6O2 ! 4CO2 þ 5H2O; ð5Þ

CH4 þ 2O2 ! CO2 þ 2H2O: ð6Þ

Column Qtot of Table 4 presents the total heat released by thecombustion of the mixtures under consideration, based on the low-er heating values of the involved fuels. Notably, the LHV for buta-nol is 33 MJ/kg and for methane 50 MJ/kg [30]. The last column ofTable 4 presents the peak temperature measured at the flameregion for each configuration.

3.2. Raman spectroscopy measurements

The raw data obtained during the Raman experiment corre-spond to curves that are in terms of arbitrary units of signal (num-ber of counts) as a function of wavelength. In order to extractRaman signal intensity information out of these plots, the area

Fig. 2. Experimental set-up.

Table 1Raman signal for the species under consideration.

Species Wavelength (nm)

C4H9OH 628CH4 629.7C2H5OH 630.2C–H bond 629O2 580N2 607.3CO2 571H2O 660.3

298 M.S. Agathou, D.C. Kyritsis / Applied Energy 93 (2012) 296–304


under the spectrum was integrated. The result equaled the energyof the signal Es:

Es ¼ Eo � n �@r@X�X � l � e: ð7Þ

In this last equation, Eo is the energy of the incident light, l is thesampling length along the beam and n is the number density ofscatterers, in our case the molecules of the gas under consider-ation. The collection efficiency is accounted by the term e and @r

@X

is the differential Raman cross-section. Clearly, if the number den-sity ni of the species at a given location is known, then the numberdensity of the species ns at any other location can be obtained interms of this known value, the intensity of the Raman signals Ei,Es and the ratio of the differential Raman cross-section at the twolocations, Eq. (8):

ns ¼ ni �Es

Ei�@r@Xi@r@Xs

: ð8Þ

The rest of the terms of Eq. (7) are constant numbers and cancelout. As the known location for this study, the exit of the fuel nozzlewas considered for N2 and C–H bond and the exit of the oxidizernozzle for O2. The number density of species j at the reference loca-tions i, is calculated in (kmol/m3) using Eq. (9), where the total vol-umetric flow rate is calculated through Eq. (1):

ni;j ¼_mj

MWj

_Vtot

: ð9Þ

The differential Raman cross-section is a temperature depen-dent term and for the temperature range of the current experi-ments its variation with temperature is plotted in Fig. 3 for thethree major species under consideration. The polynomial expres-sions used for this calculation are found in [31].

Major species concentration measurements in kmol/m3 for C–Hbond, O2, and N2, as a function of the distance from the fuel nozzle

exit in mm are shown in Fig. 4. Each figure corresponds to a differentflame configuration from Table 2. In the figures, the measured tem-perature distribution is also presented on a separate y-axis. The val-ues of measured fuel concentrations were an order of magnitudeless than the corresponding values for the rest of the measured spe-cies, such as oxygen and nitrogen. Hence, to be able to clearly illus-trate all data in the same plot, fuel concentrations were multipliedby a factor of 10. The error of the Raman measurements deservessome discussion. The image intensifier is a noisy device, hence pro-ducing significant error, which translates to lower S/N ratios forweaker signals, i.e. in the flame region. The maximum error isobserved in the flame region, where the S/N ratio for N2 is around4. The minimum error is observed at the nozzles, where for N2, O2

and C–H bond the S/N ratios are on the order of 10. S/N ratios areconsidered to represent the total uncertainty rising from the mea-surements of concentration, since their random and systematiccomponents could not be further distinguished.

In all test cases, fuel concentration has its maximum at the loca-tion closest to the fuel nozzle and starts depleting as it approachesthe flame zone. Similarly, oxidizer is maximum at the oxidizer noz-zle and its destruction begins as it advances towards the flame. Asfar as nitrogen is concerned, its signal is highly affected by the hightemperatures developed in the combustion region and thisexplains the drop-off observed in all configurations. The presenceof the other two major species of Table 1, H2O and CO2, was inves-tigated both in the flame and in the product region, but their signal

Table 2Mass flow rates for the butanol–methane flame configurations.

Test case C4H9OH (g/min) N2 for C4H9OH (g/min) CH4 (g/min) N2 for CH4 (g/min) O2 (g/min) N2 for O2 (g/min)

1 0.067 1.121 0.133 1.162 1.465 0.3882 0.135 0.697 0.052 1.093 1.151 0.7693 0.202 0.565 0.065 1.207 1.623 0.7694 0.296 0.565 0.051 1.093 1.544 0.7695 0.270 1.056 0.042 0.999 1.544 1.0746 0.337 1.056 0.011 1.139 1.937 0.388

Table 3Densities and speeds at both nozzle exits and strain rates for all test cases.

Test case Oxidizer density qox (kg/m3) Fuel density qfuel (kg/m3) Oxidizer speed uox (cm/s) Fuel speed ufuel (cm/s) K (s�1)

1 0.379 0.744 66.32 45.29 173.022 0.388 0.797 67.16 33.68 153.883 0.404 0.792 80.40 34.97 172.484 0.373 0.802 84.21 33.94 178.635 0.496 0.817 71.72 39.32 162.936 0.480 0.844 65.80 40.93 160.12

Table 4Overall equivalence ratios and total heat released for all flame configurations.

Test case FAR FARst Overall, u Qtot (W) Tpeak (K)

1 0.14 0.28 0.5 148 12182 0.16 0.33 0.5 118 12313 0.16 0.34 0.5 165 12704 0.22 0.36 0.6 206 13295 0.20 0.36 0.6 183 12166 0.18 0.38 0.5 195 1382

0

0.2

0.4

0.6

0.8

1

1.2

400 600 800 1000 1200 1400 1600 1800

N2O2C-Hbond

Temperature [K]

Nor

mal

ized

Ram

an C

ross

-sec

tion

Fig. 3. Temperature dependence of the Raman cross-section.



was not detected. This is probably due to the low sensitivity of theintensifier in the red part of the spectrum. This result indicates thatmeasurement of these species has to be pursued more effectivelywith a high-quantum-efficiency, non-intensified camera.

In cases with lower strain rates (5, 6), it was observed that oxi-dizer and fuel vanished before the two curves interact. Fuel con-sumption was initiated through pyrolysis, since the temperaturewas elevated and this was followed by combustion of the interme-diate products with oxygen. In these cases, the reaction zone was

broader as indicated by the broader temperature distributions. Asthe strain rate increased, the curves of the reactant concentrationwere brought closer (cases 3, 4) or even vanished at the same loca-tion (case 1). This happened because with increasing strain, theresidence time in the mixing layer was reduced, resulting to a thin-ner flame region. However, in case 2, even though it has the loweststrain rate, the above observations do not hold. An overlap of thereactant lines is observed indicating a fuel leakage in the oxidizerregion. This phenomenon is observed in flames close to extinction

0

0.005

0.01

0.015

0.02

0.025

0.03

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14

1.

Temperature [K]

Distance from Fuel Nozzle [mm] Distance from Fuel Nozzle [mm]

Distance from Fuel Nozzle [mm]

Spec

ies

Con

cent

ratio

ns [k

mol

/m3 ]

0

0.005

0.01

0.015

0.02

0.025

0.032.

3. 4.

5. 6.

Spec

ies

Con

cent

ratio

ns [k

mol

/m3 ]

0 2 4 6 8 10 12 14400

600

800

1000

1200

1400

1600

Temperature [K]

400

600

800

1000

1200

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1600

Temperature [K]

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600

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1600

Temperature [K]

400

600

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1000

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1600

Temperature [K]

400

600

800

1000

1200

1400

1600

Temperature [K]

0

0.005

0.01

0.015

0.02

0.025

0.03

Spec

ies

Con

cent

ratio

ns [k

mol

/m3 ]

0

0.005

0.01

0.015

0.02

0.025

0.03

Spec

ies

Con

cent

ratio

ns [k

mol

/m3 ]

0

0.005

0.01

0.015

0.02

0.025

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ies

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cent

ratio

ns [k

mol

/m3 ]

0

0.005

0.01

0.015

0.02

0.025

0.03

Spec

ies

Con

cent

ratio

ns [k

mol

/m3 ]

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14


Distance from Fuel Nozzle [mm] Distance from Fuel Nozzle [mm]

Fig. 4. Species concentration measurements for N2 (o), O2 (h) and C–H bond (+) and temperature distribution curves (x). Flame configuration cases are in accordance to cases1–6 in Table 2.



and flame 2 has the lowest heat release and low peak temperatureindicating a potential vicinity to extinction conditions.

Comparing cases with the same overall equivalence ratio (1, 2, 3and 6), the combustion behavior of the involved species follows thepattern of steeper curves with increasing strain rate (except forcase 2). Temperature distribution curves have their maximum va-lue approximately at the center of the gap between the nozzles andthis maximum increases with higher fuel energy release. The errorin these measurements is on the order of 10% and is related tooccasional instabilities of the reaction zone in the real environmentof the lab, as well as to the scatter in the seven measurements oftemperature that corresponded to each data point.

It is reminded here that the flames are highly diluted (N2 > 80%of the mixture) and as a result N2 concentration can be seen as ameasure of the total mixture density. Moreover, since N2 is practi-cally not reacting, its concentration can be seen as a conserved sca-lar. The linear relation of nitrogen concentration with temperatureacross the flame can be seen in Fig. 5. Regression coefficients R2

vary between 0.79 and 0.93 for the various mixtures.

3.3. Extinction strain rate measurements

In order to study the extinction behavior of the butanol–methane mixture flames presented in the previous sections, theamount of fuel and oxidizer was kept constant and the concentra-tion of diluent increased to achieve extinction. In this manner, thetotal heat released as well as the overall equivalence ratio of eachflame case remained unaffected. The mixture cases are namedM1–M6 and their flow rates at extinction are presented in Table5. Notably, the fuel and oxidizer mass flow rates are identical toTable 2. N2 flow rates increased to achieve extinction.

In addition, six flames of pure butanol were established. Thepure butanol flames (cases B1–B6) were selected such that theyhad the same overall equivalence ratio and total heat released withcases M1–M6 respectively. The related information is presented inTable 6. Temperature measurements necessary for the calculationof strain rate at extinction were also obtained at both nozzle exitsjust before the flame extinguished. The calculated extinction strainrates for both M1–M6 and B1–B6, are presented in Table 7. It is re-minded, that for each flame configuration the corresponding over-all equivalence ratio and total heat release can be obtained fromTable 4. In addition, Fig. 6 presents the extinction strain rate valuesas a function of the total combustion heat release of each flameconfiguration. In all six cases, pure butanol flames are more vulner-able to extinction than butanol–methane mixture flames with the

same overall equivalence ratio and total combustion heat release.Furthermore, an approximately monotonic behavior is observedwith increasing heat release, which is evident both in pure butanoland in the mixture cases.

3.4. Estimation of the scalar dissipation rate

The scalar dissipation rate, which represents the inverse of acharacteristic diffusion/convection time is defined as:

v ¼ 2DðrZÞ2; ð10Þ

where D represents the diffusivity and Z is the mixture fraction. Theimportance of the scalar dissipation rate is often compared to theone of strain rate, which represents the local velocity gradient. Un-like strain rate, it incorporates diffusion effects and therefore pro-vides a better definition for the residence time. Moreover, for non-premixed and partially-premixed flames, it constitutes the deter-mining parameter for the response of chemical reactions to suddenperturbations caused by variations of the turbulent flow field [32].

This definition points to the possibility of estimating the scalardissipation rate at the stoichiometric surface vstoich either throughthe gradient of the mixture fraction at this location rZj jstoich, orthrough an approximation of the mixing layer thicknessd, sincevstoich

scales as:

vstoich ¼ 2DðrZÞ2stoich / D=d2; ð11Þ

d / 1= rZj jstoich: ð12Þ

It is noted that for steady flames vstoich is proportional to thestrain K [33]. The estimation of the thickness d took place usingthe measured temperature distributions for the flame configura-tions of Table 2. In counter-flow diffusion flames, the temperaturehas a theoretically expected Gaussian distribution [34]. Thus, theexperimentally obtained temperature curves were curve-fittedwith a Gaussian distribution and the thickness was then approxi-mated as the ‘‘full width at half maximum’’ (FWHM) of the curve.FWHM depends solely on the standard deviation r of the Gaussiancurve and was calculated through:

FWHM ¼ 2 �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 � ln 2p

� r � 2:35482 � r: ð13Þ

The calculated flame thicknesses d are tabulated in Table 8.The inverse square root dependence of d on strain rate that is

implied by Eq. (11) is investigated in Fig. 7. The experimental find-ings are plotted on a log–log scale alongside with a curve depictingthe theoretically expected behavior. Since only the slopes of therespective curves are significant for purposes of comparison, thetheoretical curve is plotted such that the initial points of bothcurves coincide. The results of Fig. 7 show that there is a differencebetween the theoretically expected 1/K0.5 and the actually mea-sured dependence of d on strain rate which scales as approximately1/K0.91. The exponent is far from the expected value and the regres-sion constant is very poor R � 0.62.

The observed departure of the mixing layer thickness from thetheoretically expected trend renders its calculation from the tem-perature distribution curves inappropriate for this study. For thatreason, the scalar dissipation rate at the stoichiometric surfacewas approximated using the mixture fraction gradient rZj jstoich,as shown in Eqs. (11) and (12).

The definition of the mixture fraction is based on the mass frac-tion of major species and can be formulated in several ways, whichwere evaluated in [35]. A definition based on the mass fraction ofN2 can be followed, where:

Z ¼ YN2 � YN2Ox

YN2Fuel � YN2Ox: ð14Þ

0

0.005

0.01

0.015

0.02

0.025

0.03

400 600 800 1000 1200 1400

M1M2M3M4M5M6

N2 C

once

ntra

tions

[km

ol/m

3 ]

Temperature [K]

Fig. 5. Nitrogen concentrations seen as conserved scalars, as a function oftemperature for the six flame configurations.



In this last equation, Y denotes mass fraction, N2 subscript standsfor nitrogen and F, Ox for the fuel and oxidizer stream respectively.YN2 can be used as a conserved scalar since N2 is non-reactive. How-ever, in this study, due to the lack of information regarding the com-bustion products, YN2 was approximated through:

YN2 ¼CN2 �MWN2

CN2 �MWN2 þ Ci �MWi: ð15Þ

In this equation, CN2 and MWN2 are the measured concentrationand the molecular weight of nitrogen. Subscript i denotes eitherthe fuel or oxidizer side outside the flame region and Ci and MWi

are the measured concentrations and molecular weights of eitherfuel or oxygen. Hence, in both sides, the values of Ci were providedin (kmol/m3) directly from the data of Fig. 4. In the fuel side, the

molecular weight was calculated as the weighted average, on amolar basis, of the mixture under consideration. This techniquewas applied in flame configurations 2, 3 and 4 of Fig. 4, whichhad measured values of N2 concentration in the flame region aswell. Thus, for these three mixtures that had a butanol molar com-position of 35.7%, 40.4% and 55.9%, MWi was considered to be 36.7,39.4 and 48.4 respectively.

The gap between the nozzles was assumed to consist of threeregions: The reaction zone, located in the vicinity of the maximumtemperature, and the fuel and oxidizer regions where no combus-tion products were present. After calculating the mixture fraction Zin all locations, an error function was curve-fitted to the Z distribu-tion, since this is the functional form expected from the theoreticalanalyses of [35,36]. The curve fit function is of the form:

Z ¼ A�erfcx� B

C

� �; ð16Þ

Table 5Mass flow rates for the butanol–methane flame configurations at extinction.

Test case C4H9OH (g/min) N2 for C4H9OH (g/min) CH4 (g/min) N2 for CH4 (g/min) O2 (g/min) N2 for O2 (g/min)

M1 0.067 0.729 0.133 2.752 1.465 0.407M2 0.135 0.729 0.052 1.641 1.151 0.799M3 0.202 0.729 0.065 2.319 1.623 0.799M4 0.296 0.729 0.051 2.706 1.544 0.799M5 0.270 0.893 0.042 2.167 1.544 1.190M6 0.337 0.893 0.011 2.659 1.937 0.407

Table 6Mass flow rates for the pure butanol flame configurations at extinction.

Test case C4H9OH (g/min) N2 for C4H9OH (g/min) N2 for CH4 (g/min) O2 (g/min) N2 for O2 (g/min)

B1 0.270 0.729 1.944 1.465 0.407B2 0.216 0.729 1.620 1.151 0.407B3 0.301 0.729 1.963 1.623 0.407B4 0.377 0.729 2.544 1.544 0.407B5 0.333 0.729 2.283 1.543 0.407B6 0.354 0.729 2.191 1.941 0.407

Table 7Extinction strain rates for butanol–methane mixtures and for pure butanol flames.

Test case Kext (s�1) Test case Kext (s�1)

M1 128.56 B1 108.82M2 105.08 B2 93.70M3 132.45 B3 113.15M4 140.87 B4 126.10M5 141.77 B5 183.48M6 140.74 B6 195.37

0

50

100

150

200

100 120 140 160 180 200 220

Butanol-Methane MixturesButanol

Extin

ctio

n St

rain

Rat

e K ex

t [s-1

]

Total Heat Released Ptot [W]

Fig. 6. Calculations of extinction strain rate K for butanol-methane mixture flamesas well as for pure butanol flames as a function of the total heat released fromcombustion.

Table 8Calculated mixing layer thickness d as FWHM.

Test case 1 2 3 4 5 6

d (mm) 6.078 7.228 7.090 8.459 7.233 7.019

5

6

7

8

9

10

150 160 170

ExperimentalTheoretical

Strain Rate K [s-1]

Thic

knes

s de

lta [m

m]

Theoretical:delta~1/K0.5

Measured:delta~1/K0.90627

Fig. 7. Mixing layer thickness d obtained from Raman measurements vs. strain rateon a log–log scale. The experimentally measured slope (solid line acquired throughleast square fit) is compared with the theoretically expected 1/K0.5 behavior (dashedline) which is positioned such that the initial points of both lines coincide.



where A, B and C are regression constants and x is the distance fromthe fuel nozzle. The stoichiometric surface was considered to be atthe location of the maximum slope of the Z distribution and the cor-responding plot for flame configuration 3 is shown in Fig. 8.

The calculated rZj jstoich for each flame case was then used inorder to determine the mixing layer thickness d, which scales asthe inverse rZj jstoich and is shown in Fig. 8. Table 9 contains thesevalues and Fig. 9 presents the variation of d with strain rate.

The investigation of the inverse-square-root-dependence of don strain rate, is presented here similarly to Fig. 7 on a log–logscale and with the theoretical curve present. The results of Fig. 9show that there is good agreement between the theoreticallyexpected 1/K0.5 and the actually measured dependence of d onstrain rate which scales approximately as 1/K0.42. In all three cases,the stoichiometric surface was found to be located approximately2 mm away from the location of maximum temperature in the oxi-dizer side. Specifically, it was 13.2 mm for cases 2 and 4 and

12 mm for case 3. This can be attributed to the significant experi-mental error rising from the Raman measurements. A more sensi-tive camera that would capture combustion products is necessaryfor a more accurate measurement of the mixture fraction.

4. Conclusions

Pure butanol flames were found to be more vulnerable toextinction than butanol–methane mixture flames with the sameoverall equivalence ratio and total combustion heat release. Fur-thermore, a monotonic behavior of the extinction strain rate wasobserved with increasing heat release, both in pure butanol andin the mixture cases. The inverse-square-root-dependence of themixing layer thickness d on strain and scalar dissipation rate wasinvestigated, and d was estimated using the temperature distribu-tion across the flame. This was found to generate a departure of dfrom the theoretically expected square-root behavior due to thenoise of the intensified camera and the substantial shot-noise be-cause of the modest-power of the laser used for excitation. How-ever, the proposed methodology for extracting flame thicknessand essentially correlating it with scalar dissipation rate can beused in order to produce results on the vulnerability of bio-alcoholflames to extinction with rather standard laboratory equipment.The scalar dissipation rate at the stoichiometric surface was furtherapproximated through the mixture fraction gradient rZj jstoich

which was calculated using a definition based on the mass fractionof nitrogen. These results showed a better agreement with theoret-ical predictions.

Acknowledgements

The authors would like to acknowledge the support of the USDepartment of Energy through the Graduate Automotive Technol-ogy Education (GATE) Center of Excellence at the University ofIllinois.

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0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16

Flame case 3

Mix

ture

Fra

ctio

n Z

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= 0.453

IΔZstoich

I = 0.308 mm-1

δ = 0.3245


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Experimental investigation of bio-butanol laminar non-premixed flamelets