Laminar Burning Velocity of Propane/CO2/N2−Air Mixtures atElevated TemperaturesMohammad Akram,† V. Ratna Kishore,‡ and Sudarshan Kumar*,†

†Department of Aerospace Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India‡Department of Mechanical Engineering, Amrita Vishwa Vidyapeetham, Coimbatore 641105, India

ABSTRACT: The laminar burning velocity of pure and diluted high-temperature propane−air mixtures is extracted from theplanar flames stabilized in the preheated mesoscale diverging channel. The experiments were carried out for a range ofequivalence ratios of 0.7 ≤ Φ ≤ 1.3 and mixture temperatures of 370−650 K. The effect of dilution using CO2 and N2 gases (upto 40%) on C3H8−air burning velocity is also studied. Experiments complimented with computational studies of experimentalconditions confirm that the stabilized flames were planar in both transverse and depth directions, and the burning velocity withheat flux in the present case is nearly equal to the adiabatic burning velocity. The detailed uncertainty analysis shows the accuracyof the present measurement within ±5%. Computational predictions of burning velocity and detailed flame structure wereperformed using PREMIX code. The present experiments are successfully validated against existing experimental andcomputational results. The peak burning velocity was observed for slightly rich mixtures even at higher mixture temperatures.The minimum value of the temperature exponent is observed for slightly rich mixtures. The burning velocity was observed todecrease with the dilution of inert gases. The addition of CO2 shows a pronounced decrease in the burning velocity, as comparedto N2.

1. INTRODUCTION

Increasing concern over global warming has led to thedevelopment of various techniques for reducing the emissionsof greenhouse gases (GHGs). Various developments on thesequestration of CO2 are being actively pursued byresearchers.1 Propane is a major component of autogas(green fuel). It has very good potential because of its abilityto liquefy under moderate pressure conditions, thus enablingeasier onboard storage and handling. It is a known fact that theaddition of chemically inert dilutants results in better flamequenching properties, because of increased specific heats andchanges in transport properties and reaction pathways.2

Considering the possibility of using CO2 and N2 gases asdilutants, the present study is chosen to understand the effect ofdilution on the laminar burning velocity of propane−airmixtures at high temperatures. The study will help inunderstanding the effect of dilution on the mixture reactivity,diffusivity, and exothermicity in the presence of these inerts.The laminar burning velocity is a physiochemical property of

a premixed mixture of fuel and oxidizer.3 Many experimentshave been proposed to predict the accurate magnitudes of thelaminar burning velocity of premixed propane−air mixtures,including the combustion bomb method,2,4−8 counter jet orstagnation plane method,3,9−13 and heat flux method.14−17 Inthese methods, the experimental data were extrapolated toeither zero heat loss or zero strain rate conditions to obtain theaccurate magnitude. Recently, Kim and Kim18 used an annulardiverging tube to predict the laminar burning velocity of fuel−air mixtures at ambient conditions. The measured burningvelocities of fuel−air mixtures match well with the existingresults. However, they have not considered the effect of heatloss and thermal regeneration between flame and walls in suchsmall channels. Most of the available experiments provide the

burning velocity data at ambient temperature only. Therefore,accurate burning velocity data for various pure and dilutedfuel−air mixtures at high temperatures will be helpful in thedesign of various practical devices, such as internal combustion(IC) engines and gas turbines.The flame structure is largely governed by the velocity profile

in the channels and, hence, results in hydrodynamics strainrates3 depending upon the velocity gradients in the flowdomain. A planar (stretched-free) flame propagation mode wasobserved to exist for certain conditions of the flow rate andmixture equivalence ratios19−21 in high aspect ratio divergentchannels. The burning velocity extracted from these planarflames for the stoichiometric methane−air mixture19 andliquefied petroleum gas (LPG)−air mixture20 matched wellwith the existing experiments and theoretical correlations. Inthe present work, the experiments have been extended to themeasurement of the laminar burning velocity of pure anddiluted propane−air mixtures for a range of equivalence ratiosat elevated mixture temperatures.

2. EXPERIMENTAL SECTIONIn the present work, a high aspect ratio diverging mesoscale channelwith an inlet dimension of 25 × 2 mm and a diverging angle of 10° waschosen. The detailed schematic diagram of the experimental setup isshown in Figure 1. The strategy followed in the present experiment forthe stabilization of planar flames is also shown in Figure 1. The startingpoint of the diverging section is considered as the reference point andmarked as “o”. The x axis and y axis represent the axial and transversedirections, respectively. Quartz material for the channel is chosenbecause of its high heat capacity, low thermal conductivity, low thermal

Received: June 13, 2012Revised: August 23, 2012Published: August 29, 2012

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expansion, and transparency of the walls to help visualize the flamefront location in the channel. Pure and diluted propane−air premixedmixtures at ambient condition (300 K and 1.0 atm pressure) weresupplied at the inlet. A sintered metal burner (placed at thedownstream location and 20 mm below the channel) was used topreheat the channel walls, which helps in stabilizing the flame in thechannel and substantially reduces the heat loss from the flame to thesolid walls. This also helps in initial flame ignition and subsequentflame stabilization in the channel. Wall temperature measurementswere carried out using K-type thermocouples at different axial andtransverse locations. The movement of the thermocouple wascontrolled through a precisely controlled traverse, which has aminimum resolution of 0.25 mm. The measured temperatures wereaccurate within ±5 K of the actual value. The bottom wall temperatureprofiles in both directions (axial and transverse) of the inner side weremeasured in advance with airflow. Figure 2 shows the variation of thewall temperature (left half) and flow velocity (right half) in thenormalized transverse direction for an inlet velocity of 0.5 m/s.Uniform temperature and velocity distributions in transverse directionwere observed, which helped in the formation of planar flames.

3. PRELIMINARY RESULTS

The laminar burning velocity of any mixture is influenced byflame stretch and heat loss from the flame to the walls.Experiments and numerical simulations were carried out tounderstand the effect of these parameters. In computationalstudies, two-dimensional (2D) Navier−Stokes equations weresolved along with energy and species conservation equations.The laminar flow calculations were carried out to obtain thedistribution of velocity, species mass fraction, reaction rate,temperature, and heat flux in the computational domain. Thegrid was adapted to provide sufficient resolution to thepropagating flame front within the computational domain and

to obtain grid-independent results.22,23 The computations werecarried out for a 2 mm thick quartz channel with different wallheat-transfer conditions, such as preheated, adiabatic, iso-thermal, and various convective heat-transfer conditions. Moredetails of computations can be found elsewhere.20

3.1. Planar Flames. Various flame propagation modes havebeen reported inside high aspect ratio diverging channels, suchas planar, negatively stretched, and positively stretched.19

Because of the high aspect ratio of the channel, the flow fieldand wall temperature profile were uniform in transversedirection (as shown in Figure 2). This uniformity helps inthe formation of planar flames.19 Figure 3 shows a direct

Figure 1. Schematic diagram of the experimental setup and channel configuration.

Figure 2. Wall temperature and flow velocity in transverse directioninside the channel at Uairflow = 0.5 m/s.

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photograph of a typical planar flame observed during theexperiments for stoichiometric propane−air mixtures. Theflame front was observed to be very thin, and this confirms thatthe flame is planar in both transverse and depth directions.Computational simulations reported in the earlier work20 alsoconfirm that flames stabilized were planar in depth direction forthe range of mixture temperatures and flow rates studied.3.2. Adiabatic Burning Velocity. Flames stabilized in such

small-scale channels lose heat to the solid walls. However, inthe present case, the presence of external heating substantiallyreduces this heat loss. The reduction in the heat loss from theflame has been confirmed through a series of computationswith various wall heat-transfer conditions.20 Figure 4 shows thevariation of heat flux on the inner and outer sides of thechannel walls for a mixture velocity of Uinlet = 0.5 m/s withpreheated wall conditions. Such studies were carried out for arange of equivalence ratios and inlet velocities with

experimentally observed wall temperature profiles. The heatflux on the inner wall near the flame is observed to be in therange of 100−300 kW/m2 for different mixture and wall heat-transfer conditions. The heat flux on the outside wall isobserved to be in the range of 10−30 kW/m2.Botha and Spalding14 have reported the variation of the

laminar burning velocity with the heat carried away from theburner-stabilized flames. Figure 4 shows the variation of thenormalized (Su/Su,ad) burning velocity with heat flux reportedfor stoichiometric propane−air mixtures. In Figure 4, dashed(vertical) lines show the range of the heat loss from the flameto the wall in the present experiments and correspondingarrows (horizontal lines) show the respective deviation of theburning velocity from the adiabatic value. It is clear from thisfigure that, for present experiments, the deviation of the valueof the laminar burning velocity is less than 4% from theadiabatic burning velocity. A reduction in the burning velocitymagnitude with heat flux is considered in the uncertaintyanalysis and subsequently corrected.

3.3. Burning Velocity Calculation. First, the planar flameswere stabilized for different mixture velocities and knownheating rates of the bottom burner. These flames were thenused to extract the laminar burning velocity for a fixed mixtureequivalence ratio and mixture temperatures. The location of theflame was captured with a digital camera, and an accurate flameposition was obtained through detailed image processing. It isassumed that the mixture attains a temperature equal to themean temperature of the top and bottom walls of thechannel.19,20,24 The burning velocity of the given mixture at aparticular temperature is obtained using the following relationof conservation of mass of the fuel−air mixture entering theflame front.19,20,24

=⎛⎝⎜

⎞⎠⎟⎛⎝⎜⎜

⎞⎠⎟⎟S U

AA

TTu inlet

inlet

f

u

u,o (1)

Burning velocity computations are carried out for freelypropagating steady adiabatic flames to validate the presentexperimental results. The PREMIX25 code was used for thecomputations with a detailed propane−air kinetic mechanismproposed by Qin et al.26 The transport properties, such as

Figure 3. Direct photograph of a planar flame for a stoichiometricpropane−air mixture.

Figure 4. Variation of the burning velocity with heat transferred from the flame to solid walls for a stoichiometric mixture with preheated wallcondition (data taken from Botha and Spalding14).

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TRANFIT, for computations were taken from Sandia NationalLaboratories.27,28 Multicomponent diffusion and thermaldiffusion options were taken into account. The upwinddifference scheme is preferred for the study, which allows forrefined grid adaption. The grid-independent burning velocitieswere confirmed for GRAD = 0.02 and CURV = 0.1.3.4. Uncertainty Analysis. Various parameters influencing

the burning velocity, such as heat loss from the flame, thermalfeedback, boundary layer, and difference of wall and reactanttemperature, were included to calculate the overall uncertaintyin the measurement. The heat loss observed in the presentstudy reduces the burning velocity by less than 4%. Thedifference between the average wall temperature and mixturetemperature is observed to be less than 20 K. This will reducethe burning velocity by less than 5%. The effective flame area isconsidered in the calculation. The uncertainty in equivalenceratio was calculated considering the uncertainty in mass flowcontrollers of both fuel and air. Mass flow rates were in therange of ∼80% of the full scale, which reduced the uncertaintyin equivalence ratios. Some of these factors that affect theburning velocity are observed to mutually compensate for eachother. For instance, the combined influence of all of theseparameters leads to an uncertainty of less than ±5% of theactual value. The repeatability of the measurement techniquehas been confirmed by carrying out the experiment19−21 forvarious fuel−air mixtures over a period of a couple of years.

4. RESULTS AND DISCUSSION

The present work focuses on the laminar burning velocity ofpropane−air mixtures and the effect of CO2 and N2 dilution onthe mixture burning velocity. In this work, the dilution ratio isexpressed as a percentage of volume added to fuel stream. Forinstance, 20% N2 dilution means that the fuel stream consists of80% propane and 20% N2 by volume. A relative proportion ofair in the fuel−air mixture is added to define the mixtureequivalence ratio Φ. The equivalence ratio and percentagedilution are calculated using the below equations, respectively.

+ +Φ

+

→ + +Φ

+

a

x ya

(C H diluent) (O 3.76N )

CO H O 3.76N diluent

3 8 2 2

2 2 2 (2)

=+

percent dilutionvolume of diluent

volume of fuel diluent (3)

Laminar burning velocities for a range of equivalence ratios of0.7 ≤ Φ ≤ 1.3 and preheat temperatures of 370−650 K havebeen measured. The limits for the present experiments areconsidered to a temperature value near the autoignitiontemperature of the fuel. The position of the stabilized flameinside the channel depends upon the mixture equivalence ratio,flow velocity, and local temperature. The range of mixturetemperatures is different for the given range of mixtureequivalence ratios.The experiments were conducted at atmospheric (1.0 atm)

pressure only. The initial reference temperature of theunburned mixture was 300 K for the entire experiments. Thevariation of the burning velocity Su is plotted against the ratio ofthe mixture temperature and initial reference temperature, forvarious equivalence ratios. To describe the simultaneous effectof the temperature and equivalence ratio, a simple power lawcorrelation was applied to the experimental data as

= ×α⎛

⎝⎜⎜

⎞⎠⎟⎟S S

TTu u,o

u

u,o (4)

4.1. Influence of the Initial Temperature on theBurning Velocity. The variation of the laminar burningvelocity for a stoichiometric propane−air mixture with thetemperature ratio is shown in Figure 5. A continuous bold line

shows the power law trend line for the present experimentalresults. Error bars are shown for a variation of ±5% from themeasured experimental value. The burning velocity wasobserved to increase with the mixture temperature. Thepresent results are in good agreement with the values obtainedfrom the available correlations4,6 and numerical predictions ofQin et al.26 Results from Metghalchi and Keck4 show thehighest increment in the burning velocity with the temperature.However, a very small magnitude of the burning velocity isobserved at 300 K for their case. If the present results areextrapolated to 300 K, a close agreement can be observed withthe results from Takizawa et al.6 and Razus et al.7 The burningvelocity increases with the mixture temperature with Su,o =0.417 m/s and temperature exponent α = 1.636.Figure 6 shows the variation of the laminar burning velocity

for a lean mixture (Φ = 0.8) and rich mixture (Φ = 1.2) for arange of mixture temperatures. Continuous lines in Figure 6show the power law trend line for the experimental results.Error bars show the uncertainty in the present measurements.Figure 6 also shows a close agreement of the present resultswith the predictions of the Qin et al.26 mechanism. The burningvelocity for Φ = 0.8 increases with the mixture temperature,with Su,o = 0.23 m/s and temperature exponent α = 1.878. Theburning velocity for Φ = 1.2 increases with the mixturetemperature, with Su,o = 0.382 m/s and temperature exponent α= 1.694. Similar experiments were conducted for otherequivalence ratios, and the burning velocities and temperatureexponents were obtained.

4.2. Influence of the Equivalence Ratio on theTemperature Exponent. The burning velocity resultsobtained for each equivalence ratio were fitted with powerlaw correlations. The variation of the temperature exponents(α) with the mixture equivalence ratio is shown in Figure 7.The temperature exponent is a strong function of the

Figure 5. Laminar burning velocity of a stoichiometric propane−airmixture at elevated temperatures.

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equivalence ratio. The present experimental results show theexistence of a minimum value of temperature exponent α nearΦ = 1.1. A cubic polynomial appears to be well-suited todescribe the variation of the temperature exponent (α) for bothlean and rich mixtures.

α = Φ + Φ − Φ +0.888 1.057 5.23 4.913 2 (5)

Figure 7 also shows the comparison of the present results to thedata available from various experiments4,6 and numericalcomputations using the Qin et al.26 mechanism. There is alarge discrepancy in the literature4,6 regarding the magnitudeand existence of the minimum temperature exponent.Metghalchi and Keck4 reported higher values of the temper-ature exponent with a very low burning velocity at 300 K. Thecorrelation given by Takizawa et al.6 gives higher and lowermagnitudes for lean and rich mixtures, respectively, ascompared to those obtained from the Qin et al. mechanism.26

The values of the temperature exponent obtained from presentexperiments are in good agreement with numerical computa-tions of Qin et al.26 The temperature exponent reported byMetghalchi and Keck4 and Takizawa et al.6 shows a linearvariation of α with the equivalence ratio. However, such a linearvariation of the temperature exponent has been discouraged inthe existing literature for hydrocarbon fuels.29 The existence of

a minimum value of the temperature exponent near astoichiometric mixture is confirmed by various researchersearlier for methane−air29−31 and LPG−air mixtures.20

4.3. Laminar Burning Velocity at Ambient Temper-ature. The power law correlations are extrapolated to obtainthe burning velocity of propane−air mixtures at ambientconditions. A cubic polynomial correlation is obtained to showthe influence of the mixture equivalence ratio on the burningvelocity of propane−air mixtures at ambient conditions andgiven as

= − Φ + Φ − Φ +S 1.777 3.759 1.879 0.308 m/su,o3 2

(6)

Figure 8a shows the variation of laminar burning velocitiesobtained for ambient conditions from the extrapolation of high-

temperature data for propane−air mixtures. The valuesobtained are in good agreement with the recent experimentaldata available in the literature.2,8,10,13,17 Figure 8b shows thecomparison of the present data to various reaction mechanisms,i.e., Qin et al.26 and Konnov 0.5,32 at ambient conditions.Figure 9 shows that the laminar burning velocity obtained

using the present method matches well, even for high mixturetemperatures of 500 and 650 K, with the results from Qin etal.26 and Zhao et al.13 However, Zhao et al.13 have reportedslightly higher values of burning velocity at 650 K. The burningvelocity was highest for a slightly rich mixture, even at very hightemperatures.

4.4. Propane/N2−Air Flames at High Temperatures.The laminar burning velocity for pure propane−air mixturesmatches well with the predictions of Qin et al.26 and otherexperimental results available in the literature. This validates thepresent technique for measuring the burning velocities ofvarious fuel−air mixtures. Using this method, laminar burningvelocities of diluted propane−air mixtures are determined bothexperimentally and computationally. The laminar burning

Figure 6. Laminar burning velocity for rich and lean propane−airmixtures at elevated temperatures.

Figure 7. Temperature exponent variations with the equivalence ratiofor a propane−air mixture.

Figure 8. Comparison of the laminar burning velocity of a propane−air mixture at ambient pressure and 300 K with available (a)experimental and (b) computational results.

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velocity of propane/N2-diluted mixtures (up to 40%) ismeasured for a range of high temperatures. Figure 10 shows

the variation of the laminar burning velocity for the 20% N2dilution case at 300 K and higher mixture temperatures. Acomparison to the mechanism of Qin et al.26 is also shown, andno experimental comparison could be made because of thenon-availability of data for C3H8/N2−air-diluted mixtures athigh temperatures. The burning velocity peak is observedconsistently for slightly rich mixtures (Φ = 1.1). A similarvariation of the burning velocity was observed for mixtures withdifferent N2 dilutions, such as 10, 30, and 40% cases. Thepredictions of the Qin et al.26 mechanism are in reasonablygood agreement with present experiments, except at very highdilution rates. The predictions slightly overpredict the laminarburning velocity for the 40% dilution case, indicating that thereis a need to modify the mechanism for high dilution ratios.4.5. Propane/CO2−Air Flames at High Temperatures.

Figure 11 shows the variation of the burning velocity for the20% CO2 dilution case. The comparison of the experimentaldata is shown for different mixture temperatures with thepredictions of Qin et al.26 The maximum burning velocity isobserved for slightly rich mixtures (Φ = 1.1). The temperatureexponent was minimum for slightly rich mixtures, as compared

to very rich and lean mixtures for the given range of dilutionrates studied in the present work. Similar variations in theburning velocity were observed for mixtures with different CO2dilutions, such as 10, 30, and 40% cases. The predictions of Qinet al.26 are in reasonably good agreement with presentexperiments, except for very high dilution rates, where itpredicts slightly higher values.

4.6. Effect of Dilution on the Laminar BurningVelocity and Temperature Exponent. The dilution withinert gases reduces the burning velocity and increases themagnitude of the temperature exponent. Panels a and b ofFigure 12 show the variation of laminar burning velocities andtemperature exponents α for propane−air mixtures withdifferent N2 dilution rates, respectively. This will be helpfulto extract the burning velocity of diluted propane−air mixturesat elevated temperatures. Panels a and b of Figure 13 show therespective variation of laminar burning velocities and temper-ature exponents α for CO2-diluted propane−air mixtures. The

Figure 9. Laminar burning velocity of a propane−air mixture at 500and 650 K.

Figure 10. Burning velocity of a 20% N2-diluted propane−air mixtureat different temperatures.

Figure 11. Burning velocity of a 20% CO2-diluted propane−airmixture at different temperatures.

Figure 12. (a) Burning velocities and (b) temperature exponents ofN2-diluted propane−air mixtures at 300 K.

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temperature exponent was observed to vary with theequivalence ratio in a way similar to that for the purepropane−air case with both N2 and CO2 dilution. A minimumvalue of the temperature exponent was obtained for thecomplete range of N2 dilution rates studied in the presentexperiments. The value of the temperature exponent wasobserved to increase with the dilution rates for both inert gases.Figure 14 shows the influence of inert gas dilution on the

laminar burning velocity of a stoichiometric propane−air

mixture at 500 K. Experiments show a significant reductionin the burning velocity with dilution. The burning velocity withN2 dilution is in good agreement with the results from Zhao etal.13 However, a slightly higher rate of change in the burningvelocity was observed in present experiments, as compared tocomputational results, shown in Figure 14. The decrease in theburning velocity is almost linear with N2 dilution rates. Figure14 also shows a decrease in the burning velocity with CO2dilution at 500 K. The burning velocity shows a reduction of18% for the 40% N2 case and 38% for the CO2 dilution case.

This reduction in the burning velocity is small, as compared tothe reduction in the burning velocity for CO2-dilutedmethane−air mixtures, as reported by Kishore et al.16 Thereasons for the decrease in the burning velocity with inertdilution are discussed in the next section.

5. FLAME STRUCTURE STUDIESTo understand the effect of the temperature, equivalence ratio,and inert dilutions on the laminar burning velocity, detailedflame structures were computed along with burning velocityusing PREMIX25 code and the Qin et al.26 mechanism. Thevariation of total heat release with non-dimensional temper-ature [τ = (T − Tu)/(Tad − Tu)] is plotted for different cases,as shown in Figure 15. Maximum heat release rates occur for

pure propane−air mixtures at high temperatures. It can be seenthat the area under the curve for a mixture temperature of 500K is much higher than that of 300 K, which corroborates theincrease in the burning velocity because of a higher mixturetemperature. The peak heat release rate occurs at a lowertemperature with an increase in the mixture temperature. Thismay be due to the fact that initiation of chain-branchingreactions in the preheat zone as the initial mixture temperatureis high enough to generate requisite radicals to carry outreactions within the flame front. Also, with the addition ofinerts, peak heat release occurs at a higher non-dimensionaltemperature because of a delay in generating radicals. Toexplore the reason for the increase in the burning velocity, theadiabatic flame temperature and mole fractions of participatingspecies are plotted in Figures 16−18. Figure 16 shows theprofiles of the adiabatic flame temperature along the axialdirection. Because of high heat release rates, the flametemperature is maximum for a pure propane−air mixture at500 K. Figure 17 shows the variations of the mole fraction ofstable species, such as CO2, N2, and H2O, along the axialcoordinate. For a mixture with 300 K initial temperature, themole fraction of CO2 is higher, as compared to the molefraction at 500 K. The higher mole fraction of CO2 absorbs theheat energy released from the chemical reaction because of itshigh heat capacity and, hence, lowers the flame temperature by∼90 K, as compared to the pure propane case. A reduction inthe adiabatic flame temperature causes a reduction in theburning velocity. To further understand the reduction in the

Figure 13. (a) Burning velocities and (b) temperature exponents ofCO2-diluted propane−air mixtures at 300 K.

Figure 14. Influence of dilution on the laminar burning velocity of astoichiometric propane−air mixture at 500 K.

Figure 15. Heat release rate versus non-dimensional temperature, fordifferent mixture and initial temperature conditions.

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burning velocity, mole fractions of minor species, such as C3H6,OH, and O, are shown in Figure 18 along the non-dimensionaltemperature τ, which indicates that a higher initial temperatureproduces a higher mole fraction of radicals, whose peak shiftstoward a lower temperature. A similar increase in the mole

fraction with an increase in the initial mixture temperature isobserved for other radicals as well. The enhanced production ofthese species contributes significantly to an increase in the heatrelease rate and, hence, the flame temperature. As a result, thereis an increase in the burning velocity with an increase in theinitial mixture temperature.The addition of chemically inert dilutants was observed to

reduce the burning velocity. This results in better flame-quenching properties and a reduction in the reaction intensityof combustible mixtures. As shown in Figure 15, the dilutionwith both CO2 and N2 shifts the peak heat release rates towarda higher non-dimensional temperature, resulting in a delay inchain initiation. This requires additional heat to generateradicals, which leads to the combustion. The delay incombustion reduces the reaction rate and burning velocity.With the same amount of inert dilution, CO2 has a

pronounced effect on the burning velocity compared to N2dilution. Figure 17 shows that the mole fraction of CO2 andH2O is more for the CO2 dilution case. These species absorb alarge part of the heat released from the combustion, and lessamount of heat is transferred to the preheat zone. This effectcan be seen from Figure 15, where the addition of CO2 specieswith comparatively higher specific heat results in a lower heatrelease rate and a lower adiabatic flame temperature comparedto N2 dilution. The mole fraction of minor species (radicals)are lower for the CO2 dilution case compared to N2 dilution, asshown in Figure 18, which reduces the reaction rate further.The decrease in the burning velocity is due to both thermal

and chemical effects. To estimate their relative importance,simulations were carried out using CO2 and FCO2, a fake CO2molecule. This species FCO2 possesses the same thermal andtransport properties as that of CO2 but does not participate inthe reaction/dissociation. The burning velocity is thencomputed from the Qin et al.26 mechanism using PREMIX25

code. The burning velocity of a pure propane−air mixture isconsidered as a reference. The difference of the pure propaneburning velocity and propane with FCO2 dilution gives an ideaabout the thermal effect. The difference in the burning velocityof propane with FCO2 dilution and CO2 dilution provides thechemical effect, as seen from Figure 19. The relativecontributions of thermal and chemical effects are estimatedfor different dilution rates at different temperatures. Therelative decrease in the laminar burning velocity because ofthermal effects is obtained from the following relation:

Figure 16. Variation of predicted flame temperatures along the axialdirection, for various mixture and initial temperature conditions.

Figure 17. Variation of stable species mole fractions along the axialdirection, for various mixture and initial temperature conditions.

Figure 18. Variation of radical species mole fraction versus non-dimensional temperature, for various mixture and initial temperatureconditions.

Figure 19. Laminar burning velocity for pure propane and 20% FCO2and CO2 dilution at 300 K.

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=−−

×S S

S Sthermal effect

( )

( )100%u,pure u,FCO

u,pure u,CO

2

2 (7)

The relative contributions of thermal and chemical effects for20% dilution at 300 K are shown in Figure 20 for a range of

mixture equivalence ratios. In the case of dilution of fuel with20% CO2, the decrease in the laminar burning velocity becauseof thermal and chemical effects is equal for most of themixtures. However, with an increase in the dilution, thermaleffects become predominant. The thermal effect is observed tobe least for the slightly rich mixture at Φ = 1.1 for all dilutionrates and mixture temperatures.

6. CONCLUSIONIn the present paper, the burning velocity of pure and dilutedpropane−air mixtures at high temperatures is measured usingthe planar flames in an externally heated divergent channel. Themeasured burning velocity for pure propane−air mixturesmatches well with the available experimental and computationalresults for all of the ranges of mixture temperatures and mixtureequivalence ratios reported in the literature. The burningvelocity is maximum for slightly rich mixtures (even at hightemperatures), for which the temperature exponent is observedto be minimum. A linear decrease in the burning velocity ofdiluted propane−air mixtures is observed. The decrease in theburning velocity was more pronounced for the CO2 dilutioncase, as compared to the N2 dilution case. The thermal effectwas predominant at higher temperatures, and CO2 dilutionrates reduce the chemical effect. The propane reactionmechanism of Qin et al.26 slightly overpredicts the burningvelocity and slightly underpredicts the temperature exponentfor highly diluted propane−air mixtures.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +91-22-2576-7124. Fax: +91-22-2572-2602. E-mail: sudar@aero.iitb.ac.in.NotesThe authors declare no competing financial interest.

■ NOMENCLATUREAinlet = inlet cross-sectional area (m2)Af = flame cross-sectional area (m2)

Su,o = laminar burning velocity at the reference initialtemperature of 300 K (m/s)Su = laminar burning velocity at temperature Tu (m/s)Tu,o = reference initial unburned gas temperature (K)Tu = unburned gas temperature (K)Uinlet = inlet mixture velocity (m/s)Uairflow = cold flow air velocity (m/s)X = species mole fractionα = temperature exponentΦ = equivalence ratioτ = non-dimensional temperature

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Figure 20. Thermal and chemical effect on the laminar burningvelocity of propane−air mixtures at 300 K.

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