N2–Air Mixtures at Elevated

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Laminar Burning Velocity of Propane/CO2/N2−Air Mixtures at Elevated Temperatures Mohammad 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 the planar flames stabilized in the preheated mesoscale diverging channel. The experiments were carried out for a range of equivalence ratios of 0.7 ≤ Φ ≤ 1.3 and mixture temperatures of 370−650 K. The effect of dilution using CO2 and N2 gases (up to 40%) on C3H8−air burning velocity is also studied. Experiments complimented with computational studies of experimental conditions confirm that the stabilized flames were planar in both transverse and depth directions, and the burning velocity with heat flux in the present case is nearly equal to the adiabatic burning velocity. The detailed uncertainty analysis shows the accuracy of the present measurement within ±5%. Computational predictions of burning velocity and detailed flame structure were performed using PREMIX code. The present experiments are successfully validated against existing experimental and computational 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 to decrease with the dilution of inert gases. The addition of CO2 shows a pronounced decrease in the burning velocity, as compared to N2.

1. INTRODUCTION Increasing concern over global warming has led to the development of various techniques for reducing the emissions of greenhouse gases (GHGs). Various developments on the sequestration of CO 2 are being actively pursued by researchers.1 Propane is a major component of autogas (green fuel). It has very good potential because of its ability to liquefy under moderate pressure conditions, thus enabling easier onboard storage and handling. It is a known fact that the addition of chemically inert dilutants results in better flame quenching properties, because of increased specific heats and changes in transport properties and reaction pathways.2 Considering the possibility of using CO2 and N2 gases as dilutants, the present study is chosen to understand the effect of dilution on the laminar burning velocity of propane−air mixtures at high temperatures. The study will help in understanding 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 experiments have been proposed to predict the accurate magnitudes of the laminar burning velocity of premixed propane−air mixtures, including the combustion bomb method,2,4−8 counter jet or stagnation plane method,3,9−13 and heat flux method.14−17 In these methods, the experimental data were extrapolated to either zero heat loss or zero strain rate conditions to obtain the accurate magnitude. Recently, Kim and Kim18 used an annular diverging tube to predict the laminar burning velocity of fuel− air mixtures at ambient conditions. The measured burning velocities of fuel−air mixtures match well with the existing results. However, they have not considered the effect of heat loss and thermal regeneration between flame and walls in such small channels. Most of the available experiments provide the © 2012 American Chemical Society

burning velocity data at ambient temperature only. Therefore, accurate burning velocity data for various pure and diluted fuel−air mixtures at high temperatures will be helpful in the design 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 strain rates3 depending upon the velocity gradients in the flow domain. A planar (stretched-free) flame propagation mode was observed to exist for certain conditions of the flow rate and mixture equivalence ratios19−21 in high aspect ratio divergent channels. The burning velocity extracted from these planar flames for the stoichiometric methane−air mixture19 and liquefied petroleum gas (LPG)−air mixture20 matched well with the existing experiments and theoretical correlations. In the present work, the experiments have been extended to the measurement of the laminar burning velocity of pure and diluted propane−air mixtures for a range of equivalence ratios at elevated mixture temperatures.

2. EXPERIMENTAL SECTION In the present work, a high aspect ratio diverging mesoscale channel with an inlet dimension of 25 × 2 mm and a diverging angle of 10° was chosen. The detailed schematic diagram of the experimental setup is shown in Figure 1. The strategy followed in the present experiment for the stabilization of planar flames is also shown in Figure 1. The starting point of the diverging section is considered as the reference point and marked as “o”. The x axis and y axis represent the axial and transverse directions, respectively. Quartz material for the channel is chosen because of its high heat capacity, low thermal conductivity, low thermal Received: June 13, 2012 Revised: August 23, 2012 Published: August 29, 2012 5509

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Figure 1. Schematic diagram of the experimental setup and channel configuration. expansion, and transparency of the walls to help visualize the flame front location in the channel. Pure and diluted propane−air premixed mixtures at ambient condition (300 K and 1.0 atm pressure) were supplied at the inlet. A sintered metal burner (placed at the downstream location and 20 mm below the channel) was used to preheat the channel walls, which helps in stabilizing the flame in the channel and substantially reduces the heat loss from the flame to the solid walls. This also helps in initial flame ignition and subsequent flame stabilization in the channel. Wall temperature measurements were carried out using K-type thermocouples at different axial and transverse locations. The movement of the thermocouple was controlled through a precisely controlled traverse, which has a minimum resolution of 0.25 mm. The measured temperatures were accurate within ±5 K of the actual value. The bottom wall temperature profiles in both directions (axial and transverse) of the inner side were measured in advance with airflow. Figure 2 shows the variation of the wall temperature (left half) and flow velocity (right half) in the normalized transverse direction for an inlet velocity of 0.5 m/s. Uniform temperature and velocity distributions in transverse direction were observed, which helped in the formation of planar flames.

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

3. PRELIMINARY RESULTS The laminar burning velocity of any mixture is influenced by flame stretch and heat loss from the flame to the walls. Experiments and numerical simulations were carried out to understand the effect of these parameters. In computational studies, two-dimensional (2D) Navier−Stokes equations were solved along with energy and species conservation equations. The laminar flow calculations were carried out to obtain the distribution of velocity, species mass fraction, reaction rate, temperature, and heat flux in the computational domain. The grid was adapted to provide sufficient resolution to the propagating flame front within the computational domain and

to obtain grid-independent results.22,23 The computations were carried out for a 2 mm thick quartz channel with different wall heat-transfer conditions, such as preheated, adiabatic, isothermal, and various convective heat-transfer conditions. More details of computations can be found elsewhere.20 3.1. Planar Flames. Various flame propagation modes have been reported inside high aspect ratio diverging channels, such as planar, negatively stretched, and positively stretched.19 Because of the high aspect ratio of the channel, the flow field and wall temperature profile were uniform in transverse direction (as shown in Figure 2). This uniformity helps in the formation of planar flames.19 Figure 3 shows a direct 5510

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experimentally observed wall temperature profiles. The heat flux on the inner wall near the flame is observed to be in the range of 100−300 kW/m2 for different mixture and wall heattransfer conditions. The heat flux on the outside wall is observed 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 the burner-stabilized flames. Figure 4 shows the variation of the normalized (Su/Su,ad) burning velocity with heat flux reported for stoichiometric propane−air mixtures. In Figure 4, dashed (vertical) lines show the range of the heat loss from the flame to the wall in the present experiments and corresponding arrows (horizontal lines) show the respective deviation of the burning velocity from the adiabatic value. It is clear from this figure that, for present experiments, the deviation of the value of the laminar burning velocity is less than 4% from the adiabatic burning velocity. A reduction in the burning velocity magnitude with heat flux is considered in the uncertainty analysis and subsequently corrected. 3.3. Burning Velocity Calculation. First, the planar flames were stabilized for different mixture velocities and known heating rates of the bottom burner. These flames were then used to extract the laminar burning velocity for a fixed mixture equivalence ratio and mixture temperatures. The location of the flame was captured with a digital camera, and an accurate flame position was obtained through detailed image processing. It is assumed that the mixture attains a temperature equal to the mean temperature of the top and bottom walls of the channel.19,20,24 The burning velocity of the given mixture at a particular temperature is obtained using the following relation of conservation of mass of the fuel−air mixture entering the flame front.19,20,24

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

photograph of a typical planar flame observed during the experiments for stoichiometric propane−air mixtures. The flame front was observed to be very thin, and this confirms that the flame is planar in both transverse and depth directions. Computational simulations reported in the earlier work20 also confirm that flames stabilized were planar in depth direction for the 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, in the present case, the presence of external heating substantially reduces this heat loss. The reduction in the heat loss from the flame has been confirmed through a series of computations with various wall heat-transfer conditions.20 Figure 4 shows the variation of heat flux on the inner and outer sides of the channel walls for a mixture velocity of Uinlet = 0.5 m/s with preheated wall conditions. Such studies were carried out for a range of equivalence ratios and inlet velocities with

⎛ A ⎞⎛ T ⎞ Su = Uinlet⎜ inlet ⎟⎜⎜ u ⎟⎟ ⎝ A f ⎠⎝ Tu,o ⎠

(1)

Burning velocity computations are carried out for freely propagating steady adiabatic flames to validate the present experimental results. The PREMIX25 code was used for the computations with a detailed propane−air kinetic mechanism proposed by Qin et al.26 The transport properties, such as

Figure 4. Variation of the burning velocity with heat transferred from the flame to solid walls for a stoichiometric mixture with preheated wall condition (data taken from Botha and Spalding14). 5511

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TRANFIT, for computations were taken from Sandia National Laboratories.27,28 Multicomponent diffusion and thermal diffusion options were taken into account. The upwind difference scheme is preferred for the study, which allows for refined grid adaption. The grid-independent burning velocities were 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, thermal feedback, boundary layer, and difference of wall and reactant temperature, were included to calculate the overall uncertainty in the measurement. The heat loss observed in the present study reduces the burning velocity by less than 4%. The difference between the average wall temperature and mixture temperature is observed to be less than 20 K. This will reduce the burning velocity by less than 5%. The effective flame area is considered in the calculation. The uncertainty in equivalence ratio was calculated considering the uncertainty in mass flow controllers of both fuel and air. Mass flow rates were in the range of ∼80% of the full scale, which reduced the uncertainty in equivalence ratios. Some of these factors that affect the burning velocity are observed to mutually compensate for each other. For instance, the combined influence of all of these parameters leads to an uncertainty of less than ±5% of the actual value. The repeatability of the measurement technique has been confirmed by carrying out the experiment19−21 for various fuel−air mixtures over a period of a couple of years.

Su = Su,o

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

shows the power law trend line for the present experimental results. Error bars are shown for a variation of ±5% from the measured experimental value. The burning velocity was observed to increase with the mixture temperature. The present results are in good agreement with the values obtained from the available correlations4,6 and numerical predictions of Qin et al.26 Results from Metghalchi and Keck4 show the highest increment in the burning velocity with the temperature. However, a very small magnitude of the burning velocity is observed at 300 K for their case. If the present results are extrapolated to 300 K, a close agreement can be observed with the results from Takizawa et al.6 and Razus et al.7 The burning velocity 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 a range of mixture temperatures. Continuous lines in Figure 6 show 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 results with the predictions of the Qin et al.26 mechanism. The burning velocity for Φ = 0.8 increases with the mixture temperature, with Su,o = 0.23 m/s and temperature exponent α = 1.878. The burning velocity for Φ = 1.2 increases with the mixture temperature, with Su,o = 0.382 m/s and temperature exponent α = 1.694. Similar experiments were conducted for other equivalence ratios, and the burning velocities and temperature exponents were obtained. 4.2. Influence of the Equivalence Ratio on the Temperature Exponent. The burning velocity results obtained for each equivalence ratio were fitted with power law 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

(C3H8 + diluent) +

percent dilution=

volume of diluent volume of fuel + diluent

(4)

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

4. RESULTS AND DISCUSSION The present work focuses on the laminar burning velocity of propane−air mixtures and the effect of CO2 and N2 dilution on the mixture burning velocity. In this work, the dilution ratio is expressed as a percentage of volume added to fuel stream. For instance, 20% N2 dilution means that the fuel stream consists of 80% propane and 20% N2 by volume. A relative proportion of air in the fuel−air mixture is added to define the mixture equivalence ratio Φ. The equivalence ratio and percentage dilution are calculated using the below equations, respectively. a (O2 + 3.76N2) Φ a → xCO2 + y H 2O + 3.76N2 + diluent Φ

⎛ T ⎞α × ⎜⎜ u ⎟⎟ ⎝ Tu,o ⎠

(2)

(3)

Laminar burning velocities for a range of equivalence ratios of 0.7 ≤ Φ ≤ 1.3 and preheat temperatures of 370−650 K have been measured. The limits for the present experiments are considered to a temperature value near the autoignition temperature of the fuel. The position of the stabilized flame inside the channel depends upon the mixture equivalence ratio, flow velocity, and local temperature. The range of mixture temperatures is different for the given range of mixture equivalence ratios. The experiments were conducted at atmospheric (1.0 atm) pressure only. The initial reference temperature of the unburned mixture was 300 K for the entire experiments. The variation of the burning velocity Su is plotted against the ratio of the mixture temperature and initial reference temperature, for various equivalence ratios. To describe the simultaneous effect of the temperature and equivalence ratio, a simple power law correlation was applied to the experimental data as 5512

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a minimum value of the temperature exponent near a stoichiometric mixture is confirmed by various researchers earlier for methane−air29−31 and LPG−air mixtures.20 4.3. Laminar Burning Velocity at Ambient Temperature. The power law correlations are extrapolated to obtain the burning velocity of propane−air mixtures at ambient conditions. A cubic polynomial correlation is obtained to show the influence of the mixture equivalence ratio on the burning velocity of propane−air mixtures at ambient conditions and given as Su,o = −1.777Φ3 + 3.759Φ2 − 1.879Φ + 0.308

m/s (6)

Figure 8a shows the variation of laminar burning velocities obtained for ambient conditions from the extrapolation of highFigure 6. Laminar burning velocity for rich and lean propane−air mixtures at elevated temperatures.

Figure 7. Temperature exponent variations with the equivalence ratio for 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.

equivalence ratio. The present experimental results show the existence of a minimum value of temperature exponent α near Φ = 1.1. A cubic polynomial appears to be well-suited to describe the variation of the temperature exponent (α) for both lean and rich mixtures. α = 0.888Φ3 + 1.057Φ2 − 5.23Φ + 4.91

temperature data for propane−air mixtures. The values obtained are in good agreement with the recent experimental data available in the literature.2,8,10,13,17 Figure 8b shows the comparison 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 mixture temperatures of 500 and 650 K, with the results from Qin et al.26 and Zhao et al.13 However, Zhao et al.13 have reported slightly higher values of burning velocity at 650 K. The burning velocity was highest for a slightly rich mixture, even at very high temperatures. 4.4. Propane/N2−Air Flames at High Temperatures. The laminar burning velocity for pure propane−air mixtures matches well with the predictions of Qin et al.26 and other experimental results available in the literature. This validates the present technique for measuring the burning velocities of various fuel−air mixtures. Using this method, laminar burning velocities of diluted propane−air mixtures are determined both experimentally and computationally. The laminar burning

(5)

Figure 7 also shows the comparison of the present results to the data available from various experiments4,6 and numerical computations using the Qin et al.26 mechanism. There is a large discrepancy in the literature4,6 regarding the magnitude and existence of the minimum temperature exponent. Metghalchi and Keck4 reported higher values of the temperature exponent with a very low burning velocity at 300 K. The correlation given by Takizawa et al.6 gives higher and lower magnitudes for lean and rich mixtures, respectively, as compared to those obtained from the Qin et al. mechanism.26 The values of the temperature exponent obtained from present experiments are in good agreement with numerical computations of Qin et al.26 The temperature exponent reported by Metghalchi and Keck4 and Takizawa et al.6 shows a linear variation of α with the equivalence ratio. However, such a linear variation of the temperature exponent has been discouraged in the existing literature for hydrocarbon fuels.29 The existence of 5513

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Figure 9. Laminar burning velocity of a propane−air mixture at 500 and 650 K. Figure 11. Burning velocity of a 20% CO2-diluted propane−air mixture at different temperatures.

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

to very rich and lean mixtures for the given range of dilution rates studied in the present work. Similar variations in the burning velocity were observed for mixtures with different CO2 dilutions, such as 10, 30, and 40% cases. The predictions of Qin et al.26 are in reasonably good agreement with present experiments, except for very high dilution rates, where it predicts slightly higher values. 4.6. Effect of Dilution on the Laminar Burning Velocity and Temperature Exponent. The dilution with inert gases reduces the burning velocity and increases the magnitude of the temperature exponent. Panels a and b of Figure 12 show the variation of laminar burning velocities and temperature exponents α for propane−air mixtures with different N2 dilution rates, respectively. This will be helpful to extract the burning velocity of diluted propane−air mixtures at elevated temperatures. Panels a and b of Figure 13 show the respective variation of laminar burning velocities and temperature exponents α for CO2-diluted propane−air mixtures. The Figure 10. Burning velocity of a 20% N2-diluted propane−air mixture at different temperatures.

the variation of the laminar burning velocity for the 20% N2 dilution case at 300 K and higher mixture temperatures. A comparison to the mechanism of Qin et al.26 is also shown, and no experimental comparison could be made because of the non-availability of data for C3H8/N2−air-diluted mixtures at high temperatures. The burning velocity peak is observed consistently for slightly rich mixtures (Φ = 1.1). A similar variation of the burning velocity was observed for mixtures with different N2 dilutions, such as 10, 30, and 40% cases. The predictions of the Qin et al.26 mechanism are in reasonably good agreement with present experiments, except at very high dilution rates. The predictions slightly overpredict the laminar burning velocity for the 40% dilution case, indicating that there is 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 the 20% CO2 dilution case. The comparison of the experimental data is shown for different mixture temperatures with the predictions of Qin et al.26 The maximum burning velocity is observed for slightly rich mixtures (Φ = 1.1). The temperature exponent was minimum for slightly rich mixtures, as compared

Figure 12. (a) Burning velocities and (b) temperature exponents of N2-diluted propane−air mixtures at 300 K. 5514

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This reduction in the burning velocity is small, as compared to the reduction in the burning velocity for CO2-diluted methane−air mixtures, as reported by Kishore et al.16 The reasons for the decrease in the burning velocity with inert dilution are discussed in the next section.

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

Figure 13. (a) Burning velocities and (b) temperature exponents of CO2-diluted propane−air mixtures at 300 K.

temperature exponent was observed to vary with the equivalence ratio in a way similar to that for the pure propane−air case with both N2 and CO2 dilution. A minimum value of the temperature exponent was obtained for the complete range of N2 dilution rates studied in the present experiments. The value of the temperature exponent was observed 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

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

pure propane−air mixtures at high temperatures. It can be seen that the area under the curve for a mixture temperature of 500 K is much higher than that of 300 K, which corroborates the increase in the burning velocity because of a higher mixture temperature. The peak heat release rate occurs at a lower temperature with an increase in the mixture temperature. This may be due to the fact that initiation of chain-branching reactions in the preheat zone as the initial mixture temperature is high enough to generate requisite radicals to carry out reactions within the flame front. Also, with the addition of inerts, peak heat release occurs at a higher non-dimensional temperature because of a delay in generating radicals. To explore the reason for the increase in the burning velocity, the adiabatic flame temperature and mole fractions of participating species are plotted in Figures 16−18. Figure 16 shows the profiles of the adiabatic flame temperature along the axial direction. Because of high heat release rates, the flame temperature is maximum for a pure propane−air mixture at 500 K. Figure 17 shows the variations of the mole fraction of stable species, such as CO2, N2, and H2O, along the axial coordinate. For a mixture with 300 K initial temperature, the mole fraction of CO2 is higher, as compared to the mole fraction at 500 K. The higher mole fraction of CO2 absorbs the heat energy released from the chemical reaction because of its high heat capacity and, hence, lowers the flame temperature by ∼90 K, as compared to the pure propane case. A reduction in the adiabatic flame temperature causes a reduction in the burning velocity. To further understand the reduction in the

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

mixture at 500 K. Experiments show a significant reduction in the burning velocity with dilution. The burning velocity with N2 dilution is in good agreement with the results from Zhao et al.13 However, a slightly higher rate of change in the burning velocity was observed in present experiments, as compared to computational results, shown in Figure 14. The decrease in the burning velocity is almost linear with N2 dilution rates. Figure 14 also shows a decrease in the burning velocity with CO2 dilution at 500 K. The burning velocity shows a reduction of 18% for the 40% N2 case and 38% for the CO2 dilution case. 5515

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fraction with an increase in the initial mixture temperature is observed for other radicals as well. The enhanced production of these species contributes significantly to an increase in the heat release rate and, hence, the flame temperature. As a result, there is an increase in the burning velocity with an increase in the initial mixture temperature. The addition of chemically inert dilutants was observed to reduce the burning velocity. This results in better flamequenching properties and a reduction in the reaction intensity of combustible mixtures. As shown in Figure 15, the dilution with both CO2 and N2 shifts the peak heat release rates toward a higher non-dimensional temperature, resulting in a delay in chain initiation. This requires additional heat to generate radicals, which leads to the combustion. The delay in combustion 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 N2 dilution. Figure 17 shows that the mole fraction of CO2 and H2O is more for the CO2 dilution case. These species absorb a large part of the heat released from the combustion, and less amount of heat is transferred to the preheat zone. This effect can be seen from Figure 15, where the addition of CO2 species with comparatively higher specific heat results in a lower heat release rate and a lower adiabatic flame temperature compared to N2 dilution. The mole fraction of minor species (radicals) are lower for the CO2 dilution case compared to N2 dilution, as shown 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 CO2 molecule. This species FCO2 possesses the same thermal and transport properties as that of CO2 but does not participate in the reaction/dissociation. The burning velocity is then computed from the Qin et al.26 mechanism using PREMIX25 code. The burning velocity of a pure propane−air mixture is considered as a reference. The difference of the pure propane burning velocity and propane with FCO2 dilution gives an idea about the thermal effect. The difference in the burning velocity of propane with FCO2 dilution and CO2 dilution provides the chemical effect, as seen from Figure 19. The relative contributions of thermal and chemical effects are estimated for different dilution rates at different temperatures. The relative decrease in the laminar burning velocity because of thermal effects is obtained from the following relation:

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

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

Figure 18. Variation of radical species mole fraction versus nondimensional temperature, for various mixture and initial temperature conditions.

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

Figure 19. Laminar burning velocity for pure propane and 20% FCO2 and CO2 dilution at 300 K. 5516

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Article

(Su,pure − Su,FCO2) (Su,pure − Su,CO2)

× 100% (7)

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

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

mixture equivalence ratios. In the case of dilution of fuel with 20% CO2, the decrease in the laminar burning velocity because of thermal and chemical effects is equal for most of the mixtures. However, with an increase in the dilution, thermal effects become predominant. The thermal effect is observed to be least for the slightly rich mixture at Φ = 1.1 for all dilution rates and mixture temperatures.

6. CONCLUSION In the present paper, the burning velocity of pure and diluted propane−air mixtures at high temperatures is measured using the planar flames in an externally heated divergent channel. The measured burning velocity for pure propane−air mixtures matches well with the available experimental and computational results for all of the ranges of mixture temperatures and mixture equivalence ratios reported in the literature. The burning velocity is maximum for slightly rich mixtures (even at high temperatures), for which the temperature exponent is observed to be minimum. A linear decrease in the burning velocity of diluted propane−air mixtures is observed. The decrease in the burning velocity was more pronounced for the CO2 dilution case, as compared to the N2 dilution case. The thermal effect was predominant at higher temperatures, and CO2 dilution rates reduce the chemical effect. The propane reaction mechanism of Qin et al.26 slightly overpredicts the burning velocity and slightly underpredicts the temperature exponent for highly diluted propane−air mixtures.

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Su,o = laminar burning velocity at the reference initial temperature 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 fraction α = temperature exponent Φ = equivalence ratio τ = non-dimensional temperature

AUTHOR INFORMATION

Corresponding Author

*Telephone: +91-22-2576-7124. Fax: +91-22-2572-2602. Email: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE Ainlet = inlet cross-sectional area (m2) Af = flame cross-sectional area (m2) 5517

dx.doi.org/10.1021/ef301000k | Energy Fuels 2012, 26, 5509−5518

Energy & Fuels

Article

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dx.doi.org/10.1021/ef301000k | Energy Fuels 2012, 26, 5509−5518