LAMINAR BURNING VELOCITY OF LPG-AIR MIXTURE AT ELEVATED TEMPERATURES
Mohammad Akram Indian Institute of Technology Bombay
Mumbai, Maharashtra, India
Priyank Saxena Solar Turbine Inc.
San Diego, California, USA
Sudarshan Kumar Indian Institute of Technology Bombay
Mumbai, Maharashtra, India
ABSTRACT Laminar burning velocity of liquefied petroleum gas (LPG) air mixtures at high temperatures is extracted from the planar flames stabilized in the preheated meso-scale diverging channel. The experiments were carried out for a range of equivalence Computational predictions of burning velocity and detailed flame structure were performed using PREMIX code with USC mech 2.0. The present data are in very good agreement with both experimental and computational results available. Peak burning velocity was observed for slightly rich mixtures even at higher mixture temperatures. The minimum value of temperature exponent is observed for slightly rich mixtures. INTRODUCTION Increasing concerns over the fossil fuel shortage and air pollution have intensified the study on alternative fuels around the world [1]. Green fuels, such as liquefied petroleum gas (LPG) are usually preferred as clean fuels compared to the diesel and gasoline. Therefore, the introduction of these alternative fuels is beneficial for achieving higher combustion efficiency, reduced fuel consumption, and reduced exhaust emissions. Although LPG offers many advantages over gasoline and other conventional fuels, very few LPG combustion characteristics studies have been reported [1-6]. Laminar burning velocity, for instance, is an important parameter describing many features related to the reactivity, diffusivity and exothermicity of a particular fuel-air mixture. Laminar burning velocities of combustible mixtures are very useful in the validation of various chemical kinetic mechanisms, predicting the turbulent burning velocity of these fuels. Very limited experimental data are available for the burning velocity of LPG-air mixtures. A large variation in the burning
velocity magnitude of LPG-air mixtures can be observed in the literature. For instance burning velocity of stoichiometric LPG-air mixture varies from 0.16 m/s to 0.403 m/s as can be seen in Table 1. A major discrepancy in the existence of maximum burning velocity for LPG-air mixture is also observed in available literature as summarized in Table 1. Although the differences in the composition of LPG exist in literature, their adiabatic flame temperature is same as can be seen in Table 2. These adiabatic flame temperatures are close to usual hydrocarbon mixtures indicating the fact that burning velocity exist close to the burning velocity of usual hydrocarbon mixtures. It is also clear from Table 1 that the experimental data of burning velocity is available only for a very small range of temperatures varying from 298 to 400 K. In many practical devices, such as, IC engines, industrial furnaces and gas turbine engines, the local temperature of the mixture is significantly higher than the STP conditions. This necessitates the need for accurate data of LPG-air burning velocity at elevated mixture temperatures. Table 1 – Existing literature of LPG-air mixture burning velocity Authors Φrange Trange (K) Su,o (Φ=1.0) (Su,o, max) Φ
[3] 0.95-1.9 298 0.16 (0.35) 1.35
[4] 0.6-1.4 300-400 0.403 (0.405) 1.05
[5] 0.75-1.9 290-400 0.26 (0.43) 1.35
[6] 0.8-2.0 298 0.3 (0.42) 1.30 In earlier studies, authors have proposed a new technique for the extraction of burning velocity from the planar flames
Proceedings of the ASME 2012 Gas Turbine India Conference GTINDIA2012
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stabilized inside preheated diverging meso-scale channels. The technique has been successfully validated for stoichiometric methane-air mixture [7], pure and diluted propane-air mixtures [8], and LPG-air mixtures [9]. The present work reports the experimental and computational prediction of LPG-air laminar burning velocity at elevated temperatures as that in gas turbine engines. Table 2 –Composition of LPG and adiabatic flame temperature in existing literature
EXPERIMENTAL SETUP In the present work, a rectangular 10° diverging meso-scale channel with inlet dimension of 25 mm�2 mm was chosen. The schematic of the experimental set-up with channel configuration is shown in Fig. 1. Quartz material is chosen due to its high heat capacity, low thermal conductivity, low thermal expansion, and transparency of the walls to help visualize the flame front location in the channel. A premixed LPG–air mixture at ambient conditions (300 K, 1.0 atm pressure) is supplied at the inlet through precise electric mass flow controllers connected to a personal computer through a command module. The LPG in present case composes of 60 % butane and 40 % propane by volume. A porous burner was placed below the channel at a downstream location, (20 mm below the channel) to preheat the channel walls and create a positive wall temperature gradient along the direction of fluid flow. This helps in the initial flame ignition, stabilizing the flame with different preheat temperatures, and in reducing the heat loss from flame to the solid walls.
Figure 1 Schematic diagram of experimental set-up and channel configuration
Inside surface wall temperature measurements were carried out using 0.5 mm diameter K-type thermocouple. The movement of the thermocouple was controlled through a precisely controlled traverse with 0.25 mm as minimum resolution of traverse. Measured temperatures were accurate within ± 5 K of actual value. BURNING VELOCITY CALCULATIONS The planar flames were observed to stabilize inside the high aspect ratio channels for a range of inlet velocity and equivalence ratio and external heating rates [7, 10]. These planar flames were used to extract the laminar burning velocity at various mixture equivalence ratios and mixture temperatures. The burning velocities obtained with this method were stretch free and close to adiabatic values as confirmed in author’s earlier works [9]. Once a planar flame is stabilized in the channel, the location of the flame was captured with a digital camera and accurate flame position was obtained through image processing in MATLAB. For different mixture velocities and known heating rates of the bottom burner, the wall temperature profile is known apriori. For transverse direction, at a given velocity, the mass averaged temperature at the location of flame was used. The flame propagation 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 [11].
�� � ����� � � ����� � � � � �������
Premixed flat flames stabilized in present method resemble steady one-dimensional adiabatic freely propagating flames. Therefore, burning velocity computations for comparison with experimental data were performed for freely propagating steady adiabatic flames using PREMIX code [12]. Multi-component diffusion and thermal diffusion options were taken into account. On fixing the temperature at one point in the flame-fixed co-ordinate system (keyword TFIX); PREMIX calculates the adiabatic burning velocity as an eigen value. The detailed chemistry (of LPG-air mixture with 60 % Butane and 40 % Propane by volume) USC mech 2.0 [13] was used in PREMIX. The USC Mechanism version 2.0 [13] consists of 111 species and 784 reversible reactions. The upwind difference scheme is preferred for the study, which allows refined grid adaption. On using adaptive grid parameter GRAD 0.02 and CURV 0.05, the adiabatic burning velocity obtained was found to be grid independent. The detailed flame structure obtained using PREMIX code helps in understanding the effect of mixture temperature, off-stoichiometric peaking of burning velocity and variation of temperature exponent on burning velocity of LPG-air mixtures. RESULTS AND DISCUSSIONS The variation of measured laminar burning velocity is plotted with the ratio of mixture temperature, Tu to reference temperature, Tu,o for various equivalence ratios. To describe the
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simultaneous effect of temperature and equivalence ratio, a simple power law correlation was applied to the experimental values as:
�� � ���� � � �������
�
Different burning velocity correlations have been obtained for temperature dependency, α of various mixture equivalence ratios. The results were compared with the available experimental results of LPG-air mixtures. MIXTURE TEMPERATURE DEPENDENCY The variation of laminar burning velocity for a stoichiometric LPG-air mixture (Ф = 1.0) at high temperatures (370 – 520 K) is shown in Fig. 2. The correlations given by Liao et al. [4] and Huzayyin et al. [5] were used for comparison within their experimental temperature range. The predicted burning velocity with USC mech 2.0 [13] is also plotted on the same graph. The present data matches well with that of Liao et al. [4] and USC mech 2.0 [13] results. The correlation given by Huzayyin et al. [5] provides very smaller values comparatively. No experimental data are available for comparison in the high temperature range (> 400 K) as shown in Fig. 2. The burning velocity increases with the mixture temperature with ���� = 0.40 m/s and temperature exponent α = 1.589.
Figure 2 Variation of laminar burning velocity of stoichiometric LPG-air mixture with temperature ratio The variation of the laminar burning velocity for a lean mixture (Ф = 0.8) and a rich mixture (Ф = 1.2) with temperature ratio is shown in Fig. 3. A comparison is also made with the results of Liao et al. [4] and Huzayyin et al. [5] in their given temperature range of up to 400 K only and predicted burning velocities from
USC mech 2.0 [13]. Very small values of burning velocities are reported for lean mixture at high temperature by Huzayyin et al. [5]. The present results for Ф = 0.8 are also slightly lower as compare to the results of Liao et al. [4] and USC mech 2.0 [13]. The burning velocity increases with the mixture temperature with ���� = 0.298 m/s and 0.372 m/s with temperature exponent, α = 1.786 and 1.678 for Ф = 0.8 and Ф = 1.2 respectively. Similar experiments were conducted for a range of mixture equivalence ratios, 0.7 ≤ Ф ≤ 1.3. Laminar burning velocities at reference temperature, Su,o and temperature exponent, α are obtained for the given range of mixture equivalence ratios.
Figure 3 Variation of laminar burning velocity of lean and rich LPG-air mixtures with temperature ratio TEMPERATURE EXPONENT The power of temperature ratio in power law correlations is termed as temperature exponent, α and is observed to be a function of equivalence ratio at ambient pressure. Figure 4 shows the variation of temperature exponent, α with mixture equivalence ratios and its comparison with the results of Liao et al. [4] and Huzayyin et al. [5]. A continuous non-linear decrease in the value of temperature exponent is reported by Huzayyin et al. [5]. However, in many recent experiments, researchers have observed a minimum value of temperature exponent, α for near stoichiometric mixtures with a rise for both lean and rich mixtures of various hydrocarbon fuels [8, 14]. A linear correlation for temperature exponent has been discouraged by researchers [14]. Liao et al. [4] give a minimum value of temperature exponent for near stoichiometric mixture and the reported values are slightly higher than present results. The present experimental investigation shows the existence of minimum value of temperature exponent, α near Ф = 1.1.
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Figure 4 Variation of temperature exponent for LPG-air mixture with equivalence ratios To understand this peculiar behavior of variation of temperature exponent with equivalence ratios, adiabatic flame temperatures and mole fractions were obtained for two different mixture temperatures, 300 K and 600 K, over a range of equivalence ratios.
Figure 5 Computed mole fractions of participating species for LPG-air mixtures at two different temperatures As the mixture temperature is increased, a higher concentration of CO is observed for nearly stoichiometric and slightly rich mixtures as shown in Fig. 5. This is because of increased dissociation of CO2 to CO due to comparatively higher
adiabatic flame temperatures for 0.85 < Ф < 1.15. This is true for other stable species which dissociate at higher temperatures for 0.85 < Ф < 1.15 as shown in Fig. 5. This results in relatively smaller increase in adiabatic flame temperature and hence burning velocity of mixtures for mixture equivalence range of 0.85 < Ф < 1.15 increases at a smaller rate. However, the absolute values of adiabatic flame temperature and burning velocity remain highest for slightly rich mixtures even at very high temperature compared to lean and highly rich mixtures. This confirms the existence of minimum value of temperature exponent for slightly rich mixtures. This behavior has been captured well as shown by the curve corresponding to temperature ratio (ratio of adiabatic flame temperature at 600 K and adiabatic flame temperature at 300 K) in Fig. 5. MIXTURE DEPENDENCY The comparison of laminar burning velocity of LPG-air mixture at ambient conditions is important due to large scatter of their data available in existing literature as can be noted from Table 1. For instance, the burning velocity of stoichiometric LPG-air mixture varies between 0.16 m/s to 0.403 m/s in the existing literature. The existence of maximum burning velocity for LPG-air mixtures is reported for different equivalence ratios varying from Ф = 1.05 to 1.50 as shown in Table 1.
Figure 6 Comparison of laminar burning velocity of LPG-air mixtures at ambient pressure and 300 K. The correlations obtained for high temperature burning velocities are extrapolated to the ambient temperature. The variation of laminar burning velocities of LPG-air mixtures at 300 K temperature is shown in Fig. 6. The burning velocity for stoichiometric LPG-air mixture is observed to be 0.40 m/s. The burning velocity for most of the hydrocarbon stoichiometric
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mixtures is of the same order. The burning velocity first increases with equivalence ratio up to Ф = 1.1 and then decreases with equivalence ratio. The values obtained for LPG-air mixtures are in good agreement with the experimental data of Liao et al. [4] and predictions of USC mech 2.0 [13] for LPG-air mixtures with a peak at Ф = 1.05. The fact that burning velocity peaking for near stoichiometric mixture is quite reasonable, since for the off-stoichiometric mixture, the excess reactants (O2 for lean and CO, and H2 for rich mixtures) acts as a coolant and lowers the heat release rate. Also due to least amount of product dissociation, the peak heat release rate and peak adiabatic flame temperature has been observed for slightly rich mixtures. This pertains the peaking of burning velocity for very slightly rich mixtures. Laminar burning velocity at high temperatures The burning velocity variation with equivalence ratios should provide a highest value for near stoichiometric mixtures even at high temperatures. Using the power law correlations from present experiments, the variation of burning velocity for various equivalence ratios at a particular temperature can been obtained.
Figure 7 Laminar burning velocity variations with equivalence ratio at high initial temperatures Figure 7 shows the comparison between the present results and those obtained from USC mech 2.0 [13] at different mixture temperatures. The present results are in good agreement with the predicted burning velocities of LPG-air mixtures even at very high temperatures with slightly lower and higher values for lean and rich mixtures respectively. The measured burning velocity of the LPG-air mixtures is consistently highest for slightly rich mixtures.
CONCLUSIONS Experimental measurements of laminar burning velocity for LPG-air mixtures have been reported in this paper for a range of mixture equivalence ratios and mixture temperatures. Computational studies confirm to a substantial reduction in heat transfer from flame to the solid walls giving the burning velocity values near to the adiabatic values. Thermal feedback and flame structure effects were observed to be negligible. Uncertainty analysis considering the various parameters influencing the burning velocity in present set-up, gives uncertainty of ± 5 %. Although LPG is a multi-component fuel, the variation of the burning velocity with mixture equivalence ratio is similar to that of single component fuels (butane and propane). Preheating of the mixture increases its burning velocity. Burning velocity variation with equivalence ratios gives a bell shaped curve with peak for slightly rich mixture Ф = 1.1. Temperature exponent varies with equivalence ratios as inverted parabola with minimum value exist for slightly rich mixture Ф = 1.1. NOMENCLATURE
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Ainlet = Inlet cross-sectional area (m2) Af = Flame cross-sectional area (m2) Su,o = Laminar burning velocity at 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) X = Species mole fraction α = Temperature exponent Φ = Equivalence ratio τ = Non-dimensional temperature
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