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Air mixtures at elevated temperatures
Energy 176 (2019) 410e417
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Laminar burning velocity of n-butane/Hydrogen/Air mixtures at elevated temperatures E.V. Jithin a, Kadali Dinesh a, Akram Mohammad b, Ratna Kishore Velamati a, * a b
Department of Mechanical Engineering, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India Department of Aeronautical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2018 Received in revised form 9 March 2019 Accepted 1 April 2019 Available online 4 April 2019
The effect of hydrogen (H2) addition in the laminar burning velocity (LBV) of n-butane-air at elevated temperatures is described in this paper. For various equivalence ratios (f), ranging from 0.7 to 1.3, LBV was measured for 20%, 40% and 60% H2 addition to n-butane using a preheated mesoscale diverging channel technique. Using this experimental technique, LBV measurements were conducted for unburnt mixture temperature up to 450 K. The maximum burning velocity has been obtained at equivalence ratio 1.1 for all the mixture conditions. The LBV results at atmospheric condition for n-butane-hydrogen-air mixture were obtained by extrapolating the experimental data at elevated temperatures. “Heat flux method” experimental setup was used for measuring the LBV of n-butane-hydrogen-air mixture at atmospheric condition. The results obtained for LBV at atmospheric conditions with the two different methods at 0%, 20%, 40% and 60% H2 composition in n-butane were found to be in good agreement. The experimental results of LBV for n-butane were compared with the numerical predictions using USC mech II, Aramco mech 2.0 and LLNL reaction mechanisms. The numerical predictions of LBV using Aramco mech 2.0 shows good agreement with experimental data at rich, lean and stoichiometric mixture conditions. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Butane Diverging channels Heat flux method Hydrogen addition Laminar burning velocity
1. Introduction The gradual depletion of fossil fuel reserves and increasing concern for environment protection has led to a hike in demand for a shift from conventional fuels to cleaner fuels, such as hydrogen. The introduction of hydrogen (H2) to hydrocarbons (HC) seems to be a feasible solution. Hydrogen is having high energy content and wide range of flammability limit. However, the safety and storage issues have been restricting the use of pure H2. To overcome these difficulties, blending H2 with hydrocarbons is an alternative solution. The properties related to the safety of combustors and burners, such as flame blow off, flashback and ignition delay time might change when H2 is blended with hydrocarbons. The flash back phenomenon, which is related to the burner flame stability has a direct dependence on the fuel mixture property, the laminar burning velocity. Laminar burning velocity (LBV) is a fundamental combustion
* Corresponding author. E-mail addresses: (R.K. Velamati).
[email protected],
https://doi.org/10.1016/j.energy.2019.04.002 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
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property, which determines the effect of hydrogen addition in hydrocarbons. As per definition, LBV is the relative velocity with which unburnt gas moves into the flame front. The determination of LBV for various mixtures is important from a design viewpoint of combustible devices. The LBV data is essential for validating detailed reaction mechanism and for developing reduced reaction mechanisms [1e4]. For the measurement of LBV, there are several well established experimental methods such as spherical flame method, stagnation flame method, heat flux method, annular stepwise diverging tube, externally heated diverging channel method and burner method [5]. Experimental studies to understand the effect of H2 addition on LBV of hydrocarbons were performed by several researchers using different experimental techniques [6-15]. Tang et al. [33] conducted experimental and numerical study to calculate LBV of n-butane-H2air mixtures. The spherical flame experimental setup was used to calculate the LBV for various f. Detailed numerical computations were performed to investigate the cause of variation in LBV of hydrocarbons with H2 addition. Their numerical analysis revealed that global activation energy and thermal effects have the major influence on increase in LBV due to hydrogen addition. Aravind
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et al. [16] numerically investigated the effect of H2 addition on the LBV of LPG-air mixtures. This study concluded that there is a linear increase in LBV with H2 addition. A correlation for the effect of temperature and pressure on LBV was proposed in this numerical study, which is valid at elevated temperature and pressure. A detailed review on the experimental data available for the burning velocity of HC-H2 mixtures show that most of the experimental studies were reported for LBV of methane-hydrogen mixtures. The experimental data available for LBV of other HC-H2 mixtures are limited in number. Also, majority of the available experimental LBV data is for HC-H2 mixtures at atmospheric condition. In the recent history, the combustion of hydrocarbon mixtures with high initial temperature attained more importance, because of its increased thermal efficiency and reduced pollution. Moreover, the temperature of the mixture in majority of combustion devices will be raised to a higher value before the beginning of the combustion itself. Therefore, the knowledge of LBV of combustible mixtures at higher temperature is essential for design and development of better combustion devices. The present investigation aims to present LBV of n-butane-H2-air mixtures at elevated temperatures. For the present study, n-butane is chosen as target fuel, since the major constitute of commercially available LPG is n- butane. It will be useful to study the effect of H2 on the LBV of n-butane-air mixtures for the research towards the replacement of LPG gas with LPG - H2 mixtures in the near future. Accurately measured LBV has vital role in the design, testing and performance estimation of the LPG-H2 fuelled devices. In present study, the LBV of n-butane-H2-air mixtures at elevated temperatures is measured using “diverging channel setup”. The LBV at atmospheric temperature is calculated by extrapolating the data at elevated temperatures to 300 K. “Heat-flux” experimental setup was developed to calculate the LBV of n-butane-H2-air mixture at atmospheric condition in the present study. The LBV at atmospheric condition obtained from both the experimental setup has been compared and shown in results section. Numerical computations were performed to calculate the LBV and to understand the flame structure of nbutane-H2-air mixtures. The details of experimental setup and
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numerical computations are given in the next section. 2. Experimental and computational details 2.1. Experimental setup for diverging channel Diverging channel method is a technique used for measuring the LBV of a combustible mixture at elevated temperatures [17e22]. The experimental setup consists of a preheated high aspect ratio (12.5) mesoscale diverging channel with rectangular cross-section (25 mm 2 mm) through which a planar flame propagates. Quartz channel is preferred since its thermal conductivity is low, heat capacity is high and it is transparent. High aspect ratio is maintained to get a uniform flow field, which is essential for the stabilization of planar flames inside the channel [23]. The LBV is measured using this planar flame. The channel is preheated externally, using a ceramic heater to provide a positive temperature gradient in the flow direction, and also to stabilize nearly adiabatic planar flame inside the channel [24]. The channel was placed 2 cm above the top surface of the heater with an overlap of 2.5 cm. The premixed air-fuel mixture at ambient condition is given through the channel inlet, using ALICAT digital mass flow controllers (MFC's). The planar flame stabilizes at location inside the channel where the unburnt mixture velocity matches with the LBV. A Ktype thermocouple is used for the measurement of the wall temperature at the location of flame stabilization. Different MFC's were used for the precise control of fuel and air velocity according to the required mixture flow rate corresponding to various f values. The schematic diagram of the present experimental setup is shown in Fig. 1. The LBV is obtained by applying the conservation of mass principle. The channel is provided with a divergence of 10 , which creates a negative gradient of velocity along the flow direction, which resists the flame flashback [25]. After heating the channel uniformly, the required quantity of the mixture was supplied through the channel. At the channel exit, the mixture was ignited and the flame gently moves into the channel and gets stabilized at the location where the unburnt mixture
Fig. 1. Schematic diagram of the diverging channel experimental setup.
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mixture. Both the channel wall and the unburnt mixture are assumed to attain equal temperature. The temperature of the channel along the axial and transverse direction is measured using a K- type thermocouple. The conservation of mass principle is applied for calculating the LBV, as follows.
Su ¼ Uinlet
Ainlet Af
!
Tu Tu;o
(1)
LBV measured is under the influence of various parameters such as thermal feedback, boundary layer, the heat losses from flame to channel walls and the temperature difference between the reactants and wall. The inlet mass flow rates in MFC's is around 80% of the full scale. This is to reduce the uncertainties in the equivalence ratios. The temperature readings were accurate to ±5 K of the actual values. Some of the factors influencing the burning velocity are observed to be compensating each other. The effect of all these factors has been studied in overall uncertainty calculation. The uncertainty in the measured laminar burning velocity due to all these factors is expected to be around ±5% of actual value (The uncertainty calculation has been given in supplementary material). 2.2. Experimental setup for the heat-flux method Fig. 2. Direct image of planar flame stabilized in the diverging channel (10 ).
velocity becomes equal to the LBV of the mixture. The stabilized flame images were captured by a DSLR camera and the flame positions were noted. The experiment is performed at different heating rates until planar flames are obtained. Following the same procedure, experiments were conducted at different equivalence ratios and inlet velocity conditions. Akram et al. [18] reported that externally preheated meso-scale diverging channel stabilizes planar flame inside the channel. In their experimental and numerical study, it was observed that the hydrodynamic stretch has negligible influence on measured LBV, compared to various other methods employed for the measurement. Fig. 2 shows the direct image of planar flame stabilized in the channel. These stabilized flames are used for the measurement of LBV using the present experimental setup. For a particular equivalence ratio, the flame stabilizes at different locations corresponding to the inlet velocity and the temperature of the unburnt
Heat flux method is a direct method for LBV measurment introduced by de Goey et al. [26]. This measurement technique uses a nearly flat flame anchored over the burner top. The flames observed in this method are planar and stationary. The flow of unburnt gases is normal to the flame front. The schematic diagram of the setup is shown in Fig. 3. The heat-flux burner experimental setup consists of a plenum chamber, a burner head and a burner plate. The plenum chamber consisting of a distributer plate at the bottom is used to attain a uniform flow at the exit of the burner. The experimental setup was fabricated based on the details given by Bosschaart et al. [27]. A perforated burner plate of 2 mm thickness and 30 mm diameter, made of brass is positioned on top of the burner head. The plate used for the present study is having holes of 0.5 mm diameter and 0.7 mm pitch. Fine wire thermocouples are fixed into the blind holes provided at five different locations on the plate, using a thermally conductive glue to measure the burner plate temperature. The temperature at the outside perimeter and the heat fluxes at the bottom and top of the burner plate surfaces were assumed
Fig. 3. Schematic diagram of heat flux setup.
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Fig. 4. Comparison of the present experimental results for laminar burning velocity of n-butaneair mixture at ambient conditions with the published experimental data and numerical results.
constant. In heat flux method, the laminar burning velocity is determined from the knowledge of parabolic coefficient (a2) as a function of unburned gas velocity. The negative value of the parabolic coefficient corresponds to sub-adiabatic flames, whereas its positive value indicates super adiabatic condition. At a2 ¼ 0, the net amount of heat transferred from the burner plate to the unburnt mixture becomes zero, which corresponds to the adiabatic condition. The unburned gas velocity at this condition yields the laminar burning velocity of the mixture, which can be obtained by the linear interpolation. 2.3. Computational details The LBV and the flame structure of n-butane-H2-air mixtures have been studied numerically. One dimensional numerical computations were performed for predicting the LBV and the thermodynamic properties of n-butane- H2 mixture, using the PREMIX code [28]. The essential thermodynamic and transport properties were obtained from Sandia National Laboratories. Simulations were done with adaptive mesh parameters GRAD ¼ 0.02 and CURV ¼ 0.1. All the simulations were having more than 500 grid points. The mesh parameters were decided after confirming that the present parameters are sufficient for getting grid independent results. 3. Results and discussion The present experimental study aims to analyse the effect of hydrogen addition on LBV of n-butane-air mixtures using two different experimental methods such as diverging channel method and heat-flux method. Experiments were conducted for calculating the LBV of pure n-butane-air with 0%, 20%, 40% and 60% H2 addition. For each composition, burning velocities were calculated for equivalence ratios varying from 0.7 to 1.3. The diverging channel method helps to calculate the LBV over a range of unburnt mixture temperatures, 350e450 K. The measured LBV at various temperatures were used to develop following power law correlation
Su ¼ Su;o
Tu a Tu;o
(2)
In present study, experimental setup for the heat-flux method
Fig. 5. LBV of n-butane-H2-air mixtures at elevated temperatures for 0% H2 and 40% H2 for (a) f ¼ 0.9 and (b) f ¼ 1.
was developed for the measurement of LBV at atmospheric conditions, for n-butane-H2 mixtures. The LBV results obtained at atmospheric conditions from both the experimental setups were compared, for 0%, 20%, 40% and 60% H2 addition in n-butane-air. Detailed numerical computations were also performed for analysing the effect of H2 on the burning velocity of n-butane-air mixture. The results obtained from both the experimental setups were compared with the numerical predictions using PREMIX code. 3.1. Validation of the experimental setups The LBV data for n-butane-air mixture at atmospheric conditions form various experimental studies are available in literature. Therefore, to validate the present experimental setups, experiments were conducted for the measurement of LBV of n-butane-air mixtures at atmospheric condition for various f, using both diverging channel method and heat-flux method. These experimental results were compared with the available results of Bosschaart et al. [29], Kelley et al. [30], Wu et al. [31], Park et al. [32],
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Fig. 6. The temperature exponent (a) obtained for 0%, 20%, 40% and 60% H2 addition on n-butane-air mixture.
Tang et al. [33], and Dirrenberger et al. [34], as well as the numerical predictions using USC mech II [35] Aramco mech 2.0 [36e42] and LLNL mech [43] reaction mechanisms as shown in Fig. 4. The graph shows that the LBV results obtained for n-butane-air mixtures from both the experimental results agree reasonably well with the existing results in the literature. The numerical predictions show that LLNL mechanism over-predicts the laminar burning velocity for all mixture conditions. The LBV predictions using USC mech II agrees well with the experimental data at rich mixture conditions, but the mechanism over-predicts the laminar burning velocity at lean mixture conditions. The numerical predictions of LBV using Aramco mech 2.0 shows good agreement with experimental data at rich, lean and stoichiometric mixture conditions. Therefore, all the numerical results presented in the present study was based on the computations performed using Aramco mech 2.0.
3.2. Measurement of LBV at elevated temperatures The diverging channel experimental setup was used to study the effect of temperature on the LBV. Experiments were performed at various equivalence ratios to get the LBV at elevated temperatures of the unburnt mixtures. The mixture inlet temperature (Tu ¼ 300 K) is taken as the reference temperature. The power-law is fitted to the experimental results as per equation (2) which can be used for the determination of LBV at different temperatures within the specified range. The LBV obtained at various temperatures for f ¼ 0.9 and f ¼ 1 of n-butane with 0% and 40% H2, are shown in Fig. 5 (a) and (b) respectively. The symbols represent the experimental results. The dotted lines are used to represent the numerical predictions for these conditions. Solid lines show the power-law fit for the experimental data. The increase in LBV with the rise in unburnt gas temperature can be identified from Fig. 5. The figure also indicates that there is an increase in LBV for nbutane with 40% H2 in comparison with that of 0% H2. In Fig. 6, the temperature exponent (a) obtained for 0%, 20%, 40% and 60% H2 addition on n-butane-air mixture is shown for equivalence ratio varying from 0.7 to 1.3. The temperature exponent has minimum value at equivalence ratio 1.1, and show increasing trend in both rich and lean conditions. Also, the data obtained for various compositions of H2 show that the temperature exponent decreases with increase in the percentage of H2 in the mixture.
Fig. 7. The laminar burning velocities of n-butane-H2-air mixtures at ambient condition, (a) for 0%, 20%, and (b) for 40% and 60% H2 compositions obtained from two different experimental setups. Markers with: Black-Diverging channel method; RedHeat flux method. Lines represent the results obtained for 0%, 20%, 40% and 60% H2 using Aramco Mech 2.0. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.3. Laminar burning velocity at atmospheric condition In the present study, the LBV of n-butane-H2-air mixtures at atmospheric conditions were calculated from the two different experimental setups, such as heat-flux method and the diverging channel method. Using diverging channel method, the LBV at atmospheric condition for 0%, 20%, 40% and 60% hydrogen addition were calculated by extrapolating the data obtained at elevated temperature as explained in the previous section. The heat-flux method used in present study is capable of measuring LBV at atmospheric conditions. The experimental details for the measurement of LBV using this method were given in section 2.2. Following this procedure, the LBV of n-butane-air mixtures with H2 addition (0%, 20%, 40% and 60% H2) were calculated at atmospheric condition for equivalence ratios ranging from 0.7 to 1.3. These results were compared with the results obtained using the diverging channel setup, as shown in Fig. 7(a) and (b). The graph indicates that the experimental data of LBV at atmospheric
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condition from both the experiments for all the conditions are in good agreement. The peak value of LBV is obtained at f ¼ 1.1. The LBV of n-butane-H2-air mixture at atmospheric condition increases with increase in H2 addition. 3.4. Effect of hydrogen addition
Fig. 8. The laminar burning velocities of n-butane-H2 mixtures at 420 K, for 0%, 20%. 40% and 60% H2 compositions.
The laminar burning velocities calculated at 420 K using the correlation at various equivalence ratios, for 20%, 40% and 60% hydrogen addition on n-butane are shown in Fig. 8. The peak value of LBV is obtained at equivalence ratio 1.1. A significant increase in LBV can be observed when hydrogen percentage in the mixture increased from 0% to 60%. Tang et al. [33] observed that the thermal, kinetic and diffusion factors are playing a major role in the increase of laminar burning velocity with the addition of hydrogen. In Fig. 9 (a), the variation in LBV with H2 composition in C4H10-air mixture is shown at f ¼ 0.8, 1 and 1.2. The graph shows that the variation in LBV follows almost linearly increasing trend till 40%. The variation in temperature exponent with H2 composition at f ¼ 0.8, 1 and 1.2 is shown in Fig. 9 (b). The graph shows that the temperature exponent follows a decreasing trend with increase in H2 content in the mixture, for all the equivalence ratio conditions. 3.5. Chemical kinetic analysis The flame structure of n-C4H10-H2 eair mixtures have been studied by performing numerical computations using one dimensional code (PREMIX). The simulations were performed using Aramco mech 2.0 reaction mechanism to understand effect of hydrogen addition. Fig. 10 show the mole fraction profiles of H, OH and CO radicals at the flame region, plotted against the nondimensional temperature, at 0%, 40% and 60% H2 addition in nC4H10. Computations were performed for stoichiometric mixture inlet with 300 K initial temperature. It is observed from the graph that the mole fractions of both H and OH radicals increases as the H2 content in the mixture is increased from 0% to 60%. The increased production of these radicals causes an increase in heat release rate, which results in an increase in LBV. Fig. 11 (a) shows the mole fractions of C4H10 and H2 at 0% and 60% H2 compositions in the fuel mixture. From the figure it can be
Fig. 9. (a) LBV at atmospheric condition (Su,o) and (b) Temperature exponent, of nbutane-H2 mixtures at 0%, 20% and 40% H2.
Fig. 10. The mole fractions of H, OH and CO (red) radicals plotted against nondimensional temperature, for n-C4H10-H2eair mixtures at 0% H2 (black), 40% H2 (blue) and 60% H2 (red), at initial temperature of 300 K for f ¼ 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 12. (a) Net heat production rate, (b) net reaction rates of R5 and R36 at f ¼ 1, 300 K, for 0% H2 and 60% H2 mixture conditions. Fig. 11. Mole fraction profiles of (a) C4H10 and H2, (b) CH3, at f ¼ 1, 300 K, for 0% H2 and 60% H2 mixture conditions.
identified that H2 is produced in the reaction zone even for pure C4H10 (without H2 addition), as a result of chemical reactions at the reaction zone. Methyl radicals (CH3) radicals are important in hydrocarbon combustion since their oxidation produces CO. In Fig. 11 (b), CH3 radicals are plotted for 0% and 60% H2 compositions. The graph shows that peak value of CH3 radical mole fraction is higher for 60% H2 added fuel mixture. In Fig. 12 (a), the net heat production rates for 0% and 60% H2 fuel compositions are shown. The presence of hydrogen in the mixture causes an increased heat production at the reaction zone. Fig. 12 (b) shows the net reaction rates of two important reactions, R5 (H þ O2 ¼ O þ OH) and R36 (CO þ OH ¼ CO2þH). Hydrogen addition causes an increase in the net reaction rates of both R5 and R36 as shown in the graph. 4. Conclusions The laminar burning velocity of n-butane-H2 eair mixtures were measured using two different experimental setups, such as
diverging channel method and heat flux method. The diverging channel setup was used for the measurement of LBV at elevated temperatures. The present experimental setup could measure the LBV of n-butane-H2-air mixture up to an unburnt gas temperature around 450 K. The LBV at atmospheric temperature is obtained by extrapolating the data at elevated temperature to 300 K, using power-law fit. To verify the extrapolation result, heat flux method was used to measure the LBV at atmospheric condition directly, without any extrapolation. The LBV results obtained at atmospheric condition from both the experiments were compared. It was observed that the LBV at atmospheric temperature, calculated using extrapolation (diverging channel method) and that obtained from direct measurement (heat flux method) were in good agreement. To understand the effect of hydrogen enrichment in LBV, experiments were conducted for 0%, 20%, 40% and 60% H2 addition in n-C4H10-air mixture, for a range of equivalence ratio from 0.7 to 1.3. The results showed that the increase in LBV with H2 enrichment follows almost linear trend till 40%. Numerical computations were performed to study the flame structure. This study revealed that there is an increase in H and OH mole fractions due to H2 enrichment in the fuel mixture, which causes an increase in LBV. The net
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heat production rate was found to increase with increase in H2 percentage in the mixture. Acknowledgement The authors would like to thank Department of Science and Technology, Government of India for their support through grand number YSS_2015_000007 under young scientists scheme. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.04.002. Nomenclature
rinlet rf Ainlet Af Uinlet Su Tu Tu;0
a
Su,0
Density of the mixture at the channel inlet Density of the mixture at flame location Cross-sectional area of the channel inlet Flame area Mixture velocity at channel inlet Laminar burning velocity at temperature at Tu Temperature of the unburned mixture, K Initial temperature of the unburned mixture, K Temperature exponent LBV of the mixture at atmospheric conditions
References [1] Westbrook CK, Dryer FL. Simplified reaction mechanisms for the oxidation of hydrocarbon fuels in flames. Combust Sci Technol 1981;27(1e2):31e43. [2] Law CK, Sung CJ, Wang H, Lu TF. Development of comprehensive detailed and reduced reaction mechanisms for combustion modeling. AIAA J 2003;41(9): 1629e46. [3] Davis SG, Joshi AV, Wang H, Egolfopoulos F. An optimized kinetic model of H 2/CO combustion. Proc Combust Inst 2005;30(1):1283e92. [4] Wang H, Sheen DA. Combustion kinetic model uncertainty quantification, propagation and minimization. Prog Energy Combust Sci 2015;47:1e31. [5] Konnov AA, Mohammad A, Kishore VR, Kim NI, Prathap C, Kumar S. A comprehensive review of measurements and data analysis of laminar burning velocities for various fuelþ air mixtures. Prog Energy Combust Sci 2018;68:197e267. [6] Milton BE, Keck JC. Laminar burning velocities in stoichiometric hydrogen and hydrogen-hydrocarbon gas mixtures. Combust Flame 1984;58(1):13e22. [7] Yu G, Law CK, Wu CK. Laminar flame speeds of hydrocarbonþ air mixtures with hydrogen addition. Combust Flame 1986;63(3):339e47. €kalp I. Characterization of the [8] Halter F, Chauveau C, Djebaili-Chaumeix N, Go effects of pressure and hydrogen concentration on laminar burning velocities of methaneehydrogeneair mixtures. Proc Combust Inst 2005;30(1):201e8. [9] Ilbas M, Crayford AP, Yılmaz I, Bowen PJ, Syred N. Laminar-burning velocities of hydrogeneair and hydrogenemethaneeair mixtures: an experimental study. Int J Hydrogen Energy 2006;31(12):1768e79. [10] Huang Z, Zhang Y, Zeng K, Liu B, Wang Q, Jiang D. Measurements of laminar burning velocities for natural gasehydrogeneair mixtures. Combust Flame 2006;146(1e2):302e11. [11] Coppens FHV, De Ruyck J, Konnov AA. Effects of hydrogen enrichment on adiabatic burning velocity and NO formation in methaneþ air flames. Exp Therm Fluid Sci 2007;31(5):437e44. [12] Tang C, Huang Z, Jin C, He J, Wang J, Wang X, Miao H. Laminar burning velocities and combustion characteristics of propaneehydrogeneair premixed flames. Int J Hydrogen Energy 2008;33(18):4906e14. [13] Hui X, Zhang C, Xia M, Sung C-J. Effects of hydrogen addition on combustion characteristics of n-decane/air mixtures. Combust Flame 2014;161(9): 2252e62. [14] Mandilas C, Ormsby MP, Sheppard CGW, Woolley R. Effects of hydrogen addition on laminar and turbulent premixed methane and iso-octaneeair flames. Proc Combust Inst 2007;31(1):1443e50. [15] Wu F, Kelley AP, Tang C, Zhu D, Law CK. Measurement and correlation of laminar flame speeds of CO and C2 hydrocarbons with hydrogen addition at atmospheric and elevated pressures. Int J Hydrogen Energy 2011;36(20): 13171e80. [16] Aravind B, Kishore VR, Mohammad A. Combustion characteristics of the effect of hydrogen addition on LPG-air mixtures. Int J Hydrogen Energy
417
2015;40(16605):e16617. [17] Akram M, Kumar S. Measurement of laminar burning velocity of liquified petrolium gas air mixtures at elevated temperatures. Energy Fuels 2012;26(6):3267e74. [18] Akram M, Saxena P, Kumar S. Laminar Burning velocity of methaneeair mixtures at elevated temperatures. Energy Fuels 2013;27(6):3460e6. [19] Paidi SK, Bhavaraju A, Akram M, Kumar S. Effect of N 2/CO 2 dilution on laminar burning velocity of H 2eair mixtures at high temperatures. Int J Hydrogen Energy 2013;38(31):13812e21. [20] Nair A, Velamati RK, Kumar S. Effect OF CO2/N2 dilution on laminar burning velocity of liquid petroleum gas-air mixtures at elevated temperatures. Energy 2016;100:145e53. [21] Varghese RJ, Kolekar H, Hariharan V, Kumar S. Effect of CO content on laminar burning velocities of syngas-air premixed flames at elevated temperatures. Fuel 2018;214:144e53. [22] Akram M, Kishore VR, Kumar S. Laminar burning velocity of propane/CO2/ N2eair mixtures at elevated temperatures. Energy Fuels 2012;26(9):5509e18. [23] Akram M, Kumar S. Experimental studies on dynamics of methane-air premixed flame in meso-scale diverging channels. Combust Flame 2011;158(5): 915e24. [24] Akram M, Kumar S, Saxena P. Experimental and computational determination of laminar burning velocity of liquefied petroleum gas-air mixtures at elevated temperatures. J Eng Gas Turbines Power 2013;135(9). [25] Kumar S, Maruta K, Minaev S. Experimental investigations on the combustion behavior of methane-air mixtures in a new micro scale radial combustor configuration. J Micromechan Microeng. 2007;17. 900-900. [26] De Goey LPH, Van Maaren A, Quax RM. Stabilization of adiabatic premixed laminar flames on a flat flame burner. Combust Sci Technol 1993;92(1e3): 201e7. [27] Bosschaart KJ, De Goey LPH. Detailed analysis of the heat flux method for measuring burning velocities. Combust Flame 2003;132(1):170e80. [28] Kee RJ, Grcar JF, Miller JA, Meeks E, Smooke M. PREMIX users manual. San Diego, CA: Reaction Design; 1998. [29] Bosschaart KJ, De Goey LPH. The laminar burning velocity of flames propagating in mixtures of hydrocarbons and air measured with the heat flux method. Combust Flame 2004;136(3):261e9. [30] Kelley AP, Law CK. Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames. Combust Flame 2009;156(9):1844e51. [31] Wu H, Hu E, Yu H, Li Q, Zhang Z, Chen Y, Huang Z. Experimental and numerical study on the laminar flame speed of n-butane/dimethyl ethereair mixtures. Energy Fuels 2014;28(5):3412e9. [32] Yoon SH, Park J, Kwon OB, Yoon JH, Keel SI, Kim JS, Bae DS. Experimental study on self-excitations in nitrogen-diluted laminar lifted butane flames. Fuel 2012;91(1):51e60. [33] Tang CL, Huang ZH, Law CK. Determination, correlation, and mechanistic interpretation of effects of hydrogen addition on laminar flame speeds of hydrocarboneair mixtures. Proc Combust Inst 2011;33(1):921e8. [34] Dirrenberger P, Le Gall H, Bounaceur R, Herbinet O, Glaude P-A, Konnov A, Battin-Leclerc F. Measurements of laminar flame velocity for components of natural gas. Energy Fuels 2011;25(9):3875e84. [35] Wang H, You X, Joshi AV, Davis SG, Laskin A, Egolfopoulos F, Law CK, Version Ii USCM. High-temperature combustion reaction model of H2. 2007. CO/C1-C4 Compounds, http://ignis.usc.edu/USC_Mech_II.htm. [36] Li Y, Zhou C-W, Somers KP, Zhang K, Curran HJ. The oxidation of 2-butene: a high pressure ignition delay, kinetic modeling study and reactivity comparison with isobutene and 1-butene. Proc Combust Inst 2017;36(1):403e11. [37] Zhou C-W, Li Y, O'Connor E, Somers KP, Thion S, Keesee C, Mathieu O, Petersen EL, DeVerter TA, Oehlschlaeger MA. A comprehensive experimental and modeling study of isobutene oxidation. Combust Flame 2016;167: 353e79. [38] Burke U, Metcalfe WK, Burke SM, Heufer KA, Dagaut P, Curran HJ. A detailed chemical kinetic modeling, ignition delay time and jet-stirred reactor study of methanol oxidation. Combust Flame 2016;165:125e36. [39] Burke SM, Burke U, Mc Donagh R, Mathieu O, Osorio I, Keesee C, Morones A, Petersen EL, Wang W, DeVerter TA. An experimental and modeling study of propene oxidation. Part 2: ignition delay time and flame speed measurements. Combust Flame 2015;162(2):296e314. [40] Burke SM, Metcalfe W, Herbinet O, Battin-Leclerc F, Haas FM, Santner J, Dryer FL, Curran HJ. An experimental and modeling study of propene oxidation. Part 1: speciation measurements in jet-stirred and flow reactors. Combust Flame 2014;161(11):2765e84. [41] Metcalfe WK, Burke SM, Ahmed SS, Curran HJ. A hierarchical and comparative kinetic modeling study of C1 C2 hydrocarbon and oxygenated fuels. Int J Chem Kinet 2013;45(10):638e75. romne s A, Metcalfe WK, Heufer KA, Donohoe N, Das AK, Sung C-J, Herzler J, [42] Ke Naumann C, Griebel P, Mathieu O. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combust Flame 2013;160(6):995e1011. [43] Marinov NM, Pitz WJ, Westbrook CK, Vincitore AM, Castaldi MJ, Senkan SM, Melius CF. Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame. Combust Flame 1998;114(1e2):192e213.
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Laminar burning velocity of n-butane/Hydrogen/Air mixtures at elevated temperatures E.V. Jithin a, Kadali Dinesh a, Akram Mohammad b, Ratna Kishore Velamati a, * a b
Department of Mechanical Engineering, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India Department of Aeronautical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2018 Received in revised form 9 March 2019 Accepted 1 April 2019 Available online 4 April 2019
The effect of hydrogen (H2) addition in the laminar burning velocity (LBV) of n-butane-air at elevated temperatures is described in this paper. For various equivalence ratios (f), ranging from 0.7 to 1.3, LBV was measured for 20%, 40% and 60% H2 addition to n-butane using a preheated mesoscale diverging channel technique. Using this experimental technique, LBV measurements were conducted for unburnt mixture temperature up to 450 K. The maximum burning velocity has been obtained at equivalence ratio 1.1 for all the mixture conditions. The LBV results at atmospheric condition for n-butane-hydrogen-air mixture were obtained by extrapolating the experimental data at elevated temperatures. “Heat flux method” experimental setup was used for measuring the LBV of n-butane-hydrogen-air mixture at atmospheric condition. The results obtained for LBV at atmospheric conditions with the two different methods at 0%, 20%, 40% and 60% H2 composition in n-butane were found to be in good agreement. The experimental results of LBV for n-butane were compared with the numerical predictions using USC mech II, Aramco mech 2.0 and LLNL reaction mechanisms. The numerical predictions of LBV using Aramco mech 2.0 shows good agreement with experimental data at rich, lean and stoichiometric mixture conditions. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Butane Diverging channels Heat flux method Hydrogen addition Laminar burning velocity
1. Introduction The gradual depletion of fossil fuel reserves and increasing concern for environment protection has led to a hike in demand for a shift from conventional fuels to cleaner fuels, such as hydrogen. The introduction of hydrogen (H2) to hydrocarbons (HC) seems to be a feasible solution. Hydrogen is having high energy content and wide range of flammability limit. However, the safety and storage issues have been restricting the use of pure H2. To overcome these difficulties, blending H2 with hydrocarbons is an alternative solution. The properties related to the safety of combustors and burners, such as flame blow off, flashback and ignition delay time might change when H2 is blended with hydrocarbons. The flash back phenomenon, which is related to the burner flame stability has a direct dependence on the fuel mixture property, the laminar burning velocity. Laminar burning velocity (LBV) is a fundamental combustion
* Corresponding author. E-mail addresses: (R.K. Velamati).
[email protected],
https://doi.org/10.1016/j.energy.2019.04.002 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
[email protected]
property, which determines the effect of hydrogen addition in hydrocarbons. As per definition, LBV is the relative velocity with which unburnt gas moves into the flame front. The determination of LBV for various mixtures is important from a design viewpoint of combustible devices. The LBV data is essential for validating detailed reaction mechanism and for developing reduced reaction mechanisms [1e4]. For the measurement of LBV, there are several well established experimental methods such as spherical flame method, stagnation flame method, heat flux method, annular stepwise diverging tube, externally heated diverging channel method and burner method [5]. Experimental studies to understand the effect of H2 addition on LBV of hydrocarbons were performed by several researchers using different experimental techniques [6-15]. Tang et al. [33] conducted experimental and numerical study to calculate LBV of n-butane-H2air mixtures. The spherical flame experimental setup was used to calculate the LBV for various f. Detailed numerical computations were performed to investigate the cause of variation in LBV of hydrocarbons with H2 addition. Their numerical analysis revealed that global activation energy and thermal effects have the major influence on increase in LBV due to hydrogen addition. Aravind
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et al. [16] numerically investigated the effect of H2 addition on the LBV of LPG-air mixtures. This study concluded that there is a linear increase in LBV with H2 addition. A correlation for the effect of temperature and pressure on LBV was proposed in this numerical study, which is valid at elevated temperature and pressure. A detailed review on the experimental data available for the burning velocity of HC-H2 mixtures show that most of the experimental studies were reported for LBV of methane-hydrogen mixtures. The experimental data available for LBV of other HC-H2 mixtures are limited in number. Also, majority of the available experimental LBV data is for HC-H2 mixtures at atmospheric condition. In the recent history, the combustion of hydrocarbon mixtures with high initial temperature attained more importance, because of its increased thermal efficiency and reduced pollution. Moreover, the temperature of the mixture in majority of combustion devices will be raised to a higher value before the beginning of the combustion itself. Therefore, the knowledge of LBV of combustible mixtures at higher temperature is essential for design and development of better combustion devices. The present investigation aims to present LBV of n-butane-H2-air mixtures at elevated temperatures. For the present study, n-butane is chosen as target fuel, since the major constitute of commercially available LPG is n- butane. It will be useful to study the effect of H2 on the LBV of n-butane-air mixtures for the research towards the replacement of LPG gas with LPG - H2 mixtures in the near future. Accurately measured LBV has vital role in the design, testing and performance estimation of the LPG-H2 fuelled devices. In present study, the LBV of n-butane-H2-air mixtures at elevated temperatures is measured using “diverging channel setup”. The LBV at atmospheric temperature is calculated by extrapolating the data at elevated temperatures to 300 K. “Heat-flux” experimental setup was developed to calculate the LBV of n-butane-H2-air mixture at atmospheric condition in the present study. The LBV at atmospheric condition obtained from both the experimental setup has been compared and shown in results section. Numerical computations were performed to calculate the LBV and to understand the flame structure of nbutane-H2-air mixtures. The details of experimental setup and
411
numerical computations are given in the next section. 2. Experimental and computational details 2.1. Experimental setup for diverging channel Diverging channel method is a technique used for measuring the LBV of a combustible mixture at elevated temperatures [17e22]. The experimental setup consists of a preheated high aspect ratio (12.5) mesoscale diverging channel with rectangular cross-section (25 mm 2 mm) through which a planar flame propagates. Quartz channel is preferred since its thermal conductivity is low, heat capacity is high and it is transparent. High aspect ratio is maintained to get a uniform flow field, which is essential for the stabilization of planar flames inside the channel [23]. The LBV is measured using this planar flame. The channel is preheated externally, using a ceramic heater to provide a positive temperature gradient in the flow direction, and also to stabilize nearly adiabatic planar flame inside the channel [24]. The channel was placed 2 cm above the top surface of the heater with an overlap of 2.5 cm. The premixed air-fuel mixture at ambient condition is given through the channel inlet, using ALICAT digital mass flow controllers (MFC's). The planar flame stabilizes at location inside the channel where the unburnt mixture velocity matches with the LBV. A Ktype thermocouple is used for the measurement of the wall temperature at the location of flame stabilization. Different MFC's were used for the precise control of fuel and air velocity according to the required mixture flow rate corresponding to various f values. The schematic diagram of the present experimental setup is shown in Fig. 1. The LBV is obtained by applying the conservation of mass principle. The channel is provided with a divergence of 10 , which creates a negative gradient of velocity along the flow direction, which resists the flame flashback [25]. After heating the channel uniformly, the required quantity of the mixture was supplied through the channel. At the channel exit, the mixture was ignited and the flame gently moves into the channel and gets stabilized at the location where the unburnt mixture
Fig. 1. Schematic diagram of the diverging channel experimental setup.
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mixture. Both the channel wall and the unburnt mixture are assumed to attain equal temperature. The temperature of the channel along the axial and transverse direction is measured using a K- type thermocouple. The conservation of mass principle is applied for calculating the LBV, as follows.
Su ¼ Uinlet
Ainlet Af
!
Tu Tu;o
(1)
LBV measured is under the influence of various parameters such as thermal feedback, boundary layer, the heat losses from flame to channel walls and the temperature difference between the reactants and wall. The inlet mass flow rates in MFC's is around 80% of the full scale. This is to reduce the uncertainties in the equivalence ratios. The temperature readings were accurate to ±5 K of the actual values. Some of the factors influencing the burning velocity are observed to be compensating each other. The effect of all these factors has been studied in overall uncertainty calculation. The uncertainty in the measured laminar burning velocity due to all these factors is expected to be around ±5% of actual value (The uncertainty calculation has been given in supplementary material). 2.2. Experimental setup for the heat-flux method Fig. 2. Direct image of planar flame stabilized in the diverging channel (10 ).
velocity becomes equal to the LBV of the mixture. The stabilized flame images were captured by a DSLR camera and the flame positions were noted. The experiment is performed at different heating rates until planar flames are obtained. Following the same procedure, experiments were conducted at different equivalence ratios and inlet velocity conditions. Akram et al. [18] reported that externally preheated meso-scale diverging channel stabilizes planar flame inside the channel. In their experimental and numerical study, it was observed that the hydrodynamic stretch has negligible influence on measured LBV, compared to various other methods employed for the measurement. Fig. 2 shows the direct image of planar flame stabilized in the channel. These stabilized flames are used for the measurement of LBV using the present experimental setup. For a particular equivalence ratio, the flame stabilizes at different locations corresponding to the inlet velocity and the temperature of the unburnt
Heat flux method is a direct method for LBV measurment introduced by de Goey et al. [26]. This measurement technique uses a nearly flat flame anchored over the burner top. The flames observed in this method are planar and stationary. The flow of unburnt gases is normal to the flame front. The schematic diagram of the setup is shown in Fig. 3. The heat-flux burner experimental setup consists of a plenum chamber, a burner head and a burner plate. The plenum chamber consisting of a distributer plate at the bottom is used to attain a uniform flow at the exit of the burner. The experimental setup was fabricated based on the details given by Bosschaart et al. [27]. A perforated burner plate of 2 mm thickness and 30 mm diameter, made of brass is positioned on top of the burner head. The plate used for the present study is having holes of 0.5 mm diameter and 0.7 mm pitch. Fine wire thermocouples are fixed into the blind holes provided at five different locations on the plate, using a thermally conductive glue to measure the burner plate temperature. The temperature at the outside perimeter and the heat fluxes at the bottom and top of the burner plate surfaces were assumed
Fig. 3. Schematic diagram of heat flux setup.
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Fig. 4. Comparison of the present experimental results for laminar burning velocity of n-butaneair mixture at ambient conditions with the published experimental data and numerical results.
constant. In heat flux method, the laminar burning velocity is determined from the knowledge of parabolic coefficient (a2) as a function of unburned gas velocity. The negative value of the parabolic coefficient corresponds to sub-adiabatic flames, whereas its positive value indicates super adiabatic condition. At a2 ¼ 0, the net amount of heat transferred from the burner plate to the unburnt mixture becomes zero, which corresponds to the adiabatic condition. The unburned gas velocity at this condition yields the laminar burning velocity of the mixture, which can be obtained by the linear interpolation. 2.3. Computational details The LBV and the flame structure of n-butane-H2-air mixtures have been studied numerically. One dimensional numerical computations were performed for predicting the LBV and the thermodynamic properties of n-butane- H2 mixture, using the PREMIX code [28]. The essential thermodynamic and transport properties were obtained from Sandia National Laboratories. Simulations were done with adaptive mesh parameters GRAD ¼ 0.02 and CURV ¼ 0.1. All the simulations were having more than 500 grid points. The mesh parameters were decided after confirming that the present parameters are sufficient for getting grid independent results. 3. Results and discussion The present experimental study aims to analyse the effect of hydrogen addition on LBV of n-butane-air mixtures using two different experimental methods such as diverging channel method and heat-flux method. Experiments were conducted for calculating the LBV of pure n-butane-air with 0%, 20%, 40% and 60% H2 addition. For each composition, burning velocities were calculated for equivalence ratios varying from 0.7 to 1.3. The diverging channel method helps to calculate the LBV over a range of unburnt mixture temperatures, 350e450 K. The measured LBV at various temperatures were used to develop following power law correlation
Su ¼ Su;o
Tu a Tu;o
(2)
In present study, experimental setup for the heat-flux method
Fig. 5. LBV of n-butane-H2-air mixtures at elevated temperatures for 0% H2 and 40% H2 for (a) f ¼ 0.9 and (b) f ¼ 1.
was developed for the measurement of LBV at atmospheric conditions, for n-butane-H2 mixtures. The LBV results obtained at atmospheric conditions from both the experimental setups were compared, for 0%, 20%, 40% and 60% H2 addition in n-butane-air. Detailed numerical computations were also performed for analysing the effect of H2 on the burning velocity of n-butane-air mixture. The results obtained from both the experimental setups were compared with the numerical predictions using PREMIX code. 3.1. Validation of the experimental setups The LBV data for n-butane-air mixture at atmospheric conditions form various experimental studies are available in literature. Therefore, to validate the present experimental setups, experiments were conducted for the measurement of LBV of n-butane-air mixtures at atmospheric condition for various f, using both diverging channel method and heat-flux method. These experimental results were compared with the available results of Bosschaart et al. [29], Kelley et al. [30], Wu et al. [31], Park et al. [32],
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Fig. 6. The temperature exponent (a) obtained for 0%, 20%, 40% and 60% H2 addition on n-butane-air mixture.
Tang et al. [33], and Dirrenberger et al. [34], as well as the numerical predictions using USC mech II [35] Aramco mech 2.0 [36e42] and LLNL mech [43] reaction mechanisms as shown in Fig. 4. The graph shows that the LBV results obtained for n-butane-air mixtures from both the experimental results agree reasonably well with the existing results in the literature. The numerical predictions show that LLNL mechanism over-predicts the laminar burning velocity for all mixture conditions. The LBV predictions using USC mech II agrees well with the experimental data at rich mixture conditions, but the mechanism over-predicts the laminar burning velocity at lean mixture conditions. The numerical predictions of LBV using Aramco mech 2.0 shows good agreement with experimental data at rich, lean and stoichiometric mixture conditions. Therefore, all the numerical results presented in the present study was based on the computations performed using Aramco mech 2.0.
3.2. Measurement of LBV at elevated temperatures The diverging channel experimental setup was used to study the effect of temperature on the LBV. Experiments were performed at various equivalence ratios to get the LBV at elevated temperatures of the unburnt mixtures. The mixture inlet temperature (Tu ¼ 300 K) is taken as the reference temperature. The power-law is fitted to the experimental results as per equation (2) which can be used for the determination of LBV at different temperatures within the specified range. The LBV obtained at various temperatures for f ¼ 0.9 and f ¼ 1 of n-butane with 0% and 40% H2, are shown in Fig. 5 (a) and (b) respectively. The symbols represent the experimental results. The dotted lines are used to represent the numerical predictions for these conditions. Solid lines show the power-law fit for the experimental data. The increase in LBV with the rise in unburnt gas temperature can be identified from Fig. 5. The figure also indicates that there is an increase in LBV for nbutane with 40% H2 in comparison with that of 0% H2. In Fig. 6, the temperature exponent (a) obtained for 0%, 20%, 40% and 60% H2 addition on n-butane-air mixture is shown for equivalence ratio varying from 0.7 to 1.3. The temperature exponent has minimum value at equivalence ratio 1.1, and show increasing trend in both rich and lean conditions. Also, the data obtained for various compositions of H2 show that the temperature exponent decreases with increase in the percentage of H2 in the mixture.
Fig. 7. The laminar burning velocities of n-butane-H2-air mixtures at ambient condition, (a) for 0%, 20%, and (b) for 40% and 60% H2 compositions obtained from two different experimental setups. Markers with: Black-Diverging channel method; RedHeat flux method. Lines represent the results obtained for 0%, 20%, 40% and 60% H2 using Aramco Mech 2.0. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.3. Laminar burning velocity at atmospheric condition In the present study, the LBV of n-butane-H2-air mixtures at atmospheric conditions were calculated from the two different experimental setups, such as heat-flux method and the diverging channel method. Using diverging channel method, the LBV at atmospheric condition for 0%, 20%, 40% and 60% hydrogen addition were calculated by extrapolating the data obtained at elevated temperature as explained in the previous section. The heat-flux method used in present study is capable of measuring LBV at atmospheric conditions. The experimental details for the measurement of LBV using this method were given in section 2.2. Following this procedure, the LBV of n-butane-air mixtures with H2 addition (0%, 20%, 40% and 60% H2) were calculated at atmospheric condition for equivalence ratios ranging from 0.7 to 1.3. These results were compared with the results obtained using the diverging channel setup, as shown in Fig. 7(a) and (b). The graph indicates that the experimental data of LBV at atmospheric
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condition from both the experiments for all the conditions are in good agreement. The peak value of LBV is obtained at f ¼ 1.1. The LBV of n-butane-H2-air mixture at atmospheric condition increases with increase in H2 addition. 3.4. Effect of hydrogen addition
Fig. 8. The laminar burning velocities of n-butane-H2 mixtures at 420 K, for 0%, 20%. 40% and 60% H2 compositions.
The laminar burning velocities calculated at 420 K using the correlation at various equivalence ratios, for 20%, 40% and 60% hydrogen addition on n-butane are shown in Fig. 8. The peak value of LBV is obtained at equivalence ratio 1.1. A significant increase in LBV can be observed when hydrogen percentage in the mixture increased from 0% to 60%. Tang et al. [33] observed that the thermal, kinetic and diffusion factors are playing a major role in the increase of laminar burning velocity with the addition of hydrogen. In Fig. 9 (a), the variation in LBV with H2 composition in C4H10-air mixture is shown at f ¼ 0.8, 1 and 1.2. The graph shows that the variation in LBV follows almost linearly increasing trend till 40%. The variation in temperature exponent with H2 composition at f ¼ 0.8, 1 and 1.2 is shown in Fig. 9 (b). The graph shows that the temperature exponent follows a decreasing trend with increase in H2 content in the mixture, for all the equivalence ratio conditions. 3.5. Chemical kinetic analysis The flame structure of n-C4H10-H2 eair mixtures have been studied by performing numerical computations using one dimensional code (PREMIX). The simulations were performed using Aramco mech 2.0 reaction mechanism to understand effect of hydrogen addition. Fig. 10 show the mole fraction profiles of H, OH and CO radicals at the flame region, plotted against the nondimensional temperature, at 0%, 40% and 60% H2 addition in nC4H10. Computations were performed for stoichiometric mixture inlet with 300 K initial temperature. It is observed from the graph that the mole fractions of both H and OH radicals increases as the H2 content in the mixture is increased from 0% to 60%. The increased production of these radicals causes an increase in heat release rate, which results in an increase in LBV. Fig. 11 (a) shows the mole fractions of C4H10 and H2 at 0% and 60% H2 compositions in the fuel mixture. From the figure it can be
Fig. 9. (a) LBV at atmospheric condition (Su,o) and (b) Temperature exponent, of nbutane-H2 mixtures at 0%, 20% and 40% H2.
Fig. 10. The mole fractions of H, OH and CO (red) radicals plotted against nondimensional temperature, for n-C4H10-H2eair mixtures at 0% H2 (black), 40% H2 (blue) and 60% H2 (red), at initial temperature of 300 K for f ¼ 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 12. (a) Net heat production rate, (b) net reaction rates of R5 and R36 at f ¼ 1, 300 K, for 0% H2 and 60% H2 mixture conditions. Fig. 11. Mole fraction profiles of (a) C4H10 and H2, (b) CH3, at f ¼ 1, 300 K, for 0% H2 and 60% H2 mixture conditions.
identified that H2 is produced in the reaction zone even for pure C4H10 (without H2 addition), as a result of chemical reactions at the reaction zone. Methyl radicals (CH3) radicals are important in hydrocarbon combustion since their oxidation produces CO. In Fig. 11 (b), CH3 radicals are plotted for 0% and 60% H2 compositions. The graph shows that peak value of CH3 radical mole fraction is higher for 60% H2 added fuel mixture. In Fig. 12 (a), the net heat production rates for 0% and 60% H2 fuel compositions are shown. The presence of hydrogen in the mixture causes an increased heat production at the reaction zone. Fig. 12 (b) shows the net reaction rates of two important reactions, R5 (H þ O2 ¼ O þ OH) and R36 (CO þ OH ¼ CO2þH). Hydrogen addition causes an increase in the net reaction rates of both R5 and R36 as shown in the graph. 4. Conclusions The laminar burning velocity of n-butane-H2 eair mixtures were measured using two different experimental setups, such as
diverging channel method and heat flux method. The diverging channel setup was used for the measurement of LBV at elevated temperatures. The present experimental setup could measure the LBV of n-butane-H2-air mixture up to an unburnt gas temperature around 450 K. The LBV at atmospheric temperature is obtained by extrapolating the data at elevated temperature to 300 K, using power-law fit. To verify the extrapolation result, heat flux method was used to measure the LBV at atmospheric condition directly, without any extrapolation. The LBV results obtained at atmospheric condition from both the experiments were compared. It was observed that the LBV at atmospheric temperature, calculated using extrapolation (diverging channel method) and that obtained from direct measurement (heat flux method) were in good agreement. To understand the effect of hydrogen enrichment in LBV, experiments were conducted for 0%, 20%, 40% and 60% H2 addition in n-C4H10-air mixture, for a range of equivalence ratio from 0.7 to 1.3. The results showed that the increase in LBV with H2 enrichment follows almost linear trend till 40%. Numerical computations were performed to study the flame structure. This study revealed that there is an increase in H and OH mole fractions due to H2 enrichment in the fuel mixture, which causes an increase in LBV. The net
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heat production rate was found to increase with increase in H2 percentage in the mixture. Acknowledgement The authors would like to thank Department of Science and Technology, Government of India for their support through grand number YSS_2015_000007 under young scientists scheme. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.04.002. Nomenclature
rinlet rf Ainlet Af Uinlet Su Tu Tu;0
a
Su,0
Density of the mixture at the channel inlet Density of the mixture at flame location Cross-sectional area of the channel inlet Flame area Mixture velocity at channel inlet Laminar burning velocity at temperature at Tu Temperature of the unburned mixture, K Initial temperature of the unburned mixture, K Temperature exponent LBV of the mixture at atmospheric conditions
References [1] Westbrook CK, Dryer FL. Simplified reaction mechanisms for the oxidation of hydrocarbon fuels in flames. Combust Sci Technol 1981;27(1e2):31e43. [2] Law CK, Sung CJ, Wang H, Lu TF. Development of comprehensive detailed and reduced reaction mechanisms for combustion modeling. AIAA J 2003;41(9): 1629e46. [3] Davis SG, Joshi AV, Wang H, Egolfopoulos F. An optimized kinetic model of H 2/CO combustion. Proc Combust Inst 2005;30(1):1283e92. [4] Wang H, Sheen DA. Combustion kinetic model uncertainty quantification, propagation and minimization. Prog Energy Combust Sci 2015;47:1e31. [5] Konnov AA, Mohammad A, Kishore VR, Kim NI, Prathap C, Kumar S. A comprehensive review of measurements and data analysis of laminar burning velocities for various fuelþ air mixtures. Prog Energy Combust Sci 2018;68:197e267. [6] Milton BE, Keck JC. Laminar burning velocities in stoichiometric hydrogen and hydrogen-hydrocarbon gas mixtures. Combust Flame 1984;58(1):13e22. [7] Yu G, Law CK, Wu CK. Laminar flame speeds of hydrocarbonþ air mixtures with hydrogen addition. Combust Flame 1986;63(3):339e47. €kalp I. Characterization of the [8] Halter F, Chauveau C, Djebaili-Chaumeix N, Go effects of pressure and hydrogen concentration on laminar burning velocities of methaneehydrogeneair mixtures. Proc Combust Inst 2005;30(1):201e8. [9] Ilbas M, Crayford AP, Yılmaz I, Bowen PJ, Syred N. Laminar-burning velocities of hydrogeneair and hydrogenemethaneeair mixtures: an experimental study. Int J Hydrogen Energy 2006;31(12):1768e79. [10] Huang Z, Zhang Y, Zeng K, Liu B, Wang Q, Jiang D. Measurements of laminar burning velocities for natural gasehydrogeneair mixtures. Combust Flame 2006;146(1e2):302e11. [11] Coppens FHV, De Ruyck J, Konnov AA. Effects of hydrogen enrichment on adiabatic burning velocity and NO formation in methaneþ air flames. Exp Therm Fluid Sci 2007;31(5):437e44. [12] Tang C, Huang Z, Jin C, He J, Wang J, Wang X, Miao H. Laminar burning velocities and combustion characteristics of propaneehydrogeneair premixed flames. Int J Hydrogen Energy 2008;33(18):4906e14. [13] Hui X, Zhang C, Xia M, Sung C-J. Effects of hydrogen addition on combustion characteristics of n-decane/air mixtures. Combust Flame 2014;161(9): 2252e62. [14] Mandilas C, Ormsby MP, Sheppard CGW, Woolley R. Effects of hydrogen addition on laminar and turbulent premixed methane and iso-octaneeair flames. Proc Combust Inst 2007;31(1):1443e50. [15] Wu F, Kelley AP, Tang C, Zhu D, Law CK. Measurement and correlation of laminar flame speeds of CO and C2 hydrocarbons with hydrogen addition at atmospheric and elevated pressures. Int J Hydrogen Energy 2011;36(20): 13171e80. [16] Aravind B, Kishore VR, Mohammad A. Combustion characteristics of the effect of hydrogen addition on LPG-air mixtures. Int J Hydrogen Energy
417
2015;40(16605):e16617. [17] Akram M, Kumar S. Measurement of laminar burning velocity of liquified petrolium gas air mixtures at elevated temperatures. Energy Fuels 2012;26(6):3267e74. [18] Akram M, Saxena P, Kumar S. Laminar Burning velocity of methaneeair mixtures at elevated temperatures. Energy Fuels 2013;27(6):3460e6. [19] Paidi SK, Bhavaraju A, Akram M, Kumar S. Effect of N 2/CO 2 dilution on laminar burning velocity of H 2eair mixtures at high temperatures. Int J Hydrogen Energy 2013;38(31):13812e21. [20] Nair A, Velamati RK, Kumar S. Effect OF CO2/N2 dilution on laminar burning velocity of liquid petroleum gas-air mixtures at elevated temperatures. Energy 2016;100:145e53. [21] Varghese RJ, Kolekar H, Hariharan V, Kumar S. Effect of CO content on laminar burning velocities of syngas-air premixed flames at elevated temperatures. Fuel 2018;214:144e53. [22] Akram M, Kishore VR, Kumar S. Laminar burning velocity of propane/CO2/ N2eair mixtures at elevated temperatures. Energy Fuels 2012;26(9):5509e18. [23] Akram M, Kumar S. Experimental studies on dynamics of methane-air premixed flame in meso-scale diverging channels. Combust Flame 2011;158(5): 915e24. [24] Akram M, Kumar S, Saxena P. Experimental and computational determination of laminar burning velocity of liquefied petroleum gas-air mixtures at elevated temperatures. J Eng Gas Turbines Power 2013;135(9). [25] Kumar S, Maruta K, Minaev S. Experimental investigations on the combustion behavior of methane-air mixtures in a new micro scale radial combustor configuration. J Micromechan Microeng. 2007;17. 900-900. [26] De Goey LPH, Van Maaren A, Quax RM. Stabilization of adiabatic premixed laminar flames on a flat flame burner. Combust Sci Technol 1993;92(1e3): 201e7. [27] Bosschaart KJ, De Goey LPH. Detailed analysis of the heat flux method for measuring burning velocities. Combust Flame 2003;132(1):170e80. [28] Kee RJ, Grcar JF, Miller JA, Meeks E, Smooke M. PREMIX users manual. San Diego, CA: Reaction Design; 1998. [29] Bosschaart KJ, De Goey LPH. The laminar burning velocity of flames propagating in mixtures of hydrocarbons and air measured with the heat flux method. Combust Flame 2004;136(3):261e9. [30] Kelley AP, Law CK. Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames. Combust Flame 2009;156(9):1844e51. [31] Wu H, Hu E, Yu H, Li Q, Zhang Z, Chen Y, Huang Z. Experimental and numerical study on the laminar flame speed of n-butane/dimethyl ethereair mixtures. Energy Fuels 2014;28(5):3412e9. [32] Yoon SH, Park J, Kwon OB, Yoon JH, Keel SI, Kim JS, Bae DS. Experimental study on self-excitations in nitrogen-diluted laminar lifted butane flames. Fuel 2012;91(1):51e60. [33] Tang CL, Huang ZH, Law CK. Determination, correlation, and mechanistic interpretation of effects of hydrogen addition on laminar flame speeds of hydrocarboneair mixtures. Proc Combust Inst 2011;33(1):921e8. [34] Dirrenberger P, Le Gall H, Bounaceur R, Herbinet O, Glaude P-A, Konnov A, Battin-Leclerc F. Measurements of laminar flame velocity for components of natural gas. Energy Fuels 2011;25(9):3875e84. [35] Wang H, You X, Joshi AV, Davis SG, Laskin A, Egolfopoulos F, Law CK, Version Ii USCM. High-temperature combustion reaction model of H2. 2007. CO/C1-C4 Compounds, http://ignis.usc.edu/USC_Mech_II.htm. [36] Li Y, Zhou C-W, Somers KP, Zhang K, Curran HJ. The oxidation of 2-butene: a high pressure ignition delay, kinetic modeling study and reactivity comparison with isobutene and 1-butene. Proc Combust Inst 2017;36(1):403e11. [37] Zhou C-W, Li Y, O'Connor E, Somers KP, Thion S, Keesee C, Mathieu O, Petersen EL, DeVerter TA, Oehlschlaeger MA. A comprehensive experimental and modeling study of isobutene oxidation. Combust Flame 2016;167: 353e79. [38] Burke U, Metcalfe WK, Burke SM, Heufer KA, Dagaut P, Curran HJ. A detailed chemical kinetic modeling, ignition delay time and jet-stirred reactor study of methanol oxidation. Combust Flame 2016;165:125e36. [39] Burke SM, Burke U, Mc Donagh R, Mathieu O, Osorio I, Keesee C, Morones A, Petersen EL, Wang W, DeVerter TA. An experimental and modeling study of propene oxidation. Part 2: ignition delay time and flame speed measurements. Combust Flame 2015;162(2):296e314. [40] Burke SM, Metcalfe W, Herbinet O, Battin-Leclerc F, Haas FM, Santner J, Dryer FL, Curran HJ. An experimental and modeling study of propene oxidation. Part 1: speciation measurements in jet-stirred and flow reactors. Combust Flame 2014;161(11):2765e84. [41] Metcalfe WK, Burke SM, Ahmed SS, Curran HJ. A hierarchical and comparative kinetic modeling study of C1 C2 hydrocarbon and oxygenated fuels. Int J Chem Kinet 2013;45(10):638e75. romne s A, Metcalfe WK, Heufer KA, Donohoe N, Das AK, Sung C-J, Herzler J, [42] Ke Naumann C, Griebel P, Mathieu O. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combust Flame 2013;160(6):995e1011. [43] Marinov NM, Pitz WJ, Westbrook CK, Vincitore AM, Castaldi MJ, Senkan SM, Melius CF. Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame. Combust Flame 1998;114(1e2):192e213.