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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Experimental study on cellular instabilities in hydrocarbon/ hydrogen/carbon monoxideeair premixed flames Tran Manh Vu a,b, Jeong Park b,*, Jeong Soo Kim b, Oh Boong Kwon b, Jin Han Yun c, Sang In Keel c a Faculty of Civil Engineering, Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Viet Nam School of Mechanical Engineering, Pukyong National University, San 100, Yongdang-dong, Nam-gu, Busan 608-739, Republic of Korea c Environment & Energy Research Division, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea b article info abstract Article history: To investigate cell formation in methane (or propane)/hydrogen/carbon monoxideeair Received 10 August 2010 premixed flames, the outward propagation and development of surface cellular instabil- Received in revised form ities of centrally ignited spherical premixed flames were experimentally studied in 12 February 2011 a constant pressure combustion chamber at room temperature and elevated pressures Accepted 15 February 2011 Additionally, unstretched laminar burning velocities and Markstein lengths of the mixtures Available online 16 March 2011 were obtained by analyzing high-speed schlieren images In this study, hydrodynamic and diffusional-thermal instabilities were evaluated to examine their effects on flame insta- Keywords: bilities The experimentally-measured unstretched laminar burning velocities were Cell formation compared to numerical predictions using the PREMIX code with a H2/CO/C1eC4 mecha- Diffusional-thermal instability nism, USC Mech II, from Wang et al [22] The results indicate a significant increase in the Hydrocarbon unstretched laminar burning velocities with hydrogen enrichment and a decrease with the Hydrodynamic instability addition of hydrocarbons, whereas the opposite effects for Markstein lengths were Premixed flame observed Furthermore, effective Lewis numbers of premixed flames with methane addition decreased for all of the cases; meanwhile, effective Lewis numbers with propane addition increase for lean and stoichiometric conditions and increase for rich and stoichiometric cases for hydrogen-enriched flames With the addition of propane, the propensity for cell formation significantly diminishes, whereas cellular instabilities for hydrogen-enriched flames are promoted However, similar behavior of cellularity was obtained with the addition of methane, which indicates that methane is not a candidate for suppressing cell formation in methane/hydrogen/carbon monoxideeair premixed flames Copyright ª 2011, Hydrogen Energy Publications, LLC Published by Elsevier Ltd All rights reserved Introduction The significance of global climate change and the depletion of existing fossil fuels have led to the identification of replacement fuels In this sense, hydrocarbons such as methane and propane are considered to be an attractive potential fuel in spark-ignition engines [1,2] However, one of the problems is the release of carbon dioxide products if hydrocarbons are used as an alternative fuel In recent years, hydrogen has been widely used due to its advantages such as a high burning * Corresponding author Tel.: ỵ82 51 629 6140; fax: ỵ82 51 629 6126 E-mail address: jeongpark@pknu.ac.kr (J Park) 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC Published by Elsevier Ltd All rights reserved doi:10.1016/j.ijhydene.2011.02.085 6915 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 Spherical concave mirror Bypass Vacuum pump Pin hole Halogen lamp CO Pressure transducer CH4 or C3H8 1.000 Control circuit Ignition system H2 Digital display Pressure transmitter Data acquisition Charge amplifier Knife edge Air Fig e Schematic representation of the experimental setup velocity and cleanly emitted products [3,4] However, hydrogen mixtures cause cells on the flame surface to occur earlier, which can induce turbulence in the unburned mixture and cause a rapid increase in the flame propagation velocity, which can cause a gas explosion In addition, a mixture of hydrogen and carbon monoxide (i.e., syngas), which can form through the gasification process of a variety of resources such as coal, biomass, organic wastes, and refinery residuals [5], is also a potential fuel Therefore, the combustion characteristics of premixed flames combined with the use of hydrocarbons, hydrogen, and carbon monoxide as fuels have been continuously studied In premixed flames, in addition to the laminar burning velocity, a corrugated flame front due to the formation of cellular instabilities is an interesting consideration Three effects are related to the cellularity of premixed flames In this study, the cellular instabilities of hydrocarbon/hydrogen/ carbon monoxideeair premixed flames were identified and evaluated with respect to hydrodynamic and diffusionalthermal instabilities Whereas body-force effects were not significant and could be neglected because the laminar burning velocities of the flames mentioned in this study are large enough such that the flames overcome the impact of the body-force factor [4] In the early stages of flame development, the flame instabilities are primarily influenced by a diffusivethermal factor However, as the flame develops and the flame radius increases, the hydrodynamic factor becomes dominant [6] Initially, cellular instabilities are suppressed by the strong curvature associated with a small flame radius However, as the flame expands and flame stretch decreases, a state is reached in which the cell development can no longer be suppressed, and, consequently, cells will appear almost instantaneously over the entire flame surface, i.e., the onset of cellular instabilities, which is represented by the critical radius, Rcr In response to the interest in controlling the unstable behavior of cellular flames, numerous studies have been conducted regarding cell formation in hydrogeneair and hydrogen/ hydrocarboneair flames [6e10] In our previous studies, the effects of hydrocarbon additions and dilutions to the hydrodynamic and diffusive-thermal factors of the cellular instabilities in the syngaseair premixed flames were analyzed and discussed [11,12] Additionally, an understanding of the formation of cellular instabilities in hydrocarbon/hydrogen/carbon monoxideeair flames is not sufficient and should be discussed further Therefore, this study focuses on the laminar burning velocities, Markstein lengths, behavior of cell formations, and transition to cellularity of methane/hydrogen/carbon monoxideeair and propane/hydrogen/carbon monoxideeair premixed flames by enriching 40%, 60%, and 80% (by volume) of hydrogen (H2) and adding 5%, 10%, 15%, and 20% (by volume) of methane (CH4) and propane (C3H8) to the fuel blends at room temperature and elevated pressures for overall equivalence ratios of 0.8, 1.0, and 1.2 using the centrally ignited, outwardly propagating spherical flames method This method yields highly accurate results for both laminar flame speeds and cellular flame instabilities and can easily account for high initial pressures and high initial temperatures [6,7,13,14] Table e Effective volumetric fraction of hydrocarbon in the fuel blend, a XHC 5% 10% 15% 20% ¼ 0.8 ¼ 1.0 ¼ 1.2 CH4 C3H8 CH4 C3H8 CH4 C3H8 0.15 0.26 0.36 0.45 0.29 0.46 0.58 0.66 0.14 0.26 0.35 0.44 0.28 0.45 0.56 0.65 0.14 0.25 0.35 0.43 0.27 0.44 0.55 0.64 6916 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 Experimental and computational details 2.1 Experimental setup and procedure The experiments were conducted in a stainless steel, cylindrical constant-volume chamber with an inside diameter of 200 mm and a length of 220 mm Visual access was provided by two 100-mm diameter, 40-mm thickness quartz windows mounted opposite of each other on both flat sides of the chamber Two tungsten electrodes with a diameter of 0.5 mm were linked to a high voltage source (up to 10 kV) to ignite the combustible mixture at the center of the chamber The electrodes were movable, and thus the spark gap was manually adjustable The spark gap varied from 0.7 mm to 2.0 mm; larger gaps were used to ignite flames with small laminar burning velocities that required relatively large ignition energies The reactant mixtures were prepared within the chamber by adding individual component gases at corresponding partial pressures using an absolute pressure transmitter to reach the desired initial pressure, Pu A period of 15 was used to ensure complete mixing and quiescent conditions Once the spark was ignited, a flame kernel formed at the center of the chamber, propagated outward spherically, and quenched when it touched the walls of the chamber The propagating spherical flame was imaged using schlieren photography with a 300-W halogen light source and a pair of 150-mm diameter spherical concave mirrors and was recorded using a highspeed digital camera (Phantom v7.2) that operated at 10,000 frames per second with a resolution of 512  384 pixels Simultaneously, the pressure inside of the chamber during combustion was measured using a water-cooled piezo-electric pressure transducer (Kistler 6061B) along with a charge amplifier (Kistler 5011B) and was transferred to a computer via a data acquisition system (NI 9215A) After combustion, the chamber was vented to the laboratory exhaust system and purged using an air compressor to remove condensed water vapor prior to refilling for the next test The measurements were restricted to flames with radii larger than mm and smaller than 30 mm The lower bound provided sufficient time for the removal of disturbances introduced by ignition as well as minimized curvature and transient effects associated with the finite thickness of the flame, whereas the upper bound allowed the flames to avoid wall interference and limited the pressure increases during the measurement period to values less than 1.0% of the initial pressure [15] Details of the experimental setup are shown in Fig a a b Fig e Experimental (points) and calculated (lines) unstretched laminar burning velocities of various hydrocarbon/hydrogen/carbon monoxideeair mixtures at Pu [ 0.2 MPa and [ 1.0 b Fig e Markstein lengths of various hydrocarbon/ hydrogen/carbon monoxideeair mixtures at Pu [ 0.1 MPa and [ 0.8 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 a b c d 6917 Fig e Effective Lewis numbers for various equivalence ratios in CH4/H2/COeair and C3H8/H2/COeair premixed flames In this study, the volumetric fraction of each fuel is given as Xi ẳ Vi VHC ỵ VH2 ỵ VCO (1) where i ẳ CH4, C3H8, H2, and CO and VHC, VH2 and VCO are the volumes of hydrocarbon (CH4 or C3H8), hydrogen and carbon monoxide in the fuel blends, respectively To compare the effects of methane and propane to the premixed flames, due to the heavier hydrocarbon consisting of more carbon and hydrogen atoms and having more fuel content, it will have a larger effect than smaller molecular hydrocarbon for the same molar amount of addition Therefore, it is useful to define one mixture using an effective volumetric fraction of hydrocarbon in the fuel blend, which can be expressed as aẳ XHC VHC ỵVAirHC 1X 1X HC HC XHC VHC ỵVAirHC ỵ VH2 ỵVAirH2 þ VCO þVAirÀCO 2 (2) where VAireHC, VAireH2 , and VAireCO are the volumes of air corresponding to hydrocarbon, hydrogen and carbon monoxide, respectively Corresponding values of a of XHC ¼ 5%, 10%, 15%, and 20% of various hydrocarbon/hydrogen/carbon monoxideeair mixtures are shown in Table 2.2 Laminar burning velocity and Markstein length For a spherically expanding flame, the stretched flame velocity, Sn, which represents the flame propagation speed, is calculated from the instantaneous flame radius measured from the experiments using the following equation [16e18] Sn ¼ dR dt (3) where R is the instantaneous radius of the flame in the schlieren photographs and t is time Therefore, Sn can be obtained directly from the images of the flame The flame stretch rate, K, is the Lagrangian time derivative of the logarithm of the area A of any infinitesimal element of the surface [16,19,20] Kẳ dlnAị dA ẳ dt A dt (4) where A is the surface area of the flame For a spherically outwardly expanding flame front, the flame stretch rate due to the combined effects of curvature and flame motion can be simplified as K¼ dA 8pRdR dR ¼ ¼ ¼ Sn A dt 4pR2 dt R dt R (5) 6918 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 Fig e Schlieren images of C3H8/H2/COeair flames for XC3 H8 [0:15, [ 1.2 at various initial pressures From Eqs (3) and (5), the stretched flame velocity, Sn, and the flame stretch rate, K, can be calculated The stretched flame speed can be related to the flame stretch rate by the linear relationship [2,16,17,20] S1 À Sn ¼ Lb K (6) where S1 is the unstretched flame speed and Lb is the burned gas Markstein length that represents the influence of the flame speed on the flame stretch rate The unstretched flame speed, S1, can be obtained as the intercept value at K ¼ 0, in the plot of Sn against K, and the burned gas Markstein length, Lb, is the negative value of the slope of the SnÀK curve Due to S1 being known, the unstretched laminar burning velocity, which is defined as the unstretched upstream flame speed, can be determined from mass conservation as S0u ¼ S1 rb ru Fig e Schlieren images of various CH4/H2/COeair and C3H8/H2/COeair premixed flames (7) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 where ru is the density of the unburned mixtures and rb is the density of the burned products The PREMIX code [21] was used to predict the unstretched laminar burning velocity, which was then compared to experimental data The chemical mechanism in this study was the H2/CO/C1eC4 model, USC Mech Version II, which was developed by Wang et al [22] and consists of 784 elementary reactions with 111 species This model was chosen because it includes all of the species required in this study Factors that influence cellular instabilities Hydrodynamic effects have the most significant influence on the flame instability and are caused by the thermal expansion ratio through the flame front, which is defined as the ratio of the density of unburned gas (ru) to the density of burned gas (rb) at two sides of the flame front [23] The flame thickness is also an important parameter that influences hydrodynamic instability; if the flame is thin, then it will reduce the influence of curvature and enhance the baroclinic torque intensity, which is dependent on the density gradient across the flame and the transverse pressure gradient along the flame Vr  VP=r2 [24] Diffusive-thermal effects are caused by the preferential diffusion of mass versus heat and are represented by the Lewis number, Le, which is a ratio of the heat diffusivity of the mixture to the mass diffusivity of the limiting reactant relative to the abundant inert [16,25] If the Lewis number of the flame is smaller than, equal to or larger than the critical value, LeÃeff (slightly less than unity), then the flame will be unstable, neutral or stable regarding the diffusive-thermal effect, respectively 3.1 6919 where LeE and LeD are the Lewis numbers of excessive and is deficient reactants, respectively A1 ẳ ỵ bF 1ị a measurement of the mixture strength, where F is a ratio of the mass of excess-to-deficient reactants in the fresh mixture relative to their stoichiometric ratio (F ¼ 1/4 for and F ¼ for > 1) and b ẳ Ea Tad Tu ị=R0 T2ad is the Zeldovich number, where Tad the adiabatic flame temperature, Ea ẳ 2R0 pẵvlnru S0u ị=v1=Tad ị is the activation energy, R0 is the universal gas constant Tad along with ru and rb in the previous section were assumed to be in equilibrium and were calculated using the EQUIL code [27] 3.2 Thermal expansion ratio and flame thickness The laminar flame thickness, lf, is a characteristic length scale that is used to evaluate the hydrodynamic instability and to normalize the critical radius to obtain the critical Peclet number for the onset of cellular instabilities, which will be discussed in further detail later In this study, the characteristic flame thickness is given by Law et al [10] as lf ¼ l=cP ru S0u (10) where l and cP are the thermal conductivity and specific heat at 1200 K, respectively, which is an approximate average of the free stream and flame temperatures [10] s ¼ ru/rb is referred to as the thermal expansion ratio Results and discussion 4.1 Unstretched laminar burning velocities and Markstein lengths The laminar burning velocity is one of the key parameters in combustion research Thus, an accurate measurement of the Effective Lewis number In this study, three fuels (H2, CO and hydrocarbon (HC)) were used in the mixture; thus, the fuel Lewis number of the reactant is a weighted average of the Lewis numbers of the three fuels [9e11], which is given as LeF ẳ ỵ qHC LeHC 1ị ỵ qH2 LeH2 þ qCO ðLeCO À 1Þ q (8) where LeHC, LeH2 and LeCO are the fuel Lewis numbers of the hydrocarboneair mixture at fHC ẳ XHC =XA ị=XF =XA ịst , hydrogeneair mixture at fH2 ẳ XH2 =XA ị=XF =XA ịst , and carbon monoxideeair mixture at fCO ¼ ðXCO =XA Þ=ðXF =XA Þst , respectively XF and XA are the mole fractions of fuel and air in the reactant mixture [9,11], q ẳ qHC ỵ qH2 ỵ qCO is the total heat release, where qj ( j ¼ HC, H2, CO) is the nondimensional heat release associated with the consumption of species j, which is defined as qj ¼ QYj =cP Tu , where Q is the heat of reaction, Yj is the supply mass fraction of species j, cP is the specific heat of the unburned gas and Tu is the unburned gas temperature [10] The effective Lewis number is defined as the combination of the fuel and oxidizer Lewis numbers [11,12,26] Leeff LeE 1ị ỵ LeD 1ịA1 ẳ 1ỵ ỵ A1 (9) Fig e Comparison of the suppression of cellular instabilities of H2/COeair premixed flames with CH4 and C3H8 additions at similar a 6920 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 unstretched laminar burning velocity is necessary to assess combustion theories and for the validation of numerical models [1,13,28] Fig compares experimentally-measured and predicted unstretched laminar burning velocities of various hydrocarbon/hydrogen/carbon monoxideeair mixtures at Pu ¼ 0.2 MPa and ¼ 1.0; the two results are in good agreement The unstretched laminar burning velocities increase significantly along with an increase in the amount of H2, and this increasing tendency is stronger at high percentages of hydrogen concentration On the other hand, the unstretched laminar burning velocities decrease along with hydrocarbon addition to the fuel blend The primary reason for this decrease is thermal effects due to an increase in the heat release and thus an increase in the adiabatic flame temperature for hydrogen enrichment, whereas the opposite tendency was observed for hydrocarbon addition [29,30] As previously mentioned, the negative slope of the linear relationship between Sn and K is defined as the burned gas Markstein length, Lb If Lb < 0, then the flame speed increases along with an increase in the flame stretch rate In this case, if any protuberance occurs on the flame front, then the flame speed increases, which increases the instability of the flame On the other hand, for the case of a positive Lb, the flame front instabilities will be restricted, and the flame will be stable [2,20] Fig shows that the Markstein length decreases along with an increase in hydrogen enrichment, whereas it increases along with an increase in hydrocarbon addition This effect indicates that the flame instability becomes more susceptible to an increase in the hydrogen fraction, and the flame front becomes more stable with the addition of hydrocarbons in the reactant mixture 4.2 Flame stability and cellular structure Flame stability is another important characteristic of premixed flames As previously mentioned, two instabilities of premixed flame were observed in this study: diffusionalthermal instability and hydrodynamic instability The effective Lewis numbers, Leeff, which represent the influence of diffusive-thermal effects on premixed flames, are analyzed Fig shows the effective Lewis numbers of CH4/H2/COeair and C3H8/H2/COeair flames for different equivalence ratios This figure indicates that the effective Lewis numbers of all of the mixtures are larger than unity; therefore, the diffusionalthermal instability can be sufficiently suppressed As the content of H2 concentration increases in the fuel blends, as shown in Fig 4a and b, the effective Lewis numbers increase for rich and stoichiometric mixtures and decrease for lean mixtures Conversely, the effective Lewis numbers of premixed flames with propane addition increase for lean and stoichiometric mixtures and decrease for rich mixtures, as shown in Fig 4d This effect is due to opposite tendencies of the effective Lewis number of hydrogeneair flames and propaneeair flames [8,31] This indicates that a modulation of the a b c d Fig e Experimentally-measured critical radii for the onset of cellular instabilities for various equivalence ratios in CH4/H2/ COeair and C3H8/H2/COeair premixed flames i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 a b c d 6921 Fig e Experimentally-measured critical Peclet numbers for various equivalence ratios in CH4/H2/COeair and C3H8/H2/ COeair premixed flames diffusional-thermal instability is obtained for rich and stoichiometric flames of the amount of hydrogen increases, whereas this modulation is attained for lean and stoichiometric mixtures for the propane addition cases Fig 4c shows that the effective Lewis numbers in the premixed flames with methane addition always slightly decrease along with an increase in the methane fraction; thus, the addition of methane may not be effective to diminish the diffusionalthermal instability for syngaseair flames The effects of initial pressure to destabilize the flame front of the mixtures are observed by analyzing schlieren images of the premixed flames by adding 15% propane by volume, as shown in Fig Three important parameters (Leeff, s, lf) are tabulated at the bottom of the figure For Pu ¼ 0.1 MPa, the flame front remains smooth; however, the cells form earlier, and the size of the cells is smaller at higher Pu As the initial pressure increases, the diffusive-thermal effect does not affect the destabilization of the flame front because the effective Lewis number maintains the same amount of pressure change This effect can be attributed to the hydrodynamic instability, which is related to the thermal expansion ratio, s, and the flame thickness, lf The augmentation of cellular instabilities along with an increase in the initial pressure results from an enhancement of the hydrodynamic instability due to a substantial decrease in the flame thickness, whereas the thermal expansion ratio maintains nearly the same value The flame instability characteristics of hydrocarbon/ hydrogen/carbon monoxideeair flames with hydrogen enrichment as well as methane and propane additions are shown in Fig For hydrogen enrichment, Fig 6a and b indicates that the flame front destabilizations are promoted due to an enhancement in both the diffusional-thermal instability due to the decrease in the effective Lewis number and the hydrodynamic instability caused by the decrease in the flame thickness and the small changes in the thermal expansion ratio For the methane addition, Fig 6c indicates that there are no differences in the sequences of the flame front surfaces between the premixed flames with and without methane addition As the amount of methane addition in the fuel blend increases, the thermal expansion ratio maintains a nearly constant value, and the flame thickness slightly increases; thus, they only slightly affect the hydrodynamic effect However, the effective Lewis number slightly decreases to promote the diffusive-thermal effect The two effects combine and cause the premixed flames with methane addition to exhibit flame surfaces similar to the original flame Schlieren images of the premixed flames with propane added to the reactant mixtures are shown in Fig 6d, which indicate that the propensity of stabilization tends to be progressively promoted For propane addition, the thermal expansion ratio increases, and the flame thickness also increases Thus, the net effect of the two factors related to hydrodynamic instability is negligible The remaining parameter that affects the 6922 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 suppression of cellular instabilities is the effective Lewis number In this case, the effective Lewis number significantly increases along with an increase in the propane concentration; therefore, it will affect the flame front instabilities due to the suppression of the diffusive-thermal effect To examine the effects of methane and propane for suppression of cellular instabilities of H2/COeair flames, Fig shows a comparison of schlieren images of the flame surfaces in H2/COeair flames with methane and propane additions for ¼ 0.8 and Pu ¼ 0.2 MPa at similar a As shown in this figure, similar behavior of the flame front instabilities is observed for the H2/COeair and CH4/H2/COeair flames Furthermore, the cells form for the methane addition case, whereas wrinkles not form for the C3H8/H2/COeair flame, and only a few large cracks are observed The thermal expansion ratio and the flame thickness of H2/COeair flames with methane and propane additions are larger compared to those of the H2/ COeair flame Therefore, the net effect of hydrodynamic instability is negligible However, for the propane addition case, the effect of diffusional-thermal instability diminishes due to a significant increase in the effective Lewis number As a result, a combination of the two effects causes the H2/ COeair flame with propane addition to affect the cellular instabilities compared to the CH4/H2/COeair flame Therefore, the H2/COeair flame can be suppressed by adding propane to the fuel blend 4.3 Fig shows the experimentally-measured critical Peclet number, Pecr, which is defined as the critical radius normalized by the flame thickness, Pecr ¼ Rcr/lf, for various equivalence ratios of CH4/H2/COeair and C3H8/H2/COeair premixed flames As shown in Fig 9a and b, the critical Peclet number significantly decreases along with hydrogen enrichments, which indicates that the flame front instabilities appear at smaller radii Fig 9c indicates that the critical Peclet number maintains almost the same values for CH4/H2/COeair flames Meanwhile, the critical Peclet number increases significantly along with an increase in propane in the fuel blends, as shown in Fig 9d, which indicates that the stabilizing effect becomes stronger if propane is added to the H2/COeair mixtures For a given reactant mixture and overall equivalence ratio, the initial chamber pressure gradually increases in increments of 0.01 MPa The chamber pressure at which the flame loses its stability at a critical radius around 23e25 mm is defined as the critical initial pressure, Pcr [11,12] Fig 10a shows variations in critical initial pressures for ¼ 1.2 of hydrogen enrichment flames, and the critical initial pressures of H2/COeair flames with methane and propane additions are compared in Fig 10b The critical initial pressure decreases fairly linearly with hydrogen enrichment, which indicates that cells appear earlier and results in a smaller Pcr Fig 10b a Onset of cellular instabilities In this study, the onset condition of flame instabilities was obtained from the plot of the stretched flame velocity versus the flame stretch rate (Eq (6)) and is represented by the critical radius, Rcr In this case, the critical radius was detected at the moment when the burning velocity increased significantly and lost the linear relationship between Sn and K [9] The critical radius was also determined from the sequence of flame images in which the cells appeared spontaneously and uniformly over the entire flame surface Both methods had similar results Fig shows the critical radius at the onset of instabilities of the CH4/H2/COeair and C3H8/H2/COeair flames for different equivalence ratios For hydrogen enrichment, as shown in Fig 8a and b, the critical radii significantly decrease, which indicates that cells form at earlier stages, whereas the critical radius of the CH4/H2/COeair flames is slightly larger, as shown in Fig 8c For propane addition, Fig 8d shows that the critical radius significantly increases As a result, the larger critical radius of the C3H8/H2/COeair flame compared to that of the CH4/H2/COeair flame is in good agreement with the result of the comparison of the suppression for cellular instabilities between methane and propane additions to the H2/COeair flames shown in Fig This indicates that the onset of instabilities will be delayed if the propane concentration increases, whereas a similar situation occurs in the methane addition case The influence of diffusive-thermal effects on the results of flame instabilities is also shown in Fig 8d The critical radii of lean flames are smaller or larger than those of rich flames at small or large percentages of propane addition, respectively, because of an increase or decrease in the effective Lewis numbers in lean/rich H2/COeair flames with propane addition, as shown in Fig 4d b Fig 10 e (a) Critical initial pressures of CH4/H2/COeair and C3H8/H2/COeair with hydrogen enrichment at [ 1.2 and (b) Comparison of critical initial pressures of H2/COeair flames with methane and propane additions for [ 1.2 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 shows that the critical initial pressure of the flame with methane addition increases slightly, whereas the critical initial pressure in the H2/COeair flame with propane addition increases significantly This indicates that, for the initiation of cellular instabilities at a similar critical radius, the C3H8/H2/ COeair flame requires a higher initial pressure of the reactant mixture compared to the CH4/H2/COeair flame Conclusions An experimental study was conducted regarding the flame characteristics of CH4/H2/COeair and C3H8/H2/COeair premixed flames The following conclusions can be made: (1) The unstretched laminar burning velocities increase along with an increase in the hydrogen fraction and decrease along with the addition of hydrocarbons Conversely, the Markstein length decreases along with an increase in hydrogen enrichment, whereas it increases along with an increase in hydrocarbon addition, which indicates that the flame becomes stable or unstable along with an increase in the percentage of hydrocarbon or hydrogen, respectively (2) The modulation of diffusional-thermal instability is obtained for rich and stoichiometric mixtures of hydrogen enrichment flames as well as lean and stoichiometric mixtures of C3H8/H2/COeair flames due to an increase in the effective Lewis number Meanwhile, the addition of methane did not diminish the diffusional-thermal instability due to a decrease in the effective Lewis number (3) For an increase in the initial pressure, the onset of cellular instabilities is obtained at an earlier stage, and the flame is more unstable because of the enhancement of hydrodynamic instability due to a significant decrease in the flame thickness (4) The three parameters, Rcr, Pecr, Pcr, decrease for hydrogen enrichment, significantly increase for propane addition and maintain almost the same values for methane addition For hydrogen enrichment, this indicates that the flame will be more unstable and cellular instabilities will appear earlier However, the opposite tendency occurs such that cellular instabilities will be suppressed with propane addition Meanwhile, the H2/COeair flames could not be diminished for methane addition to the reactant mixtures Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0021916) references [1] Metghalchi M, Keck JC Laminar burning velocity of propaneeair mixtures at high temperature and pressure Combust Flame 1980;38:143e54 6923 [2] Gu XJ, Haq MZ, Lawes M, Woolley R Laminar burning velocity and Markstein lengths of methaneeair mixtures Combust Flame 2000;121:41e58 [3] Aung KT, Hassan MI, Faeth GM Flame stretch interactions of laminar premixed hydrogen/air flames at normal temperature and pressure Combust Flame 1997;109:1e24 [4] Qiao L, Kim CH, Faeth GM Suppression effects of diluents on laminar premixed hydrogen/oxygen/nitrogen flames Combust Flame 2005;143:79e96 [5] Park J, Lee DH, Yoon SH, Vu TM, Yun JH, Keel SI Effects of Lewis number and preferential diffusion on flame characteristics in 80% H2/20% CO syngas counterflow diffusion flames diluted with He and Ar Int J Hydrogen Energy 2009;34:1578e84 [6] Law CK, Kwon OC Effects of hydrocarbon substitution on atmospheric hydrogeneair flame propagation Int J Hydrogen Energy 2004;29:867e79 [7] Tse SD, Zhu DL, Law CK Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atmospheres Proc Combust Inst 2000;28:1793e800 [8] Tang C, He J, Huang Z, Jin C, Wang J, Wang X, et al Measurements of laminar burning velocities and Markstein lengths of propaneehydrogeneair mixtures at elevated pressures and temperatures Int J Hydrogen Energy 2008;33: 7274e85 [9] Hu E, Huang Z, He J, Zheng J, Miao H Measurements of laminar burning velocities and onset of cellular instabilities of methaneehydrogeneair flames at elevated pressures and temperatures Int J Hydrogen Energy 2009;34:5574e84 [10] Law CK, Jomaas G, Bechtold JK Cellular instabilities of expanding hydrogen/propane spherical flames at elevated pressures: theory and experiment Proc Combust Inst 2005; 30:159e67 [11] Vu TM, Park J, Kwon OB, Kim JS Effects of hydrocarbon addition on cellular instabilities in expanding syngaseair spherical premixed flames Int J Hydrogen Energy 2009;34: 6961e9 [12] Vu TM, Park J, Kwon OB, Bae DS, Yun JH, Keel SI Effects of diluents on cellular instabilities in outwardly propagating spherical syngaseair premixed flames Int J Hydrogen Energy 2010;35:3868e80 [13] Dowdy DR, Smith DB, Taylor SC, Williams A The use of expanding spherical flames to determine burning velocities and stretch effects in hydrogen/air mixtures Proc Combust Inst 1990;23:325e32 [14] Hassan MI, Aung KT, Faeth GM Measured and predicted properties of laminar premixed methane/air flames at various pressures Combust Flame 1998;115:539e50 [15] Tseng LK, Ismail MA, Faeth GM Laminar burning velocities and Markstein numbers of hydrocarbon/air flames Combust Flame 1993;95:410e26 [16] Clavin P Dynamic behavior of premixed flame fronts in laminar and turbulent flows Prog Energy Combust Sci 1985; 11:1e59 [17] Bradley D, Gaskell PH, Gu XJ Burning velocities, Markstein lengths, and flame quenching for spherical methaneeair flames: a computational study Combust Flame 1996;104: 176e98 [18] Law CK, Sung CJ Structure, aerodynamic, and geometry of premixed flamelets Prog Energy Combust Sci 2000;26:459e505 [19] Williams FA Combustion theory 2nd ed Redwood City, CA: Addison-Wesley; 1985 [20] Bradley D, Hicks RA, Lawes M, Sheppard CGW, Woolley R The measurement of laminar burning velocities and Markstein numbers for iso-octaneeair and iso-octane en-heptaneeair mixtures at elevated temperatures and pressures in an explosion bomb Combust Flame 1998;115: 126e44 6924 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y ( 1 ) e6 [21] Kee RJ, Grcar JF, Smooke MD, Miller Ja A Fortran program for modeling steady laminar one-dimensional premixed flames Report No SAND85e8240 Sandia National Laboratories; 1993 [22] Wang H, You X, Joshi AV, Davis SG, Laskin A, Egolfopoulos F, Law CK High temperature combustion reaction model of H2/ CO/C1eC4 compounds Available at [23] Kadowaki S, Suzuki H, Kobayashi H The unstable behavior of cellular premixed flames induced by intrinsic instability Proc Combust Inst 2005;30:169e76 [24] Kwon OC, Rozenchan G, Law CK Cellular instabilities and self-acceleration of outwardly propagating spherical flames Proc Combust Inst 2002;29:1775e83 [25] Bechtold JK, Matalon M Hydrodynamic and diffusion effects on the stability of spherically expanding flames Combust Flame 1987;67:77e90 [26] Addabbo R, Bechtold JK, Matalon M Wrinkling of spherically expanding flames Proc Combust Inst 2002;29:1527e35 [27] Kee RJ, Dixon-Lewis G, Warnatz J, Coltrin ME, Miller Ja A Fortran computer code package for the evaluation of gasphase, multi-component transport properties Report No SAND86e8246 Sandia National Laboratories; 1992 [28] Brown MJ, McLean IC, Smith DB, Taylor SC Markstein lengths of CO/H2/air flames, using expanding spherical flames Proc Combust Inst 1996;26:875e81 [29] Yu G, Law CK, Wu CK Laminar flame speeds of hydrocarbon air mixtures with hydrogen addition Combust Flame 1986; 63:339e47 [30] Wang J, Huang Z, Tang C, Miao H, Wang X Numerical study of the effect of hydrogen addition on methaneeair mixtures combustion Int J Hydrogen Energy 2009;34:1084e96 [31] Jomaas G, Law CK, Bechtold JK On transition to cellularity in expanding spherical flames J Fluid Mech 2007;583:1e26 ... flame front due to the formation of cellular instabilities is an interesting consideration Three effects are related to the cellularity of premixed flames In this study, the cellular instabilities. .. flames combined with the use of hydrocarbons, hydrogen, and carbon monoxide as fuels have been continuously studied In premixed flames, in addition to the laminar burning velocity, a corrugated flame... Rcr In response to the interest in controlling the unstable behavior of cellular flames, numerous studies have been conducted regarding cell formation in hydrogeneair and hydrogen/ hydrocarboneair