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Experimental and numerical investigation on diluted DME flames: Thermal and chemical kinetic effects on laminar flame speeds
Fuel 102 (2012) 567–573
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Experimental and numerical investigation on diluted DME flames: Thermal and chemical kinetic effects on laminar flame speeds Zhaoyang Chen, Chenglong Tang ⇑, Jin Fu, Xue Jiang, Qianqian Li, Liangjie Wei, Zuohua Huang ⇑ State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
h i g h l i g h t s " Experimental and numerical study on laminar flame speeds with CO2, N2 and Ar dilution is conducted. " Thermal and chemical kinetic effects on flame suppression are clarified. " Sensitivity analysis provides most important reactions of CO2 suppression on flame speeds.
a r t i c l e
i n f o
Article history: Received 5 September 2011 Received in revised form 18 April 2012 Accepted 2 June 2012 Available online 15 June 2012 Keywords: Dimethyl ether Laminar flame speed Dilution
a b s t r a c t Experiments and computational simulations were used to study effects of diluents on laminar flame speeds of stoichiometric, laminar, premixed dimethyl ether (DME)/air flames. The experiments were conducted in a constant volume bomb under initial temperature of 298 K and initial pressure of 0.1 MPa. Outwardly propagating spherical flames were used to measure the laminar flame speeds of mixtures with varying concentration of Ar, N2 or CO2 addition. Laminar flame speeds were also computed using the steady, one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics. Predictions showed a good agreement with experiments. For all the three diluting agents invested in this work, laminar flame speed decreases with the increase of dilution ratio, while for a given dilution ratio, the significance of suppression effect on laminar flame speed is in the order of CO2, N2 and Ar. The suppression effects of N2 and Ar on flame propagation are mainly caused by reduced reactant concentration and modified heat capacity, both of which result in a decreased flame temperature, while besides that, CO2 further influences the chemical kinetics because it is a major product of combustion. Further experimental study on flame speed and simulation study on flame structure at fixed adiabatic flame temperature were conducted to evidence this speculation and the possible reaction pathways which contribute to the chemical retarding effect by CO2 addition were recognized through sensitivity analysis. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Limited fossil fuel availability and increasingly stringent emission regulations are driving studies on alternative fuels. DME is the simplest aliphatic ether and it is considered as one of the most promising substitutes for natural gas, liquid petroleum gas, and diesel fuel because of its soot-free emission due to its high oxygen content [1], easy massive production through coal, oil residual, or bio-mass, safe on board transportation due to its low saturated vapor pressure, and excellent autoignition characteristics due to its high cetane number [2]. There are extensive studies on DME engine emissions and they show that DME produces much lower particulates than other fuels such as diesel or biodiesel when ⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (Z. Huang).
(C.
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.003
Tang),
operating under compression ignition conditions [3–6]. However, depending on the engine conditions, DME may produce comparative NOx emissions as diesel engine does [7,8], and it is even possible that DME may produce even higher NOx than diesel engine [6]. The decrease in NOx emission could be realized when DME fueled engines are operated combined with exhaust gas recirculation [5,7,9], which is a well recognized approach to reduce the flame temperature [10]. Laminar flame speed represents the overall reactivity, diffusivity and exothermicity of a combustible mixture [11] and the accurate determination of laminar flame speed is important for engine design, turbulent combustion modeling, and chemical kinetics validation [12]. Laminar flame speed of DME/air mixtures has been detected by researchers with different methods. Daly et al. obtained the burning velocity of dimethyl ether/air mixtures both by using photoelectric method and by high speed camera method [13], Qin et al. by using the spherical flame method [14] and Zhao
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Nomenclature Af
j P / t T0 P0 /r Le
flame front area stretch rate pressure equivalence ratio time initial temperature initial pressure dilution ratio Lewis number
et al. by using stagnation flame method [12]. With the development of computation capability, chemical kinetic mechanism simulation is extensively used to investigate 1D freely-propagating laminar flame [15–20], through which, the flame structure and flame speed can be simultaneously determined. However, there are few reports on laminar combustion studies of exhaust gas/ DME/air mixtures, neither experimentally, nor computationally. Thus one objective of the present study is to use different kinds of diluents such as Ar, N2, and CO2 as the EGR simulation gases and determine the laminar flame speed of diluents/DME/air mixtures experimentally and numerically so as to provide fundamental data for computational simulation of engine EGR. Mechanistically, the addition of chemically stimulating agents such as hydrogen into a fuel/air system may exert positive influence on the total thermodynamic properties represented by adiabatic flame temperature, the diffusion characteristics represented by Lewis number, and the chemical kinetics represented by the overall activation energy [21]. Oppositely, the addition of chemically passive agents may also influence these three aspects of the premixed system, but the mechanism of this influence is quite different from that reported in Ref. [21]. Thus the second objective of the present work is to study the suppression effect of the different dilution agents on laminar flame speed of DME/air mixtures. To simplify the system, stoichiometry mixtures were investigated as to exclude the influence of Lewis number. Finally, since the diluents such as N2 and Ar typically act as inert gases and do not participate in reactions, however, CO2 is a major product involved in the reactions and it’s inert as well. Thus the third objective of the present study is to examine whether CO2 will influence the flame structure and accordingly flame speed through chemical kinetics. To do so, experiments and computations were conducted for stoichiometry mixtures with three inert gases addition under a fixed adiabatic flame temperature. In this way, the coupled effect of thermal diffusivity and mass diffusivity on laminar flame speed, as represented by unity Lewis number, is excluded. Thus we can examine if the laminar flame speed is influenced by possible chemical kinetics under this condition. The possible reaction pathways that contribute, either positively, or negatively to flame speed, will be recognized. In the next section, the experimental and numerical specifications will be given and followed by presentations of experimental observations and numerical predictions of laminar flame speed for various diluting agents/DME/air mixture conditions. Finally, the details of suppression effects of different diluting agents would be discussed.
rf Lb Tad Sb S0b S0u S0u sim
q0b qu
instantaneous flame radius Markstein length of the burned gas adiabatic flame temperature stretched flame propagation speed unstretched flame propagation speed laminar flame speed, by experiment laminar flame speed, by calculation density of the burned gas density of the unburned gas
combustion chamber according to their corresponding partial pressures. Both sides of the chamber have optical assess for schlieren photography. After been left for at least 5 min, the mixtures were then ignited by the spark electrodes located in the center of the chamber. The signal that triggers the spark is synchronized with the signal that triggers the high speed camera operating at 2000–10,000 frames per second as well as the pressure data acquisition oscilloscope with sampling frequency of 10,000 Hz. In this way, the flame radius history rf t and pressure history p–t can be readily recorded for further analyses. All the experiments were conducted at initial temperature of 298 K and initial pressure of 1 atmosphere. The initial temperature of mixtures in the vessel was measured by a thermocouple placed inside the chamber with accuracy of 1 K and controlled around 298 K with a maximum variation of ±3 K. The pressure gauge has an uncertainty of less than 1%. Dilution ratio, /r , is defined as the volumetric fraction of diluents (Ar/N2/CO2) addition in the combustible mixtures.
/r ¼
V diluents V air þ V fuel þ V diluents
where Vdiluents, Vfuel and Vair are the volume fractions of diluents, fuel and air in the combustible mixtures, respectively. The stretched flame speed Sb, was obtained from the raw data of flame radius history rf(t) and the stretch rate were then evaluated, through Sb ¼ dr f =dt and j ¼ d ln Af =dt ¼ 2=r f Sb , where Af is the flame front area and the subscript ‘‘b’’ designates the burned gas. The unstretched flame speed, S0b , was obtained by linearly extrapolating Sb to zero flame stretch rate [24,25] through Sb ¼ S0b Lb j, where j is the flame stretch rate and Lb is the burned gas Markstein length. Finally, the upstream laminar flame speed was determined
2. Experimental and numerical specifications Details of the experimentations can be found in Refs. [22,23]. Here only a brief introduction will be given. Mixtures of DME/air with or without diluents were prepared in a cylindrical
ð1Þ
Fig. 1. Stretched flame speed and the relative pressure rate.
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Z. Chen et al. / Fuel 102 (2012) 567–573 Table 1 The test conditions and major results. S0b (mm/s)
Lb (mm)
qu =q0b
S0u (cm/s)
Error of S0u (cm/s)a
S0u
367.9 318.1 286.1 255.8 214.7 305.1 255.0 196.3 165.2 226.6 169.1 114.6 66.6
0.1037 0.1022 0.1072 0.1175 0.1364 0.1149 0.1352 0.1697 0.2126 0.1268 0.1587 0.1904 0.2918
8.3 8.1 8.0 7.8 7.6 8.0 7.8 7.6 7.3 7.9 7.5 7.1 6.8
47.2 42.8 38.9 35.3 31.4 40.9 35.0 29.5 25.8 33.7 25.1 18.3 12.9
1.0 0.9 1.2 0.9 0.8 1.2 0.8 1.1 0.9 1.1 1.3 1.5 1.8
48.0 44.2 40.3 36.7 33.2 42.2 36.9 31.2 25.9 35.1 25.6 18.2 12.1
P0 = 0.1 MPa, T0 = 298 ± 0.5 K, / ¼ 1:0, Tad = 2185 ± 0.5 K Ar 0.15 2185 255.8 N2 0.10 2185 255.0 CO2 0.059 2184 252.8
0.1175 0.1352 0.1305
7.8 7.8 7.9
35.3 35.0 32.0
0.9 0.8 1.2
36.7 36.9 33.5
Diluents
/r
Tad (K)
P0 = 0.1 MPa, T0 = 298 ± 0.5 K, / ¼ 1:0 – 0.00 2295 Ar 0.05 2265 Ar 0.10 2227 Ar 0.15 2185 Ar 0.20 2146 N2 0.05 2247 N2 0.10 2190 N2 0.15 2125 N2 0.20 2053 CO2 0.05 2204 CO2 0.10 2102 CO2 0.15 2003 CO2 0.20 1893
a
sim
(cm/s)
The uncertainty reported here is standard deviation.
Fig. 3. Comparison of flame speeds with other literatures.
Fig. 2. Stretched flame speed as function of stretch rate.
according to mass conservation across the flame surface, i.e. S0u ¼ q0b =qu S0b . The flame radius in the vertical direction was used for data processing because horizontal flame propagation was constantly affected by electrode pair, as shown by the inset picture in Fig. 1. It is further noted that when the spherically propagating flame
Fig. 4. Laminar flame speeds as functions of dilution ratios in case of Ar, N2 or CO2 dilution.
method is used for determination of laminar flame speeds, the raw flame radius data should be carefully selected for meaningful data processing. First, the initial radius should be masked because
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Fig. 5. Adiabatic flame temperature versus dilution ratio.
Fig. 8. Production rate of H, O and OH free radicals.
Fig. 6. Concentration of reactants and specific heat of mixtures as functions of dilution ratio.
Fig. 9. Mole fraction profiles of radicals for different diluents added mixtures under the same flame temperature.
Fig. 7. Laminar flame speed as function of adiabatic flame temperature.
those data are affected by spark ignition energy, evidenced by the abnormally high flame speed at the initial flame propagation in Fig. 1. As the flame is propagating to a certain flame radius, the influence of spark energy gradually vanishes. Bradley et al. [24] and Huang et al. [26] showed that this flame radius limit is about 5–6 mm. In this work, we have examined the effects of ignition energy and it shows that the flame radius trajectories with different
Fig. 10. Sensitivity coefficients of the most important reactions on the mass burning rate of diluted stoichiometric mixtures under the same adiabatic flame temperature condition.
Z. Chen et al. / Fuel 102 (2012) 567–573
ignition energy do overlap after around 5–6 mm, which is defined as the lower flame radius limit in Fig. 1. On the other hand, the final stage of flame propagation is affected by increased pressure and chamber confinement. The upper flame radius limit for meaningful data processing was selected to be 26 mm, as also shown Fig. 1. For flame radius less than this upper limit, the pressure rise was less than 4%, thus the influence of pressure rise was negligible. Furthermore, this upper flame radius limit is less than 30% of the wall radius, thus the effects of chamber confinement on flame propagation can also be neglected, as proposed in Ref. [27]. The 1D freely propagating adiabatic laminar premixed flames were simulated by using the PREMIX code [28] in conjunction with the CHEMKIN package [29] and TRANSPORT package. Mechanism of Daly et al. [13], Kaiser et al. [30] and Zhao et al. [15] were used for the computation. To solve the energy equation, upwind differencing on the convective term was used and the transport properties use the mixture averaged model. The converged solution was obtained for a large number of grid points by considering the gradient and curvature to be 0.05 and 0.02 respectively, further refinement has been tried, but no appreciable difference was found. Detailed kinetic mechanism of DME oxidation had been prevalently developed. Comparisons showed that the predictions based on these three mechanisms gave higher values than the measurements of this study. However, the performance of Zhao’s mechanism [15] shows the closed agreement with the present measurement. This mechanism was also proved to be good at predictions for high temperature flow reactor data [16], for jet-stirred reactor data [18], for high-temperature shock tube ignition [17] and for laminar flame speeds of DME–air mixtures [20]. The numerical simulations of flames reported here are based on the DME oxidation chemical kinetic mechanism of Zhao et al. [15]. This mechanism consists of 55 species and 290 reversible elementary reactions. The test conditions and major results (adiabatic flame temperature, unstretched flame propagating speed, burned gas Markstein length, density ratio, laminar flame speed and its error by experiment, and laminar flame speed by simulation) of this study are summarized in Table 1.
3. Results and discussion 3.1. Stretch affected flame propagating Fig. 2 shows the stretched flame speed as functions of stretch rate for (a) Ar/DME/air mixtures with increasing dilution ratio, and for (b) different diluting agents/DME/air mixtures with ur = 0.1. It is seen that for all the mixture conditions here, stretched flame speed is approximately linearly related to stretch rate, and it decreases with increasing stretch rate, indicating that the flame is propagating gradually faster. Fig. 2a shows that with the increase of ur, stretched flame speed decreases significantly, but the slope of Sb–j line, the negative value of which is the burned gas Markstein length reflecting the stretch effect of the flame, does not change much. This experimental observation indicates that the Ar addition significantly suppresses flame propagation but does not necessarily change the flame stretch effect. This is reasonable since the mixtures considered here are all stoichiometric thus have a near unity Lewis number, which minimizes the effect of stretch on flame propagation speed. Fig. 2b shows that for a given dilution ratio, suppression effect of diluents is in the order of significance of CO2, N2 and Ar, and again, different diluting agents do not show much difference in the slope of flame propagation speed and stretch rate, which means dilution addition slightly influences the Markstein length.
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3.2. Results of DME/air mixtures Present experimental measurements, experimental results of previous literatures, and numerically computation of laminar flame speeds of DME/air mixture as a function of equivalence ratio were shown in Fig. 3. Measurements of Zhao et al. [12] with stagnation flame configuration agree well with most of the literatures on the lean side, but on the rich side, their measurements were consistently 7–8 cm/s higher than most of the literature values and the present work. Rich stagnation flames are unsteady due to the interference of the diffusion flame with the solid wall or main jet [31]. In Daly et al.’s measurements, two techniques were used. One was the photoelectric method, in which, neither flame stretch nor flame front information was considered and only luminosity information at three fixed points were converted to the flame speed. The other one was the high speed camera method, however, with this method, the flame was initiated by the spark between the electrode and the wall and thus the flame was not symmetric. Measurements of Qin et al. [14] with spherical flame method showed good agreement with Daly’s results for lean mixtures, while remarkable differences were also existed in the rich side. Computed laminar flame speed of DME/air mixtures with mechanism of Zhao et al. [15] agree best with most of the existed literature values as well as the present measurements. Thus this mechanism was selected for computing laminar flame speed of different diluting agents/DME/air mixtures. 3.3. Effects of diluting agents on laminar flame speed and adiabatic flame temperature Fig. 4 shows the laminar flame speeds of different diluting agents/DME/air mixtures as functions of dilution ratio at stoichiometric condition. Measurements and predictions of laminar flame speed using Zhao’s mechanism agree well for DME/air/diluents mixture. Both measurements and computations show that laminar flame speed of Ar/DME/air and N2/DME/air mixture decrease approximately linearly with the increase of dilution ratio, while for CO2/DME/air mixtures the laminar flame speed decreases nonlinearly. For a given dilution ratio, the suppression effect of CO2 on laminar flame speed is the most significant, followed by N2 and finally Ar. Tang et al. stated that by adding chemically stimulating hydrogen into a fuel/air system, the laminar flame speed increases due to the positive influence of modified diffusivity represented by Lewis number, thermal property represented by adiabatic temperature and chemical kinetics represented by activation energy [21]. Oppositely, it is supposed that by addition of chemically passive agents selected here will also influence these three aspects, but negatively. It is noticed that the mixtures investigated here were stoichiometric, thus the influence of Lewis number vanished. So, only the influence of the diluting agents on adiabatic temperature will be studied next. The adiabatic flame temperatures of mixtures as a function of dilution ratio for three diluting agents were shown in Fig. 5. Similar behavior as that of laminar flame speed was observed. The suppression effect of CO2 is again the most significant while that of Ar is the weakest. The reason is as follows. If the initial diluents/fuel/air system is assumed at temperature Tu, and goes through complete combustion as shown in the following equation
nF F þ nO O þ nD D ! nP P þ nD D Assuming constant specific heat, the adiabatic flame temperature can be simply evaluated through
T ad ¼ ðnF hF þ nO hO nP hP Þ=cp ðnD þ nP Þ þ T u
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where n and h is respectively the mole fraction and enthalpy of the species, subscripts F, O, D, P respectively designate fuel, oxidizer, diluents and product. It is then seen that the addition of chemically passive agents into a fuel/oxidizer system will dilute the fuel and oxidizer concentration through increased nD (thus decreased nF), as shown in Fig. 6a. In addition, the specific heat cp is modified, as shown in Fig. 6b. For Ar dilution case, the specific heat actually decreases, which favors the increase of adiabatic temperature, however, the concentration of the fuel decreases, leading to the decrease in the total heat of combustion, thus the decrease of adiabatic temperature with the increase of Ar dilution ratio is resulted from the competition between reduced reactant concentration and decreased specific heat. For N2 dilution case, the specific heat is only weakly dependent on dilution ratio, thus the decreased flame temperature is dominantly affected by decreased reactant concentration. For CO2 dilution case, the specific heat increases with the increase of dilution ratio, which favors the decrease in adiabatic temperature. 3.4. Chemical kinetic effects of diluting agents Ar and N2 are all chemically passive agents and have been constantly treated as non reactive bulk gases that do not participate in combustion. However, CO2 is a major product of combustion while it is chemically passive as well. Adding CO2 into fuel/air system may possibly influence the chemical kinetics and thus the laminar flame speed. CO2 modifies the flame kinetics in two main ways. First, the reverse of the reaction, CO + OH = CO2 + H, decreases the H atom concentration and weakens the flame. Second, dilution with CO2 results in an overall stronger third-body efficiency of the mixture than dilution with Ar or N2 [32,33]. Furthermore, compared to the chain branching reactions, the third-body reactions are comparatively more important than the chain branching reactions only when the flame is very weak or pressure is high [33–37]. Flames considered here are under atmosphere pressure and even with 20% CO2 as diluents, are still far away from low flame temperature of flammability limit. Therefore, in present study, the role of CO2 as a third-body is not as important as that of CO2 as a reactant. To examine the possible chemical kinetic effect, experiments and computations were conducted for all three different diluting agents, and the adiabatic temperatures were fixed at stoichiometric condition, so that diffusivity and thermal effects were excluded. Fig. 7 shows the measured and predicted laminar flame speed as a function of adiabatic temperature for three diluting agents/ DME/air mixtures at stoichiometric condition. The computed results show that flame speed of Ar and N2 almost completely overlapped while the laminar flame speed of CO2 is systematically lower. Experimental measurement at adiabatic temperature of 2185 K shows that the laminar flame speed of Ar/DME/air and N2/DME/air is almost the same, while the laminar flame speed of CO2/DME/air is about 3–4 cm/s lower. Since the influence of Lewis number and adiabatic temperature were excluded, the only effect that could result in the lower flame speed might be the retarded chemical kinetics by CO2. It is noted in Tang et al.’s work [21] that the chemical kinetic effect was represented by overall activation energy. This activation energy approach can’t be used here because the one-step overall reaction assumption for extracting activation energy is not valid anymore for stoichiometric condition. However, the flame structure can still be examined to identify the chemical kinetic effects. Figs. 8 and 9 show the computed rate of production and mole fraction of H, O and OH as functions of distance through the flame for three diluting agents/DME/air mixtures at adiabatic temperature of 2185 K. It should be noticed that the origins of the distance in these figures do not correspond to the central ignition point. The x-axis represents the computational domain, which is read from
the PREMIX solution file. It is seen that, the production rates and mole fractions of OH, H and O radicals of mixtures with Ar and N2 dilution almost over lapped, while the production rate curve in the CO2 dilution case was systematically lower and shifted to post reaction zone. This phenomenon is corresponding to the lower laminar flame speed of CO2 diluted mixtures than that of mixtures with Ar or N2 addition, displayed in Fig. 7. Chemical kinetic effects of CO2 in terms of decreased production rate and concentration of typical free radicals were believed to be the main reason for the further decrease in laminar flame speed because those parallel computations with Ar or N2 diluting were conducted under the same thermal and diffusional conditions. To further identify the chemical kinetic effects of CO2, sensitivity analysis was performed at adiabatic flame temperature of 2185 K and the most important chemical reactions on the mass burning rate under different kinds of dilution were scrutinized, as shown in Fig. 10. The difference between Ar case and N2 case was negligible while the sensitivity coefficients were more significantly affected with CO2 dilution. The two most sensitive chemical reactions are R1: H + O2 ? O + OH and R29: CO + OH ? CO2 + H. For R29, which produces H radical, the sensitivity coefficients on mass burning rate in the CO2 case was lower than that of Ar and N2 case. With the addition of external CO2, less H was produced and this H production reaction was less sensitive. Thus though the chain branching reaction R1 has a higher sensitivity coefficients in the CO2 diluting case, because the reaction was progressed under a lower concentration level of H radical, the net effect is that the overall radical pool concentration was reduced, as manifested by Fig. 9, leading to the further decrease in laminar flame speed as shown in Fig. 7. 4. Conclusions Different suppression effects of argon, nitrogen or carbon dioxide dilution on laminar flame speeds were investigated experimentally and numerically. The main conclusions are summarized as follows: (1) Predictions and measurements of laminar flame speeds are in good agreement for all the determined conditions, no matter what equivalence ratios, dilution ratios and kinds of dilution are. (2) For all the three diluting agents invested, laminar flame speed decreases with the increase of dilution ratio. For given dilution ratio, the significance of suppression effect on flame speed decreases in the order of CO2, N2 and Ar. (3) Adiabatic flame temperatures of mixture have large effluence on the laminar flame speeds. For a specific adiabatic flame temperature, laminar flame speed and free radicals concentrations of flames with Ar or N2 dilution have the approximate values, while those of flames with CO2 dilution have lower values, and the flame structure shows that CO2 addition reduces the typical radical concentrations. (4) Further discussions on sensitivity analysis at given adiabatic temperature shows that reaction H + O2 ? O + OH and CO + OH ? CO2 are the most sensitive on laminar burning flux and the addition of CO2 chemically reduced H radical concentration, leading to the decrease in concentration of other free radicals such as O and OH and thus CO2 addition reduces laminar flame speed through chemical kinetics was recognized.
Acknowledgments This work is supported by National Natural Science Foundation of China (51136005, 51121092).
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Experimental and numerical investigation on diluted DME flames: Thermal and chemical kinetic effects on laminar flame speeds Zhaoyang Chen, Chenglong Tang ⇑, Jin Fu, Xue Jiang, Qianqian Li, Liangjie Wei, Zuohua Huang ⇑ State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
h i g h l i g h t s " Experimental and numerical study on laminar flame speeds with CO2, N2 and Ar dilution is conducted. " Thermal and chemical kinetic effects on flame suppression are clarified. " Sensitivity analysis provides most important reactions of CO2 suppression on flame speeds.
a r t i c l e
i n f o
Article history: Received 5 September 2011 Received in revised form 18 April 2012 Accepted 2 June 2012 Available online 15 June 2012 Keywords: Dimethyl ether Laminar flame speed Dilution
a b s t r a c t Experiments and computational simulations were used to study effects of diluents on laminar flame speeds of stoichiometric, laminar, premixed dimethyl ether (DME)/air flames. The experiments were conducted in a constant volume bomb under initial temperature of 298 K and initial pressure of 0.1 MPa. Outwardly propagating spherical flames were used to measure the laminar flame speeds of mixtures with varying concentration of Ar, N2 or CO2 addition. Laminar flame speeds were also computed using the steady, one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics. Predictions showed a good agreement with experiments. For all the three diluting agents invested in this work, laminar flame speed decreases with the increase of dilution ratio, while for a given dilution ratio, the significance of suppression effect on laminar flame speed is in the order of CO2, N2 and Ar. The suppression effects of N2 and Ar on flame propagation are mainly caused by reduced reactant concentration and modified heat capacity, both of which result in a decreased flame temperature, while besides that, CO2 further influences the chemical kinetics because it is a major product of combustion. Further experimental study on flame speed and simulation study on flame structure at fixed adiabatic flame temperature were conducted to evidence this speculation and the possible reaction pathways which contribute to the chemical retarding effect by CO2 addition were recognized through sensitivity analysis. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Limited fossil fuel availability and increasingly stringent emission regulations are driving studies on alternative fuels. DME is the simplest aliphatic ether and it is considered as one of the most promising substitutes for natural gas, liquid petroleum gas, and diesel fuel because of its soot-free emission due to its high oxygen content [1], easy massive production through coal, oil residual, or bio-mass, safe on board transportation due to its low saturated vapor pressure, and excellent autoignition characteristics due to its high cetane number [2]. There are extensive studies on DME engine emissions and they show that DME produces much lower particulates than other fuels such as diesel or biodiesel when ⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (Z. Huang).
(C.
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.003
Tang),
operating under compression ignition conditions [3–6]. However, depending on the engine conditions, DME may produce comparative NOx emissions as diesel engine does [7,8], and it is even possible that DME may produce even higher NOx than diesel engine [6]. The decrease in NOx emission could be realized when DME fueled engines are operated combined with exhaust gas recirculation [5,7,9], which is a well recognized approach to reduce the flame temperature [10]. Laminar flame speed represents the overall reactivity, diffusivity and exothermicity of a combustible mixture [11] and the accurate determination of laminar flame speed is important for engine design, turbulent combustion modeling, and chemical kinetics validation [12]. Laminar flame speed of DME/air mixtures has been detected by researchers with different methods. Daly et al. obtained the burning velocity of dimethyl ether/air mixtures both by using photoelectric method and by high speed camera method [13], Qin et al. by using the spherical flame method [14] and Zhao
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Nomenclature Af
j P / t T0 P0 /r Le
flame front area stretch rate pressure equivalence ratio time initial temperature initial pressure dilution ratio Lewis number
et al. by using stagnation flame method [12]. With the development of computation capability, chemical kinetic mechanism simulation is extensively used to investigate 1D freely-propagating laminar flame [15–20], through which, the flame structure and flame speed can be simultaneously determined. However, there are few reports on laminar combustion studies of exhaust gas/ DME/air mixtures, neither experimentally, nor computationally. Thus one objective of the present study is to use different kinds of diluents such as Ar, N2, and CO2 as the EGR simulation gases and determine the laminar flame speed of diluents/DME/air mixtures experimentally and numerically so as to provide fundamental data for computational simulation of engine EGR. Mechanistically, the addition of chemically stimulating agents such as hydrogen into a fuel/air system may exert positive influence on the total thermodynamic properties represented by adiabatic flame temperature, the diffusion characteristics represented by Lewis number, and the chemical kinetics represented by the overall activation energy [21]. Oppositely, the addition of chemically passive agents may also influence these three aspects of the premixed system, but the mechanism of this influence is quite different from that reported in Ref. [21]. Thus the second objective of the present work is to study the suppression effect of the different dilution agents on laminar flame speed of DME/air mixtures. To simplify the system, stoichiometry mixtures were investigated as to exclude the influence of Lewis number. Finally, since the diluents such as N2 and Ar typically act as inert gases and do not participate in reactions, however, CO2 is a major product involved in the reactions and it’s inert as well. Thus the third objective of the present study is to examine whether CO2 will influence the flame structure and accordingly flame speed through chemical kinetics. To do so, experiments and computations were conducted for stoichiometry mixtures with three inert gases addition under a fixed adiabatic flame temperature. In this way, the coupled effect of thermal diffusivity and mass diffusivity on laminar flame speed, as represented by unity Lewis number, is excluded. Thus we can examine if the laminar flame speed is influenced by possible chemical kinetics under this condition. The possible reaction pathways that contribute, either positively, or negatively to flame speed, will be recognized. In the next section, the experimental and numerical specifications will be given and followed by presentations of experimental observations and numerical predictions of laminar flame speed for various diluting agents/DME/air mixture conditions. Finally, the details of suppression effects of different diluting agents would be discussed.
rf Lb Tad Sb S0b S0u S0u sim
q0b qu
instantaneous flame radius Markstein length of the burned gas adiabatic flame temperature stretched flame propagation speed unstretched flame propagation speed laminar flame speed, by experiment laminar flame speed, by calculation density of the burned gas density of the unburned gas
combustion chamber according to their corresponding partial pressures. Both sides of the chamber have optical assess for schlieren photography. After been left for at least 5 min, the mixtures were then ignited by the spark electrodes located in the center of the chamber. The signal that triggers the spark is synchronized with the signal that triggers the high speed camera operating at 2000–10,000 frames per second as well as the pressure data acquisition oscilloscope with sampling frequency of 10,000 Hz. In this way, the flame radius history rf t and pressure history p–t can be readily recorded for further analyses. All the experiments were conducted at initial temperature of 298 K and initial pressure of 1 atmosphere. The initial temperature of mixtures in the vessel was measured by a thermocouple placed inside the chamber with accuracy of 1 K and controlled around 298 K with a maximum variation of ±3 K. The pressure gauge has an uncertainty of less than 1%. Dilution ratio, /r , is defined as the volumetric fraction of diluents (Ar/N2/CO2) addition in the combustible mixtures.
/r ¼
V diluents V air þ V fuel þ V diluents
where Vdiluents, Vfuel and Vair are the volume fractions of diluents, fuel and air in the combustible mixtures, respectively. The stretched flame speed Sb, was obtained from the raw data of flame radius history rf(t) and the stretch rate were then evaluated, through Sb ¼ dr f =dt and j ¼ d ln Af =dt ¼ 2=r f Sb , where Af is the flame front area and the subscript ‘‘b’’ designates the burned gas. The unstretched flame speed, S0b , was obtained by linearly extrapolating Sb to zero flame stretch rate [24,25] through Sb ¼ S0b Lb j, where j is the flame stretch rate and Lb is the burned gas Markstein length. Finally, the upstream laminar flame speed was determined
2. Experimental and numerical specifications Details of the experimentations can be found in Refs. [22,23]. Here only a brief introduction will be given. Mixtures of DME/air with or without diluents were prepared in a cylindrical
ð1Þ
Fig. 1. Stretched flame speed and the relative pressure rate.
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Z. Chen et al. / Fuel 102 (2012) 567–573 Table 1 The test conditions and major results. S0b (mm/s)
Lb (mm)
qu =q0b
S0u (cm/s)
Error of S0u (cm/s)a
S0u
367.9 318.1 286.1 255.8 214.7 305.1 255.0 196.3 165.2 226.6 169.1 114.6 66.6
0.1037 0.1022 0.1072 0.1175 0.1364 0.1149 0.1352 0.1697 0.2126 0.1268 0.1587 0.1904 0.2918
8.3 8.1 8.0 7.8 7.6 8.0 7.8 7.6 7.3 7.9 7.5 7.1 6.8
47.2 42.8 38.9 35.3 31.4 40.9 35.0 29.5 25.8 33.7 25.1 18.3 12.9
1.0 0.9 1.2 0.9 0.8 1.2 0.8 1.1 0.9 1.1 1.3 1.5 1.8
48.0 44.2 40.3 36.7 33.2 42.2 36.9 31.2 25.9 35.1 25.6 18.2 12.1
P0 = 0.1 MPa, T0 = 298 ± 0.5 K, / ¼ 1:0, Tad = 2185 ± 0.5 K Ar 0.15 2185 255.8 N2 0.10 2185 255.0 CO2 0.059 2184 252.8
0.1175 0.1352 0.1305
7.8 7.8 7.9
35.3 35.0 32.0
0.9 0.8 1.2
36.7 36.9 33.5
Diluents
/r
Tad (K)
P0 = 0.1 MPa, T0 = 298 ± 0.5 K, / ¼ 1:0 – 0.00 2295 Ar 0.05 2265 Ar 0.10 2227 Ar 0.15 2185 Ar 0.20 2146 N2 0.05 2247 N2 0.10 2190 N2 0.15 2125 N2 0.20 2053 CO2 0.05 2204 CO2 0.10 2102 CO2 0.15 2003 CO2 0.20 1893
a
sim
(cm/s)
The uncertainty reported here is standard deviation.
Fig. 3. Comparison of flame speeds with other literatures.
Fig. 2. Stretched flame speed as function of stretch rate.
according to mass conservation across the flame surface, i.e. S0u ¼ q0b =qu S0b . The flame radius in the vertical direction was used for data processing because horizontal flame propagation was constantly affected by electrode pair, as shown by the inset picture in Fig. 1. It is further noted that when the spherically propagating flame
Fig. 4. Laminar flame speeds as functions of dilution ratios in case of Ar, N2 or CO2 dilution.
method is used for determination of laminar flame speeds, the raw flame radius data should be carefully selected for meaningful data processing. First, the initial radius should be masked because
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Fig. 5. Adiabatic flame temperature versus dilution ratio.
Fig. 8. Production rate of H, O and OH free radicals.
Fig. 6. Concentration of reactants and specific heat of mixtures as functions of dilution ratio.
Fig. 9. Mole fraction profiles of radicals for different diluents added mixtures under the same flame temperature.
Fig. 7. Laminar flame speed as function of adiabatic flame temperature.
those data are affected by spark ignition energy, evidenced by the abnormally high flame speed at the initial flame propagation in Fig. 1. As the flame is propagating to a certain flame radius, the influence of spark energy gradually vanishes. Bradley et al. [24] and Huang et al. [26] showed that this flame radius limit is about 5–6 mm. In this work, we have examined the effects of ignition energy and it shows that the flame radius trajectories with different
Fig. 10. Sensitivity coefficients of the most important reactions on the mass burning rate of diluted stoichiometric mixtures under the same adiabatic flame temperature condition.
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ignition energy do overlap after around 5–6 mm, which is defined as the lower flame radius limit in Fig. 1. On the other hand, the final stage of flame propagation is affected by increased pressure and chamber confinement. The upper flame radius limit for meaningful data processing was selected to be 26 mm, as also shown Fig. 1. For flame radius less than this upper limit, the pressure rise was less than 4%, thus the influence of pressure rise was negligible. Furthermore, this upper flame radius limit is less than 30% of the wall radius, thus the effects of chamber confinement on flame propagation can also be neglected, as proposed in Ref. [27]. The 1D freely propagating adiabatic laminar premixed flames were simulated by using the PREMIX code [28] in conjunction with the CHEMKIN package [29] and TRANSPORT package. Mechanism of Daly et al. [13], Kaiser et al. [30] and Zhao et al. [15] were used for the computation. To solve the energy equation, upwind differencing on the convective term was used and the transport properties use the mixture averaged model. The converged solution was obtained for a large number of grid points by considering the gradient and curvature to be 0.05 and 0.02 respectively, further refinement has been tried, but no appreciable difference was found. Detailed kinetic mechanism of DME oxidation had been prevalently developed. Comparisons showed that the predictions based on these three mechanisms gave higher values than the measurements of this study. However, the performance of Zhao’s mechanism [15] shows the closed agreement with the present measurement. This mechanism was also proved to be good at predictions for high temperature flow reactor data [16], for jet-stirred reactor data [18], for high-temperature shock tube ignition [17] and for laminar flame speeds of DME–air mixtures [20]. The numerical simulations of flames reported here are based on the DME oxidation chemical kinetic mechanism of Zhao et al. [15]. This mechanism consists of 55 species and 290 reversible elementary reactions. The test conditions and major results (adiabatic flame temperature, unstretched flame propagating speed, burned gas Markstein length, density ratio, laminar flame speed and its error by experiment, and laminar flame speed by simulation) of this study are summarized in Table 1.
3. Results and discussion 3.1. Stretch affected flame propagating Fig. 2 shows the stretched flame speed as functions of stretch rate for (a) Ar/DME/air mixtures with increasing dilution ratio, and for (b) different diluting agents/DME/air mixtures with ur = 0.1. It is seen that for all the mixture conditions here, stretched flame speed is approximately linearly related to stretch rate, and it decreases with increasing stretch rate, indicating that the flame is propagating gradually faster. Fig. 2a shows that with the increase of ur, stretched flame speed decreases significantly, but the slope of Sb–j line, the negative value of which is the burned gas Markstein length reflecting the stretch effect of the flame, does not change much. This experimental observation indicates that the Ar addition significantly suppresses flame propagation but does not necessarily change the flame stretch effect. This is reasonable since the mixtures considered here are all stoichiometric thus have a near unity Lewis number, which minimizes the effect of stretch on flame propagation speed. Fig. 2b shows that for a given dilution ratio, suppression effect of diluents is in the order of significance of CO2, N2 and Ar, and again, different diluting agents do not show much difference in the slope of flame propagation speed and stretch rate, which means dilution addition slightly influences the Markstein length.
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3.2. Results of DME/air mixtures Present experimental measurements, experimental results of previous literatures, and numerically computation of laminar flame speeds of DME/air mixture as a function of equivalence ratio were shown in Fig. 3. Measurements of Zhao et al. [12] with stagnation flame configuration agree well with most of the literatures on the lean side, but on the rich side, their measurements were consistently 7–8 cm/s higher than most of the literature values and the present work. Rich stagnation flames are unsteady due to the interference of the diffusion flame with the solid wall or main jet [31]. In Daly et al.’s measurements, two techniques were used. One was the photoelectric method, in which, neither flame stretch nor flame front information was considered and only luminosity information at three fixed points were converted to the flame speed. The other one was the high speed camera method, however, with this method, the flame was initiated by the spark between the electrode and the wall and thus the flame was not symmetric. Measurements of Qin et al. [14] with spherical flame method showed good agreement with Daly’s results for lean mixtures, while remarkable differences were also existed in the rich side. Computed laminar flame speed of DME/air mixtures with mechanism of Zhao et al. [15] agree best with most of the existed literature values as well as the present measurements. Thus this mechanism was selected for computing laminar flame speed of different diluting agents/DME/air mixtures. 3.3. Effects of diluting agents on laminar flame speed and adiabatic flame temperature Fig. 4 shows the laminar flame speeds of different diluting agents/DME/air mixtures as functions of dilution ratio at stoichiometric condition. Measurements and predictions of laminar flame speed using Zhao’s mechanism agree well for DME/air/diluents mixture. Both measurements and computations show that laminar flame speed of Ar/DME/air and N2/DME/air mixture decrease approximately linearly with the increase of dilution ratio, while for CO2/DME/air mixtures the laminar flame speed decreases nonlinearly. For a given dilution ratio, the suppression effect of CO2 on laminar flame speed is the most significant, followed by N2 and finally Ar. Tang et al. stated that by adding chemically stimulating hydrogen into a fuel/air system, the laminar flame speed increases due to the positive influence of modified diffusivity represented by Lewis number, thermal property represented by adiabatic temperature and chemical kinetics represented by activation energy [21]. Oppositely, it is supposed that by addition of chemically passive agents selected here will also influence these three aspects, but negatively. It is noticed that the mixtures investigated here were stoichiometric, thus the influence of Lewis number vanished. So, only the influence of the diluting agents on adiabatic temperature will be studied next. The adiabatic flame temperatures of mixtures as a function of dilution ratio for three diluting agents were shown in Fig. 5. Similar behavior as that of laminar flame speed was observed. The suppression effect of CO2 is again the most significant while that of Ar is the weakest. The reason is as follows. If the initial diluents/fuel/air system is assumed at temperature Tu, and goes through complete combustion as shown in the following equation
nF F þ nO O þ nD D ! nP P þ nD D Assuming constant specific heat, the adiabatic flame temperature can be simply evaluated through
T ad ¼ ðnF hF þ nO hO nP hP Þ=cp ðnD þ nP Þ þ T u
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where n and h is respectively the mole fraction and enthalpy of the species, subscripts F, O, D, P respectively designate fuel, oxidizer, diluents and product. It is then seen that the addition of chemically passive agents into a fuel/oxidizer system will dilute the fuel and oxidizer concentration through increased nD (thus decreased nF), as shown in Fig. 6a. In addition, the specific heat cp is modified, as shown in Fig. 6b. For Ar dilution case, the specific heat actually decreases, which favors the increase of adiabatic temperature, however, the concentration of the fuel decreases, leading to the decrease in the total heat of combustion, thus the decrease of adiabatic temperature with the increase of Ar dilution ratio is resulted from the competition between reduced reactant concentration and decreased specific heat. For N2 dilution case, the specific heat is only weakly dependent on dilution ratio, thus the decreased flame temperature is dominantly affected by decreased reactant concentration. For CO2 dilution case, the specific heat increases with the increase of dilution ratio, which favors the decrease in adiabatic temperature. 3.4. Chemical kinetic effects of diluting agents Ar and N2 are all chemically passive agents and have been constantly treated as non reactive bulk gases that do not participate in combustion. However, CO2 is a major product of combustion while it is chemically passive as well. Adding CO2 into fuel/air system may possibly influence the chemical kinetics and thus the laminar flame speed. CO2 modifies the flame kinetics in two main ways. First, the reverse of the reaction, CO + OH = CO2 + H, decreases the H atom concentration and weakens the flame. Second, dilution with CO2 results in an overall stronger third-body efficiency of the mixture than dilution with Ar or N2 [32,33]. Furthermore, compared to the chain branching reactions, the third-body reactions are comparatively more important than the chain branching reactions only when the flame is very weak or pressure is high [33–37]. Flames considered here are under atmosphere pressure and even with 20% CO2 as diluents, are still far away from low flame temperature of flammability limit. Therefore, in present study, the role of CO2 as a third-body is not as important as that of CO2 as a reactant. To examine the possible chemical kinetic effect, experiments and computations were conducted for all three different diluting agents, and the adiabatic temperatures were fixed at stoichiometric condition, so that diffusivity and thermal effects were excluded. Fig. 7 shows the measured and predicted laminar flame speed as a function of adiabatic temperature for three diluting agents/ DME/air mixtures at stoichiometric condition. The computed results show that flame speed of Ar and N2 almost completely overlapped while the laminar flame speed of CO2 is systematically lower. Experimental measurement at adiabatic temperature of 2185 K shows that the laminar flame speed of Ar/DME/air and N2/DME/air is almost the same, while the laminar flame speed of CO2/DME/air is about 3–4 cm/s lower. Since the influence of Lewis number and adiabatic temperature were excluded, the only effect that could result in the lower flame speed might be the retarded chemical kinetics by CO2. It is noted in Tang et al.’s work [21] that the chemical kinetic effect was represented by overall activation energy. This activation energy approach can’t be used here because the one-step overall reaction assumption for extracting activation energy is not valid anymore for stoichiometric condition. However, the flame structure can still be examined to identify the chemical kinetic effects. Figs. 8 and 9 show the computed rate of production and mole fraction of H, O and OH as functions of distance through the flame for three diluting agents/DME/air mixtures at adiabatic temperature of 2185 K. It should be noticed that the origins of the distance in these figures do not correspond to the central ignition point. The x-axis represents the computational domain, which is read from
the PREMIX solution file. It is seen that, the production rates and mole fractions of OH, H and O radicals of mixtures with Ar and N2 dilution almost over lapped, while the production rate curve in the CO2 dilution case was systematically lower and shifted to post reaction zone. This phenomenon is corresponding to the lower laminar flame speed of CO2 diluted mixtures than that of mixtures with Ar or N2 addition, displayed in Fig. 7. Chemical kinetic effects of CO2 in terms of decreased production rate and concentration of typical free radicals were believed to be the main reason for the further decrease in laminar flame speed because those parallel computations with Ar or N2 diluting were conducted under the same thermal and diffusional conditions. To further identify the chemical kinetic effects of CO2, sensitivity analysis was performed at adiabatic flame temperature of 2185 K and the most important chemical reactions on the mass burning rate under different kinds of dilution were scrutinized, as shown in Fig. 10. The difference between Ar case and N2 case was negligible while the sensitivity coefficients were more significantly affected with CO2 dilution. The two most sensitive chemical reactions are R1: H + O2 ? O + OH and R29: CO + OH ? CO2 + H. For R29, which produces H radical, the sensitivity coefficients on mass burning rate in the CO2 case was lower than that of Ar and N2 case. With the addition of external CO2, less H was produced and this H production reaction was less sensitive. Thus though the chain branching reaction R1 has a higher sensitivity coefficients in the CO2 diluting case, because the reaction was progressed under a lower concentration level of H radical, the net effect is that the overall radical pool concentration was reduced, as manifested by Fig. 9, leading to the further decrease in laminar flame speed as shown in Fig. 7. 4. Conclusions Different suppression effects of argon, nitrogen or carbon dioxide dilution on laminar flame speeds were investigated experimentally and numerically. The main conclusions are summarized as follows: (1) Predictions and measurements of laminar flame speeds are in good agreement for all the determined conditions, no matter what equivalence ratios, dilution ratios and kinds of dilution are. (2) For all the three diluting agents invested, laminar flame speed decreases with the increase of dilution ratio. For given dilution ratio, the significance of suppression effect on flame speed decreases in the order of CO2, N2 and Ar. (3) Adiabatic flame temperatures of mixture have large effluence on the laminar flame speeds. For a specific adiabatic flame temperature, laminar flame speed and free radicals concentrations of flames with Ar or N2 dilution have the approximate values, while those of flames with CO2 dilution have lower values, and the flame structure shows that CO2 addition reduces the typical radical concentrations. (4) Further discussions on sensitivity analysis at given adiabatic temperature shows that reaction H + O2 ? O + OH and CO + OH ? CO2 are the most sensitive on laminar burning flux and the addition of CO2 chemically reduced H radical concentration, leading to the decrease in concentration of other free radicals such as O and OH and thus CO2 addition reduces laminar flame speed through chemical kinetics was recognized.
Acknowledgments This work is supported by National Natural Science Foundation of China (51136005, 51121092).
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