Effects of hydrogen addition on the standoff distance of premixed

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Effects of hydrogen addition on the standoff distance of premixed burner stabilized flames of various hydrocarbon fuels Lei Xu, Fuwu Yan, and Yu Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03089 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on January 10, 2018

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Effects of hydrogen addition on the standoff distance of premixed burner stabilized flames of various hydrocarbon fuels Lei Xua,b, Fuwu Yana,b, Yu Wanga,b,* a.

Hubei Key Laboratory of Advanced Technology for Automotive Components, School of Automotive Engineering, Wuhan University of Technology, Wuhan 430070, P.R. China

b.

Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan 430070, P.R. China

*Corresponding author: Yu Wang School of Automotive Engineering Wuhan University of Technology Wuhan, 430070, P.R. China Email: [email protected]

Full length article Prepared for submission to Energy & Fuels ACS Paragon Plus Environment

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Effects of hydrogen addition on the standoff distance of premixed burner stabilized flames of various hydrocarbon fuels Lei Xua,b, Fuwu Yana,b, Yu Wanga,b,* a.

Hubei Key Laboratory of Advanced Technology for Automotive Components, School of Automotive Engineering, Wuhan University of Technology, Wuhan 430070, P.R. China

b.

Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan 430070, P.R. China

Abstract The quenching distance of premixed hydrocarbon flames is of significant importance for studying flame/wall interactions and for understanding the unburned hydrocarbon emissions of internal combustion engines. Motivated by the fact that the standoff distance of premixed burner-stabilized flames could be used to investigate the head-on quenching distance of freely-propagating flames, a parametric investigation on the standoff distances of methane, ethane and propane burner-stabilized flames were conducted numerically using a detailed chemical kinetic mechanism, with a focus on the effects of hydrogen addition. Specifically, the minimum standoff distance was found to correlate with the head-on quenching distance of premixed flames. The variations of the minimum standoff distance as a function of hydrogen fractions were then investigated in detail. The results showed that as hydrogen fraction increased, the minimum standoff distances decreased monotonously for all the hydrocarbon-air flames, with the reduction being most significant for methane fuel. Accompanying kinetic analysis showed than hydrogen addition enhanced the heat release process, which promotes the reduction of minimum standoff distance. Subsequently, the dependence of the minimum standoff distance on fuel dilution, equivalence ratios, unburned gas temperatures and pressures were explored. In addition, the potential to study the parametric dependence on unburned hydrocarbons emissions induced by near-wall flame quenching using the burner-stabilized flame model was discussed. The current study provides a useful approach to quantify the quenching distance of premixed flames, which has practical applications in internal combustion engines. Moreover, the dependence of standoff distance / quenching distance on hydrogen addition and other varying flame parameters can now be more fundamentally understood with the help of detailed chemical kinetics. In addition, the potential to study the parametric dependence on unburned hydrocarbons emissions induced by near-wall flame quenching using the burner-stabilized flame model was discussed.

Keywords: Standoff distance; Burner-stabilized flames; Hydrogen addition; Hydrocarbon emissions

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1. Introduction Flat premixed flames can be stabilized on a water-cooled porous burner plate with a wide range of reactant inlet velocities and are therefore used as a convenient one-dimensional flame configuration for fundamental combustion research1-3. The standoff distance, which is defined as the distance between the intensive reaction zone and the burner surface, is an important property in that it is directly related to the stabilization mechanism of burner-stabilized flames 4-8

. For a given fuel/air reactant inlet velocity, the reaction zone would automatically stabilize at a position such that

the heat loss to the burner surface accounts for the reduction of the flame speed from its theoretical adiabatic value to the prescribed reactant inlet velocity. Of course, the inlet velocity needs to be smaller than the adiabatic laminar flame speed, as the flame would be blown out if otherwise. Previous studies have shown an interesting phenomenon that the standoff distance exhibits a U-shaped relation with the reactant inlet velocity (or actual flame speed, noting that in un-stretched flat burner-stabilized flames the actual flame speed always self-adjusts to match with the reactant inlet velocity, which is smaller than adiabatic flame speed) in that the standoff distance first decreases and then increases. This implies that each standoff distance corresponds to two different flame speeds. The existence of both an upper and a lower branches of flame speed solution for a given standoff distance has been discussed in some detail in the literature9-13. In particular, Ferguson et al.

12

conducted an experimental and numerical investigation on the standoff

distances of burner-stabilized premixed flames of several fuels including methane, ethylene and hydrogen. The experimental results showed that with the increase of the reactant inlet velocity, the flame standoff distance exhibited a non-monotonic variation in that it first decreased, reaching a minimum value and then started to increase. This observation experimentally confirmed the two flame speed solutions for a fixed standoff distance. In addition, the existence of a critical flame speed corresponding to the minimum standoff distance was suggested. Recognizing that the stabilization mechanism for the flat premixed flames is the heat loss to the burner, we can interpret the physical meaning of the dual flame speed solution, at least partially, by considering the thermal characteristics of the flame. When the inlet velocity and the corresponding flame speed is relatively high (e.g.

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approaching the adiabatic laminar flame speed), only a small fraction of the combustion heat is to be transferred to the burner as heat loss. Furthermore, since the primary heat loss mechanism is heat conduction to the burner which scales with the temperature gradient near the burner surface, it may not be surprising to find the standoff distance to be large under these high flame speed conditions, to ensure a low temperature gradient. As the flame speed decreased, the heat loss rate will be increased and thus a higher near-burner temperature gradient is required, translating to a smaller standoff distance. When the flame speed is excessively small, although a larger fraction of the combustion heat needs to be removed through the burner, the absolute value of heat loss rate would be rather small. This is because the heat generation rate itself is small for low flame speed conditions (i.e. low mass burning flux) and as a result, the standoff distance tends to increase again to reduce absolute heat loss. Therefore, as flame speed increases gradually a minimum standoff distance could be obtained. Moreover, since very little heat loss is needed to quench the flame at the minimum state, Ferguson et al. 12, 14 suggested that the turning point, namely the minimum standoff distance, could be considered as a quasi-steady approximation of quenching distance of premixed flames. In this regard, the burner-stabilized premixed flame with the minimum standoff distance corresponds to the end state of a transient flame quenching process induced by a cold boundary. An interesting implication is that the much simpler steady burner-stabilized premixed flames may be used to investigate the transient head-on quenching behavior, which occurs when a freely propagating flame approaches a cold wall (with the direction normal to the wall surface, i.e. head-on) beyond a critical distance, resulting from both the excessive heat loss and the extinction of reactive radicals to the wall. In practical combustion devices such as spark ignition internal combustion engines, flame quenching could lead to reduced engine thermal efficiency due to incomplete combustion. Furthermore, as the unburned fuel and intermediate hydrocarbon species left in the quenching layer cannot be fully oxidized, they will be exhausted and thus lead to increased engine hydrocarbon (HC) emissions 15

. Recognizing the importance of quenching distances in understanding the flame/wall interactions

16

and modeling

engine unburned HCs, a number of experimental 17-25 and numerical studies 23, 24, 26, 27 have been conducted. However,

3


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experimental data is still very limited, especially for head-on quenching distances, and this may be partially due to the fact that relevant experimental systems are generally rather complex and costly. In relevant numerical studies, the quenching distance was generally obtained by solving the energy conservation equations in the quenching layer

23, 24

.

One-step global reaction was frequently used to simplify the analysis. More detailed treatments include directly simulating the transient quenching process of premixed flames 26, 27 such that transient CFD models need to be adopted. However, the computational costs of these models are significantly higher, especially in cases where detailed chemical mechanisms are required. This prevents a comprehensive parameter investigation on the variation of quenching distance with various flame parameters such as temperature, pressure and fuel composition. Fortunately, the above-mentioned one-dimensional burner-stabilized premixed flame model may provide an alternative approach, which is especially useful for comprehensive parametric investigations on the flame quenching characteristics. It is to be noted that although this simple model is only an approximation of the actual head-on quenching process, it will be shown that the minimum flame standoff distances have comparable values and, more importantly, the same dependence on all the parameters we investigated (fuel type, equivalence ratios, pressure, temperature and etc.) with quenching distance. Furthermore, the minimum standoff distance is the closest position that a flame can approach to burner surface and thus the fuel and intermediate HCs that are trapped in this near-wall region cannot be fully oxidized. This situation is indeed similar with the flame quenching induced HC emission in spark ignition engines. Hydrogen, as a fuel additive to enhance engine’s combustion performance and reduce carbon emissions, has received intensive research interest. In particular, spark ignition hydrogen-enriched natural gas engines have been widely studied. The effects of hydrogen addition on both the fundamental combustion properties of premixed methane-air mixtures 28-30 and natural gas engine performance and emissions have been extensively studied kinetic influence of hydrogen addition on methane-air premixed flame have also been discussed

33, 34

31, 32

. The

. However,

fundamental studies focusing on the influence of hydrogen addition on the quenching distance of premixed flames are rather scarce. As far as the authors are aware of, the only work was conducted by Fukuda and co-workers

4


35

who

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experimentally measured the quenching distances of CH4/H2/air premixed flames. However, Fukuda’s study

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only

presented certain experiment results for methane-air flame. Besides, the detailed kinetic analysis on the variation of quenching distance was also not elaborated. In the current study, we intend to perform a comprehensive parametric study on the quenching distance of various premixed hydrocarbon/H2/air flames, by numerically investigating the minimum standoff distance of the burner-stabilized flames while emphasizing its dependence on hydrogen fraction and other important flame conditions such as equivalence ratio, unburned gas temperature and pressures. The relationship between the minimum standoff distance and corresponding quenching distance will be clarified. In addition, the effects of hydrogen addition on the unburned HCs and other intermediate pollutant species trapped in the near-wall quenching layer (i.e. minimum standoff distance) will also be discussed. The current work present an example of a systematical investigation of the minimum standoff distance of burner-stabilized flame with detailed gas-phase chemistry. As will be shown in later sections, although the current study focused on the effects of hydrogen addition, the computational methodology we have proposed succeeded in capturing all the trend regarding the fuel, diluent, pressure and temperature effects on quenching distance and may be generalized, after further validations, to the studies of other fuels (e.g. oxygenated fuel addition in gasoline surrogate) of practical importance. It is expected that this methodology can become a useful tool to quantify the quenching distance and conduct parametric study of the quenching distance behavior of premixed flames, which have practical applications in spark ignition internal combustion engines.

2. Computational methodology The standoff distance of burner-stabilized flames can be obtained by solving the governing equations for steady, one-dimensional laminar premixed flame. The non-monotonic (U-shaped) variation of standoff distance with flame speed, which can be obtained with analytic method assuming one-step global reaction 9, 11, can be seen in Fig. 1.

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To present an explanation for the U-shaped relation, a simplified analysis conducted by Ferguson et al.

is

briefly reviewed here. In their study, the standoff distance was obtained by considering energy conservation for the region between the burner surface and the flame sheet: = +

( )

(1)

where D refers to the standoff distance (i.e. the distance between the burner surface and the flame sheet), Su is the flame speed, λ, Cp, ρ and T denote thermal conductivity, constant-pressure specific heats, density and temperature, respectively. The subscripts o, b and u represent the adiabatic state, burned products and unburned fresh mixture, respectively. Equation (1) simply means that the total heat released from chemical reaction (the term of ) goes to two parts: 1) to raise the temperature of the reactants from Tu to Tb and 2) heat loss to the burner surface. Note that Eq. (1) is much simplified by assuming 1) linear axial temperature distribution from the burner surface to the flame sheet; 2) one-step reaction with negligible flame sheet thickness and 3) constant and equal transport properties for all species. For burner-stabilized flame, the flame speed Su can be empirically correlated with flame temperature as:

= − "#

(2)

Substitute Su in Eq. (1) and rearrange: $ =

& exp % '

−

"#

(3)

Equation (3) shows that the standoff distance D depends on the maximum flame temperature Tb, as all the other parameters are known under a specified condition. The solution of Eq. (3) would give the U-shaped relationship between standoff distance and flame temperature (or flame speed). By solving d$⁄d = 0, the minimum standoff distance can be obtained.

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Fig. 1 Typical variations of standoff distance with flame speed Su for premixed burner-stabilized flames. The schematic graph indicates a flat premixed flame stabilized above the burner surface for a horizontally placed McKenna type burner. As the flame speed Su (i.e. the reactant inlet velocity of pre-mixtures) gradually increases, the standoff distance will show a U-shaped variation in that it first decreases, reaching a minimum and then increase again. The minimum standoff distance (i.e. the turning point in the figure) and its dependence on various flame parameters are the research foci of the present study. Note that at sufficient high inlet velocity (e.g. approaching adiabatic flame speed Su0), the flame cannot be stabilized on the burner surface and thus shall be blown off (standoff distance increase drastically).

The above analysis is based on the one-step chemistry and simplified transport properties

11, 12

, which have

significant limitations in resolving the detailed flame structures. Besides, such one-step reaction approach cannot be used to model intermediate HCs

36, 37

or to investigate the underlying chemical effects of flame conditions on the

standoff distances. In contrast, the solutions of the standoff distance in the present study were obtained by solving the fully-coupled species and energy equations with detailed chemistry using the Sandia PREMIX code 38. Considering the general feature that the standoff distance will firstly decrease and then increase as flame speed increases, a special computation procedure, as schematically shown in Fig. 2, was proposed to find out the minimum standoff distance. For each case, the computation was started by specifying the reactant compositions, temperature (Tu), pressure (P) and an initial inlet velocity (e.g. mass burning rate) u1, then a corresponding standoff distance D1 could be obtained. As the inlet velocity increased gradually with a certain interval (2 cm/s in current study), the corresponding standoff distance was calculated and compared with the previous one. Once the standoff distance started to increase with the increase of inlet velocity, the minimum standoff distance could be found and the computation was completed.

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Fig. 2 Flow chart for the computation of minimum standoff distance.

It is necessary to point out here that there are various ways to locate the flame sheet numerically. For example, the position where flame temperature reaches 1500 K (DT1500), where maximum heat release rate (DHRR), maximum OH concentration (DOH) or maximum temperature gradient (DdT/dx) occurs have all been used to define the reaction zone

26, 39

. To evaluate such definition dependence, the standoff distance as determined by the above four definitions

were computed and compared with the experimental data of Ferguson et al. 12 and the results are shown in Fig. 3a. As can be seen, although their absolute values varied with different flame sheet definitions, all the standoff distance curves shared similar qualitative varying trend with equivalence ratios. This indicates that the choice of the flame position definition may not affect the conclusions regarding the qualitative trend in the parametric study. As the primary objective of the current study is to obtain the parametric dependence of the standoff distance, it is more important to concentrate on the relative trend of the standoff distances rather than their absolute values 40. As such, in the following discussions we chose the position where maximum OH concentration occurs as the location of the flame sheet. By using this definition, the computed results of standoff distance for both CH4 and C2H4 fuels with two

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different mechanisms, i.e. USC Mech 2.0 41 and AramcoMech 1.3 42 were predicted and compared with experimental data in Fig. 3b. Again, the variation trend of experimental standoff distance with equivalence ratio are well captured for both CH4 and C2H4 fuels with those two mechanisms. This result may indicate that current method for standoff distance calculation can apply to different fuels and the results are mechanism-independent, provided the reaction mechanism is well-validated.

Fig. 3 The variation of standoff distances with equivalence ratios for (a) CH4/air flames using several definitions computed with USC Mech 2.0 and (b) CH4/air and C2H4/air flames computed with both USC Mech 2.0 and AramcoMech 1.3 mechanism using the definitions of DOH. Experimental data are taken from Ferguson et al. 12.

In subsequent computations, the well-validated detailed high temperature reaction mechanism for CO/H2/C1-C4 fuels ( USC Mech 2.0 41 ) consisting of 111 species and 784 reactions were used to describe the chemical reactions in the flame. This was in contrast to previous studies

12

where one-step global reaction was assumed. In addition, the

assumptions of constant thermodynamic and transport properties were discarded. Instead, the mixture-averaged formula was applied to describe the diffusion coefficient of each species and temperature-driven mass diffusion (Soret effect) was also considered. Note that in the present study the standoff distance behavior was considered to be

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dominated by thermal effects and therefore, the burner surface was considered chemically inert, neglecting the potential radical quenching on the burner surface.

3. Results and discussions In this section, we will first confirm the quantitative correlations between the minimum standoff distance of burner-stabilized flames and the head-on flame quenching distance of freely propagating flames. Afterwards, the effects of hydrogen addition and/or inlet diluents on the minimum standoff distance behavior of hydrocarbons-air flames will be discussed. Throughout the text, the hydrogen fraction (XH2) in the hydrogen-hydrocarbon blends is defined as XH2= QH2 / (QH2+QHC), where QHC and QH2 are the volumetric flow rates of hydrocarbons and hydrogen, respectively. The oxidizer (air) used in the simulation is composed of 21% of oxygen and 79% of nitrogen in volume and the ambient temperature is 298 K.

3.1 Correlation with quenching distance

Following the computational procedures described in the preceding section, the minimum standoff distance Dm under different flame conditions could be obtained. Before discussing in detail the various factors that may affect Dm, it is desirable to examine the numerical relationship between the minimum standoff distance and the corresponding flame quenching distances which have implications for S.I. engines. After all, it is the authors’ wish that the current study on standoff distance could provide useful information on how the quenching distance may be affected by various operating conditions. Available experimental quenching distance was frequently obtained by measuring the minimum gap between two parallel plates that a flame can propagate without extinction

35, 43

. The calculated Dm are

not expected to be directly compared with such two-plate quenching distance data due to the different physical processes. Fortunately, researchers have shown that they are indeed correlated after some numerical manipulation. In particular, Ishikawa et al. 44 recommended to scale the Dm up with a factor of 2.5 before comparing with the quenching distance data. We accepted such recommendation and the comparisons for premixed CH4/air flame under various

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equivalence ratios are shown in Fig. 4a. As can be seen, the value of Dm (after scaling up) agrees reasonably well with the measured quenching distance. More importantly, Dm could well reproduce the relationship between quenching distances and equivalence ratios. That is, the effects of equivalence ration on quenching distance can be obtained by studying its impact on the computed Dm of burner-stabilized flames, rendering the current study of practical importance. Note in Fig. 4a that even for the same condition, the measured quenching value may vary between different researchers

35, 43

. In fact, such discrepancy is not uncommon due to the rather small value of quenching

distance which require high experimental accuracy. Therefore, the differences between their absolute value are deemed acceptable considering the experimental uncertainties. Apart from the effects of equivalence ratio, Ferguson et al.

14

further reported that Dm had a good fitting with

quenching distance for flames with different single-component fuels. However, no comparison has been made for flame with binary fuel blends. In this regard, we conducted additional comparisons for flames of methane/hydrogen blends and the results are presented in Fig. 4b. As can be seen, similar to the result for CH4/air flame, a good correlation still exists between computed Dm and experimental quenching distance.

Fig. 4 Comparison of calculated minimum standoff distances (lines) with experimental quenching distance of (a) methane-air flame and (b) hydrogen-methane-air flame, at pressure of 1 atm and Tu of 298 K.

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Furthermore, previous works

40, 44, 45

have observed that the Peclet number, defined as the ratio of the flame

quenching distance Dq to the laminar flame thickness δF, is a nearly constant for a fixed flame quenching geometry, not strongly dependent on flame parameters such as fuel type, equivalence ratio, pressure and temperatures. It is interesting to check if such independence holds for the calculated Dm and δF. Here, flame thickness δF is defined as 1: ∆

./ = (1 /13)

(4)

456

where ∆ is the difference between the peak flame temperature and the unburned gas temperature; (7/7)893 is the maximum gradient of the temperature profile. The computation of flame thickness were conducted at the state where minimum standoff distance occurs. Figure 5 demonstrates the relations between ./ and Dm for various CH4/H2 blends under different conditions. As can be seen, both ./ and Dm decrease as hydrogen fraction increases from 0% to 60%. However, all the data points shown fall onto the same line, indicating that the ratio of Dm / δF is constant under all these conditions. This result further confirms the positive correlation between minimum standoff distance of burner-stabilized flames and head-on quenching distance.

Fig. 5 Correlation of minimum standoff distance with flame thickness for different CH4/H2 fuel blends at several flame conditions. Case 1: Φ=1.0, Tu =298 K; Case 2: Φ=1.0, P=1 atm; Case 3: Tu=298 K, P=1 atm.

We may now conclude that the minimum standoff distance of burner-stabilized flames could well represent the transient head-on quenching distance. It is reasonable to treat the minimum standoff distance as a sound

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approximation of practical quenching distance, especially in cases where the parametric dependence of quenching distance, rather than their absolute values, is the primary research objective. The current study falls into this category.

3.2 Effects of hydrogen addition

We next move on to a detailed parametric study of the minimum standoff distance (Dm). The first variable to investigate is the hydrogen fraction (XH2). Figure 6 presents the effects of XH2 on Dm for stoichiometric methane-air flames. As expected, Dm decreases monotonously as XH2 increases from 0% up to 80%. This overall variation trend is consistent with the experimental observation by Fukuda et al.

35

who showed that the quenching distance of

CH4/H2/air mixture decreased with increasing XH2.

Fig. 6 Effects of hydrogen fraction on minimum standoff distance of stoichiometric methane-air flame. Calculated at Tu of 298 K and pressure of 1 atm.

To understand the underlying mechanism of the dependence of Dm on hydrogen addition, the effects of hydrogen on the fuel oxidation chemistry and the overall heat release rates of premixed methane-air flame were analyzed. Rate of production (ROP) analysis in the present study suggests that the most important reactions promoting methane oxidation are: CH4 + OH ⇔ CH3 + H2O, CH4 + O ⇔ CH3 + OH and CH4 + H ⇔ CH3 + H2. Since the impacts of hydrogen addition on OH, H and O radicals are qualitatively similar, we first take a look at OH radical whose

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concentration profiles along the axial direction are shown in Fig. 7a, for cases with XH2 = 0, 20% and 40%. It can be seen that as XH2 increases, the OH concentration profile tends to shift to the left, with its peaks becoming closer to the burner surface. This indicates that the flame reaction zone moves closer to burner surface and the main fuel oxidation reactions of methane-air flame are accelerated after hydrogen addition. In addition, the peak location where maximum methane consumption rate occurs (through CH4 + OH ⇔ CH3 + H2O) also moves towards the burner surface after hydrogen addition, as shown in Fig. 7b. Meanwhile, the heat release rates are also seen to be promoted by hydrogen addition, as evident from Fig. 7c. As XH2 increases, the peak location of the heat release rate profiles shifts towards the burner surface. Since the standoff distance depends on the balance between the heat release and the heat loss rate, for flames with rapid heat release the reaction sheet will tend to move closer to the burner surface in order to increase the near-burner temperature gradient and thus heat loss (see Fig. 7d), such that a new energy balance would be achieved at a smaller standoff distance. As a consequence, the hydrogen promoting effect on reducing minimum standoff distance may be attributed to the facts that flame heat release process is enhanced due to increasing concentration of OH, H and O radical after hydrogen addition.

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Fig. 7 Effects of hydrogen addition on: (a) OH concentration profile, (b) CH4 consumption rate through OH, (c) Heat production rate and (d) flame temperature distribution profile of stoichiometric CH4/H2/air flames. Calculated at Tu of 298 K and pressure of 1 atm.

It may seem confusing from Fig. 7 that although OH radical concentration (also O and H, not shown) shows a more rapid increase, their peak values even decrease as XH2 increases. This is actually caused by the increased heat loss to the burner surface for higher XH2 cases. Hydrogen addition can increase the adiabatic laminar flame speed of methane-air, while for the different XH2 cases shown in Fig. 7, the reactant inlet velocities and thus the actual flame speeds are kept the same. Therefore, the heat loss would be higher for higher XH2 cases, resulting in their relatively lower peaks of radical concentrations. In fact, if we look at adiabatic flames where no heat loss occurs, hydrogen addition will certainly lead to an increase of peak radical concentrations. This is confirmed in Fig. 8, where the radical concentration profiles for adiabatic freely-propagating flames are presented.

Fig. 8 Effects of hydrogen addition on OH, O and H radical concentration of freely propagating stoichiometric CH4/H2/air flames. Calculated at Tu of 298 K and pressure of 1 atm.

We now have a general understanding regarding the effects of hydrogen addition on standoff distance of premixed burner-stabilized flames, by using methane as a representative fuel. However, the presence of hydrogen may exhibit different thermal / chemical effects on flames of other larger hydrocarbons due to the differences in fuel molecular structures, which will certainly influence the flame standoff distance. To explore such differences, ethane

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and propane flames were further studied for comparison with methane flame. The choice of ethane and propane as additional test hydrocarbons was based on the fact that they are also vital components of natural gas 46, which is an important fuel for IC engines

47, 48

. Figure 9 shows the Dm of ethane-air, propane-air and the baseline methane-air

flames, with hydrogen addition up to 50% in both volume (a) and fuel energy fraction (b). Note here the standoff distance is defined as the position where maximum heat production rate (DHRR) occurs, considering that the definition of DOH may not be proper for comparisons among different hydrocarbon fuels.

Fig. 9 Effects of hydrogen addition on the minimum standoff distance of stoichiometric CH4, C2H6 and C3H8 flames, with Tu of 298 K and pressure of 1 atm.

As can be seen in Fig. 9a, for the various neat hydrocarbon/air mixture flames (XH2 = 0), Dm follows the decreasing order of methane > propane > ethane. For all the hydrocarbon fuels, Dm shows a decreasing trend with increasing hydrogen volume fraction XH2. We also note that the quantitative extent of the effects of hydrogen addition on Dm is not universal but dependent on the base fuel. For instance, Dm of ethane-air and propane-air flame can be seen to decrease by 13.3% and 13.1% respectively, as XH2 increases from 0% to 50%, while that for methane-air flame decreases by 30.6%. Although it is to be noted that, due to the differences in the volumetric lower heating values of these hydrocarbon fuels, the same XH2 addition actually accounts for a varying energy fraction in the different hydrogen/hydrocarbon mixtures. To exclude such discrimination, the effects of hydrogen addition on Dm was further

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investigated in terms of hydrogen energy fraction (βH2) defined as: βH2 = QH2 LH2 / (QH2LH2 + QHCLHC) 49, where Q is the volumetric flow rate (at 1 atm and 298 K), L denotes the volumetric lower heating value which is 10.6, 36, 64.4 and 92.7 MJ/m3 for hydrogen, methane, ethane and propane, respectively 50. As shown in Fig. 9b, it is clearly seen that hydrogen addition has a more pronounced effect on Dm of methane-air flame, even on an energy fraction basis. These results confirms the facts that the effect of hydrogen addition on decreasing Dm is fuel-dependent. As discussed above for the methane-air flame, the reduction of standoff distance after hydrogen addition is related to the enhanced fuel consumption and heat released rates, due to increased concentrations of OH, O and H radicals. Therefore, the different decreasing ratio of Dm by hydrogen addition may be explained by considering the different fuel oxidation processes.

Fig. 10 Rate of fuel consumption with OH, O and H radicals for stoichiometric methane-air, ethane-air and propane-air flame respectively, with and without 20% hydrogen addition. Conducted at Tu of 298 K and pressure of 1 atm.

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Figure 10 presents the rates of fuel consumption for methane (a), ethane (b) and propane (c) via reactions involving OH, O and H. It is found that with 20% hydrogen added, all the peaks of consumption rate profiles appear closer to burner surface, consistent with the results that hydrogen addition leads to smaller Dm for all hydrocarbon/air flames. Moreover, the location of peak fuel consumption rate has a larger shift after hydrogen addition for the methane flame than propane-air and ethane-air flames, which may explain the more obvious effect of hydrogen addition on Dm for the methane flame. It is interesting to note that the differences in Dm and the relative effects of hydrogen addition on Dm, among CH4, C2H6 and C3H8 flames can also be interpreted from the perspective of adiabatic laminar flame speed. Lavoie et al.

51

noticed that the quenching distance of premixed flames were inversely proportional to laminar flame speed. As the laminar flame speed for the three hydrocarbon fuels follow the order of CH4 < C3H8 < C2H6 52, it is not surprising to find Dm of C2H6 flames to be the smallest. Specifically, that stoichiometric C2H6-air mixture was found to have highest heat release rate which promote the reduction of the minimum standoff distance, as discussed in the preceding. With hydrogen addition, the laminar flame speed will be increased due to enhanced concentrations of OH, O and H radicals 53

and therefore Dm is expected to become smaller. The relatively weaker effect of hydrogen addition for C2H6/C3H8

flames in terms of reducing Dm, as compared to CH4 flame, can be understood from the fact that C2H6/C3H8 has higher adiabatic laminar flame speeds and also less sensitive to hydrogen addition 54 than CH4. However, it should be emphasized that although the reduction of Dm could be qualitatively correlated with adiabatic laminar flame speed, the quantitative relationship between the Dm and adiabatic laminar flame speed is not straightforward. For instance, Chen et al.55 reported that the laminar flame speed of CH4/H2 mixture showed a non-liner increase with hydrogen fraction, while the result of Fig. 6 in this study presents an approximately linear decrease of Dm as XH2 increases. This result indicates the different quantitative variation of Dm and adiabatic laminar flame speed. Indeed, the physics involved are essentially different in that as the stabilizing mechanism of the present burner-stabilized flames is heat loss, burner-stabilized flames are never adiabatic.

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3.3 Effects of diluents

Apart from hydrogen addition, it is also of practical importance to understand the effects of diluents on the quenching distance of premixed flames. For instance, in addition to N2, CO2 is frequently used as an diluent to reduce the excessive flame temperature in oxy-fuel combustion

56

through flue/exhaust gas recirculation

(FGR/EGR)57, 58. In this regard, the impacts of diluent (CO2, N2 and Ar) on the minimum standoff distance of CH4, C2H6 and C3H8 flames were investigated. In the present study, the diluted fuel is composed of y% diluents and (100-y) % hydrocarbons in molar ratios, with y varying from 0 to 50. The diluted fuels were burned with air at stoichiometric conditions. The results are shown in Fig. 11 and it is found that the minimum standoff distance exhibits similar variation trends for the three hydrocarbon flames investigated. The increase of dilution ratio results in an increasingly larger Dm 59. Moreover, CO2 addition leads to the highest increase of Dm, followed by N2 and Ar. From Fig. 11a-c, no obvious difference is found regarding the variation behavior of Dm when the diluents are added to different hydrocarbons.

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Fig. 11 Effects the addition of various diluents on Dm of stoichiometric (a) CH4/air, (b) C2H6/air and (c) C3H8/air flames. Conducted at Tu of 298 K and pressure of 1 atm.

We may infer from the above results that the diluents play primarily a thermal effect, behaving as a heat sink. As compared to the high reactivity of hydrogen, the studied diluents are rather chemically-inert and therefore minimum chemical effects may exist for diluent-hydrocarbon interaction. It is noted that in the present computation, the chemical effect of CO2 was included while Ar and N2 were treated as chemically-inert species. To isolate the chemical effects of CO2, a fictitious species FCO2 (defined to have exactly the same thermal and transport properties with CO2 but is not allowed in any chemical reactions, similar to the work of Wang et al. 57) was devolved for comparison. The results showed that the difference of Dm between CO2 and FCO2 addition was rather small (about 4-9.5%), indicating the negligible chemical role of CO2 on Dm, compared to dilution effects. As a consequence, the increase of Dm with diluent addition can be explained by the fact that the diluents absorb a fraction of the energy released from the

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combustion process, leading to a reduction in flame temperature and therefore decreased fuel oxidation and heat release rates. The different quantitative effects between these three additives (CO2, N2 and Ar) are due to the facts that CO2 has the largest heat capacity and therefore absorb the most combustion-released heat, leading to a more significant increase of Dm 59.

3.4 Effects of equivalence ratio

In addition to fuel additives or diluents, various other flame parameters also have significant impacts on the flame minimum standoff distance. For a comprehensive parametric study, we shall discuss in the following sections the dependence of minimum standoff distance on various other flame conditions, i.e. equivalence ratio, unburned temperature and pressure conditions. These studies will be on the hydrogen enriched methane-air flame only, as similar qualitative trends are observed for other hydrocarbon fuels. Figure 12 shows the effects of equivalence ratio on the Dm of CH4/H2/air flames with XH2 varying in the range between 0 to 60%. For all the XH2 cases, the Dm exhibit a non-monotonic variation with equivalence ratio in that it first decreases, reaching a minimum value and then starts to increase again with the reactants become progressively richer. This equivalence ratio dependence of Dm is similar to that of quenching distance value as reported by Saika et al.

43

and Lavoie et al.51. Furthermore, the critical equivalence ratio at which minimum Dm occurs is dependent on XH2. For instance, Dm reachs its minimum value at Φ of 1.0 for the XH2 = 0 cases (i.e. neat methane-air flame) but shifts to Φ of around 1.1 for hydrogen-enriched flames. These effects of fuel/air mixture on Dm can again be interpreted from the perspective of flame thermal characteristics. Since near-stoichiometric flame has the fastest heat release rate as compared to fuel-rich or fuel-lean mixture, the flame reaction zone would approach closer to the burner surface and exhibits lower standoff distance. Theoretically, flames with stoichiometric fuel/air mixture should have the smallest Dm because of its highest heat production rate. The observed shift to richer conditions after hydrogen addition may be attributed to the preferential diffusion effect of hydrogen

60

. It is also noticeable that the extent to which hydrogen

addition can decrease Dm is not universal but depends on the equivalence ratio. For example, compared with the XH2 =

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0 case, 60% hydrogen addition decrease the minimum standoff distance by 23%, 20.4% and 48.4% respectively, at Φ of 0.8, 1.0 and 1.3. This indicates that hydrogen addition has a stronger influence on Dm at relatively fuel-rich conditions.

Fig. 12 Minimum standoff distance versus equivalence ratio Φ for different CH4/H2 fuels. Calculated at Tu of 298 K and pressure of 1 atm.

3.5 Effects of pressures

The propagation of premixed flames in most practical combustion devices proceeds under elevated pressure conditions, while available experimental data on flame standoff distance or quenching distance were generally obtained at atmospheric environments. To extend our knowledge on flame quenching to higher pressure ranges, the dependence of minimum standoff distance on pressure was investigated. Figure 13 presents the effects of pressure on Dm for CH4/H2/air flames, again with different hydrogen fractions. Much as expected, the minimum standoff distance decreases with the increase of XH2, regardless of the pressure conditions. As to the effects of pressure for a fixed XH2, it is shown that as pressure increases, Dm decreases continuously but in a nonlinear fashion. In particular, Dm exhibits a rapid rate of decrease at the lower end of the investigated pressure range but tends to level out as the pressure continues to increase. Quantitatively, take the

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methane-air flame (XH2 = 0) for example, Dm decreases dramatically from 1.63 mm to 0.817 mm (by 49.9%) as the pressure increases from 0.5 to 1.0 atm, while it just showed a slight drop from 0.484 mm to 0.414 mm (by 14.5%) as the pressure increased from 2.0 to 2.5 atm. Note Sotton et al. 24 observed similar pressure dependence of the transient head-on quenching distance for methane-air flames at a pressure range of 0.5-17 atm. They reported that the quenching distance decreased more notably within the pressure range of 0.5 - 4 atm than that beyond 4 atm.

Fig. 13 Effects of pressure on the minimum standoff distance of stoichiometric CH4/H2 flames blended with varying hydrogen fraction, conducted at Tu of 298 K.

The pressure-dependence of Dm can be attributed to the enhancement of fuel oxidation and thus heat release process as pressure increases. Although not shown, the increase of pressure lead to the shifts of heat production curves towards the burner surface which indicates a more rapid chemical reaction and heat release rate for these conditions. As a consequence, the flame reaction zone would approach the surface, resulting in a smaller standoff distance.

3.6 Effects of unburned temperatures

As another important parameter that influences flame standoff distance, the effects of different reactant unburned temperatures (Tu) on Dm of CH4/H2/air flames were also investigated and the results are shown in Fig. 14. Consistent with previous findings, Dm decreases with increasing XH2 at a fixed Tu. While for a specified XH2, Dm exhibits a

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monotonously decreasing trend with the increase of Tu, which is in agreement with the experimental observations of Cleary et al.

61

regarding the effects of initial reactant temperature on the head-on quenching distances of premixed

flames. Moreover, it can also be noticed in Fig. 14 that the rate of decrease of Dm with Tu is almost a constant over the temperature range investigated (from 298 K to 800 K).

Fig. 14 Effects of unburned gas temperature on the minimum standoff distance of stoichiometric CH4/H2/air flames for different hydrogen fractions addition. Conducted at pressure of 1 atm.

With higher unburned gas temperature, the temperature-dependent chemical reactions are enhanced which lead to a faster heat release process and thus a smaller standoff distance. However, it should be emphasized here that for burner-stabilized flames, the final maximum flame temperature could even be decreased with the increase of the initial unburned temperature, as demonstrated in Fig. 15a. This is because that the actual flame speeds (i.e. inlet velocity) are kept fixed for different Tu cases and to maintain such speed, more heat is required to be removed for higher initial temperature cases. Such enhanced heat transfer loss to the burner surface is achieved through higher temperature gradient near the burner surface and thus smaller standoff distance. As a contrast, for freely-propagating flame that no fixed physical boundary and heat loss exits, the maximum flame temperature increases as Tu increases, as shown in Fig. 15b. It is easily understood from the viewpoint of energy conservation: when combustion released energy are the same, the maximum flame temperature will certainly increase with larger energy input.

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Fig. 15 Near-wall temperature distribution profile at different initial wall temperature for (a) 20% H2 / 80% CH4 mixture burner-stabilized flame and (b) freely-propagating flame, conducted at pressure of 1 atm.

3.7 Incomplete combustion products in the quenching layer

The results shown in the preceding sections support the correlation between the minimum standoff distance of burner-stabilized flames and the corresponding transient head-on quenching distances. For these two flame configurations the chemical reaction will be much slowed down in the near wall region due to the rather low temperature and concentrations of active free radicals. As a consequence, the remaining fuel and other intermediate pollutant species that cannot be fully oxidized will be trapped in this region. Considering such trapped incomplete combustion products in the near-wall flame quenching region is one of major source of unburned HC emission in SI engines, it is of practical interests to investigate the emissions based on the minimum standoff distance Dm of burner-stabilized flames. It should be emphasized here that compared to head-on quenching distances, Dm may be more relevant to engine-out emissions considering in typical quenching distance experiments the wall temperature does not have enough time to heat up when a flame collides with the wall. Previous experimental studies have shown that the addition of hydrogen as fuel additive could reduce the unburned HC emissions of S. I. natural gas engines

62, 63

, which has been partially attributed to hydrogen’s effects in

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reducing flame quenching distances. In this section, the effects of hydrogen addition on the incomplete combustion products of methane/air flame is numerically investigated. By integrating the species concentration profile from the burner surface to the minimum standoff distance, the total unburned species trapped in the near-wall quenching layer can be obtained as:

U? = @C 4 AB ()7

(5)

AB () denotes the concentration distribution of species A as functions of distance x. The trapped amounts of unburned fuel CH4 under varying flame conditions was firstly studied with and without 20% hydrogen addition and the results are shown in Fig. 16. Note all the emission data (as calculated using Eq. 5) was normalized to a reference value, which is defined as the emission of the corresponding species for stoichiometric CH4/air flame at Tu of 298 K and a pressure of 1 atm. As expected, the trapped CH4 emission shows a decreasing trend under all investigated flame conditions after hydrogen addition. Quantitatively, 20% hydrogen addition leads to a 35.8% reduction of CH4 emission for the Φ = 1.3 case, compared to 22.7% and of 20.0% for the Φ = 0.8 and Φ =1.0 cases, respectively. Such reduction can be attributed to both the dilution and the chemical effects of hydrogen. With a portion of CH4 replaced by H2, the unburned CH4 will certainly decrease due to fuel dilution. In addition, the chemical promoting effects of hydrogen contribute to an enhancement of fuel consumption rate and thus a reduction of unburned HC emissions. Regarding the effects of various other flame parameters, it was found that the emission level decreased monotonously with increasing unburned temperature (Fig. 16b) and pressures (Fig. 16c). It also reaches its minimum value at stoichiometric fuel/air conditions and increases for both fuel-lean and fuel-rich conditions (Fig. 16a).

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Fig. 16 Normalized CH4 emissions at (a) different equivalence ratio, (b) unburned gas temperature and (c) pressure conditions with and without 20% hydrogen addition.

We may conclude from the above discussions that unburned CH4 emissions and minimum standoff distance share the same dependence on hydrogen fraction and other flame parameters. In other words, the reduction of emissions in the near-wall region always accompanies the decrease of standoff distance. However, it does not mean that lower Dm can directly translate to lower emissions. Since almost all unburned CH4 is distributed within the standoff distance regime and according to Eq. (5), it may seem that the slight reduction of Dm has only little influence on the calculated emissions. In fact, the reductions of both Dm and emissions are the results of improved fuel oxidation chemistry. Apart from unburned fuel (CH4), other intermediate pollutant species such as C2H2 that cannot be fully oxidized in the near wall region also exist. Considering the facts that CH2O and CO have negative impact on human health while C2H2 is an important soot precursor, their emission characteristic were also investigated. Compared to the spatial distribution of CH4, CO, CH2O and C2H2 have their peaks some distance away from the burner surface, with a reduction of 11%, 25.3% and 11.3%, respectively when 20% hydrogen was added.

4. Concluding remarks In this study, the effects of hydrogen addition on the standoff distances of various premixed burner-stabilized hydrocarbon flames was investigated numerically using a detailed chemical kinetic mechanism. The correlation between the minimum standoff distance and the head-on quenching distance of premixed flames was discussed. The results showed that the minimum standoff distance Dm and the head-on quenching distance are quantitatively related and, more importantly, share the same dependence on varying flame conditions. A comprehensive parametric study was conducted to determine the effects of hydrogen addition, fuel type / dilution, equivalence ratio, unburned temperature and pressure on the variation of the minimum standoff distance. The R3C2 results show that the minimum standoff distance of tested hydrocarbon-air flames all decreased monotonously as hydrogen fraction increased. Furthermore, hydrogen addition leads to the most significant reduction of Dm in methane-air flame

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as compared to ethane and propane flames. The addition of diluents (CO2, N2 and Ar) result in the increase of Dm. with CO2 being the most effective. The Dm had its smallest value at near-stoichiometric flame condition compared to fuel-lean and fuel-rich condition, while exhibits a monotonously decreasing trend as unburned gas temperature and pressure increased. The application of minimum standoff distance on modelling unburned HCs trapped in the near-wall region was presented. A substantial decrease of unburned CH4 of methane-air flame was observed when 20% hydrogen was added, due to both the dilution and chemical effects of hydrogen.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51606136) and the Fundamental Research Funds for the Central Universities (WUT: 2016IVA038, 2017VB032).

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(12) Ferguson, C. R.; Keck, J. C., Stand-off distances on a flat flame burner. Combust. Flame 1979, 34, 85-98. (13) Chao, B. H.; Law, C. K., Duality, Pulsating Instability, and Product Dissociation in Burner-Stabilized Flames. Combust. Sci. Technol. 1988, 62, 211-237. (14) Ferguson, C. R.; Keck, J. C., On Laminar Flame Quenching and Its Application to Spark Ignition Engines. Combust. Flame 1977, 28, 197-205. (15) Chauvy, M.; Delhom, B.; Reveillon, J.; Demoulin, F.-X., Flame/Wall Interactions: Laminar Study of Unburnt HC Formation. Flow. Turbul. Combust. 2010, 84, (3), 369-396. (16) Demesoukas, S.; Caillol, C.; Higelin, P.; Boiarciuc, A.; Floch, A., Near wall combustion modeling in spark ignition engines. Part A: Flame–wall interaction. Energ. Convers. Manag. 2015, 106, 1426-1438. (17) Takizawa, K.; Igarashi, N.; Takagi, S.; Tokuhashi, K.; Kondo, S., Quenching distance measurement of highly to mildly flammable compounds. Fire Safety. J. 2015, 71, 58-68. (18) Bidabadi, M.; Mohebbi, M.; Poorfar, A. K.; Hochgreb, S.; Lin, C.-X.; Biouki, S. A.; Hajilou, M., Modeling quenching distance and flame propagation speed through an iron dust cloud with spatially random distribution of particles. J. Loss. Prevent. Proc. 2016, 43, 138-146. (19) Liu, Z.; Kim, N. I., An assembled annular stepwise diverging tube for the measurement of laminar burning velocity and quenching distance. Combust. Flame 2014, 161, (6), 1499-1506. (20) Jung, Y.; Lee, M. J.; Kim, N. I., Direct prediction of laminar burning velocity and quenching distance of hydrogen-air flames using an annular stepwise diverging tube (ASDT). Combust. Flame 2016, 164, 397-399. (21) Palecka, J.; Julien, P.; Goroshin, S.; Bergthorson, J. M.; Frost, D. L.; Higgins, A. J., Quenching distance of flames in hybrid methane–aluminum mixtures. Proc. Combust. Inst. 2015, 35, (2), 2463-2470. (22) Bellenoue, M.; Kageyama, T.; Labuda, S. A.; Sotton, J., Direct measurement of laminar flame quenching distance in a closed vessel. Exp. Therm. Fluid. Sci. 2003, 27, (3), 323-331. (23) Boust, B.; Sotton, J.; Labuda, S. A.; Bellenoue, M., A thermal formulation for single-wall quenching of transient laminar flames. Combust. Flame 2007, 149, (3), 286-294. (24) Sotton, J.; Boust, B.; Labuda, S. A.; Bellenoue, M., Head-on Quenching Of Transient Laminar Flame: Heat Flux And Quenching Distance Measurements. Combust. Sci. Technol. 2005, 177, (7), 1305-1322. (25) Karrer, M.; Bellenoue, M.; Labuda, S.; Sotton, J.; Makarov, M., Electrical probe diagnostics for the laminar flame quenching distance. Exp. Therm. Fluid. Sci. 2010, 34, (2), 131-141. (26) Westbrook, C. K.; Adamczyk, A. A.; Lavoie, G. A., A Numerical Study of Laminar Flame Wall Quenching. Combust. Flame 1981, 40, 81-97. (27) Turcios, M.; Ollivier-Gooch, C.; Huang, J., Numerical Calculation of Quench Distance for Laminar Premixed Flames Under Engine Relevant Conditions. SAE Technical Paper 2011-01-1997, 2011. (28) Donohoe, N.; Heufer, A.; Metcalfe, W. K.; Curran, H. J.; Davis, M. L.; Mathieu, O.; Plichta, D.; Morones, A.; Petersen, E. L.; Güthe, F., Ignition delay times, laminar flame speeds, and mechanism validation for natural gas/hydrogen blends at elevated pressures. Combust. Flame 2014, 161, (6), 1432-1443. (29) Zhang, Y.; Huang, Z.; Wei, L.; Zhang, J.; Law, C. K., Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures. Combust. Flame 2012, 159, (3), 918-931. (30) Miao, H.; Lu, L.; Huang, Z., Flammability limits of hydrogen-enriched natural gas. Int. J. Hydrogen. Energ 2011, 36, (11), 6937-6947. (31) Verma, G.; Prasad, R. K.; Agarwal, R. A.; Jain, S.; Agarwal, A. K., Experimental investigations of combustion, performance and emission characteristics of a hydrogen enriched natural gas fuelled prototype spark ignition engine. Fuel 2016, 178, 209-217. (32) Korb, B.; Kawauchi, S.; Wachtmeister, G., Influence of hydrogen addition on the operating range, emissions and efficiency in lean burn natural gas engines at high specific loads. Fuel 2016, 164, 410-418. (33) Ying, Y.; Liu, D., Detailed influences of chemical effects of hydrogen as fuel additive on methane flame. Int. J. Hydrogen. Energ 2015, 40, (9), 3777-3788. (34) Li, Q.; Hu, G.; Liao, S.; Cheng, Q.; Zhang, C.; Yuan., C., Kinetic Effects of Hydrogen Addition on the Thermal Characteristics of Methane−Air Premixed Flames. Energ. Fuel 2014, 28, 4118-4129.

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Effects of hydrogen addition on the standoff distance of premixed

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