Laminar Flame Calculations for Analyzing Trends in Autoignitive Jet

Energy Fuels , 2016, 30 (10), pp 8680–8690. DOI: 10.1021/acs.energyfuels.6b01264. Publication Date (Web): September 6, 2016. Copyright © 2016 Ameri...
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Laminar Flame Calculations for Analyzing Trends in Autoignitive Jet Flames in a Hot and Vitiated Coflow Paul R. Medwell,*,† Michael J. Evans,† Qing N. Chan,‡ and Viswanath R. Katta§ †

School of Mechanical Engineering, The University of Adelaide, Adelaide, Australia School of Mechanical Engineering, University of New South Wales, Sydney, Australia § Innovative Scientific Solutions Inc., Dayton, Ohio 45459, United States ‡

ABSTRACT: Experiments of autoignitive jet flames in a hot and vitiated coflow have previously shown various flame behaviors, spanning lifted flames to moderate or intense low oxygen dilution (MILD) combustion. For better understanding the behavior of flames in this configuration, regime diagrams and ignition delay results are presented from well-stirred reactor calculations across a wide range of operating conditions for methane and ethylene fuels. In conjunction with two-dimensional calculations, the importance of flame precursors and oxygen penetration across the reaction zone is revealed. It is found that widely accepted definitions and regime diagrams are inadequate to classify and reconcile the different flame behaviors that are observed experimentally. For accurate prediction of the ignition process, it is necessary to incorporate boundary conditions that capture minor species in the oxidizer. The role of fuel type also has a major impact on the ignition process and flame appearance.

1. INTRODUCTION The jet in hot co-/cross-flow (JHC) burner configuration has been widely used for fundamental studies of autoignitive lifted flames1−7 and moderate or intense low oxygen dilution (MILD) combustion.8−14 A key feature of both of these types of flames is that the stabilization process is governed by autoignition due to the high temperature of the oxidant stream.15 Autoignition occurs in both flame types, and they tend to be classified as either MILD or lifted. Despite these classifications, there remains uncertainty about the precise definition between these types of flames.11,16 Irrespective of the classification of the flames, the basic JHC configuration of fuel issuing into a hot environment is relevant to practical systems, such as compression-ignition engines17 and inter-turbine burner gas turbines.18,19 However, the link between the studies in JHC burners and practical applications is limited by a lack of knowledge of the flame behavior over a range of operating conditions. The objective of this paper is to provide further insight into the fundamental behavior of such flames with a longer-term view of enabling increased predictive capability under more applied-combustion conditions in future studies. A large number of experiments and computations have been performed in the JHC flame configuration.1−12,14 For the purpose of the current work, the focus will be on a narrow subset of similar experiments that show very different flame behaviors. In a vitiated coflow burner (VCB), Cabra et al.1,2 showed that CH4/air and H2/N2 jet flames demonstrate conventional lift-off behavior, albeit with strong sensitivity to the temperature of the coflow. Gordon et al.20−22 extended the work in a VCB, demonstrating that flame stabilization is by autoignition, consistent with other similar work.15 The base of these flames visually shows a steep gradient from non-burning to burning and thus appears similar to conventional lifted jet flames that occur in ambient air coflows. The autoignitive lifted flames in the VCB have not previously been classified as MILD © XXXX American Chemical Society

combustion but bear many resemblances due to the hot and vitiated coflow and are therefore considered to be complementary to the current study. In the current work, the focus is only on the CH4/air in the VCB2 and is herein referred to as VCB-MA. In a series of experimental campaigns, Medwell and Dally8,9,23 have shown that CH4/H2 and C2H4 (with various diluents) flames under MILD combustion conditions stabilize at the jet exit plane in the JHC burner. Increasing the coflow oxygen level (from 3 to 9% O2) causes the flames to visually appear lifted if H2 is not added to the fuel stream. However, a contiguous OH structure (“OH-tail”) that extends towards the jet exit plane from what looks like the lift-off height indicates that these flames are not in fact lifted but are instead referred to as “transitional”.23 The C2H4 flames in the JHC burner23 are referred to in the current paper as JHC-E3 and JHC-E9 for the flames in the 3 and 9% O2 coflow cases, respectively. In contrast to the JHC-E3 and JHC-E9 flames, Dutch natural gas flames by Roekaerts and co-workers10,24−26 in the Delft JHC burner (DJHC) visually appear lifted. They feature distinct ignition kernels and therefore more closely resemble the conventional autoignitive lifted behavior seen in a VCB than the typical gradual ignition characteristics of MILD combustion. Flame I in the DJHC burner,10 referred to as DJHC-I in the current paper, may be classified as MILD combustion according to one definition of MILD combustion27 but has since been shown to not meet all the criteria for MILD combustion.28 It is therefore apparent that the measurements and analysis of these types of flames are open to a degree of interpretation. This paper seeks to analyze in greater detail the requirements for a more robust classification. Received: May 26, 2016 Revised: September 5, 2016

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is unclear, especially how effects other than O2 concentration and initial temperature are incorporated. For addressing the issue of developing regime diagrams for nonpremixed flames, other approaches have been used previously in hot oxidant diluted oxidant (HDDO)38 and hot diluted diffusion ignition (HDDI)37,44 configurations. The focus of the current study is on the validity of simplified reactors for analysis of lifted-flame autoignition. In this configuration, ignition occurs in kernels with some premixed mixture of fuel and oxidant.15,21 This analysis is, hence, better represented by a WSR rather than a nonpremixed configuration for lifted flames. Furthermore, the use of WSR models has been demonstrated for attached flames in the JHC burner45 based on the distributed nature of MILD combustion.27,37,46,47 Regime diagrams using the WSR approach may differ from those of HDDI and HDDO configurations; however, assessing the validity of the WSR configuration in predicting flame behavior is what this work aims to examine. The applicability of regime diagrams based on autoignition processes in simplified reactors to turbulent flames in JHC burners requires reliable determination of the autoignition delay. It has been shown that turbulent DNS and laminar onedimensional flame calculations give the same ignition delay; however, it is critical that the boundary conditions are accurately specified; otherwise, the differences in ignition delay can be significant.48,49 Therefore, if the boundary conditions are accurately specified, regime diagrams based on autoignition have the potential to be valid. Although often overlooked, one of the important parameters to consider is the presence of minor species that are introduced from the oxidant stream consisting of combustion products.29,39,50 Although this may seem obvious, it remains unclear what particular chemical and physical features need to be included in flame calculations to ensure accurate a priori prediction of flame behavior. Without knowledge of the required boundary conditions, there remains uncertainty over the usefulness of generalized regime diagrams. To this end, the current paper presents regime diagrams and assesses the performance of these maps to classify four experimental flames. Autoignition has been shown to be the stabilization mechanism of flames in the JHC10,49 and VCB configurations.2,7,21 Under these conditions, the ignition delays predicted by WSR analyses are known to under-predict ignition delays of individual kernels due to heat losses and species transport.15,31,51 Analyses of simplified reactors can, however, be used to compare the ignition processes in isolated autoignitive kernels31,41 and in the simulation of MILD combustion.45 The focus of this paper is, therefore, not about the applicability of autoignition delay time but rather how metrics derived from autoignition can be combined in a generalized sense to capture the flame behavior in experimental flames. Despite progress in the successful modeling of many flames in the JHC configuration,3,21,25,52−67 predictive modeling of the flames has tended to be unreliable with parameters within the model typically requiring optimization for each flame condition to match the experimental measurements. In addition, there is no reliable methodology to distinguish between autoignitive lifted flames and MILD combustion. In light of these gaps, the aim of the present paper is to characterize the behavior of four JHC flames based on dominant effects that are independent of the burner geometry. This paper does not intend to resolve all of the parameters required to demarcate a flame under every

Drawing meaningful comparisons between previous JHC and VCB burner studies is difficult because each burner has a slightly different geometry with no overlap between fuel stream and coflow conditions. What is particularly unclear is the underlying reasons for the different observations noted between each study. In particular, whether the differences are governed by physical or chemical processes. The focus of the current paper is on the chemical effects, to see if they alone are capable of explaining the differences that are observed in the experimental flames. In an effort to reconcile the differences between JHC flames with different coflow oxygen levels, a series of strained laminar flame calculations for a CH4/H2 mixture have been presented previously.29 It was shown that flames with 21% O2 extinguish at a lower strain rate than with either 3 or 9% O2 in the coflow. This points towards a fundamentally different stabilization mechanism at these low O2 concentrations, especially as the conditions of MILD combustion are approached. In particular, it has been shown that the formation of a pool of precursors (notably H2CO) helps to stabilize the flame.30 This pool of precursors under MILD combustion conditions intensifies at higher strain rates and enables stable flames at high strain rates.29 These observations are consistent with direct numerical simulation (DNS) studies by Chen and co-workers,31−33 which suggested that flames in the JHC configuration can combine features of both lifted-flame behavior as well as precursory reactions upstream of the transition height. However, what happens under other conditions remains largely unknown. Each of the previous experimental investigations has been conducted with a different set of conditions (fuel, coflow temperature, coflow composition, Reynolds number, etc.) and what happens in the transition between these conditions remains unresolved. Medwell and Dally11 attempted to investigate this issue through visual observation and CH* chemiluminescence imaging across a wider range of conditions. It was indicated that lift-off height has a non-monotonic behavior with either coflow O2 or coflow temperature. On either side of the peak lift-off height, flames seem to exist as either lifted flames or in the MILD combustion regime. Nevertheless, defining exact bounds for reliable flame classification from that experimental parametric study was not possible because it was limited to visual observation. Indeed, despite many previous studies using various burners, it has not yet been identified whether it is possible to classify flames based solely on the operating conditions. The lack of a reliable classification of flames is the result of different metrics between studies as well as differing interpretations of the measurements. Identifying suitable metrics for classification of these types of flames is one of the objectives of the current paper. For broadening the relatively narrow range of operating conditions achieved in the previous JHC burners, a number of simplified zero- and one-dimensional analyses have been performed. 34−41 In particular, from well-stirred reactor (WSR) calculations, a widely adopted definition of MILD combustion was provided by Cavaliere and de Joannon27 based on the inlet, equilibrium, and autoignition temperatures. Other regime diagrams spanning conventional to MILD combustion have previously been reported in terms of dilution by exhaust gases.42,43 Although these regime diagrams provide a good framework, the parameters on which they are based may be difficult to identify, especially in nonpremixed flames. Furthermore, the criteria on which these definitions are based B

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lations based on the composition of the inlet gases of the coflow burners (Table 1). 2.2. Chemkin Simulations of Simplified Reactors. For the purpose of producing regime diagrams, a series of simulations of simplified reactors are performed. In the calculations, the reactant temperature and oxygen concentration are varied independently, consistent with the controlled parameters of the coflow in the JHC burner. The resultant regime diagrams use similar definitions proposed by Cavaliere and de Joannon27 but differ from many others in the literature because they are presented as a function of the controlled parameters in a JHC burner (namely, reactant temperature and oxygen concentration) rather than the temperature increase,27 C/O ratio,68 or recirculation rate.42 In the current work, the autoignition temperature is determined from transient calculations for each O2 level rather than a single value taken from the literature as assumed previously.27 The autoignition temperature is defined as the required reactant temperature of a particular mixture to achieve an ignition delay time of less than 0.5 seconds. This time was chosen to be consistent with previous studies.34,35,68 The reported ignition delay times are determined from the WSR calculations by the maximum rate of change of temperature consistent with the concept of thermal runaway used previously.31,51 This approach has also previously been shown to yield similar ignition delay times to the 10 K temperature increase used in other studies.41 Well-stirred reactor (WSR) models are widely used as a tool to investigate lifted flames.15,33,41 Although the flames considered in this paper are predominately nonpremixed, previous attempts at replicating experimental trends using opposed-flow calculation have not performed well due to fundamentally different assumptions in the flow field.29,41 One of the key objectives of the paper is to identify alternative approaches to predict laminar flame behavior without resorting to detailed two-dimensional time-dependent simulations; however, such detailed simulations are also performed for comparative purposes in this paper. Therefore, the use of wellstirred reactor models to identify trends in the nonpremixed experimental flames is a first-step toward reconciling the differences and assessing the utility of the resultant regime diagrams. For a detailed assessment of the chemical kinetics of interest to this study, the AURORA module of the Chemkin software suite (version 4.1) is used. Transient simulations were run to determine both the steady-state value of the WSR but also enable the ignition delay to be calculated. Both the GRI-Mech 3.0 and the University of California at San Diego (UCSD, version CK_2014-07-31) kinetic mechanism schemes are included in the Chemkin simulations to assess the sensitivity of the findings to the kinetic model. Calculations are performed for methane (CH4) and ethylene (C2H4) fuels. The initial reactant temperature and the O2 concentration are independently varied, and the fuel/O2 ratio is adjusted to ensure stoichiometry. Two sets of runs are presented. In one, the mixture is assumed to only have fuel/ O2/N2 with a total of 240 conditions for each of the two fuels with 15 O2 levels and 16 inlet temperatures. Another complete set of calculations was performed with the inclusion of constant volumetric levels of CO2 and H2O at 3 and 10% respectively, representing the typical composition of the coflow.9 Additional calculations are also reported with the addition of minor species at equilibrium concentrations to the mixture. Stoichiometric

possible scenario. Instead, it puts forward an alternative means of identifying and classifying flames over a wide range of conditions. For this study, the focus is on the chemistrydominated effects to distinguish different operating regimes. The influence of turbulence is recognized as important and will be the focus of subsequent studies.

2. METHODOLOGY 2.1. Flame Conditions of Interest. In this study, four flames that have been experimentally measured have been identified and given a designation as shown in Table 1. The Table 1. Experimental Jet Flames in a Hot and Vitiated Coflow Considered in the Present Study

a

designation

fuel

coflow T (K)

coflow O2 (vol/vol)

VCB-MA DJHC-I JHC-E3 JHC-E9

CH4/air DNGa C2H4 C2H4

1350 1540 1100 1100

12% 7.3 % 3% 9%

ref 2 10 23 23

DNG = Dutch natural gas.

four flames are produced with three different burners and given the designations VCB (vitiated coflow burner2), DJHC (Delft jet-in-hot-coflow10), and JHC (jet-in-hot-coflow23). All of the burners consist of a central fuel jet (djet ≈ 4.5 mm), which issues into a hot coflow from a lean premixed secondary burner mounted upstream of the jet exit plane. The four flames in Table 1 span autoignitive lifted flames (VCB-MA2) to MILD combustion (JHC-E323). Two intermediate cases are also considered: DJHC-I10 and JHC-E9.23 These both fall under one definition of MILD combustion27 but not another.28 The DJHC-I flame10 is classified as MILD combustion but also displays visual features more typically associated with a conventional lifted flame, such as a clearly defined flame base. The JHC-E9 flame23 is characterized by a transitional behavior, giving lifted flame appearance but with OH and H2CO upstream of the apparent flame base. In addition to coflow temperature, one of the key defining parameters distinguishing the different flames considered in Table 1 is the O2 concentration in the coflow. Moreover, in addition to variation of the O2 level, the composition and ratio of the other gases in the coflow are different as well. In all cases, the coflow consists of products of a premixed flame upstream of the jet exit plane. The premixed flame for each of the burners listed in Table 1 consists of a different fuel mixture: H2/air for VCB-MA, DNG/air for DJHC-I, and CH4/H2/N2/air for JHCE3 and JHC-E9. In addition to the major species products of combustion (O2, N2, CO2, and H2O), the coflow also includes a range of minor species (notably OH). These minor species are known to play an important role in the stabilization of these flames.29,39,50 In the present study, this impact is further assessed computationally. To expand the operating domain of the four key flames (Table 1), this study also investigates CH 4 and C 2 H 4 combustion in several different oxidizer mixtures. A simple O2/N2 oxidizer is considered to solely investigate the role of O2 level. The addition of other major species encountered in the coflow of the four target flames are also considered (O2, N2, CO2, and H2O). Furthermore, additional analysis investigates the role of minor species (listed in section 3.6) in the oxidant stream at concentrations determined by equilibrium calcuC

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Energy & Fuels conditions have been chosen rather than the “most reactive mixture”41,48,69,70 for three reasons: (1) this is consistent with previous attempts to generate regime maps;27 (2) although the distance between the most reactive and the stoichiometric mixture fraction increases with increased dilution,70 the actual difference in ignition delay time is quite small under MILD combustion conditions;28 and (3) computational convenience dictates that only a single stoichiometry be chosen for each condition on each of the maps (determining the most reactive mixture at every point in the regime map would be excessive and negate the advantage of trying to develop a simplified map). 2.3. Unsteady Ignition and Combustion Using Reactions (UNICORN) Calculations. Opposed-flow onedimensional laminar flame calculations of JHC flames have been reported previously.29 Such calculations can provide detailed information about ignition processes but have some inherent weaknesses when used as an analogy for lifted jet flames. First, the mixing that occurs in lifted flames creates premixed flame pockets, which are believed one of the primary stabilization mechanisms for these types of autoignitive flames.15 Second, the time-dependent nature of the ignition processes are not readily captured accurately in most opposedflow simulations. For circumventing these issues, the UNICORN solver is used to perform axisymmetric twodimensional time-dependent laminar flame calculations. Further details of the UNICORN code have been presented previously.71−74 In brief, it is a time-dependent mathematical model for the simulation of unsteady reacting flows. For the current study, however, only steady laminar flows are considered. Under these conditions, UNICORN performs direct numerical simulations. For consistency with the Chemkin calculations, the University of California at San Diego (UCSD) kinetic mechanism is used. The composition of the mixtures is consistent with the four flames (Table 1) and includes both major and minor species at equilibrium concentrations. The geometrical configuration used for the UNICORN simulations consists of a coflowing stream of fuel and oxidant. A simplified case of a fuel jet in a depleted oxygen hot coflow is considered, where the velocity of both streams is 1.0 m/s and the flow within the domain is laminar. The focus on laminar flames is to decouple the chemistry from the turbulence while still retaining the spatial and temporal evolution of the reaction zone.

Figure 1. Regime diagram for a CH4/O2/N2 mixture (Φ = 1) showing four combustion domains (refer Table 2): N.R., no Reaction; Conventional, conventional combustion; Hot, hot flames; and MILD, MILD combustion. Two chemical kinetic mechanisms are considered: (a) GRI-Mech 3.0 and (b) University of California San Diego (UCSD).

temperature, which gives an ignition delay time of less than 0.5 seconds (as defined in section 2.2). The four combustion domains are defined in Table 2 based on definitions previously suggested by Cavaliere and de Table 2. Definition of Combustion Domains Based on Reactant (Tinlet), Equilibrium (Tequil), and Auto-Ignition (Tai) Temperatures from Well-Stirred Reactor Calculations27 ΔT = Tequil − Tinlet

Tinlet < Tai

Tinlet > Tai

ΔT < Tai ΔT > Tai

no reaction conventional

MILD combustion hot flames

Joannon.27 The boundary between adjacent regimes is shown by linear interpolation between adjoining points in the regime maps. In these intermediate regions, calculations have not been performed but will fall into one of the neighboring regimes. Comparison of the results in Figure 1 from the two kinetic mechanisms indicates that the ignition temperature predicted by each mechanism is slightly different (lower by ∼100 K with UCSD). Nevertheless, the basic shape and trends are the same. Both kinetic mechanisms give the same qualitative regime maps for CH4 fuel and indicate that the ignition temperature is around 1000 K, which is consistent with previous studies.27 Although the GRI-Mech 3.0 mechanism includes C2H4 and has previously been used for ethylene combustion,62,63 it is developed primarily for methane and natural gas combustion. In contrast, the UCSD mechanism has been specifically designed to include ethylene;75 hence, it will be used for all subsequent results presented in this paper. The regime diagrams presented in Figure 1 are a simplified map to give an indication of the combustion domains encountered under these conditions. This approach does not include other major and/or minor species that would be encountered in actual flames. It also assumes stoichiometric conditions and not the “most reactive mixture”.41,48,69,70 Furthermore, this diagram does not reveal the unsteady/lifted flame region that occurs between the “hot” flames and the MILD combustion domains.42,43 Detailed analyses of the transient solutions that occur in this transitional region have been presented elsewhere.34,36,39 Despite the limitations, this type of regime map has been reported previously to indicate the conditions required to achieve MILD combustion. From Figure 1, it is deduced that the methane flames considered in this

3. RESULTS 3.1. Combustion Regimes from WSR: Methane Fuel. Figure 1 demonstrates the MILD combustion operating regime in the domain of oxygen (O2) concentration and reactant temperature. This figure has been generated by a series of WSR simulations for a stoichiometric mixture of CH4/O2/N2 using two different chemical kinetic mechanisms (GRI-Mech 3.0 and UCSD). Such regime maps have previously been used to classify flames; although the validity of this approach is considered in the current work, the primary purpose of these diagrams is to visually indicate the conditions required to establish the various regimes. The reactant temperature and O2 concentration are independently varied (N 2 is adjusted to balance the composition). For each combination of reactant temperature and O2 concentration, the equilibrium and auto-ignition temperatures are determined. The auto-ignition temperature (Tai) is based on calculations for each O2 level as the reactant D

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the lift-off height of some of the DJHC-I flames depending on the jet Reynolds number.10 Therefore, the stabilization of this particular flame could be influenced by the third stream (surrounding air) rather than autoignition in these experiments; an influence that has been studied in greater detail by Oldenhof et al.24 3.3. Experimental Observations: Ethylene Fuel. The methane-based VCB-MA and DJHC-I flames are autoignitive20 and display a typical lifted flame base. In contrast, Figure 3

paper (Table 1) fall within the MILD combustion operating regime. 3.2. Ignition Delay from WSR: Methane Fuel. The two methane-based flames in Table 1 (VCB-MA and DJHC-I) both visually appear lifted.2,10 One approach to compare the lift-off behavior is to look at the ignition delay from the well-stirred reactor calculations, especially because these flames are considered autoignitive.20 Figure 2 reports the ignition delay times for a stoichiometric CH4/O2/N2 mixture over a range of reactant temperatures and

Figure 2. Calculated ignition delay times for a CH4/O2/N2 mixture (Φ = 1) across a range of O2 levels (3−30% by volume).

O2 levels. For a particular reactant temperature, the ignition delay typically increases by an order of magnitude between conventional air and the depleted oxygen environment associated with MILD combustion. For a specified temperature, the influence of O2 concentration is seen to have a monotonic effect on the ignition delay of the mixture. This is in contrast to the non-monotonic behavior observed in previous experiments.11 However, these previous experimental findings were only by visual observation, so it is unclear whether the flames were actually lifted or instead transitional (refer to section 3.3). Nevertheless, the appearance of the flames revealed that the apparent lift-off (or transition) height was seen to increase with a reduction of coflow O2 from 9 to 6%, but the location of the transition point subsequently decreased from 6 to 3% O2 (i.e., the apparent flame base was higher at 6% as compared with that at either 3 or 9% O2).11 If the results in Figure 2 are taken at face value, the autoignition delay time for the VCB-MA flame is ∼6 ms. Even though many physical effects encountered in the experimental flame are not accounted for in the well-stirred reactor calculation (such as assuming stoichiometric conditions rather than the most reactive mixture), this ignition delay is similar to that determined by an unsteady flamelet/progress variable (UFPV) model of this flame.57 Further insight into the unsteady ignition processes and their impact on lift-off height prediction has been subsequently presented.28 The DJHC-I flame is described as MILD10 as a result of the low O2 level in the coflow. It is therefore expected to have a relatively long ignition delay time. However, in comparison with the VCB-MA flame, the higher coflow temperature counteracts the role of the low O2 level such that the DJHCI ignition delay is almost an order of magnitude less than for the VCB-MA flame (0.7 compared with 6 ms). This highlights the fact that ignition delay alone is insufficient to differentiate between autoignitive lifted flames and MILD combustion. A potentially complicating factor in the comparison of the flames is that surrounding air entrainment is known to occur in the experimental measurements at around the 120 mm downstream location in the JHC burner,8,9,24 which corresponds to

Figure 3. Experimental measurements of JHC-E3 and JHC-E9 flames.23 (a) Experimental photographs and (b) laser diagnostic images of OH-LIF, H2CO-LIF, and temperature (via Rayleigh scattering) at a measurement height of 35 mm above the jet exit plane. The laser diagnostic images are 30 × 8 mm, and the jet centreline is marked by the vertical dashed line.

presents experimental observations for ethylene flames (JHCE3 and JHC-E9). Figure 3(a) shows photographs of the flames, whereas Figure 3(b) are sample instantaneous measurements of OH and H2CO (measured with laser-induced fluorescence (LIF)) and temperature (deduced from laser Rayleigh scattering).23 The visual observations of these flames can be misleading. Although a more detailed analysis has been presented previously,23 an interpretation of these flames is provided in the current paper to aid in the interpretation of the new results that are presented in subsequent sections. It should be noted that the influence of the coflow on the jet flame only extends to an axial location around 120 mm; beyond this height, the entrainment of surrounding air leads to uncontrolled oxidant stream temperature and composition. The experimental photograph of the JHC-E3 flame in Figure 3(a) has very low luminosity, consistent with the “flameless” nature of MILD combustion.42 The low level of luminosity gives a false impression of a lifted flame, but in reality, it is possible to distinguish a faint reaction zone to the jet exit plane. This is supported by laser diagnostic images of the OH radical, formaldehyde, and temperature. The images of each scalar presented in Figure 3(b) are 30 × 8 mm, centered at the 35 mm axial location with the jet centerline marked by a vertical white dashed line. An OH layer is clearly present between the jet centerline and the coflow oxidant stream, indicative of a E

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Energy & Fuels reaction zone in the JHC-E3 flame at the 35 mm location despite the low luminosity at this height. Further evidence of a reaction in this region is the measurement of a strong formaldehyde (H2CO) signal. Subsequent computational modeling of this flame also predicts an attached jet flame.62 Therefore, despite the appearance of Figure 3(a), it is surmised that the JHC-E3 flame is attached. The JHC-E9 flame, shown in Figure 3, has some unusual characteristics that make its classification and description open to some level of interpretation. This flame has previously been described as “transitional” because it displays a transition in the intensity of the reaction zone at a height of approximately 35 mm above the jet exit plane. This transition gives the flame the visual appearance that it is lifted by approximately 35 mm; however, laser diagnostic measurements of OH and H2CO reveal evidence of a reaction zone both upstream and downstream of this height. The OH images collected at the 35 mm location frequently show a strong reaction zone at the top of the image. Upstream of the strong OH signal, it is possible to identify a weak OH tail that extends to the jet exit plane. The presence of OH upstream of the transition point, coupled with high levels of formaldehyde in the lower regions of these flames, tends to indicate that they are not lifted according to the typical definition. It is therefore inappropriate to classify them with other (lifted) flames in other combustion regimes that do not display this type of behavior. Highly resolved DNS studies support the assertion that there is an intense buildup of precursors in the region upstream of the transition point of flames in a hot coflow.32,33,76 Therefore, the most apt description of these flames seems to be “attached” but with a “transition” point, which visually manifests itself as an apparent lift-off height. 3.4. Ignition Delay from WSR: Ethylene Fuel. To compare the observation that the JHC-E3 flame appears attached but the higher O2 JHC-E9 flame appears transitional, Figure 4 presents the calculated ignition delay times for a

approximately 1100 K (1000/Tinlet ≈ 0.91). This feature is expected to be the result of crossover between low- and hightemperature chemistry that occurs at approximately 1100 K,77 although further work would be needed to explore this phenomenon in further detail. This inlet temperature (1100 K) is the same coflow temperature of both JHC-E3 and JHCE9 (refer to Table 1), and thus, the experimental flames are expected to demonstrate artifacts of operating close to the crossover temperature. 3.5. Combustion Regime Diagram: Ethylene Fuel. Measurements of the C2H4 flames (section 3.3) reveal significant differences in their appearance. From the regime diagram for a C2H4/O2/N2 mixture presented in Figure 5(a), it

Figure 5. Regime diagram for a C2H4 mixture (Φ = 1) showing four combustion domains (refer to Table 2): N.R., no reaction; Conventional, conventional combustion; Hot, hot flames; and MILD, MILD combustion. Two oxidant compositions are considered: (a) C2H4/O2/N2 and (b) C2H4/O2/N2/CO2/H2O.

is identified that both JHC-E3 and JHC-E9 flames are in the MILD combustion regime. The different appearance and behavior between the two flames, despite a similar classification, indicates there may be some limitations using this simplified analysis for these flames. The composition of the mixture in Figure 5(a) neglects the presence of other species present in the oxidant stream of the experimental flames (JHC-E3 and JHC-E9). In the experiments, the composition of the coflow oxidant stream is adjusted such that the CO2 and H2O concentrations are constant (3 and 10%, respectively), whereas the temperature and O2 level are varied. For the influence of these major species to be examined, Figure 5(b) presents the same regime map as Figure 5(a) but includes the addition of CO2 and H2O into the mixture. Comparison of the regime map with/without the inclusion of CO2 and H2O (Figure 5(a) and (b)) shows only minor differences, even though previous studies have shown that N2 and CO2 dilution have both physical and chemical effects.78,79 Even with a better representation of the oxidant composition, the regime map still indicates that the JHC-E3 and JHC-E9 flames are both classified as MILD combustion despite their different appearances. It should be noted that other studies have looked at the detailed kinetic differences with different inert species,68,79,80 but for the purpose of the present analysis, these differences are not significant enough to affect the regime map. 3.6. Role of Minor Species. The regime diagrams and ignition delay calculations presented in this paper have indicated that all four flames under consideration fall within the MILD combustion regime even though they each have distinctive flame appearances and behaviors. One important

Figure 4. Calculated ignition delays for a C2H4/O2/N2 mixture (Φ = 1) across a range of O2 levels (3−30% by volume).

C2H4/O2/N2 mixture. A reduction in the O2 concentration is seen to increase the ignition delay across the entire range of conditions. Although expected, this observation contradicts the trends noted in the experimental flames where the 3% O2 (JHC-E3) flame seems to ignite at the jet exit plane rather than further downstream for the 9% O2 (JHC-E9) case (refer to section 3.3). The ignition delay from Figure 4 is 8 and 5.3 ms for the JHC-E3 and JHC-E9 flames, respectively. Although the difference is small, the trend is not exactly consistent with the experimental observations, perhaps indicating a different stabilization mechanism between these two flames. In comparison with the methane case (Figure 2), the ethylene ignition delay curve in Figure 4 shows a trend towards a collapse in ignition delay time at an inlet temperature of F

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between the 3 and 9% O2 cases. At 9% O2, the OH profile is seen to undergo a continuous increase in OH concentration until peak OH, whereas at 3% O2, after an initial increase, the OH concentration remains relatively constant for a prolonged period ahead of the rise to the peak OH concentration. In both cases, the initial rise in OH starts almost instantaneously and has a similar rate of increase to the first peak. This is consistent with the OH measurements near the jet exit plane of JHC-E3 and JHC-E9, where both flames show evidence of OH near the exit plane (refer to section 3.3). For both flames, the concentration of OH is limited by consumption through the reaction C2H4 + OH ⇋ C2H3 + H2O, demonstrating that the minor species in the radical pool serves to consume the fuel directly. Production of OH during this first ignition stage is predominantly from the reactions HO2 + H ⇋ 2OH and, to a lesser extent, H + O2 ⇋ OH+O in the 3% O2 flame. In the 9% O2 flame, however, OH is mostly produced through the reaction CH3 + O2 ⇋ H2CO + OH, enhancing the conversion of CH3 to H2CO (as seen in CH4 combustion near the MILD regime39). The importance of this reaction is likely due to the increased availability of O2, which also increases the production of OH from the H + O2 reaction to above that of HO2 + H. The slow but steady increase in OH following the first ignition stage provides evidence supporting the assertion that the flames in Figure 6 are attached. During this phase, at approximately 0.6 ms into the reaction, the OH pathways in both flames are similar to the H + O2 responsible for producing the majority of OH. The bulk of OH consumption in this phase is still through direct reactions with C2H4 and, to a lesser extent, H2CO (forming HCO and H2O) and H2CCO (forming H2COH and CO). The significantly different rates of OH production at this stage are due to increased O2 in the 9% O2 flame, which pushes forward the production of HO2 (through HCO + O2 ⇋ CO + HO2) and, hence, OH. Both flames show evidence of two-stage ignition, but this effect is far more pronounced in the 3% O2 flame, where the temporal distribution of the MILD combustion reaction is apparent in Figure 6. The long ignition process of the 3% O2 flame supports the common description of MILD combustion being a “distributed” reaction27,37,45−47 accompanied by flame thickening and spreading.38 The temporal profile of the 3 and 9% O2 flames are different, yet both occur within the MILD combustion regime in the definition from Table 2. A fundamental difference between jet flames in 3 and 9% O2 environments has been discussed previously,30 but the consideration of temporal differences has not. It is therefore apparent that, within the MILD combustion regime proposed by Cavaliere and de Joannon,27 there exists multiple flame stabilization mechanisms and flame behaviors in the JHC configuration. Although this has been reported previously,11 there has been no attempt to relate these differences to the definition and classification of MILD combustion in the JHC burner instead of the well-stirred reactor configuration. 3.8. Two-Dimensional Structure. The well-stirred reactor calculations presented in this paper indicate that single-valued global metrics have limited value in mapping the flames in Table 1 to the simplified combustion operating regime diagrams. Investigating the time-evolution of a well-stirred reactor adds some insight but still neglects the nonpremixed nature of the experimental flames. To provide more detailed analysis while still focusing on chemistry-dominated effects and avoiding the complexity of turbulent flows, a two-dimensional

aspect that has been omitted from the calculations is the inclusion of minor species that are present at equilibrium concentrations in the oxidant stream of the experimental flames. Previous studies have reported that minor species can have a large influence on the ignition of these flames29,39,50 and affect the lift-off height.54 To account for this, minor species at equilibrium levels from the auxiliary burner have been added to the oxidant stream for each of the flames (Table 1). Table 3 Table 3. Ignition Delay Time for Different Flames (Refer to Table 1) and Three Different Oxidizer Compositions: Simple, O2/N2; Major Species, O2/N2/CO2/H2O; and Minor Species, O2/N2/CO2/H2O/H2/CO/OH/O/H flame

simple

major species

minor species

VCB-MA DJHC-I JHC-E3 JHC-E9

6.8 ms 1.27 ms 6.2 ms 5.0 ms

7.5 ms 1.38 ms 4.8 ms 5.3 ms

6.2 ms 1.29 ms 0.69 ms 0.84 ms

shows the ignition delay at stoichiometric conditions for the different oxidant compositions at the corresponding temperature from Table 1, assuming an oxidant of O2/N2 (simple), O2/N2/CO2/H2O (major species), and O2/N2/CO2/H2O/H2/ CO/OH/O/H (minor species). Consistent with the comparison of the regime diagrams in Figure 5, the addition of CO2 and H2O to the O2/N2 oxidizer mixture has little effect on the ignition delay time for any of the four flames. Adding minor species at the levels encountered in the methane-based flames (VCB-MA and DJHC-I) is also quite insignificant to the ignition delay times. However, the ethylene flames (JHC-E3 and JHC-E9) are both very sensitive to the addition of minor species with the ignition delay almost an order of magnitude faster. These ignition delay calculations are therefore much closer to the actual conditions encountered in the JHC-E3 and JHC-E9 flames, and because the ignition delay is faster for these flames, it begins to explain why the ethylene flames show indications of being attached, as distinct from the methane-based flames, which are lifted. 3.7. Temporal Evolution: Ethylene Fuel. For looking further into the ignition process, Figure 6 presents the time

Figure 6. Temporal profile of OH mole fraction for C2H4 flames with minor species (Φ = 1) at two O2 levels.

evolution of the OH radical in the JHC-E3 and JHC-E9 flames from well-stirred reactor calculations with minor species added to the oxidant. This analysis starts to provide greater insight into the ignition and reasoning as to why simple metrics based on a single ignition delay or autoignition temperature fail to capture the flame features seen experimentally due to the complex time-varying nature of the ignition process. Comparison of the temporal profile of OH presented in Figure 6 demonstrates that the ignition process is very different G

DOI: 10.1021/acs.energyfuels.6b01264 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels laminar axisymmetric streaming flow CFD model has been generated using the UNICORN code. This configuration enables an investigation of the chemical and diffusive timescales without turbulent transport and with minimal shear. This investigation is not intended to replicate the experimental flame conditions but instead provide insight into the chemical-driven processes in this streaming flow configuration. Figure 7

Figure 8. Calculated OH, H2CO, HCO, and O2 mass fraction for a laminar C2H4 flame in a 3% O2 coflow (JHC-E3 and JHC-E9). Note that OH and HCO mass fractions have been multiplied by 200 and 105, respectively, to highlight low concentration regions with Tjet = 305 K, Tcoflow = 1100 K, and U̅ jet = U̅ coflow = 1.0 m/s. Dimensions are in millimeters.

equilibrium OH in the coflow (from the secondary burner). Radially, the OH in the coflow is seen to be consumed on the lean side of the reaction zone. These results are consistent with autoignitive flames and support the presence of a buildup of radicals to the jet exit plane.54 Of note in Figure 8(b) is that, although there is no O2 in the fuel jet stream, significant O2 is seen to occur along the jet centerline, especially up to 50 mm above the jet exit plane. Although this flame features low strain, this observation is consistent with previous reports of high levels of O2 diffusion across the reaction zone under these conditions, which becomes even more pronounced under high strain rate conditions.30 The additional role of turbulence in the experimental flames is expected to enhance mixing and O2 entrainment. In combination, the effects of strain and mixing observed previously will tend to promote even stronger partial premixing in the experimental flames with turbulence. This may explain why the measurements tend to indicate an attached flame as distinct from the transitional flame structure noted in the low velocity, low strain rate, laminar calculations presented here. The enhanced effect of increased jet velocity on reducing the apparent lift-off height is consistent with that of previous experimental studies.10,11 In addition to OH and H2CO that are seen to extend to the exit plane for the 3% O2 flame in Figure 8(a), the HCO species distribution shown in Figure 8(b) provides further evidence of reactions in the region upstream of the strong OH concentration. Although the formyl (HCO) radical is widely accepted as a marker of heat release rate,81 this may not be the case under MILD combustion conditions.82 Nevertheless, the significant buildup of radicals in these types of flames is consistent with previous observations.20,22,54 An important difference is the relative strength of the build-up in the flames at 3% O2 presented here; the HCO concentration is higher in the “lifted” region of the flame than further downstream in the flame where the OH concentrations increase significantly.

Figure 7. Calculated temperature and OH mass fraction for a laminar C2H4 flame in 3 and 9% O2 coflows (JHC-E3 and JHC-E9, respectively) with Tjet = 305 K, Tcoflow = 1100 K, and U̅ jet = U̅ coflow = 1.0 m/s. Dimensions are in millimeters.

presents the temperature and OH mass fraction for laminar flames with compositions consistent with JHC-E3 and JHC-E9, and thus the same naming convention will be retained despite different flow fields. It it essential to emphasize that the flow conditions between the calculations and the experiments are very different. As outlined in section 2.3, the simulations presented in Figure 7 are performed with the jet and oxidant stream at the same velocity (namely, both jet and coflow at 1.0 m/s) and under laminar flow conditions. This is to ameliorate the effects of shear and turbulence, which are prevalent in the experiments and enable a focus on the chemistry. Despite the different flow conditions, the OH profile from the simulations in Figure 7 highlights that both the JHC-E3 and JHC-E9 flames appear attached. Although the JHC-E3 flame shows a transitional behavior at around 60 mm above the jet exit plane, there are reactions that occur upstream of this height. This transitional behavior is analogous to the profile in Figure 6. To more clearly compare the well-stirred reactor trends to those from the two-dimensional calculations for the JHC-E3 flame, Figure 8(a) presents the same OH mass fraction results from Figure 7(b) but with the color scale chosen to be more appropriate for the low OH concentration encountered under MILD combustion conditions. For further comparison with the experimental results, Figure 8(a) also presents the results of H2CO mass fraction from the calculations. It is clear that the trend of a relatively constant OH concentration for a prolonged period ahead of the transition to fully-fledged combustion is apparent, consistent with Figure 6. It is noteworthy that, in the region near the jet exit plane, the OH concentration in the reaction zone is of a similar order of magnitude to the H

DOI: 10.1021/acs.energyfuels.6b01264 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels These observations support the notion of a spatially distributed reaction zone under MILD combustion conditions.37,45 The 3% O2 C2H4 flame gives an initial appearance of a lifted flame but shows strong reactions all the way to the exit plane, indicating that it is burner stabilized, consistent with the expected behavior in the MILD combustion regime. This behavior is also consistent with the experimental observations in the JHC-E9 flame. Therefore, although both C2H4 flames individually appear different in either the presented experiments or the CFD modeling, it seems that the fundamental behavior is consistent, just that the process is stretched or compressed spatially and/or temporally. In both cases, the flame ignition is a gradual process that starts at/near the jet exit plane. The structure of these flames is in contrast to the methane-based cases (VCB-MA and DJHC-I), which are truly autoignitive, where the first ignition kernel spontaneously initiates at a downstream location in the flame.

experimental measurements. However, for the purpose of developing regime diagrams, they are not practical. In summary, provided that the chemical boundary conditions are sufficiently well-defined, flame behavior that has been observed experimentally can be replicated by simplified calculations. For the flames to be classified accurately, temporal/spatial evolution of species profiles needs to be considered, not simple global parameters (such as the increase in temperature of a well-stirred reactor). Future work is required to examine the role of turbulence, but laminar flame calculations are adequate to predict the overall trends of turbulent flames in a hot and vitiated coflow.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS Four flames that consist of a fuel jet, which issues into a hot, low-oxygen environment, have been compared. The flames have previously been described as either autoignitive lifted flames or MILD combustion. In this work, the flames have all been classified as MILD combustion based on regime maps developed using a widely accepted criterion for MILD combustion. However, all four flames demonstrate different flame behaviors and appearances. In an effort to classify these flames, a series of well-stirred reactor models, autoignition delay calculations, regime diagrams, inclusion of minor species, and temporal/spatial evolution simulations have been presented. It has been determined that the differences in flame behavior are not directly linked to global predictions from well-stirred reactors or autoignition delay time calculations. The nonmonotonic behavior that has been observed in other previous experiments is also not captured in these calculations. Therefore, characterization of flames in the jet in hot co-/ cross-flow (JHC) configuration requires detailed analysis to reproduce trends observed experimentally. Regime diagrams from well-stirred reactor calculations using simplified metrics are not well suited to the JHC configuration. Such diagrams tend to classify flames as MILD combustion even if their appearance and behavior is not consistent with the expectations of MILD combustion. Therefore, prediction of flame behavior in these types of flames requires a more thorough analysis. In particular, it is essential to take into consideration the presence of minor species that occur in the oxidizer streams of these types of flames. With the inclusion of minor species into a well-stirred reactor mixture, it is possible to replicate the temporal trends in ignition that are observed to occur in experimental flames. Although incorporating minor species indicates that it is possible to obtain useful information from well-stirred reactor models, determining the local composition may be challenging and therefore negates the advantages of simplified maps. Similarly, interpretation of the time-dependent solutions requires additional analysis beyond simple definitions based on increasing temperature of the system. Laminar axisymmetric models of the flames show similar trends to the temporal ignition of the well-stirred reactors and agree with the behavior observed experimentally. Importantly, these provide greater insight into the ignition processes than is possible from either the well-stirred reactor calculations or

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of The University of Adelaide, the Australian Research Council (ARC), the Asian Office of Aerospace Research and Development (AOARD) and the Air Force Office of Scientific Research (AFOSR).

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b01264 Energy Fuels XXXX, XXX, XXX−XXX