Combustion Characteristics of a Methane Jet Flame in Hot Oxidant

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Combustion characteristics of a methane jet flame in hot oxidant coflow diluted by H2O versus the case by N2 Chong Dai, Ziyun Shu, Pengfei Li, and Jianchun Mi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03060 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Combustion characteristics of a methane jet flame in hot oxidant coflow diluted by H2O versus the case by N2 C. Daia, Z. Shua, P. Lib and J. Mia* a

Department of Energy & Resource Engineering, College of Engineering, Peking University, Beijing 100871, China b

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China *

Corresponding author: E-mail: [email protected]; Fax: +86-10-62767074

Abstract This numerical study investigates combustion characteristics of a CH4 jet flame in hot oxidant coflow (JHC) diluted by H2O versus the case by N2. Both MILD oxy-fuel combustion and traditional combustion are generated by varying the coflow oxygen level from 3% to 85% in volume. To distinguish physical and chemical effects of changing the diluent from N2 to H2O, the JHC flame diluted by XH2O (artificial H2O without reactivity) are also simulated. It is demonstrated that both the temperature and dimension of the JHC flame reduce substantially when changing the diluent from N2 to H2O. Such reductions derive primarily from different fluid properties of N2 and H2O: i.e., H2O has a lower fluid density and a higher thermal capacity. The change from N2 to H2O is also found to impact more chemically on the formations of the main intermediate species (i.e. H, O, CO, OH) and simultaneously to increase the flame lift-off distance due to longer ignition delay. Hence, the MILD combustion is expected to occur more feasibly for the JHC flame diluted by steam than that by nitrogen. The differences noted above lessen as the coflow oxygen is increased. In addition, kinetic oxidation pathways are analyzed to improve our understanding of the CH4 JHC combustion diluted by N2 and H2O.

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1 Introduction Both the environment deterioration and global warming are serious issues that our human being has long fought with.1 As a result, it is an urgent task to seek practical approaches of achieving both high efficiency and low pollutant emissions from burning fossil fuels. The oxy-fuel combustion is such a promising technology that has been developed for decades, particularly, for coal-fired plants and Carbon Capture and Storage (CCS).2 Commonly, fuel combustion occurs with air that consists approximately of 21% O2 and 79% N2 in volume. For the oxy-fuel combustion, the nitrogen in air is replaced by CO2, so that the final exhaust gas is mainly composed of CO2 and H2O. This combustion takes place in the mixture of oxygen and recycled exhaust gas, instead of air. By condensation to separate the steam from the recirculated exhaust gas, or with the dry exhaust gas, more than 90% of CO2 in mass can be reached in the exhaust gas. The oxy-fuel combustion technologies in conjunction with CCS have be developed for the ‘near zero-emissions’ of fossil-fuel combustion but face several challenges. For example, high NOx emissions result from air-ingress3 and poor flame stabilities arise from delayed ignition4,5. However, those challenges may vanish when the oxy-fuel combustion operates under moderate or intense low-oxygen dilution (MILD), i.e. when using MILD oxy-combustion. The practical advantages of this oxy-combustion technology has been well demonstrated by The Linde Group in steel industry for practical furnaces of firing gases such as walking beam furnace and pit furnace.6 There has been another ‘zero-emission’ system of combustion that occurs with oxygen diluted by steam (H2O), instead of CO2.7 This combustion can also be considered as one form of the oxy-fuel combustion, whose exhaust gas consists almost only of CO2 and H2O, when firing CxHy-type fuels. However, such two forms of oxy-fuel combustion are likely to produce considerably different combustion characteristics, because the concentrations of CO2 and H2O in their individual flue gases are very distinct. Different physical properties and chemical activities of CO2 and H2O at high temperatures should affect chemical reactions and heat transfer in combustion process. Indeed, some special thermodynamic properties of steam have been found to make the H2O-diluted oxy-fuel combustion system to have several advantages over the CO2-diluted counterpart, such as the compact configuration and energy saving.7 In this context, it is interesting to identify their different combustion characteristics and, more practically, to examine whether the recycled steam should be removed for the oxy-fuel combustion technology. Fundamentally, a more general issue should be considered as to how combustion behaviors are affected by dilution of either CO2 or H2O or both. To distinguish the CO2 oxy-fuel combustion from the N2-diluted combustion, several experimental


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studies have been conducted. Li et al.8 investigated by experiment the MILD oxy-combustion of gaseous fuels at furnace and obtained several significant findings. For instance, the stable MILD combustion was found to occur over a wider range of equivalence ratio for dilution by CO2 than by N2. They also found more uniform temperature distributions and lower emissions of NOx and CO in the MILD oxycombustion than in the conventional one. Heil et al.9 carried out an experimental investigation of methane MILD combustion with different inert gases (N2/O2 and O2/CO2) at the O2 volume fractions of 21% and 18%. It was revealed that the participation of CO2 in chemical reactions enhances the effect of high CO2 concentration on combustion reaction rates and weakens the physical influence of CO2. By varying the flow velocities of fuel and oxidizer, Kim et al.10 experimentally investigated the combustion characteristics, flame structure, and lengths of the air-fuel and oxy-fuel combustion zones. Moreover, Sabia et al.11 measured auto-ignition delay times of propane in diluted and pre-heated conditions in a one-dimensional flow reactor. They found that the CO2-diluted MILD oxy-combustion has the longest auto-ignition time. Numerically, there are a number of previous studies concerning the effect of varying diluents on conventional oxy-fuel combustion. Mei et al.12 simulated the JHC combustion of CH4 diluted by N2 and CO2, and revealed that the CO2-diluted MILD combustion produces a far larger reaction zone, lower temperatures, and higher CO concentrations than in the CO2-diluted case. The fundamentals behind those differences were sought by Zhang et al.13 through chemical kinetic calculations of well-stirred reactor (WSR). These authors revealed that a high concentration of CO2 from the CO2 dilution delays ignition process, slows oxidations and reduces the temperature rise, thus facilitating the establishment of MILD combustion. All the above results mainly from a greater heat capacity of CO2 than N2; relatively, the chemical effect is minor. Following Zhang et al.13, Tu et al.14 studied the effects of CO2 addition on CH4/H2 JHC flames at low oxygen conditions, reporting that the chemical effect of CO2 is responsible for the ignition delay and the increased CO emission. Moreover, Wang et al.15 studied the effects of CO2 and H2O diluents on a counter-flow methane flame. They found that both chemical and thermal effects of CO2 dilution play compatible roles in reducing the flame temperature, yet, on which the H2O dilution has little chemical effect. In addition, Zou et al.16 compared the temperature profiles of steam oxy-fuel and air-fuel counter-flow diffusion flames. Their work focused on those cases of relatively high steam concentration (mole fraction of steam ranges from 0.5 to 0.8). They found that the overall chemical effect of the steam reaches the maximum when the mole fraction of H2O ranges between 0.60 and 0.66.


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Despite the previous studies noted above, the main attention was paid to the effects of CO2 and H2O dilutions on local temperature profiles and local distributions of major species. No distinct physical and chemical effects of CO2 and H2O have been systematically examined on both global and local combustion characteristics in terms of the entire reaction zone. In addition, the previous work was normally undertaken for a couple of either high or low oxygen levels. Notably as well, they were conducted generally for the CO2 oxy-fuel combustion. It is a bit unfortunate that the H2O oxy-fuel combustion has not been investigated sufficiently and so not understood as much as for the CO2 case. Consequently, systematic comparisons are lacking between combustion characteristics of CH4 flames under dilutions by N2 and H2O for a wide range of oxygen concentration. The above deficit has stimulated this work, which is designated to investigate distinct combustion characteristics of a methane jet in hot oxidant coflow (JHC) diluted, respectively, by H2O and N2. The dilution is varied by adjusting the coflow oxygen level between 3% and 85% in volume. The main objective is three-fold: (1) To reveal the discrepancies in both global and local characteristics of the JHC flames between the two cases diluted by N2 and H2O; (2) To differentiate physical and chemical effects of changing the diluent from N2 to H2O on the JHC flame; (3) To find reasons behind distinct combustion characteristics due to using different diluents of H2O and N2. Both MILD and traditional combustion processes are considered by taking the coflow oxygen at fO2 = 3%, 9%, 21%, 40% and 85%. A number of simulations through RANS modelling are performed using the Eddy Dissipation Concept (EDC) model with the detailed chemistry-reaction mechanism (DRM22). See the next section for more computational details. The rest of this paper is organized as follows. Computational details of the present modeling are given in Section 2 while Section 3 offers an experimental validation of the modeling. Computational results are presented in Section 4 orderly as the influences of coflow oxygen fraction and changing the diluent from N2 to H2O on combustion characteristics, and then the relative importance of chemical and physical effects. Of note, a virtual diluent (XH2O) is introduced to quantify either physical or chemical effect of H2O dilution. Moreover, chemial characteristics including intermediate radical formation and reaction routes of the CH4 oxidation are analyzed in Section 5, which may advance our understanding to the H2O-diluted oxy-combustion. Finally, the conclusions are summarized in Section 6.


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Fig. 1 Schematic diagrams of the JHC burner and computational domains of present simulations for (a) the JHC case of Dally et al.17 and (b) the modified JHC case.

2 Computational details 2.1 JHC Configuration and computational domain The present model of JHC combustion, Fig. 1(b), is modified from the experimental combustor of Dally et al.17 shown in Fig. 1(a). The device consists of an insulated and cooled central jet nozzle, which has an inner diameter D = 4.25 mm and a wall thickness of 0.2 mm. The fuel jet nozzle was placed at the center of a perforated disc in an annulus nozzle, with inner diameter of 82 mm and wall thickness of 2.8 mm. In their experiment17, nearly uniform hot combustion products were provided in the annulus nozzle and then mixed with air and nitrogen via oxidant inlets to control the temperature and O2 levels in the hot coflow. Dally et al.17 mounted the entire burner in a wind tunnel, which provided a co-axial surrounding air stream at room temperature (≈ 300K) but with the same velocity as the hot coﬂow (≈ 3.2 m/s). The fuel was a mixture of CH4 and H2 equally in volume and ejected with a velocity of ≈ 69.5 m/s.


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Therefore, the fuel-jet Reynolds number was kept at Re (≡ Vf D/ν) = 9482 for all the cases, where Vf is the fuel-injection velocity and ν the kinematic viscosity. Table 1 gives the detailed inlet conditions of the experiment17. Table 1 Experimental inlet conditions of Dally et al.17 Fuel jet (CH4/H2)

Coflow velocity & temperature

Re

T(K)

Vcof (m/s)

Tcof (K)

9482 9482 9482

305 305 305

3.2 3.2 3.2

1300 1300 1300

Coflow compositions (mass fraction) YO2 (%) 3 6 9

YN2 (%) 85 82 79

YH2O (%) 6.5 6.5 6.5

YCO2 (%) 5.5 5.5 5.5

To validate the present modeling, simulations for the experimental cases of Table 1 are performed, using the computational domain in Fig. 1(a). However, our previous studies12,18 have revealed that the tunnel air can be entrained into the central fuel jet and then influence the combustion regime of the JHC flame. That is, the invisible flame in the burner-near region turns to be a very traditional flame above the axial position of x = 100 mm, especially for YO2 = 3% in the coflow. In order to eliminate the influence, the present study uses a different JHC configuration in which a central jet flame is surrounded only by hot oxidant stream, i.e., the outer cold air stream is removed from the JHC system of Dally et al.17. This removal ensures that YO2 and Tcof remain constant across the computational domain, see Fig. 1(b), thus able to properly mimic the effects of local O2 level, temperature and velocity of the hot flue gas on the flame characteristics in furnace. Considering the symmetry of the JHC configuration, a geometrically simplified two-dimensional (2D) axisymmetric computational model is constructed, with the computational domain of 4500 mm × 800 mm. This choice was based on the work of Galletti et al.19 who found that the 3-D axisymmetric furnace combustion could be simplified to a 2-D model, which can reduce the amount of computation and predict as accurately as the 3-D simulation. A primary orthogonal structured mesh with about 50000 cells is used to simulate the JHC flames after verifying the grid-independency using a finer grid with near 100000 cells and a coarser grid with around 25000 cells. Detailed comparison will be made in Section 3. The minimum and maximum cells are dimensionally 0.30 mm × 0.35 mm for the fuel jet inlet and 10.0 mm × 9.0 mm for the coflow inlet, respectively. To differentiate physical and chemical effects of the H2O dilution on the CH4 flame characteristics, the artificial water XH2O is utilized as a diluent that has the same physical properties of H2O but does


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not participate in any chemical reactions. In this sense, it is expected that different flame characteristics between the O2/XH2O and O2/N2 cases result from the physical effect of the H2O dilution. Then, the flame differences between the O2/H2O and O2/XH2O cases can be measured for the chemical effect of the H2O dilution. Table 2 lists all the 15 cases with the different diluents and various oxygen concentrations in the hot coflow whose velocity and temperature are 3.2m/s and 1300K, respectively. In addition, the fuel-jet temperature and Reynolds number are also taken at 305K and 9482 for all the cases.

Table 2 Computational conditions of the present work (The mixture compositions are all given by volume).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Cases O2 / N2 O2 / N2 O2 / N2 O2 / N2 O2 / N2 O2 / H2O O2 / H2O O2 / H2O O2 / H2O O2 / H2O O2 / XH2O O2 / XH2O O2 / XH2O O2 / XH2O O2 / XH2O

fO2（%） 3 9 21 40 85 3 9 21 40 85 3 9 21 40 85

fN2（%） 97 91 79 60 15 0 0 0 0 0 0 0 0 0 0

fH2O（%） 0 0 0 0 0 97 91 79 60 15 0 0 0 0 0

fXH2O（%） 0 0 0 0 0 0 0 0 0 0 97 91 79 60 15

2.2 Computational models The present simulations use ANSYS Fluent 16.2 to solve the seven transport equations, i.e., the mean conservation of mass, axial and radial momentums, turbulence kinetic energy and its dissipation rate, energy and radiative intensity. The SIMPLE algorithm method is utilized for the pressure-velocity coupling. It has been shown how the turbulence model plays an important role in achieving good modeling results.20 According to the previous studies12,21-23, a modified k-ε model is used as the turbulent model by adjusting the coefficient Cε1 from 1.44 to 1.6 in the eddy dissipation equation, with the standard wall function. As indicated by the previous work21,24, the accuracy of the numerical solution is highly sensitive to boundary conditions, especially the turbulent kinetic energy at fuel inlet. Therefore,


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the mean turbulent kinetic energy (k) is set to be 40 m2/s2 (i.e., the fuel turbulence intensity ≈ 7%) and 1.8 m2/s2 at the fuel and hot coflow inlets, to achieve the best agreement between the calculated and measured results.18 The downstream and side-boundary conditions of the computational domain are taken to be the pressure outlet and zero-shear stress25, respectively. The EDC model coupled with the reduced mechanism DRM2226 is applied for the turbulence/chemistry interactions. The EDC model is an extension of the Eddy Dissipation Model (EDM) and widely used when the assumption of fast chemistry is invalid. In this model, reactions are assumed to occur in the small turbulent structures, the size of which is determined by the length fraction of the fine scales ξ = Cξ (νε/k2)1/4, where ε is the turbulent dissipation rate. Species are then assumed to react in the fine structures as constant pressure reactors over a residence time scale τ = Cτ (ν/ε) 1/2. Here, the volume fraction constant Cξ and the time scale constant Cτ are both set as the default values. The NOx emission characteristics are not investigated herein. DRM22 is a reduced version of the full chemical mechanism GRI-Mech 1.227. It consists of 22 species, apart from Ar and N2, and 104 reversible reactions. The previous studies26,28-30 have identified its good performance in capturing ignition time delay, laminar flame speed and oxidation pathways of methane combustion. Besides, Wang et al.18 also reported that predictions for the temperature and species profiles using DRM22 are as good as those using GRI-Mech 2.1131 and GRI-Mech 3.032, except for the NOx emission. Thus, the DRM22 mechanism, eliminating the reactions of N, is chosen to improve the calculation efficiency. To reduce the huge computational cost of time integration, the in situ adaptive tabulation (ISAT) model of Pope33 is used. The ISAT error tolerance is set to be 10-4 finally, ensuring the quality of the calculated results. Considering the high temperature gradient across the flame front, the specific heat of any species (Cp) is set as a polynomial function of temperature. Moreover, Christo et al.21 found that the differential diffusion plays an important role in predicting temperature and concentrations of H2O, CO and CO2. The current simulations hence consider the full multicomponent diffusion by representing the molecular diffusion coefficient of each species as a four–order polynomial function of temperature. Together with the refined weighted sum of gray gas model (WSGGM) derived by Yin et al.34, the discrete ordinate (DO) radiation model is utilized to correctly predict the absorption and the emission under oxy-fuel conditions with much higher levels of H2O that can strongly promote radiative heat transfer. Yin et al.34 demonstrated that the modified WSGGM, with new parameters calculated from the exponential wide band model (EWBM), is applicable for modeling both air-fuel and oxy-fuel


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combustion processes. In Yin et al.34, the emissivity databases were generated for ten partial pressures of CO2 and H2O vapor. In the same way that the WSGGM of Smith et al.35 (three gray gases plus one clear gas) is used in Fluent, the new WSGGM (four gray gases plus one clear gas) is implemented in Fluent via a user-defined function. Note that the thickness of MILD combustion is usually not clear; the DO model is applicable due to its wide range of optical thickness. For the DO mode, each octant of the angular space is discretized into 3×3 solid angles and hence 72 discrete solid angles across the computational domain are solved through the radiative transfer equation. Moreover, all species used currently are assumed to obey the incompressible ideal gas law. The second-order upwind scheme is employed for discretizing the equations to improve the accuracy of the calculations. The solution convergence is reached when two criteria are met: (a) the residuals are less than 10-6 for the energy and radiation intensity and 10-5 for all the other variables, and (b) the variations of the area-weighted average outlet temperature and total pressure are allowed to be within 1.0 K and 0.001 Pa, respectively.

3 Validation of the modeling 3.1 Check of grid independency The grid sizes of the original and modified JHC flames for the present study are 27000 and 48300, respectively. Our previous JHC modeling work18 found that a grid size of about 30000 cells is sufficiently accurate for predictions of the measured results by Dally et al.17 To check the grid independency of the modified computation domain (Fig. 1(b)), calculations are carried out with three different structured meshes, i.e., 25400, 48300 and 103600 cells. Fig. 2 shows the predicted centerline temperature, axial velocity, species profiles, turbulent kinetic energy and dissipation at fO2 = 9% for the three grids. Apparently, no obvious differences exist for the three grids. This approves the adequacy of the mesh with 48300 cells for the present modeling of the JHC flame.


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Fig. 2 Comparisons of the centerline properties of the JHC flame at fO2 = 9% for three grids of 25400, 48300 and 103600 cells. Part (a), temperature; (b), axial velocity; (c), OH mole fraction; (d), CO mole fraction; (e), turbulent kinetic energy; and (f), turbulent dissipation rate. 3.2 Present predictions versus previous measurements of Dally et al.17 The present simulations for the modeling validation were conducted under the experimental conditions of Dally et al.17. User defined functions are adopted to import the inlet coflow conditions of YO2, i.e., the measurements at 4 mm downstream from the jet exit. The average mass fractions of O2 in the coflow are approximately 3%, 6% and 9% as given in Table 1. Figs. 3(a-d) compare the temperature T and mass fractions of species CO, OH and O2 (i.e., YCO, YOH and YO2) between our predictions and the measurements of Dally et al.17 obtained at x = 30 mm. Overall, the radial profiles of the predicted temperature and mass fraction of species agree quite well with the experimental data. In particular, T and YO2 are predicted very well across the entire field. In contrast, the obvious difference in YCO between the predicted and measured results is seen at r/D = 4 ~12 for all the three cases. Over that range of r/D, YCO is expected to be nearly zero but the measured value is not so for some reason. Dally et al.17 used an internal burner (Fig. 1(a)) to provide combustion products that were mixed with air and nitrogen via two side inlets at the bottom of the annulus to control the O2


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levels. Christo et al.21 claimed that in the experiments of Dally et al.17, the carbon monoxide product initially existed in the coflow due to the cooling and extinction effects of the secondary flame outside the primary burner and near the burner outer wall. However, there is no carbon monoxide present in any calculated coflow of the present investigation. Christo et al.21 also believed that such level of YCO ≈ 0.0025 has minor effects on the reaction zone. Moreover, the simulation predicts well for the mixture fraction (ξ), obtained from Bilger’s formula36. This is demonstrated in Fig. 4, which shows the mean mixture fraction for YO2 = 9% along the axis and its radial distributions at different axial locations. In other words, the current modeling approach can predict the JHC flames of present investigation with good accuracy. In addition, a check is also given to the present prediction of the JHC flame lift-up height (H). Similar to Mei et al.22, we take the lift-off distance of the contour of fCO/fCOmax = 0.01 as H, where fCO is the mole concentration of CO and fCOmax the maximum. This method was validated to be as effective as using the OH radical and flame photographs.22 Fig. 5 compares the predicted values of H with the measurements of Dally et al.17 for YO2 = 3%, 6% and 9%. Apparently, the current simulations predict quite well, despite a slight underprediction.

Fig. 3 Mean temperatures and mass fractions of species CO, OH and O2 obtained at x = 30 mm from the


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present predictions and the measurements of Dally et al.17 for YO2 =3%, 6%, and 9%.

Fig. 4 Present predictions and previous measurements17 of (a) axial profiles and (b) radial profiles of mixture fraction at axis and various axial locations of x = 30, 60 and 120 mm for YO2 = 9%.

Fig. 5 Comparison of the present predictions and the experimental measurements of Dally et al.17 of flame lift-off heights.

Fig. 6 Center-plane contours of the normalized CO concentration fCO/fCOmax in the JHC flame diluted by N2, XH2O and H2O for fO2 = 9% (-----, fCO/fCOmax = 0.01). Definitions of length (L), width (W) and lift-


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off height (H) of the reaction zone are provided.

4 Results and discussion Following Mei et al.12, we define the contour of the ratio fCO/fCOmax = 0.01 as the border of the reaction zone, where fCO and fCOmax are the mole concentration of CO and its maximum, respectively. The rationality is that carbon monoxide may be regarded as the last intermediate product of hydrocarbon combustion.12 This definition was confirmed by Mei et al.12 to be effective for the methane JHC combustion at different combustion modes. Fig. 6 shows the center-plane continuous contours of fCO/fCOmax in the JHC flames diluted respectively by N2, XH2O and H2O for fO2 = 9%. Here, the length (L), width (W) and the lift-off height (H) of reaction zone are defined based on the contour of fCO/fCOmax = 0.01. Fig. 6 also illustrates the physical and chemical effects on the length L when changing the diluent from N2 to H2O.

Fig. 7 Center-plane contour distributions of the mean temperature of JHC flames for fO2 = 9% and 21%: (a) T, (b) T/Tmax.


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Fig. 8 Center-plane contours of the CO concentrations for fO2 = 9% and 21% (white dotted line: fCO/fCOmax = 0.01): (a) fCO, (b) fCO/fCOmax. Logarithm of fCO/fCOmax is taken to better view low contours.

Fig. 9 Center-plane contours of the OH concentrations for fO2 = 9% and 21%: (a) fOH, (b) fOH/fOHmax.

Fig. 10 Normalized reaction zone lengths (left) and widths (right) of the JHC flames versus fO2 (= 3%85%).


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Fig. 11 Effects of the coflow oxygen concentration fO2 on (a) ∆Tmax, (b) fCOmax and (c) fOHmax. In part (a), the auto-ignition temperatures for the N2 and H2O dilutions (Tai, N2 and Tai, H2O) are calculated following Wang et al.37.

Fig. 12 Effects of the coflow oxygen concentration fO2 on the reaction-zone-volume-weighted averages of (a) temperature rise ∆, (b) thermal capacity and (c) heat absorbed per unit volume at fO2 = 3%-85%. Fig. 7 shows the center-plane continuous contours of the mean temperature T for fO2 = 9% and 21%, whereas Figs. 8 and 9 display the continuous contours of the mean concentrations of CO and OH, respectively. In Figs. 7-9, part (a) is for the absolute values of T, fOH and fCO while part (b) for their values normalized by their maxima. Logarithms of the concentrations are taken in order to better view low contours. Note also that the results for XH2O diluent are displayed to reveal how differently chemical and physical effects are on the combustion characteristics, as indicated by black/yellow and red arrows across the plots. To more effectively view the discrepancies between the different cases, Figs. 10


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and 11 present, respectively, the dimensionless reaction zone length (L/D), the maximum temperature rise (∆Tmax) and mole fractions of CO (fCOmax) and OH (fOHmax) against fO2 varying between 3% and 85%. Moreover, Fig. 12 shows the reaction zone volume-weighted averages of the temperature rise ∆ from Tcof =1300K, thermal capacity and heat absorbed per unit volume . A number of interesting observations can be made directly or indirectly from those figures and are given below successively as the effects of coflow oxygen fraction and changing the diluent from N2 to H2O on combustion characteristics, and the relative importance of chemical and physical factors.

4.1 Influence of coflow oxygen fraction Comparisons of contour plots in Figs. 7-9 and particularly Figs. 10 & 11 reveal for all the cases that: as the coflow oxygen fraction fO2 increases, (1) the reaction zone decreases dramatically in size; (2) the zone temperatures and concentrations of CO and OH increase (thus the inhomogeneity of their distributions rises); (3) the maximum temperature rise ∆Tmax due to combustion and the maximum concentrations of CO and OH (fCOmax and fOHmax) all increase significantly. These observations can be well explained. When a fuel jet issues into the hot oxidant coflow that is highly diluted for fO2 = 3%-9%, combustion reactions occur less rapidly and combustion heat releases at lower rates, thus taking a larger space to burn out, over which the temperature rise must be smaller, than in the cases slightly diluted for fO2 = 21%-85%. Significantly, in the six cases with fO2 = 3% and 9%, all the maximum temperature rises (e.g., ≈ 135K and 505K under the H2O dilution) are well below the corresponding auto-ignition temperatures (Tai) of methane (≈ 1200K and 1020K, e.g., under the H2O dilution), see Fig. 11(a). In other words, these low oxygen cases satisfy the condition of ∆Tmax < Tai and Tc > Tai for MILD combustion proposed by Cavaliere and de Joannon38. However, for fO2 = 21%-85%, ∆Tmax is greater than the auto-ignition temperature. Namely, the MILD combustion occurs in the cases of fO2 = 3%-9% but not in those of fO2 = 21%-85%. Next, interesting is to look at the effect of fO2 on Damköhler number Da ≡ τmix/τchem, i.e., the ratio of turbulent to chemical time scales, considering that the reacting JHC flow is turbulent. Define Da ≡ (νcr2/ρ2ε)1/2, where ρ and ν are the local mixture density and turbulent viscosity while ε and cr denote the mean dissipation rate of the kinetic energy and the constant of the maximal chemical reaction rate among chemical reactions.39 The Damköhler numbers for the cases diluted by both N2 and H2O are obtained and shown in Table 3. Clearly, as fO2 rises from 3% to 85%, Da grows significantly from ~1.0


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to ~200, no matter which diluent is used. This implies that the coflow oxygen level determines the relative importance of turbulent mixing and chemistry. For fO2 = 3% and 9%, the JHC flame takes the MILD mode, where Da = 0.4 ~ 4.0, so that both chemistry and mixing play a significant role in combustion process. However, for fO2 = 21%-85%, Da = 37 ~ 200, the flame is totally controlled by turbulent mixing. Besides, it is interesting to note that the Damköhler number for the steam-diluted JHC flame is slightly lower when fO2 = 3% and 9% (MILD regime) but higher for fO2 = 21%-85% (conventional regime) than that for the nitrogen-diluted case.

Table 3 Damköhler numbers of JHC flames diluted by N2 and H2O versus fO2 = 3%-85% fO2 N2 H2O

3% 0.44 0.40

9% 4.14 3.63

21% 36.85 43.35

40% 105.79 116.62

85% 195.00 199.26

4.2 Influence of varying the diluent from N2 to H2O Figs. 8(b) and 9(b) properly compare the CO and OH related reaction zones of the JHC combustion diluted by N2, H2O and XH2O for the coflow fO2 = 9% and 21%. It is observed immediately that the dilution by H2O causes the reaction zone to be significantly smaller than that diluted by N2 due to physical factors (see Sect. 4.4). Interestingly, this is highly contrasting to the corresponding temperature reduction, observed from Figs. 11(a) and 12(a). For instance, at fO2 = 9%, the maximal temperature rise for the H2O dilution is ∆Tmax = 505K that is considerably smaller than ∆Tmax = 638K for the N2 dilution. Also due to changing the diluents, significant variations of the CO and OH concentrations occur, as seen in Figs. 11(b, c). The discrepancies observed in the temperatures become larger when fO2 varies from 3% to 21%, but less substantial as fO2 increases from 21% to 85%. This tendency also happens to the mole fractions of CO, OH, H and O radicals as revealed in Figs. 11(b, c) and 17. As fO2 increases, the volumeaveraging flame temperature increases, see Fig. 12(a), and concurrently the physical properties of the diluent gases change, see Fig. 13. This change of the thermal capacity against the oxygen mole fraction is revealed in Fig. 12(b). That is, the discrepancies between the N2 and H2O dilutions are largest around fO2 = 21%. However, as fO2 continues to increase, the degree of dilution drops and so the dilution effects of N2 and H2O reduce. The above results indicate that chemical reactions of combustion occur with lower temperatures and simultaneously over a smaller volume under the H2O dilution than under the N2 dilution. This looks inconsistent since rationally a smaller reaction zone should correspond to a higher zone-averaged


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temperature. Nonetheless, this ‘inconsistence’ can be explained below. There are two likely reasons for the JHC combustion diluted by H2O to have a smaller reaction zone than that diluted by N2. The first is associated with chemical reaction R59: CO + OH CO2 + H + heat, whose reaction rate depends on the concentrations of CO and OH. This reaction is the main pathway to convert CO to CO2 and is responsible for a major fraction of heat release derived in the oxidation of methane40. When replacing N2 by H2O for dilution, the OH radical is produced more rapidly, mainly due to the inhibitory effect of H2O on the forward reaction of R49 (OH +H2 H+H2O), and so the forward (CO to CO2) reaction rate of R59 increases. Consequently, the fuel burns out more rapidly and thus heat releases at a higher rate, with a reduced reaction-occupied volume.

Fig. 13 Thermal capacity (a) and fluid density (b) of gases N2, O2 and H2O with temperature ranging from 400 K to 3000 K.

The second reason is related with the physical entrainment of coflow fluid by the central fuel jet. Ricou and Spalding41 investigated by experiment the entrainment of round turbulent jets including combustion cases and found that the jet entrainment rate (Rent) can be formulated generically as

0.32 .

(1)

Here, m0 and m represent the jet mass flux at exit and at the downstream distance x, respectively, while ρ0 and ρcof denote the center jet and surrounding coflow fluid densities. Consider the two cases: (i) a fuel jet issuing into a coflow of O2/N2 at the density ρcof1 and (ii) a fuel jet into a coflow of O2/H2O at the density ρcof2. The volume ratio of the entrained fluid of the two cases can be approximated, viz.


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.

(2)

When fO2 = 9% and Tcof = 1300K, it is estimated from Fig. 13(a) that ρcof1 = 0.296 kg/m3 for the O2/N2 coflow and ρcof2= 0.181 kg/m3 for the O2/H2O coflow; i.e. (ρcof1/ρcof2)1/2 ≈ 1.28. Accordingly, it is obtained from Eq. (2) that the mole fraction of the entrained oxidant from the O2/H2O coflow is about 28% higher than that from the O2/N2 coflow. Equally, the entrained oxygen from the O2/H2O coflow exceeds that from the O2/N2 coflow by 28%. Since the increased oxygen concentration accelerates chemical reactions, the reaction zone of the JHC combustion is thus reduced significantly in size from the N2 dilution to the H2O dilution, as demonstrated by Figs. 8-10. Now we explore the possible reasons behind the lower temperature for the H2O dilution than for the case diluted by N2. Fig. 13(b) compares the values of the thermal capacity ρCp of gases O2, N2 and H2O with temperature ranging from 400 K to 3000 K. Evidently, the nitrogen ρCp is considerably lower than that of H2O. This implies that the nearly identical heat release from the two cases will lead to a lower rise of the average temperature ∆ per unit volume for the H2O dilution than for the N2 case. Indeed, Fig. 11(a) and Fig. 12(a) show that for fO2 = 9%, ∆Tmax ≈ 505K and ∆ ≈ 280K for the former whereas they are approximately 638K and 379K for the latter. To summarize for the above explanation, the JHC combustion diluted by H2O simultaneously produces lower values of ∆Tmax and ∆ and a smaller reaction volume than does the N2 dilution. This is due primarily to the different fluid properties or physical factors: i.e., a lower density of H2O than N2 leads to a smaller reaction zone of combustion diluted by H2O whereas a higher thermal capacity of H2O than N2 plays a dominant role in decreasing the reaction temperature. Nevertheless, those discrepancies weaken as fO2 is increased or the dilution is reduced.

4.3 Influence of varying the diluent from N2 to H2O on JHC flame lift-off Careful checks to Figs. 6-9 find that the JHC flame diluted by N2 for fO2 = 9% starts at some distance downstream from the nozzle exit. This indicates that the flame lift-off occurs in the N2 case. The same figures also show the difference in the JHC flame lift-off due to varying the diluent from N2 to XH2O (physical effect) and then to H2O (chemical effect). We can quantify the effect of replacing N2 by H2O on the JHC flame lift-off below. The experimental flame lift-off height is often estimated by luminance17, OH radical42 and temperature40. Several identification methods for numerical simulations have been proposed and


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validated as well.43-46 The present work uses the method developed by Mei et al.22, i.e., takes the lift-off distance of the contour of fCO/fCOmax = 0.01 as the JHC flame lift-off height (H). The effectiveness of this method is also well validated early in Fig. 5 and by Mei et al.22. Relative to the N2 case, the onset of MILD combustion diluted by XH2O takes place over a shorter distance while that by H2O moves farther downstream from the jet nozzle. Indeed, the lift-off height (H) is H ≈ 1.0 mm, 1.0 mm and 34 mm, respectively, for N2, XH2O and H2O dilutions at fO2 = 9%. That is, the physical mixing appears to have far less influence on the ignition process than does the chemical effect. Adding more H2O to lower the O2 level chemically delays the ignition11 and so move the stable position of flame farther downstream. To confirm the ignition delay mainly due to the chemical effect, chemical kinetics calculations of well-stirred reactor (WSR) are conducted for fO2 = 9%. Namely, the ignition delay time τd is obtained for the MILD combustion diluted by N2 and H2O. Spadaccini and Colket47 related the ignition delay time closely to the local concentrations of CH4 and O2 as τd = Aexp (E/T)[O2]-1.05[CH4]0.66, where A and E are experimental constants. For the present work, as analyzed before, when switching the diluent from N2 to H2O, the oxygen concentration locally in the jet mixing layer is increased by about 20%. Taking it into account, the ignition delay time τd is estimated under p = 1 atm, Φ = 1.0 and T = 1300 K for the two WSR cases: the MILD combustion diluted by N2 at 9% O2 and the MILD combustion diluted by H2O at 10.8% O2. Here, τd is defined as the period from the beginning of calculation to the occurrence of the maximum OH production. The calculations have given that τd = 0.032s and 0.044s, respectively, for the N2 and H2O dilution cases. It thus manifests that, relative to the N2 case, the H2O dilution chemically slow down the ignition of CH4 remarkably, consistent with the different values of H for the two JHC flames. This finding also supports that the H2O dilution chemically lowers the conditions for achieving MILD combustion from the N2 case.

4.4 Relative importance of chemical and physical effects One objective of the present study is to differentiate and compare chemical and physical effects on combustion characteristics when changing the diluent from N2 to H2O. For this, our calculations also use the dilution by XH2O, which has the same thermodynamic coefficients as the real H2O but does not participate in any chemical reactions. As a result, only the chemical factor plays a role in differing combustion characteristics between JHC flames diluted by H2O and XH2O, as marked in Figs. 6-9 by


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red arrows. Black/yellow arrows represent the physical factor for the N2 dilution to that for the XH2O dilution. It is immediately observed from Figs. 8 and 9 that both physical and chemical factors act consistently to size down the reaction zone when switching the diluent from N2 to H2O at fO2 = 9% and 21%. Quantitatively, Fig. 14 shows that the contribution of physical factor (≈ 20.4%) is far higher than that of chemical factor (6.8%), both acting to decrease the flame length approximately by 27.2% in the combustion diluted by H2O at fO2 = 9%. The total effect of the two factors is also reflected indirectly in the volume-weighted average from Fig. 12(c). Given that the total heat release is Q, the global volume of reaction zone may be expressed as V=Q/. In other words, a greater value of corresponds to a smaller value of V. It is obtained that ≈ 117354 and 125665 J/m3, respectively, for the N2 and H2O dilutions achieving fO2 = 9%. That is, the global volume of reaction zone is smaller for the H2O case, which is consistent with the results of Figs. 10 and 14. Figs. 8-10 together suggest that the physical effect of H2O dilution plays a far more important role than the chemical effect in reducing the reaction zone size. Fig. 14 also quantifies the physical and chemical effects of H2O versus N2 dilution on Tmax, fCOmax, fOHmax, fHmax and fOmax over the reaction zone for fO2 = 9%. The variation of the flame maximal temperature results primarily from the physical effect whereas the chemical effect is minor. On the other hand, the chemical effect is more significant on the presence of active radicals and particularly on reducing the concentrations of O and H; as the diluent N2 is replaced by H2O, fHmax and fOmax physically grow 36.2% and 0.5% but chemically drop 117.7% and 92.4%, respectively. This is mainly because the lower flame temperature and higher H2O concentration for the H2O dilution inhibit the forward reaction of R49 (OH +H2 H+H2O) and produce a higher OH and lower H concentrations. Further, the forward reaction of R24 (H +O2 O+OH) also slows down and thus produce a lower O concentration. Moreover, it is observed that the maximum temperature (thus temperature in general) is apparently lest sensitive, while the maximum concentrations of O and H are most sensitive, to the change of diluent. These observations suggest that some dominant reactions of firing methane may be altered by changing the diluent from N2 to H2O. In addition, from Figs. 10-12, both physical and chemical effects weaken as the coflow oxygen increases from fO2 = 3% to fO2 = 85%.


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Fig. 14 Physical and chemical effects of changing the diluent from N2 to H2O on L, Tmax, fCOmax, fOHmax, fHmax and fOmax for fO2 = 9%.

5. Further discussion: oxidation pathways of CH4 combustion To better understand the influence of changing the diluent from N2 to H2O on the CH4 oxidation, the chemical pathways of JHC flames diluted by N2, XH2O and H2O for the coflow fO2 = 9%, 21% and 85% are analyzed. Figs. 15(a-c) display the results. In calculations, for each elementary reaction, the average kinetic reaction rate ri is obtained by

ri

r !", i

where A is the total area of the computational domain and ri is the local reaction rate. For Fig. 15, it is necessary to make the following notes: (i) arrows start from reactants and point to products with their width and color indicating the relative importance of each pathway, neglecting those pathways with reaction rates less than 1.0×10-8 (kgmol/m3·s); (ii) the reaction number and species refer to DRM22; (iii) for the convenience of comparing the physical and chemical effects, reaction rates for the three cases are juxtaposed; (iv) reaction rates in square brackets are for the N2 dilution, angle parentheses for the XH2O dilution and round parentheses for the H2O dilution.


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Fig. 15 Physical and chemical effects on the major CH4 oxidation pathways of JHC combustion diluted by N2 [brackets], XH2O and H2O (round parentheses): (a) fO2 = 9%, (b) fO2 = 21%, and (c) fO2 = 85%. Reaction numbers refer to DRM22 while reaction rates are shown in the brackets or parentheses. Note that, e.g., 2.9-7 implies 2.9×10-7 kgmol/m3s.

5.1 The oxidation of CH4 Figs. 15(a-c) illustrate the main oxidation progresses of CH4 to CO2 respectively for fO2 = 9%, 21% and 85%. As expected, changing the diluent from N2 to H2O causes considerable alternations of the kinetic combustion process: e.g., most reactions slow down because of a lower temperature rise due to combustion. However, relative to the length and temperature of the reaction zone, the properties of H2O have subtler or more complex effects on various combustion reactions for the H2O dilution. For the CH4 pyrolysis to methyl (CH3) of all the cases investigated, the top three significant reactions are R7 (O+CH4OH+CH3), R35 (H+CH4CH3+H2) and R58 (OH+CH4CH3+H2O). In these reactions, H, OH, and O radicals attack on the CH4 molecule most fiercely, and thus together make the most important contribution to the methane pyrolysis. Fig. 16 shows the relative contributions of the above radicals to the CH4 consumption. Here, the relative contribution (θi) is calculated through θi (%) = 100 r/ i , i ∑& r where i denotes the reaction number related to the main process from CH4 to CH3. Fig. 16 demonstrates that the dilution of H2O instead of N2 raises the relative importance of R58 or the OH-related reaction


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for the CH4 pyrolysis and weakens the significance of R7 & R35. This is mainly because higher fOHmax and lower fHmax and fOmax are generated when diluting the coflow by H2O, as seen in Figs. 11(c) and 17. More specifically, the OH-related reaction R58 contributes mostly to the CH4 pyrolysis at fO2 ≤ 9% for the N2 dilution and at fO2 ≤ 21% for the H2O dilution while R7 and R35 become more important as fO2 is increased.

Fig. 16 Relative contributions to the CH4 consumption of (a) OH, (b) H and (c) O radicals through the reactions R7, R35, R58 only.

Fig. 17 Dependences of (a) fHmax and (b) fOmax in JHC combustion processes on fO2 (= 3%-85%).


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Fig. 18 Physical and chemical effects of changing the diluent from N2 to H2O on the kinetic reaction rates for significant reactions of the methane JHC flames: (a) fO2 = 9%, (b) fO2 = 21% and (c) fO2 = 85%. Note: the diluent N2 to XH2O, physical effect; XH2O to H2O, chemical effect. The relatively important reactions in Figs. 18(a-c) are considered from the order of their reaction rates for fO2 = 9%, 21% and 85%. Careful inspection and comparison can identify the physical and chemical


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impacts of changing the diluent from N2 to H2O on each reaction step. Among all the reactions displayed in Fig. 18, only reactions R49 and R58 are observed to enhance chemical effects on the reaction rates. The lower flame temperature due to the higher thermal capacity of H2O and higher H2O concentration, for the H2O dilution, inhibit the forward reaction of R49 (OH +H2 H+H2O) and R2 (O +H2 H+OH). On the other hand, R58 has the lowest activation energy among the three reactions (R7, R35, and R58) from CH4 to CH3. Therefore, R58 dominates the consumption of CH4 in low temperature flames at fO2 ≤ 9%. When the oxygen is enriched and the temperature becomes higher, the net production rates of H and O grow relatively faster than that of OH, see Figs. 16(a, b). The larger pre-exponential factors and temperature exponents in the Arrhenius expressions of R7 and R35 overcome their high activation energy and gain higher kinetic reaction constants. The superposition of the above two factors causes R35 to overtake R58 as the most important reaction in the CH4 pyrolysis for fO2 = 85%, see Fig. 18(c). To summarize, the chemical effect of the H2O dilution always promote the reaction rates of R49 and R58. Note that R49 is the most important chain-branching reaction while R58 is the most important chain-initializing reaction for the methane combustion. Despite suppressing the physical effect of H2O, the H2O dilution causes R58 to react faster than the N2 dilution by 38%, 20% and 5%, respectively, for fO2 = 9%, 21% and 85%. The increased chemical effects can reduce the ignition delay time under low oxygen conditions for the H2O diluted cases.

5.2 The oxidation of CH3 The pyrolysis of CH3 is not the same as that of CH4. To summarize from Figs. 15(a-c), the following four routes are mainly responsible for the CH3 oxidation: (I) CH4→CH3→CH2O→HCO→CO→CO2; (II) CH4→CH3→CH2(s) →CH2→HCO→CO→CO2; (III) CH4→CH3→C2H6→C2H5→C2H4→C2H3→C2H2→CO→CO2; (IV) CH4→CH3→CH3O/CH2O→HCO→CO→CO2. The pyrolysis route of CH3 to CO2 is the rest of CH4 oxidation to CO2, as highlighted above. To quantify the chemical and physical effects of the H2O dilution on those pathways, the relative contribution ratios of different routes to the CH3 pyrolysis are calculated. By comparison, the route IV just has a contribution maximally of 1.3% and is thus not considered below.


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Fig. 19 Relative contribution ratios of important reaction routes I, II and III to the CH3 pyrolysis under the different diluents for fO2 = 9%, 21% and 85%. Figs. 19(I-III) present the relative contributions of routes I, II and III, respectively. Evidently, when changing the diluent from N2 to H2O, the relative importance of the route I is weakened while that of the route II is enhanced, mainly due to chemical factors for fO2 = 9% to 85%. For the route III, the negative contributions imply that the reverse reactions from C2H6 to CH3 occur at higher rates than do the forward reactions. Fig. 19(III) suggests that the dilution of H2O instead of N2 enhances the reverse reacti on of C2H6 from CH3 for fO2 = 9% and inhibit the ethane production for fO2 = 21% and 85%. Further analyzing finds that the chemical effect of the H2O dilution plays a dominant role in making the discrepancies between the cases diluted by N2 and H2O. Specifically, under the H2O dilution, the higher OH concentration enhances R56 (OH+CH3CH2+H2O) and R57 (OH+CH3CH2(s) +H2O), so that the route II is boosted. Consequently, the reaction path from CH2 to CH2O, HCO and then CO is enhanced. Since a third body for H2O occurs frequently in R92 (2CH3 + M C2H6+ M), the H2O dilution thus enhances the reverse reaction of R92 for fO2 = 21% and 85%. Besides, the weakening effect of H2O on the route I is likely to derive mainly from lower O radical concentrations for the H2O dilution. In DRM22, the kinetic reaction constant of R6 (O+CH3H+CH2O) does not vary with temperature. So, a lower O radical concentration corresponds to a lower reaction rate. Therefore, the route I from CH3 to CH2O is highly probably weakened under the H2O dilution. Overall, when the diluent is changed from N2 to H2O, the chemical effect contributes more to the CH3 pyrolysis than does the physical effect. The higher OH concentration makes the route II more important, while a higher third body efficiency for H2O in the reaction R92 enhances the route III. Under the H2O dilution, the route I weakens considerably because of a lower O concentration.


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6. Conclusions The present work has compared the combustion characteristics of a central CH4 jet flame issuing into a hot oxidant coflow diluted by H2O with those from diluted by N2 for the oxygen molar fractions of 3% ~ 85%. The artificial gas XH2O was used to separate physical and chemical effects of changing N2 to H2O. Based on the results reported in Sections 4 and 5, several conclusions can be drawn below: (1) When changing the diluent from N2 to H2O, physical and chemical effects act together to reduce the size of reaction zone and decrease the reaction temperature. In the joint action, the physical effect plays a far more important role than the chemical one. In particular, a lower density of H2O than N2 leads to a significantly smaller reaction zone of combustion whereas a higher thermal capacity of H2O than N2 plays a dominant role in decreasing the reaction temperature. (2) The dilution of H2O instead of N2 greatly boosts the chemical effect, and so weakens the physical effect, on the formations of main radicals CO, OH, H and O. Also, the chemical effect of the H2O dilution plays a dominant role in determining the pathway direction of the methane oxidation, e.g., significantly enhancing the route: CH4→CH3→CH2(s)→CH2→HCO→CO→CO2 and weakening the route: CH4→CH3 →CH2O →HCO→CO→CO2. (3) As the coflow oxygen drops to a sufficiently low level by dilution of H2O instead of N2, the ignition delay becomes noticeable and thus the fuel-jet flame lifts up significantly. (4) As the coflow oxygen fraction increases from 3% to 85% in volume, both physical and chemical effects from the diluent change weaken, and so all the discrepancies noted above narrow. (5) All the above findings suggest that the MILD regime of combustion can be more feasibly established under the dilution of H2O than N2.

Acknowledgment The support of Nature Science Foundation of China (Nos. 51276002 and 51776003) is gratefully acknowledged.

References ACS Paragon Plus Environment

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