Effects of diluents on the lifted flame characteristics in laminar nonpremixed coflow propane jets
Item Type Article
Authors Oh, Suhyeon;Van, Kyu Ho;Yoo, Chun Sang;Chung, Suk Ho;Park, Jeong
Citation Oh, S., Van, K. H., Yoo, C. S., Chung, S. H., & Park, J. (2020).
Effects of diluents on the lifted flame characteristics in laminar nonpremixed coflow propane jets. Combustion and Flame, 222, 145–151. doi:10.1016/j.combustflame.2020.08.044
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DOI 10.1016/j.combustflame.2020.08.044
Publisher Elsevier BV
Journal Combustion and Flame
Rights NOTICE: this is the author’s version of a work that was accepted for publication in Combustion and Flame. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Combustion and Flame, [222, , (2020-09-04)] DOI: 10.1016/j.combustflame.2020.08.044 . © 2020. This manuscript version is made available under the CC-BY- NC-ND 4.0 license http://creativecommons.org/licenses/by-nc- nd/4.0/
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Link to Item http://hdl.handle.net/10754/665095
Effects of diluents on the lifted flame characteristics in laminar nonpremixed coflow propane jets
Suhyeon Oha, Kyu Ho Vanb, Chun Sang Yooa,∗, Suk Ho Chungc, Jeong Parkb,∗
aDepartment of Mechanical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
bDepartment of Mechanical Engineering, Pukyong National University, Busan 48513, Republic of Korea
cClean Combustion Research Center (CCRC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
Abstract
The effects of diluents including CO2, N2, and He on the lifted flame characteristics of laminar coflow propane jets are experimentally investigated by measuring their liftoff heights, HL, and visualizing flow fields with oil mist. For lifted flames at a specified diluent mole fraction, XD, HL increases in the order of He, N2, CO2 when the diluent is added to the coflow stream. However, the order of HL becomes opposite when the diluent is added to the fuel stream. In addition, HL increases with increasing XD for all cases. From the visualization of nonreacting flow fields, it is found that the negative buoyancy represented by the density difference between the fuel jet and the coflow induces a stagnation flow near the fuel nozzle by decelerating the fuel jet. As such, the lifted flame is found to be stabilized further upstream with increasing negative buoyancy. In addition to the buoyancy effect, the effects of diluents on HL via the edge flame speed of the lifted flames, Se, are estimated by evaluating the laminar burning velocity,SL0. Among the dilution, thermal, and chemical effects, the dilution effect is found to be dominant in reducing SL0 for all cases, while the chemical effect is negligible. Finally, a correlation forHLis formulated using the ratios of the positive to negative buoyancy and the fuel jet velocity,U0, toSL0, which shows a satisfactory agreement with the experimental data.
Keywords: Laminar lifted flame, liftoff height, dilution effect, positive/negative buoyancy, flow visualization
1. Introduction
A laminar lifted jet flame typically features a tribrachial (or triple) flame structure, composed of lean/rich premixed flame wings and a trailing diffusion flame in-between, all extending from the tribrachial edge [1–4]. Numerous experimental and numerical studies have been performed to elucidate stabilization mechanisms with regard to both laminar and turbulent lifted jet flames [4–18]. The stabilization mechanism of a tribrachial edge flame can be explained based on balance between its propagation speed, Se, and local flow velocity, uL. In general, Se is influenced by mixture strength, mixture fraction gradient, flame curvature, and Lewis number, while uL is affected by jet momentum, buoyancy, and flow redirection effect. Therefore, the liftoff characteristics of lifted flames are governed by those factors affecting Se and uL [7–9, 11–14, 19].
Based on the similarity solutions in an axisymmetric steady laminar jet, Chung and Lee [1, 2] developed a stabilization theory for the laminar lifted flame in a free jet that the liftoff height, HL, is determined by the diameter, d, the velocity, U0, and the Schmidt number, ScF, of the fuel jet, which predicts the existence of a stationary lifted flame only for ScF >
1. However, Won et al. [20] found that a stationary lifted flame can exist for N2-diluted methane jets in coflow, of which ScF is less than unity, identifying an important role of buoyancy in stabilizing the lifted flame by modifying flow field. Recently, Van et al. [21]
further identified the importance of buoyancy effect by observing a ‘U’-shapedHL behavior of lifted flames in N2-diluted methane jets in coflow, which occurs due to decreasing buoyancy effect with increasingU0 at relatively-smallU0. In addition, Van et al. [18] investigated the buoyancy effects on oscillating lifted flames in laminar N2-diluted fuel jets. They found that the flame oscillation occurs due to competition between the positive buoyancy represented by (ρco −ρb) and the negative buoyancy of the fuel stream heavier than the coflow air represented by (ρf −ρco), where ρf, ρco, and ρb are the densities of the fuel stream, the coflow stream, and the burnt gas, respectively. The positive buoyancy by the burnt gas
∗Corresponding authors.
Email addresses: [email protected](Chun Sang Yoo),[email protected](Jeong Park)
exerts upwards by entraining coflow while the negative buoyancy by the fuel stream does downwards by decelerating the fuel jet momentum.
The role of the positive (negative) buoyancy in stabilizing (destabilizing) the oscillating lifted flames has been identified in [18], while the liftoff height behavior of stationary lifted jet flames in coflow has never been investigated under such conditions that the effects of both positive and negative buoyancies by diluent addition can appear. Moreover, once diluent is added to the fuel and/or coflow streams, its thermal, diluent, and chemical effects can also affect the liftoff height through Se. Therefore, the objective of the present study is to elucidate the effects of diluents on HL of stationary laminar lifted propane jet flames in coflow by examining how the diluents change Se and uL through the positive/negative buoyancy and the laminar burning velocity,SL0, respectively. For this purpose, we measure HL of lifted flames in laminar coflow propane jets by systematically varying the amount of N2, CO2, or He in the fuel and coflow streams and visualize reacting/nonreacting flow fields with oil mist.
2. Experiment setup
The experimental apparatus consists of a coflow burner, a mass flow system, and mea- surement setups as shown in Fig. 1. A fuel jet, for which the mean velocity isU0, issues from a fuel nozzle with 4.0 mm inner diameter, d, and 400 mm length, L. U0 varies in a range of (18 ≤U0 ≤ 60 cm/s) to ensure a fully-developed laminar jet. The fuel is surrounded by coflow issuing through by a cylinder (100 mm i.d. and 600 mm length). To ensure a uniform flow at the coflow exit, a layer of glass beads and a ceramic honeycomb are installed at the bottom of the coflow section and near the exit of the nozzle, respectively. An acrylic cylin- der is placed on the coflow nozzle having four rectangular quartz windows for measuringHL and preventing from ambient air entrainment. The coflow velocity, Vco, is set to 5 cm/s to minimize the coflow effect on HL while sufficient enough to maintain overall lean condition.
The fuel is propane (C3H8) and the oxidizer is a mixture of oxygen and nitrogen, both of which are diluted by various amounts of diluent gases such as CO2, N2, and He. Thus,
Figure 1: Schematic of the experimental setup.
the thermodynamic and transport properties of the fuel jet and the oxidizer coflow can be readily changed by varying their compositions with the diluents. Commercially-pure grade gases (> 99.95%) are used for the experiments. To mix fuel/oxidizer with diluents, two mixing chambers are installed for the streams, of which flow rates are controlled by flow controllers with 99% accuracy.
In a diluent-added stream (XK+XD+XN2,0 = 1.0), the nitrogen mole fraction,XN2,0, is fixed to 0.62 regardless of diluents such thatXK+XD= 0.38, where the subscript K denotes propane (F) in the fuel jet or oxygen (O2) in the oxidizer coflow, and the subscript D does a diluent including CO2, N2, and He. In a stream without diluent, XK = 0.205 andXN2,0 = 0.795. Since the purpose of this study is to investigate the diluent effects on HL, XD varies in a range of (0.155 ≤ XD ≤ 0.195) so that lifted flames in diluent-added coflow/fuel can exist within the burner.
HL is measured using a digital camcorder (30 fps) attached to a cathetometer. HL is defined as the distance between the flamebase and the nozzle exit and the error in repet- itive measurements is approximately 0.5 – 1.2% for all cases. The experimental setup for visualizing reacting/nonreacting flow fields is composed of a high-speed camera (Olympus;
iSPEED3, 60 fps) and a diode laser (Laser Lab; 532 nm, 4W). An oil mist generator (LaV- ision) supplies seed particles to the coflow to visualize the flow fields.
Figure 2: Effect of diluent addition to the coflow withXD= 0.175 (top) and to the fuel jet withXF= 0.155 (bottom) onHL. The diluents are (a) CO2, (b) N2, and (c) He, respectively, andU0 = 18 cm/s.
3. Results and discussion
3.1. Diluent effect on flame and flow characteristics
The effects of diluent addition to the coflow and the fuel jet on the characteristics of the lifted flames, especially on HL are first investigated by visualizing them with oil mist as shown in Fig. 2. It is readily observed that the lifted flames exhibit a tribrachial edge flame structure (especially to the naked eye) regardless of the diluents and the streams in which the diluent is added. However, diluent addition by 15 – 18% significantly changes HL trend depending on the streams in which the diluent is added: for the diluent-added coflow, HL increases in the order of He, N2, CO2, while the trend becomes opposite for the diluent-added fuel.
Since the heat capacity of CO2 (He) is the largest (smallest) among the three, it is expected that the maximum flame temperature of the lifted flames in CO2-(He-)added coflow/fuel is the lowest (highest), leading to the lowest (highest)Se and the largest (small-
Figure 3: Effect of diluent addition to the coflow with XD = 0.175(top) and to the fuel jet with XF = 0.155(bottom) on nonreacting flow fields. The diluents are (a) CO2, (b) N2, and (c) He, respectively, andU0
= 18 cm/s. The white dots and arrows represent the stagnation points and local flow directions, respectively.
est) HL. This may explain the HL trend for the diluent-added coflow cases, while contra- dicting that for the diluent-added fuel cases.
As mentioned above, positive/negative buoyancy occurs by the density difference and changes HL by varying flow field at relatively-small U0 [18]. To understand the effect of buoyancy especially on nonreacting flow fields corresponding to the lifted flames in Fig. 2, we visualize them with oil mist as shown in Fig. 3. In addition, we evaluate and summarize the density differences for reacting/nonreacting flows in Table 1. Note that ρb and SL0 are estimated from the equilibrium state and 1-D premixed flame of the corresponding stoichio- metric mixture based on the compositions of the fuel jet and the coflow, respectively. In the present study, SL0 is calculated using the PREMIX code [22] with the USC-II mechanism [23]. The Richardson number, Ri, is defined by the ratio of the positive buoyancy to the jet momentum, orRi ≡(ρco−ρb)gd/(ρfU02), and hence,Ri of O(1) in this study indicates that
Table 1: Density differences, laminar burning velocity, and Richardson number for lifted flames in (a) diluent-added coflow and (b) diluent-added fuel in Fig. 2.
(a) Diluent in coflow
Diluent mole fraction,
XD
Fuel mole fraction,
XF
Positive buoyancy, (ρco−ρb) [kg/m3]
Negative buoyancy, (ρf−ρco) [kg/m3]
Laminar burning velocity, SL0 [m/s]
Richardson number,
Ri
CO2
0.175 0.205
1.098 -0.013 0.148 1.045
N2 1.008 0.101 0.254 0.959
He 0.862 0.271 0.345 0.819
(b) Diluent
in fuel XD XF (ρco−ρb) [kg/m3]
(ρf−ρco) [kg/m3]
SL0 [m/s] Ri
CO2
0.155 0.225
1.005 0.215 0.241 0.878
N2 1.009 0.114 0.266 0.951
He 1.013 -0.037 0.278 1.081
the lifted flames are affected by both the buoyancy and the fuel jet momentum.
The fuel jet in He-added coflow/CO2-added fuel (see Figs. 3c-i and 3a-ii) loses its mo- mentum more readily right after issuing from the fuel nozzle than the other diluent cases.
This is because the negative buoyancy represented by(ρf −ρco)has its maximum value for He-added coflow and CO2-added fuel cases. The negative buoyancy is more likely to restrain the fuel jet from developing downstream and induce a stagnation flow by decelerating the fuel jet. Although the magnitude of the negative buoyancy is smaller than those of the corresponding positive buoyancy, it can break the fuel jet structure by inducing backflow (see Figs. 3a-ii and 3b-ii), which consequently helps the lifted flames in N2-/He-added coflow (CO2-/N2-added fuel) in Fig. 2 to be stabilized further upstream than that in CO2-added coflow (He-added fuel).
On the contrary, the ‘minus’ negative buoyancy in CO2-added coflow/He-added fuel (see Table 1) can help the fuel jet to keep its jet structure even further downstream without losing momentum by exerting a buoyant force in the downstream direction like positive buoyancy.
This leads to the largest HL in CO2-added coflow/He-added fuel among the three diluent
Figure 4: Variations ofHL as a function of XD in (a) oxidizer coflow and (b) fuel jet forU0 = 18 cm/s. dq
represents the quenching distance under stoichiometric condition from [24]. For CO2-added fuel case, two symbols at givenXD represent theHL range of the oscillating lifted flame.
cases.
3.2. Overall characteristics of HL
To further identify the characteristics of HL depending on the degree of dilution, we measure HL by varying XD as shown in Fig. 4. Several points are noted. First, for both diluent-added coflow/fuel cases, the order of HL magnitude at specified XD remains the same as those in Fig. 2 except that the lifted flame in He-added coflow comes to extinction when XHe > 0.175 and the one in CO2-added fuel oscillates when XCO2 ≥ 0.160. The negative buoyancy and resulting stagnation point height increase in the order of CO2, N2, He regardless of XD for diluent-added coflow cases while they do in the order of He, N2, CO2 for diluent-added fuel cases. Therefore, HL follows the tendency of the order of the negative buoyancy values. Readers are referred to Supplementary Material for the details of the positive/negative buoyancy values,SL0, and Ri for each case.
Note that the negative buoyancy in CO2-added fuel is larger than those in He-/N2- added fuel and remains the same regardless of XCO2, while the lifted flame becomes weak with increasing XCO2. As such, the negative buoyancy can trigger flame oscillation when XCO2 becomes larger than a critical value. Then, the lifted flame continues to oscillate due to competition between the positive and negative buoyancies [18]. For He-added coflow case, however, flame extinction is observed rather than flame oscillation whenXHe exceeds a critical value. Once the fuel jet is broken by the negative buoyancy, it is quickly mixed with the oxidizer coflow due to the high mass diffusivity of He as shown in Fig. 2c-i, from which any traces of the fuel jet cannot be observed downstream of the stagnation point.
Consequently, the lifted flame cannot survive such a fuel-lean condition, leading to flame extinction.
Second, for diluent-added coflow cases (see Fig. 4a),HL increases with increasingXDfor all diluent cases. As XD increases, the maximum flame temperature, Tmax, at the stoichio- metric mixture fraction,ξst, based on the fuel and oxidizer free stream conditions is decreased simply because XF is reduced at ξst. Consequently, this leads to a reduction ofSe. Besides the decrease ofSewithXD, the negative buoyancy in He-/N2-added coflow increases slightly with increasingXD due to reduction of ρco with XD, which further increases HL. Note that for lifted flames with relatively-small XHe and XN2, say, less than 0.16 (see Fig. 4a), their HL becomes smaller than the quenching distance at the stoichiometric condition, dq [24], indicating that they are attached flames rather than lifted flames.
Third, for He-/N2-added fuel cases (see Fig. 4b),HL also increases with increasing XD. Since Tmax at ξst is reduced with increasing XD, similar to that in diluent-added coflow, Se is also decreased, which leads to the increase of HL with increasing XD. The negative buoyancy decreases with increasing XD, which also helps to increase HL with increasing XD. Note that the slope of HL increase in N2-added fuel is relatively smaller than that in N2-added coflow. This is becauseHL is more affected bySL0 than the negative buoyancy in N2-added cases, which will be further discussed in the next section.
Figure 5: Variations of laminar burning velocity,SL0, as a function of XD for three different diluent-added coflow/fuel.
3.3. Effects of diluent addition on SL0
As discussed above, HL variation with different diluents can be reasonably estimated by considering the variation of Se and the dominance of positive or negative buoyancy in the lifted flames. In this section, the effects of diluent addition are further elucidated by examining SL0 at the stoichiometry as a measure of the corresponding Se. Although SL0 is different fromSedue to several reasons such as flow redirection, concentration gradient, and flame curvature, it can qualitatively represent the behavior of Se with which a lifted flame moves along theξst isoline [4, 25].
Figure 5 showsSL0 as a function of XD for the three diluents. As expected, SL0 decreases with increasing XD for all cases. Since the decrease of SL0 with XD implies the reduction of the correspondingSe, this result is consistent with the increasing tendency ofHL withXD in Fig. 2. As expected, SL0 is found to be the largest (smallest) in the He-(CO2-)added stream at given XD, which explains that HL in diluent-added coflow increases in the order of He, N2, CO2. However, thisSL0 trend is opposite to the decreasingHLtendency in diluent-added fuel in the order of He, N2, CO2, which is primarily attributed to the change of the flow field by the negative buoyancy as explained above.
We further elucidate the effects of diluents onSL0 by decomposing them into three different ones: dilution, thermal, and chemical effects as in [26–31]. They are quantitatively measured
by introducing the contribution factor to the reduction of SL0, which is defined as:
SL0[0]−SL0[D]
SL0[0]−SL0[D] = SL0[0]−SL0[PN2]
SL0[0]−SL0[D] +SL0[PN2]−S0L[PD]
SL0[0]−SL0[D] +SL0[PD]−S0L[D]
SL0[0]−SL0[D] , (1)
where 0 denotes no dilution and D refers to diluent. PN2 and PD denote pseudo N2 and pseudo diluent, respectively. The first, second, and third terms indicate the contribution factors of the dilution, thermal, and chemical effects, respectively. The dilution effect repre- sents a reduction ofSL0 due to the decrease of fuel or oxygen amount by diluent addition; the thermal effect does a reduction of SL0 caused by the specific heat capacity, cp, and thermal diffusivity, α, of the diluent. The chemical effect denotes a reduction of SL0 by the diluent participating in chemical reactions such that it can be measured by replacing a real diluent with a pseudo diluent. Here, the pseudo diluent has the same thermal and transport prop- erties as the real diluent but does not participates in any reactions [26, 29, 31]. For details of the contribution factors, readers are referred to [31]. Note that SL0[0] are 71.7 and 31.2 cm/s for diluent-added coflow and fuel, respectively.
Figure 6 shows the contribution factors of the dilution, thermal, and chemical effects to the reduction of SL0 as a function ofXD for diluent-added coflow/fuel cases. Two points are noted. First, the dilution effect is the largest contributor to the reduction ofSL0 for all cases regardless ofXD while the chemical effect is negligible for all cases except for CO2, of which chemical dissociation reduces SL0 slightly. In addition, the contributions of each effect to the reduction ofSL0 remain nearly the same for all cases regardless of XD even though SL0 is actually decreased with increasing XD as shown in Fig. 5.
Second, for He addition cases, the thermal effect increases SL0 by approximately 20%
while for CO2 addition cases, both thermal and chemical effects reduceSL0 by approximately 10%. For N2 addition cases, the thermal and chemical effects have little influence on SL0. Therefore, the contribution of the dilution effect to the reduction of SL0 becomes the largest (smallest) for He (CO2) addition cases even though the actual reduction ofSL0,SL0[0]−SL0[D], is the smallest (largest) in He (CO2) addition cases (see Fig. 5). Note that for He addition
Figure 6: Variations of contribution factors of the dilution, thermal, and chemical effects to reduction ofSL0 as a function ofXDin (a) the oxidizer coflow and (b) the fuel jet.
cases, the increase of SL0 by the thermal effect actually means that SL0[PHe] is faster than SL0[PN2] because thermal diffusivity of He is much larger than that of N2 [31].
3.4. Effects of U0
In this section, it is further elucidated if the effects of diluents on flame and flow char- acteristics remain the same even in the momentum-driven regime with large U0. Figure 7 shows the variation ofHL as a function ofXD,K atU0 = 60 cm/s. It is readily observed that HL increases in the order of CO2, N2, He for given XD for diluent-added coflow cases while the order becomes opposite for diluent-added fuel cases. The trends of HL are identical to those at U0 = 18 cm/s even though HL atU0 = 60 cm/s are much higher than HL at U0 = 18 cm/s. Note that for cases with U0 = 60 cm/s, Ri ≤ 0.1 such that the buoyancy effect is relatively small compared to the jet momentum effect.
For He-added coflow cases, flame oscillation is observed instead of flame extinction when XHe > 0.165. In addition, for CO2-added fuel cases, steady lifted flames exist for the whole
Figure 7: Variations ofHL as a function ofXD in (a) the oxidizer coflow and (b) the fuel jet for U0 = 60 cm/s. For He-added coflow case, two symbols at givenXD represent theHL range of the oscillating lifted flame.
range of XCO2 rather than oscillating lifted flames. These characteristics of the lifted flames at high U0 indicates that the effect of the negative buoyancy is reduced because the lifted flames are mainly dominated by the fuel jet momentum. Note that similar flame character- istics are also observed for lifted flames at U0 = 40 cm/s (see Supplementary material).
3.5. Correlation for HL
From the above results, it can be summarized that HL is increased with increasing U0 and/or increasing positive buoyancy while being decreased with increasing SL0 and/or increasing negative buoyancy. Therefore, by assuming the opposite effects of the positive buoyancy vs. the negative buoyancy and U0 vs. SL0 as simple fractional expressions, we can come up with a functional form of HL:
HL ∼ (ρco−ρb) (ρf −ρco) + ∆ρ0
(U0 SL0
)n
, (2)
where∆ρ0 is a coefficient introduced to render the buoyancy ratio term to be positive even for the minus negative buoyancy. We set ∆ρ0 to unity so that the buoyancy ratio term in Eq. 2 represents only the positive buoyancy when the negative buoyancy vanishes. HL is also determined by the competition between the fuel jet momentum and the buoyancy since they are comparable to each other for U0 = 18 cm/s. Thus, it is further assumed that HL is more affected by flow momentum such that the velocity ratio term in Eq. 2 is set to a quadratic form orn = 2.
Figure. 8 shows experimental HL and its linear best fit function of Eq. 2. Even if the model equation is one of the simplest forms, its prediction is in good agreement with the experimental data, which substantiates that both positive and negative buoyancies play a critical role in determiningHLin diluent-added coflow/fuel together withU0 andSL0. It is of interest to note that even if∆ρ0 changes in the range of 0.5 – 1.5, the correlation coefficient, R, for the linear best fit does not change much, which implies that the functional form of the buoyancy ratio term in Eq. 2 is proper to delineate the HL variation. Moreover, R for the linear best fit does not change much with n either if n ranges from 1.5 to 3.0, which
Figure 8: Correlation ofHL with the ratio of the positive to negative buoyancy and(U0/SL0)2 for the lifted flames in Fig. 2.
implies that the functional form for the velocity effect may be more complicated than a simple fractional formula to the power of two. Note that Eq. 2 holds only for HL at U0 = 18 cm/s because under high U0 condition, the jet momentum overwhelms the buoyancy.
4. Conclusions
The effects of diluent addition to the fuel and coflow streams on HL of the laminar non-premixed propane jet flames in the oxidizer coflow were experimentally investigated by visualizing flow fields with oil mist and estimating the positive/negative buoyancies andSL0. The liftoff height behavior of the stationary lifted jet flames in coflow by the influences of the positive and negative buoyancies was investigated for the first time and characterized with a functional dependence on the ratio of the positive to negative buoyancy andU0 scaled bySL0. The following results are obtained from the present study.
1. For lifted flames in diluent-added coflow, HL increases in the order of He, N2, CO2 at givenXD, while for those in diluent-added fuel, the order of HL becomes opposite. In addition, HL increases with increasingXD for all cases.
2. From the visualization of nonreacting flow fields, it was found that the negative buoy- ancy can induce stagnation flow near the fuel jet nozzle by pulling down the fuel jet,
which helps the lifted flames to be stabilized further upstream with increasing negative buoyancy.
3. For the lifted flames in He-added coflow, when XHe exceeds a critical value, flame extinction occurs due to local fuel-lean condition induced by the high mass diffusivity of He. For the lifted flames in CO2-added fuel, however, flame oscillation is observed when XCO2 is larger than a critical value. The flame oscillation occurs by the competition between the positive and negative buoyancies as found in [18].
4. In addition to the buoyancy effect, the effects of diluent addition on HL via Se were elucidated by estimating their contribution to SL0. The effects are classified into the dilution, thermal, and chemical effects, among which the first is found to reduce SL0 the most for all cases and the second significantly increases SL0 in He addition cases, while the third is negligible. Moreover, the contribution of each effect to reduction of SL0 remains nearly the same regardless of XD.
5. Finally, a correlation equation forHL is formulated using the ratios of the positive to negative buoyancy andU0 toSL0, which shows a good agreement with the experimental data.
Declaration of Competing Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF- 2018R1A2A2A05018901). SHC was supported by the King Abdullah University of Science and Technology (KAUST). JP was supported by the Research and Development Program of the Korea Institute of Energy Research (B9-2431).
References
[1] S. H. Chung, B. J. Lee, On the characteristics of laminar lifted flames in a nonpremixed jet, Combst.
Flame 86 (1991) 62–72.
[2] B. J. Lee, S. H. Chung, Stabilization of lifted tribrachial flames in a laminar nonpremixed jet, Combst.
Flame 109 (1997) 163–172.
[3] T. Plessing, P. Terhoeven, N. Peters, M. Mansour, An experimental and numerical study of a laminar triple flame, Combst. Flame 115 (1998) 335–353.
[4] S. H. Chung, Stabilization, propagation and instability of tribrachial triple flames, Proc. Combust. Inst 31 (2007) 877–892.
[5] N. Peters, Laminar diffusion flamelet models in non-premixed turbulent combustion, Prog. Energy Combust. Sci. 10 (1984) 319–339.
[6] Y. S. Ko, S. H. Chung, Propagation of unsteady tribrachial flames in laminar non-premixed jets, Combust. Flame 118 (1999) 151–163.
[7] J. W. Dold, Flame propagation in a nonuniform mixture: Analysis of a slowly varying triple flame, Combust. Flame 76 (1989) 71–88.
[8] J. Daou, A. Liñán, The role of unequal diffusivities in ignition and extingtion fronts in strained mixing layers, Combust. Theory Model. 2 (1998) 449–477.
[9] T. Echekki, J. H. Chen, Structure and propagation of methanol-air triple flames, Combust. Flame 114 (1998) 231–245.
[10] Y. C. Chen, R. W. Bilger, Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows, Combust. Flame 122 (2000) 377–399.
[11] J. Buckmaster, M. Matalon, Anomalous lewis number effects in tribrachial flames, Symp. (Int.) Com- bust. 22 (1988) 1527–1535.
[12] L. J. Hartely, J. W. Dold, Flame propagation in a nonuniform mixture: analysis of a propagating triple-flame, Combust. Sci. Techol. 80 (1991) 23–46.
[13] G. R. Reutsch, L. Vervisch, A. Liñán, Effects of heat release on triple flames, Phys. Fluids 7(6) (1995) 1447–1454.
[14] P. N. Kioni, B. Rogg, K. N. C. Bray, A. Liñán, Flame spread in laminar mixing layers: the triple flame, Combust. Flame 95 (1993) 276–290.
[15] M. Füri, P. Papas, P. A. Monkewitz, Non-premixed jet flame pulsations near extinction, Proc. Combust.
Inst 28 (2000) 831–838.
[16] S. H. Won, S. H. Chung, M. S. Cha, B. J. Lee, Lifted flame stabilization in developing and developed regions of coflow jets for highly diluted propane, Proc. Combust. Inst 28 (2000) 2093–2099.
[17] V. N. Kurdyumov, M. Matalon, Stabilization and onset of oscillation of an edge-flame in the near-wake
of a fuel injector, Proc. Combust. Inst 31 (2007) 909–917.
[18] K. H. Van, J. Park, S. H. Yoon, S. H. Chung, M. S. Cha, Mechanism on oscillating lifted flames in nonpremixed laminar coflow jets, Proc. Combust. Inst 37 (2019) 1997–2004.
[19] J. Lee, S. H. Won, S. H. Jin, S. H. Chung, O. Fujita, K. Ito, Propagation speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions, Combust. Flame 134 (2003) 411–420.
[20] S. H. Won, J. Kim, K. J. Hong, M. S. Cha, S. H. Chung, Stabilization mechanism of lifted flame edge in the near field of coflow jets for diluted methane, Proc. Combust. Inst. 30 (2005) 339–347.
[21] K. Van, K. S. Jung, C. S. Yoo, S. Oh, B. J. Lee, M. S. Cha, J. Park, S. H. Chung, Decreasing liftoff height behavior in diluted laminar lifted methane jet flames, Proc. Combust. Inst 37 (2019) 2005–2012.
[22] R. J. Kee, J. F. Grcar, M. D. Smooke, J. A. Miller, PREMIX: A Fortran program for modeling steady laminar one-dimensional premixed flames, Tech. Rep. SAND-85-8240, Sandia National Labs., Livermore, CA, USA (1985).
[23] H. Wang, X. You, A. V. Joshi, S. G. Davis, A. Laskin, F. Egolfopoulos, C. K. Law, USC Mech Version II. High-Temperature Combustion Reaction Model of H2/CO/C1-C4 Compounds, http://ignis.usc.edu/USC_Mech_II.htm.
[24] A. M. Kanury, Introduction to Combustion Phenomena, Gordon and Breach, 1987.
[25] R. W. Bilger, The structure of turbulent nonpremixed flames, Symp. (Int.) Combust. 22 (1988) 475–488.
[26] D. X. Du, R. L. Axelbaum, C. K. Law, The influence of carbon dioxide and oxygen as additives on soot formation in diffusion flames, Symp. (Int.) Combust. 23 (1990) 1501–1507.
[27] F. Liu, H. Guo, G. J. Smallwood, Ö. L. Gülder, The chemical effects of carbon dioxide as an additive in an ethylene diffusion flame: Implications for soot and NOx formation, Combust. Flame 125 (2001) 778–787.
[28] F. Halter, F. Foucher, L. Landry, C. Mounaïm-Rousselle, Effect of dilution by nitrogen and/or carbon dioxide on methane and iso-octane air flames, Combust. Sci. Technol. 181 (2009) 813–827.
[29] B. Galmiche, F. Halter, F. Foucher, P. Dagaut, Effects of dilution on laminar burning velocity of premixed methane/air flames, Energy Fuels 25 (2011) 948–954.
[30] J. Min, F. Baillot, A. Wyzgolik, E. Domingues, M. Talbaut, B. Patte-Rouland, C. Galizzi, Impact of CO2/N2/Ar addition on the internal structure and stability of nonpremixed CH4/air flames at lifting, Combust. Sci. Technol. 182 (2010) 1782–1804.
[31] E. Hu, X. Jiang, Z. Huang, N. Iida, Numerical study on the effects of diluents on the laminar burning velocity of methane-air mixtures, Energy Fuels 26 (2012) 4242–4252.
