7.2 Flame stabilization and extinction
7.2.4 Flame extinction dynamics
re/Rbecause the radial velocity is linearly proportional tor (see Fig. 7.4). Therefore, the counterflow flame achieves flame stabilization by shrinking its flame length toward the center of the flow where the reactivity of the mixture is high enough to sustain the flame.
However, this is feasible only when the positive effect (i.e., increasing reactivity of mixture at the flame edge) can compensate for the negative effects (i.e., increasing/decreasing conductive/convective heat and mass transfer) with decreasing the flame length.
Figure 7.12: Temporal evolutions of counterflow nonpremixed flames of CH4/He versus air at ag = 10 s−1 for the experiment (left) and 2-D simulations in normal (middle) and zero gravity (right).
Residence time, t [ms]
Speed [cm/s] re/R
0 50 100 150
-60 -40 -20 0 20
0.0 0.5 1.0
XHe = 0.6 → XHe = 0.65 w/g
-Ue,n re/R
Se
(a)
w/o g
Time, t [ms]
Transport budget [kg/m3 s]
0 50 100 150
-0.08 0.00 0.08
D × 0.2 R × 0.2
C
D + R
(b)
w/g w/o g XHe = 0.6 → XHe = 0.65
Figure 7.13: Temporal evolutions of (a) Se, −Ue,n, re/R, and (b) convection, diffusion, chemical reaction terms at the flame edge of counterflow nonpremixed flames CH4/He versus air with XF,0 = 0.35 at ag = 10 s−1 in normal (dashed-dot lines)and zero (solid lines with symbols) gravity. For both cases, the simulations start from the steady cases with XHe = 0.60.
of XHe are qualitatively consistent with those related to Fig. 7.11. However, the increase of R no longer dominates over the decrease of D, resulting in a monotonic decrease of the netD + Rin time. In other words, the magnitude of negative Se keeps increasing in time, while the magnitudes of−Ue,n andCat the flame edge decrease, leading to a large inward movement of the edge flame. As a result, the net displacement speed expressed by Se+Ue,n keeps decreasing in time until the flame is completely extinguished by the shrinkage of the outer edge flame toward the flame center. Therefore, the present results indicate that whenXHeexceedsXHe,cr, the shrinkage of the flame induced by the increase of XHe destabilizes the flame, thereby accelerating the flame extinction.
Chapter 8
Conclusion and future work
8.1 Conclusions
This study focuses on the fundamental liftoff, flame structure, and stabilization character- istics of laminar CH4/H2, DME, and n-heptane jet flames in heated coflow air, turbulent H2 jet flames near the autoignitive limit, and the stabilization and extinction character- istics of negatively propagating CH4/He versus air nonpremixed flames in a counterflow burner by using high-fidelity numerical simulations.
The key findings of the study on the characteristics of laminar/turbulent jet flames are summarized as follows.
• From the simulations with normal and modified DH2, it was verified that the high diffusive nature of hydrogen molecules or the differential diffusion between methane and hydrogen is primarily attributed to the unusual decreasing HL behavior with increasing U0 for autoignited laminar CH4/H2 jet flames. The species transport budget, autoignition index, displacement speed and chemical explosive mode anal- ysis revealed that the stabilization of the autoignited lifted methane/hydrogen jet flames is affected by both autoignition and flame propagation. The role of flame propagation on their stabilization increases with increasingU0 due to theRHchar- acteristics depending onU0and the flamebase locations: a lifted flame in the MILD combustion regime is primarily stabilized by autoignition while it in the tribrachial edge flame regime is stabilized by autoignition-assisted flame propagation.
• It is verified that the U-shaped HL behavior observed in the autoignited DME jet flame is primary attributed to the fast diffusion rate of H2. Since the DME jet is readily decomposed into small species such as CH4, H2, and CO in the heated fuel tube, the different mass diffusivities of the species generated from the pyrolysis of DME cause the unusual U-shaped HL behavior. CEMA identified the important variables and reactions contributing to the autoignition of the DME jet. For the increasingHL regime, the overall autoignition characteristics are not much affected by U0. For the decreasing HL regime, on the other hand, the autoignition charac- teristics are highly affected by the differential diffusion effect, and thus, the ignition
delay and resultantHLchange withU0. The results indicate that 0-D homogeneous ignition delay time,τig,st, does not always represent the autoignition characteristics of 2-D jet flames, especially for the highly diffusive fuels.
• For the laminarn-heptane jet flame atT0 = 1025 K, the steep variation inHLat the transition from the tribrachial edge flame to the MILD combustion mode observed from a previous experimental study is numerically reproduced, from which it was verified that autoignition helps the MILD combustion mode to exist further down- stream ofzξst at relatively-highU0. It was also found that for the MILD combustion mode, there exists a region between the flamebase of the lifted flame andzξst where temperature increase by autoignition is marginal, which ultimately leads to the significant change inHL at the transition from the tribrachial edge flame to MILD combustion mode. Based on the characteristics of flame structures and stabiliza- tion mechanisms, a flame regime diagram for the lifted jet flames was constructed in the U0/SR,0|st−Da space. The autoignited laminar n-heptane lifted jet flames under atmospheric pressure can exhibit the MILD combustion, partially-premixed edge flame, tribrachial edge flame, and tetrabrachial flame structure depending on the inlet temperature and velocity.
• The ignition dynamics and stabilization characteristics of turbulent lifted hydrogen jet flames in heated coflows near the ignition limit were numerically investigated using 3-D DNS with a detailed chemical kinetic mechanism for hydrogen oxidation.
The numerical simulations were performed for different Tc of 750 K (Case L), 850 K (Case M), 950 K (Case H), and 1100 K (Case Ig). Here, Case H represents a turbulent lifted jet flame near the ignition limit, serving as a baseline case for the present study. We examined the instantaneous and time-averaged profiles of the variables relevant to autoignition for the 3-D DNS cases. The local flamebase dynamics were examined by evaluatingSe/SLat the flamebases. In addition, a local combustion mode indicator elucidated the detailed ignition characteristics of the cases, from which it was found that vortices play a different role in the autoignition of hydrogen/air mixture depending on the proximity ofTc to the ignition limit. To
further clarify the ignition characteristics of a nonpremixed H2/air mixture within a vortex, an additional parametric study was performed in a 2-D domain with different Ta, Umax, andτv.
• The flame structure, edge flame stabilization, and extinction characteristics of coun- terflow nonpremixed flames of CH4/He versus air at low strain rates were experi- mentally and numerically investigated. By adopting a novel experimental method- ology of He curtain flow in the counterflow burner, we could locate the flames in the middle of the burner, and hence, measure the critical He mole fraction,XHe,cr, for flame extinction even at very-low strain rates. The comparison between XHe,cr of the experiments and 2-D numerical simulations in normal and zero gravity re- vealed that the He curtain flow can effectively reduce the buoyancy effect on the flame characteristics such that the extinction dynamics of counterflow nonpremixed flames at low strain rates can be reasonably investigated even in normal gravity.
The dynamics of the outer edge flame for flame stabilization was numerically in- vestigated by performing 2-D transient numerical simulations of counterflow non- premixed flames of CH4/He versus air at low strain rates of 10 and 30 s−1, from which the transition of the edge flame from a bibrachial edge flame with positiveSe to a monobrachial edge flame with negativeSewas observed. The species transport budget analysis was performed to better understand the flame stabilization and extinction mechanism. The following results are obtained in the present study.