5.1 Overall characteristics
5.1.1 Autoignition-affected flame stabilization
Figure 5.1 shows the isocontours of temperature, mass fractions of OH, CH2O, and heat release rate of autoignited laminar lifted n-heptane jet flames for various U0 with T0 = 1025 K andXF,0 = 0.02. The detailed flame structures for differentU0 are highlighted in the insets of Fig. 5.1b.
For relatively-lowU0 (i.e., 4.0 ≤U0 ≤ 6.0 m/s), the lifted flames exhibit a tribrachial edge flame structure consisting of lean/rich premixed wings and a trailing diffusion flame along the stoichiometric mixture fraction isoline,ξst, which is denoted by the dashed lines in Fig. 5.1. Here, the mixture fraction is evaluated based on the Bilger’s formula [66].
Note that the flamebase of the tribrachial edge flame slightly deviates from ξst isoline due to the existence of velocity gradient upstream of the tribrachial edge flame [111]. In addition, any trace of CH2O formation, which is one of the most important intermediate species in hydrocarbon oxidation, and heat release process upstream of the tribrachial edge flame are not observed. These results imply that the contribution of autoignition process to liftoff height is marginal even if it may exist.
Figure 5.1: Isocontours of (a) T [K] (left half) and YOH (right half) and (b) YCH2O (left half) and heat release rate [J/m3s] (right half) for autoignited laminar n-heptane jet flames for various fuel exit velocities,U0, with XF,0 = 0.02 andT0 = 1025 K. The dashed line represents the stoichiometric mixture fraction,ξst (= 0.494), isoline; the value of the mixture fraction at the flamebase,ξfb, for each U0 is shown in (a).
AsU0 increases to 6.5 m/s,ξst isoline becomes closed near the flamebase such that the trailing diffusion flame vanishes and the lifted flame features a flat flame front. According to the conventional blowout criterion for the nonpremixed laminar jet flame under non- autoignitive conditions [15], it is blown out when its triple point or flamebase is detached fromξst isoline. As such, U0 of 6.5 m/s could be the blowout limit for the corresponding non-autoignited laminar lifted jet flame. However, the lifted flames with U0 > 6.5 m/s are found to stabilize further downstream from the most downstream point ofξst isoline, zξst, with a significant increase of HL compared to that of U0 = 6.5 m/s case, which is contradictory to the stabilization theory of the non-autoignited laminar lifted jet flames [12, 13, 15]. It is readily observed from Fig. 5.1b that for cases with U0 > 6.5 m/s, relatively-large heat is released and relatively-large amount of CH2O is formed upstream of the flamebases, which indicates that the lifted jet flames with U0 > 6.5 m/s can be stabilized by autoignition process in the mixing layer, similar to those in the previous studies [96, 110].
To quantitatively identify the response of the lifted flames to U0, the variations in HL and (Tmax − T0)/Tig as a function of U0 are shown in Fig. 5.2. For comparison purposes, HL variation in the previous experiments [8] is also shown in the figure. The characteristics of HL variation in the experiments featuring two steep changes in its magnitude is qualitatively well captured by the present numerical simulations. While the first HL transition is due to the reattachment from a lifted flame to a nozzle-attached flame, the second HL transition is mainly attributed to the change of combustion mode from the tribrachial edge flame to the MILD combustion. The latter can be substantiated by the drop of (Tmax−T0)/Tig below unity at the second HL transition (i.e. at U0 = 6.7 m/s in the simulation). The MILD combustion mode is usually observed when its stabilization is highly affected by autoignition [96, 110]. As such, the existence of lifted flames withHL being far downstream of zξst implies that autoignition can play a critical role in extending the blowout limit of the lifted flame. In addition, it is of interest to note that the lifted flames with U0 = 6.7 and 7.0 m/s can be stabilized along the ξst isoline rather than exhibiting MILD combustion mode whenU0 is increased from the steady case with U0 = 6.5 m/s. It indicates that there exists a hysteresis in the response ofHL toU0
U0 [m/s]
HL [cm] (Tmax- T0)/Tig
2 4 6 8 10 12
0 10 20 30 40 50
0.8 1.0 1.2
MILD Tribrachial
HL (experiment) HL (simulation)
U0 increases (hysteresis)
Figure 5.2: Variations in HL and (Tmax −T0)/Tig as a function of U0 for autoignited laminar n-heptane jet flames with XF,0 = 0.02 and T0 = 1025 K. The black square symbols represent HL variation in previous experiments atT0 = 980 K [8].
and the details will be discussed later.
One of the main roles of autoignition in stabilizing a lifted flame is to enhance its propagation speed by increasing unburned mixture temperature upstream of its flamebase [17, 38, 96, 110]. Therefore, the contribution of autoignition to the flame stabilization can qualitatively be measured by comparing the propagation speed of the flame edge, Se, with the flame propagation speed of a 1-D unstrained laminar premixed flame, SR,0, in the limit of an unreacted upstream composition atT0 [110]. Here, Se is estimated by the density-weighted displacement speed, Sd∗. We adopt OH forSeevaluation since it is often used as a flame marker [30, 34, 91].
Note that SR,0 is estimated from the method proposed by Krisman et al. [17]. Also note that for SR,0 calculation, we use the mixture composition at the flamebase of a lifted flame that is converted into the fresh reactants at the fuel and oxidizer streams (i.e. n-heptane, O2, and N2) while its ξ remains identical to ξfb. In addition, the inlet temperature is set toT0 such that the thermo-chemical effect of autoignition process up- stream of the flamebase is ruled out forSR,0 calculation. In this regard, we can reasonably expect that for the conventional non-autoignited lifted flames,Se exhibits a value similar toSR,0, while Sebecomes considerably larger thanSR,0 when autoignition contributes to the stabilization of the lifted flames.
Figure 5.3 shows Se/SR,0 and SR,0 for the autoignited laminar n-heptane lifted jet flames for various U0. It is readily observed that Se/SR,0 ≈ 1 for the tribrachial edge flames, demonstrating that the stabilization mechanism of the present lifted flames with
U0 [m/s]
Se/SR,0 SR,0 [m/s]
5 6 7 8 9 10 11 12
1 6 11
0 1 MILD 2 (concave) Tribrachial MILD
(convex)
Figure 5.3: Se/SR,0 andSR,0 for the autoignited laminarn-heptane lifted jet flames with XF,0 = 0.02 and T0 = 1025 K for various fuel tube exit velocities, U0.
tribrachial edge flame mode follows the conventional stabilization theory for the non- autoignited lifted flames [12, 13]. Here, SR,0 increases with increasingU0 simply because the mixture condition at the flamebase,ξfb, becomes closer to the stoichiometric mixture, ξst (=0.494), with increasing U0 (see Fig. 5.1a). For the lifted flame with the MILD combustion mode at U0 = 6.7 m/s, Se becomes significantly larger than SR,0, which indicates that autoignition starts to contribute to its stabilization by enhancing Se.
It is also readily seen thatSe/SR,0 increases slightly with increasing U0 for the MILD combustion mode with a convex flame shape along the jet centerline while it significantly increases toO(10) for the MILD combustion mode with a concave flame shape along the jet centerline (see III and IV in Fig. 5.1b). This result implies that the effect of autoigni-