According to a previous experiment on self-igniting laminar DME jet flame, an unusual lift height behavior is observed such that lift height of lifted flame decreases with increasing fuel jet velocity. From a series of numerical studies, it is elucidated that the decreasing lift height behavior is mainly due to the auto-ignition of DME, so that lift height variations can be well correlated with ignition delay time. All the results show consistently decreasing lift height with the increase in fuel jet velocity for a relatively low fuel velocity regime.
Finally, the pyrolysis effect on the ignition and lift height characteristics of self-igniting laminar DME jet flame is investigated by varying the fuel nozzle length while the fuel jet velocity is fixed at 5 m/s. However, in transitional and tribrachial edge flame regimes, the rise height trend is no longer consistent with ignition delay time.
Introduction
The height of rise of laminar raised flames is generally proportional to the velocity of the fuel jet, all other conditions remaining the same. However, from an experimental study of self-ignited laminar lifted DME jet flames in heated coflow air, an unusual HL behavior was observed; HL is inversely proportional to the fuel jet velocity, U0 [18]. In general, this decreasing HL behavior only occurs when two types of fuels are mixed in the fuel jet (e.g. methane/hydrogen jet flames) [19], and the differential diffusion effect between the two fuels appears to determine the unusual launch altitude. behaviour.
Numerical methods
Tig is the lowest temperature for autoignition of a stoichiometric mixture given the inlet conditions. To better understand the pyrolysis of a DME fuel jet through a fuel nozzle, 0-D simulations were performed. Additional numerical simulations were performed by artificially changing the mass diffusivity of the hydrogen molecule, DH2, to that of DME.
To investigate the different ignition characteristics of the lifted DME jet flames with various pyrolysis levels and flame structures, we performed the CEMA for several cases. Second, we investigate the effects of the pyrolysis on HL and auto-ignition characteristics of the DME jet flame by adjusting Lres with fixed U0 of 5 m/s. These results clearly show that the pyrolysis of DME not only affects the lift-off height, but also the flame structure, and as such we can suspect that the self-ignition characteristics of the lifted DME jet flames may also change depending on the degree of pyrolysis of DME.
Therefore, the ignition characteristics of raised flames in the MILD combustion regime follow DME ignition, so that the variation of HL can be well related to 𝜏ig0. In summary, as the pyrolysis rate increases, the autoignition properties of the DME jet change from those of DME to hydrogen. However, the take-off height depends not only on the characteristics of the ignition delay, but also on the characteristics of the flame spread.
The ignition of the DME can be attributed to the unusual behavior of the raised flame at Lres = 1.5 and 3 m.
Characteristic of lifted flame under experimental condition
To accurately measure the lift, we use the chemical explosive mode analysis (CEMA) and determine the flame base as the most upstream location of the Re(𝜆exp) = 0 isoline, where 𝜆exp is an eigenvalue of the Jacobian of the chemical source term. To categorize the distinct combustion feature of the lifted flames, we measure (Tmax−T0) / Tig as shown in fig. We determine Tig considering that autoignition can occur within the computational domain if the 0-D ignition delay time, 𝜏ig0, at T = Tig is less than the one-jet flow time of the coflow air.
5b, all the flames under this condition are categorized in the MILD combustion regime, which implies that 𝜏ig0 will dominantly affect the stabilization of the lifted flame [19]. That is, as Lres becomes longer, the pyrolysis of the DME jet is enhanced, and as such, more CH4 and H2 are injected from the nozzle. H2O2 is subsequently decomposed to OH via R16, which is one of the exothermic reactions of hydrocarbons.
Furthermore, the U-shaped behavior of HL is irrelevant to DH2 because the U-shaped behavior always occurs regardless of the value of DH2. Therefore, we can summarize that due to the pyrolysis process, the DME jet through the fuel nozzle is relatively - a large amount of H2. The EI of HO2 and H2O2 (Figures 12d and 12e) associated with DME ignition, as shown in the 0-D calculations, are already completed before the flame base and after. On the other hand, the activation of HO2 and H2O2 (Figures 18d and 18e), which are important in DME ignition, as shown in the 0-D calculation, is already completed before the flame region for the cases with Lres.
In the tribrachial edge flame regime, where DME pyrolysis takes place significantly, the ignition characteristics behave more like hydrogen. Therefore, in order to clearly understand the take-off characteristics of the auto-ignited flames of the DME jet, a further study will be carried out in the future using the flame speed analysis and the budget expression. From simulations with different DH2, it was verified that the high diffusion characteristic of the hydrogen molecule is mainly attributed to the difference of HL from the 𝜏ig0 trend.
In the tribrachial edge flame regime, where the pyrolysis of DME proceeds significantly, the ignition properties behave more like hydrogen flame. Sato, “Development and popularization of heavy-duty vehicles powered by dimethyl ether (DME) as new clean alternative energy,” in Proceedings of the 9th WSEAS International Conference on POWER SYSTEMS.
Correlation between ignition delay time and H L
Ignition Characteristics in 2-D jet flame: CEMA
Therefore, the auto-ignition characteristics of lifted flame follow the ignition of DME, so that HL variations can be well correlated with 𝜏ig0 and unusual HL behavior can be explained by the ignition of DME. In this section, we investigate how the pyrolysis of DME jet through the fuel nozzle affects flame characteristics. To understand this behavior, additional numerical simulations are performed to identify pyrolysis effect on the lifted flame's properties.
To identify the effect of pyrolysis on the raised flame, additional numerical simulations were performed by varying Lres from 3 cm to 23 m, while U0 is fixed at 5 m/s. When a significant amount of DME is pyrolyzed, a tribrachial edge flame appears at Lres = 14 m and an attached flame appears at Lres =17 m. Tmax – T0) / Tig repeats the behavior of increase and decrease depending on the degree of pyrolysis and the transition occurs with the increase of the amount of pyrolysis.
In contrast, for the tribrachial edge flame regime (i.e. Lres = 14 m), DME ignition has nothing to do with the stabilization of the flame base. To identify the effect of ignition inhomogeneity on the unusual HL, additional numerical simulations with modified DH2 were also performed. It was verified by CEMA that the auto-ignition characteristics of raised flame follow the ignition of DME, so that HL variations can be well correlated with 𝜏ig0 under Lres = 0.75 m condition.
To further identify the pyrolysis effect on the lifted flame, additional numerical simulations were performed by changing Lres from 3 cm to 23 m. Based on the flame structure and (Tmax – T0) / Tig, the flames can be classified into four different regimes. Law, “Effects of NO on the Ignition of Hydrogen and Hydrocarbons by Heated Countercurrent Air,” Combust.
Pyrolysis effects
U-shaped behavior for various nozzle length
Pyrolysis effects on the lifted flame characteristics
As can be expected, the variation of HL with increasing Lres shows a non-monotonic behavior. To quantitatively analyze the lift characteristics and flame structure, HL and (Tmax – T0) / Tig are shown in Fig. 16 as a function of Lres. Since the HL of the raised laminar self-igniting flame is greatly affected by 𝜏ig0, HL and 𝜏ig0 are shown in Fig.
17 as a function of Lres, where Lres is defined in the 0-D domain as Lres = U0res. First, it is easy to see that the variations of 𝜏ig0 with different degree of pyrolysis also show non-monotonic behavior. 17 can be classified into four different regimes based on their flame types: MILD combustion, transition, tribrachial edge flame, and attached flame regimes. The reason for this is that the ignition inhomogeneity is large due to the high degree of pyrolysis of DME, or the ignition characteristic may change during the course of pyrolysis.
Ignition Characteristics in 2-D jet flame: CEMA
To further identify different ignition characteristics for each regime, isocontours of PIs for several important reactions are shown in Figure 19 . For the MAGTE combustion (i.e. Lres = 1.5 m), hydrogen-related reactions do not affect the upstream of flame base. Due to a large amount of H2 generated from the pyrolysis, the ignition characteristics in this regime are dominated by the ignition of H2 as demonstrated in Fig.
In this regard, the HL behavior for this regime is inconsistent with the variations of 𝜏ig0. Isocontours of PI of (a) chain branching reaction of hydrogen, (b) heat release step of hydrogen, (c) HO2 formulation, (d) DME H abstraction, (e) CH2O to HCO reactions, and (f) heat release step of hydrocarbon for self-igniting laminar DME jet flames with different pyrolysis level, Lres m cases.
Conclusions
Chung, “Autoignited laminar lifted flames of propane in coflow jets with tribrachial rim and mild combustion,” Combust. Chung, “Autoignited laminar lifted flames of methane, ethylene, ethane, and n-butane jets in elevated-temperature coflow air,” Combust. Chung, “Autoignited and non-autoignited lifted flames of vaporized n-heptane in coflow jets at elevated temperatures,” Combust.
Chung, “Simulation of non-auto-ignited and auto-ignited laminar non-premixed jet flames of syngas in heated confluent air,” Combust. Chung, “Autoignition characteristics of laminar lifted jet flames of prevaporized isooctane in heated confluent air,” Fuel , vol. Chung, “Numerical study of laminar nonpremixed methane flames in coflow jets: Autoignited flames with tribrachial edges and mild combustion at elevated temperatures,” Combust.
Ranzi, “A computational tool for the detailed kinetic modeling of laminar flames: application to C2H4/CH4 coflow flames,” Combust. Yoo, “Direct numerical simulations of the ignition of a lean n-heptane/air mixture with temperature and composition inhomogeneities relevant to HCCI and SCCI combustion,” Combust. Law, "Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: a chemical-explosive mode analysis", J.
Lu, “Chemical explosive mode analysis for a turbulent lifted ethylene jet flame in highly heated coflow,” Combust.
