OH and CH₂O Laser-Induced Fluorescence Measurements for Hydrogen Flames and Methane, n-Butane, and Dimethyl Ether Weak Flames in a Micro Flow Reactor with a Controlled Temperature Profile

Laser-induced fluorescence (LIF) measurements were applied for the first time to a micro flow reactor with a controlled temperature profile to investigate general combustion and ignition characteristics of hydrogen (H₂/O₂/N₂ mixture at O₂/N₂ = 1:9), methane, n-butane, and dimethyl ether (DME) (fuel/air mixtures). For the hydrogen case, overall flame responses of the H₂/O₂/N₂ mixture against inlet flow velocity were investigated on the basis of the OH-LIF measurement. The existence of the three kinds of flame responses, such as normal flames in the high inlet flow velocity, flames with repetitive extinction and ignition (FREI) in the intermediate inlet flow velocity, and weak flames in the low inlet flow velocity were confirmed at φ = 0.6, 1.0, and 1.2. Experimental identification of the hydrogen weak flame was established for the first time. However, the OH-LIF signal level from fuel-rich hydrogen weak flames are quite low, and that at φ = 3.0 could not be detected. The reason for the low-level OH-LIF signal from the fuel-rich hydrogen weak flame was examined computationally, and it was found to be due to the drastic reduction of the maximum OH mole fraction in hydrogen weak flames at a fuel-rich condition. For methane, n-butane, and DME weak flames, CH₂O- and OH-LIF measurements in addition to the simple observation of the chemiluminescence with a CH filter were conducted to investigate their weak flame structures. Experimental results were compared to computations with several detailed chemical kinetics. Results showed that experimental methane weak flame structure was qualitatively reproduced by GRI-Mech 3.0 and NUIG CH₄/DME 2014 Mech, but the position of the main reaction zone of the methane weak flame was not well predicted by these two mechanisms. For the n-butane weak flame case, computations by Natural Gas III implied significant formation of formaldehyde in the n-butane weak flame, while computations by Aramco Mech 1.3 showed no significant heat release in the low-temperature region at atmospheric pressure. Experiments showed that the CH₂O-LIF signal of the n-butane weak flame was distributed from 900 to 1200 K, which supports the computational results with Aramco Mech 1.3. For the DME weak flame case, clear CH₂O-LIF signals distributed from 500 to 1150 K from the cool weak flame and broad dual-zoned chemiluminescence from blue and hot weak flames were observed. Computations for the DME weak flame with DME 2000 and NUIG CH₄/DME 2014 Mech reproduced the significant formation of formaldehyde. Detection limits of the chemiluminescence and LIF signals were examined and summarized by comparing the experimental and computational results of the present micro flow reactor. Results showed that OH in weak flames on the order of magnitude larger than 10^-10 mol/cm³ would be observable by the present OH-LIF measurements.