OH-PLIF investigation of wall effects on the flame quenching in a slit burner
Introduction
To apply MEMS technology, the need to develop a stable and high-efficiency hydrocarbon-fueled micro-combustor is an urgent task for researchers. This kind of micro-combustor could theoretically provide high-density energy, approximately 100 times greater than that obtained from state-of-the-art lithium-ion batteries [1], [2], [3], [4]. However, gas-phase combustion is usually unstable and is difficult to sustain in micro-combustors, which are usually on the order of a few millimeters [5], [6]. Thermal and chemical quenching, which are caused by an increased heat loss and an elevated radical recombination on the wall and are associated with an increase in the surface-to-volume ratio, are two dominant mechanisms attributed to flame quenching. Thermal quenching can be suppressed by reducing the heat loss through thermal management in the form of Swiss-roll-type, excess enthalpy, and recirculation combustors [7], [8], [9], [10]. However, chemical quenching is influenced, not only by the surface temperature, but by surface properties and treatment methods for defect removal as well [2], [11]. Moreover, heterogeneous radical recombinations on the wall can significantly affect the flame stability both thermally and chemically [12]. Surface processing, including annealing in oxygen, can decrease chemical quenching by eliminating the radical traps from the surface [11], [13].
To determine the relative roles of thermal and chemical quenching mechanisms in micro-scale flames, some studies have been conducted recently [11], [14], [15]. By measuring the quenching distance of a standard methane/oxygen torch between two parallel walls with different wall materials/treatments, Miesse et al. [11] concluded that quenching distances are only a function of the wall temperature over lower temperature ranges (∼200 °C), but the quenching distance becomes a function of the surface material at higher temperature ranges (∼1000 °C). By investigating a two-dimensional slit burner flame, Kim et al. [14] identified three different wall temperature ranges from 100 to 800 °C in which different mechanisms controlled the flame quenching. Specifically, the flame quenching becomes dependent on the radical removal at the surface for wall temperatures between 400 and 600 °C. Yang et al. [15] analyzed the quenching mechanism by measuring the surface properties and found that the ability of adsorbing hydroxyl groups on the surface will play an important role in the effect of chemical quenching. However, there is lack of detailed data regarding radicals close to wall to validate those assumptions because the behavior of radicals is the key to understanding chemical quenching. Some investigations of radical behavior near the surface have been carried out using new techniques. By measuring the molecular fractions of CH4, CO, H2, CO2, OH, and H for catalytic (platinum) and non-catalytic (gold) wall surfaces using a molecular beam mass spectrometer, Sloane et al. [16], [17] demonstrated that the radical recombination at the surface affects the flame quenching within 1 mm of the surface. With the help of planar laser-induced fluorescence (PLIF), Davis et al. [18] measured the OH concentration near the wall and found that the absolute OH concentration decreases with an increase in the catalytic activity. To further study this effect, Prakash et al. [13] measured the OH concentrations in low-pressure flames at 0.76 mm from the substrate walls at wall temperatures of ∼700 to 1100 K using laser-induced fluorescence. These researchers showed the importance of surface chemistry and suggested that surface reactions affect the OH profiles in the gas phase near the substrate wall. Although we have not measured the OH concentrations near the walls in our previous work [15], the same surface reaction mechanisms may occur on the surfaces of oxides, i.e., zirconia, silica, and ferric oxide at greater wall temperatures. Additionally, Fan et al. [19] examined premixed CH4/air combustion in quartz channels with heights of 0.7, 1.0, and 1.5 mm by measuring CH∗/OH∗ chemiluminescence using a high-speed intensified charged-coupled device camera. Although they focused on the investigation of flame instability in narrow channels, they found that the thermal quenching mechanism still plays an important role even at high wall temperatures of up to 800 °C, which contradicts previously reported results [11], [14]. However, these experiments are conducted on a large scale and cannot capture the OH fluorescence intensity close to wall, which might be very helpful for the simulation of the flame–wall interaction and the quenching process in a detailed kinetic scheme.
In this paper, to further understand the effect of the wall on OH radical and flame quenching, we measure the OH fluorescent intensity of a slit burner flame between parallel walls using a planar laser-induced fluorescence technique (PLIF) and investigate the effects of the material type and the wall temperature on the OH intensity. The maximum OH fluorescent intensity of the flame and the OH intensity close to wall are calculated and compared with the quenching characteristics for three types of wall materials, namely stainless steel 304 (STS 304), silicon, and zirconia ceramics. Finally, the chemical action of the surface is calculated and analyzed.
Section snippets
Slit burner
Figure 1(a) shows the experimental schematic used in the present study. A methodology similar to that used in the flame quenching study by Kim et al. [14] was employed, and the detailed experimental setup can be found in our previous work [15]. Briefly, the nozzle of a slit burner consists of 10, 1 × 1 mm2 square cells in one row through which a premixed stoichiometric methane/air mixture that was calibrated using mass flow controllers was supplied at a fixed velocity of 1 m/s (Re = 70). The flame
Results and discussion
The OH flame quenching images, which are processed by decreasing the gap between the two walls, were captured using ICCD, and some instability was observed. Figure 4 shows the quenching process for three types of materials at T = 300 °C. As observed in Fig. 4, the flame is initially in a stable mode and is anchored on the burner without pulsation or fluctuation. With a decrease in the space between the walls, the flame gradually becomes unstable and the anchoring points pulsate at the two sides of
Conclusions
We have measured the OH fluorescence intensity of a slit burner flame between two parallel walls constructed from three types of materials at different wall temperatures. Due to the different properties of the materials, the results demonstrate different critical distances for a flame changing from a stable mode to an unstable mode during the quenching process. By comparing the maximum OH fluorescence intensity of the flame with the quenching distance, we found that the maximum value can only
Acknowledgements
The current work was supported by the National Science Foundation of China (No. 51006109), by the NSFC-JST Major International (Regional) Joint Research Project (No. 50721140651), and by SRF for ROCS, SEM.
References (22)
- et al.
Prog. Energy Combust. Sci.
(2011) Proc. Combust. Inst.
(2002)Proc. Combust. Inst.
(2011)- et al.
Proc. Combust. Inst.
(2002) Combust. Flame
(2003)- et al.
Energy Convers. Manage.
(2009) - et al.
Combust. Flame
(2006) - et al.
Combust. Flame
(1982) - et al.
Combust. Flame
(1983) - et al.
Combust. Flame
(2000)
Proc. Combust. Inst.
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