OH-PLIF investigation of wall effects on the flame quenching in a slit burner

https://doi.org/10.1016/j.proci.2012.07.038Get rights and content

Abstract

To examine the effects of the wall on flame quenching, an OH-PLIF investigation of a premixed methane/air flame was conducted with a slit burner between two parallel walls. Three types of materials, i.e., stainless steel 304, silicon, and zirconia ceramics, were tested at wall temperatures of 300 and 600 °C. The quenching process, captured by an intensified charge-coupled device (ICCD) camera, showed different critical distances for stable flames that became unstable and were quenched for the three materials at the same temperature. It is interesting to note that, at a higher wall temperature, the flame is lifted before finally quenching, while this did not happen at 300 °C. By analyzing the maximum OH fluorescence intensity in the flame extracted from the OH image, we have not found a distinct relation between the maximum OH fluorescence intensity and the quenching distance. In some cases, the flame can sustain a very weak OH intensity. Conversely, we also obtained the OH fluorescence intensity close to wall and within 0.6 mm from the surface. We found that the trend of the OH fluorescence intensity close to wall correlates with the quenching characteristics very well. At the same wall temperature, a greater OH intensity close to wall results in a shorter quenching distance and vice versa. The wall made of zirconia ceramics demonstrates the greatest OH intensity close to the wall and, thus, the shortest quenching distance, while of the quenching distance in the case of STS 304 demonstrates the opposite trend. Additionally, we also calculated the non-dimensional chemical action of the surface based on the OH-PLIF data, which can demonstrate the differences in the chemical quenching characteristics for the three types of materials.

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)

  • Y. Ju et al.

    Prog. Energy Combust. Sci.

    (2011)
  • A.C. Fernandez-Pello

    Proc. Combust. Inst.

    (2002)
  • K. Maruta

    Proc. Combust. Inst.

    (2011)
  • F.J. Weinberg et al.

    Proc. Combust. Inst.

    (2002)
  • P.D. Ronney

    Combust. Flame

    (2003)
  • L.Q. Jiang et al.

    Energy Convers. Manage.

    (2009)
  • K.T. Kim et al.

    Combust. Flame

    (2006)
  • T.M. Sloane et al.

    Combust. Flame

    (1982)
  • T.M. Sloane et al.

    Combust. Flame

    (1983)
  • M.B. Davis et al.

    Combust. Flame

    (2000)
  • Y. Fan et al.

    Proc. Combust. Inst.

    (2009)
  • Cited by (45)

    • Effect of CO-to-H<inf>2</inf> ratio on syngas flame behaviors in a micro flow reactor with controlled wall temperature gradient

      2023, Fuel
      Citation Excerpt :

      However, micro-/meso-scale combustion faces severe challenges which differ from the combustion under macro-scale [4,5]. Specifically, with the decrease of combustor scale, the surface-to-volume ratio of combustion chamber augments drastically, which makes flame stability imperiled by thermal and radical quenching effects [6–8]. Therefore, many technologies have been developed to improve flame stability under small scales [9], represented by bluff body [10,11], wall cavity [12–14], backward facing step [15,16], porous media [17–20], and other thermal management methods [21–23].

    • One zirconia-based ceramic coating strategy of combustion stabilization for fuel-rich flames in a small-scale burner

      2022, Fuel
      Citation Excerpt :

      The channel widths were arbitrarily adjustable in tests. This combustor structure was similar to the used apparatus by Kim et al. [31] and Yang et al. [42]. The nozzle was made of cast iron material and its location was three-dimensional adjustable.

    View all citing articles on Scopus
    View full text