Convection-Diffusion Controlled Laminar Micro Flames

Abstract

Small laminar diffusion flames (flame height ≃2–3 mm) established by a fuel jet issuing into a quiescent medium are investigated. It was found that for these flames buoyancy effects disappeared as the flame size decreased (Fr≫1), and diffusive transport of the fuel was comparable to the convective transport of the fuel. The effect of buoyancy on these flames was studied by examining the flame shape for horizontally oriented burners. A phenomenological model was developed (based on experimentally determined flame shapes) to compare diffusion and convection transport effects. Finally, the flame shapes were theoretically determined by solving the conservation equations using similarity methods. It was seen that when the axial diffusion (in momentum and species equations) terms are included in the conservation equations, the calculated flame shape is in better agreement (as compared to without the axial diffusion term) with the experimentally measured flame shape.

Experimental study on the unique stability mechanism via miniaturization of jet diffusion flames (microflame) by utilizing preheated air system

“…Interestingly, all existing combustors/burners utilizing the heat recirculation for further stabilization have been only considered in the premixed flame, although the same strategy could also be applied to the diffusion (i.e., non-premixed) flame. More specifically, the similar stability mechanism would exist also in small-scale diffusion flames, so called microflame [12][13], in which the flame-burner (wall) interaction becomes prominent. If so, this additional stabilized mechanism has potential to cover up unknowns in miniaturizing flames as pointed in [2].…”

“…< 1.0 mm) [12][13]. So far, a number of studies have been conducted in order to elucidate the stability of the microflame.…”

Progress in small-scale combustion

“…One of the earliest micro-or small-scale non-premixed (diffusion) flame (so called microflame) studies was conducted by Ban et al (Ban, et al, 1994). Flame structures of ethane, ethylene, and acetylene were investigated using circular-port stainless-steel burners with inner diameters of 0.15, 0.25 and 0.40 mm.…”

“…Numerous researches have been performed thus far, accordingly, a sufficient number of review articles were already available (e.g., Fernandez-Pello;2002;Ju and Maruta, 2011;Maruta, 2011;Shirsat and Gupta, 2011;Kaisare and Vlachos, 2012). Key articles were published in the early 1990s from a group of U. Kentucky (e.g., Ban, et al, 1994), however, little attention was paid at that time because it was a purely fundamental study on the scale effect for a conventional laminar jet diffusion flame. Later, the small-scale effect on combustion stability suddenly became the center of interest in late 90s, at which time Defense Advanced Research Projects Agency (DARPA) started to support the development of micro-power devices as a promising power source for microelectromechanical system (MEMS).…”

Development of "Tiny" Droplet Flame Simulator: "Super-Stabilized" Micro-Jet Flame in a Hot Air

“…Microflame is known as a tiny diffusion flame formed over a small needle jet [14], holding Fr >> 1 and Re ~ O(1). Fig.…”