Extinction of laminar jet diffusion microflames

2013

Combustion and Flame

In this work, we study the near-extinction behavior of micro-jet diffusion (i.e. non-premixed) flame, so called microflame, formed in a preheated air (up to 1020 K) in order to elucidate the unique and promising stability mechanism due to miniaturization of the jet diffusion flame. Effects of fuel flow rate and preheated air temperature on overall flame shape, flame temperature and the burner tip temperature are examined experimentally. Furthermore, the slight premixing effect on the near-extinction character is also investigated in order to support the stability mechanism suggested by this study. Methane is used as fuel and the several kinds of burner material are employed in order to examine the role of the burner. It turns out that the increasing the preheated air temperature decreases the limiting minimum flow rate effectively to simulate well the ideal condition of miniaturization of jet flame. This allows the flame to stay close to the burner and suppress the heat loss to the ambient, accordingly, the burner tip is substantially heated up. Then, the fuel flowing through the burner "receives" the heat from the burner (heated by flame) effectively to enhance the reactivity, resulting in improving the stability. It is also suggested that the endothermic radical-chain reactions are promoted 2 near the exit of the burner when the burner temperature is substantially heated, at which the back-diffused oxygen is penetrated. Our experimental observations convince the existence of the unique and promising stability mechanism apparently found in the miniaturization of the jet diffusion flame, where the flame and burner scale are almost identical and their thermal interaction becomes prominent.

Section: Fundamental Characteristics Of Microflames In Preheated Airsupporting

confidence: 69%

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

Fujiwara

2013

Combustion and Flame

“…(4) obtained by theory or not. If so, it means that the actual flame behavior observed here is predictable by aforementioned theory [20]. First, let us assume that the temperature dependency of kinematic viscosity (ν ) is expressed as an exponential function, i.e., exp(E T /T air ) with an appropriate fitting constant of E T .…”

Section: Limiting Behavior Of Microflame Established In High-temperatmentioning

confidence: 97%

“…Kuwana et al [20] have proposed the theoretical description of lower-limit condition to sustain the microflame, as shown in Eq. (3), under the assumption of unity of Schmidt number (Sc = 1).…”

Section: Limiting Behavior Of Microflame Established In High-temperatmentioning

confidence: 99%

“…This difference is somewhat acceptable in terms of order comparison. Thus, although above-discussion includes ambiguity so that further concern would be necessary, it is possible to claim that the limiting theory developed by Kuwana et al [20] would be applicable to the case even for the microflame formed in high-temperature air. We will continuously work on this topic in future in order to refine this conclusion.…”

Section: Limiting Behavior Of Microflame Established In High-temperatmentioning

confidence: 99%

See 1 more Smart Citation

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

Fujiwara

Hirasawa

2011

JTST

An attempt to form a "tiny" spherical flame over the small jet burner (so called microflame) as a model of a tiny droplet flame was made experimentally without any assistance brought by large facilities which could eliminate/minimize buoyancy effect. Using ceramic burner and high-temperature air effectively suppresses the quenching distance and such "super-stabilized" micro-jet flame would be fairly close to 1-D (droplet) flame. The temperature of air (up to 770 K) and the fuel (methane) flow rate were varied as experimental parameters. Fundamental characteristics of limiting and near-extinction behavior of methane-air microflame in high-temperature air are investigated. Results show that the flame shape under high-temperature air condition is hardly affected by the external disturbance and the quenching distance is minimized due to the increase of the temperature at burner tip. It is found that the theory developed by Kuwana et al., to predict the limiting behavior of small-scale flame would be applicable even for the one formed in high-temperature air. Minimum flame size achieved in high-temperature air is predicted as an order of hundreds micron; interestingly, this is identical to what is predicted with ideal adiabatic burner (i.e., no conductive and radiative heat loss to/from the burner) in our previous numerical work.

“…The Damköhler number concept was used to explain the extinction criterion, suggesting that the limiting feature can be well-summarized by the diffusion flame mode. Later, Kuwana, et al (2009) developed a theory to predict the extinction of such a small diffusion flame based on the extinction of a Burke-Schumann flame. It was concluded that such a theory works to predict the extinction of methane and butane flames with reasonable accuracy.…”

Section: Non-premixed Combustion 331 Lower-limiting Flame Behaviormentioning

confidence: 99%

Progress in small-scale combustion

Gao

Matsuoka

2017

JTST

This article briefly reviews the recent works related to small-scale combustion and its potential impact into combustion science and engineering is presented. Followed by a simple description of the scale effect on combustion to highlight its "unique" feature, past related works are then summarized. The impact of heat recirculation appearing in combustion systems, which is the most prominent feature of a micro-or small-scale combustion system, is focused upon and is understood that exactly the same strategy promising better combustion performance is confirmed irrespective of flame type (either premixed or non-premixed). With respect to this, paying attention to the entire combustor design, to optimize within the target working range is a crucial matter when micro-scale combustion is adopted. Potential subjects to be covered to further promote these aspects in this field are then presented.