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

Abstract

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.

Progress in small-scale combustion

“…Interestingly, such excess heating at the tip is hardly observed in a normal methane-air microflame, however, it Nakamura, Gao and Matsuoka, Journal of Thermal Science and Technology, Vol.12, No.1 (2017) appears when ambient oxidizer is preheated (Fujiwara and Nakamura, 2013). This is because the quenching distance could be shorter under a preheated oxidizer environment so that a large amount of heat can be transferred.…”

“…Fig. 12 depicts a hydrogen-air non-premixed microflame (Cheng, et al, 2005) and a methane-air (preheated) non-premixed microflame (Fujiwara and Nakamura, 2013), showing that both flames clearly substantially preheat the burner. Because the flame is formed over the burner, a part of the combustion heat can be transferred to the burner as illustrated in Fig.…”

“…12 Microflame over "hot" jet-tip. The left hydrogen flame figure comes from the work by Cheng, et al (2005), and the right methane flame comes from Fujiwara and Nakamura (2013). Fig.…”

Effect of Burner Wall Material on Microjet Hydrogen Diffusion Flames near Extinction: A Numerical Study

“…Hence, flame quenches at the tube wall for lower fuel flow velocities. Many studies [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] were conducted to analyze the flame structure and characteristics of micro diffusion flames, which were recently reviewed by Maruta [26] and Nakamura et al [27].…”

Effects of electric field on micro-scale flame properties of biobutanol fuel

“…Since the energy density of typical hydrocarbon fuels is about 100 times higher than that of the most advanced batteries, hydrocarbon fuel-based microcombustors are regarded as the optimal substitution of traditional batteries 5 6 7 8 . Since the characteristics of diffusion flames is the critical concern in the development of microscale combustion systems 9 , many studies have investigated the characteristics of gaseous fueled flames 10 11 12 13 . However, very few studies focus on analyzing micro diffusion flames of liquid fuels 14 15 16 .…”