Cup-burner에서 불활성가스의 유속 및 농도변화에 따른 소화농도와 화염불안정성에 관한 연구

Abstract

In this study, the effects of the coflow velocity and the concentration on the flame extinction and stability were investigated experimentally in the cup burner flames. Heptane used as fuel and four inert gases (N2, CO2, Ar, He) were added into the air in oxidizer stream, of which velocity was varied from 2 cm/s to 16 cm/s. The concentrations of nitrogen, argon, and carbon dioxide at blow out are increased with increase of oxidizer velocity but that of helium is almost constant regardless of oxidizer velocity. This result might be attributed to the fact that the velocity effect is not noticeable relatively because of the higher thermal conductivity and diffusivity of helium. Also, the higher coflow velocity induces the lower burning rates of heptane. The flickering frequency decreases with increasing the coflow velocity. The higher oxidizer velocity induces the lower fuel velocity because aerodynamic effect results in the reduction of heat transfer near flame base. The frequency-buoyancy relation with non-dimensional variables coincides with that of buoyant flume and pool fires under ambient air when the characteristic velocity is defined to the difference between fuel and oxidizer velocities. This supports the fact that the origin of the gravitational instability is Kelvin-Helmholtz instability in shear layer. Also, the flickering frequency has an irregular pattern as nitrogen is added on the oxidizer stream. The irregularity of the flickering according to the concentration of nitrogen might be due to change of flame structure, which is not only influenced on the buoyancy forces by the density field change based on the heat release rate but also coupled with a different stability mode such as radial directional instability. Under near extinction condition, two kinds of instability are observed at anchoring base of the flame as well as at downstream. The occurrence of flicking motion according to the velocity can be understood by introducing the Richardson number for buoyant force and Kelvin-Helmholtz instability in shear layer. The instability observed at flame base can be classified into four modes, i.e. cell, radial oscillation, swing and rotating. However, it is difficult to find a governing parameter.