Experimental study on flame stability and thermal performance of an n-heptane-fueled microscale combustor

In this work, we study the stabilization behavior of micro-diffusion flame of n-heptane formed in a combustor with the inside diameter of 4 mm, in order to elucidate the unique stability mechanism due to miniaturization of diffusion flame downstream porous medium. Effects of incoming Reynolds number and fuel flow rate on overall flame shape, exhaust temperature, and wall temperature are examined experimentally. Furthermore, an energy balance model of the micro combustor is established and optimal working conditions are proposed. Liquid n-heptane is used as fuel and two types of outer tubes are employed in order to examine the role of the heat recirculation. It turns out that the outer tube increases the wall temperature and broadens the flame stability limits. The incoming Reynolds number changes flame position and energy balance in the micro combustors. At low Reynolds number, the outer tube allows the flame to stay close to the porous medium and, accordingly, the porous medium is substantially heated up. Then, the fuel flowing through the porous medium “receives” the heat from the burner (heated by flame) effectively to enhance the reactivity, resulting in improving the stability. At high Reynolds number, the outer tube allows the flame to stay close to the bottom wall of the outer tube and, hence, more radiative heat is transferred through the outer tube, ideal for micro-photovoltaic systems. Additionally, in the case of fixed equivalence ratio, with increasing of the fuel flow rate, combustion releases more heat and the flame is blown toward the bottom of the outer tube. More energy is transferred to the surroundings via the outer tube wall and the maximum value is up to 72.7% of the total combustion heat release.