Wall Thermal/Chemical Effects on Small-scale Flames

Abstract

Since energy density of hydrocarbon fuels is tens of times larger than that of lithium-ion batteries, micro-scale combustion attracts much attention to develop mobile high-added-value energy source with prolonged operation time. In micro-scale combustion, however, surface to volume ratio becomes large, so that gas-phase combustion is difficult to sustain due to both wall thermal and chemical effects. In this paper, recent progress on our study of the wall thermal/chemical quenching effects is presented. First, oscillating flames induced by the thermal effect in micro quartz plate channels are introduced. Second, effects of wall material on the chemical quenching behavior in narrow quartz plate channels are reported. Quartz, alumina and platinum are chosen as the wall surface materials. Alumina and platinum thin films ∼100 nm in thickness are deposited on quartz substrates using atomic layer deposition or vacuum arc plasma gun techniques to establish equivalent thermal boundary condition with different wall chemical reactions. Through the OH-PLIF measurement, it is shown that the chemical effect starts to take over the thermal effect at high wall temperature at around 800 °C, resulting in the highest OH^* concentration in the vicinity of the alumina surface. By using a radical quenching model, the initial sticking coefficient associated with radical adsorption is estimated to be 0 and 0.01 for the alumina and the quartz surfaces, respectively. Thus, radical quenching does exist on the quartz surface, while the alumina works as an inert surface.