THE MEASUREMENT OF SURFACE TEMPERATURE AND VAPOR CONCENTRATION PROFILES FOR A SPREADING FLAME OVER LIQUID FUELS

摘要

Long controversial discussions on mechanism of pulsating flame spread over liquid fuels has been continued since 1970's. This paper might put an end to this problem. Mainly, two hypothesis has been proposed on mechanism of pulsating flame spread, one is "the formation and destruction of a recirculation cell in gas phase", the other is "the formation of flammable vapor layer due to hot surface tension flow". Recently, experiments in micro-gravity revealed that in spite of a recirculation cell in gas phase which existed in front of flame edge, no pulsation occurs. Instead, slow pseudo-uniform spread has been observed in n-butanol deep pool. This fact arise the new problems, which are the role of liquid phase buoyancy and three-dimensional effect of both gas expansion and surface tension flow. In our previous studies, we measured the detailed velocity and fuel concentration profiles in a n-propanol pool by using particle track velocimetry and dual wavelength holographic interferometry, respectively. Then, we first succeed to measure a series of detailed velocity and vapor concentration profiles in pulsating regime, which consists of five stages, (a) onset of pulsating, (b) development of surface tension flow, (c) formation of recirculation flow, (d) jumping, and (e) end of pulsating. This paper focused on obtaining detailed measurements of fuel surface temperature profiles by using infrared camera in each pulsating stage. We used both n-propanol and n-butanol for fuels in 2.5 cm-deep Pyrex pool. Then, we found that low temperature liquid; so-called "temperature valley", exists in front of pulsating flame for both fuels as shown in Fig. A-1. This cold area clearly obstruct crawling flame from shifting to jumping stage, because the surface temperature in this area is below the flash point. The jumping stage occurs after the temperature valley disappears and flammable vapor layer is generated due to heating by out-coming flame. The emergence of temperature valley is derived by the interaction of surface tension flow and buoyancy. This suggests that as there is no buoyancy effect in micro-gravity condition, the flow field in liquid phase and subsequent surface temperature might be different with normal-gravity condition. The results of dual wavelength holographic interferometry confirmed that low concentration region exists at several millimeter ahead of a flame leading edge. This hypothesis can also explain phenomenology of pseudo-uniform region. We observed that very large temperature valley, which is larger than that of pulsating flame, always proceeds to spreading flame in both n-propanol and n-butanol pools.