Measurement of Gas Temperature Around a Burning Droplet: Thin Filament and Soot Pyrometry

A common setup for the study of liquid fuel combustion involves hanging a droplet from one or several tiny fibers. This paper explores the capabilities and limitations of thin filament pyrometry (TFP) when applied to the fiber(s) holding the fuel droplet. A 2D, two-color pyrometer is used to obtain the temperature along the fiber with low uncertainty and good spatial resolution. The derivation of the gas temperature profile from that of the fiber, however, entails a number of assumptions regarding the flow pattern around the droplet in the radiation/convection balance and also estimating the heat conduction along the fiber. Both aspects and the associated uncertainties are discussed in detail in a test with a 750 μm eicosane droplet hanging from two 15 µm SiC fibers and burnt in air; the “corrections” needed to estimate the maximum flame temperature sum up to ~ 300 K for a final value around 2000 K. The pyrometer is then applied to the soot cloud above the eicosane droplet, an alternative approach whose uncertainty arises mainly from that regarding the emissivity of soot. A reasonable agreement with the corrected TFP results is found. Finally, TFP is applied in a worst-case scenario, that is the case of a glycerol droplet, with no noticeable soot and a short distance between the flame and the droplet surface.

液体燃料燃烧研究的经典实验装置，通常是将燃料液滴悬挂于一根或多根超细纤维之上。本文针对承载燃料液滴的纤维，探讨了细丝高温计（thin filament pyrometry, TFP）的适用范围与局限性。本研究采用二维双色高温计，以低不确定度与优异空间分辨率获取纤维沿线的温度分布。然而，由纤维温度分布反演气体温度分布时，需基于液滴周边辐射-对流平衡条件下的流场形态，以及纤维沿线热传导估算等多项假设。本文针对由两根15微米碳化硅纤维悬挂、于空气中燃烧的750微米二十烷液滴开展实验，对上述两方面及其相关不确定度展开详细讨论；为得到约2000开尔文的最终火焰最高温度，所需各项修正量的总和约为300开尔文。随后，将该高温计应用于二十烷液滴上方的炭烟云团，这一替代测量方法的不确定度主要源于炭烟发射率的估算误差，且该方法与修正后的细丝高温计测量结果吻合良好。最后，本文将细丝高温计应用于极端工况：即无明显炭烟生成、火焰与液滴表面间距极短的甘油液滴燃烧场景。