Phase changes in burning precursor-laden single droplets leading to puffing and micro-explosion

IF 2.3 3区 工程技术 Q2 ENGINEERING, MECHANICAL Experiments in Fluids Pub Date : 2024-11-07 DOI:10.1007/s00348-024-03895-w
Benjamin A. Südholt, Arne Witte, Greg J. Smallwood, Sebastian A. Kaiser, Lutz Mädler, Niklas Jüngst
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Abstract

When producing metal-oxide nanoparticles via flame spray pyrolysis, precursor-laden droplets are ignited and undergo thermally induced disintegration, called ‘puffing’ and ‘micro-explosion’. In a manner that is not fully understood, these processes are associated with the formation of dispersed phases inside the droplets. This work aims at visualizing the interior of precursor-laden burning single droplets via diffuse back illumination and microscopic high-speed imaging. Solutions containing iron(III) nitrate nonahydrate (INN) and tin(II) 2-ethylhexanoate (Sn-EH) were dispersed into single droplets of sub-100 μm diameter that were ignited by passing through a heated coil. At low precursor concentration, 50% of the INN-laden droplets indicate a gas bubble of about 5 μm diameter in the center of the droplet. The bubble persists for several hundred microseconds at a similar size. In almost all of these cases, the bubble expands at some point and the droplet ends up in a micro-explosion. In some of these instances, the droplet’s surface shows spatial brightness modulations, i.e., surface undulations, indicating the formation of a viscous shell. With increasing INN concentration, the fraction of droplets showing surface undulations, gas bubbles, and micro-explosions drastically decreases. This may be associated with a more rigid viscous shell and reduced mobility of bubbles. Bright incandescent streaks originating from the disrupting INN-laden droplets, may indicate sub-micrometer droplets or particles from within the droplets or formed in the gas phase. In contrast, Sn-EH-laden droplets show very fast disruptions, typically less than 10 μs from first visible deformation to ejection of secondary droplets. Bubbles and surface undulations were not observed.

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燃烧前体单液滴中的相变导致膨化和微爆炸
通过火焰喷射热解法生产金属氧化物纳米粒子时,充满前驱体的液滴会被点燃并发生热诱导分解,即所谓的 "膨化 "和 "微爆"。这些过程与液滴内部分散相的形成有关,但人们对其方式尚不完全了解。这项研究旨在通过漫反射照明和显微高速成像技术,对含有前驱体的燃烧单液滴内部进行可视化观察。将含有一水硝酸铁(III)(INN)和 2-乙基己酸锡(II)(Sn-EH)的溶液分散成直径小于 100 μm 的单液滴,通过加热线圈点燃。在前驱体浓度较低时,50% 含有 INN 的液滴会在液滴中心出现直径约为 5 μm 的气泡。类似大小的气泡会持续几百微秒。几乎在所有这些情况下,气泡都会在某个点膨胀,液滴最终会发生微爆炸。在其中一些情况下,液滴表面会出现空间亮度调节,即表面起伏,表明形成了粘性外壳。随着 INN 浓度的增加,出现表面起伏、气泡和微爆炸的液滴比例急剧下降。这可能与粘性外壳更加坚硬和气泡流动性降低有关。从含有干扰性 INN 的液滴中产生的明亮炽热条纹,可能表示来自液滴内部或在气相中形成的亚微米液滴或颗粒。相比之下,含 Sn-EH 的液滴显示出非常快的破坏速度,从第一次可见变形到喷射出次级液滴的时间通常不到 10 μs。没有观察到气泡和表面起伏。
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来源期刊
Experiments in Fluids
Experiments in Fluids 工程技术-工程:机械
CiteScore
5.10
自引率
12.50%
发文量
157
审稿时长
3.8 months
期刊介绍: Experiments in Fluids examines the advancement, extension, and improvement of new techniques of flow measurement. The journal also publishes contributions that employ existing experimental techniques to gain an understanding of the underlying flow physics in the areas of turbulence, aerodynamics, hydrodynamics, convective heat transfer, combustion, turbomachinery, multi-phase flows, and chemical, biological and geological flows. In addition, readers will find papers that report on investigations combining experimental and analytical/numerical approaches.
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