A Numerical Study of Dripping on the Ignitability of a Vertically Oriented Thermoplastic Material Locally Heated by an Irradiation Source
A Numerical Study of Dripping on the Ignitability of a Vertically Oriented Thermoplastic Material Locally Heated by an Irradiation Source
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DOI：
10.1007/s10694-021-01137-7
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发表时间：
2021-05
期刊：
影响因子：
3.4
通讯作者：
Shashank Singh;Yuji Nakamura
中科院分区：
文献类型：
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作者：
Shashank Singh;Yuji Nakamura
本研究数值研究的效果上的垂直取向的热塑性材料进行局部辐射加热内的二维域的可燃性滴。热塑性材料被建模为具有规定的凝固/熔化温度的相变材料，并且其气化（热解）过程的速率由阿克里尼乌斯定律描述。熔融物质可以由于重力而向下移动，因此，气化（热解）表面积随时间而变化。利用FLUENT软件，结合作者开发的自定义函数（UDF），求解了包括全局一步热解反应在内的含时热质输运过程。为了简化问题，气相动力学不被考虑，相反，（预期）可燃性估计的基础上，从接口演变的燃料质量通量。熔融物质的粘度作为一个数值参数变化，以修改的程度随时间变化的变形的熔融物质，其滴落的可燃性的影响进行了讨论。数值结果清楚地表明，当施加较低的粘度时，滴液发生得很快，尽管其质心的轨迹产生类似的趋势，而与粘度无关（尽管其移动速度相差很大）。由于滴落明显，加热区中熔融物质的厚度变得更薄，导致其快速加热，此时立即发生气化，导致点火延迟时间变得更短。有趣的是，由于在非常低粘度的情况下进一步促进滴落，熔融物质在发生实质性气化之前迅速流出，导致抑制点火的机会。在这方面，滴落表现出两个竞争的影响，对可燃性，这意味着有最佳条件的点火与最短的延迟时间。一个简单的策略来模仿这样的滴水效果在一个传统的数值模型（而不开发一个滴水模型）进行了讨论。
This study numerically investigates the effect of dripping on the ignitability of a vertically-oriented thermoplastic material subjected to localized radiant heating within a 2D domain. Thermoplastic material is modeled as a phase-change material with the prescribed solidification/melting temperature, and its rate of gasification (pyrolysis) process is described by the Arrhenius law. Molten matter can move downward due to the gravitational force, and accordingly, the gasification (pyrolyzed) surface area vary over time. Time-dependent heat and mass transport processes, including global one-step pyrolysis reaction are solved using FLUENT combined with appropriate user-definition functions (UDFs) developed by the authors. In order to simplify the problem, gas-phase kinetics were not considered, instead, the (expected) ignitability was estimated on the basis of the fuel mass flux evolved from the interface. The viscosity of the molten matter was varied as a numerical parameter in order to modify the degree of time-dependent deformation of the molten matter, and the influence of its dripping on ignitability is discussed. The numerical results clearly indicate that dripping occurs quickly when lower viscosity is imposed, although the trajectory of its mass-center yields a similar trend, irrespective of the viscosity (its moving speed greatly differs, though). As the dripping was pronounced, the thickness of the molten matter in the heated zone became thinner causing it to heat up quickly, at such point immediate gasification occurred, resulting in the ignition delay time becoming shorter. Interestingly, as the dripping was further promoted in a very-low viscosity case, the molten matter quickly flows-off prior to substantial gasification is occurring, resulting in the chance of ignition being inhibited. In this respect, dripping exhibited two competing effects on ignitability, implying that there were optimal conditions for the ignition with the shortest delay time. A simple strategy to mimic such a dripping effect in a conventional numerical model (without developing a dripping model) is also discussed.