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.

本研究以数值方式研究了滴落对在二维域内受到局部辐射加热的垂直取向热塑性材料的可燃性的影响。热塑性材料被建模为具有规定凝固/熔化温度的相变材料，其气化（热解）过程的速率由阿伦尼乌斯定律描述。由于重力，熔融物质可以向下移动，因此，气化（热解）表面积随时间变化。使用 FLUENT 结合作者开发的适当用户定义函数 (UDF) 解决了与时间相关的热量和质量传输过程，包括全局一步热解反应。为了简化问题，没有考虑气相动力学，取而代之的是，（预期的）可燃性是根据从界面演化出来的燃料质量通量来估计的。熔融物质的粘度作为数值参数变化，以改变熔融物质随时间变化的变形程度，并讨论了其滴落对可燃性的影响。数值结果清楚地表明，当施加较低的粘度时，滴落会迅速发生，尽管其质心轨迹产生类似的趋势，而与粘度无关（尽管其移动速度有很大差异）。随着滴落的明显，受热区熔融物的厚度变薄，使其升温迅速，此时立即气化，导致点火延迟时间变短。有趣的是，由于在极低粘度的情况下进一步促进滴落，熔融物质在发生大量气化之前迅速流出，导致点火机会被抑制。在这方面，滴落对可燃性表现出两种相互竞争的影响，这意味着存在具有最短延迟时间的最佳点火条件。还讨论了在传统数值模型（不开发滴水模型）中模拟这种滴水效应的简单策略。