Laminar burning velocity measurements of refrigerants under terrestrial and microgravity conditions

下一代制冷剂具有较低的全球变暖潜力，用于制冷和供暖应用，由于其高度易燃，导致火灾危险增加。物质火灾危险的分类不仅包括可燃性，而且还包括对其层流燃烧速度的评估，这是一个额外表示反应性和放热性的量。确定层流燃烧速度的传统方法测量混合物的火焰传播速度，例如，通过评估球形火焰。由于大多数制冷剂火焰的传播速度很慢，两种物理现象的影响显著增加：(1)浮力引起的火焰变形，(2)辐射热损失，使实验方法所需的基本假设无效。因此，标准的数据提取假设，如球形火焰形状，失败和关于这些制冷剂火焰速度的准确文献数据很少。为了对制冷剂的燃烧行为有一个基本的了解，并提供准确的火焰速度数据，必须使用高保真实验详细研究火焰的结构、传播和周围气体的动力学。在这个项目中，将开发稳健的方法来可靠地表征制冷剂的燃烧行为。首先，利用粒子图像测速仪(PIV)研究了慢传播制冷剂的浮力变形效应。火焰锋面的局部曲率和应变效应是决定燃烧速度的关键因素。微重力实验在不来梅大学应用空间技术和微重力中心(ZARM)的落塔设施中进行，将提供无浮力火焰传播数据，隔离辐射热损失效应。这些热损失将通过使用高速瑞利散射获得的空间和时间分辨的火焰温度场来量化。这些发现将有助于开发和修改现有的辐射修正模型，这些模型是为碳氢化合物开发的，但尚未对制冷剂火焰进行验证。最后，基于多步反应方案的渐近方法进行火焰结构分析，以揭示制冷剂/氧化剂混合物的着火、熄灭和传播所涉及的基本物理化学过程。将研究现有渐近方法的适用性，并将应用专门针对制冷剂火焰的修改。这将有助于开发一种准确和健壮的简化建模方法，并使用近似公式来计算制冷剂火焰的燃烧速度。

Next-generation refrigerants with lower global warming potential for cooling and heating applications pose increased fire hazards due to their high flammability. Classification of a substance’s fire hazard potential includes not only flammability but also the evaluation of its laminar burning velocity, a quantity additionally representing reactivity and exothermicity. Conventional methods for determining laminar burning velocities measure a mixture's flame propagation speed, for instance, by assessing spherical flames. Due to the slow propagation speed of most refrigerant flames, the impact of two physical phenomena significantly increases: (1) The buoyancy-induced deformation of the flames, and (2) the radiation heat losses, invalidating the underlying assumptions required for experimental methods. Hence, standard data extraction assumptions, such as spherical flame shape, fail and accurate literature data on flame velocities for these refrigerants are rare. To obtain a fundamental understanding of the refrigerant’s combustion behavior and to provide accurate flame velocity data, the flame structure, propagation, and surrounding gas dynamics must be studied in detail using high-fidelity experiments. In this project, robust methods will be developed to reliably characterize the combustion behavior of refrigerants. First, the buoyancy deformation effects of slow-propagating refrigerants will be investigated using Particle Image Velocimetry (PIV). The flame front's local curvature and strain effects are key factors in determining burning velocities. Microgravity experiments, conducted in the drop tower facility of the Center of Applied Space Technology and Microgravity (ZARM) of the University of Bremen, will provide buoyancy-free flame propagation data, isolating the radiation heat loss effect. These heat losses will be quantified by the spatially and temporally resolved flame temperature fields obtained using the high-speed Rayleigh scattering. The findings will help develop and modify existing radiation correction models, that were developed for hydrocarbons but have not yet been validated for refrigerant flames. Finally, a flame structure analysis based on an asymptotic approach with a multi-step reaction scheme will be performed to reveal the underlying physicochemical processes involved in the ignition, extinction, and propagation of refrigerant/oxidant mixtures. The applicability of the existing asymptotic approaches will be studied, and modifications tailored explicitly for refrigerant flames will be applied. This will contribute to developing an accurate and robust simplified modeling approach with approximation formulas for the burning velocities of refrigerant flames.