高速横流作用下液氮喷雾液滴碰撞热动力特性研究

Liquid nitrogen spray cooling is a crucial technique for acquiring accurate aircraft testing results in wind tunnels via elevating the flying Reynolds number by decreasing the gas temperature. Under the influence of a high-speed gas flow, liquid nitrogen droplet collisions are highly possible to occur, and the resultant droplet coalescence, separation or splashing, and the droplet new trajectories greatly affect the overall nitrogen vaporization efficiency and the possible liquid impacts on the apparatus at downstream. To understand and resolve the problem, we plan to investigate the thermodynamics of liquid nitrogen droplet collisions in high-speed gas flows. Through examining the effects of high-speed gas flow, droplet properties and collision characteristics on the dual-droplet post-collision results, this study aims to provide a comprehensive understanding on the liquid nitrogen droplet collisions with and without considering nitrogen vaporization, and based on which to develop a monogram for depicting collision regimes by relevant dimensionless numbers. The effect of fast nitrogen vaporization on dual-droplet collisions will be probed and highlighted for revealing the thermodynamics of the unsteady liquid-vapor droplet interface in the high-speed gas flow. Numerical models will be established for dual nitrogen droplet collisions with or without liquid-to-vapor phase change to analyze the coupled fluid flow and heat and mass transfer throughout the collisions. On the basis of the aforementioned thermodynamics of dual nitrogen droplet collisions at a mesoscopic view, we will further study the development of nitrogen droplet population in dual/multi-stream spray cooling process to enhance the utilization of liquid nitrogen and refrigeration efficiency, and to ensure the safety operation of cryogenic wind tunnels.

液氮喷雾冷却是通过降低风洞内气流温度提高飞行雷诺数以确保飞行器模拟实验准确性的关键技术。受风洞内高速横流的影响，喷雾流中氮液滴撞击发生的聚合、破碎等行为以及新液滴的运动轨迹决定了液氮的整体汽化效率和低温风洞的安全运行。针对这一问题，拟开展高速横流作用下氮液滴碰撞热动力特性研究，建立对单相及汽化氮液滴碰撞现象的全面认知，考察高速横流、液滴参数和碰撞参数对单相及汽化双氮液滴碰撞结果的影响，利用准则数建立低温环境氮液滴碰撞结果划分准则。探索高汽化速率对液滴碰撞的作用，揭示高速横流中气液非稳态界面动力及相变特性。建立高速横流中单相/汽化双氮液滴碰撞数学模型，分析碰撞过程中流动及传热传质耦合机制。以高速横流中介观氮液滴碰撞特性为基础，探索双/多股液氮喷雾流液滴群发展规律，建立离散相液滴群与连续相高速气流耦合模型，为快速降低气流温度、提高工质利用率、保障低温风洞安全高效运行提供理论指导。

氮液滴碰撞动力学行为是液氮喷雾冷却过程的核心现象，对低温风洞的安全运行至关重要。本项目针对液滴碰撞的动力学行为进行了深入研究。首先，通过设计和搭建液滴碰撞实验平台，获得了不同的液滴碰撞模式，揭示了液滴表面张力和粘度对碰撞的影响。通过对液滴飞溅行为发生的临界状态进行能量分析，建立了可预测飞溅发生的数学模型，提出了飞溅行为的临界韦伯数的实验关联式。通过构建氮液滴碰撞数值模型，获得了低温环境下氮液滴的多种碰撞模式，并阐释了各碰撞模式的形成机理和特征。通过分析液滴碰撞过程中的能量分配规律，揭示了粘性耗散和表面能对液滴碰撞结果的影响。通过构建横流中液滴碰撞数值模型，研究了高速横流作用下氮液滴碰撞后产生的形变以及破碎等行为，获得了不同横流参数下液滴碰撞产生的子液滴的尺寸分布规律，揭示了横流流场分布对液滴碰撞的影响。针对复杂环境下液滴碰撞的实验及理论研究为深入理解低温风洞高速横流作用下液氮喷雾冷却特性提供了指导。

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