热作用下低温流体两相射流雾化机理研究

Liquid nitrogen（LN2）jet spray cooling has great potential applications in the cooling down for cryogenic wind tunnels, cold storage and transportation for food, high-flux chip cooling and cryotherapy, etc. In spray cooling process, the atomization effect is the prerequisite of improving the cooling performance and flow field uniformity..This project aims to clarify the atomization mechanism of LN2 spray with the consideration of characteristics that gas-liquid spray easily occurs and the surface of the jet evaporates. Firstly, based on linear instability theory, a three dimensional non-axisymmetric dispersion equations with the effect of void fraction for the surface wave will be derived. Based on the results, the relation between jet instability and atomization will be built. Then the influence of void fraction on atomization and the expression of Sauter mean diameter (SMD) of atomized droplets under the effect of bubbly flow could be obtained. Secondly, a three dimensional non-axisymmetric dispersion equations with the thermal effect for the surface wave will be derived. Based on the equations, the thermophysical properties of LN2 on the jet thermostability will be studied. Lastly, visualization experiments of gas-liquid spray with nitrogen flow will be carried out to obtain spray characteristics (e.g. droplet mean diameters, droplets distribution and spray cone angle). Besides, the experimental results will validate the above theoretical models and amend the expression of SMD..The achievements will help the further understanding of atomization mechanism with LN2, and provide a solid foundation for improving the cooling performance and spray uniformity.

液氮射流喷雾冷却在低温风洞降温、食品冷藏运输、高热流芯片冷却和低温冷冻疗法等诸多领域有着广阔的应用前景。喷雾冷却过程中，良好的雾化效果是提高换热能力和流场均匀性的先决条件。. 针对液氮射流极易出现的气液两相射流和射流体表面蒸发的特点，本项目以明晰其射流雾化机理为目标。首先基于射流线性不稳定理论，推导出含空泡率的三维非轴对称表面波的色散方程，进而构建射流不稳定和雾化之间的联系，获得空泡率对雾化形态的影响规律和粒径分布的表达式。然后基于热作用下的三维非轴对称表面波的色散方程，分析低温流体物性对射流热稳定性的影响。最后开展射流雾化可视化实验，研究含气泡液氮的雾化特性，验证理论模型，并修正雾滴平均直径表达式。从而揭示气液两相和热作用下射流失稳过程的影响规律。. 本项目的研究成果可进一步加深对液氮射流喷雾机理的理解，并为控制喷射雾化效果，提高喷雾冷却能力和流场均匀性奠定基础。

低温流体射流雾化现象存在于低温风洞、液体火箭燃烧室、超导等国防和大科学装置中，同时因其具有较高的热流密度，在高热流芯片冷却、食品冷藏运输和低温冷冻疗法等领域有着广阔的应用前景。对射流雾化过程的深刻理解是提高喷雾冷却过程传热性能和流场均匀性的先决条件。基于上述背景，本项目开展了以下研究内容：（1）基于线性稳定性和小扰动理论，构建了低速射流雾化的理论模型，研究了低温流体射流破碎过程，得到了低速射流的最不稳定波波长、破碎液滴直径等参数；（2）基于流体体积法和界面位移算法，建立了高速射流雾化的直接数值模型，分析了初次破碎形态演变，进而获得了雾化形成的微小液滴粒径分布。（3）建立了气泡微爆模型，描述了二次雾化过程中热作用的影响机制，得到了低温流体物性参数和气化相变对射流雾化的作用。（4）建立了描述液滴运动行为的直接数值模型，描述了二次雾化液滴在喷雾冷却表面上的铺展规律。（5）通过搭建的低温可视化液氮射流雾化实验平台，研究了不同工况下液氮射流喷雾锥角、雾化直径等参数。并探索研究了强化液氮喷雾冷却能力的方法，结果表明直翅化微结构被冷却表面相比光滑表面，最大热流密度提高了20.18%，换热系数提高了64.2%。