过冷水滴凝固机理及凝固组织特征

Freezing mechanism and microstructure characteristics in solidification of supercooled water droplets

针对飞机过冷水滴结冰的精细化预测需求,基于相变热力学与相变动力学相关理论,采用示差扫描量热法(DSC)、结冰风洞试验及微结构测试相结合的方法,研究了过冷水滴凝固过程的热力学机理及凝固组织特征。基于示差扫描量热法,研究了冷却速率及形核条件对结晶凝固特性的影响规律;基于结冰风洞试验开展了不同温度条件下冰相的宏观形貌及微结构特征研究。结果表明,过冷条件及冷却速率是影响过冷水滴结晶速率及结晶完善程度的重要因素。降温速率越大,结晶速率常数增大、结晶速率相应提高。同时,结晶峰变宽,结晶初始温度向低温方向移动,过冷效应相对显著;反之亦然。过冷度及冷却速率对冰相的宏观及微观形貌均有着重要影响。过冷度越大则相同时间内冷却速率越大,晶体生长过程越不充分,晶体不规则程度相对较高,同时晶粒密度变大、尺度变小,冰相表观透明度相对降低;反之,过冷度越小,则晶粒密度变小、尺度变大,冰相表观透明度相对较高。异相形核条件对加速结晶过程有重要促进作用,晶种的存在可有效加速二次结晶的触发,使过冷效应显著减弱。相关研究可为飞机结冰速率、冰相物理特征及冰形宏观形貌的精细化预测提供参考。

To better meet the demand of refined prediction of the aircraft icing process,the freezing mechanisms and microstructure characteristics in solidification of supercooled water droplets are studied combining the theory of phase-change thermodynamics and kinetics with tests.Based on the Differential Scanning Calorimetry(DSC)test,the effects of cooling rate and nucleation conditions on the solidification characteristics are studied.Icing tunnel tests are carried out at different temperature conditions,and the macroscopic morphology and microstructure characteristics of ice are obtained.The results indicate that icing conditions and cooling rate have great influence on the crystallization behavior of supercooled water.The greater the cooling rate,the greater the crystallization rate.At the same time,the crystallization peak becomes wider,and the initial temperature of crystallization becomes lower,and vice versa.The subcooling and cooling rate have important influence on the macroscopic and microscopic morphology of ice.The greater the subcooling and cooling rate,the more irregular the grains,the larger density and the smaller the grains,and the ice phase looks more opaque.On the contrary,the smaller the subcooling,the lower density and the larger the grains,and the ice phase looks more transparent.Heterogeneous nucleation conditions play an important role in accelerating the crystallization process.The addition of crystal seeds can trigger the crystallization effectively,significantly reducing the supercooling effect.The results lay an important foundation for better understanding the mechanisms of aircraft icing,which has important value for predicting the icing process more accurately.