Mechanism And Fundamental Theory Of Supercooled Water Solidification On The Abnormal Aircraft Icing

The aircraft icing always cause severe flight accidents and is a great threat to aircraft. The flight safty in icing conditions is an important part of airworthness authorizing of civil transport aircraft. Since 1994 a series of accidents revealed similar abnormal icing phenomenon that the severe runback ice accreted after the de-icing device and could not be normaly removed. In the investigation about those accidents the icing conditions of Supercooled Large Droplet(SLD) are found to cause the icing phenonmenon, which indicate that the regulation of FAR25 1419 and Appendix C are inadequate to ensure the flight safty of aircraft. Many researches about the SLD icing and and lead to the modification of airworthness regulation of icing-FAR25 Appendix O. However, the existing researches about the dynamic process of large droplets cound not well interpret thet mechanism of abnormal icing. Also the anti-/de-icing method against those icing phenomenon has not been developed. The current investigations indicate that the abnormal icing can not be interpreted or predicted by adding the droplet dynamic models to traditional icing theory. Researches into the fundamental icing mechanism and theory of supercooled water are required to understand the physics of abnormal icing, and to support the airworthness authorizing of aircraft in those icing conditions. The research into the abnormal icing phenomenon would does not only improve the safty of civil aircraft, but also valuable to the aviation industry of China and relevant scientific researches.Based on the physical properties of supercooled water, this thesis investigate:(1) The mechanism of ice growth rate and pattern evolution on the surface of aircraft, and the coherence of large supercooled water droplet impingement and icing, to understand the mechanism of abnormal icing. With the investigation about the icing mechanism, a coherenced icing evolution theory is presented to describe the icing of supercooled water film/large droplets.(2) With this theory the abnormal icing phenomenon is explained by the growth of dendritic ice crystal and the effect of SLD impingement on unsteady heat transfer.(3) The aircraft icing numerical simulation method are applied to validate the models given by the researches of icing mechanism, and to investigate the macroscopic law of aircraft icing phenomenon. The content of this thesis is:(1) Based on the focus of this theory the method of investigating the icing mechanisms(droplet breakup, droplet impingement and freezing of supercooled water) are presented. The multi-phase numerical technique are used to investigate the criterions of different droplet breakup pattern, the size of son-droplet and the position of breakup, in order to accurately predict the diameter distribution of impinging droplets. In the theory two breakup patterns are included by sorting the condisions of those breakup patterns. The impinging and icing of supercooled droplet are observed in the vertical droplet windtunnel. By analyzing the observed icing process of supercooled water droplets, the coherence of droplet impinging, heat transfer and ice growth are theoretically investigated.(2) In the investigation of supercooled icing properties, the transient icing of supercooled water on a substrate is studied as a fundamental physcics of the freezing of large droplet on aircraft. The evolution of ice growth on a substrate is observed and the ice growth velocity/pattern in different directions as well as the influence of substrate are investigated experimentally. The icing evolution is analyzed theoretically and the stability criterion of solidification interface in unsteady temperature is given. The ice growth in flowing water film is studied with Lattice Boltzmann-phase field method to understand the impact of flowfield on ice growth.(3) Based on the researches on the mechanism of aircraft icing, a coherented icing evolution theory is presented to describe the icing of SLDs, which includes the icing evolution of supercooled water and the coherence of droplet impingement with icing. In this theory, the icing of supercooled water on aircraft is a multi-stage evolution process in which the temperature distributions in water and ice define the stage of ice growth. The impingement of droplets does not only lead to the splash and reimpingement, but also change the evolution process of icing. The droplet breakup can affect the impingement and icing process by produced droplet diameters.(4) Numerical models and algorithm are presented based on the icing mechanisms and theory. By calculating the ice shapes in different conditions, the variation of ice shapes with conditions are analyzed and possible reasons of abnormal icing are discussed.The results show that the icing calculation algorithm and physical models can well predict the ice shape in many icing conditions. By considering the icing evolution and impingement –icing coherence the formation of spawise-ridge ice shape is well interpreted, which is the typical icing phenomenon of abnormal icing.The significance and innovation of this thesis are: find the mechanism of icing evolution and the criterion of icing pattern in unsteady temperature field; Keep the properties of supercooled water in experiment to make sure the quality of experiememtal results;give out the algorithm and models which can predict a part of abnormal icing phenomenon.