Study On The Coupling Freezing Mechanism Of SLD Impact-flow For Aircraft Abnormal Icing

Aircraft icing is one of the serious threats to the safety of airplane.In recent years,the number of aircraft icing disasters caused by supercooled water droplets keeps increasing.The abnormal run-back ridged ice formed by supercooled large droplets on wing surface will destroy the aerodynamic appearance of the aircraft,reduce the lift force,increase the resistance,and seriously affect the aircraft safety.However,the icing mechanism of SLD is not fully understood yet,making it hard to effectively explain the phenomenon of abnormal run-back ice with supercooled large droplet,and to support the development of aircraft anti-icing and de-icing technology under SLD conditions.Although the FAA has issued new amendments in 2014,the icing environment of SLD in appendix O still can not be realized in the icing wind tunnel test.Meanwhile,the numerical simulation technology mainly considers the influence of dynamics on the collection rate of water droplets,which can not satisfy the effective SLD anti-icing technologies.As for SLD freezing mechanism,the nucleation and the icing evolution of supercooled water are not considered in the exsiting researches on the run-back ridged ice.Besides,the study on nucleation and icing is rarely carried out under the SLD impact conditions.However,in actual situation,it is the coupling of impact-flow and icing that results in the SLD run-back ridged ice.Therefore,the research on the impact freezing of supercooled drolplet can help effectively explain the phenomenon of abnormal run-back ridged ice.So,we conducted the experiment of supercooled large droplet impact freezing on surface.By observing the retraction-freezing process of droplet on horizontal surface and the flow-freezing process of droplet on inclined surface,the coupling of impact dynamics and freezing is discussed.And the run-back icing of supercooled large droplet is also analyzed theoretically.In this experimental research,new frozen morphologies of supercooled droplet are found.Except for previous findings that supercooled droplet freezes in the shape of semisphere,pancake and basin after impinging on the horizontal surface,new frozen shapes such as poached egg and irregular shape are found under the condition of low growth rate of vertical ice and low growth rate of horizontal ice.Besides,different from room-temperature droplet which freezes in the form of two satellites,supercooled droplet freezes in the shape of ellipse and slender strip after impinging on the inclined surface.With the increase of nucleation time and the decrease of ice growth rate,the frozen morphology of supercooled droplet impinging on inclined surface will gradually evolve from ellipse to slender strip.And different impact freezing modes of supercooled large droplet determined by nucleation time and ice growth rate are found.The freezing of droplet is nucleation time-dominated when the ice growth rate is higher than the flow rate of droplet.And it is determined by ice growth rate in the opposite case.It is also found that the impact freezing modes of supercooled large droplet will evolve into the large-overflow distance occasion when the nucleation time is long enough,especially with adequate water.Based on the coupling of impact dynamics and freezing,the flow freezing model of supercooled droplet is illustrated theoretically,and the abnormal runback icing on wing surface in actual situation is discussed.Under the condition of low supercooling,droplet nucleation time is long and ice growth rate is slow,which will result in the continuous flowing of supercooled water droplet.Finally,the droplets accumulate on the back of de-icing and the run-back ridged ice is formed,which poses a serious threat to aircraft safety.The researches and discussions on supercooled large droplet flow freezing after impinging on surface,are of great significance to the prediction of icing environment and the design of aircraft anti-icing and de-icing system.