Supercooled Large Drop Icing Mechanism And Airworthiness Safety For Transport Category Airplanes

Supercooled Large Droplet,unlike those normal icing eviornment limited within Appendix C,is an extremely dangerous environment in aerospace engineering.The interial mechanism and basic theory for airworthiness safety in this condition needs to be further improved due to its complex dynamics,thermodynamics and aerodynamic characteristics.Viewing the “drop size” as the breakthrough point,the distinct characteristics are interpreted from two aspects including the freezing mechanism and the airworthiness specially for SLDs.On theoretical side,the "SLD impact freezing" experiment is presented as first sight representative of the real-world droplets which in-flight aircrafts might encounter.The results show SLDs impinge on the wall with a high speed and exhibit a high diversity of freezing characteristics.The phenomena reflect the high-coupling relationship between the impact(dynamic)and heat transfer(thermodynamic)process.An interacted-freezing type last only 0.001 second,which is far away from traditional acknowledgement of supercooled freezing.Combined with theoretical analysis,the rapid freezing phenomenon due to larger size is attributed as the increments of supercooled diffusive rate Bi and effective heat transfer time Fo.Based on the experimental data,this paper also proposed an Impinging and Heating semi-empirical model for estimation of unsteady convective heat transfer quantity during the supercooled droplet collision.Simulations illustrate that this Imping Heating model can improve the accuracy in ice shape prediction,by which it increases the ice accretion rate at the leading edge of the airfoil,especially the upstream and downstream sides behind the stationary point,and therefore the horn angle of ice accretion clearer.On airworthiness side,using SJTUICE procedure for investigation,this paper focuses on the characteristics of aerodynamic degradations upon the complex airfoil with control surface or high-lift devices,in order to correspond to the freezing requirement upon different flight conditions(following Amendment 25-121).Comparisons are presented between “horn ice” and “ridge ice”(static),between “SLD” and “non-SLD” respectively.Researches on “control surface effect” show that the variation of coefficient of lift is determined by two critical factors,including the effect of ice shape and the relative position between the control surface and its generated vortex.Studies on multi-element airfoil show that the leading edge of the flap can be realized as one of the most sensitive regions to drop size,since the secondary-acceleration near the gap takes different effects to the local collection efficiency and impact thermodynamics and therefore freezing characteristics with diameter variations.Through aerodynamic analysis,the thicken boundary layer on the upper surface of slat,the earlier separation of flap,and the vortex developing mode in the wake all make more adverse effect on the aerodynamic performance.The aerodynamic degradation rate under SLD condition is much larger than that with normal particle size at the same time scale.Finally,the calculation is applied to aerodynamic analysis upon a commercial aircraft.The loss rates with different chord lengths are compared on wings and tail-wings,which has pointed out the potential risk the aircraft may encounter under SLD conditions.In this study,SLD icing links physical congition and ice airworthiness together,based on the innovations on unsteady heat transfer theory ranging large-sized diameters and continuous icing and aerodynamic effects on complex geometries.The specificity of harm in SLD environments have been deeply interpreted from multi-levels,including physics,ice accretion simuation,aerodynamic analysis and flight safety airworthiness.This paper presents solutions for SLD airworthiness which is about to face,and lies the foundation for security upgrade of civil aviation airworthiness.