Study on the Heat Transfer Characteristics in aircraft icing

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Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 00 (2014) 000 000 www.elsevier.

Yi,Ye-Wei Gui, Yan-Xia du State Key Laboratory of Aerodynamics, Aerodynamics Research and Development Center, Mianyang, 621000, China Abstract Investigation on the heat transfer mechanisms of liquid

1 Available online at ScienceDirect Procedia Engineering 00 (2014) APISAT2014, 2014 Asia-Pacific International Symposium on Aerospace Technology, APISAT2014 Study on the Heat Transfer Characteristics in aircraft icing Zhi-Hong Zhou*,Xian Yi,Ye-Wei Gui, Yan-Xia du State Key Laboratory of Aerodynamics, Aerodynamics Research and Development Center, Mianyang, , China Abstract Investigation on the heat transfer mechanisms of liquid and solid is important to predicting the icing process. Messige model is a typical model for the process of mass and heat transfer, but the effect of surface tension of water droplet is been ignored in the model. We established a model which was add the infection of surface energy based on traditional thermodynamic model. Some cases at NACA0012 are simulated with our model in order to verify its correctness. The calculated results show that, our model was consistent with the traditional thermodynamic model in Low temperature, but our model have better results in high temperatures 2014 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA). Keywords: Ice, Thermodynamic model, supercooled droplets, surface energy. 1. Introduction Aircraft often fly under subfreezing temperatures,if encounter cloud containing supercooled droplets,and supercooled droplets of cloud impact on the plane,ice accretion phenomenon will occur Ice accretion and its subsequent build-up is one of the potential hazards in airplane flight, which lead to deterioration of aircraft aerodynamics performance. Prediction of ice accretion is thus an important part of airplane design[1]. We know that, depending on the structure of ice and the physical changes that occur in the process of freezing, ice on the aircraft surface is usually divided into three types: rime ice, glaze ice and mixed ice. Icing on the wing of aircraft will have a significant influence on the safety of the aircraft. For example, it will lead to decreasing of the lift coefficient and increasing of the drag coefficient. Since the shape of glaze ice is * Corresponding author. Tel.: ; fax: address: zhouzhihong029@163.com The Authors. Published by Elsevier Ltd. Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).

2 2 Zhi-Hong Zhou / Procedia Engineering 00 (2014) irregular, and it is widely distributed along the surface of the wing chord, and there have no air bubbles in the glaze ice, therefore it is transparent and dense. It is difficult to separate from the ice surface. It has the most serious damage on aerodynamics, and will significantly affect flight safety. It is necessary to research priorities. Investigation on the heat transfer characteristics of ice growth on aircraft surfaces is the premise and foundation for studying the icing mechanisms and anti-icing or deicing techniques. During the icing process, the accretion rate and ice type depend not only on the airflow parameters but also on the inner heat transfer characteristics. Investigation on the heat transfer mechanisms of liquid and solid is therefore important to predicting the icing process. A number of works have studied the influence of the mass and heat transfer on ice growth, some thermodynamic models is established, Messige model is a typical one. 1953, Messiger analysis of aircraft surface heat and mass transfer processes, established a thermodynamic model freezing process based on view of the energy and mass balance. This model is the foundation of research on heat and mass transfer characteristics of aircraft icing, until now it has been widely adopted. Research of the heat transfer in aircraft icing process are essentially based on the Messiger model or models with a certain degree of modification. Although Messiger model and improved models are widely used in icing research of aircraft, however, these models describe the actual physical process of freezing is still not accurate enough. There are still some limitations in the simulation of graze ice and other complex types with numerical simulation methods based on Messiger model. The effect of surface tension of water droplet is been ignored in those works, researchers had thought that all of the supercooled water droplet s kinetic energy translate into heat energy while it hit the surface of the object. This assumption ignores the effects of approximate surface tension of water droplets. We found that, not all of its kinetic energy is converted into heat when the drop impact on the airplane, most of the energy will be converted into water droplets shape changes of increased surface energy. The surface energy of the surface tension caused by changes of the shape of drops have the same order of magnitude with the droplet kinetic energy. This energy can not be ignored during icing simulation. In order to accurately predict the growth pattern of ice formation and to flow under different weather conditions and to adapt to different requirements forecast icing conditions, the paper surface tension effects into account, establish a more general freezing heat transfer model, and uses the model analyzes the influencing factors and the role of the law of aircraft icing during the ice growth. 2. Method of aircraft icing calculation Aircraft icing is an unsteady process, the freezing process is usually treated as a number of quasi-steady process in the actual calculation process. Every quasi-steady process is usually carried out with three steps. First, solve the basic equations of air flow field to get air flow field flow around the aircraft. Then, solving equations of motion of water droplets to get collection efficiency based on the results of air flow field. Finally, consider freezing by solving the phase change heat transfer model to determine the aircraft icing Calculation method for flowfield Numerical simulations using the Reynolds-averaged Navier-Stokes equations are conducted to calculate the air flow field. Our numerical approach to solve the NS equations is based on the finite volume form of the integral equations. The equations are the expression of the conservation principle for mass, momentum and energy. In a domain of volume Ω with boundary S, the equations may be written in the following form: U Ω+ F S = 1 F V (1) t Re Ω S S As used for the present computations, the cell-centered finite volume methods are employed to solve the Navier- Stockes equations. The flux-vector splitting (FVS) method of van Leer is implemented as Reference 2. A timeaccurate, fully implicit method based on LU-SGS has been used to solve the viscous flow problems. At the wall surface boundary the immovability boundary condition is applied, i.e., the wall velocity is zero. At the far-field boundary some special boundary conditions must be imposed. The viscosity coefficient computed as the sum of

3 Zhi-Hong Zhou / Procedia Engineering 00 (2014) laminar and turbulent viscosity coefficients, which are evaluated by the Sutherland s law. The Spalart-Allmaras model is implemented to handle turbulence flows Calculation method for droplet collection efficiency The collection efficiency on the structure surface is computed based on the distribution of air field with an Eulerian method.the governing equations of the water phase are composed of continuum and momentum equations[3,4],shown as: (ρ d α) + (ρ d αu d ) = 0 (2) t (ρ d αu d ) + (ρ d α u dud ) = ρ d αk( u a u d )+ ρ d αg (3) t Where a is the droplet volume fraction,and K the called inertia factor.eq.(2) is called continuum equation and Eq.(3) is called momentum equation.in order to obtain water collection efficiency and impingement characteristics,the same numerical method with air field calculation is used to discrete and solve Eq.(2) and Eq.(3).The droplet volume fraction, a,can be obtained after the equation is solved.the droplet collection efficiency,β,which shows the distribution of liquid water on structure surface,can be calculated as v r a u n dr b = a v (4) u Where a and u v are the droplet volume fraction and air velocity of far field,and n r is the unit vector normal to body surface Thermodynamic model Based on the results of water droplets collection rate,we can calculate the icing on the plane with the opinion of mass balance and energy balance[5]. (1) Mass balance equations m + m = m + m + m (5) im in so va out m The mass flow go into the control volume contains the following sections: in is the water flow which is not m freezes from upstream of the control body, im is supercooled droplets impact on the airplane; the mass flow out of m control bodies contains the following sections: va is the amount of water change to steam by evaporation or m m sublimation, out is the mass which is not freezes and flow into downstream, so is the water mass which was change to ice. (2) Energy balance equation Q 1 Q 2 +Q 3 Q 4 Q 5 +Q 6 = 0 (6) Messinger model set the surface of the infinitesimal control volume on anti-icing components under the quasisteady conditions as research subjects,, according to the first law of thermodynamics to establish thermal equilibrium equation for each component surface heat flow items. Messinger considered infinitesimal surface heat Q flux are the following items: 1 is the convective hot flow of wall and the environment; Aerodynamic heating heat Q Q flow; 2 is the heat of the wall heat flux aerodynamic heating; 3 is the heat of evaporation of water droplets or ice Q Q Q surface; 4 is the energy of droplets into the body; 5 is the impact kinetic energy into the water heat flux; 6 is the phase change heat flow during freezing.

4 4 Zhi-Hong Zhou / Procedia Engineering 00 (2014) The improved thermodynamic model 3.1. The influence of roughness Thermodynamic model is the key of the ice type simulation, and solving of the surface and air convection heat transfer coefficient is very important to the ice simulation accuracy. The surface will become rough while aircraft surfaces is icing, Flow transition forward position by the disturbance tissue of rough surface to the air flow. This will enhance convection heat transfer of the surface and the surrounding air. Currently, the mechanism of roughness generated still in the research stage, the general study of empirical formula. We learn from Glenn Research Center icing software LEWICE approach, according to the experimental data fit the empirical formula, used to calculate the equivalent sand surface roughness height to measure the roughness of the ice surface[6] c = [ / c 3.2. The influence of surface energy k ] MVD [ s / c ] LWC [ / c base / c base / c ] T [ / c base c ] (7) base The icing process is closely related to the impact of water droplets. Spread,stick, rebound and splash phenomena is happened in droplet impact process. The process is influenced by many factors, such as the inertial force, capillary forces, viscous forces, surface tension, surface properties of solid and so on. The great mass of kinetic energy is converted into surface energy when the droplet impact on the surface of the airplane, and part of the energy translate into heat energy. The surface energy and water droplet s kinetic energy within the same order of magnitude. There are a part of the energy dissipated by the flow of a viscous liquid, called the viscous dissipation of energy, this energy is converted into heat. The thermal energy conversion by viscous dissipation is[7]: t c W 1 = φdωdt φωt c (8) 0 Ω And f is the function of viscous dissipation φ = µ( v i x j + v j x i ) v i x j µ( V 0 δ 0 ) 2 (9) When we add in the improved thermodynamic model of the effects of surface energy, the study found that when added icing simulation, better type of consistent ice surface can affect the thermodynamic model, the closer zero degrees Celsius, the effect is more obvious. 4. The calculation examples (1) The calculation of heat transfer coefficient We simulated the convective heat transfer coefficient with NACA0012 airfoil surface. Case: V=130m/s, P=90748Pa. T=-12ºC,L=0.3m,MVD=20μm,LWC=0.5g/m3,α=0º and 4º. Our calculated results compared with the results calculated LEWICE were shown as the Figure 1 (a) and (b). The minimum, maximum, and the trend curve are good agree with the result of LEWICE, it indicating that the method we used in this article is correct.

5 Zhi-Hong Zhou / Procedia Engineering 00 (2014) (a) α=0º (b) α=4º Fig 1 The calculation of heat transfer coefficient (2) The influence of surface energy We simulated ice shape on NACA0012 airfoil surface. Case: V=130m/s, P=90748Pa, L=0.3m, MVD=20μm, LWC=0.5g/m3, t=360s, α= 4º, T=-12ºC and -4ºC. We calculated results in the improved model and the Message model 'y' Foil Tensility Experiment No_Tensility 'y' Foil Tensility Experiment No_Tensility 'x' (a) T=-12ºC (a) T=-4ºC 'x' Fig 2 The calculation of heat transfer coefficient Our model was developed based on traditional thermodynamic model,we add the infection of surface energy. Some cases at NACA0012 are simulated with our model in order to verify its correctness. The calculated results show that, our model was consistent with the traditional thermodynamic model in Low temperature, but our model have better results in high temperatures. 5. Conclusion We get the following conclusions based on our studies: (1) During the icing process, the accretion rate and ice type depend not only on the airflow parameters but also on the inner heat transfer characteristics. (2) We found that, not all of its kinetic energy is converted into heat when the drop impact on the airplane, most of the energy will be converted into water droplets shape changes of increased surface energy. The surface energy of the surface tension caused by changes of the shape of drops have the same order of magnitude with the droplet kinetic energy. This energy can not be ignored during icing simulation.

6 6 Zhi-Hong Zhou / Procedia Engineering 00 (2014) Acknowledgements This research was sponsored by The National Natural Science Foundation ( ), National Postdoctoral Foundation(2012M512065),and Research Foundation of State Key Laboratory of Aerodynamics. References [1] Zhou, Zhihong,Li, Fengwei,Li, Guangning, Applying Eulerian droplet impingement model to numerically simulating ice accretion but with some improvements, Journal of Northwestern Polytechnical University, Vol.28,pp , February [2] E.Iuliano,V. Brandi, G. Mingione,"Water Impingement Prediction on Multi-Element Airfoils by Means of Eulerian and Lagrangian Approach with Viscous and Inviscid Air Flow",AIAA [3] Jameson, A., et al, Numerical Solutions of Euler Equations by Finite Volume Methods with Runge-Kutta Time Stepping Schemes, AIAA Paper, , [4] Yves Bourgaultk, A Finite Element Method Study of Eulerian Droplets Impingement Models, International Journal for Numerical Methods in Fluids,29: ,1999. [5] Thomas S K, Cassoni R P, Charles D M."Aircraft anti-icing and de-icing techniques and modeling",journal of Aircraft,1996,33(5): [6] Zhihong Zhou, Fengwei Li, Icing Numerical Simulation for Single and Multi-Element Airfoils, AIAA [7] WANG Shi-meng, WANG Jia-chun, CHEN yan-wen. The Thermodynamic Analysis on the Surface Tension and Specif ic Surface Free Energy of Liquid. Journal of Shenyang Arch. and Civ. Eng. Univ. Vol.18, No.4,2002

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