Aircraft Icing Icing Physics

Transcription

1 Aircraft Icing Icing Physics Prof. Dr. Dept. Aerospace Engineering, METU Fall 2015

2 Outline Formation of ice in the atmosphere Supercooled water droplets Mechanism of aircraft icing Icing variations Ice growth Precipitation type and icing Precipitation and type of cloud 2

3 Outline-continued Physical factors effecting aircraft icing Icing intensity (severity) Liquid water content Temperature Droplet diameter Collection efficiency Airspeed Ice accretion modeling Flow field calculation Droplet trajectory calculation Thermodynamic analysis Ice accretion calculation 3

4 Formation of ice in the atmosphere Water is the only substance that can be found in all three phases (gas, liquid, solid) in the atmosphere depending on the temperature and pressure. The concentration of water vapor may range from near zero in desert regions to as high as 4% in tropical regions. Saturation occurs when water vapor is added to the air or, when the air is cooled. 4

5 Formation of ice in the atmosphere Condensation nuclei: microscopic particles present in the atmosphere, such as salt crystal, dust, and smoke particles. Hygroscopic particles: particles which contribute to transform the water vapor into liquid or ice. Condensation nucleus: 1 m Droplet: 10 m (10 seconds) Droplet: 100 m (500 seconds) Rain drop > 200 m (3 hours) Collision/coalescence mechanisms 5

6 Supercooled water droplets Cloud droplets do not freeze until they reach temperatures far below the freezing temperature. When temperature approaches -40 o C the droplets freeze rapidly and transform to ice crystals. Supercooled droplets: the droplets that stay liquid at temperatures below 0 o C. Small droplets do not freeze at 0 o C because their molecules do not line up in the proper order to form ice crystals. Supercooled droplets are unstable and may rapidly change from liquid to ice whenever their stability is perturbed. 6

7 Mechanism of aircraft icing Two conditions must be present: Ambient temperature must be below below 0 o C, Supercooled water droplets must be present. As the impacting water droplets freeze, heat is released so that their temperature rises until 0 o C is reached. After this freezing stops and the unfrozen water starts to run back along the surface of the aircraft or along existing ice and freeze downstream (runback ice). At cold temperatures a large part of a droplet freezes (rime ice), At higher temperatures only a small part freezes while the remaining part freezes slowly (glaze ice). 7

8 Rime ice Rime ice is a dry, milky and opaque ice deposit which usually occurs at low airspeed, low temperature and low liquid water content. Characterized by instantaneous freezing of the incoming supercooled water droplets upon impact trapping the air inside. Freezing fraction = 1. As a consequence, the shape of the surface is altered generating performance penalties due to the loss in the aerodynamic characteristics and to the added weight which introduces an unbalance of the aircraft components during the flight. 8

9 Rime ice 9

10 Glaze ice Glaze is a wet growth ice formed at a temperature around 0 o C and a high liquid water content. It has a clear appearance and a density closer to that of the cloud water. It occurs when a fraction of the water droplets freezes upon impact while the remainder droplets run back along the surface and freeze downstream. Freezing fraction < 1. This ice accretion process produces different ice shape deposit: such as double horned, beak or a rounded glaze ice. Glaze ice accretion dangerously affects and alters the shape of the original surface body producing aerodynamic penalties much more severe than rime ice accretion can cause. 10

11 Glaze ice 11

12 Icing variations Icing intensity may be severe in some regions and light in some other regions depending on the structure, horizontal and vertical extents, and the contents of the clouds. Icing conditions varies with altitude, season and geographical regions. Icing is more serious in winter season at altitudes of 7000 to 9000 ft above ground level. At high altitudes, above ft (6 km) icing is rare and have light intensity. 12

13 Icing variations The minimum low temperature for icing is -40 o C, for low temperatures all water droplets transform to ice crystals. Icing may also vary with horizontal extent of clouds due to the variation of liquid water content. 13

14 Icing variations 14

15 Ice growth There are two mechanisms by which cloud droplets grow: coalescence process which occurs because of the different fall velocities of the cloud droplets, growth of ice crystals (Bergeron process) due to the coexistence side by side of both ice crystals and cloud droplets. 15

16 Ice growth - coalescence 16

17 Ice growth Bergeron process 17

18 Ice growth Bergeron process Bergeron process is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions where the ambient vapor pressure is between the saturation vapor pressure over water and the (lower) saturation vapor pressure over ice. This is a subsaturated environment for liquid water but a supersaturated environment for ice resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition. If the density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough. Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. 18

19 Precipitation type and icing 19

20 Precipitation type and icing 20

21 Physical factors affecting icing Meteorological and aerodynamical factors affecting aircraft icing: liquid water content, temperature, relative humidity, droplet diameter, rate of catch. 21

22 Icing intensity 22

23 Icing intensity 23

24 Icing intensity 24

25 Liquid water content - cumulus clouds 25

26 Liquid water content - stratiform clouds 26

27 Liquid water content 27

28 Liquid water content 28

29 Temperature 29

30 Collection efficiency Is the ratio of the mass of droplets impinging on an obstacle, such as wing or airfoil, in unit time to the mass of droplets which would impinge if the droplets were following straight line trajectories. 30

31 Collection efficiency 31

32 Collection efficiency (effect of airspeed) 32

33 Airspeed When the airspeed is high water droplets do not have enough time to deviate from the airfoil and follow ballistic trajectories, thus more droplets impact on the airfoil. As a consequence, the impingement zone will be wider and the icing is expected to occur in a wider region. In addition, the velocity has an effect on the type of icing due to aerodynamic heating effect. For high velocity we usually have a glaze ice with horns which may cause separation over the airfoil. 33

34 Collection efficiency (effect of airframe size) 34

35 Collection efficiency (effect of droplet size) 35

36 Airframe and droplet size A larger airframe will constitute a larger obstacle for the incoming droplets causing them to deviate significantly away from itself. As a result fewer droplets impact the surface over a narrow impingement zone. Increasing droplet size has the same effect as increasing the airspeed as far as the droplet trajectories are concerned since the kinetic energy of the droplets increase. 36

37 Ice accretion modeling The main objectives of ice accretion simulation: calculation of the impingement pattern of the particles on the wing which determines the droplet impingement regions, mass of liquid (ice) on the body surface, the tangent or limit trajectories used to determine catch distributions or the global and local collection efficiency. 37

38 Ice accretion modeling The main applications of ice accretion simulation: predict the shape and the rate of ice growth on the airframe, predict aerodynamic performance degradations, use in the design of anti/deicing systems. 38

39 Ice accretion modeling There are four main steps in an icing simulation, flow field calculation, particle trajectories, thermodynamic analysis, ice accretion calculation. The computational procedure is an iterative process with a time-stepping procedure where successive thin ice layers are formed on the surface and followed by flow field and droplet impingement recalculations. 39

40 Flow field calculation The flow field calculation is needed to determine the velocity of air at any point in the flow field so that the droplet trajectory calculations can be proceeded. Navier-Stokes (sophisticated but accurate) or Panel method (simpler but less accurate) can be used. Icing calculation accuracy does not improve significantly with sophisticated methods. 40

41 Droplet trajectory calculation Droplets are released upstream of the wing and followed until impact on the airfoil surface. The droplet equation of motion takes into account the drag, buoyancy and gravitational forces. At each integration step, the local velocity needed to solve the droplet equation of motion is obtained from the flow field solution while the integration is continued following droplets until they impinge on the wing surface or pass downstream of the wing. Collection efficiency distribution is obtained. 41

42 Droplet trajectory calculation (single element airfoil) 42

43 Droplet trajectory calculation (airfoil with flap) 43

44 Droplet trajectory calculation (wing) 44

45 Collection efficiency distribution (wing) 45

46 Thermodynamic analysis The model is based on the first law of thermodynamics stating that the mass and energy must be balanced in a control volume. The mass balance will take into account the mass flow rate of the impinging water, the mass flow rate of water flowing into the control volume (runback water from previous CV), the mass flow rate of water flowing out of the control volume (runback water to next CV), the mass flow rate due to evaporation or sublimation, the mass flow rate of the freezing water. 46

47 Thermodynamic analysis The energy balance will take into account: the convective heat losses, the heat gain by friction, the enthalpy associated with impinging water, energy associated with runback water entering the control volume, the enthalpy associated with evaporation or running back to neighboring control volumes, the internal energy, calculated in relation to a given reference state depending on the type of surface involved: dry, wet or liquid. 47

48 Ice accretion calculation Messinger model (1953). 1-D phase change (Stefan) problem. Extended to handle 2 and 3-D problems by Myers (2001). Consists of one mass conservation, one phase change and two energy equations (one for the water and one for the ice layer). 48

49 Computed ice shape (wing) 49

50 Ice thickness distribution (wing) 50