HYBRID AIRFOIL DESIGN FOR FULL-SCALE ICE ACCRETION TEST

A hybrid airfoil inverse design method according to the target pressure distribution and the impingement efficiency is presented.The method is developed to design hybrid airfoils that simulate the droplet impingement and ice accretion of full-scale airfoil.Flow field and droplet impingement around the full-scale airfoil are calculated to obtain pressure distribution and impingement efficiency firstly.The Navier-Strokes(N-S)solver is used in flow field calculation to improve calculation precision.The droplet impingement and ice accretion on the airfoil are performed by FENSAP-ICE.Once the target chord or original airfoil is given,the hybrid airfoil geometries can be computed.The designed hybrid airfoil consists of full-scale leading edges and redesigned aft-section.The hybrid airfoil can be tested under full-scale conditions to produce full-scale ice accretion in the exiting icing tunnels which are too small to perform ice accretion testing of full-scale airfoils.Moreover,the ice shapes formed on the full-scale and hybrid airfoils are compared at various attack angles.The results demonstrate that ice shapes between hybrid and full-scale airfoils match well and the developed method is effective.

Numerical analysis of ice accretion effects at super-cooled large droplet conditions on airfoil aerodynamics

Changes in flow field around NACA23012 airfoil from a clean condition to a super-cooled large droplet （SLD） condition were simulated, and variations in aerodynamic parameters were calculated using FLUENT. In the case of numerical simulation for a clean airfoil, flow field characteristics simulated agreed well with theory analysis, indicating that turbulence models and parameters setting are feasible. Aerodynamic parameters for iced airfoil were calculated using the same method and agreed with those measured test data under the same environment in icing wind tunnels by S. Lee. Conclusion is made that the numerical simulation is valid, and it can be an alternative to study ice accretion effects at the SLD condition on airfoil aerodynamics, leading to reduction in research cycle time and cost.

Wind Tunnel Test and Numerical Computation on Ice Accretion on Blade Airfoil for Straight-bladed VAWT

To invest the condition of ice accretion on the blade used for straight-bladed vertical axis wind turbine （SB-VAWT）, wind tunnel tests were carried out on a blade with NACA0015 airfoil by using a small simple icing wind tunnel. Tests were carried out at some typical attack angles under different wind speeds and flow discharges of a water spray with wind. The icing shape and area on blade surface were recorded and measured, Then the numerical computation was carded out to calculate the lift and drag coefficients of the blade before and after ice accretion according to the experiment result, the effect of icing on the aerodynamic characteristics of blade were discussed.

Analysis on Aerodynamic Performance of Finite Swept Wing with Glaze Ice Accretions

A computational investigation was performed to predict the effects of aerodynamic performance degradation on aircraft swept taper wing with and without 10 minutes and 22.5 minutes glaze ice accretions. In this study, the three-dimensional simulated glaze ice shapes were defined from a series of two-dimensional ice sections. The aerodynamic performances of glaze iced swept wings with C-H structure multi-block grid were analyzed and evaluated. The steady Reynolds- Averaged Navier-Stokes （RANS） equations are employed to compute solutions with implementation of two equation Shear-Stress Transport （SST） turbulence model and second-order upwind differencing for entire iced wing flow field. Computed results were compared with available experimental data. The CFD computation can also accurately predict the aerodynamic performance degradation of lift, drag and pressure coefficients of finite swept wing with glaze ice accretions which have two big upper and lower horn.

Numerical simulation of rime ice accretion on a threedimensional wind turbine blade using a Lagrangian approach

The accreted ice on wind turbine blades significantly deteriorates the blade aerodynamic performance and consequently the power production.The existing numerical simulations of blade icing have mostly been performed with the Eulerian approach for two-dimensional(2D)blade profiles,neglecting the possible three-dimensional(3D)rotating effect.This paper conducts a numerical simulation of rime ice accretion on a 3D wind turbine blade using the Lagrangian approach.The simulation results are validated through previously published experimental data.The icing characteristics along the blade radial direction are then investigated in detail.Significant radial airflow along the blade is observed,which demonstrates the necessity of 3D simulation.In addition,more droplets are found to impinge on the blade surface near the tip region,thereby producing severer ice accretion there.The accreted ice increases almost linearly along the blade radial direction in terms of both ice mass and maximum ice thickness.

Estimating and Mapping Extreme Ice Accretion Hazard and Load Due to Freezing Rain at Canadian Sites

The ice accretion load in Canadian structural design codes is developed based on an operational ice accretion prediction model.In the present study,three models are employed to predict the ice accretion amount on a flat surface and horizontal wire at Canadian sites.The results confirm that the model used by Canadian practice for predicting ice accretion leads to a conservative estimate as compared to the remaining two models.The results also indicate that the use of the Gumbel distribution for the annual maximum ice accretion is adequate for regions prone to ice accretion and that the lognormal distribution may be considered for regions with a moderate or negligible amount of ice accretion.Maps of the ice accretion hazard at five selected Canadian sites are developed.Statistical analysis of an equivalent wind speed that is concurrent with the iced wire is carried out,showing that the concurrent wind speed for the 50-year return period value of the annual maximum ice accretion amount is smaller than the 50-year return period value of the annual maximum wind speed.It is shown that the statistical characteristics of the annual maximum concurrent wind speed on iced wire differ from that of the annual maximum wind speed.

Torrential Rainfall Responses to Ice Microphysical Processes during Pre-Summer Heavy Rainfall over Southern China

In this study, the effects of key ice microphysical processes on the pre-summer heavy rainfall over southern China during 3-8 June 2008 were investigated. A series of two-dimensional sensitivity cloud-resolving model simulations were forced with zonally uniform vertical velocity, zonal wind, horizontal temperature, and water vapor advection data from the National Centers for Environmental Prediction （NCEP）/Global Data Assimilation System （GDAS）. The effects of key ice microphysical processes on the responses of rainfall to large-scale forcing were analyzed by comparing two sensitivity experiments with a control experiment. In one sensitivity experiment, ice crystal radius, associated with depositional growth of snow from cloud ice, was reduced from 100 #m in the control experiment to 50 #m, and in the other sensitivity experiment the efficiency of the growth of graupel from the accretion of snow was reduced to 50~ from 100% in the control experiment. The results show that the domain-mean rainfall responses to these ice microphysical processes are stronger during the decay phase than during the onset and mature phases. During the decay phase, the increased mean rain rate resulting from the decrease in ice crystal radius is associated with the enhanced mean local atmospheric drying, the increased mean local hydrometeor loss, and the suppressed mean water vapor divergence. The increased mean rain rate caused by the reduction in accretion efficiency is related to the reduced mean water vapor divergence and the enhanced mean local hydrometeor loss.

Numerical Simulation on Ice Shedding Phenomena in Turbomachinery

In the jet engine, icing phenomena occur primarily on the fan blades, the FEGVs （fan exit guide vanes）, the splitter, and the low-pressure compressor. Accreted ice disturbs the inlet flow and causes large energy losses. In addition, ice accreted on a fan rotor can be shed from the blade surface due to centrifugal force and can damage compressor components. This phenomenon, which is typical in turbomachinery, is referred to as ice shedding. Although existing icing models can simulate ice growth, these models do not have the capability to reproduce ice shedding. In the present study, we develop an icing model that takes into account both ice growth and ice shedding. Furthermore, we have validated the proposed ice shedding model through the comparison of numerical results and experimental data, which include the flow rate loss due to ice growth and the flow rate recovery due to ice shedding. The simulation results for the time at which ice shedding occurred and what were obtained using the proposed ice shedding model were in good agreement with the experimental results.

The microphysics of ice accretion on wires:Observations and simulations

The weather system, meteorological conditions, and microphysics of cloud, fog, and rain droplets are studied during the formation, growth, maintenance, and shedding periods of ice accretion on wires in Enshi, Hubei Province in China using 2008/2009 and 2009/2010 winter observations. The comprehensive observations include data of visibility, microphysics of fog and rain droplets, and ice thickness, as well as data from an automated weather station and other routinely recorded meteorological data. The results show that icing occurred during the passage of a cold front, with a high-pressure system and a cold temperature trough at 850 hPa, and a southeasterly at 500 hPa that provided abundant moisture. Ice formation usually started in the evening or early morning, and ice shed around noon the following day when the temperature was -1℃ to 0℃. The averaged liquid water content of the fog droplet was distinctly greater during the growth period than during the other three periods, and there was precipitation during the growth period in each case of ice accretion. The growth rate of the ice thickness was clearly correlated with the liquid water content, with a correlation coefficient of 0.62. Simulations using empirical equations were carried out, and the simulated ice thickness agreed with observations fairly well.

Numerical simulation of icing effect and ice accretion on three-dimensional configurations

It is common for an aircraft to encounter icing weather conditions, which would be dangerous to the flight. Thus, there is a need to study the detail of icing effect and the process of ice accretion on the aircraft. In this paper, considering three different icing models according to weather conditions, i.e., sharp-angled ice, blunt-nosed ice and double horn ice, the Reynolds-averaged N-S equations and the S-A turbulence model are used to analyze the flow field for an iced wing/body configuration with a multi-block strategy and structured grid technique. The numerical result is compared with the experimental data. A flow solver is developed based on the Euler equations to investigate the ice accretion process. The droplets are tracked by using the Lagrangian method. In addition, a revised Messinger model is proposed to simulate the ice accretion. This numerical simulation is conducted for the ice accretion on an M6 wing and a wing/body/tail configuration. The presented results preliminarily show that the numerical methods are feasible and effective.

Numerical simulation of ice accretion prediction on multiple element airfoil

For studying ice accretion on aircraft and helicopter airfoils,a modified model of the mass and heat transfer on icing surface was first proposed based on the classical Messinger model.Then an approach for predicting ice accretion on multi-element airfoils was set up through introducing the interpolation calculation of airflow field around the multi-element airfoils.Consid-ering the equivalent thermal power from anti-ice system,a method of the prediction of ice accretion under anti-ice situation was proposed.In order to study the prediction of ice accretion on helicopter rotor,a numerical simulation method combining the computational fluid dynamics (CFD) technique with helicopter aerodynamics theory was set up.The agreement between the results of numerical simulation and the experimental data indicates that the model and methods proposed in this paper are feasible and effective,and that they can lay the foundation of the research on the dynamics in icing condition and design of anti/de-ice system.

Numerical simulation of ice accretion in supercooled large droplet conditions

The simulated methods for ice accretion on two-dimensional airfoil surface are established completely under the Eulerian framework in supercooled large droplet(SLD) conditions. The two-dimensional code to solve the partial differential equations(PDEs) of droplet phase is derived to simulate the impingement characteristic of SLD. Also, several semi-empirical models which explain the droplet-wall interaction are compared and discussed to show respective features when simulating the splashing phenomenon. In particular, a new boundary condition for wall called penetrable wall for splashing droplet(PWSD) is proposed to deal with the impingement of SLD on solid surface, which efficiently improves the accuracy of simulation. Then the improved impingement characteristic of SLD is input into the extended mass and heat transfer model to simulate the ice growth on airfoil surface. The multistep advanced method is carried out to better match the physical phenomenon of ice growth. At last, the simulated results of critical parameters: local droplet collection efficiency and the height of ice growth are compared with the experimental data which verify the applicability of proposed models.

The Influence of Freezing Drizzle on Wire Icing during Freezing Fog Events

Both direct and indirect effects of freezing drizzle on ice accretion were analyzed for ten freezing drizzle events during a comprehensive ice thickness, fog, and precipitation observation campaign carried out during the winter of 2008 and 2009 at Enshi Radar Station （30°17′N, 109°16′E）, Hubei Province, China. The growth rate of ice thickness was 0.85 mm h-1 during the freezing drizzle period, while the rate was only 0：4 mm h-1 without sleet and freezing drizzle. The rain intensity, liquid water content （LWC）, and diameter of freezing drizzle stayed at low values. The development of microphysical properties of fog was suppressed in the freezing drizzle period. A threshold diameter （Dc） was proposed to estimate the influence of freezing drizzle on different size ranges of fog droplets. Fog droplets with a diameter less than Dc would be affected slightly by freezing drizzle, while larger fog droplets would be affected significantly. Dc had a correlation with the average rain intensity, with a correlation coefficient of 0.78. The relationships among the microphysical properties of fog droplets were all positive when the effect of freezing drizzle was weak, while they became poor positive correlations, or even negative correlations during freezing drizzle period. The direct contribution of freezing drizzle to ice thickness was about 14.5%. Considering both the direct and indirect effects, we suggest that freezing drizzle could act as a ＂catalyst＂ causing serious icing conditions.

APPLICATION OF CFD TECHNOLOGY IN WIND TURBINE ICING PROBER DESIGN

A series of numerical methods,which are suitable to design the shape and configuration of the icing prober for the horizontal axis wind turbine,are presented.The methods are composed of a multiple reference frame(MRF)method for calculating flow field of air,a Lagrangian method for computing droplet trajectories,an Eulerian method for calculating droplet collection efficiency,and an arithmetic for fast computing ice accretion.All the numerical methods are based on the computational fluid dynamics(CFD)technology.After proposing the basic steps and ideas for the design of the icing detection system,the shape and configuration of the icing prober for a 1.5 MW horizontal axis wind turbine are then obtained with the methods.The results show that the numerical methods are efficient and the CFD technology plays an important role in the design process.

Numerical Simulation of the Wake Generated by a Helicopter Rotor in Icing Conditions

The wake generated by the rotor of a helicopter can exert a strong interference effect on the fuselage and the horizontal/vertical tail.The occurrence of icing on the rotor can obviously make this interplay more complex.In the present study,numerical simulation is used to analyze the rotor wake in icing conditions.In order to validate the overall mathematical/numerical method,the results are compared with similar data relating to other tests;then,different simulations are conducted considering helicopter forward flight velocities of 0,10,20,50,and 80 knots and various conditions in terms of air temperature(atmospheric temperature degrading from−12°C to−20°C or from−20°C to−26°C).The results indicate that the rotor aerodynamic performance(i.e.,the lift-to-drag ratio distribution of the rotor disc)drops significantly once the rotor undergoes ice accretion.More importantly,the icing exerts a different influence of the wake dynamics depending on the atmospheric conditions.Interestingly,the rime-ice firstly occurs on the inner portion of rotor blades and then diffuses outward along the blade radial direction with the decrease in atmospheric temperature.

Thermal Feature Analysis of a New Hot-Air Anti-Icing Structure

Ice accretion on surfaces of the aircraft and engine is a serious threat to the flight safety.In this paper,a novel hot air anti-icing method is proposed based on the porous foam.Taking the NACA0012 airfoil as an example,the traditional thermal protection structure is proved to exist the deficiency in balancing the heat exchange caused by route loss of the heat.By dividing the hot chamber into multiple regions to fill with various foam metal,flow resistance characteristics and heat transfer characteristics for this protection mode are analyzed in order to derive the maximized benefit in anti-icing process.The calculation results reveal that,under the same condition,the region filled with foamed copper not only improves the temperature uniformity on the anti-icing area,but also achieves a better protection effect for enhancing heat transfer between the tube and the hot gas,averagely above 20℃higher than it without porous foam filling in surface temperature.Additionally,the minimum mass flow rate of the protection hot air is reduced by 16.7%.The gratifying efficiency of the porous filler in fortifying heat transfer confirms the potential of replacing the efficient but complex heat transfer design with simple structure filled with foam metal.

A spongy icing model for aircraft icing

Researches have indicated that impinging droplets can be entrapped as liquid in the ice matrix and the temperature of accreting ice surface is below the freezing point. When liquid entrapment by ice matrix happens, this kind of ice is called spongy ice. A new spongy icing model for the ice accretion problem on airfoil or aircraft has been developed to account for entrapped liquid within accreted ice and to improve the determination of the surface temperature when enter- ing clouds with supercooled droplets. Different with conventional icing model, this model identifies icing conditions in four regimes： rime, spongy without water film, spongy with water film and glaze. By using the Eulerian method based on two-phase flow theory, the impinging droplet flow was investigated numerically. The accuracy of the Eulerian method for computing the water collection efficiency was assessed, and icing shapes and surface temperature distributions predicted with this spongy icing model agree with experimental results well.

Numerical Investigation on Super-cooled Large Droplet Icing of Fan Rotor Blade in Jet Engine

Icing(or ice accretion) is a phenomenon in which super-cooled water droplets impinge and accrete on a body.It is well known that ice accretion on blades and vanes leads to performance degradation and has caused severe accidents.Although various anti-icing and deicing systems have been developed,such accidents still occur.Therefore,it is important to clarify the phenomenon of ice accretion on an aircraft and in a jet engine.However,flight tests for ice accretion are very expensive,and in the wind tunnel it is difficult to reproduce all climate conditions where ice accretion can occur.Therefore,it is expected that computational fluid dynamics(CFD),which can estimate ice accretion in various climate conditions,will be a useful way to predict and understand the ice accretion phenomenon.On the other hand,although the icing caused by super-cooled large droplets(SLD) is very dangerous,the numerical method has not been established yet.This is why SLD icing is characterized by splash and bounce phenomena of droplets and they are very complex in nature.In the present study,we develop an ice accretion code considering the splash and bounce phenomena to predict SLD icing,and the code is applied to a fan rotor blade.The numerical results with and without the SLD icing model are compared.Through this study,the influence of the SLD icing model is numerically clarified.