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Support of Icing Tests (Runback-Ice behaviour of surfaces) and Icing Mechanisms

Final Report Summary - ICE-TRACK (Support of icing tests (runback-ice behaviour of surfaces) and icing mechanisms)

Executive summary:

Test subject was specimens of coatings intended for reducing the adhesion force to run-back ice.

Work objective was to obtain data on run-back ice adhesion to coatings, developed by the IFAM firm, and to choose among the tested coatings those, which has the minimal shear adhesion force to run-back ice.

In progress of investigations the test procedure was being worked out, the process of run-back ice formation was observed by eye and recorded with video camera, the parameters of water airflow were registered. The run-back ice thickness and adhesion force to coating surface were measured.

As a result of the research the comparative characteristics of different coatings in the runback ice conditions have been determined.

A review of basic icing mechanisms was performed. On the basis of the experimental results, the processes, which are concomitant to run-back ice formation, were analysed and physicomathematical modelling of these processes was done. The criteria for selection of anti-icing coatings were worked out.

The significance of the work consists in the fact that the worked out procedure allows developing research guide lines to creation of the materials with low adhesion force to run-back ice.

Project context and objectives:

The project concerns the problem of aircraft in-flight icing. This dangerous phenomenon often occurs when an airplane flies through the clouds which contain water droplets (SCDs). Thou their temperature is below 0 degrees of Celsius, yet these super-cooled droplets are liquid, but in a metastable state. When such droplets strike the aircraft surface, they freeze in a flash to produce ice on forward-facing edges of the wings, tail, engine inlet and so on. The probability of icing depends on a combination of a number of parameters characterising the surrounding atmospheric conditions: liquid water content (LWC), water droplets temperature, size range and size distribution, length and height of the clouds. Despite that specialists throughout world actively work on the aircraft ice protection problem, the current level of understanding of icing physics is still insufficient.

Ice masses emerged on the surfaces of aircraft elements lead to deterioration of aircraft lift-drag ratio and to increase of the whole air vehicle weight. Aircraft icing takes place in one in a ten flight on average. Experts attribute to icing about 7 % of air accidents.

In practice, in order to ensure air flight safety, it is necessary to provide aircrafts with anti-icing protection, i.e. to minimise ice growth on surface of aircraft / aeroengine elements. The thermal anti-icing systems are widespread. Such systems melt ice on the protected surface due to heat supply. These systems allow for preventing ice formation in the areas of water droplets precipitation. However, the liquefied water runs back along the surface and freezes in unheated areas. Thus, the barrier of runback ice forms along the path of the water stream and air-water flow.

The following processes require serious attention of researchers in order to improve ice protection of flying vehicles:

(a) ice formation on surfaces without anti-icing systems;
(b) run-back ice formation on surfaces with thermal anti-icing systems;
(c) ice adhesion to surfaces.

Depending on the adhesion force to the surface, run-back ice is either thrown off by airflow or keeping on the surface it grows in size and weight due to both freezing water and drops of air-water flow. Excessive growth of run-back ice may lead to violation of aircraft control and evoke a crash. Therefore, the problem of creating materials and coatings with low adhesion force to run-back ice is very important.

In the framework of the Clean Sky programme, the IFAM firm is developing the anti-icing coatings with low adhesion to run-back ice. If such coatings were developed, it would result in the sufficient energy consumption decrease of anti-icing systems and flight safety improvement.

The ICE-TRACK project emerged as an answer to the tasks defined in the 'Support of icing-tests (runback-ice behaviour of surfaces) and icing mechanisms' topic of the 3rd additional call for proposals of the Clean Sky Joint Technology Initiative (JTI). Its useful role is to make an advance towards a better understanding of the runback ice formation mechanisms on aircraft wings, empennage and engine air intakes through experimental and theoretical research and to provide recommendations on the choice of efficient ice protection coatings for flying vehicles. This will help to increase safety of air flights in icing conditions. Thus, the project aims to collaborate its effort towards achieving the double and five-fold reduction of aircraft accident rate in the short and long terms respectively, which is one of the goals set for the year 2020 by the Strategic Research Agenda (SRA) provided by the Advisory Council for Aeronautics Research in Europe (ACARE).

In order to solve the problems related to runback ice formation on the aerodynamic surfaces and aeroengine air intakes, the joint team of the two leading Russian institutes, namely Central Aerohydrodynamical Institute (TsAGI) and Central Institute of Aviation Motors (CIAM), was to perform the following work within the frames of the ICE-TRACK project:

(a) to conduct tests of approximately 30 profile samples with and without coatings provided by the relevant SFWA member; the tests will be accomplished in artificial icing conditions in the range of outside temperatures between 0 and -30 degrees of Celsius, LWC 0.2-2.5 g / m3, air flow speed up to 100 m / s and with given water droplets sizes;
(b) to measure iced area on the tested profiles is measured;
(c) to measure forces of ice adhesion to the surfaces of the profiles;
(d) to investigate runback ice formation processes in case of heated leading edge;
(e) to provide recommendations concerning the choice of the most promising surfaces.

The contribution of teams in the ICE-TRACK project was that TsAGI's wind tunnel provided cheap preliminary test results, while CIAM using its more advanced U-9M test bench wind tunnel concentrated on deeper studies of the profile samples chosen after the preliminary test campaign. This approach was proposed to increase the economic effectiveness of the project while maintaining a high level of scientific results. Both teams have researchers with experience in theoretical works on icing problems and understand well the problem posed by the runback ice forming in the regions behind the leading edge.

Project results:

Introduction

In the framework of the Clean Sky programme, the IFAM firm developed the anti-icing coatings with different physical and chemical properties that can provide the low shear adhesion force to run-back ice. If such coatings were developed, it would result in the sufficient energy consumption decrease of anti-icing systems and flight safety improvement. Work objective was to obtain data on run-back ice adhesion to developed coatings, and to choose amongst the tested coatings those, which has the minimal shear adhesion force to run-back ice.

The test procedure for specimens of the developed coatings has been worked out and the comparative characteristics of 32 specimens have been determined. The test of coating specimenes was carried out in the wind tunnel, which reproduced the natural conditions of icing for the model of an airfoil with the heated leading edge in the air-water flow. The coating specimens were placed on the airfoil surface behind the zone of heating.

During the tests the process of run-back ice formation was observed by eye and recorded with video camera, the parameters of water-air flow were registered. The run-back ice thickness and adhesion force to coating surface were measured.

A review of basic mechanisms of icing was performed. On the basis of the experimental results, the processes, which are concomitant to run-back ice formation, are analysed and physicomathematical modelling of these processes is done. The criteria for selection of anti-icing coatings are worked out.

Description of the experimental set-up and research object

Experimental investigations on icing of the different coatings were carried out in the ejector type wind tunnel, which generated water-air flow of cold air.

The tunnel is equipped with the water spraying system and measurement tools for measuring both the technological parameters of the set-up and physical characteristics of the flow as well as the investigated models.

The wind tunnel is the ejector, which has 25 nozzles. The ejecting gas is compressed air at the pressures from 1.5 to 5 ga atm. The ejected air at the temperatures from -5 to -25 degrees of Celsius is captured from outdoor in the cold season. The ejection coefficient is k 7. The total length of wind tunnel channel equals 2 910 mm. The cross-section sizes are 200 x 200 mm. The flow velocity in the channel is controlled in the range from 10 to 100 m / s due to changing the ejecting air pressure.

The water spraying system of the set-up contains the water tank of 25 l capacity, water filter, pipeline with demountable injectors and 40 l tank of compressed nitrogen. The injectors and pipeline are equipped with electric heaters to prevent water freezing. Water is supplied to the injector by forcing it out the tank with nitrogen at the pressure 5 - 7 ga atm.

The water-air flow in the wind tunnel channel is generated due to water injection in the air stream through the swirl injector located at the distance 2335 mm from the tunnel exit cross-section.

Coating specimens are aluminium plates of 130 x 150 x 3 mm sizes coated from one side. In testing two airfoil models are used: No 1 and No 2. Both airfoils are identical in design. Airfoil No 2 is tested in the horizontal position only. The specimen is mounted from above, meanwhile a duralumin plate is always placed from below. Airfoil No. 1 is tested both in horizontal and vertical positions. In most test cases airfoil No. 1 was used.

The model design allows the various coating specimens to be mounted and removed. The leading edge of the airfoil is heated by a resistance heater, its power is approximately 300 W.
The bottom of airfoil model No 1 is perforated. It allows avoiding heat transfer from the heater along the airfoil, thus the model remains unheated.
Three thermocouples are mounted in the airfoil bottom to control the model temperature in testing.

Measurement tools

In testing the icing process in the wind tunnel it is measured the water-air flow characteristics (velocity, temperature, LWC, droplet sizes) and adhesion force of ice to coating.

The water-air flow velocity was measured with the rake of Pitot tubes in the preliminary test. Based on its results, the dependencies of the flow velocity from the pressure of compressed ejecting air were plotted. Then these dependencies were used to establish the flow velocity.

The temperature of the water-air mixture at the exit of the tunnel channel was measured with a alcohol thermometer and controlled by a temperature sensor of resistance type. The LWC was estimated from the injector discharge characteristic plotted beforehand as the dependency of the discharge from pipeline pressure. In testing the swirl injector was used, its outlet hole diameter was 0.45 mm. Before testing the uniformity of LWC distribution was checked from inspection of grate icing, the rate being mounted in the tunnel exit section.

The sizes of the drops, sprayed by the injector, were measured in the preliminary test using the special drop sampler (impactor). In this device the trapping drops are deposited on the glass slides coated with polymethyloxan (silicon) fluid and then they are photographed through microscope.

For measuring shear adhesion force of run-back ice to coating it was designed and fabricated the device consisting of an adhesiometer itself and thermal knife.
The thermal knife is assigned for separating a small portion of specified ice area from the whole ice body formed on the coating.
The construction arrangement of the thermal knife has a form of tetragonal frame made from highly heat-conducting material (brass). The frame supports the resistance heaters, which heat the frame bottom up to specified temperature (10 - 30 degrees of Celsius). The thermal knife melts off the rectangular in plan ice portion with the sizes of 20 to 40 mm.

Adhesiometer measures the shear force of ice adhesion to the coating area selected with the thermal knife. The designed adhesiometer is a mechanical spring device, which measures shear force of adhesion in the range of 10 to 100 N. The device is calibrated prior to measurements. If ice of adhesion area equalled 20 x 40 mm is not shifted in the course of measuring under driving force up to 100 N, the selected ice area is diminished half of quarter as much.

Test procedure
The adhesion properties of the developed coatings was tested in the wind tunnel, with the natural conditions for run-back ice formation being simulated.
The test strategy was as follows: The airfoil model with coating specimens was mounted in the tunnel. A coating specimen was placed on the airfoil upper surface, while, as a rule, a polished aluminium plate was downside for comparison. Voltage was applied to the heater, which warmed airfoil edge up to approximately 8 to 10 degrees of Celsius. Then compressed air under pressure required for specified flow velocity was supplied in the wind tunnel. Coatings were tested at flow velocity of approximately 40 and 80 m / s. On stabilising the airflow parameters the injector sprayed water to the wind tunnel channel. From the moment of water supply the process of run-back ice formation was supervised. The run-back ice position on the coating specimen was controlled due to adjustment of the airfoil edge temperature. The thermal conditions were selected so that the ice barrier was located near the middle of the specimen.

In testing the process of run-back ice formation was recorded with video camera and the temperatures of the water-air flow and airfoil edge were registered. It was registered the time period of run-back ice growth up to the moment of ice shedding due to water-air flow pressure. If ice was not shed and its thickness was not practically changed, water spraying through the injector was ceased and the airfoil edge heater was switched off. After equalising airfoil edge and airflow temperatures, air supply to the wind tunnel channel stopped and ice thickness was measured. For measurement of the ice adhesion force to surface, the ice thickness reached approximately 5 - 10 mm.

Test results

The results of carried out researches are obtained in the wind tunnel at the following parameters of water-air flow:
(1) Flow velocity in the tunnel channel was approximately 40 or 80 m / s.
(2) Flow temperature in the channel was the same as in the atmosphere (from -5 to -20 degrees of Celsius).
(3) LWC was approximately 0.75 g / m3 at flow velocity of 80 m / s and 1.4 g / m3 at 40 m / s.
(4) Arithmetical mean diameter of drops was approximately 30 µm.

The process of run-back ice formation was recorded with a video camera. Photos show, as an example, the successive time stages of run-back ice formation on a certain coating. Visual observations indicated that the process of run-back ice formation corresponded to the natural one.

Conclusions

(1) The following devices were designed to investigate the influence of the various coatings on the adhesion force of run-back ice to the surface: the heated airfoil, which make it possible to form run-back ice and to settle specimens under research; and the thermal knife for separating the ice fragment to measure the adhesion force. It was measured the parameters of airflow and icing (LWC, droplet sizes, uniformity of flow velocity LWC fields). The test procedure was worked out.

(2) On producing ice at the flow velocities of 65 to 80 m / s and outdoor temperatures of -10 to -18 degrees of Celsius, the specific force of ice adhesion to the main number of coatings was in the range of 12 to 92 N / cm2 (shear force of ice along the surface. These values are sufficient for the absence of spontaneous ice shedding at the ice thickness of about 30 to 40 mm and the length (along the stream) of 70 to 90 mm for the flow velocity of 65 to 80 m / s. If the spontaneous ice shedding occurred for these specimens in some cases, the same occurred for the reference specimens: duralumin and polished duralumin without special coatings.

(3) During tests carried out at V = 76 m / s and temperatures of -5 to -5.5 degrees of Celsius it was observed the spontaneous shedding of run-back ice at its thicknesses of 13 to 25 mm and lengths (along the flow) of 70 to 90 mm. At this, the similar spontaneous ice shedding occurred for the reference specimens: duralumin and polished duralumin without special coatings.

(4) For specimen with specific adhesion force of produced ice to coating less than 12 N / cm2 and outdoor temperature of -15 degrees of Celsius, the run-back ice thickness grew considerably slower than for downside duralumin plate. Perhaps, it was due to more intensive water blow-off for the coating. At the outdoor temperature of -17 degrees of Celius the periodic spontaneous ice shedding occurred at the ice thickness of about 10 to 12 mm.

(5) A simple mathematical model is developed, which illustrate tendencies of dependences of the hydrothermodynamic parameters of different liquid fragments, moving along on a solid surface (fluid film, rivulet, drop) on a flow speed rate and a wetting angle. It is shown that the increase of a wetting angle significantly increases a drops motion velocity along a surface and prevent them from freezing.

Potential impact:

The ICE-TRACK project useful role was to make an advance towards a better understanding of the runback ice formation mechanisms on aircraft wings, empennage and engine air intakes through experimental and theoretical research and to provide recommendations on the choice of efficient ice protection coatings for flying vehicles. This will help to increase safety of air flights in icing conditions. During flights in harsh atmospheric conditions (large SCDs, ice rain and ice drizzle), ice masses emerge on surface of aircraft elements, which leads to deterioration of aircraft lift-drag ratio and to increase of the whole air vehicle weight.

The probability of icing depends on a combination of a number of parameters characterising the surrounding atmospheric conditions: LWC, water droplets temperature, size range and size distribution, length and height of the clouds. The most probable combinations of these parameters are summarised in normative documents. The United States and the European Union use FAR and CS documents, in Russia currently there exist aviation rules.

Despite that specialists throughout world actively work on the aircraft ice protection problem, the current level of understanding of icing physics is still insufficient. In practice, in order to ensure air flight safety, it is necessary to provide aircrafts with anti-icing protection, i.e. to minimise ice growth on surface of aircraft / aeroengine elements. This goal can be attained through the following means:

(a) structural solutions for minimisation of water drops precipitation onto the protected surfaces;
(b) methods providing periodical spontaneous shaking the ice off in small portions and preventing excessive ice growth;
(c) anti-icing systems preventing ice growth or providing its periodic removal;
(d) anti-icing coating;
(e) combined methods.

Experience of aviation industry throughout the world shows that in general the existing methods of ice protection provide solution to the air vehicle anti-icing protection effectiveness problem.

In the cases where thermal anti-icing systems are used, such systems usually are built to have incomplete water vaporisation in order to save energy. These systems allow for preventing ice formation in the areas of water droplets precipitation. However, the liquefied water flows back along the surface and freezes in typical runback ice shapes in unheated areas.

Run-back ice appearing on aerodynamic surfaces can substantially lower aerodynamic characteristics of an aircraft, while runback ice inside air intake channels can lead to intolerable damages of the aeroengine elements. Thus, the following processes require serious attention of researchers in order to improve ice protection of flying vehicles:

(a) ice formation on surfaces without anti-icing systems;
(b) runback ice formation on surfaces with thermal anti-icing systems;
(c) ice adhesion to surfaces.

The main scientific results of the ICE-TRACK project are the newest results in:

(a) characteristics of provided coatings in term of their behaviour in icing conditions;
(b) a deeper insight into physics of icing processes based on the latest available relevant knowledge from outside the project and the experimental results obtained during the project;
(c) development of recommendations for necessary characteristics of materials with anti-icing properties on the basis of physical and chemical understanding of icing process.

The above scientific results will help to bring about more effective methods of ice protection, relieving aircrafts from runback ice unevenness on aerodynamics surfaces and from additional weight. This will lead to increasing aircraft aerodynamic stability in icing conditions, lower drag losses and therefore decrease fuel consumption.

As regards to the goals stated by the (ACARE) towards the year 2020, the project results will contribute their part to the following corresponding tasks:

(a) reduction of air traffic accident rate by 50 % in the short term and by 80 % in the long term;
(b) reduction of aircraft direct operating costs by 20 % in the short term and by 50 % in the long term through improved aircraft performance;
(c) reduction of fuel burn and carbon dioxide (CO2) emissions by 50 % per passenger kilometer in the long term through improved efficiency of aircraft operation.

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