Improvement of an altitude test facility capability in glaciated icing conditions at DGA Aero-engine testing.
The A06 test facility designed for combustor testing in altitude has been modified to be converted in an icing facility for probe testing. The objective was to be able to simulate ice crystals conditions at high altitude, high Mach number and low temperature. This facility has been upgraded in several steps extending the median size of the ice crystals produced and the ice water content range. The aero-thermal and icing capabilities have been assessed during commissioning tests. Finally, in order to prepare the calibration of the facility, some measurement techniques for cloud characterization have been selected or developed, especially for cloud uniformity measurement.
CITATION: Hervy, F., Maguis, S., Virion, F., Esposito, B. et al., "Improvement of an Altitude Test Facility Capability in Glaciated Icing Conditions at DGA Aero-engine Testing," SAE Int. J. Aerosp. 8(1):2015, doi:10.4271/2015-01-2154.
Ice crystals have been identified as a threat for airplane affecting engines and probes . To understand and reproduce these phenomena, experimental tools have been developed.
Prior to this interest for glaciated icing conditions, some icing wind tunnels had the capability to produce ice crystals with median size of about one millimeter as defined in the CS requirements. These facilities were limited to ground level altitude. As ice crystal icing is associated to high altitude condition and engine compressor environment, it was useful to develop facilities able to simulate variable pressure.
Only two facilities could produce ice crystals conditions in altitude: RaTF at NRC  and PSL at NASA . The first facility is able to perform test on small component or specific set-up like compressor cascade and the second one is able to test engines of medium sizes. Such capabilities were missing in Europe.
In 2010, DGA Aero-engine Testing decided to develop this kind of capability in order to perform tests on probes. The center has a long experience in icing testing on probes, airfoils and engines in various facilities . The objective was to select a small test facility able to simulate low temperature and high altitude. A test facility, named A06, has been identified as the best candidate and has been upgraded to be able to simulate high Mach number and to generate ice crystals.
With the future integration of glaciated icing conditions and mixed phase in a new appendix of the requirements, means of compliance are needed for certification. At the same time, however, the representativeness of such conditions in facilities has to be improved to reproduce ice crystals with relevant properties like size, shapes and density as close as possible to natural ice crystals.
In 2012, the European project High Altitude Ice Crystals (HAIC) started . One of its objectives aims to improve some facilities in order to get acceptable means of compliance and useful experimental tools for the understanding of icing from glaciated and mixed phase conditions. As partner of this project, DGA Aero-engine Testing is involved in the upgrading of its A06 test facility.
Among the set of facilities available at DGA Aero-engine Testing, A06 is a small test facility designed for combustor testing in altitude and at low temperature. The tests performed are relight, flame out and stability limits of combustors. A new configuration has been proposed for icing. The overall facility scheme is described in Figure 1. It is an open circuit configuration.
A new cooling machinery with a two stages heat exchanger able to cool the airflow down to -48[degrees]C in total temperature and to adjust the humidity has been implemented. The maximum temperature difference between the dew/frost point and the air temperature is 24 [degrees]C allowing a humidity range from around 50% to 100% within a part of the total temperature operating range.
Two branches equipped with flowmeters allow working on two air flow rate ranges, below and above 1 kg/s, with good accuracy.
The ice generator is installed between the upstream valve and the test cell.
A small scale altitude test cell used for engineering research has been adapted because its interfaces are compatible with the pipe diameters of the facility. This test cell is 2 meters in length and 0.5 meter in diameter (Figure 2).
A converging nozzle has been designed and manufactured to be able to reproduce high Mach number up to 0.9. Due to the limitation of the exhaust compressor mass flow rate, the exit diameter of this nozzle is 100 millimeters.
The air flow is blown out by an exhaust compressor able to work down to 15 kPa without pressure losses. Its maximum standard mass air flow rate is of 4.2 kg/s.
IMPROVEMENTS OF ICE CRYSTALS GENERATION SYSTEM
There are two basic methods to produce ice crystals. One consists in shaving or grinding ice blocks. This method is used in ground level test facilities because it requires to store ice blocks and to feed the grinding machine regularly. The second method uses pneumatic atomizers with high atomization pressure ratio to freeze out the droplets produced. This system can operate continuously but the median size of the ice crystals is small (< 100 microns). The main drawback is that the time for complete freezing of the droplets must be shorter than the residence time between the spray bars and the test section, leading to some restrictions in the achievable maximum median size and maximum static temperature. In addition, another possible issue is the built-up of ice accretion if partially frozen or super-cooled droplets impinged on the facility walls.
In a first step, the technical solution selected has been the freeze out technique.
A straightener and a screen were installed upstream the ice crystal generator to get a good flow quality.
The spray grid is composed of four spray bars with one pneumatic atomizer each (Figure 4) Radial position can be adjusted to avoid ice accretion on the pipe or optimize cloud uniformity. It is also possible to use only one atomizer placed on the centerline if low ice water content is required or if there is some ice deposit accreted on the pipe wall with four atomizers (Figure 5).
Ice crystal generation is controlled by the air atomization pressure and temperature.
The distance between the ice crystal generator and the test section has been maximized to increase as far as possible the residence time for a complete freezing of the droplets (Figure 3).
Second step was the improvement of the ice crystal generator device to increase the sizes of the ice crystals produced.
Downstream the spray bars, partially frozen and/or super-cooled droplets are collected on rotating cylinders and the ice layer formed is scratched by a saw blade placed into the cylinder's wake. There are two alternated rows of cylinders to collect the droplets without generating important pressure losses.
It is possible to change the proportion between large and small ice particles by removing some cylinders and blades.
During commissioning, main performances have been explored to define the operating envelope of the facility.
Aero-Thermal Operating Envelope
The altitude-Mach number domain in Figure 8 shows that maximum Mach number is 0.9 for altitude from 15 kft to 36 kft. At higher altitude, maximum Mach number decreases because of the pressure loss and the minimum pressure achievable with the exhaust compressor. Even if low Mach numbers are achievable, the minimum value depends on the minimum air mass flow rate which can be measured accurately.
Static temperature and humidity envelopes are presented in Figure 9.
Static temperature in the test section is calculated from the total temperature, measured in the same plane than the spray bars and the Mach number.
Humidity is measured upstream in the branches where the flowmeters are located by capacitive or chilled mirror hygrometer. Static humidity in the test section is calculated using the total and static conditions.
Ice Particle Sizes
In order to measure the particles size in the test cell and because existing instrumentation could not be used due to size constraints and limited optical access, DGA Aero-engine Testing chose a non-intrusive technique based on shadowgraphy. The measuring device combine a high-resolution video camera and a 20 ns duration flash: the use of a ultra-short light source is essential for shooting sharp pictures of high speed moving particles. In order to calculate the droplet size distribution, an image processing software package computes a number versus particle size histogram. The main issue was to be able to measure particle sizes on a wide range from 10 microns to 1 mm at a distance between 300 and 500 mm. Such a range cannot be covered by the same optics so a macro-objective has been selected for large size (> 100 [micro]) and a long distance microscope for small size. The sensitivity is 25 microns/pixel for macro-objective and 3 microns/pixel for long distance microscope. First of all, shadowgraphy has been used to study the effectiveness of the full ice crystal generator.
Particles with size larger than 100 microns are not observed when spray bars are used alone (Figure 10). The shapes are close to a sphere even if there are not fully round. The ice crystal density is estimated to be the bulk density of ice.
When the full ice crystal generator is used, large particle are observed with maximum size of about 1 mm (Figure 11). The shapes are complex with aspect ratio different from one. The ice crystal density is not known but seems to be lower than the bulk density of ice.
The ice crystal generator has been tested for several total temperature from -40[degrees]C to -15[degrees]C and Mach number from 0.5 to 0.85 to define the envelope where it can operate continuously and the maximum bulk IWC that can be achieved. The criterion for success was the stability of the pressure loss of the ice crystal generator. Each condition was maintained during at least five minutes. The absence of super-cooled droplet has been checked by means of an accretion grid placed in the test section (Figure 3).
With the spray bars alone, the maximum IWC is close to 12 g/[m.sup.3] because only one atomizer can be used.
With the full system, spray bars and cylinders, the maximum IWC at low temperature is about 20 g/[m.sup.3]. This maximum decreases with increasing temperature. The limit is defined by the run back which leads to ice accretion forming on the saw blade supports.
Trials were done by using the spray bars with low air atomization pressure in order to produce super-cooled droplets. Unfortunately, super-cooled droplets, and then mixed phase, cannot be simulated because it is not possible to maintain stationary conditions due to the ice accretion forming on the pipe walls and the downstream valve.
PREPARATION TO CALIBRATION
The calibration of the cloud consists in three steps: particle size distribution, total water content and cloud uniformity.
Particle size distribution will be measured using modular High Speed Imaging. Total water content will be measured with the isokinetic probe developed by Cranfield University. The main challenge for DGA Aero-engine Testing was to find a technique that could be used for qualitative cloud uniformity measurement because it is not possible to use icing grid: with ice crystals, there is no ice accretion on the bars of the grid.
Because optical access is limited and it is difficult to perform a 2D mapping using a TWC probe, an alternative non-intrusive solution based on a laser sheet scattering imagery technique has been set up. Similar technique has been developed by NASA .
The set-up is described on Figure 13. The laser sheet plane is perpendicular to the axis of the A06 test facility and is created by means of the hemi-cylindrical lens from a laser beam. A camera is placed in front of the laser with a small angle off-axis plane is order to get the best contrast depending on the intensity of the scattered light.
The raw images (Figure 14) are corrected taking into account the laser sheet intensity profile and the view angle from the camera. A mask is applied to remove light interferences outside the exit nozzle section. In the example on Figure 15, the corrected intensities are relative to the intensity value on the centerline.
Improvements done on the A06 facility leads to extend the ice crystals size range capability from 10 microns to 1 millimeter for high Mach number, high altitude, high IWC and low temperature. Nevertheless, the maximum operating static temperature depends on the ice water content which decreases with increasing temperature. In addition, mixed phase cannot be achieved with current configuration.
Basic technique based on shadowgraphy has been used to characterize the ice particle size and shapes. A non-intrusive technique has been developed for cloud uniformity measurement.
Next challenge is the calibration.
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As part of the HAIC project, this work has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement n[degrees]ACP2-GA-2012-314314.
ATF - Altitude test facility
CS - Certification Specification
HAIC - High Altitude Ice Crystals
HSI - High speed imaging
IKP - Isokinetic probe
IWC - Ice water content
NASA - National Aeronautics and Space Administration
NRC - National Research Council
PSD - Particle size distribution
PSL - Propulsion system laboratory
RaTF - Research altitude test facility
Franck Hervy, Severine Maguis, and Francois Virion DGA Essais Propulseurs
Biagio Esposito CIRA Scpa
Hugo Pervier Cranfield Univ.
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|Author:||Hervy, Franck; Maguis, Severine; Virion, Francois; Esposito, Biagio; Pervier, Hugo|
|Publication:||SAE International Journal of Aerospace|
|Date:||Sep 1, 2015|
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