Dan Fuleki

Fundamental Ice Crystal Accretion Physics Studies

Ice accretion within an engine due to ice crystal ingestion is being investigated because of numerous engine power-loss events associated with high-altitude convective weather. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. Two weeks of testing occurred at the NRC Research Altitude Facility in November 2010. The tests utilized a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15 °C. Ice and liquid water contents ranged up to 20 and 3 g/m3, respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leading-edge ice growth to a thickness greater than 15 mm in 3 min. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared correlate with the wet-bulb temperature, which was estimated to be above 0 °C in tests at 93 kPa and below 0 °C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic performance of a compressor component such as a stator blade to degrade significantly, and could damage downstream components if shed.

Fundamental Study of Mixed-Phase Icing with Application to Ice Crystal Accretion in Aircraft Jet Engines

This paper describes experiments performed in an altitude chamber at the National Research Council of Canada (NRC) as the first phase of a joint NRC/NASA program investigating ice crystal accretion in aero engines. The principal objective was to explore the effect of wet bulb temperature Twb (dependent on air temperature, humidity and pressure) on accretion behavior, since preliminary results published in an earlier paper indicated that well-adhered accretions are only possible at Twb 0°C in all tests. The limited test results confirmed that accretion behavior is very sensitive to Twb, which is in turn strongly related to pressure since evaporative cooling increases with decreasing pressure. Humidity and total temperature did not appear to have an independent effect on accretion behavior. Accretions, often resembling glaze ice, formed at Twb 0°C ice deposits were observed to be slushy, poorly adhered and shed frequently. The size of such deposits appeared to be a non-linear function of the freestream ice water content (IWC), becoming much larger at high IWC.

Experimental Studies of Mixed-Phase Sticking Efficiency for Ice Crystal Accretion in Jet Engines

Aircraft jet engines operating at high altitudes in ice crystal clouds can experience operational problems and/or damage resulting from accretion of ingested ice crystals within the compressor. It is believed the ice crystals partially melt, allowing them to stick to internal components. A method for modelling such mixedphase accretion is required to de-risk new engine designs, modify existing designs with such icing issues and define critical operating points for scrutiny in proposed ice crystal certification tests. This paper presents preliminary results for a modelling approach which treats the accretion process as strictly a sticking phenomenon, largely ignoring heat transfer, phase change, runback and other location-dependent effects commonly used in the analysis of supercooled water icing. Ice-on-ice growth is described by a sticking efficiency, defined as the fraction of the mixed-phase impinging mass flux which remains on the surface (i.e. sticks). Experimental results are presented for 3 test articles tested in a small mixed-phase icing tunnel located in an altitude chamber (Research Altitude Test Facility or RATFac) at the National Research Council of Canada. These results show that the sticking efficiency is highly correlated with the ratio of liquid water content (LWC) to total water content (TWC) in the freestream, reaching a maximum value of 0.4-0.5 at melt (LWC/TWC) ratios in the approximate range 10-20%, as measured with a multi-element probe. It is shown that sticking efficiencies are largely independent of TWC, Mach number (M) and particle size at normal incidence (i.e. at the stagnation point) at these melt ratios, at least in the limited ranges of these variables investigated, but are strongly dependent on these parameters at oblique impingement angles. It is also shown that accretions can grow to a very large size at an almost constant rate at high levels of TWC. The experimental results are used to develop an erosion-based semi-empirical accretion model which at least partially explains this super-growth phenomenon and predicts most of the experimental results with acceptable fidelity. The model predicts that the almost unlimited growth observed in the experiments is possible at lower Mach numbers (e.g. 0.25) for TWC levels exceeding ~10g/m, when the sticking efficiency remains finite at all particle impingement angles. The model also predicts that such growth is unlikely for higher Mach numbers (e.g. 0.4), at least for the 45μ (MVD) particles to which the model is applied. Smaller particles will likely extend the Mach number range over which the sticking efficiency remains finite.

Altitude Scaling of Ice Crystal Accretion

This paper describes experiments performed in an altitude chamber at the National Research Council of Canada (NRC) as the first step towards developing altitude scaling laws and procedures that will possibly allow aero-engines to be certified for operation in ice crystal clouds at high altitude by testing in sea level facilities. The principal objective was to test the hypothesis that accretion within a compressor due to ice crystal ingestion occurs when the local ratio of freestream liquid water content (LWC) to total water content (TWC) lies within a critical range at an accretionsusceptible location. If this hypothesis is correct, the local LWC/TWC ratio is the key parameter that must be matched in tests at low and high pressures to match accretions. Experiments were conducted in a small wind tunnel with an axisymmetric test article, consisting of a hemispherical nose attached to a conical afterbody, at a fixed TWC over a range of LWC/TWC ratios at (absolute) pressures of 34.5 kPa and 69 kPa to test the hypothesis. The LWC/TWC ratio was varied by changing the wet bulb temperature. Accretion steady-state volumes and growth rates measured at the two pressures were compared at conditions which were analytically predicted to produce matched LWC/TWC ratios. Good agreement was achieved in all cases. Accretion growth was greatest for LWC/TWC ratios in the range 10-25%. Additional tests demonstrated that wet bulb temperature, which was identified as an important variable in earlier studies, had little influence on accretion growth beyond its effect on LWC/TWC (i.e. ice particle melting). Tests were also conducted to determine whether accretion growth scales linearly with TWC at constant LWC/TWC. Those tests confirmed that not only does the accretion growth rate in the early growth phase scale in direct proportion to TWC , but so does the final size of the accretion. A simple semi-empirical model for predicting this behavior is described. While most of the tests were conducted with an ice particle median volumetric diameter of 45μ, some of the scaling tests were repeated with larger particles, which produced smaller accretions.

Particle Size Effects on Ice Crystal Accretion

This paper describes the commissioning of a new test apparatus intended to simulate an inner-compressor duct bleed slot. It also identifies, for the first time, that ice crystal particle size plays an important role in the ice crystal phenomenon. Data and sample images of accretion are presented for wet bulb temperatures near freezing. The effect of wet bulb temperature and particle size on the natural melting of ice crystals is investigated. In addition, the erosion of surface accretion by ice crystal particles is discussed.

Preparation for Scaling Studies of Ice-Crystal Icing at the NRC Research Altitude Test Facility

This paper describes preparation for ice-crystal icing scaling work utilizing the Cascade rig at the National Research Council (NRC) of Canada’s Research Altitude Test Facility (RATFac). Tests supporting this work and continuing the collaboration between NASA and NRC on ice-crystal icing took place between March 26 and April 11, 2012. The focus was on several aspects but emphasized characterization of the RATFac cloud including watercontent and test-section uniformity as well as particle-size measurements. Water content measurements utilized the Science Engineering Associates (SEA) Multi-Element probe while cloud uniformity measurements used light scattering from particles passing through a laser sheet. Finally, particle size-spectra measurements used two developmental shadowgraph systems. Details of these measurements as well as selected results are presented. An analysis algorithm is presented that interprets mixed-phase measurements from the SEA probe using calibrations from individual water and ice clouds. The analysis is applied to one mixedphase data set generated with a glaciated cloud combined with supplemental water. The test section temperature was below freezing to prevent the natural melting of the ice crystals. The analysis algorithm relies on the measurement of test-section humidity to account for cloud evaporation. Results of the cloud-uniformity measurements using scattered light suggest that the measured intensity is a good first-order measurement of concentration, independent of the water phase. Steeper intensity gradients across the test section are observed with increasing ice-water content. For particle-size measurements, both shadowgraphy methods provide high-quality images of the particles. These images will be processed to establish particle-size distributions and morphology characteristics. The results from this work will help guide future ice-crystal icing research including scaling studies.

Particle Size Effects on Ice Crystal Accretion - Part II

This paper describes ongoing research intended to simulate ice accretion in an intercompressor duct bleed slot resulting from the ingestion of altitude ice crystals. The authors have previously shown that ice crystal particle size plays an important role in the ice crystal accretion phenomenon. It was also shown that ice crystal particle size affects the degree of natural melt that occurs for a given aerodynamic condition. The data presented herein decouples the effects of ice particle melt and particle size distribution to generate accretions with the same ratio of freestream liquid-to-total water fraction. The effects of wet bulb temperature and ice particle size on the natural melting of ice crystals are discussed. An ice preservation procedure is followed to allow tracings of the accretion to be taken along the test article. Ice crystal particle size distribution is characterized using a shadowgraphy imaging technique. Finally, the reduction in accretion rate relative to the theoretical maximum rate of surface accretion by ice crystal particles is discussed. The test article simulates a forward facing, inclined endwall bleed slot in a gas turbine compressor as a simplified two-dimensional representation. The geometry, having a surface inclined 20° to the incoming flow, proved to be susceptible to mixed phase ice crystal accretion. Particle size and particularly the large particle tail of the distribution had a significant impact on the magnitude of accretion under mixed phase test conditions for wet bulb temperatures above and below 0°C. The leading edge growth rates were found to be 1/4 to 1/9 of the theoretical growth rate suggesting that erosion, splashing, particle bounce and other loss mechanism rates are significant. The ice tracings were used to estimate an accretion mass for a hypothetical large bypass ratio gas turbine. It was found that approximately 4kg of ice could be generated should the inter-compressor duct be exposed to the conditions tested for 5 minutes.

Development of a Sensor for Total Temperature and Humidity Measurements under Mixed-Phase and Glaciated Icing Conditions

This paper presents a critical development to a prototype sensor that is capable of measuring total air temperature and humidity in a mixed-phase environment, consisting of liquid water droplets and ice crystals. The sensor has fast and stable measurement response under particularly challenging mixed-phase conditions. Total temperature and humidity levels measured with the probe are in good agreement with the results of analytical energy and moisture balances.

Ice Crystal Accretion Test Rig Development For A Compressor Transition Duct

Ingestion of ice crystals into aircraft gas turbine engines have been shown to trigger partial or complete power loss. Although the ice crystal phenomenon has been recognized since the early 1950’s, it was not until the mid-1990’s that significant attention had been given to it with a key event being a flight campaign conducted with a small commuter aircraft which demonstrated ice crystals to be responsible for engine power loss. Although flight and engine testing have revealed ice crystals to be detrimental to gas turbine engine operation, these are not ideal test platforms to observe the ice crystal phenomenon due to limited access for instrumentation and visual observations. This paper discusses the development of an ice crystal test system used to simulate the ice crystal environment seen in a gas turbine compressor while maintaining visual accessibility and ease of instrumentation. This test system consists of a method to produce a range of ice crystal environments, the ability to vary airflow conditions in the rig and a static test section which simulates a gas turbine compressor transition duct. This system has been successful in producing a wide range of ice crystal test conditions and has shown significant ice accretion can occur on surfaces above 0 o C while allowing for visual observations and recording of temperature data during the accretion phenomenon.

Ice Accretion Measurements on an Airfoil and Wedge in Mixed-Phase Conditions

This paper describes ice accretion measurements from experiments conducted at the National Research Council (NRC) of Canadas Research Altitude Test Facility during 2012. Due to numerous engine power loss events associated with high altitude convective weather, potential ice accretion within an engine due to ice crystal ingestion is being investigated collaboratively by NASA and NRC. These investigations examine the physical mechanisms of ice accretion on surfaces exposed to ice crystal and mixed phase conditions, similar to those believed to exist in core compressor regions of jet engines. A further objective of these tests is to examine scaling effects since altitude appears to play a key role in this icing process.