Peter M. Struk

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.

Inverse influence of initial diameter on droplet burning rate in cold and hot ambiences: a thermal action of flame in balance with heat loss

Abstract Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K.

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.

Single droplet combustion of decane in microgravity: experiments and numerical modelling

This paper presents experimental data on single droplet combustion of decane in microgravity and compares the results to a numerical model. The primary independent experiment variables are the ambient pressure and oxygen mole fraction, pressure, droplet size (over a relatively small range) and ignition energy. The droplet history (D2 history) is non-linear with the burning rate constant increasing throughout the test. The average burning rate constant, consistent with classical theory, increased with increasing ambient oxygen mole fraction and was nearly independent of pressure, initial droplet size and ignition energy. The flame typically increased in size initially, and then decreased in size, in response to the shrinking droplet. The flame standoff increased linearly for the majority of the droplet lifetime. The flame surrounding the droplet extinguished at a finite droplet size at lower ambient pressures and an oxygen mole fraction of 0.15. The extinction droplet size increased with decreasing pressure. The model is transient and assumes spherical symmetry, constant thermo-physical properties (specific heat, thermal conductivity and species Lewis number) and single step chemistry. The model includes gas-phase radiative loss and a spherically symmetric, transient liquid phase. The model accurately predicts the droplet and flame histories of the experiments. Good agreement requires that the ignition in the experiment be reasonably approximated in the model and that the model accurately predict the pre-ignition vaporization of the droplet. The model does not accurately predict the dependence of extinction droplet diameter on pressure, a result of the simplified chemistry in the model. The transient flame behaviour suggests the potential importance of fuel vapour accumulation. The model results, however, show that the fractional mass consumption rate of fuel in the flame relative to the fuel vaporized is close to 1.0 for all but the lowest ambient oxygen mole fractions.

Development of a Coupled Air and Particle Thermal Model for Engine Icing Test Facilities

• Air and water vapor are treated as ideal gases• Air is continually well mixed• No supersaturation• 1-D air and particle flow• Dilute system (no particle interaction)• Particles are spherical• Discrete particle size distribution (bins)• Uniform temperature within the particle• Supercoolingcan occur • Mixed phase particles are not spatially resolved• Phase change occurs at particle surface at particle temperature• Adiabatic tunnel walls• The flow of particles and air is a continuous stream• The fundamental CV is adiabatic and mass is conserved

A Model to Assess the Risk of Ice Accretion due to Ice Crystal Ingestion in a Turbofan Engine and its Effects on Performance

4The occurrence of ice accretion within commercial high bypass aircraft turbine engines has been reported under certain atmospheric conditions. Engine anomalies have taken place at high altitudes that were attributed to ice crystal ingestion, partially melting, and ice accretion on the compression system components. The result was degraded engine performance, engine roll back, compressor surge and stall, and flameout of the combustor. As ice crystals are ingested into the fan and low pressure compression system, the air temperature increases and a portion of the ice crystals melt. This allows the ice-water mixture to stick to the metal surfaces of the compressor components. The resulting accretion causes a blockage on stationary components such as the stator vanes, and subsequently results in the deterioration in performance of the compressor and engine. The main focus of this research is the development of a computational tool that can estimate whether there is a risk of ice accretion by tracking key parameters through the compression system blade rows at all engine operating points within the flight trajectory. The tool has an engine system thermodynamic cycle code, coupled with a compressor flow analysis code, and an ice particle melt code that has the capability of determining the rate of sublimation, melting, and evaporation through the compressor blade rows. Assumptions are made to predict the complex physics involved in engine icing. Specifically, the code does not directly estimate ice accretion and does not have models for particle breakup, or erosion. Two key parameters have been suggested as conditions that must be met at the same location for ice accretion to occur: the local wet-bulb temperature to be near freezing and below, and the minimum local melt ratio must be above 10%. These parameters were deduced from analyzing normalized laboratory icing test data. These two parameters are the criteria that are used to determine whether ice accretion due to ice crystals is possible in an engine, and are used to identify the specific blade row where it could occur. Once the possibility of accretion is determined from these parameters, the degree of blockage due to ice accretion on the local stator vane can be estimated from an empirical model of ice growth rate and time spent at that operating point in the flight trajectory. The computational tool can be used to assess specific turbine engines to their susceptibility to ice accretion in an ice crystal environment.

Experimental Investigation of Solder Joint Defect Formation and Mitigation in Reduced-Gravity Environments

This paper documents a research effort on reduced gravity soldering of plated through hole joints which was conducted jointly by the National Center for Space Exploration Research, NASA Glenn Research Center, and NASA Johnson Space Center. Significant increases in joint porosity and changes in external geometry were observed in joints produced in reduced gravity as compared to normal gravity. Multiple techniques for mitigating the observed increase in porosity were tried, including several combinations of flux and solder application techniques, and demoisturizing the circuit board prior to soldering. Results were consistent with the hypothesis that the source of the porosity is a combination of both trapped moisture in the circuit board itself, as well as vaporized flux that is trapped in the molten solder. Other topics investigated include correlation of visual inspection results with joint porosity, pore size measurements, limited pressure effects (0.08 MPa - 0.1 MPa) on the size and number of pores, and joint cooling rate.

Ice Particle Impacts on a Moving Wedge

This work presents the results of an experimental study of ice particle impacts on a moving wedge. The experiment was conducted in the Adverse Environment Rotor Test Stand (AERTS) facility located at Penn State University. The wedge was placed at the tip of a rotating blade. Ice particles shot from a pressure gun intercepted the moving wedge and impacted it at a location along its circular path. The upward velocity of the ice particles varied from 7 to 12 meters per second. Wedge velocities were varied from 0 to 120 meters per second. Wedge angles tested were 0 deg, 30 deg, 45 deg, and 60 deg. High speed imaging combined with backlighting captured the impact allowing observation of the effect of velocity and wedge angle on the impact and the post-impact fragment behavior. It was found that the pressure gun and the rotating wedge could be synchronized to consistently obtain ice particle impacts on the target wedge. It was observed that the number of fragments increase with the normal component of the impact velocity. Particle fragments ejected immediately after impact showed velocities higher than the impact velocity. The results followed the major qualitative features observed by other researchers for hailstone impacts, even though the reduced scale size of the particles used in the present experiment as compared to hailstones was 4:1.

An Initial Study of the Fundamentals of Ice Crystal Icing Physics in the NASA Propulsion Systems Laboratory

This paper presents results from an initial study of the fundamental physics of ice-crystal ice accretion using the NASA Propulsion Systems Lab (PSL). Ice accretion due to the ingestion of ice-crystals is being attributed to numerous jet-engine power-loss events. The NASA PSL is an altitude jet-engine test facility which has recently added a capability to inject ice particles into the flow. NASA is evaluating whether this facility, in addition to full-engine and motor-driven-rig tests, can be used for more fundamental ice-accretion studies that simulate the different mixed-phase icing conditions along the core flow passage of a turbo-fan engine compressor. The data from such fundamental accretion tests will be used to help develop and validate models of the accretion process. The present study utilized a NACA0012 airfoil. The mixed-phase conditions were generated by partially freezing the liquid-water droplets ejected from the spray bars. This paper presents data regarding (1) the freeze out characteristics of the cloud, (2) changes in aerothermal conditions due to the presence of the cloud, and (3) the ice accretion characteristics observed on the airfoil model. The primary variable in this test was the PSL plenum humidity which was systematically varied for two duct-exit-plane velocities (85 and 135 m/s) as well as two particle size clouds (15 and 50 μm MVDi). The observed clouds ranged from fully glaciated to fully liquid, where the liquid clouds were at least partially supercooled. The air total temperature decreased at the test section when the cloud was activated due to evaporation. The ice accretions observed ranged from sharp arrow-like accretions, characteristic of ice-crystal erosion, to cases with doublehorn shapes, characteristic of supercooled water accretions.