Jen-Ching Tsao

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.

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.

Application of triple-deck theory to the prediction of glaze ice roughness formation on an airfoil leading edge

Abstract A viscous–inviscid interaction triple-deck structure is developed to describe the thermomechanical interaction of an air boundary layer with ice sheets and liquid films. Linear stability results are compared with nonlinear triple-deck computations, and a number of nonlinear simulations of air–water–ice interactions are presented. An icing instability is encountered in regimes with simultaneous wall and air cooling that is believed to admit small scale and highly irregular surface roughness. The stabilization of the smallest scale icing disturbances is obtained through the Gibbs–Thomson relation. This local thermodynamic condition relates the freezing temperature of a pure substance to the surface tension and the mean curvature of the interface and provides a short scale stabilizing mechanism for icing instability modes. Comparison with available experimental data on glaze ice roughness diameters, accreted on NACA 0012 airfoil leading edges under glaze icing conditions, is provided. It is also found in all cases computed in this study that water beads can be formed on a wetted ice surface once the water film is locally ruptured by ice roughness elements.

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.

Comparisons of Mixed-Phase Icing Cloud Simulations with Experiments Conducted at the NASA Propulsion Systems Laboratory

This paper presents the evaluation of a numerical model for simulation of the icing cloud development at NASA Glenn Research Center’s Propulsion Systems Laboratory (PSL). The model is helping the icing facility and the fundamental ice-crystal icing physics research team to better understand the complex interactions between the test parameters and have greater confidence in the conditions at the test section of the PSL tunnel. The model attempts to explain the observed changes in test conditions by coupling the conservation of mass and energy equations for both the cloud particles and flowing air, while accounting for compressibility and the variable PSL geometry. A subroutine has been added to more accurately simulate the tunnel when water vapor conditions potentially exceed saturation. The model simulation results are compared to experimentally measured values that were taken during the first fundamentals of ice-crystal icing physics tests conducted at PSL in March 2016. The tests simulated ice-crystal and mixed-phase icing that relate to ice accretions within turbofan engines. Experimentally measured air temperature, humidity, total water content, liquid and ice water content, as well as cloud particle size, are compared with model predictions. The model showed good trend agreement with experimentally measured values, but often over-predicted aero-thermodynamic changes. This discrepancy is likely attributed to radial variations that this one-dimensional model does not address. One of the key findings of this work is that greater aero-thermodynamic changes occur when humidity conditions are low. In addition a range of mixed-phase clouds can be achieved by varying only the tunnel humidity conditions, but the range of humidities to generate a mixed-phase cloud becomes smaller when clouds are composed of smaller particles. In general, the model predicted melt fraction well, in particular with clouds composed of larger particle sizes.

Plans and Preliminary Results of Fundamental Studies of Ice Crystal Icing Physics in the NASA Propulsion Systems Laboratory

This presentation accompanies the paper titled Plans and Preliminary Results of Fundamental Studies of Ice Crystal Icing Physics in the NASA Propulsion Systems Laboratory. NASA is evaluating whether PSL, 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. This presentation (and accompanying paper) presents data from some preliminary testing performed in May 2015 which examined how a mixed-phase cloud could be generated at PSL using evaporative cooling in a warmer-than-freezing environment.

Numerical Analysis of Mixed-Phase Icing Cloud Simulations in the NASA Propulsion Systems Laboratory

This paper describes the development of a numerical model that couples the thermal interaction between ice particles, water droplets, and the flowing gas of an icing wind tunnel for simulation of NASA Glenn Research Center’s Propulsion Systems Laboratory (PSL). The ultimate goal of the model is to better understand the complex interactions between the test parameters and have greater confidence in the conditions at the test section of the PSL tunnel. The model attempts to explain the observed changes in test conditions by coupling the conservation of mass and energy equations for both the cloud particles and flowing gas mass. The model uses isentropic relations to relate gas temperature, velocity, density and pressure with respect to the PSL geometry. Measurements were taken at the PSL during wind tunnel tests simulating ice-crystal and mixed-phase icing that relate to ice accretions within turbofan engines in May 2015. The model was compared to experimentally measured values, where test conditions varied gas temperature, pressure, velocity and humidity levels, as well as the cloud total water content, particle initial temperature, and particle size distribution. Wet-bulb temperatures were generally within a few degrees of freezing. The model showed good agreement with experimentally measured values, to within approximately 30% of the measured change in gas temperature and humidity at the tunnel test section. The model did reasonably well in predicting melt content (liquid mass to total mass) at the test section, especially for clouds with larger particle sizes. In addition, the model predicted particle size at the tunnel exit with good agreement, however, the comparison was limited to clouds consisting of a small particle size distribution. One of the key findings from this work is that there was a nearly constant but slight increase in total wet-bulb temperature when the spray cloud was activated for every test and simulation. In addition, the total wet-bulb temperature in the tunnel plenum was a large factor in determining cloud phase.

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.

Time-Sequence Observations of the Formation of Ice Accretions on Swept Wings

This work presents the results of two complementary experiments, one conducted in the Icing Research Tunnel (IRT) at NASA Glenn Research Center and the other in the Goodrich Icing Wind Tunnel (IWT). The experiments were designed to study in real time the process by which ice accretions are formed on swept wings. In the IRT experiment a time sequence imaging technique (TSIT) was used to obtain real time data during the ice accretion formation. The time sequence photographic data was used to study the process frame by frame and to create movies of how the process developed in real time. A nonactivated heater located on the leading edge was used to study its effect on the formation of the ice shape. In the IWT experiment an improved TSIT was tested and additional data was taken at a baseline condition to clarify and complement the data from the IRT. The smaller IWT allowed greater optical access than the IRT providing increased field of view and an alternate grazing angle view. The data from the two experiments led to a more detailed conceptual model of how ice accretions develop.