James MacLeod

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.

Isokinetic total water content probe in a naturally aspirating configuration: initial aerodynamic design and testing

Recent measurements in wind tunnel and flight experiments have demonstrated that water loss prior to complete evaporation is possible for the hot-wire total water content probes. Other cloud water content probes similarly have their own operating problems. To enhance the efficiency of water particle capture and water mass retention, isokinetic sampling is appealing. The probe described in this paper represents an extension of a concept that was developed and tested through the then RAE Farnborough, UK. The intention is to quantify and minimize possible departures from isokinetic sampling during operation - something that is difficult to achieve in the original RAE configuration. This paper describes the design methodologies and the experiments that have been performed to characterize the aerodynamic performance of the prototype.

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.

Naturally Aspirating Isokinetic Total Water Content Probe: Evaporator Design and Testing

A total water content probe for flight and ground based testing is under development. Previous work has demonstrated the feasibility of a naturally aspirating isokinetic probe. Current work has concentrated on developing an evaporator to vaporize the liquid and solid water content of the inlet air, while maintaining isokinetic flow and conforming to the aerodynamic, power and geometric constraints in place for flight testing. Development has involved aerodynamic modeling of the system to ensure isokinetic flow at flight conditions and thermal modeling to produce a first iteration of the evaporator design. Wind tunnel testing verified the isokinetic properties of the system and the ability to vaporize the ice particles with a concentration expected to be encountered in flight.

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.