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When a gas turbine operates in a particle laden environment, such as a desert, small solid particles are ingested into the engine. The ingested sand particles can cause damage to engine components and reduce the service life of the engine. Particle ingestion causes the erosion of metal blades and vanes, and, if the firing temperature is hot enough, deposition of molten particles in the hot sections of the engine. Both deposition and erosion phenomena can severely reduce overall engine performance. The Coefficient of Restitution (COR) is a measure of the particle-wall interaction, and has been widely used to quantify particle rebound characteristics in past particle impact studies. This work investigates the effects of temperature on sand particle impact characteristics by measuring the COR and other deposition related impact parameters.
The first study presented as part of the dissertation contains a description of a novel method used to measure COR using a Particle Tracking Velocimetry (PTV) method. This is combined with Computational Fluid Dynamics (CFD) flow field to allow for an accurate determination of the particle impact velocity. The methodology described in this paper allows for measurement of the COR in a wide range of test conditions in a relatively simple manner. The COR data for two different sizes of Arizona Road Dust (ARD) and one size of glass beads are presented in this paper. Target material was stainless steel 304 and the impact angle was varied from 25 to 85 degrees.
The second study details the first quantification of the COR of san particles at elevated temperatures. Temperatures used in this study were 533 K, 866 K, and 1073 K. In this study the mass flow rate through the experimental setup was fixed. This meant that velocity and temperature were coupled. Target material for this study was stainless steel 304 and the impact angle was varied from 30° to 80°. The COR was found to decrease substantially at the temperatures and velocity increased. It was determined that the decrease in COR was almost certainly caused by the increase in velocity, and not the decrease in temperature.
The third study contains COR results at elevated temperatures. Significant improvements from the method used to calculate COR in the first paper are described. The particle used for these tests was an ARD sand of 20-40 μm size. Target materials used were stainless steel 304 and Hastelloy X. The particles impinged on the target coupon at a velocity of 28m/s. Tests were performed at three different temperatures, 300 K (ambient), 873 K, and 1073 K to simulate temperatures seen in gas turbine cooling flows. The angle of impingement of the bulk flow sand on the coupon was varied between 30° and 80°. A substantial decrease in COR was discovered at the elevated temperatures of this experiment. Hastelloy X exhibited a much larger decrease in COR than does stainless steel 304. The results were compared to previously published literature.
The final study also used the ARD size of 20-40 μm. The target material was a nickel alloy Hastelloy X. Experiments for this study were performed at a constant velocity of 70m/s. Various temperatures ranging from 1073 K up to and including 1323 K were studied. Particle angle of impact was varied between 30° and 80°. Significant deposition was observed and quantified at the highest two temperatures. The COR of the ARD sand at the highest temperatures was found not to change despite the occurrence of deposition. At elevated temperatures, many of the particles are not molten due to sand's non-homogenous and crystalline nature. These particles rebound from the target with little if any change in COR. |
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