Experimental Investigation of Microparticle Sand Sticking Probability from 1000°C to 1100°C

Experimental Investigation of Microparticle Sand Sticking Probability from 1000°C to 1100°C Andrew James Boulanger Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Mechanical Engineering Wing F. Ng Srinath V. Ekkad Todd Lowe Walter O’Brien Gary R. Pickrell October 30, 2017 Blacksburg, Virginia, USA Keywords: Dust Ingestion, Deposition, Arizona Road Dust, Sticking Probability Copyright 2017 Experimental Investigation of Microparticle Sand Sticking Probability from 1000°C to 1100°C Andrew James Boulanger ABSTRACT Increasing commercial and military aircraft operations in arid environments are increasing the likelihood of sand and dust ingestion. Turbine engines are particularly susceptible to the ingestion of sand and dust, which can erode cold-section components and deposit onto hot-section components. Ultimately, the erosion and deposits will shorten the operational lifespan of these engines and limit their availability thereby increasing maintenance costs and risking safety. Mitigating these risks has become more prevalent in recent years due to increasing combustion temperatures in effort to increase fuel efficiency. Increasing combustion temperatures directly increases deposit formation onto hot-section components. Monitoring deposit formation on existing turbine engine platforms and improving deposit resilience on new designs has been the industry focus for the last two decades. This study focused on statistically modeling the initial onset of microparticle deposits onto an analogous hot-section surface. Generally, as deposits accumulate onto a hot-section surface, the existing deposit formation is more likely to bond with incoming particulate at a faster rate than an exposed bare surface. Predicting the initial deposits onto a bare surface can determine the accelerated deposition rate depending on subsequent particulate impinging onto the surface. To emulate the initial deposits, a HASTELLOY® X test coupon was exposed to 20 µm to 40 µm samples of Arizona Road Test Dust (ARD) at varying loadings and aerosol densities. The Virginia Tech Aerothermal Rig was used for all test scenarios at flow-particle temperatures between 1000°C to 1100°C. Several statistical models were developed as a function of many independent variables, culminating with a final sticking probability (SP) model. Overall, the SP of individual ARD particulate is a primary function of flow-particle temperature and normal impact momentum. Tangential impact momentum of a particle will decrease the SP, while surface temperatures reaching isothermal conditions with the flow will increase SP. However, there are specific cases where lower surface temperatures and high particle temperatures result in a high SP. Particle size was a strong predictor of SP where particles between 10 µm to 19 µm were 5 to 10 times greater than the 19 µm to 40 µm range. Additional studies will be necessary to examine some additional parameters that become more prominent with smaller particle sizes. Ultimately, the intention of the models is to assist turbine engine designers to improve resilience to deposit formation on hot-section components. Experimental Investigation of Microparticle Sand Sticking Probability from 1000°C to 1100°C Andrew James Boulanger GENERAL ABSTRACT Dust ingestion by propulsion turbine engines can have severe negative implications on the operational safety of an aircraft. Recently, increased air traffic, both military and commercial, in desert regions has caused many aircraft engine designers to improve the resilience to dust ingestion effects. One of the detrimental mechanisms is hot particle deposits in the combustion and exhaust sections. This dissertation evaluates deposit formation using carefully developed high temperature experiments. In general, deposit formation can negatively change flow characteristics inside the engine that can limit available power and safety margins. Likewise, deposits can reduce or stop cooling needed for hot-section parts inside a jet engine. Hot-section components need cooling since the main gas path operation temperatures of a jet engine typically exceed the melting points of common high temperature metals. During dust ingestion events, deposits will initially adhere to a hot metallic or ceramic surface inside the engine. Subsequent deposit accumulation will occur at a faster rate since incoming particles will more readily adhere to existing deposits than to a metallic or ceramic surface. The experimental work in this dissertation focused only on quantifying the initial individual particle deposits on a HASTELLOY®-X surface between 1000°C to 1100°C. Arizona Road Dust was the particulate selected for all testing. The dust has sizes ranging between 10 µm to 40 µm. The sticking probability or the likelihood a particle would deposit per an impact was less than 5% for all tests performed. Particles smaller than 19 µm had a sticking probability up to 5% while larger particles were generally less than 3%. Effectively, this implies that the initial deposits onto a hot engine surface are strongly dependent on the smallest particles. Propulsion turbine engine designers can utilize this information to develop mitigation methods against deposit formation of the smallest particles that are ingested. Ultimately, the research presented in this work is intended to improve operational safety of current and future aircraft. PREFACE Deposit formation on turbine hardware in propulsion turbine engines can occur in many arid regions globally. Characterizing crystalline deposits on metallic substrates can aid in component resilience and health monitor algorithms during particle ingestion. This dissertation is written in manuscript format and contains three primary papers with an additional chapter intention of the analysis to be submitted to the ASME Journal of Turbomachinery. The author is the first author for all three papers and the final chapter presented in this dissertation. There is an Appendix which contains supplementary analytical information regarding the last chapter that was not deemed necessary for publication purposes. The first paper, presented in Chapter 1, was for the 2016 ASME Turbo Expo in Seoul, South Korea [1]. It was a preliminary investigation of Arizona Road Dust (ARD) deposits onto a HASTELLOY® X (HX) test coupon using the Virginia Tech Aerothermal Rig (VTAR). The investigation was able to establish a multi- linear regression of deposition as a function of flow temperature and test coupon angle. The resulting correlation had an of 0.67, which implied that the remaining 0.33 could be explained by other factors such as coupon temperature and injection rates. The experimental setup impinged combusted air and 20-40 µm ARD particles onto the test coupon for a variety of angles and temperatures. Gas temperatures ranged from 975°C to 1075°C with a constant bulk velocity of 70 m/s. Coupon angles were varied between 30°, 50°, 80°, and 90°. The regression developed indicated that ARD deposition increased linearly from 975°C to 1075°C for all coupon angles. Overall, this study was a preliminary investigation to establish individual ARD particle deposit response at high gas temperatures. Chapter 2 contains the second paper that was published in The Aeronautical Journal 2017 by the Royal Aeronautical Society [2]. It expanded on the first paper’s analysis by performing tests with a consistent ARD injection rate and incorporating surface temperature instead of gas path temperature. Similar to the previous study [1], coupon angles were varied between 20°, 50°, and 80° for flow temperatures of 1000°C, 1050°C, and 1100°C. Averaged deposits were methodically quantified through normalized particle deposit tallies per area and Coverage Ratio (CR) or percent coverage of the surface using microscopic imaging and object recognition scripts. Multi-linear regression models of bulk deposits were developed as a function of coupon angle and flow temperature. The multi linear regression models had values between 0.96 to 0.99 depending on the deposition metric employed. The prediction models can be used to estimate deposit accumulation on similar scenarios in gas turbine applications. Chapter 3 contains the third paper that was presented at the 2017 ASME Turbo Expo in Charlotte, North Carolina, USA [3] in conjunction with a complimentary paper by Barker et. al. [4]. Compared to the second paper [2], this analysis used similar test conditions using ARD and HX but evaluated deposits as a function of local aerothermal conditions on the test coupon and particle impact trajectories. All experiments use 20 µm to 40 µm ARD on a bare HX coupon from 1000°C to 1100°C bulk flow temperature with a constant flow velocity of 70 m/s. As previously indicated, a multi linear regression was fit to CR data as a function of surface temperature and impact trajectories. The resulting value of the model is 0.855. The model is able to predict the initial onset of deposits as a quadratic function of local surface temperature and impact velocity components (normal and tangential). A prominent observation was that tangential impact velocity has a significant nonlinear, independent effect on deposits relative to normal impact velocity and local surface temperatures. The final CR model can be used to predict when rapid deposit accumulation may begin under similar conditions tested. The final chapter is a paper intended to be submitted for journal

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