Monitoring Damage in Non-Oxide Composites at High Temperatures Using Carbon-Containing CVD SiC Monofilament Fibers As Embedded Electrical Resistance Sensors

Electrical resistance has become a technique of interest for monitoring SiC-based ceramic composites. The typical constituents of SiC fiber-reinforced SiC matrix composites, SiC, Si and/or C, are semi-conducive to some degree resulting in the fact that when damage occurs in the form of matrix cracking or fiber breakage, the resistance increases. For aero engine applications, SiC fiber reinforced SiC, sometimes Si-containing, matrix with a BN interphase are often the main constituents. The resistivity of Si and SiC is highly temperature dependent. For high temperature tests, electrical lead attachment must be in a cold region which results in strong temperature effects on baseline measurements of resistance. This can be instructive as to test conditions; however, there is interest in focusing the resistance measurement in the hot section where damage monitoring is desired. The resistivity of C has a milder temperature dependence than that of Si or SiC. In addition, if the C is penetrated by damage, it would result in rapid oxidation of the C, presumably resulting in a change in resistance. One approach considered here is to insert carbon “rods” in the form of CVD SiC monofilaments with a C core to try and better sense change in resistance as it pertains to matrix crack growth in an elevated temperature test condition. The monofilaments were strategically placed in two non-oxide composite systems to understand the sensitivity of ER in damage detection at room temperature as well as elevated temperatures. Two material systems were considered for this study. The first composite system consisted of a Hi-Nicalon woven fibers, a BN interphase and a matrix processed via polymer infiltration and pyrolysis (PIP) which had SCS-6 monofilaments providing the C core. The second composite system was a melt-infiltrated (MI) pre-preg laminate which contained Hi-Nicalon Type S fibers with BN interphases with SCS-Ultra monofilaments providing the C core. The two composite matrix systems represent two extremes in resistance, the PIP matrix being orders of magnitude higher in resistance than the Si-containing pre-preg MI matrix. Single notch tension-tension fatigue tests were performed at 815°C to stimulate crack growth. Acoustic emission (AE) was used along with electrical resistance (ER) to monitor the damage initiation and progression during the test. Post-test microscopy was performed on the fracture specimen to understand the oxidation kinetics and carbon recession length in the monofilaments.

Micromechanics Approach for the Overall Elastic Properties of Ceramic Matrix Composites Incorporating Defect Structures

Initial mechanical behavior of Ceramic Matrix Composites (CMCs) is linear until the proportional limit. This initial behavior is characterized by linear elastic properties, which are anisotropic due to the orientation and arrangement of fibers in the matrix. The linear elastic properties are needed during various phases of analysis and design of CMC components. CMCs are typically made with ceramic unidirectional or woven fiber preforms embedded in a ceramic matrix formed via various processing routes. The matrix processing of interest in this work is that formed via Polymer Impregnation and Pyrolysis (PIP). As this process involves pyrolysis process to convert a pre-ceramic polymer into ceramic, considerable volume shrinkage occurs in the material. This volume shrinkage leads to significant defects in the final material in the forms of porosity of various size, shape, and volume fraction. These defect structures can have a significant impact on the elastic and damage response of the material. In this paper, we develop a new micromechanics modeling framework to study the effects of processing-induced defects on linear elastic response of a PIP-derived CMC. A combination of analytical and computational micromechanics approaches is used to derive the overall elastic tensor of the CMC as a function of the underlying constituents and/or defect structures. It is shown that the volume fraction and aspect ratio of porosity at various length-scales plays an important role in accurate prediction of the elastic tensor. Specifically, it is shown that the through-thickness elastic tensor components cannot be predicted accurately using the micromechanics models unless the effects of defects are considered.

CFD Modeling of Pulverized Coal Combustion Using Relax to Chemical Equilibrium Model With Turbulence-Chemistry Interaction

A comprehensive numerical investigation of 2.4 MW IFRF swirl-stabilized coal furnace is conducted. A novel Relax to Chemical Equilibrium (RTCE) model with turbulence-chemistry interaction is used for the gas-phase combustion and the results are compared with the standard Eddy Break-Up (EBU) model. In the RTCE model, the species compositions are relaxed towards the local chemical equilibrium at a characteristic time scale determined by the local flow and turbulence. The turbulence-chemistry interaction is treated using the Eddy Dissipation Concept (EDC) model. The simulation uses a Lagrangian-Eulerian framework to treat the particle transport and the fluid-particle interactions. In all, fifteen species have been included in the RTCE model. For coal particles, a one-step devolatilization, first-order char oxidation, particle porosity, and particle radiation models are employed. The NOx emissions model includes both thermal and fuel NOx pathways. It was found that RTCE model performs well in predicting the overall temperature distribution in the IFRF coal furnace. The predicted temperature, NOx and CO at the outlet match very well with the experimental data, showing marked improvement over the EBU model. The overall NOx profile is also predicted better by the RTCE model.

Comparative Life Cycle Assessment of Different Gas Turbine Axial Compressor Water Washing Systems

Nowadays the climate change is widely recognized as a global threat by both public opinion and industries. Actions to mitigate its causes are gaining momentum within all industries. In the energy field, there is the necessity to reduce emissions and to improve technologies to preserve the environment. LCA analyses of products are fundamental in this context. In the present work, a life cycle assessment has been carried out to calculate the carbon footprint of different water washing processes, as well as their effectiveness in recovering Gas Turbine efficiency losses. Field data have been collected and analyzed to make a comparison of the GT operating conditions before and after the introduction of an innovative high flow online water washing technique. The assessments have been performed using SimaPro software and cover the entire Gas Turbine and Water Washing skids operations, including the airborne emissions, skid pump, the water treatment and the heaters.

Comparison of Tabulated and Complex Chemistry Turbulent-Chemistry Interaction Models With High Fidelity Large Eddy Simulations on Hydrogen Flames

Hydrogen micromix combustion is a promising concept to reduce the environmental impact of both aero and land-based gas turbines by delivering carbon-free and ultra-low-NOx combustion without the risk of autoignition or flashback. The EU H2020 ENABLEH2 project aims to demonstrate the feasibility of such a switch to hydrogen for civil aviation, within which the micromix combustion, as a key enabling technology, will be matured to TRL3. The micromix combustor comprises thousands of small diffusion flames where air and fuel are mixed in a crossflow pattern. This technology is based on the idea of minimizing the scale of mixing to maximize mixing intensity. The high-reactivity and wide flammability limits of hydrogen in a micromix combustor can produce short and low-temperature small diffusion flames in lean overall equivalence ratios. In order to mature the hydrogen micromix combustion technology, high quality numerical simulations of the resulting short, thin and highly dynamic hydrogen flames, as well as predictions of combustion species, are essential. In fact, one of the biggest challenges for current CFD has been to accurately model this combustion phenomenon. The Flamelet Generated Manifold (FGM) model is a combustion model that has been used in the past decades for its predicting capabilities and its low computational cost due to its reliance on pre-tabulated combustion chemistry, instead of directly integrating detailed chemistry mechanisms. However, this trade for a lower computational cost may have an impact on the solution, especially when considering a fuel such as Hydrogen. Therefore, it is necessary to compare the FGM model to another combustion modelling approach which uses more detailed complex chemistry. The main focus of this paper then, is to compare the flame characteristics in terms of position, thickness, length, temperature and emissions obtained from LES simulations done with the FGM model, to the results obtained with more complex chemistry models, for hydrogen micromix flames. This will be done using STAR-CCM+ to determine the most suitable numerical approach required for the design of injection systems for ultra-low NOx.

Design of Stress Constrained SiC/SiC Ceramic Matrix Composite Turbine Blades

The structural and aerodynamic performance of a a low aspect ratio SiC/SiC CMC High Pressure Turbine blade was determined. The application was a NASA notional single aisle aircraft engine to be available in the N+3, beyond 2030, time frame. The notional rpm was maintained, and to satisfy stress constraints the annulus area was constrained. This led to a low span blade. For a given clearance low span blade are likely to have improved efficiency when shrouded. The efficiency improvement due to shrouding was found to strongly depend on the axial gap between the shroud and casing. Axial gap, unlike clearance or reaction, is not a common parameter used to correlate the efficiency improvement due to shrouding. The zero clearance stage efficiency of the low aspect ratio turbine was 0.920. Structural analyses showed that the rotor blade could be shrouded without excessive stresses. The goal was to have blade stresses less than 100 MPa (14.5 ksi) for the unshrouded blade. Under some not very restrictive circumstances, such as blade stacking, a one-dimensional radial stress equation accurately predicted area averaged Von Mises stress at the blade hub. With appropriate stacking radial and Von Mises stresses were similar.

Environmental Barrier Coating Oxidation and Adhesion Strength

Plasma Spray-Physical Vapor Deposition (PS-PVD) environmental barrier coatings (EBCs) of Yb2Si2O7 were deposited on SiC and exposed in a steam environment (90% H2O/O2) at 1426°C to form a thermally grown oxide (TGO) layer between the substrate and EBC. In advanced ceramic material systems such as coated ceramic matrix composites (CMCs), the TGO layer is the weak interface in coated CMC systems and directly influences component lifetimes. The effect of surface roughness and TGO thickness on the adhesion strength were evaluated by mechanical testing of the coatings after exposure. Morphology and oxide layer thickness were analyzed with electron microscopy while the composition and crystal structure were tracked with X-ray diffraction. The strength of the system is evaluated with respect to oxidation rate to give a qualitative understanding of coating durability.

Thermal History Coatings: Part II — Measurement Capability up to 1500°C

To improve the efficiency of gas turbines, the turbine inlet temperature needs to be increased. The highest temperature in the gas turbine cycle takes place at the exit of the combustion chamber and it is limited by the maximum temperature turbine blades, vanes and discs can withstand. A combination of advanced cooling designs and Thermal Barrier Coatings (TBCs) are used to achieve material surface temperatures of up to 1200°C. However, further temperature increases and materials that can withstand the harsh temperatures are required for next-generation engines. Research is underway to develop next-generation CMCs with 1480 °C temperature capability, but accurate data regarding the thermal load on the components must be well understood to ensure the component life and performance. However, temperature data is very difficult to accurately and reliably measure because the turbine rotates at high speed, the temperature rises very quickly with engine startup and the blades operate under harsh environments. At the operating temperature range of CMCs, typically platinum thermocouples are used, however, this material is incompatible with silicon carbide CMCs. Other temperature techniques such as infrared cameras and pyrometry need optical access and the results are affected by changes in emissivity that can take place during operation. Offline techniques, in which the peak temperature information is stored and read-out later, overcome the need for optical access during operation. However, the presently available techniques, such as thermal paint and thermal crystals cannot measure above ∼1400°C. Therefore, a new measurement technique is required to acquire temperature data at extreme temperatures. To meet this challenge, Sensor Coating Systems (SCS) is focused on the development of Thermal History Coatings (THC) that measure temperature profiles in the 900–1600 °C range. THC are oxide ceramics deposited via air plasma spraying process. This innovative temperature profiling technique uses optically active ions in a ceramic host material that start to phosphoresce when excited by light. After being exposed to high temperatures the host material irreversibly changes at the atomic level affecting the phosphorescence properties which are then related to temperature through calibration. This two-part paper describes the THC technology and demonstrates its capabilities for high-temperature applications. In this second part, the THC is implemented on rig components for a demonstration on two separate case studies for the first time. In one test, the THC was implemented on a burner rig assembly on metallic alloys instrumented with thermocouples, provided by Pratt & Whitney Canada. In another test, the THC was applied to environmental barrier coatings developed by NASA, as part of a ceramic-matrix-composite system and heat-treated up to 1500°C. The results indicate the THC could provide a unique capability for measuring high temperatures on current metallic alloys as well as next-generation materials.

Defining Key Environmental Performance Factors (KEPF) for Gas Turbines Eco-Design and Production Through Life Cycle Assessment

Nowadays environmental impact assessment of a new product is necessary to meet rising sustainability requirements also in the Oil & Gas and Power Generation markets, especially for industrial gas turbines. From the conceptual phase to the detailed design, engineer’s work is supported by a wide range of tools aimed to define and evaluate typical parameters such as performances, life and costs, etc. However, considering environmental impact aspects from the early stages of product development may not be easy if the involved engineers are not provided by a specific Life Cycle Assessment (LCA) knowledge. Scope of this paper is to introduce and explain the development of a methodology aimed to define and evaluate the Key Environmental Performance Factors (KEPF) during the whole design process. The proposed methodology enables easy and fast eco-design evaluations and supports sustainable design assessments. Preliminary analysis of the entire processes involved in gas turbine (GT) design and production as well as testing and commissioning phases were performed to evaluate which factors affect mostly the Carbon Footprint of each process, referred to their specific functional unit. Extrapolating the KEPF from Cradle-to-Gate LCA they can be combined with case-specific qualitative and quantitative information such as material selection, manufacturing processes, mass quantity, presence of coatings etc. to provide environmental assessments. A case study of LCA applied to a heavy-duty GT is presented to outline the relative weight of each KEPF.

Detailed Description of the Fluidized Bed Mixing and Heat Transfer by Means of Eulerian Multi-Fluid Numerical Simulations

The hydrodynamics and heat transfer of a binary mixture of sand and biomass in a fluidized bed have been numerically investigated. The Eulerian multi-fluid model MFM incorporating kinetic theory of granular flow was used to numerically investigate fluidized bed. A commercial code has been used together with user-defined functions to correctly predict the hydrodynamics and the heat transfer. Numerical results were validated against the experiment in terms of pressure drop across the bed and concentration of biomass at different heights of the bed. Influence of additional parameters, such as superficial gas velocity and sand sizes on hydrodynamics were investigated. Additionally, heat transfer in the fluidized bed was also studied highlighting the influence of the temperature dependent properties of air on the results. The present results reveal that better mixing is achieved for smallest sand size, also promoting more uniform temperature of biomass.