Publications

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The development of advanced thermal barrier coatings (TBCs) of yttria stabilized zirconia (YSZ) that exhibit lower thermal conductivity through better control of electron beam - physical vapor deposition (EB-PVD) processing is of prime interest to both the aerospace and power industries. This report summarizes the work performed under a two-year Lab-Directed Research and Development (LDRD) project (38664) to produce lower thermal conductivity, graded-layer thermal barrier coatings for turbine blades in an effort to increase the efficiency of high temperature gas turbines. This project was sponsored by the Nuclear Fuel Cycle Investment Area. Therefore, particular importance was given to the processing of the large blades required for industrial gas turbines proposed for use in the Brayton cycle of nuclear plants powered by high temperature gas-cooled reactors (HTGRs). During this modest (~1 full-time equivalent (FTE)) project, the processing technology was developed to create graded TBCs by coupling ion beam-assisted deposition (IBAD) with substrate pivoting in the alumina-YSZ system. The Electron Beam - 1200 kW (EB-1200) PVD system was used to deposit a variety of TBC coatings with micron layered microstructures and reduced thermal conductivity below 1.5 W/m.K. The use of IBAD produced fully stoichiometric coatings at a reduced substrate temperature of 600°C and a reduced oxygen background pressure of 0.1 Pa. IBAD was also used to successfully demonstrate the transitioning of amorphous PVD-deposited alumina to the -phase alumina required as an oxygen diffusion barrier and for good adhesion to the substrate Ni₂Al₃ bondcoat. This process replaces the time consuming thermally grown oxide formation required before the YSZ deposition. In addition to the process technology, Direct Simulation Monte Carlo plume modeling and spectroscopic characterization of the PVD plumes were performed. The project consisted of five tasks. These included the production of layered periodic microstructures in the coating, the Direct Simulation Monte Carlo (DSMC) modeling of particle transport in the PVD plume, functional graded layer development, the deposition of all layers to form a complete coating, and materials characterization including thermal testing. Ion beam-assisted deposition, beam sharing through advanced digital rastering, substrate pivoting, hearth calorimetry, infrared imaging, fiber optic-enabled optical emission spectroscopy and careful thermal management were used to achieve all the milestones outlined in the FY02 LDRD proposal.

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The Quantum-Kinetic (Q-K) chemical reaction model is implemented in a Navier-Stokes solver, US3D, and tested on the Bow Shock UltraViolet flight experiments. The chemical reaction rates predicted by the Q-K model are compared to a commonly used Park model for flows in thermal non-equilibrium. The results show that in thermal equilibrium the reaction rates between these two models are comparable. The Q-K model predicts greater rates for some chemical reactions and lesser rates for other reactions in an five species air chemistry model. In thermal non-equilibrium, the Q-K model maintains comparable rates near thermal equilibrium, while avoiding issues of strong thermal non-equilibrium seen in the Park model. The application of the Q-K model to the Bow Shock UltraViolet flight experiments show that the model remains consistent with previous Navier-Stokes and DSMC computations over altitudes ranging from 53:5 km up to 87:5 km despite the enforcement of translational-rotational equilibrium. The commonly used Park model was unable to match this performance.

The effect of non-equilibrium internal energy excitation on the reaction rates predicted by Bird's Quantum-Kinetic (Q-K) model for dissociation and exchange reactions is analyzed. The effect of vibrational non-equilibrium is treated explicitly by the Q-K model. The effect of rotational non-equilibrium is introduced as a perturbation to the effect of vibrational non-equilibrium in chemical reactions. For dissociation reactions, a small but measurable improvement in the rates is observed. For exchange reactions, the change is negligible. These findings are in agreement with experimental observations and theoretical predictions. The results from one-dimensional stagnation-streamline and two-dimensional axi-symmetric DSMC code implementations of the original and modified Q-K models are compared for a typical re-entry flow. The influence of rotational non-equilibrium in promoting chemical reactions is seen to be small for this type of flow. © 2012 American Institute of Physics.

The effect of non-equilibrium internal energy excitation on the reaction rates predicted by Bird's Quantum-Kinetic (Q-K) model for dissociation and exchange reactions is analyzed. The effect of vibrational non-equilibrium is treated explicitly by the Q-K model. The effect of rotational non-equilibrium is introduced as a perturbation to the effect of vibrational non-equilibrium in chemical reactions. For dissociation reactions, a small but measurable improvement in the rates is observed. For exchange reactions, the change is negligible. These findings are in agreement with experimental observations and theoretical predictions. The results from one-dimensional stagnation-streamline and two-dimensional axi-symmetric DSMC code implementations of the original and modified Q-K models are compared for a typical re-entry flow. The influence of rotational non-equilibrium in promoting chemical reactions is seen to be small for this type of flow. © 2012 American Institute of Physics.

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The Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics is used to simulate the steady flow of an ideal gas through a long thin isothermal microscale tube connecting two infinite reservoirs at different pressures. The tube wall is at the reservoir temperature, and molecules reflect from the walls according to the Maxwell model (i.e., a linear combination of specular reflections and diffuse reflections at the wall temperature). The computed mass flow rates approach the known expressions in the near-continuum and free-molecular regimes and agree reasonably with recent experimental measurements in microscale tubes and channels. Approximate closed-form expressions for the mass flow rate and the pressure profile along the tube are developed and are in reasonable agreement with the DSMC results in all regimes and for all values of the accommodation coefficient. © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

The Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics is used to simulate the steady flow of an ideal gas through a long thin isothermal microscale tube connecting two infinite reservoirs at different pressures. The tube wall is at the reservoir temperature, and molecules reflect from the walls according to the Maxwell model (i.e., a linear combination of specular reflections and diffuse reflections at the wall temperature). The computed mass flow rates approach the known expressions in the near-continuum and free-molecular regimes and agree reasonably with recent experimental measurements in microscale tubes and channels. Approximate closed-form expressions for the mass flow rate and the pressure profile along the tube are developed and are in reasonable agreement with the DSMC results in all regimes and for all values of the accommodation coefficient. © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

Moving-boundary algorithms for the Direct Simulation Monte Carlo (DSMC) method are investigated for a microbeam that moves toward and away from a parallel substrate. The simpler but analogous one-dimensional situation of a piston moving between two parallel walls is investigated using two moving-boundary algorithms. In the first, molecules are reflected rigorously from the moving piston by performing the reflections in the piston frame of reference. In the second, molecules are reflected approximately from the moving piston by moving the piston and subsequently moving all molecules and reflecting them from the moving piston at its new or old position. © 2011 American Institute of Physics.

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