Katharine L. Harrison

COMPARISON OF RANS TURBULENCE MODELS FOR PREDICTION OF FILM COOLING PERFORMANCE

The realizable k-e, standard k-ω, and RSM turbulence models were used to simulate flat plate film cooling experiments that are commonly described in literature. Adiabatic effectiveness simulations revealed that using the standard k-ω model resulted in the closest agreement with experimentally determined laterally averaged adiabatic effectiveness, but the worst agreement with centerline adiabatic effectiveness. Conversely, the realizable k-e model agreed worst with experimental laterally averaged adiabatic effectiveness values and best with centerline values. Use of the anisotropic RSM model was not found to predict more realistic coolant spreading than the other models. Simulations to find heat transfer coefficients without film cooling showed good agreement with correlations for all three models, and the closest agreement resulted from using the realizable k-e model. Heat transfer coefficient augmentation was also examined for two configurations: unit density ratio with and without upstream heating. Laterally averaged heat transfer coefficient augmentation simulations using all three turbulence models agreed well with experiments. However, the spanwise variation in heat transfer coefficient augmentation in all cases was greater than is typically seen experimentally.Copyright

Microwave-assisted Low-temperature Growth of Thin Films in Solution

Thin films find a variety of technological applications. Assembling thin films from atoms in the liquid phase is intrinsically a non-equilibrium phenomenon, controlled by the competition between thermodynamics and kinetics. We demonstrate here that microwave energy can assist in assembling atoms into thin films directly on a substrate at significantly lower temperatures than conventional processes, potentially enabling plastic-based electronics. Both experimental and electromagnetic simulation results show microwave fields can selectively interact with a conducting layer on the substrate despite the discrepancy between the substrate size and the microwave wavelength. The microwave interaction leads to localized energy absorption, heating, and subsequent nucleation and growth of the desired films. Electromagnetic simulations show remarkable agreement with experiments and are employed to understand the physics of the microwave interaction and identify conditions to improve uniformity of the films. The films can be patterned and grown on various substrates, enabling their use in widespread applications.

Microwave-Assisted Solvothermal Synthesis and Characterization of Metastable LiFe1−x(VO)xPO4 Cathodes

Vanadyl ion substituted LiFePO(4) cathodes of the form LiFe(1-x)(VO)(x)PO(4) for 0 ≤ x ≤ 0.25 have been synthesized by a rapid microwave-solvothermal process at <300 °C within 10 min. Clear evidence of vanadyl ion substitution is demonstrated, despite a large size difference between Fe(2+) and (VO)(2+), by characterizing the products structurally, spectroscopically, and electrochemically. The vanadyl ion substitution is accompanied by the formation of iron vacancies in the lattice and Fe(3)O(4) impurity phase, which increases with increasing (VO)(2+) substitution for Fe(2+) and could be removed with a magnetic stir bar. The formation of iron vacancies, along with the oxidation of some Fe(2+) to Fe(3+) to maintain charge neutrality, results in a decrease in the unit cell volume with increasing x despite the substitution of larger (VO)(2+) for Fe(2+). Charge-discharge data of the vanadyl ion substituted samples suggest suppression of the two-phase plateau behavior that is characteristic of LiFePO(4). Electrochemical data collected without any carbon coating reveal that the capacity and rate capability decreases, but the capacity retention improves with (VO)(2+) substitution.

Turbine Airfoil Net Heat Flux Reduction With Cylindrical Holes Embedded in a Transverse Trench

Film cooling adiabatic effectiveness and heat transfer coefficients for cylindrical holes embedded in a 1d transverse trench on the suction side of a simulated turbine vane were investigated to determine the net heat flux reduction. For reference, measurements were also conducted with standard inclined, cylindrical holes. Heat transfer coefficients were determined with and without upstream heating to isolate the hydrodynamic effects of the trench and to investigate the effects of the thermal approach boundary layer. Also, the effects of a tripped versus an untripped boundary layer were explored. For both the cylindrical holes and the trench, heat transfer augmentation was much greater for the untripped approach flow. A further increase in heat transfer augmentation was caused by use of upstream heating, with as much as a 180% augmentation for the trench. The tripped approach flow led to much lower heat transfer augmentation than the untipped case. The net heat flux reduction for the trench was found to be significantly higher than for the row of cylindrical holes.

CFD Predictions of Film Cooling Adiabatic Effectiveness for Cylindrical Holes Embedded in Narrow and Wide Transverse Trenches

Recent studies have shown that film cooling adiabatic effectiveness can be significantly improved when holes are embedded in shallow, transverse trenches. In this study computational simulations were made using the commercial CFD code FLUENT to determine if the dramatic improvement in film cooling performance was predictable. Simulations were made of a baseline cylindrical hole configuration, and narrow and wide trench configurations. Simulations correctly predicted that the narrow trench outperformed the baseline row of cylindrical holes and the wide trench at all blowing ratios. Furthermore, the simulations showed that enhanced performance with the trench could be attributed to decreased separation of the coolant jets. The success of these predictions show that computational simulations can be used as a tool for studying and identifying promising film cooling configurations.Copyright

A Rapid Microwave-Assisted Solvothermal Approach to Lower-Valent Transition Metal Oxides

A green, rapid microwave-assisted solvothermal process using tetraethylene glycol (TEG) as a reducing agent has been explored as a soft-chemistry route for the preparation of various lower-valent transition metal oxides. To demonstrate the feasibility of the approach, lower-valent binary oxides such as V4O9, Mn3O4 or MnO, CoO, and Cu2O have been obtained within a short reaction time of 30 min by reducing, respectively, V2O5, MnO2, Co3O4, and CuO with TEG at <300 °C. Moreover, the approach has been used to extract oxygen from ternary oxides such as LaFeO3, SrMnO3, LaCoO3, LaNiO3, and La4Ni3O10. The oxidation state of the transition metal ions and the oxygen content in these ternary oxides could be tuned by precisely controlling the reaction temperatures from 160 to 300 °C. The products have been characterized by X-ray powder diffraction and iodometric titration. The versatility of this novel technique is demonstrated by the facile synthesis of V4O9, which has only been produced recently in single-phase form.

Use of the Adiabatic Wall Temperature in Film Cooling to Predict Wall Heat Flux and Temperature

The adiabatic wall temperature is generally assumed to be the driving temperature for heat transfer into conducting gas turbine airfoils. This assumption was analyzed through a series of FLUENT simulations using the standard k-ω turbulence model. Adiabatic effectiveness and heat transfer experiments commonly documented in literature were mimicked computationally. The results were then used to predict both the heat flux and temperature distributions on a conducting flat plate wall and the predictions were compared to the heat flux and temperature distributions found through a flat plate conjugate heat transfer simulation. The heat flux analysis was compared to previously published work using the realizable k-e turbulence model. The same conclusions could be drawn for both turbulence models despite differences in simulated adiabatic effectiveness and heat transfer coefficient distributions. Agreement between heat flux predictions and the heat flux from the conjugate simulations correlated well with how closely the adiabatic wall temperature approximated the over-riding gas driving temperature for heat transfer into the wall. In general, the driving temperature for heat transfer was represented well by the adiabatic wall temperature and the heat flux was well predicted. However, in some locations, the heat flux was over-predicted by up to 300%. Since wall temperature is ultimately the parameter of interest for industrial gas turbine design, the conducting flat plate temperature distribution was also predicted. This was done by using the adiabatic effectiveness and heat transfer coefficients found with the standard k-ω turbulence model as boundary conditions in a three dimensional solid conduction simulation. Then metal temperatures predicted in the solid conduction simulation were compared to those found through conjugate analysis. Despite deviations in predicted heat flux and the conjugate model heat flux of up to 300%, deviations in the predicted and the conjugate model non-dimensional metal temperatures were less than 10%. Thus, use of the adiabatic wall temperature as the driving temperature for heat transfer to predict temperature on the surface of a conducting wall results in relatively small errors.Copyright