PATRICK MENSAH

... Trans. Patrick Mensah. Stanford: Stanford UP, 1998. Djebar, Assia. ... Donadey, Anne. Recasting Postcolonialism. Portsmouth: Heinemann, 2001. Elia, Nadia. Trances, Dances and Vociferations: Agency and Resistance in Africana Women's Narratives. New York: Garland, 2001.

Plant-based complementary foods are the main source of nutrients for many young children in developing countries. They may, however, present problems in providing nutritionally adequate and safe diets for older infants and young children. The high starch content leads to low-nutrient diets that are bulky and dense, with high levels of antinutritive factors such as phytates, tannins, lectins, and enzyme inhibitors. Phytates impair mineral bioavailability, lectins interfere with intestinal structure, and enzyme inhibitors inhibit digestive enzymes. In addition, there is often microbial contamination, which leads to diarrhea, growth-faltering, and impaired development, and the presence of chemical contaminants may lead to neurological disease and goiter. The fact that some fruits containing carotenoids are only available seasonally contributes to the vulnerability of children receiving predominantly plant-based diets. Traditional household food technologies have been used for centuries to improve the quality and safety of complementary foods. These include dehulling, peeling, soaking, germination, fermentation, and drying. While modern communities tend to reject these technologies in favor of more convenient fast-food preparations, there is now a resurgence of interest in older technologies as a possible means of improving the quality and safety of complementary foods when the basic diet cannot be changed for economic reasons. This paper describes the biology, safety, practicability, and acceptability of these traditional processes at the household or community level, as well as the gaps in research, so that more effective policies and programs can be implemented to improve the quality and safety of complementary foods.

Microstructural effects on heat transfer in a thermal barrier coatings system subjected to a pulsed surface heating is presented in this paper within the framework of dual phase lag heat conduction model. The effects of thermo-physical properties on the energy transport for the simplified two-layered thermal barrier coating and substrate system (TBC-substrate system) are presented. Interfacial thermal resistance is specified as a function of temperature differential on both sides of the imperfect thermal contact. Due to the difference in properties between the two dissimilar materials on both sides of the interface, a strong nonlinear interfacial boundary condition is introduced into the problem. A robust numerical scheme, named Mean Value Finite Volume Method (MVFVM), which is capable of solving conservation equations, is used to analyze the present problem. Results show that the lagging thermal behavior is affected by the thermal properties of the padding material (substrate) and ...

In this research the design of a class of aircraft wing with a constant taper angle is formulated as a constrained optimization problem. The design variables for the optimization problem are the placements of the internal components and geometric dimensions of the wing. The cost function is the weight of the wing consisting of the weights of the front spar, the rear spar and the reinforced skin. The design constraints imposed on the optimization process are the bending stresses in the spars, the shear stresses and the angle of twist in the ribs. All the design calculations are non-dimensionalized with a corresponding calculation of a rectangular planform wing, which is a member of the class of wings studied. The optimization of the dimensionless weight of the wing, defined in terms of ten design variables, is implemented using Nelder and Mead technique. Simplified versions of the optimization problem for 1D and 2D cases are presented to serve as the basis for the validation of the s...

A study in collaboration between investigators at Southern University and Louisiana State University in Baton Rouge, Louisiana and NASA/MSFC is examining materials for modeling and analysis of heat-activated thermal coupling for joining composite to composite/alloy structures. The short-term objectives of this research are to develop a method for joining composite or alloy structures, as well as to study the effects of thermal stress on composite-to-alloy joints. This investigation will result in the selection of a suitable metallic alloy. Al-Li alloys have potential for this purpose in aerospace applications due to their excellent strength-to-weight ratio. The study of Al-Li and other alloys is of significant importance to this and other aerospace as well as offshore related interests. Further research will incorporate the use of computer aided design and rapid prototype hardware for conceptual design and verification of a potential composite piping delivery system.

ABSTRACT Yttria-Stabilized-Zirconia (YSZ) is the most common material used in the fabrication of Thermal Barrier Coatings (TBCs) for gas turbine applications. Due to the low thermal conductivity and a small mismatch of the thermal expansion coefficient to the high temperature alloy, YSZ is commonly used as the top coat layer to provide a thermal barrier effect. The aim of this work is to study the thermo-physical properties of standard (STD) and vertically cracked (VC) thermal barrier coatings fabricated by Atmospheric Plasma Spray (APS) for two different thicknesses, 400 and 600 μm respectively. This paper reports the thermal diffusivity, thermal conductivity, specific heat, porosity, and scanning electron microscope (SEM) images of STD-TBC samples and VC-TBC samples. In addition, a heat transfer model is presented for the STD-TBC and VC-TBC microstructures. The results show an increase in both thermal diffusivity and conductivity for the VC-TBC samples, compared to the STD-TBC sample over the temperature range tested (400°C to 800°C). In addition, there is a temperature dependence of the thermal diffusivity and the thermal conductivity for both VC-TBC and STD-TBC samples. The change of thermo-physical properties is directly linked to the microstructure of the samples, demonstrated by the porosity measurements, SEM images, and the heat transfer model.

ABSTRACT Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve a higher working temperature and thus lead to a better efficiency. Yttria-Stabilized-Zirconia (YSZ), a material with low thermal conductivity, is commonly used as the TBC top coat to provide the thermal barrier effect. In this paper, an analytical model is proposed to estimate the effective thermal conductivity of the TBCs based on the microstructures. This model includes the micro structure details, such as grain size, pore size, volume fraction of pores, and the interfacial resistance. To validate the model, two sets of TBC samples were fabricated and tested for thermal conductivity and associated microstructures. The first set of samples were disk shaped YSZ-Al2 O3 samples fabricated using a pressing machine. The YSZ-Al2 O3 powder mixture was 0, 1, 2, 3, 4 and 5 wt% Al2 O3 /YSZ powder ratio. The second set of samples were fabricated by Atmospheric Plasma Spray process for two different microstructure configurations, standard (STD) and vertically cracked (VC), at two different thicknesses, 400 and 700 urn respectively. A laser flash system was used to measure the thermal conductivity of the coatings. Experiments were performed over the temperature range from 100°C to 800°C. The porosity of the YSZ samples was measured using a mercury porosimetry analyzer, POREMASTER 33 system. A Scanning Electron Microscope (SEM) was used to study the microstructure of the samples. It is observed that the microstructure and the porosity are directly linked with the thermal conductivity values. The relationship of the properties to the real microstructure determines the validity of the proposed model.

ABSTRACT Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve higher turbine inlet temperatures (TITs), improve turbine operating temperatures, reduce fuel consumption, increase components lives and thus lead to better turbine efficiency. Yttria-stabilized zirconia (YSZ), is an ideal candidate for TBCs as it has good thermal shock resistance, high thermal stability, low density, and low thermal conductivity. Traditionally, there are two main methods of fabricating TBCs: air plasma spray (APS) TBCs and electron beam physical vapor deposition (EBPVD) TBCs. It is the objective of this paper to study the effects of APS TBC microstructures in comparison with EBPVD TBCs deposited on NiCoCrAlYHf bond coated In738 substrate material for applications in advanced gas turbines. The bond coat NiCoCrAlY contains 0.25w% Hf which is expected to improve the reliability of standard (STD) and vertically cracked (VC) APS TBC material. TBC top coatings of 300 μm and 600 μm thickness for both standard and VC APS TBC and 300 μm EBPVD TBC were further investigated to determine the effect of coating thickness of TBC performance. Selected test specimens were evaluated for dry and wet thermal cyclic oxidation performance. Thermal property determination of select samples was achieved using a laser flash system that measures the thermal diffusivity and specific heat capacity from which the thermal conductivity is calculated. Lastly, select YSZ-Al2 O3 composite structures were analyzed in addition to APS and EBPVD TBC microstructure, porosity, and thermal conductivity determination using a variety of analytical techniques. A laser flash system was used to measure the thermal diffusivity for all the samples. A POREMASTER 33 system was used to measure the porosity of the APS and EBPVD samples.

ABSTRACT Thermal barrier coatings (TBCs) are applied to blades, vanes, combustion chamber walls, and exhaust nozzles in gas turbines not only to limit the heat transfer through the coatings but also to protect the metallic parts from the harsh oxidizing and corrosive thermal environment. There is a growing interest in operating these hot gas path (HGP) components at optimal conditions which has resulted in a continuous increase of the turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially in the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBC such as electron beam physical vapor deposition thermal barrier coatings (EBPVD TBCs) and Air Plasma Sprayed thermal barrier coatings (APS TBCs). This study focuses on the estimation of temperature distribution in the turbine metal substrate (IN738) and coating materials (EBPVD TBC and APS TBC) subjected to isothermal conditions (1573 K) around the turbine blade. The heat conduction in the turbine blade and TBC systems necessary for the evaluation of substrate thermal loads are assessed. The steady state 2D heat diffusion in the turbine blade is modeled using ANSYS FLUENT computational fluid dynamics (CFD) commercial package. Heat transfer by radiation is fully accounted for by solving the radiative transport equation (RTE) using the discrete ordinate method. The results show that APS TBCs are better heat flux suppressors than EBPVD TBCs due to differences in the morphology of the porosity present within the TBC layer. Increased temperature drops across the TBC leads to temperature reductions at the TGO/bond coat interface which slows the rate of the thermally induced failure mechanisms such as CTE mismatch strain in the TGO layer, growth rate of TGO, and impurity diffusion within the bond coat.

ABSTRACT High Temperature exposure and the corresponding thermo-mechanical behavior of cylindrical polymer composite pipe using CFD simulation has been investigated in this study. The software FLUENT was employed for the analysis of heat transfer, by coupling equations of energy and motion. Analysis was done based on applied external boundary temperature profile, change in internal energy, the total surface heat flux and surface heat transfer rate in order to evaluate the extent of thermal damage. FLUENT compatible program written in C++ language in the form of user define functions (UDF) has been developed and used to specify the time dependent heat flux generated temperature as well as temperature dependent thermal properties of density, thermal conductivity and specific heat. Available furnace test experimental data from (ASTM 1173-95) database were used as outer surface boundary condition in the model setup by developing it into UDF correlation equations. The outputs of the FLUENT simulations are predictions of transient temperature distribution through the thickness of the pipe wall that were then used in evaluating the thermal stresses of the composite pipe. Validation of the simulation results is done with existing data available in the literature. Using the wall generated temperatures, internal energy, the rate of change of the temperature dependent properties and the heat transfer rate, the thermal endurance of each of the coatings materials has been predicted in this work. At the same time knowledge of the thermal performance of these materials is essential for the optimum design of protection based on the composite application.

ABSTRACT Traditionally, thermal barrier coatings (TBCs) are used in gas turbine engines to create an insulation layer between the metallic components and the gases in the hot section. Atmospheric plasma spray (APS) is a common method used to produce TBCs. The goal of this study is to study the porosity and thermal cycling behavior of standard (STD) and vertically cracked (VC) thermal barrier coatings (TBCs) fabricated by Atmospheric Plasma Spray (APS) for two different thicknesses, 300 and 600 μm respectively. Electron Beam Physical Vapor Deposition (EBPVD) coatings with 300 micron thickness prepared under tumbled and non-tumbled conditions were studied. For this study, mercury porosimeter equipment (POREMASTER 33) by Quantachrome Instruments was used to measure porosity, and pore size distribution. Scanning Electron Microscopy (SEM) images were obtained for all the samples. The images showed clear microstructural difference between the APS and EBPVD coatings. All the coatings were thermal cycled to 1200°C and the conventional APS-STD (300μm) performed the best followed by APS-VC coatings and EBPVD coatings which performed similarly.

ABSTRACT Thermo-mechanical properties and thermal cycling behavior of gadolinium zirconate Gd2Zr2O7 (GZ) based thermal barrier coatings (TBCs) was investigated in this study in comparison to conventional yttria-stabilized zirconia (YSZ) coatings. This paper presents results focusing on coefficient of thermal expansion (CTE) measurements, thermal cycling tests, measured elastic properties and porosity of the multilayered GZ/YSZ TBCs deposited by atmospheric plasma spraying (APS) on an Inconel 738 (IN738) superalloy substrate. SEM microstructural images of failed TBC specimens are also presented. Samples of different double layer combinations with one layer being either 100% YSZ or 100% GZ and the second containing varying amounts of the two compounds were prepared to determine optimum combination that maintains good insulating properties while reducing the CTE mismatch between the TBC layers. The temperature range of the tests was 25°C to 1300°C. The samples are processed by APS on (Ø −12.7 mm, thickness 3 mm) IN738. Using a dilatometer, CTEs of the as sprayed top coat (TC) combinations are measured and compared. The elastic properties are measured using a Hysitron nanoindenter. Results showed that the 10%GZ/ 90%YSZ+ 100YSZ double layered structure was the best among the tested GZ based TBCs and the delamination of GZ/YSZ coating first initiated in the GZ layer close to the interface of GZ and YSZ layers, which was mainly caused by the sintering effect of the GZ layer.

Thermal barrier coatings (TBCs) that can be suitable for use in industrial gas turbine engines have been processed and compared with electron beam physical vapor deposition (EBPVD) microstructures for applications in advanced gas turbines that use coal-derived synthesis gas. Thermo-physical properties have been evaluated of the processed air plasma sprayed TBCs with standard APS-STD and vertically cracked APS-VC coatings samples

ABSTRACT This paper presents forced convection studies on a flat plate and NACA 0010 section airfoil surfaces. The surface temperatures of the models at given stations are measured with K-type thermocouples and used to assess the heat transfer characteristics of the models. The model surfaces are subjected to constant heat fluxes of 1.45kW/m2 (for the flat plate) and 0.60kW/m2 (for the airfoil) using KH Kapton flexible heaters. The temperature readings are then utilized to determine the heat transfer coefficients on the surfaces over a range of Reynolds numbers, from which Nusselt number correlations are deduced. The experiments were conducted in an open circuit wind tunnel, powered by a 37 kW motor, capable of generating air velocities of up to about 41 m/s in the 24 square-inch test section. For the flat plate, the Nusselt number correlations obtained agree well with what is reported in literature. The plate length (0.25m) used for the experiment was just enough for the initiation of turbulent thermal boundary layer at 35.60 m/s air speed. The flow phenomena and Nusselt number correlations on the NACA 0010 airfoil surface are also evaluated and found to fit correlations similar to that of a cylinder in cross flow for laminar case. However, turbulence on the airfoil surface has a significant influence on the average Nusselt number relation. Correlations for the laminar and turbulent flow regimes have been presented using a modified Hilpert and Churchill correlation for a cylinder in crossflow.

ABSTRACT Thermal efficiency of energy conversion systems such as gas turbines can be increased greatly with an increase in the turbine inlet temperature of combustion gases. However, this necessitates the use of efficient cooling techniques in addition to thermal barrier coatings (TBCs) to help significantly improve the life expectancy of gas turbine blades. The effect of TBC use is the formation of oxides, particularly alumina, at the interface of the ceramic top coat and bond coat material during in-service application. This effect is well known to cause failure of TBCs exposed to extreme high temperature environments. The objective of this paper is to present a micro-scale finite difference thermal model for the TBC-Substrate system that considers growth of the TGO layer and predicts in-situ thermal gradients. The governing equation is the transient heat diffusion equation discretized over a 1-D domain using mean value finite volume method with grid adaptation for zones involving depletion of bond coat (BC) material and TGO growth; hence, necessitating a moving interfacial boundary problem. The resulting algebraic equations are simultaneously solved in MATLAB to produce temperature distributions and BC/TGO interfacial locations. The model has utility in studying the evolution of residual stresses and hence prediction of TBC durability and failure.

ABSTRACT This paper presents the results of an experimental study of the high-temperature isothermal oxidation behavior and micro-structural evolution in plasma sprayed thermal barrier coatings (TBCs) at temperatures between 900 and 1200 °C. Two types of specimens were produced for testing. These include a standard and vertically cracked (VC) APS. High temperature oxidation has been carried out at 900, 1000, 1100 and 1200 °C. The experiments have been performed in air under isothermal conditions. At each temperature, the specimens are exposed for 25, 50, 75 and 100 hours. The corresponding microstructures and microchemistries of the TBC layers are then examined using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy EDS. Changes in the dimensions of the thermally grown oxide (TGO) layer are determined as functions of time and temperature. The evolution of bond coat microstructures/interdiffusion zones and thermally grown oxide (TGO) layers are compared in TBCs with standard (STD) and vertically cracked (VC) microstructures.

ABSTRACT Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve a higher working temperature, and thus lead to a better efficiency. Yttria-stabilized-zirconia (YSZ), a material with low thermal conductivity, is commonly used as the top coat layer to provide the thermal barrier effect. Recent studies demonstrated that YSZ-Al2 O3 composite layer could reduce the oxygen diffusion through the TBC, thus YSZ-Al2 O3 composite layer potentially could be used to mitigate the spalling induced failure of a TBC coating. The goal of this study is to investigate the effect of the addition of Al2 O3 on the thermal properties of YSZ based TBCs. In this study, a stainless steel die was used to make disk shaped samples with 0, 1, 2, 3, 4 and 5 wt% Al2 O3 /YSZ powder ratios under uniaxial pressure. A laser flash system was used to measure the thermal diffusivity for all samples and the porosity of the samples is measured using mercury porosimetry. It is found that adding Al2 O3 to YSZ decreases the thermal conductivity and increases the porosity of the ceramic composites.

ABSTRACT Non-equilibrium parallel molecular dynamics simulation is used to determine the thermal conductivity of Alumina in the [2 1 1 0]direction at 1200 K: When thermal expansion is not allowed it is found to be 3.45 W/mK, while with thermal expansion it is 2.95 W/mK. A short ranged empirical potential for Yttria stabilized Zirconia (YSZ) is developed by fitting to available ab initio and experimentally derived data for Zirconia. With this potential, simulations of YSZ at 2073.16 K, with 4.9–23.1 mol% of Yttria in Zirconia, shows diffusing Oxygen and non-diffusing Zirconium and Yttrium atoms as expected. However, the diffusion constant of oxygen increases with the Yttria content, inconsistent with simulations with long range interactions showing a peak around 10 mol% of Yttria and also inconsistent experiment at 923 K. Visualizing the dynamics of atoms in Alumina, when driven by a heat-current forcing perturbation, shows phonon-like modes indicating the need for smaller perturbation or an alternate method to determine thermal properties.

... Trans. Patrick Mensah. Stanford: Stanford UP, 1998. Djebar, Assia. ... Donadey, Anne. Recasting Postcolonialism. Portsmouth: Heinemann, 2001. Elia, Nadia. Trances, Dances and Vociferations: Agency and Resistance in Africana Women's Narratives. New York: Garland, 2001.

The results of an experimental study of the high-temperature isothermal oxidation behavior and microstructural evolution in two variations of air plasma sprayed ceramic thermal barrier coatings (TBCs) are discussed in the paper. Two types of TBC specimens were produced for testing. These include a standard and vertically cracked APS. High temperature oxidation was carried out at 900, 1000, 1100 and

ABSTRACT This paper presents a numerical solution of the hyperbolic heat conduction equation in a thermal barrier coating (TBC) structure under an imposed heat flux on the exterior of the TBC. The non-Fourier heat conduction equation is used to model the heat conduction in the TBC system that predicts the heat flux and the temperature distribution. This study presents a more realistic approach to evaluate in-service performance of thin layers of TBCs typically found in hot sections of land based and aircraft gas turbine engines. In such ultrafast heat conduction systems, the orders of magnitude of the time and space dimensions are extremely short which renders the traditional Fourier conduction law, with its implicit assumption of infinite speed of thermal propagation, inaccurate. There is, therefore, the need for an advanced modeling approach for the thermal transport phenomenon taking place in microscale systems. A hyperbolic heat conduction model can be used to predict accurately the transient temperature distribution of thermal barrier structures of turbine blades. The hyperbolic heat conduction equations are solved numerically using a new numerical scheme codenamed the mean value finite volume method (MVFVM). The numerical method yields minimal numerical dissipation and dispersion errors and captures the discontinuities such as the thermal wave front in the solution with reliable accuracy. Compared with some traditional numerical methods, the MVFVM method provides the ability to model the behavior of the single phase lag thermal wave following its reflection from domain boundary surfaces. In addition, parametric studies of properties of the substrate on the temperature and the heat flux distributions in the TBC revealed that relaxation time of the substrate material, unlike the thermal diffusivity and thermal conductivity has very little effect on the transient thermal response in the TBC. The study further showed that for thin film structures subject to short time durations of heat flux, the hyperbolic model yields more realistic results than the parabolic model. [DOI: 10.1115/1.4006976]

Adhesive bonded single lap joint has been used extensively in laminated composite structures. Using neat resin adhesives, however, the joint strength is comparatively low and the fabrication time is long. In order to increase the joint strength and reduce the fabrication ...

A typical blade material is made of Nickel super alloy and can bear temperatures up to 950°C. But the operating temperature of a gas turbine is above the melting point of super alloy nearly at 1500°C. This could lead to hot corrosions, high temperature oxidation, creep, ...