Patrick F. Mensah

Investigation of prepreg bonded composite single lap joint

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 time, two types of fiber pre-impregnated (prepreg) composites were used to bond composite single lap joints. Test specimens were prepared per ASTM D 3165-95 standard. Ninety days of accelerated conditioning using seawater and ultraviolet radiation were conducted to investigate the long-term performance of prepreg bonded single-lap joint in an offshore environment. The shear strength of various specimens was obtained using tension tests. Two types of neat resin bonded specimens were also used for comparisons. Finite element analysis was implemented to justify test results. Parameters affecting the load carrying capacity of prepreg bonded composite single lap joints were investigated based on finite element analysis results.

Thermo-Mechanical Study of the Role of Gd2Zr2O7 (GZ) in Improving Life of YSZ and GZ Double Layered Thermal Barrier Coatings

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 (O −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.Copyright

Design, Integration, and Initial Testing of an Integrated System for Thermal Conductivity Measurement of Insulation Materials at High Pressure

A hyperbaric chamber integrated with thermal conductivity measurement apparatus has been designed and constructed. The apparatus was designed using the ASTM c177 – the guarded hot plate method. The apparatus is a steady-state thermal conductivity measuring device with an operational pressure range up to 100 bar. This newly-constructed pressure vessel has the capability of containing pressures up to 100 bar. The thermal conductivity measurement method and the chamber are designed together, allowing a much improved measurement system over previous high-pressure measurements. The thermal conductivity of a calcium silicate block insulation has been measured at pressures up to 60 Bar. High pressure measurements are performed in a nitrogen gas environment. The hermal conductivity of the material shows a dependence on pressure. The thermal conductivity curve shows a rising trend for increasing pressure. There is a significant increase in the thermal conductivity when the nitrogen gas is in the supercritical thermodynamic state.

Pressure and Thermophysical Property of Gas Dependence of Effective Thermal Conductivity of a Porous Silica Insulator

The thermal conductivity of a high-temperature calcium silicate block insulation product was measured in gaseous environments at pressures up to 100 bar at room temperature. The thermal conductivity of the porous material was tested in nitrogen, argon, and carbon dioxide gaseous environments. These tests were performed in a newly-constructed pressure chamber integrated with a thermal conductivity testing device. A standardized testing method was employed in the design of the apparatus. The test method used was based on the ASTM c177, the guarded-hot-plate method [1]. Tests performed in a carbon dioxide pressure medium have produced data with thermal conductivity as a nonlinear function of pressure. The results of tests conducted using nitrogen and argon show that the variations of thermal conductivity of the porous silica insulating material are linear functions of pressure and specific heat (Cv) of the fill gas. Tests performed in a nitrogen gaseous environment have relatively higher thermal conductivity values than thermal conductivity values at corresponding pressures in argon gaseous environment. This trend is attributable to the higher thermophysical property values of nitrogen than those of argon. This observation suggests that the thermophysical properties of the fill gas have significant effect on the effective thermal conductivity of the porous material. Thermal conductivity data collected in both nitrogen and argon pressure media have coefficients of determination (r2) of 0.9955 and 0.9956, respectively. An exponential function fitted to the carbon dioxide data produced a coefficient of determination of 0.9175. A precision study for the newly-constructed steady-state thermal conductivity measuring apparatus was performed in atmospheric air. With a standard deviation of 0.00076 W/m · K and a mean thermal conductivity value of 0.07294 W/m · K, a 95% confidence interval was assumed for a sample space size of 13 for the baseline tests in air. This produced a precision error of ±0.00046 W/m · K (±0.63%), a mean bias error of ±0.00955 W/m · K (±13.09%), and a mean steady-state error of ±1.67%. Hence, the total uncertainty in the mean thermal conductivity value of the baseline tests in atmospheric air could be reported as 0.07294W/m · K ± 13.22% with 95% confidence. The result of the precision study is indicative of the reliability of the apparatus. The single-sample precision uncertainty in thermal conductivity values at varying pressures in the various fill gases were estimated based on the standard deviation of the repeated tests in atmospheric air as 0.001166W/m · K.Copyright

Study of Heat Transfer in a Thermal Barrier Coatings System Using the Dual Phase Lag Model Based on Mean Value Finite Volume Method

This study presents a more realistic approach to evaluate in-service performance of thin layers of materials such as thermal barrier coatings (TBCs) typically found in hot sections of air, land and sea based gas turbine engines. The results of this study can also be used to analyze thermal damage to biological tissues caused by lasers in the treatment of certain diseases. The governing differential equation for the DPL model, which is second order in time and space, is reduced to a system of first order equations by the introduction of an intermediate function. The system of equations is 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 discontinuities in solution domains very well. The study further showed that for thin film structures subject to short time durations, the DPL model is a better model for heat transfer as it is suitable for both microscopic and macroscopic systems.© 2013 ASME

Effects of Thermal Radiation on Air Plasma Spray (APS) Coated Gas Turbine Blade

Thermal barrier coatings (TBCs) are used to protect hot gas path (HGP) components such as the first two stages of turbine blades and vanes of land-based turbine engines against high temperature environment, corrosion and oxidation. The continuing thrust towards higher thermal efficiencies of gas turbines has resulted in a continuous increase of turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBCs such as air plasma spray (APS) TBCs which are transparent and reflective to radiation. This paper focuses on the combined effects of radiation and conduction heat transfer in the semitransparent yttria-stabilized zirconia (YSZ) coatings used to offer thermal protection to turbine blade. The temperature distribution in the turbine blade depends on the surface convection, reflectivity and refractive index of the grey semitransparent YSZ coatings. The temperature distributions in the metal substrate and the TBC systems are determined by solving the steady state heat diffusion equation and the radiative transport equation simultaneously using ANSYS FLUENT 12.0 CFD commercial package. Preliminary results indicate that substrate metal temperature reduction of about 100K results with the use of the TBC. This temperature drop reduces the thermally activated oxidation rate of the bond coat in the TBC and so delays failure of TBC by oxidation. Furthermore, by taking into account the effect of radiation, the temperature distribution in the metal substrate with TBC exceeds the temperature distribution without radiation by about 40 K, signifying the importance of including radiation in the thermal modeling of TBCs for high temperature applications.Copyright

Shear Strength Characteristics of an Ultrasonic Welded Lap Shear Joint

Experimental attempts were carried out on ultrasonically joining glass fiber composite materials using fiber reinforced adhesive. Two sets of specimens with different energy guides were investigated. All the samples failed by shear at the interface of the lap joint. Finite element analysis was conducted to justify the test results and the effect of adherend surface treatment.Copyright

Engine Performance and Emission Products of Pure Diesel and Multi-Feedstock Blended Biodiesel

Due to the ever-reducing conventional petroleum resources, considerable research on renewable energy sources such as biodiesel as a possible “greener” substitute fuel for internal combustion engines is needed. This study aims to compare the engine performance and emission results of various blends of pure diesel and a multi-feedstock (MFS) biodiesel when used in a naturally aspirated air-cooled, single-cylinder direct injection diesel engine. The engine was coupled to a dynamometer for torque measurement and output data transmitted to a PC for post-processing and displayed using customized programs in the computer. Engine combustion products — Nitrogen Oxide emissions (NOx), Hydrocarbons (HCs), Carbon monoxide (CO) and Carbon dioxide (CO2) — were measured and are presented alongside performance properties including brake-specific fuel consumption (BSFC), engine efficiency, torque and power. The experimental results show that, relative to diesel, biodiesel had approximately 3–24% decrease in torque, 4–11% decrease in power, 11–32% increase in BSFC and 8–29% general reduction in engine efficiency. However, biodiesel reduced the emissions of CO (1.5–6%), CO2 (13–34%) and unburned HCs (3–25%), while NOx emissions were increased significantly (12–48%). These results indicate that smaller percentages of biodiesel (20% or less) could be blended with pure diesel and used in a diesel engine, without any engine modifications, as an alternative and environmentally friendly fuel and without significantly compromising engine performance.Copyright

Modeling of Thermally Grown Oxide Layer Growth as a Moving Boundary Problem

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.Copyright

Parallel Molecular Dynamics Simulations and Immersive Visualization of Thermal Barrier Coating Components: Thermally Growing Oxide and Yttria Stabilized Zirconia

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.Copyright