Kevin R. Ellwood

A finite-element model for an electrostatic bell sprayer

Abstract Electrostatic rotary atomizers are receiving considerable interest for the application of paint systems, which include primer, basecoat, and clearcoat. Within this paper, a model is described providing some insights into the dynamics and couplings of the physics driving the process. The foundation of the model was built around a Lagrangian particle tracking scheme to simulate the formation of spray patterns for charged droplets. Steady-state spray patterns were computed using an iterative particle source in cell (PSIC) approach, which represents momentum exchange between the droplet and gas phases as a body force. The flow solver is based on a finite-element formulation incorporating the streamlined upwind Petrov Galerkin (SUPG) method for stabilization. Space charge on the surrounding electric field, caused by the motions of charged particles, was computed as a natural extension of the PSIC method.

Application of Fourier-Based Transforms to Impedance Spectra of Small-Diameter Tubular Solid Oxide Fuel Cells

Recent demonstrations of direct utilization of hydrocarbon fuels have stimulated an automotive interest in solid oxide fuel cells for reformerless auxiliary power units with high power density, high chemical-to-electrical efficiency, and low exhaust emissions. Furthermore, recent designs with small-diameter oxide tubes appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. To understand the limiting transient processes in these small-tube fuel cell designs, we applied an analysis approach which requires no a priori equivalent circuit model assumptions. This approach was applied to the electrochemical impedance spectroscopy (EIS) data measured from such cells in the temperature range from 585 to 888°C In this way, the complex, overlapping arc EIS details (seen in Cole-Cole plots) were transformed in a network-model-independent way into a spectrum of relaxation times. We extended the deconvolution method to allow peak fitting and integration to calculate the resistances of individual processes within the cathode polarization, which becomes limiting in comparison to either anode or electrolyte at temperatures below about 700°C. With the new results, the process with the highest apparent activation energy can be targeted to improve cathode development.

Accelerated aging of tires, Part II

Abstract The purpose of this research is to determine the conditions whereby a new tire can be artificially aged in an accelerated manner, in order to duplicate the actual mechanism of chemical agi...

Correlation of rubber properties between field aged tires and laboratory aged tires

The kinetics of aging of key tire properties both in the field and in oven exposures at different temperatures has been interpreted by using a combination of empirical models and accelerated shift factors. Crosslink density and rubber modulus increase with aging while peel strength and elongation-to-break decrease. In the case of oven aging, the rate of property change increases from 40 °C to 70 °C and then decreases. In the case of field aging, the rate of property change is greatest in hotter climates such as Phoenix and is slower in cooler climates such as Detroit. Spare tires age at a rate that is ∼70% as fast as on-road tires. Below 70 °C, the rate data for all of the aging changes can be fit to an Arrenhius relationship with an activation energy of ∼69 kJ/mole, a value that is consistent with the aging process resulting from diffusion limited oxidation. The measured acceleration factor of oven aging at 70 °C relative to on-road aging in Phoenix is independent of the property change measured confirming that it is possible to chemically age tires in ovens. It takes 6-7 weeks of oven aging at 70 °C to produce a tire that is aged 4 years in Phoenix. Field results show that the rate of tire aging varies by over a factor of 5 for the different tire types and brands studied in this work. The implications for tire durability testing are discussed.

Numerical Simulation of Thermal Oxidation in Automotive Tires

Abstract A finite element kinetic model has been developed to interpret issues related to accelerated aging of tires. The model is based on the Basic Autoxidation Scheme and incorporates mass transport limitations related to diffusion of oxygen into the layered elastomer system. The extent of oxidation is calculated at different locations within the tire as a function of time and temperature. Results for aging rates due to thermal oxidation predicted by the model are compared to experimentally derived data such as crosslink density, elongation-to-break and modulus variation. Comparative results for aging rates in different regions of the tire suggest that a reasonable accelerated oven testing temperature range is 60 °C to 70 °C. Additionally, initial results for predicted aging rates for a rolling tire are presenting indicating the thermal oxidation plays a large role in tire aging.

A simplified analysis method for correlating rotary atomizer performance on droplet size and coating appearance

The dominant method of atomizing automotive paint is through the use of rotating bell sprayers. For this class of atomizer, the problem of paint thickness across the bell has been theoretically solved on a representative geometry that includes factors such as fluid flow rate, bell speed, bell cup radius, and fluid properties. It was assumed that the paint film eventually forms uniform ligaments at the bell cup edge that break due to hydrodynamic stability during the paint spray process; thus, creating a characteristic particle size distribution for the spray. These particle size distributions will vary as the spray parameters, specifically fluid flow rate, bell speed, and bell cup radius, vary. The theoretical model that has been developed strongly correlates to the literature data available for paint droplet size from rotary bell atomizers. Expansion of the correlation of the theoretical model to paint appearance wavelength measurements, Wc and Wd, in place of droplet size provides further understanding of the effect of paint spray parameters on paint appearance. Use of these correlations can help to optimize paint appearance and improve paint spray simulation results.

A Finite Element Model for Oven Aged Tires

Abstract A finite element kinetic model has been developed to interpret issues related to accelerated aging of tires. The model is based on the Basic Autoxidation Scheme and incorporates mass trans...

Correlation of Laboratory Tire Endurance and Rubber Aging

Abstract Tire endurance as measured by performance on the National Highway Traffic Safety Administration (NHTSA) Stepped Up Load (SUL) test is shown to be a function of both tire construction and the extent of oxidation in the skim and wedge rubber regions of the tire, as measured by peel strength or elongation to break retention. Tire constructions can be distinguished by speed rating. Tires with higher speed ratings (> S) tend to have relatively high times-to-failure (TTF) in the SUL test and are relatively insensitive to rubber oxidation. SUL TTFs for tires with speed rating of S and lower tend to be much more sensitive to rubber oxidation. For these tires, the SUL TTF decreases linearly with aging time in the field. The rate of loss of SUL performance is proportional to the rate of loss of rubber properties. The large variability observed in the SUL results from field aged tires can be explained by the natural variability in oxidation aging rates observed for these tires. For oven aged tires, the corr...

A hybrid ray-wave optics model to study the scattering behavior of silver metallic paint systems

The gonioapparent lightness of a metallic paint system depends strongly on the 3D microstructure of the platelet-containing basecoat layer, and on the platelet roughness. Current optical models which are used to simulate the paint’s appearance, however, ignore the multi-scale and multi-dimensional microstructural features, which limits their predictive power. Here, we describe a hybrid, ray-wave optics model for metallic paints. This model incorporates the ray-optics of the 3D platelet microstructure and the wave effects that result from the surface roughness of the platelets. This model is used to link the roughness to the reflection lobes of the aluminum platelets, and to the gonioapparent lightness of the paint system. Predicted lightness values from our model matched, at most viewing angles, measurements from physical paint samples. This model can be used to explore the effect of platelet surface roughness on the lightness of the complete paint system and predict the appearance of paints with different platelet microstructures.

A statistical method for the relative comparison of automotive paint colours using digital imaging analysis

Relative colour comparisons were performed using digital imaging techniques and analysis. A statistical method was used to quantify how well a test colour matched a colour standard. Colour histogram comparisons were performed by incorporating a combination of control limits (based on prediction intervals) and threshold limits that were calculated for each curve set. Test colours were imaged and compared to colour standards by calculating the per cent match for each of the RGB curves. Colours that did not show a per cent match of 60 per cent or greater in all three colour curves were considered failures. Some colour families, e.g. reds, required larger control limits to account for colour variability.