Shear-induced migration and clustering mechanisms of kaolinite clay particles under the oscillating face seal by Jinchul Hong
The migration and clustering mechanisms of clay particles under the contact band of an oscillating face seal are investigated. There are two seperate stages in the wear, which are called the break-in period and the aggressive wear period. First small particles in the slurry penetrate through a short distance under the seal lip, where a thin oil film is present and there is no sign of significant wear. This period is called breakin period. However, after a number of cycles, the particles aggregate and start to form clusters. These clusters grow and the seal wears rapidly. Thus cluster formation plays a critical role in the whole wear process. As a first step, we have compared the possible forces which drive particles inwards. These include the effects of Van der Waals interactions, hydrodynamic shear, Brownian diffusion, etc. In the initial stages of the process, when the particle concentration is not high, prior experiments are consistent with shear-induced migration as the primary mechanism. Particles migrate inwards due to gradients in concentration, shear rate, viscosity and the effects of curved-streamline. Van der Waals force are likely to be dominant when the concentration is high enough so that the distance between the particles is small. In order to confirm these mechanisms, a new experiment using steel seals was carried out and corroborates earlier studies. Finally, some clustering and dispersion mechanisms such as the effects of dissolved ions (leading to aggregation) and thermal expansion of the seal lip (leading to dispersion) were also studied. Thesis Supervisor: L.Mahadevan Title: Associate Professor
Rheology and microstructure of dense suspensions of plate-shaped colloidal particles
The flow properties and microstructure of dense kaolin clay suspensions are explored for volume fractions, φ, as large as 0.39. To avoid flocculation these particles were suspended in pH 10 buffer solutions where edge–face interactions are negligible. These plate-shaped particles have an aspect ratio of 10–12 and, thus, are likely to show alignment above a critical volume fraction φ* > 0.10. At low φ, the suspensions are Newtonian but show substantial orientation with increased shear rate. As φ is increased, the suspensions develop a yielding type of behavior. In the concentrated region φ > φ*, elastic moduli are a function of the previously applied shear rate, γ, decreasing from a low shear rate plateau of G′(0) to a high shear rate plateau of G′(∞). We interpret the modulus behavior as being a consequence of changes in alignment of domains in the suspension. Conductivity and x-ray scattering measurements confirm that particle alignment increases with increasing shear rate. The normalized modulus value, Gnorm′=[G′(γ)−G′(∞)]/[G′(0)−G′(∞)], is independent of φ indicating that the characteristic alignment shear rate is independent of volume fraction. Conductivities and x-ray scattering intensities normalized in the same manner are also independent of the volume fraction, and superimpose on the modulus data. Surprisingly, these data indicate that fractional particle alignment at a given shear rate is independent of φ for 0.05   0.10. At low φ, the suspensions are Newtonian but show substantial orientation with increased shear rate. As φ is increased, the suspensions develop a yielding type of behavior. In the concentrated region φ > φ*, elastic moduli are a function of the previously applied shear rate, γ, decreasing from a low shear rate plateau of G′(0) to a high shear rate plateau of G′(∞). We interpret the modulus behavior as being a consequence of changes in alignment of domains in the suspension. Conductivity and x-ray scattering measurements confirm that particle alignment increases with increasing shear rate. The normalized modulus value,...
A Critical Assessment on Functional Attributes and Degradation Mechanism of Membrane Electrode Assembly Components in Direct Methanol Fuel Cells
Direct methanol fuel cells (DMFC) are typically a subset of polymer electrolyte membrane fuel cells (PEMFC) that possess benefits such as fuel flexibility, reduction in plant balance, and benign operation. Due to their benefits, DMFCs could play a substantial role in the future, specifically in replacing Li-ion batteries for portable and military applications. However, the critical concern with DMFCs is the degradation and inadequate reliability that affect the overall value chain and can potentially impede the commercialization of DMFCs. As a consequence, a reliability assessment can provide more insight into a DMFC component’s attributes. The membrane electrode assembly (MEA) is the integral component of the DMFC stack. A comprehensive understanding of its functional attributes and degradation mechanism plays a significant role in its commercialization. The methanol crossover through the membrane, carbon monoxide poisoning, high anode polarization by methanol oxidation, and operating parameters such as temperature, humidity, and others are significant contributions to MEA degradation. In addition, inadequate reliability of the MEA impacts the failure mechanism of DMFC, resulting in poor efficiency. Consequently, this paper provides a comprehensive assessment of several factors leading to the MEA degradation mechanism in order to develop a holistic understanding.