The physiological mechanisms that give rise to the inception and development of a cerebral aneurysm are accepted to involve the interplay between the local mechanical forces acting on the arterial wall and the biological processes occurring at the cellular level. In fact, the wall shear stresses (WSSs) that act on the endothelial cells are thought to play a pivotal role. A computational framework is proposed to explore the link between the evolution of a cerebral aneurysm and the influence of hemodynamic stimuli that act on the endothelial cells. An aneurysm evolution model, which utilizes a realistic microstructural model of the arterial wall, is combined with detailed 3D hemodynamic solutions. The evolution of the blood flow within the developing aneurysm determines the distributions of the WSS and the spatial WSS gradient (WSSG) that act on the endothelial cell layer of the tissue. Two illustrative examples are considered: Degradation of the elastinous constituents is driven by deviations of WSS or the WSSG from normotensive values. This model provides the basis to further explore the etiology of aneurysmal disease.

A fluid-solid-growth (FSG) model of saccular cerebral aneurysm evolution is developed. It utilises a realistic two-layered structural model of the internal carotid artery and explicitly accounts for the degradation of the elastinous constituents and growth and remodelling (G&R) of the collagen fabric. Aneurysm inception is prescribed: a localised degradation of elastin results in a perturbation in the arterial geometry; the collagen fabric adapts, and the artery achieves a new homeostatic configuration. The perturbation to the geometry creates an altered haemodynamic environment. Subsequent degradation of elastin is explicitly linked to low wall shear stress (WSS) in a confined region of the arterial domain. A sidewall saccular aneurysm develops, the collagen fabric adapts and the aneurysm stabilises in size. A quasi-static analysis is performed to determine the geometry at diastolic pressure. This enables the cyclic stretching of the tissue to be quantified, and we propose a novel index to quantify the degree of biaxial stretching of the tissue. Whilst growth is linked to low WSS from a steady (systolic) flow analysis, a pulsatile flow analysis is performed to compare steady and pulsatile flow parameters during evolution. This model illustrates the evolving mechanical environment for an idealised saccular cerebral aneurysm developing on a cylindrical parent artery and provides the guidance to more sophisticated FSG models of aneurysm evolution which link G&R to the local mechanical stimuli of vascular cells.

This work investigates numerically a catalytic postcombustor for a micro-solid oxide fuel cell (SOFC) system. The postcombustor oxidizes toxic and explosive carbon monoxide (CO) and hydrogen exiting a solid oxide fuel cell to carbon dioxide and water. A single 1 mm diameter monolith reactor channel coated with platinum catalyst is modeled in this work. The inlet stream composition is provided by a semi-analytical 2D model of a detailed SOFC system. The model of the postcombustor includes the 2D axisymmetric Navier–Stokes equations, heat conduction in the channel wall, and a multistep finite-rate mechanism for the surface reactions. It is shown that under the operation conditions considered, the influence of homogeneous (gas phase) reactions can be neglected. The model predicts the expected adiabatic temperatures at the postcombustor outlet correctly and can be used for dimensioning and optimization. Postcombustor performance varies significantly with the choice of the operating parameters of the fuel cell. The most critical molecule at the SOFC outlet is shown to be CO because its depletion is slower than that of H2 for the entire operating range of the SOFC. It can be shown that the postcombustor is able to reduce the level of CO below the toxicity threshold of 25 ppm. Although higher voltages of the fuel cell lead to faster CO conversion in the postcombustor, they also result in a significant increase in wall temperature of the catalyst device. Furthermore, the percentage of SOFC power output used for pump work is lowest for the voltage where the maximum power is reached. For postcombustion the optimal operation point of the SOFC is at the voltage for maximum power of the SOFC system.

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