Ahmed Taha

A micromechanics approach to the study of hydrogen transport and embrittlement

Abstract The mechanisms of hydrogen related fracture are briefly outlined. Previous investigations on the physics and treatment of the hydrogen transport processes are reviewed. A hydrogen diffusion model based on the interaction of hydrogen induced strain in the lattice with local material elastoplasticity is presented. Finite element studies were carried out to analyze the hydrogen distribution in the neighborhood of a blunting crack tip under small scale yielding conditions and in the neighborhood of a rounded notch in a four-point bend specimen. The calculated hydrogen concentration profiles and experimental observations of embrittlement in high strength steels are used to make evaluative statements on the occurrence of the first microcracking event.

Alya: Multiphysics engineering simulation toward exascale

Alya is a multi-physics simulation code developed at Barcelona Supercomputing Center (BSC). From its inception Alya code is designed using advanced High Performance Computing programming techniques to solve coupled problems on supercomputers efficiently. The target domain is engineering, with all its particular features: complex geometries and unstructured meshes, coupled multi-physics with exotic coupling schemes and physical models, ill-posed problems, flexibility needs for rapidly including new models, etc. Since its beginnings in 2004, Alya has scaled well in an increasing number of processors when solving single-physics problems such as fluid mechanics, solid mechanics, acoustics, etc. Over time, we have made a concerted effort to maintain and even improve scalability for multi-physics problems. This poses challenges on multiple fronts, including: numerical models, parallel implementation, physical coupling models, algorithms and solution schemes, meshing process, etc. In this paper, we introduce Alyas main features and focus particularly on its solvers. We present Alyas performance up to 100.000 processors in Blue Waters, the NCSA supercomputer with selected multi-physics tests that are representative of the engineering world. The tests are incompressible flow in a human respiratory system, low Mach combustion problem in a kiln furnace, and coupled electro-mechanical contraction of the heart. We show scalability plots for all cases and discuss all aspects of such simulations, including solver convergence.

Effect of subsonic microjets on cold flow in dump combustors

Fluidic control of shear layers in air-breathing liquid-fuel combustors is considered as a viable means for improving performance. The increasing demand for compact combustion accompanied by low drag, high turndown ratio, and a reliable flame anchor calls for concurrent application of advanced control strategies such as counter-current shear and microjets. Traditional flame holders, such as the backward-facing step dump combustor, provide a protective environment for the flame to reside. However, these systems also carry a significant drag penalty. The focus of this research is to explore the role of microjets as a shear layer control strategy in dump combustors via numerical simulation. The simulations have been conducted within the Reynolds-averaged Navier-Stokes (RANS) framework. Results for cold flow have shown that increasing the momentum of the microjets leads to greater non-uniformity in the turbulent flow field. Microjets can also affect the recirculation zone, thus influencing flame stabilization. Many parameters such as the location, size, mass flow rate ratio, and momentum ratio of the microjets determine the overall performance. A detailed parametric study is conducted to investigate the effect of various parameters.

Hydrogen, Biodiesel and Ethanol for Internal Combustion Engines: A Review Paper

Alternative fuels research has been on going for well over many years at a number of institutions. Driven by oil price and consumption, engine emissions and climate change, along with the lack of sustainable fossil fuels, transportation sector has generated an interest in alternative, renewable sources of fuel for internal combustion engines. The focus has ranged from feed stock optimization to engine-out emissions, performance and durability. Biofuels for transportation sector, including alcohols (ethanol, methanol…etc.), biodiesel, and other liquid and gaseous fuels such as methane and hydrogen, have the potential to displace a considerable amount of petroleum-based fuels around the world. First generation biofuels are produced from sugars, starches, or vegetable oils. On the contrary, the second generation biofuels are produced from cellulosic materials, agricultural wastes, switch grasses and algae rather than sugar and starch. By not using food crops, second generation biofuel production is much more sustainable and has a lower impact on food production. Also known as advanced biofuels, the second-generation biofuels are still in the development stage. Combining higher energy yields, lower requirements for fertilizer and land, and the absence of competition with food, second generation biofuels, when available at prices equivalent to petroleum derived products, offer a truly sustainable alternative for transportation fuels. There are main four issues related to alternative fuels: production, transportation, storage, handling and usage. This paper presents a review of recent literature related to the alternative fuels usage and the impact of these fuels on fuel injection systems, and fuel atomization and sprays for both spark-ignition and compression-ignition engines. Effect of these renewable fuels on both internal flow and external flow characteristics of the fuel injector will be presented.Copyright

Parametric study of effect of geometric configurations of micro jets in an axisyimnetric dump corabustor flow

Present day engineering and environment challenges demand the design of combustion systems to be lean and clean. Various flow control strategies have been studied to make them more efficient. In this study, microjets are used as a flow control measure and a parametric study of the geometric configurations is performed. Increasing the number of microjets decreases the outlet temperature, but increases the temperature near the inlet. Compared with no microjets configuration, outlet temperatures are higher with microjets. Species concentration at the outlet is also affected by the number of microjets. Least amount of unburnt fuel at the outlet was for the configuration with the highest number of microjets.