Zürich, Switzerland
Zürich, Switzerland

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Varna A.,ETH Zurich | Spiteri A.C.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Wright Y.M.,ETH Zurich | Wright Y.M.,Combustion and Flow Solutions GmbH | And 2 more authors.
Applied Energy | Year: 2015

This study presents a joint experimental and numerical investigation of a spray used in selective catalytic NOx reduction (SCR) after-treatment systems for Diesel engine exhaust gases. The focus lies on the impingement conditions and distribution and mixing mechanisms of the spray in cross-flowing exhaust gases. These aspects are vital to system performance but still not well described. A pressure-driven SCR injector is characterized in an optically accessible flow test rig at different temperatures and mass flow rates representative of Diesel engine exhaust gas conditions by means of Mie scattering and Phase Doppler Anemometry (PDA). The effects of cross-flow velocity and temperature on spray structure, droplet size and velocity distributions are assessed resulting in a comprehensive characterization of the spray. Results show that the dense spray core, with droplets up to 200 μm, only moderately reduces in density as smaller droplets are entrained at high exhaust flows. Wall impingement conditions however vary substantially as impingement angles are shallower at higher cross-flow velocities. A detailed assessment of the numerical model is presented and validation is carried out at different measurement locations of interest. The predicted droplet size distributions and velocities follow the observed trends and impingement angles as well as spray film areas on the channel floor are also in agreement with the experimental data. The validated model is subsequently used to numerically study the mixing dynamics. The findings suggest that at low cross-flow conditions two counter-rotating kidney vortices are formed which entrain reflected droplets of the spray impinging on the channel floor, which leads to improved mixing. Vapor concentrations increase close to the side walls and reflected droplets lead to film-formation on the side walls. At higher cross-flow velocities, vortex formation is not evident and spray-wall interaction is less pronounced - impairing mixing and leading to a reduction in concentration uniformity at down-stream locations. Film formation and high vapor concentrations are restricted to the bottom channel centerline. © 2015 Elsevier Ltd.

Wietek M.,VSH Hagerbach Test Gallery Ltd. | Berweger C.,Xirrus GmbH | Lammle C.,Combustion and Flow Solutions GmbH
Underground - The Way to the Future: Proceedings of the World Tunnel Congress, WTC 2013 | Year: 2013

Today, fire detection in road tunnels by gas analysis has not yet reached notable spread. Thus, the goal of the gas analytics project is to identify such a gas sensor and optimize its placement inside the tunnel to serve as a reliable early detection system for evolving fires, even prior to visible smoke or heat formation. Fundamental insight into mechanisms leading to vehicle fires shall be developed, supported by computer simulations and validations by experiments in the test gallery. After successful calibration of the simulation models, other situations shall be extrapolated in order to be able to supply various tunnels with warning sensors in an efficient way. Although the main focus in the project is on safety, environmental aspects and further knowledge of very different materials that could catch fire, e. g. vehicle components or cargo, and the distribution of combustion products, are expected as a by-product of the project. The project work includes identification of (gaseous) components which form at the very early phase of a fire, evaluation of a sensor for reliable detection these components, evaluation of an optimal positioning of sensors in the tunnel and finally the validation of the findings through pilot experiments at VSH. © 2013 Taylor & Francis Group.

Schmitt M.,ETH Zurich | Hu R.,ETH Zurich | Wright Y.M.,ETH Zurich | Wright Y.M.,Combustion and Flow Solutions GmbH | And 2 more authors.
Flow, Turbulence and Combustion | Year: 2015

In this work the flow field evolution, mixture formation and combustion process in an engine with methane Direct Injection (DI) is investigated using Large Eddy Simulations. The supersonic methane injection is modeled according to Müller et al. (2013) and combustion by a level set approach. The flame propagation showed to be dependent on the grid resolution. Higher grid resolutions have two opposing effects: first the fraction of unresolved turbulence is reduced, which decrease the flame speed and second flame wrinkling is increased resulting in faster flame propagation. For the observed setup the wrinkling effect was stronger. The average in-cylinder pressure traces as well as the cyclic variability thereof were compared to experimental data and very good agreement was found. During the supersonic gaseous injection the turbulence level in the cylinder is significantly increased, which dissipates quickly and thus has only a minor effect on the flame propagation. The introduced momentum showed a larger impact, since it enhances the tumble motion resulting in increased turbulence levels as the tumble decays shortly before ignition. During DI the cyclic differences in the tumble motion are preserved, but the impact on the average tumble level results in changing relative differences of the cyclic turbulence levels at ignition timing. Thus an injection direction supporting the tumble flows is expected to reduce the Cycle-to-Cycle Variations (CCV), while a reduction of the tumble strength could increase the CCV level. Compared to the fluctuations in the turbulence levels, the cyclic variability of the equivalence ratio at the injection location with DI showed a minor effect on the simulated CCVs. © 2015 Springer Science+Business Media Dordrecht

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