A comparative study of different reaction models for turbulent methane/hydrogen/air combustion

Reaction modelling of methane/hydrogen combustion has two important aspects. First, such mixtures may be used in future in combustion devices like gas turbines and gas engines in the frame of the demand for efficient energy storage systems, where the amount of hydrogen in natural gas delivering systems may vary according to varying hydrogen production from renewable energies. Second, this can be an important aspect for safety, as such mixtures may occur in disastrous situations and calculations may allow the prediction of safety issues. Modelling of such mixed fuel combustion processes is non-trivial due to the involved preferential diffusion effects, coming from the different diffusivities of methane and hydrogen. In turbulent flame modelling, this topic is of special interest, as also thermo-diffusive instabilities and local influence of the local burning velocity near leading edges of the flame seem to be of importance even for highly turbulent flames. This numerical work deals therefore with a comparative study of five different turbulent combustion models - Bray-Moss-Libby, Linstedt-Vaos (LV), a modified version LV, Turbulent Flamespeed Closure, and Algebraic Flame Surface Wrinkling model - to the situation of turbulent methane/hydrogen/air flames. Validation is done with extensive experimental data obtained by a low swirl burner in the group by Cheng. Besides a basic case with pure methane/air, special emphasis is laid on flames with 40 to 100 % hydrogen content by volume. It is shown that for such methane/hydrogen fuel mixtures common reaction rate models are not sufficient where the fuel effects are included only via a laminar flame speed. Instead, a recently proposed reaction model with the incorporation of an effective Lewis number of the fuel mixture is found to work rather well. This is of both, practical as well as theoretical importance, as for the latter it confirms controversially discussed assumptions of the influence of preferential diffusion

DI Diesel Engine Combustion Visualized by Combined Laser Techniques

In this work we demonstrate that the progress of the combustionccycle in a four-cylinder (in-line) 1.9 1 direct injection Diesel engine can be studied effectively using different laser visualization techniques. Direct optical access to the piston bowl was facilitated by inserting quartz windows in one of the pistons. The flow field at the time of injection was characterized by seeding the flow and illuminating the piston bowl with a laser light sheet. Fuel spray development, auto-ignition and flame propagation in a Diesel cycle were followed by laser shadowgraphy and high speed cinematography while simultaneous laser induced fluorescence (LIF) and Mie scattering images were taken to distinguish the fuel distribution in the liquid and vapor phase. In addition, two dimensional distributions of OH and NO, formed during n-heptane/air combustion in the same engine, were recorded in the pressure range 5 to 50 bar by LIF following narrowband excitation using tunable excimer lasers. Finally, further work, designed to obtain quantitative images and hence data for comparison with model calculations, is outlined.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86790/1/Sick50.pd

Onset of fluidization in vertically shaken granular material

When granular material is shaken vertically one observes convection, surface fluidization, spontaneous heap formation and other effects. There is a controversial discussion in literature whether there exists a threshold for the Froude number $\Gamma=A_0\omega_0^2/g$ below which these effects cannot be observed anymore. By means of theoretical analysis and computer simulation we find that there is no such single threshold. Instead we propose a modified criterion which coincides with critical Froude number $\Gamma_c=1$ for small driving frequency $\omega_0$.Comment: 7 pages, 5 figure

A model for collisions in granular gases

We propose a model for collisions between particles of a granular material and calculate the restitution coefficients for the normal and tangential motion as functions of the impact velocity from considerations of dissipative viscoelastic collisions. Existing models of impact with dissipation as well as the classical Hertz impact theory are included in the present model as special cases. We find that the type of collision (smooth, reflecting or sticky) is determined by the impact velocity and by the surface properties of the colliding grains. We observe a rather nontrivial dependence of the tangential restitution coefficient on the impact velocity.Comment: 11 pages, 2 figure

Static Friction Phenomena in Granular Materials: Coulomb Law vs. Particle Geometry

The static as well as the dynamic behaviour of granular material are determined by dynamic {\it and} static friction. There are well known methods to include static friction in molecular dynamics simulations using scarcely understood forces. We propose an Ansatz based on the geometrical shape of nonspherical particles which does not involve an explicit expression for static friction. It is shown that the simulations based on this model are close to experimental results.Comment: 11 pages, Revtex, HLRZ-33/9

Deficits in Long-Term Recognition Memory Reveal Dissociated Subtypes in Congenital Prosopagnosia

The study investigates long-term recognition memory in congenital prosopagnosia (CP), a lifelong impairment in face identification that is present from birth. Previous investigations of processing deficits in CP have mostly relied on short-term recognition tests to estimate the scope and severity of individual deficits. We firstly report on a controlled test of long-term (one year) recognition memory for faces and objects conducted with a large group of participants with CP. Long-term recognition memory is significantly impaired in eight CP participants (CPs). In all but one case, this deficit was selective to faces and didn't extend to intra-class recognition of object stimuli. In a test of famous face recognition, long-term recognition deficits were less pronounced, even after accounting for differences in media consumption between controls and CPs. Secondly, we combined test results on long-term and short-term recognition of faces and objects, and found a large heterogeneity in severity and scope of individual deficits. Analysis of the observed heterogeneity revealed a dissociation of CP into subtypes with a homogeneous phenotypical profile. Thirdly, we found that among CPs self-assessment of real-life difficulties, based on a standardized questionnaire, and experimentally assessed face recognition deficits are strongly correlated. Our results demonstrate that controlled tests of long-term recognition memory are needed to fully assess face recognition deficits in CP. Based on controlled and comprehensive experimental testing, CP can be dissociated into subtypes with a homogeneous phenotypical profile. The CP subtypes identified align with those found in prosopagnosia caused by cortical lesions; they can be interpreted with respect to a hierarchical neural system for face perception