Characterisation of cyclic variability in an optically accessible IC Engine by means of phase-independent POD

Investigation of cyclic variability in engine operation has recently received new impulse with the widespread application of advanced numerical and experimental techniques. The present work attempts to shed some light on the existence and nature of correlations between coherent structures dynamics and cyclic variability in IC engines by means of phase-independent Proper Orthogonal Decomposition applied to highly-resolved PIV measurements obtained in an optically accessible, motored engine. Analysis of the conditional means and variances of the reconstruction coefficients reveal interesting patterns in the break-up of coherent structures which are also confirmed by experimental observation and leave room for speculation on the true nature of the flow field at different crank angles. A first attempt has also been carried out to reconstruct missing information from available measurements, with encouraging results: the development of such interpolation/reconstruction technique could obviously have a great impact on the reduction of the cost normally involved in experimental and computational campaigns

Experimental and theoretical investigation of the flashback of a swirling, bluff-body stabilised, premixed flame

Flashback of an open turbulent, premixed flame in a swirl burner with central bluff-body is considered. The aim is to obtain further understanding of the physical mechanisms responsible for the upstream flame propagation. Previous studies on the same configuration hypothesised that there is an adverse pressure gradient in the direction of flame propagation. In this paper this is further investigated experimentally and theoretically. Static gauge pressure is measured on the surface of the bluff-body during flame flashback. Simultaneously, flame luminosity is imaged at 5 kHz. The results indicate that the static pressure rises downstream of the propagating reactive front. This is, then, discussed in the context of the theory of vortex bursting. An existing theory of flame propagation in the core flow is extended to a configuration similar to that investigated experimentally. The theory, although highly simplified, explains the generation of adverse pressure gradient across the flame and is qualitatively consistent with the experiment

Generation of adverse pressure gradient in the circumferential flashback of a premixed flame

Circumferential upstream propagation of a premixed flame in a region confined between two concentric tubes is considered. The cold flow in this configuration features rotational motion and the flame is modelled as an interface separating the burned and unburned gases. Through an analytical solution of the integral form of the governing equations, it is shown that the static pressure increases across the flame. Hence, the circumferential propagation of the flame is associated with the generation of an adverse pressure gradient. The theoretical prediction of the pressure increase is, further, supported by the experimental observations and discussed in the context of the theory of flame back pressure. The results extend the recent findings on the generation of adverse pressure gradient during the axial propagation of swirling flames, to the circumferential direction. It is argued that the demonstrated pressure gain across the flame can significantly facilitate flame flashback

Non-LTE treatment of molecules in the photospheres of cool stars

We present a technique to treat systems with very many levels, like molecules, in non-LTE. This method is based on a superlevel formalism coupled with rate operator splitting. Superlevels consist of many individual levels that are assumed to be in LTE relative to each other. The usage of superlevels reduces the dimensionality of the rate equations dramatically and, thereby, makes the problem computationally more easily treatable. Our superlevel formalism retains maximum accuracy by using direct opacity sampling (dOS) when calculating the radiative transitions and the opacities. We developed this method in order to treat molecules in cool dwarf model calculations in non-LTE. Cool dwarfs have low electron densities and a radiation field that is far from a black body radiation field, both properties may invalidate the conditions for the common LTE approximation. Therefore, the most important opacity sources, the molecules, need to be treated in non-LTE. As a case study we applied our method to carbon monoxide. We find that our method gives accurate results since the conditions for the superlevel method are very well met for molecules. Due to very high collisional cross sections with hydrogen, and the high densities of H_2 the population of CO itself shows no significant deviation from LTE.Comment: AASTeX v50, 35 pages including 12 figures, accepted by Ap

Development and Characterization of a LT-PEMFC Stack with an Extended Operating Temperature Range up to 120 °C

Nowadays, the operation temperature of a polymer electrolyte fuel cell (PEFC) stack for automotive application is about 80 °C. The presented work concerns the characterization of a 30-cell PEFC stack (2.5 kWel) developed at the German Aerospace Center, designed for operating temperatures up to 120 °C for a limited time span. Short-term operation of the stack at higher temperatures would contribute to the improvement of cooling system components with scaled-down dimensions. This concept helps to reduce the vehicle weight and thus to save fuel. In this contribution we present a proof-of-concept of the feasibility of short-term operation for excess temperature events. For this purpose, the stack behaviour was investigated through a series of 20 temperature cycles from 90 to 120 °C at galvanostatic conditions of 70 A (approx. 0.5 A·cm–2 and 1.5 kWel) and without adaption of the gas dew points. The stack power decreased by 21 ± 1 %, with a fully reversible performance recovery at the end of every thermal cycle (see Fig. 1). The higher irreversible degradation rate under these harsh conditions was attributed to the enhanced mechanical stress, which is also correlated to the cycling of the membrane humidity. Furthermore, the results of a long-term steady-state test of 1200 hours under automotive relevant conditions at 80 °C are presented. An end-of-life characterization of the individual cells helped to identify possible causes for performance losses due to catalyst, electrode and membrane degradation. A reduction of electrochemically active surface areas in some cells was ascribed to a platinum catalyst particle growth. Membrane degradation and carbon corrosion occurred additionally, evidenced by increased high frequency resistances and hydrogen crossover rates of the membranes. As result of carbon corrosion, the hydrophobicity of the carbon-based components decreased, causing water accumulation in individual cells

Full Configuration Interaction wave function as a formal solution to the Optimized Effective Potential and Kohn-Sham models in finite basis sets

Using finite basis sets, it is shown how to construct a local Hamiltonian, such that one of its infinitely many degenerate eigenfunctions is the ground state full configuration interaction (FCI) wave function in that basis set. Formally, the local potential of this Hamiltonian is the optimized effective potential and the exact Kohn-Sham potential at the same time, because the FCI wave function yields the exact ground-state density and energy. It is not the aim of this paper to provide a new algorithm for obtaining FCI wave functions. A new potential is the goal

Experimental Investigation of Droplet Injections in the Vicinity of the Critical Point: A comparison of different model approaches

[EN] The disintegration process of liquid fuel within combustion chambers is one of the most important parameters for efficient and stable combustion. Especially for high pressures exceeding the critical value of the injected fluids the mixing processes are not fully understood yet. Recently, different theoretical macroscopic models have been introduced to understand breakdown of the classical two phase regime and predict the transition from evaporation to a diffuse-mixing process. In order to gain deeper insight into the physical processes of this transition, a parametric study of free-falling n-pentane droplets in an inert nitrogen atmosphere is presented. Atmospheric conditions varied systematically from sub- to supercritical values with respect to the fluid properties. An overlay of a diffuse lighted image with a shadowgram directly in the optical setup (front lighted shadowgraphy) was applied to simultaneously detect the presence of a material surface of the droplet as well as changes in density gradients in the surrounding atmosphere. The experimental investigation illustrates, that the presence of a material surface cannot be shown by a direct shadowgram. However, reflections and refractions caused by diffuse ambient illumination are able to indicate the presence of a material surface. In case of the supercritical droplet injections in this study, front lighted shadowgraphy clearly revealed the presence of a material surface, even when the pre-heated droplets are released into a supercritical atmosphere. This detection of the droplet interface indicates, that the droplet remains subcritical in the region of interest, even though it is injected into a supercritical atmosphere. Based on the adiabatic mixing assumption recent Raman-scattering results in the wake of the droplet are re-evaluated to compute the temperature distribution. Presented experimental findings as well as the re-evaluation of recent Raman scattering results are compared to thermodynamic models to predict the onset of diffuse-mixing and supercritical disintegration of the droplet. Additionally, a one dimensional evaporation model is used to evaluate the validity of the adiabatic mixing assumption in the estimation of the droplet temperature. The presented findings contribute to the understanding of recent theoretical models for prediction of spray and droplet disintegration and the onset of diffuse-mixing processes.The authors gratefully acknowledge the German Research Foundation (DFG) for the financial support through the collaborative research centre SFB/Transregio 75.Steinhausen, C.; Lamanna, G.; Weigand, B.; Stierle, R.; Groß, J.; Preusche, A.; Dreizler, A. (2017). Experimental Investigation of Droplet Injections in the Vicinity of the Critical Point: A comparison of different model approaches. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 830-837. https://doi.org/10.4995/ILASS2017.2017.4635OCS83083