Applications of the LIF method for the diagnostics of the combustion process of gas-IC-engines

Within the underlying project, the task was to develop methods for optical measurements in a hydrogen-fuelled engine with direct-injection, with the goal of measuring the jet patterns during injection, the stratification of the charge at ignition point and the propagation of the flame during combustion. Therefore, the method of planar laser-induced-fluorescence (PLIF) was chosen. In order to apply this technique for the named tasks, particular methods the visualisation of fuel distribution and the flame front were developed. The measurements were carried out on a single cylinder research engine installed at the Institute for Internal Combustion Engines at Graz University of Technology. This engine features optical access through a quartz-glass liner and a window in the piston while providing a layout equivalent to modern passenger car engines and the possibility to operate in fired mode. As it is hardly feasible to directly excite molecular hydrogen by means of laser light, it is necessary to add a tracer substance to the fuel that provides high fluorescence intensity while not changing the properties of the fuel. Consequently, Triethylamine was chosen as a tracer to be mixed with hydrogen at 200 ppm, which allows it to be used up to a maximum pressure of 200 bar while still providing a strong LIF signal. Due to the excellent linearity of the signal to the local air/fuel-ratio it was possible to develop a method for the calibration of the images in order to compensate for inhomogeneities of the laser beam and staining of the optical access and to ultimately allow a quantification of the fuel distribution. The results are images scaled on air/fuel-ratio which can be used for a direct optimisation of mixture formation processes and the validation of CFD-models. For the analysis of the combustion process the method was adapted with two different approaches. For homogeneous charges a new method was applied by marking the flame front using the tracer within the fuel, so that both are burned together. However, as this method is limited to measurements with a homogeneous distribution of tracer within the measured volume, an alternative technique had to be applied for the measurement of stratified charges. In this case, a direct visualisation of the flame front was achieved by exciting the OH-radicals formed during combustion. As this method has significantly increased demands on measuring equipment and is more time consuming, both methods are used in parallel on specific measuring tasks.     

Effective plastic properties of laminates

In this contribution, we make use of the fact homogenization of elastic properties of laminates [1] can be extended relatively easy to the plastic case [2] to derive exact expressions for the effective yield limit and the ultimate loading in monotonic tension test. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effective equations for two-phase flow in porous media: the effect of trapping on the microscale

In this article, we consider a two-phase flow model in a heterogeneous porous column. The medium consists of many homogeneous layers that are perpendicular to the flow direction and have a periodic structure resulting in a one-dimensional flow. Trapping may occur at the interface between a coarse and a fine layer. Assuming that capillary effects caused by the surface tension are in balance with the viscous effects, we apply the homogenization approach to derive an effective (upscaled) model. Numerical experiments show a good agreement between the effective solution and the averaged solution taking into account the detailed microstructure.     

Experimental Characterization of Premixed Flame Instabilities of a Model Gas Turbine Burner

In recent years, the NO _x emissions of heavy duty gas turbine burners have been significantly reduced by introducing premixed combustion. These highly premixed burners are known to be prone to combustion oscillations. In this paper, investigations of a single model gas turbine burner are reported focusing on thermo-acoustic instabilities and their interaction with the periodic fluctuations of the velocity and pressure. Phase-locked optical measurement techniques such as LDA and LIF gave insight into the mechanisms.Detailed investigations of a gas turbine combustor rig revealed that the combustor as well as the air plenum oscillate in Helmholtz modes. These instabilities could be attributed to the phase lag of the pressure oscillations between the air plenum and the combustor, which causes an acceleration and deceleration of the air flow through the burner and, therefore, alternating patterns of fuel rich and lean bubbles. When these bubbles reach the reaction zone, density fluctuations are generated which in turn lead to velocity fluctuations and, hence, keep up the pressure oscillations.With increasing the equivalence ratio strong combustion oscillations could be identified at the same frequency. Similarly as with weak oscillations, Helmholtz mode pressure fluctuations are present but the resulting velocity fluctuations in the combustor can be described as a pumping motion of the flow. By the velocity fluctuations the swirl stabilization of the flame is disturbed. At the same time, the oscillating pressure inside the combustor reaches its minimum value. Shortly after the flame expands again, the pressure increases inside the combustor. This phenomenon which is triggered by the pressure oscillations inside the air plenum seems to be the basic mechanism of the flame instability and leads to a significant increase of the pressure amplitudes.     

Existence of Asymptotic Values for Nonexpansive Stochastic Control Systems

We consider an optimal stochastic control problem for which the payoff is the average of a given cost function. In a non ergodic setting, but under a suitable nonexpansivity condition, we obtain the existence of the limit value when the averaging parameter converges (namely the discount factor tends to zero for Abel mean or the horizon tends to infinity for the Cesàro mean). The main novelty of our result lies on the fact that this limit may depend on initial conditions of the control system (in contrast to what is usually obtained by other approaches). We also prove that the limit does not depend of the chosen average (Abel or Cesàro mean).     

Double isotope dilution surface-enhanced Raman scattering as a reference procedure for the quantification of biomarkers in human serum

Double isotope dilution surface-enhanced Raman scattering (double IDSERS) is qualified as a method for accurate and precise determination of biomarkers in human blood serum. Providing a full evaluation of the measurement uncertainty as well as traceability to a reference material sets the procedure in line with the requirements of a primary ratio method. Data evaluation is based on a partial least squares (PLS) model, whose prediction ability is validated from quantifying the uric acid concentration in both an artificial reference solution and a real human blood serum sample. With the proposed approach, the uric acid serum concentration can be determined with an uncertainty of 1.6% at a confidence level of 95%.     

Chemical alterations to murine brain tissue induced by formalin fixation: implications for biospectroscopic imaging and mapping studies of disease pathogenesis

Understanding biochemical mechanisms and changes associated with disease conditions and, therefore, development of improved clinical treatments, is relying increasingly on various biochemical mapping and imaging techniques on tissue sections. However, it is essential to be able to ascertain whether the sampling used provides the full biochemical information relevant to the disease and is free from artefacts. A multi-modal micro-spectroscopic approach, including FTIR imaging and PIXE elemental mapping, has been used to study the molecular and elemental profile within cryofixed and formalin-fixed murine brain tissue sections. The results provide strong evidence that amino acids, carbohydrates, lipids, phosphates, proteins and ions, such as Cl(-) and K(+), leach from tissue sections into the aqueous fixative medium during formalin fixation of the sections. Large changes in the concentrations and distributions of most of these components are also observed by washing in PBS even for short periods. The most likely source of the chemical species lost during fixation is the extra-cellular and intra-cellular fluid of tissues. The results highlight that, at best, analysis of formalin-fixed tissues gives only part of the complete biochemical "picture" of a tissue sample. Further, this investigation has highlighted that significant lipid peroxidation/oxidation may occur during formalin fixation and that the use of standard histological fixation reagents can result in significant and differential metal contamination of different regions of tissue sections. While a consistent and reproducible fixation method may be suitable for diagnostic purposes, the findings of this study strongly question the use of formalin fixation prior to spectroscopic studies of the molecular and elemental composition of biological samples, if the primary purpose is mechanistic studies of disease pathogenesis.     

Simultaneous Determination of 12 Sulfonylurea Herbicides in Drinking Water after SPE by LC-DAD

A liquid chromatographic method (LC) with diode array detection (DAD) for the routine screening and quantification of highly applicated polar herbicides in drinking water samples was developed. The investigated herbicides consisted of 12 sulfonylurea herbicides (amidosulfuron, flazasulfuron, foramsulfuron, iodosulfuron-methyl Na, mesosulfuron-methyl, metsulfuron-methyl, nicosulfuron, prosulfuron, thifensulfuron-methyl, triasulfuron and tritosulfuron) together with 6 polar pesticides of relevance (atrazine, desethylatrazine, desisopropylatrazine, chlortoluron, diuron, fluoxypyr). The herbicides were extracted and concentrated by off-line solid-phase extraction and subsequently eluates were analyzed by LC-DAD. Recoveries obtained from fortified water samples at 100 ng L^?1 were in the range of 84–107% with RSD’s <20%. The limit of detection varied from 2 to 16 ng L^?1.     

Numerically exact, time-dependent treatment of vibrationally coupled electron transport in single-molecule junctions

The multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) theory within second quantization representation of the Fock space, a novel numerically exact methodology to treat many-body quantum dynamics for systems containing identical particles, is applied to study the effect of vibrational motion on electron transport in a generic model for single-molecule junctions. The results demonstrate the importance of electronic-vibrational coupling for the transport characteristics. For situations where the energy of the bridge state is located close to the Fermi energy, the simulations show the time-dependent formation of a polaron state that results in a pronounced suppression of the current corresponding to the phenomenon of phonon blockade. We show that this phenomenon cannot be explained solely by the polaron shift of the energy but requires methods that incorporate the dynamical effect of the vibrations on the transport. The accurate results obtained with the ML-MCTDH in this parameter regime are compared to results of nonequilibrium Green's function theory.