Robert Schießl

Spark ignited hydrogen/air Mixtures : Two dimensional detailed modeling and laser based diagnostics

This study reports a detailed 2-D model which describes the spark ignition of an initially quiescent hydrogen/air mixture. The model includes the compressible Navier-Stokes equations, detailed chemistry and molecular transport in the gas phase as well as heat conduction to the electrodes. The spark is modeled for the phases subsequent to breakdown using the Maxwell equations for quasi-stationary conditions for the electric field. Initial and boundary conditions necessary for the simulations are chosen in accordance with experimental values. Heat conduction to the electrodes and different electrode shapes are investigated. The influence of these parameters on the shape of the initial flame kernel is discussed and compared qualitatively to experimental results. Spark ignition experiments are performed using a highly reproducible ignition system. Shapes of the early flame kernels are monitored by 2-D laser-induced fluorescence (PLIF) imaging of OH radicals produced during the ignition and the combustion process. The investigations are performed for different equivalence ratios. In addition, for a central position within the flame kernel, temperatures are measured at different times after ignition using vibrational coherent anti-Stokes Raman spectroscopy (CARS) of nitrogen.

TWO-DIMENSIONAL TEMPERATURE MEASUREMENTS IN AN SI ENGINE USING TWO-LINE TRACER LIF

Transient two-dimensional temperature distributions in the compression stroke and in the unburned end-gas of an SI engine were measured employing laser-induced fluorescence (LIF) of a fuel marker that possesses strongly temperature-dependent spectroscopic properties. The use of two different excitation wavelengths simplifies the otherwise complicated relation between LIF signal intensity and system parameters. The temperature fields obtained in this manner can be used to correct measured tracer-LIF maps and thus help to determine fuel distributions. Averaged temperature fields are compared to model calculations based on a homogeneous reactor assumption.

Double-Pulse PLIF Imaging of Self-Ignition Centers in an SI Engine

In this study, the occurrence of auto-ignition centers in a two-stroke SI engine was investigated using planar laserinduced fluorescence (PLIF). An experimental SI engine equipped with glass windows to enable full optical access into the combustion chamber was operated under knocking conditions. The pulsed output of two XeCl excimer lasers was formed into planar light sheets (300 μm × 4 cm), which were spatially overlapped and directed into the combustion chamber of the operating engine. Unburned fuel components fluoresce strongly when illuminated with XeCl laser radiation; burned regions display no fluorescence. Self-ignited regions therefore show up as dark sites in the fluorescence images, indicating local consumption of the fuel. The resulting PLIF images were recorded using fast-gated ICCD cameras. By delaying the second laser pulse a specific time (100 ns—600 μs), image pairs were acquired which allowed the temporal development and mutual influence of hot-spots to be studied. The short laser pulse duration (20 ns), the two-dimensional nature of the imaged region and the strong signals obtained from our detection scheme yield very detailed information of the self-ignited regions. Approximately 20 000 image pairs were recorded. As important quantities, spatial distribution and expansion velocities were extracted from the PLIF images and investigated statistically.

A Detailed Two-Dimensional Numerical Study of Spark Ignition Including Ionization

In this work, the spark–ignition (SI) of a methane/air mixture contained in a constant–volume chamber is investigated by numerical simulations. A cylinder–shaped vessel filled with a methane/air mixture containing two electrodes is used as simulation model. The impact of an electrical discharge at the electrodes on the surrounding gas is simulated, with detailed treatment of the ignition process involvig chemical kinetics, transport phenomena in the gas– phase and electrodynamical modeling of the interaction between spark and fuel/air mixture. For the calculations, a 2D–code to simulate the early stages of flame development, shortly after the breakdown discharge, has been developed. Computational results are shown for ignition of a methane air–mixture.

Experimental and Simulative Modeling of Drilling Processes for the Compensation of Thermal Effects

This work focuses on the prediction of phase transformations and shape deviations for drilling of demonstrator workpieces. In a first step, a 2D chip formation simulation was developed with all physical effects. Based on this simulation a simplified workpiece geometry with only one drilling hole was 3D modeled and tested for its predictive capability of phase transformations and shape deviations. Therefore, the feed rate, the rotation of the drilling tool and the material removal were considered for the process kinematics. Using these results the modeling approach was optimized and transferred into a simulation model with a demonstrator workpiece to minimize shape deviations and control phase transformations using different compensation strategies. The simulations were validated by experiments.

Temperature Fluctuations in the Unburned Mixture: Indirect Visualisation Based on LIF and Numerical Simulations

We apply a method for the visualization and semiquantitative estimation of small spatial temperature fluctuations in internal combustion engines with premixed loads. It is based on laser-induced fluorescence (LIF) of formaldehyde (CH2O), which is formed in the unburned gas near the end of the compression stroke. The chemical reactions leading to formaldehyde formation during the phase before auto-ignition are strongly temperaturedependent. The concentration of CH2O therefore acts as a natural, very sensitive tag for local gas temperature variations. A correlation between temperature fluctuation and formaldehyde concentration fluctuation is assessed by using numerical simulations involving a detailed treatment of chemical reactions leading to formaldehyde formation in the unburned gas. Formaldehyde is detected in the unburned gas of an optically accessible test SI engine by laser-induced fluorescence (LIF) along a line. The LIF-signal trace displays pronounced spatial fluctuations, which can not be attributed to inhomogeneities of the fuel/air ratio or to measurement noise, but which must be due to local temperature inhomogeneities in the unburned gas. By combining the observed spatial LIF-signal fluctuations with the computed formaldehyde/temperature correlations, temperature fluctuation amplitudes can be estimated. It is shown that the technique is capable of detecting fluctuations well below ±10 K before a mean temperature background of 800 K and higher. The results obtained by applying the method in a simple test engine give evidence that considerable temperature fluctuations exist in the unburned gas, both with respect to their amplitude and to their geometrical size. The information delivered by the method may be important especially in the field of auto-ignition in HCCI-engines, since local temperature inhomogeneities in the unburned gas can be of paramount influence for the subsequent development of self-ignition and combustion.

Ignition by Hot Free Jets

Abstract This paper describes some of our experimental studies on the re-ignition caused by jets of hot gas that interact with unburned fuel/air mixtures. The problem is approached from two complementary sides: On the one hand, phenomenological studies are conducted, which ask for the conditions under which a hot jet may cause ignition. A dedicated experiment is described which allows to create well-controlled exhaust gas jets and ambient conditions. In this experiment, parameters influencing the ignition process are varied, and the dependence of jet behavior on these parameters (i.e. pressure ratio, diameter and length of the gap through which the exhaust gas has to pass before getting into contact with ambient fuel/air) is studied. In particular, the frequency of a jet causing re-ignition in the ambient gas is studied. On the other hand, we also perform studies which are more “analytical” in nature. These attempt a more in-depth understanding, by first decomposing the hot jet ignition phenomenon into the underlying physical processes, and then studying these processes in isolation. This approach is applied to measurements of mixture fraction fields. First, non reacting isothermal variable density jets are studied. Here, the density of the gas mixture varies as to mimic the density of hot exhaust gas at varying temperatures. A laser-based non-intrusive method is introduced that allows to determine quantitative mixture fraction fields; although applied here to cold jets only, the method is also applicable to hot jets. The results show the effect of turbulence on the mixing field in and at the free jet, and allow to derive quantities that describe the statistics of the turbulent jet, like probability density functions (PDFs) and geometrical size of fluctuations.

Effects of nozzle geometry on the reignition by hot gas jets

ABSTRACT In this paper, the ignition of a hydrogen–air mixture by a jet of burned hot gas (often termed reignition in the field of explosion protection) is studied in a two-chamber experimental configuration. This experiment allows systematic creation of hot gas jets emerging from a thin, long nozzle and them to impinge into an unburned, cold hydrogen–air mixture, possibly causing (re)ignition. In an extensive parametric study, hot exhaust gas jets originating from different nozzle diameters (0.6 mm–1.2 mm) and lengths (25 mm and 70 mm), as well as different pressure ratios along the nozzle (up to 5.5) were tested for their ability to reignite the hydrogen–air mixture. It was investigated how often reignition occurred within 10 tests under each condition. Dependencies between the occurrence of reignition and the mentioned nozzle and jet parameters were determined. It was found that the outcome (reignition or no reignition) can differ for nominally identical experimental paramete rs, which demonstrates the parametric sensitivity of the process. The statistical frequency of reignition occurrence depends on the nozzle diameter as well as on its length. For pressure ratios above the critical value, reignition occurred more often for the longer nozzle (l = 70 mm) at a given diameter. Some of these findings, notably the dependence on the nozzle length, pose a challenge to conventional models.

Effects of Mixture Quality on Controlled Auto-Ignition

Controlled auto-ignition in petrol engines is a promising concept for the reduction of exhaust emissions and fuel consumption. Experimental and numerical investigations have been carried out at the Institute for Reciprocating Engines (IFKM) and the Institute of Technical Thermodynamics (ITT) to investigate the implementation of this combustion process in passenger car engines.

Controlled auto-ignition — Reduction of engine emissions by controlled auto-ignition in gasoline engines

The controlled auto-ignition in gasoline engines is a promising concept to reduce both emissions and fuel consumption simultaneously. Within the FVV-Vorhaben „Benzinselbstzun dung“ experimental and numerical investigations have been carried out at the Institut fur Kolbenmaschinen (IFKM) and at the Institut fur Technische Thermodynamik (ITT) to estimate the realisation of this combustion process in passenger car engines.