Philipp Adomeit

Hydrodynamics of three-dimensional waves in laminar falling films

Experiments were performed to investigate the flow and surface structure in laminar wavy films over a Reynolds number range from Ref=ρūδf/η=27–200. Measurements of the velocity distribution by particle image velocimetry and film thickness by a fluorescence technique enabled to gain detailed information on the transient conditions within the three-dimensional wavy flow. In the entire range of Reynolds numbers, the flow in the wave crest is in a decelerated state, as its momentum is partially transferred into the near wall region, which results in acceleration of the wave back above the equilibrium state. This also affects the residual film behind the waves and causes subsequent waves to collide with their predecessors. The three-dimensional effects and the wave collision frequency increase with increasing flow rate. Transitions from streak-like to surge-like waves and the development of turbulent spots are first observed to occur at Ref≈75. The wave shapes at Ref≈200 become completely unsteady and approximately every second wave collision causes the formation of a turbulent spot.

Deposition of fine particles from a turbulent liquid flow: Experiments and numerical predictions

A theoretical model is developed to predict the deposition rate of fine particles from turbulent, non-isothermal liquid flow. The model accounts for the relevant transport mechanisms, describing particle adhesion by the interaction forces calculated from the DLVO-theory. Predictions reasonably agree with the experimental data obtained in a simple plate heat exchanger. The influence of chemical and thermal conditions on adhesion is adequately described by the equilibrium and reaction enthalpy data of the surface ionisation reactions. Transport is found to be dominated by hydrodynamic lift which is often referred to as a secondary effect. The hydrodynamic lift suppresses deposition of 1.2 μm-particles even under laminar flow conditions. The experimental results show that thermophoretic transport is important, but overpredicted by theories developed for stagnant fluids. Apparently particle rotation induced by flow shear diminishes temperature gradients within the particle and in the surrounding fluid and leads to a considerable decrease of the thermophoretic migration velocity.

Analysis of Cyclic Fluctuations of Charge Motion and Mixture Formation in a DISI Engine in Stratified Operation

The processes of an internal combustion engine are subject to cyclic fluctuations, which have direct consequence on the operational and emission behavior of the engine. Direct injection gasoline engines have fluctuations that are induced and superimposed by the flow and the injection. In stratified operation they can cause serious operating problems, such as misfiring. Currently, the state of knowledge on the formation and causes of cyclic fluctuations is rather limited, which can be attributed to the complex nature of flow instabilities.

Virtual Tubelets-efficiently visualizing large amounts of particle trajectories

The depiction of particle trajectories is an effective means for the visualization of fluid flows. However, standard visualization techniques suffer from a variety of weaknesses, ranging from ambiguous depth perception for simple line drawings to a high geometrical complexity and decreased interactivity for polygonal tubes. This paper addresses these problems by introducing a novel approach to pathline visualization, which we call Virtual Tubelets. It employs billboarding techniques in combination with suitable textures in order to create the illusion of solid tubes, thus efficiently and unambiguously depicting large amounts of particle trajectories at interactive frame rates. By choosing an appropriate orientation for the billboards, certain issues concerning immersive displays with multiple projection screens are resolved, which allows for an unrestricted use in virtual environments as well. Using modern graphics hardware with programmable vertex and pixel pipelines results in an additional speed-up of the rendering process and a further improvement of image quality. This creates a nearly perfect illusion of tubular geometry, including plausible intersections and consistent illumination with the rest of the scene. The efficiency of our approach is proven by comparing rendering speed and visual quality of Virtual Tubelets to that of conventional, polygonal tube renderings.

Optical investigation of fuel and in-cylinder air-swirl effects in a high-speed direct-injection engine

The primary aim of this study is to investigate current issues of combustion in high-speed direct-injection diesel engines in detail by optical diagnostics. Both fuel and engine design are considered. Recently, measurements of engine-out emissions demonstrated that approximately soot-free combustion can be achieved using a newly designed two-component fuel named BLT. It was composed of 70% butyl levulinate and 30% n-tetradecane (by volume). In this work, the underlying mechanism is clarified by in-cylinder visualization of OH* radicals and soot. In particular, it turns out that in-cylinder soot formation is avoided almost completely, that is, the soot oxidation process is much less important. This is basically achieved by both the oxygen content (about 21%) and the low cetane number (approximately 33) of BLT. The latter leads to enhanced pre-mixing of fuel and air. Consequently, soot formation can be greatly reduced because it depends on the air–fuel ratio of the mixture shortly before high-temperature combustion. The influence of in-cylinder air swirl on combustion and soot formation is also studied for both BLT and conventional diesel fuel, respectively, using the same optical diagnostic. The measurements show that the combustion zone strongly depends on the local gas-flow velocity for diesel fuel. Soot formation decreases with increasing swirl because of enhanced air entrainment and pre-mixing of fuel and air. In addition, the results indicate that soot oxidation is improved under high-swirl conditions for diesel fuel. In contrast, the influence of swirl on the combustion of BLT is overall found to be weak, suggesting that swirl could be reduced in future high-speed direct-injection engines for BLT-like fuels.

Spray propagation and mixture formation in an air guided direct injection gasoline engine

Abstract Numerical analysis is used to gain information on the spray propagation and mixture formation in tumble guided gasoline direct injection (DI) engines. In order to achieve reliable predictions an atomization model for high-pressure swirl injectors is described and verified by comparison to experimental data. The approach is capable of adequately predicting the most important effects, such as nozzle orifice diameter, cone angle or injection pressure on spray development. Furthermore, it is found that the pre-jet generated at the beginning of the injection strongly affects the overall spray development. The temporal development of the pre-jet is described empirically. The in-cylinder computational fluid dynamics (CFD) analysis reveals that the tumble charge motion strongly affects spray propagation and mixture formation in the stratified operation mode, as it transports the fuel vapour cloud towards the spark plug. The CFD simulation improves understanding of the interaction between the flow field, spray propagation and evaporation and enables guidance of the optimization of the flow control and of the injection parameters for tumble guided gasoline DI engines.

Experimental investigation of in-cylinder wall wetting in GDI engines using a shadowgraphy method

This paper discusses an experimental approach to compare the amount of gasoline on the liner for different engine setups. This is done in a non-fired motored gasoline direct injection (GDI) test engine with transparent liner walls. The main goal is a planar observation and detection of the liner wetting using a shadowgraphy method. The area of impinged fuel on the liner is visualized. After one injection cycle the decay of the area due to evaporation can be described over the next running cycles without injection. The evaporation rate is a function of the wetted area. The amount of impinged fuel is estimated with a combination of the measured wetted area and theory of evaporation behavior.In this study three different injectors are tested under full load conditions. The injection strategies are varied. Big differences are observed between the injectors and injection strategies. Furthermore the advantages and drawbacks of the measurement method are discussed.

Numerical investigation of the effect of swirl flow in-homogeneity and stability on diesel engine combustion and emissions

The present study is aimed at numerically investigating the effect of in-cylinder charge motion on mixture preparation, combustion and emission formation in a high-speed direct-injection diesel engine. Previous investigations have shown that different valve-lift strategies nominally lead to similar in-cylinder filling and global swirl levels. However, significant differences in engine-out emissions, especially soot emission, give rise to the assumption that the flow structure and local differences of the swirl motion distribution have a noticeable effect on emission behaviour. In this work, different swirl generation strategies applying different intake valve actuation schemes are numerically investigated by applying transient in-cylinder computational fluid dynamic simulations using both the Reynolds-averaged Navier–Stokes model and the multi-cycle large-eddy simulation approach. Two operating points within the operating range of current diesel passenger cars during federal test procedure 75 and new European driving cycles are simulated. The injection and combustion simulations of different valve strategies show that an in-homogeneity in the in-cylinder flow structure leads to a significant increase in soot emissions, and agree with the observed trends of corresponding experimental investigations.

Glow-plug Ignition of Ethanol Fuels under Diesel Engine Relevant Thermodynamic Conditions

The requirement of reducing worldwide CO 2 emissions and engine pollutants are demanding an increased use of bio-fuels. Ethanol with its established production technology can contribute to this goal. However, due to its resistive auto-ignition behavior the use of ethanol based fuels is limited to the spark ignited gasoline combustion process. For application to the compression ignited Diesel combustion process advanced ignition systems are required. In general, ethanol offers a significant potential to improve the soot emission behavior of the Diesel engine due to its oxygen content and its enhanced evaporation behavior. In this contribution the ignition behavior of ethanol and mixtures with high ethanol content is investigated in combination with advanced ignition systems with ceramic glow-plugs under Diesel engine relevant thermodynamic conditions in a high pressure and temperature vessel. The investigation focuses on optimizing the injection conditions, especially injection pressure and rate. Optical measurements are performed by high speed imaging of the fuel injection and ignition, and evaluated in terms of ignition and flame propagation. The high speed imaging technology furthermore enables to gain information on the statistical behavior of the ignition process and thus provides a direct assessment of the repeatability of the ignition and combustion process. The results of the ignition investigation aim at improving the understanding of the glow-plug induced ignition process in order to provide a reliable ignition strategy for the Diesel engine operation with fuel with high ethanol content. The results show that the favorable spray targeting relative to the glow-plug depends on the glow-plug design. Furthermore a moderate injection pressure improves the ignition reliability of the ethanol fuels. The latter leads to the hypothesis that reduced injection induced shear rates improve the ignition behavior by diminishing shear induced quenching in the ignition zone in the direct vicinity of the hot glow-plug surface.

Modeling and Simulation of Gasoline Auto Ignition Engines

Abstract Both the customer demand for increasing mobility and the emission legislation lead to a challenge for engine researchers and developers in order to reduce emissions and fuel consumption. One approach that is presently under extensive investigation is to implement auto-ignition combustion in gasoline engines. This combustion mode offers the possibility to reduce emissions and fuel consumption during part load operation. Furthermore it offers the advantage that it does not need an expensive exhaust gas after treatment due to nearly zero NOx-emissions in contrast to stratified direct injection operation. The auto-ignition depends strongly on stratification of air, residual gas and fuel. Furthermore, the thermodynamic state of the charge is of major importance to control the combustion process. Detailed knowledge of ignition and its dependency on operating conditions is necessary to develop efficient control strategies. This paper gives a summary on modeling strategies for gasoline auto-ignition developed within the collaborative research centre “SFB 686 – Modellbasierte Regelung der homogenisierten Niedertemperatur-Verbrennung” [1]. The auto-ignition process is simulated with two different approaches. 3D CFD calculation of flow, injection and mixture formation, which is bi-directional coupled to a multi-zone reaction kinetics solver. This 3D approach enables to analyze the thermodynamic conditions in the combustion chamber that lead to the auto-ignition. Thus, the temporal and spatial occurrence of exothermic reactions and their influence on the engine process are specified in detail. To reduce the computational costs and enable multi-cycle calculations, a second simulation approach was developed to analyze the process under steady state and transient operating conditions. The approach uses 1D gas exchange calculation with embedded burn function calculations based on reaction kinetics. The simulation shows good correlation to the test bench results, but requires a computational time of approximately 5 min per cycle. The calculation time can be further reduced with an approach based on a polynomial combustion model. Multi-cycle calculations are performed and compared to test bench results. Due to the small computational effort, this approach offers the possibility of a coupling to a controller design environment for synchronous simulation and control.