Benjamin Böhm

Influence of intake geometry variations on in-cylinder flow and flow–spray interactions in a stratified direct-injection spark-ignition engine captured by time-resolved particle image velocimetry

Time-resolved particle image velocimetry and Mie-scattering of fuel droplets at 16 kHz were used to capture simultaneously the temporal evolution of the in-cylinder flow field and spray formation within a direct-injection spark-ignition engine. The engine was operated in stratified combustion mode, with stratified mixtures created by a triple injection late in the compression stroke. The impact of geometric variation of the intake port on in-cylinder flow and flow–spray interactions was investigated, focusing on the second injection, since it provides ignitable mixtures at the time of ignition and is subject to strong fluctuations, rather than the first injection, which is very reproducible. Flow field statistics conditioned on the spray shape of the second injection revealed regions with macroscopic cycle-to-cycle flow variations, which correlated with the spray for all recorded cycles. The flow–spray interaction was traced back to before the first injection using correlation analysis, which revealed that cycle-to-cycle fluctuations of the large-scale tumble vortex had a big impact on the spray shape of the second injection, while the first injection was unaffected. This indicates that the origin of the spray fluctuations may be during intake. Despite significant flow modifications due to the intake port geometry variation, fluctuation levels of the second injection were the same for both geometries, that is, spray fluctuations were not sensitive to the geometric change.

On the turbulent flow in piston engines: Coupling of statistical theory quantities and instantaneous turbulence

Planar particle image velocimetry (PIV) and tomographic PIV (TPIV) measurements are utilized to analyze turbulent statistical theory quantities and the instantaneous turbulence within a single-cylinder optical engine. Measurements are performed during the intake and mid-compression stroke at 800 and 1500 RPM. TPIV facilitates the evaluation of spatially resolved Reynolds stress tensor (RST) distributions, anisotropic Reynolds stress invariants, and instantaneous turbulent vortical structures. The RST analysis describes distributions of individual velocity fluctuation components that arise from unsteady turbulent flow behavior as well as cycle-to-cycle variability (CCV). A conditional analysis, for which instantaneous PIV images are sampled by their tumble center location, reveals that CCV and turbulence have similar contributions to RST distributions at the mean tumble center, but turbulence is dominant in regions peripheral to the tumble center. Analysis of the anisotropic Reynolds stress invariants reve...

Single Acetone Droplets at Supercritical Pressure: Droplet Generation and Characterization of PLIFP

Abstract An experimental investigation of isolated acetone droplets under supercritical pressure is presented. An optically accessible pressure chamber was set up for pressures ranging up to 60 bar and temperatures of up to 530 K. A novel method for the generation of acetone droplets on demand in the vicinity of the critical point is presented. High speed shadowgraphy imaging revealed remarkable reproducibility of the droplet generation regarding their size and shape and the timing of the detachment. The photo-physical properties of acetone in nitrogen as well as nitrogen-oxygen mixtures were investigated using planar laser induced fluorescence and phosphorescence (PLIFP). The measurements revealed a strong sensitivity and selectivity of the phosphorescence-fluorescence-ratio relative to the oxygen concentration. This demonstrates the potential of PLIFP to distinguish between the liquid and gaseous phase of acetone injected into mixtures of nitrogen and oxygen.

Evaluation of the flame propagation within an SI engine using flame imaging and LES

This work shows experiments and simulations of the fired operation of a spark ignition engine with port-fuelled injection. The test rig considered is an optically accessible single cylinder engine specifically designed at TU Darmstadt for the detailed investigation of in-cylinder processes and model validation. The engine was operated under lean conditions using iso-octane as a substitute for gasoline. Experiments have been conducted to provide a sound database of the combustion process. A planar flame imaging technique has been applied within the swirl- and tumble-planes to provide statistical information on the combustion process to complement a pressure-based comparison between simulation and experiments. This data is then analysed and used to assess the large eddy simulation performed within this work. For the simulation, the engine code KIVA has been extended by the dynamically thickened flame model combined with chemistry reduction by means of pressure dependent tabulation. Sixty cycles have been simulated to perform a statistical evaluation. Based on a detailed comparison with the experimental data, a systematic study has been conducted to obtain insight into the most crucial modelling uncertainties.

Advanced Laser Diagnostics for Understanding Turbulent Combustion and Model Validation

This contribution is not an original publication but a report of cumulative work that was carried out within the framework of SFB 568. The work was published in different archival journals and figures and text passages have been taken from different journal articles as indicated by the references. The aim of this report is to present experiments in projects B1 and B3 for improving our understanding in turbulent combustion with a focus of turbulent flow and scalar fields as well as their mutual interactions. The report is restricted to generic gaseous turbulent flames that feature different characteristics important to practical applications. The methods presented here are feasible to study boundary conditions, flow and scalar fields and are based all on interactions between laser light and matter. Following a brief introduction, two target flames are discussed in Sect. 4.2. Sections 4.3 and 4.4 exemplify flow and scalar measurements. Section 4.5 discusses combined scalar/flow measurements that can significantly improve our understanding of turbulence-chemistry interactions. In Sect. 4.6 new developments based on high-repetition-rate imaging are highlighted. These diagnostics complement methods at low repetition rate commonly used to generate an understanding by statistical moments and probability density functions. High repetition rate imaging techniques presently are an emerging field. Although the most recent developments achieved in the funding period of the Collaborative Research Center are included to this report, near-future progress in this field will lead to even more interesting insights into combustion phenomena.

Cause-and-effect chain from flow and spray to heat release during lean gasoline combustion operation using conditional statistics

This work comprises of experimental and numerical investigations of the volumetric flow in a direct injection spark-ignition engine to analyse the origin of cycle-to-cycle variations during stratified engine operation. High-speed two-dimensional two-component particle image velocimetry measurements are carried out simultaneously in the central tumble and mid-intake valve plane of an optically accessible engine, to capture the three-dimensional characteristic of the in-cylinder flow. Early investigation showed spray formation of stratified operation to be sensitive to cyclic fluctuations of the flow in a specific region below the spark within the central plane prior the first injection. Conditional statistics are used to track the origin of these variations back to the tumble flow in the valve plane during early compression underlining the three-dimensional structure of the flow. Moreover, conditional statistics reveals that the combustion performance is sensitive to the same specific flow region. According to this, computational fluid dynamics simulations are used to describe the in-cylinder flow with a map of four dominant flow structures. A new intake port geometry is derived from computational fluid dynamics simulations, optimized for high tumble generation. High-speed two-dimensional two-component particle image velocimetry in the central tumble plane is utilized to compare the optimized intake port with the original geometry. The new intake ports yield a considerable increased tumble movement with a favourable combustion performance and less cycle-to-cycle variation.

Flame/flow dynamics at the piston surface of an IC engine measured by high-speed PLIF and PTV

Resolving fluid transport at engine surfaces is required to predict transient heat loss, which is becoming increasingly important for the development of high-efficiency internal combustion engines (ICE). The limited number of available investigations have focused on non-reacting flows near engine surfaces, while this work focuses on the near-wall flow field dynamics in response to a propagating flame front. Flow-field and flame distributions were measured simultaneously at kHz repetition rates using particle tracking velocimetry (PTV) and planar laser induced fluorescence (PLIF) of sulfur dioxide (SO2). Measurements were performed near the piston surface of an optically accessible engine operating at 800 rpm with homogeneous, stoichiometric isooctane-air mixtures. High-speed measurements reveal a strong interdependency between near-wall flow and flame development which also influences subsequent combustion. A conditional analysis is performed to analyze flame/flow dynamics at the piston surface for cycles with weak and strong flow velocities parallel to the surface. Faster flame propagation associated with higher velocities before ignition demonstrates a stronger flow acceleration ahead of the flame. Flow acceleration associated with an advancing flame front is a transient feature that strongly influences boundary layer development. The distance from the wall to 75% maximum velocity ({\delta}75) is analyzed to compare boundary layer development between fired and motored datasets. Decreases in {\delta}75 are strongly related to flow acceleration produced by an approaching flame front. Measurements reveal strong deviations of the boundary layer flow between fired and motored datasets, emphasizing the need to consider transient flow behavior when modelling boundary layer physics for reacting flows.

High-Speed Laser Diagnostics for the Investigation of Cycle-to-Cycle Variations of IC Engine Processes

The work presented in this report was conducted within the Collaborative Research Center 568 funded by the Deutsche Forschungsgemeinschaft over a period of eleven years. The aim of project T4 was the transfer of laser based measurement techniques, which were used and improved in the context of stationary gas turbine combustion, to the investigation of intermittent processes in internal combustion (IC) engines. The focus was on cycle-to-cycle fluctuations as they appear in recent direct injection IC engines. High-speed measurement techniques were applied to investigate the temporal evolution of the in-cylinder flow, fuel distribution and flame propagation. Charge motion was investigated by particle image velocimetry (PIV) and spray by imaging of Mie-scattering. Mixture distribution was captured qualitatively by means of planar laser induced fluorescence (PLIF) of a fluorescing fuel. OH-PLIF was used to investigate the development of the early flame kernel and turbulent flame propagation.

Cinematographic Laser Combustion Diagnostics

Cinematographic PIV and OH PLIF operated at repetition rates up to 5 kHz are applied simultaneously to identify flow structures that entail extinction in a turbulent opposed jet flame.