Sebastian A. Kaiser

VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines

We report the first application of a vertical-cavity surfaceemitting laser (VCSEL) for calibration- and sampling-free, high-speed, in situ H2O concentration measurements in IC engines using direct TDLAS (tunable diode laser absorption spectroscopy). Measurements were performed in a single-cylinder research engine operated under motored conditions with a time resolution down to 100 μs (i.e., 1.2 crank angle degrees at 2000 rpm). Signal-to-noise ratios (1σ) up to 29 were achieved, corresponding to a H2O precision of 0.046 vol.% H2O or 39 ppm · m. The modulation frequency dependence of the performance was investigated at different engine operating points in order to quantify the advantages of VCSEL against DFB lasers.

REACTION-RATE, MIXTURE-FRACTION, AND TEMPERATURE IMAGING IN TURBULENT METHANE/AIR JET FLAMES

Instantaneous two-dimensional measurements of reaction rate, mixture fraction, and temperature are demonstrated in turbulent partially premixed methane/air jet flames. The forward reaction rate of the reaction CO OH ⇒ CO2 H is measured by simultaneous OH laser-induced fluorescence (LIF) and two-photon CO LIF. The product of the two LIF signals is shown to be proportional to the reaction rate. Temperature and fuel concentration are measured using polarized and depolarized Rayleigh scattering. A three-scalar technique for determining mixture fraction is investigated using a combination of polarized Rayleigh scattering, fuel concentration, and CO LIF. Measurements of these three quantities are coupled with previous detailed multiscalar point measurements to obtain the most probable value of the mixture fraction at each point in the imaged plane. This technique offers improvements over two-scalar methods, which suffer from decreased sensitivity around the stoichiometric contour and biases in fuel-rich regions due to parent fuel loss. Simultaneous reaction-rate, mixture-fraction, and temperature imaging is demonstrated in laminar (Re 1100) and turbulent (Re 22,400) CH4/air (1/3 by volume) jet flames. The turbulent jet flame is the subject of multiple numerical modeling efforts. A primary objective for developing these imaging diagnostics is to provide measurements of fundamental quantities that are needed to accurately model interactions between turbulent flows and flames.

POLARIZED/DEPOLARIZED RAYLEIGH SCATTERING FOR DETERMINING FUEL CONCENTRATIONS IN FLAMES

Rayleigh scattering has been shown to be a useful diagnostic technique for two-dimensional imaging studies of reacting and non-reacting flows. For example, by combining Rayleigh scattering with a simultaneous measurement of the fuel concentration (e.g., using Raman scattering), mixture fraction and temperature can be determined in flames. In this work, it is demonstrated that the fuel concentration can be obtained by measuring the polarized and depolarized components of the Rayleigh signal and taking their difference or a suitable linear combination. While the depolarized Rayleigh signal is smaller than the polarized signal by a factor of ≈100, this is still a factor of ≈10 larger than the Raman scattering. Application of the technique requires that one of the primary constituents of the fuel stream possess a depolarization ratio sufficiently different from that of the oxidizer. Methane is a convenient candidate as it has no measurable depolarization. Results are shown for methane flames diluted by argon as well as air.

Use of Rayleigh imaging and ray tracing to correct for beam-steering effects in turbulent flames

Laser Rayleigh imaging has been applied in a number of flow and flame studies to measure concentration or temperature distributions. Rayleigh cross sections are dependent on the index of refraction of the scattering medium. The same index of refraction changes that provide contrast in Rayleigh images can also deflect the illuminating laser sheet. By applying a ray-tracing algorithm to the detected image, it is possible to correct for some of these beam-steering effects and thereby improve the accuracy of the measured field. Additionally, the quantification of the degree of beam steering through the flow provides information on the degradation of spatial resolution in the measurement. Application of the technique in a well-studied laboratory flame is presented, along with analysis of the effects of image noise and spatial resolution on the effectiveness of the algorithm.

The effects of laser-sheet thickness on dissipation measurements in turbulent non-reacting jets and jet flames

The effects of laser-sheet thickness on planar laser measurements of scalar gradients in turbulent flows are studied. Experiments are performed in the near field of a turbulent, non-premixed, axisymmetric jet flame and in the near field of a non-reacting, isothermal turbulent jet. Laser Rayleigh scattering provides two-dimensional measurements of the instantaneous temperature and mixture fraction fields in the flame and non-reacting jet, respectively. The effect of spatial resolution on measurements of the mean dissipation and the power spectral density of axial temperature and mixture-fraction gradients is examined. The effect of varying the laser-sheet thickness is compared to that of spatial filtering within the image plane. Measurements of the mean dissipation and power spectral density are significantly less sensitive to resolution degradation in the non-differentiated dimensions than in the differentiated dimension. For example, on the jet flame centreline, the dissipation-cut-off microscale, which is determined from the measured power spectral density, is overestimated by 9% when the beam-waist thickness is increased from a 1/e-squared width of 160 ?m to 624 ?m. In contrast, spatial filtering along the direction of differentiation with a smoothing kernel of 624 ?m width produces a bias of 76% in the cut-off microscale. These results experimentally confirm the theoretical analysis of previous studies. A simple spatial model illustrates the origin of this difference and approximately predicts its magnitude for both planar and line measurements. A criterion for matching in-plane and out-of-plane resolution is established. For many planar gradient measurements, considerably less out-of-plane resolution is needed than in-plane resolution. The combined effects of noise and spatial averaging on the dissipation measurements are also briefly examined.

Quantitative two-dimensional measurement of oil-film thickness by laser-induced fluorescence in a piston-ring model experiment

This paper describes advances in using laser-induced fluorescence of dyes for imaging the thickness of oil films in a rotating ring tribometer with optical access, an experiment representing a sliding piston ring in an internal combustion engine. A method for quantitative imaging of the oil-film thickness is developed that overcomes the main challenge, the accurate calibration of the detected fluorescence signal for film thicknesses in the micrometer range. The influence of the background material and its surface roughness is examined, and a method for flat-field correction is introduced. Experiments in the tribometer show that the method yields quantitative, physically plausible results, visualizing features with submicrometer thickness.

Schlieren measurements in the round cylinder of an optically accessible internal combustion engine.

This paper describes the design and experimental application of an optical system to perform schlieren measurements in the curved geometry of the cylinder of an optically accessible internal combustion engine. Key features of the system are a pair of cylindrical positive meniscus lenses, which keep the beam collimated while passing through the unmodified, thick-walled optical cylinder, and a pulsed, high-power light-emitting diode with narrow spectral width. In combination with a high-speed CMOS camera, the system is used to visualize the fuel jet after injection of hydrogen fuel directly into the cylinder from a high-pressure injector. Residual aberrations, which limit the systems sensitivity, are characterized experimentally and are compared to the predictions of ray-tracing software.

LES of Flow Processes in an SI Engine Using Two Approaches: OpenFoam and PsiPhi

In this study two different simulation approaches to large eddy simulation of spark-ignition engines are compared. Additionally, some of the simulation results are compared to experimentally obtained in-cylinder velocity measurements. The first approach applies unstructured grids with an automated meshing procedure, using OpenFoam and Lib-ICE with a mapping approach. The second approach applies the efficient in-house code PsiPhi on equidistant, Cartesian grids, representing walls by immersed boundaries, where the moving piston and valves are described as topologically connected groups of Lagrangian particles. In the experiments, two-dimensional two-component particle image velocimetry is applied in the central tumble plane of the cylinder of an optically accessible engine. Good agreement between numerical results and experiment are obtained by both approaches. Introduction Direct injection, downsizing and advanced combustion modes are key fuel-saving technologies in gasoline engines. To further decrease the fuel consumption and pollutant emissions and to increase the power output, a better understanding of the in-cylinder processes is crucial. Currently, advanced combustion modes cannot be used over the full operating range, often due to turbulence-induced flame quenching or as a result of poor fuel-air mixing near the spark. In-cylinder phenomena are commonly studied in single-cylinder research engines with optical access for laser diagnostics. On the other hand, engines are investigated by numerical techniques like CFD, which is often less expensive and more flexible than an experiment. As the state of the art, U-RANS simulations are successfully applied by industry to gain an understanding of the engine, but U-RANS will normally fail to predict cyclic variations. A promising alternative are large eddy simulations (LES) that resolve smaller flow structures, enabling them to capture cyclic variations. However, LES is computationally more expensive and requires high-quality meshes on which high-order numerical schemes must be applied. In the context of LES, several CFD codes like AVBP [5,6], KIVA [7], FLUENT [8], or Star-CD [9] have shown at least partial ability to predict some relevant phenomena in internal combustion engines. A critical problem with the application of LES is that any discretization of less than second order accuracy and CFL numbers greater than one lead to artificial dissipation – causing slow mixing, insufficient flame wrinkling, and hence slow flame propagation. Unfortunately, it is very hard to satisfy these accuracy requirements with CFD codes that have been optimized for RANS on unstructured grids. In this study, two different approaches are compared, which satisfy the stringent requirements for mesh quality and numerical accuracy for the LES of internal combustion engines. Both methods require very limited effort for the grid generation (less than one personnel hour for meshing). The first approach (OpenFOAM) [1] uses unstructured grids with deformable meshes. The second approach (PsiPhi) [2] is based on a structured grid with a combination of Lagrangian particles [3] and Page 2 of 13 immersed boundaries [4] to represent the moving parts of the engine. Both codes, OpenFOAM and PsiPhi (in-house, developed at the chair of Fluid Dynamics, University DuisburgEssen) were available without excessive cost for licensing and have demonstrated good parallel scaling, in the case of PsiPhi beyond 4000 cores. So far, the simulations have concentrated on a motored case, for which the velocity fields obtained in both approaches will be compared to each other and to measurements in an optically accessible engine. At the end of the paper, preliminary results are presented for a fired case.

CFD and Optical Investigations of Fluid Dynamics and Mixture Formation in a DI-H2ICE

This paper reports the validation of a three-dimensional numerical simulation of the in-cylinder processes during gas-exchange, injection, and compression in a direct-injection, hydrogen-fueled internal combustion engine. Computational results from the commercial code Fluent are compared to experimental data acquired by laser-based measurements in a corresponding optically accessible engine. The simulation includes the intake-port geometry as well as the injection event with its supersonic hydrogen jet. The cylinder geometry is typical of passenger-car sized spark-ignited engines. Gaseous hydrogen is injected from a high-pressure injector with a single-hole nozzle. Numerically and experimentally determined flow fields in the vertical, central symmetry plane are compared for a series of crank angles during the compression stroke, with and without fuel injection. With hydrogen injection, the fuel mole-fraction in the same data plane is included in the comparison as well. The results show that the simulation predicts the flow field without injection reasonably well, with increasing numerical-experimental disagreement towards the end of the compression stroke. The injection event completely disrupts the intake-induced flow, and the simulation predicts the post-injection velocity fields much better than the flow without injection at the same crank-angles. The two-dimensional tumble ratio is evaluated to quantify the coherent barrel motion of the charge. Without fuel injection, the simulation significantly over-predicts tumble during most of the compression stroke, but with injection, the numerical and experimental tumble ratio track each other closely. The evolution of hydrogen mole-fraction during the compression stroke shows conflicting trends. Jet penetration and jet-wall interaction are well captured, while fuel dispersion appears under-predicted. Possible causes of this latter discrepancy are discussed.Copyright

Development of laser-induced fluorescence to quantify in-cylinder fuel wall films

Laser-induced fluorescence of a fuel tracer is a very sensitive technique to image in-cylinder liquid fuel films, but quantification of the measured film thickness has proven difficult so far. This article describes improvements in the quantification procedure and presents an example application in a motored, optically accessible spark-ignition engine with direct injection. We designed a calibration tool that could be pressurized and heated, allowing investigation of the laser-induced fluorescence intensities at temperatures exceeding the liquid’s standard-pressure boiling point. The fluorescence intensity of liquid toluene and 3-Pentanone dissolved in isooctane upon excitation with a pulsed laser at 266 nm was investigated as a function of temperature and pressure. Consistent with the literature results on gas-phase laser-induced fluorescence, the signal from toluene was much stronger than from 3-Pentanone, about two orders of magnitude for films thinner than 50 μm. Laser-induced fluorescence from both tracers decreased with increasing temperature but that of toluene significantly more. The response to pressure was less pronounced. For imaging across a large field of view, the spatial non-uniformity of laser excitation and detection efficiency was taken into account using a solid fluorescing substrate, an inexpensive Schott-glass WG280 filter. Isooctane with 0.5 vol.% toluene was used for application in the motored engine, imaging the liquid film on the piston-top window after direct injection from a central multi-hole injector. Air as a bulk gas was found to be advantageous over nitrogen in that gas-phase fluorescence was quenched by oxygen. The imaged film distributions and thicknesses and the derived total fuel film mass were physically plausible. Consistent with the recent literature results from a constant pressure vessel, increasing injection pressure from 50 to 100 bar did not decrease wall wetting but further increase to 200 bar did.