The first application of a microlens array beam homogenizer to planar laser measurement techniques in combustion diagnostics is demonstrated. The beam homogenizing properties of two microlens arrays in combination with a Fourier lens for widespread applications are presented. An uniform line profile with very little temporal fluctuations of the spatial intensity distribution was generated resulting in a significant reduction of measurement noise and enabling an easier and faster signal processing.

Although the fluorescence behavior of acetone has already been examined widely, the amount of data is still not sufficient for the quantification of signals over the parameter field relevant for combustion engines. This leads to large uncertainties when new excitation wavelengths are applied or in cases where temperature and pressure and bath gas composition dependences of the fluorescence yield must be extrapolated from models. This work presents calibration results of the fluorescence signal intensities in nitrogen, air, and an exhaust-gas-air mixture in the wide range from 298 to 748 K and from 0.2 bar (0.02 MPa) to 20 bars for the two important excitation wavelengths 308 and 248 nm. Based on this data, measurements of temperature and exhaust gas concentrations in a fired spark ignition engine were performed with high accuracy in single-shot images also.

Abstract:The combustion process inside a direct injection spark ignition engine with charge stratification is a very complex process with a large number of variables existing for the definition of the engine characterization map. In this work, two different tracer-based laserinduced fluorescence techniques were applied to analyse the charge motion during the mixing process. For two different operation points with and without a complex in-cylinder flowfield, the two-line excitation LIF technique was applied for temperature and residual gas fraction imaging, and the FARLIF technique for fuel/air ratio imaging. It was found that the mixing of air and residual gas was very fast, and hence no inhomogeneities were found at ignition time. For the mixture preparation process, the swirl makes it difficult to position the fuel vapour cloud near the spark plug at ignition. As a solution to overcome this, a double-injection strategy was applied. With careful parameter choice, depending on the second injection mass and timing, the desired charge situation could be achieved.

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This work presents an experimental approach to the direct determination of the sub-grid scale scalar flux used in Large Eddy Simulation (LES). Two non-reacting jet-flow configurations were examined, one with the jet temperature similar to the ambient air temperature and one preheated jet. Hence data were obtained for varying viscosities. Each case was investigated for two different, high Reynolds numbers of 17,471 and 31,714. For the measurements of mixture fraction and flow velocity field a finest resolution of almost 100 μm was achieved. Thus both the sub-grid scale scalar flux as well as the spatially filtered quantities can be obtained from the same data set. The data were acquired by the simultaneous application of planar laser-induced fluorescence (LIF) and particle image velocimetry (PIV). Abbreviations LIFlaser-induced fluorescence PIV particle image velocimetry RANS Reynolds-averaged-Navier-stokes LES large eddy simulation SGS sub-grid scale Flow Turbulence Combust (2008)