Volker Sick

Temperature and pressure dependences of the laser-induced fluorescence of gas-phase acetone and 3-pentanone

Laser-Induced Fluorescence (LIF) from the S1 state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O2, and N2 respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N2 but decreased markedly for O2. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (≈9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.

A practical guide for using proper orthogonal decomposition in engine research

Proper orthogonal decomposition has been utilized for well over a decade to study turbulence and cyclic variation of flow and combustion properties in internal combustion engines. In addition, proper orthogonal decomposition is useful to quantitatively compare multi-cycle in-cylinder measurements with numerical simulations (large-eddy simulations). However, the application can be daunting, and physical interpretation of proper orthogonal decomposition can be ambiguous. In this paper, the mathematical procedure of proper orthogonal decomposition is described conceptually, and a compact MATLAB® code is provided. However, the major purpose is to empirically illustrate the properties of the proper orthogonal decomposition analysis and to propose practical procedures for application to internal combustion engine flows. Two measured velocity data sets from a motored internal combustion engine are employed, one a highly directed flow (each cycle resembles the ensemble average), and the other an undirected flow (no cycle resembles the average). These data are used to illustrate the degree to which proper orthogonal decomposition can quantitatively distinguish between internal combustion engine flows with these two extreme flow properties. In each flow, proper orthogonal decomposition mode 1 is an excellent estimate of ensemble average, and this study illustrates how it is thus possible to unambiguously quantify the cyclic variability of Reynolds-averaged Navier–Stokes ensemble average and turbulence. In addition, this study demonstrates the benefits of comparing two different samples of cycles using a common proper orthogonal decomposition mode set derived by combining the two samples, the effect of spatial resolution, and a method to evaluate the number of snapshots required to achieve convergence.

Laser-induced-fluorescence detection of nitric oxide in high-pressure flames with A–X(0, 2) excitation

Laser-induced fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. The commonly applied D-X(0, 1) or A-X(0, 0) schemes are restricted to atmospheric-pressure flames and engines driven with gaseous fuels because of strong attenuation of the exciting laser beam by combustion intermediates. The properties of a detection scheme for which excitation in the nitric oxide A-X(0, 2) band was used were investigated. We discuss the advantages of the A-X(0, 2) system (excited at 247.95 nm) based on measurements in laminar premixed methane/air flames at 1-40 bars.

On the use and interpretation of proper orthogonal decomposition of in-cylinder engine flows

The proper orthogonal decomposition (POD) has found increasing application for the comparison of measured and computed data as well as the identification of instantaneous and time varying flow structures, particularly cyclic variability in reciprocating internal combustion engines. The patterns observed in the basis functions or modes are sometimes interpreted as coherent structures, though justification of this is not obvious from the mathematical derivations. Similarly, there is no consensus about whether or not the ensemble mean should be subtracted prior to performing POD on a data set. Synthetic flow fields are used here to reveal POD properties otherwise ambiguous in real stochastic flow data. In particular, each POD mode includes elements of all flow structures from all input snapshots and in general, several modes are needed to reconstruct physical flow structures. POD analysis of two experimental in-cylinder engine data is done: one flow condition where every cycle resembles the ensemble-averaged flow pattern, and the other with large cyclic variability such that no cycles resemble the ensemble average. The energy and flow patterns of the POD modes, derived with and without first subtracting the mean, are compared to each other and to the Reynolds decomposed flow to reveal properties of the POD modes.

Quantitative 2D single-shot imaging of no concentrations and temperatures in a transparent SI engine

The formation of nitric oxide in a transparent spark-ignition (SI) engine was studied experimentally using a combination of planar laser-induced fluorescence and Rayleigh scattering. Simultaneous images were obtained after excitation with a single pulse from a tunable KrF excimer laser. Tuning the laser to 247.9 nm allows excitation of the NO A-X (0,2) band, and subsequent fluorescence emission is spectrally well separated to image Rayleigh scattering from the same laser pulse. The advantages of using the (0,2) band for excitation of NO under engine conditions are demonstrated, and quantitative distributions of NO concentrations are shown along with temperature fields calculated from the Rayleigh images. We investigated various engine operation conditions with propane and iso -octane as fuels. Cycle-resolved measurements show strong cycle-to-cycle variations of the NO formation with peaks in the spatial NO distribution, whereas the temperature field is nearly homogeneous, leading to the conclusion that small variations in the temperature distributions lead to peak NO concentrations via the zeldovich mechanism.

A LASER-INDUCED FLUORESCENCE SCHEME FOR IMAGING NITRIC OXIDE IN ENGINES

Abstract Laser-induced fluorescence of nitric oxide is commonly used for the detection of this major pollutant in combustion processes. Mostly, excitation within the A-X(0, 0) band is used next to excitation of the D-X(0, 1) band. However, for real engine applications strong absorption of the laser beam prevents the application of these schemes. Exciting in the A-X(0, 2) band circumvents these problems and allows the sensitive two-dimensional detection of nitric oxide in engines fueled with iso-octane or diesel fuel. The spectroscopy of this detection scheme and the experimental apparatus used to generate the required laser energy are reported.

Laser in situ monitoring of combustion processes

Several examples of laser in situ monitoring of combustion processes are presented. Using a frequency modulated (13)CO(2) waveguide laser, in situ concentrations of NH(3) down to 1 ppm were measured at temperatures up to 600 degrees C in waste incinerators and power or chemical plants. Following ignition of CH(3)OH-O(2) mixtures by a TEA CO(2) laser, gas temperature profiles were measured using rapid scanning tunable diode laser spectroscopy of CO molecules. In laminar CH(4)-air counterflow diffusion flames at atmospheric pressure absolute concentrations, temperatures, and collisional lifetimes of OH radicals were determined by 2-D and picosecond LIF and absorption spectroscopy. Two-dimensional LIF and Mie scattering were used to observe fuel injection and combustion in a diesel engine.

Attenuation effects on imaging diagnostics of hollow-cone sprays.

Detrimental effects to quantitative interpretation of Mie and laser-induced fluorescence images of hollow-cone sprays were investigated. The attenuation of the laser beam leads to locally unknown intensities rendering it impossible to obtain high-fidelity images of these sprays. Two strategies that use bidirectional illumination of the spray are discussed and evaluated. Conditions for which a bidirectional illumination, single-image detection will allow good recovery of the spray structure are identified. Furthermore, the attenuation of laser-induced fluorescence signals on their path through the spray is quantified.

Invited Review: Combustion instability in spray-guided stratified-charge engines: A review

This article reviews systematic research on combustion instabilities (principally rare, random misfires and partial burns) in spray-guided stratified-charge (SGSC) engines operated at part load with highly stratified fuel -air -residual mixtures. Results from high-speed optical imaging diagnostics and numerical simulation provide a conceptual framework and quantify the sensitivity of ignition and flame propagation to strong, cyclically varying temporal and spatial gradients in the flow field and in the fuel -air -residual distribution. For SGSC engines using multi-hole injectors, spark stretching and locally rich ignition are beneficial. Combustion instability is dominated by convective flow fluctuations that impede motion of the spark or flame kernel toward the bulk of the fuel, coupled with low flame speeds due to locally lean mixtures surrounding the kernel. In SGSC engines using outwardly opening piezo-electric injectors, ignition and early flame growth are strongly influenced by the sprays characteristic recirculation vortex. For both injection systems, the spray and the intake/compression-generated flow field influence each other. Factors underlying the benefits of multi-pulse injection are identified. Unresolved questions include (1) the extent to which piezo-SGSC misfires are caused by failure to form a flame kernel rather than by flame-kernel extinction (as in multi-hole SGSC engines); (2) the relative contributions of partially premixed flame propagation and mixing-controlled combustion under the exceptionally late-injection conditions that permit SGSC operation on E85-like fuels with very low NOx and soot emissions; and (3) the effects of flow-field variability on later combustion, where fuel-air-residual mixing within the piston bowl becomes important.

Quantitative laser-based measurements and detailed chemical kinetic modeling of nitric oxide concentrations in methane-air counterflow diffusion flames

The formation and destruction of nitric oxide in diffusion flames remains a topic of particular relevance to practical applications. In the present work, the chemistry of NO in atmospheric methane-air counterflow diffusion flames is investigated by quantitative laser spectroscopic measurements and detailed chemical kinetic modeling. Highly resolved spatial NO concentration profiles have been obtained usinglaser-induced fluorescence (LIF), and the influence of temperature and collisions on the quantitative interpretation of LIF signals is described. Experimental nitric oxide profiles obtained in pure CH4-air flames and CH4-air flames seeded with NO and NH3 are presented. Comparisons are made with computations featuring a detailed chemical kinetic mechanism with 74 species and 506 reactions. It is shown that acceptable agreement is obtained, and the main areas of uncertainty are outlined. Thus, three current recommendations (GRI Mech. 2.11, Dean et al., and Lindackers et al.) for the “prompt” NO formation channel CH+N2 are assessed. In apparent agreement with the theoretical study of Miller and Walch, the present work tentatively supports the determination of Dean et al., and it is suggested that the computer-optimized rate used in GRI-Mech. 2.11 is significantly (≈250%) too slow. It is further shown that methane diffusion flames differ from their premixed counterparts and that reaction channels such as CH with H2O and NO with HCCO exert a primary influence on computed results.