Alfred Leipertz

Experimental Vibrational Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide and 1-Ethyl-3-methylimidazolium Ethylsulfate

The vibrational structure of two room-temperature ionic liquids with the cation 1-ethyl-3-methylimidazolium [EMIM] and the respective anions bis(trifluoromethylsulfonyl)imide [TFSI] and ethylsulfate [EtOSO3] is investigated. In particular, attenuated total reflection (ATR) infrared (IR) as well as Raman spectra in the spectral range from 500 to 3500 cm−1 have been recorded and analyzed. Moreover, the depolarization ratios of the Raman lines are determined. The individual peaks are assigned to the corresponding vibrational modes of the molecules. While the CH stretching region around 3000 cm−1 is dominating in Raman spectra, it is remarkably weak in IR spectra. Finally, the results for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared to previous studies of single ions available in the literature. This comparison shows good agreement.

Two-dimensional soot-particle sizing by time-resolved laser-induced incandescence

The evaluation of the temporal decay of the laser-induced incandescence (LII) signal from soot particles is introduced as a technique to obtain two-dimensional distributions of particle sizes and is applied to a laminar diffusion flame. This novel approach to soot sizing exhibits several theoretical and technical advantages compared with the established combination of elastic scattering and LII, especially as it yields absolute sizes of primary particles without requiring calibration.

Performance characteristics of soot primary particle size measurements by time-resolved laser-induced incandescence

A detailed analysis of various factors that influence the accuracy of time-resolved laser-induced incandescence for the determination of primary soot particles is given. As the technique relies on the measurement of the signal ratio at two detection times of the enhanced thermal radiation after an intense laser pulse, guidelines are presented for a suitable choice of detection times to minimize statistical uncertainty. An error analysis is presented for the issues of laser energy absorption, vaporization, heat conduction, and signal detection. Results are shown for a laminar ethene diffusion flame that demonstrate that concurring results are obtained for various laser irradiances, detection characteristics, and times of observation.

Soot Temperature Measurements and Implications for Time-Resolved Laser-Induced Incandescence (TIRE-LII)

Emission spectroscopy has been used to determine soot particle temperatures in an ethene diffusion flame both under normal combustion conditions and also after irradiation with an intense laser pulse. On the basis of these measurements, a check on the models and an improvement of parameters underlying time-resolved laser-induced incandescence (TIRE-LII) was performed. With this technique a two-dimensionally resolved measurement of soot primary particle sizes is feasible in a combustion process from the ratio of emission signals obtained at two delay times after a laser pulse, as the cooling behavior is characteristic of particle size. For accurate measurements, local gas temperatures must be known, which can be derived from the temperatures of the soot particles themselves. These have been measured by fitting full Planck curves to line-of-sight emission spectra after an inversion algorithm. The temperature and heat of vaporization of soot, which govern the energy and mass loss at high temperatures, were obtained by measurements of maximum particle temperature for various laser irradiances and a fit procedure to the theoretical dependence. Finally, the temperature decay of laser-heated soot was measured with high temporal resolution. Comparisons with model predictions show that soot temperatures are roughly 300 K higher than expected after the onset of vaporization, which indicates deficiencies in the present models of vaporization. It is demonstrated that the TIRE-LII performance is essentially unaffected by these shortcomings if LII signals are detected in a period where conductive heat transfer dominates and an appropriate correction is performed.

Experimental comparison of single-shot broadband vibrational and dual-broadband pure rotational coherent anti-Stokes Raman scattering in hot air

Broadband vibrational and dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS) have been compared in a high-temperature oven, in which the accuracy and single-shot precision of gas temperature and relative O(2)- and N(2)-concentration measurements in hot air were probed over a temperature range that is typical for many combustion processes. To ensure a realistic comparison, we used nearly the same experimental setup for both CARS techniques. Besides temperature information, dual-broadband pure rotational CARS offers the possibility of achieving simultaneous single-shot concentration measurements. The comparison shows that this technique also has significant advantages in temperature evaluation over a large temperature range in comparison with vibrational CARS.

Structure of locally quenched highly turbulent lean premixed Flames

The local flame structure of a premixed swirl-stabilized gas turbine burner has been investigated, where “thickened flames” or flames in the “stirred reactor regime” are expected from the Borghi diagram. Simultaneous 2-D OH and temperature measurements show that the flame structure fluctuates between four typical flame modes: (1) flamelet-like burning, (2) modified preheat zone with sharp reaction zone, (3) locally quenched reaction zone, and (4) hot nonreacted holes. In mode (3), evidence for local quenching processes is found with no superequilibrium OH between fresh gas and recirculating burned gas. In about 10% of the obtained images, regions without detectable OH but with temperatures above 1300 K are seen, surrounded by sharp reaction zones with low thermal gradients (mode 4). Although the explanation is not clear yet, significant modifications of the reaction processes due to strain combined with transient effects seem to occur here. From 2-D and 3-D measurements, gradients and flame thickness distributions have been determined, showing strong fluctuation of the thermal gradients, but on average, no significantly broadened instantaneous flame fronts, contrary to the expectation of thick flames. Instead, mean gradients conditioned to the local reaction progress variable c=(T−T 0 )/(T max −T 0 ) =0,50 result in a slightly thinned thermal thickness, and within the preheat zone at c =0.25 a thinning of about 30% is found. For an explanation. strained laminar calculations are helpful, indicating the same trend for lean CH 4 -air mixtures. Obviously, strain effects from medium- and large-scale eddies have much more influence than diffusive effects from entrained small eddies. While the thermal thickness distribution shows large fluctuations, the OH thickness distribution (deduced from the OH ascent) is narrow. Comparing the Oh ascent thickness (similar to the calculated width of the CH peak) with the inner layer Kolmogorov size, a ratio of order 1 is found, when local quenching occurs.

Two-dimensional temperature determination in sooting flames by filtered Rayleigh scattering

We present what to our knowledge are the first filtered Rayleigh scattering temperature measurements and use them in sooting f lame. This new technique for two-dimensional thermography in gas combustion overcomes some of the major disadvantages of the standard Rayleigh technique. It suppresses scattered background light from walls or windows and permits detection of two-dimensional Rayleigh intensity distributions of the gas phase in the presence of small particles by spectral filtering of the scattered light.

Design and characterization of a Raman-scattering-based sensor system for temporally resolved gas analysis and its application in a gas turbine power plant

A sensor system for fast gas composition analysis is presented. Using linear Raman scattering the simultaneous detection of virtually all components of fuel gas mixtures such as natural gas and biogas can be achieved. The system consists of commercially available hardware components, in detail a frequency doubled continuous wave laser at 532 nm and a compact spectrometer with an embedded charge coupled device chip. For the evaluation of the Raman spectra a fast software module based on a contour fit algorithm is developed. Moreover, modules for controlling the hardware components are implemented in the sensor software ensuring simple operability of the entire system. In this paper the sensor is characterized in terms of, e.g., accuracy, reproducibility, detection limits and temporal performance. Finally its application for natural gas analysis in a gas turbine power plant is demonstrated, and the results obtained are compared to gas chromatography results.

Pressure influence on the flame front curvature of turbulent premixed flames: comparison between experiment and theory

Abstract In gas turbines, lean premixed combustion is executed in strongly turbulent flow fields and under high-pressure to allow large thermal loads within small-size combustors. Previous research on turbulent premixed flames has revealed the vital importance of flame-vortex interactions, but most of these investigations have been performed only at atmospheric pressure disregarding the large pressure dependency of the flame front dynamics. We report about spatially high-resolved laser-induced predissociation fluorescence imaging of OH ( OH - LIPF ) in premixed, high-pressure bluff-body stabilized methane/air flames. For each of the two measurement series with different equivalence ratio (φ = 0.7 and φ = 1.0), the planar flame topology at different pressures (0.1 to 1.1 MPa) but constant exit velocity was detected and stored for analysis. As the pressure was increased, the flame front contour of both equivalence ratios became strongly wrinkled with formation of highly curved flame front elements. For quantification of this phenomenon, the probability density function of flame curvature was evaluated with definition of the mean curvature radius as representative folding scale. To discuss different mechanisms of flame front disturbances according to their relevance, the flame curvature is compared with characteristic turbulence scales of the flow field and with the expected folding scale derived with Sivashinsky‘s formulation of linear flame instability theory. Significant changes become obvious especially if the pressure is increased up to 0.5 MPa. The mean curvature radius decreases distinctly and can be linked to the decreasing size of the Taylor length. Additionally, the formation of highly convoluted flame front elements is enforced by the increasing flame instability behavior. As the results show, the flame stoichiometry has a strong impact on the flame front topology at increasing pressures due to the differences of their flame dynamics.

Measurement of the resolved flame structure of turbulent premixed flames with constant reynolds number and varied stoichiometry

Wire-stabilized premixed methane-air flames have been studied in a grid-generated homogeneous turbulent flow field in order to identify different burning regimes. The planar Rayleigh scattering technique was used with two parallel laser light sheets, which allows the detection of three-dimensional temperature gradients. For a detailed investigation of the flame structure and topology, the modification of the local temperature gradients at different progress variables c due to the turbulent motion was studied by varying the flame stoichiometry and thereby the Karlovitz number Ka while keeping the turbulent Reynolds number Ret constant at 87 or 134. Because of a nearly Gaussian shaped statistical distribution of the thermal gradients, the 50% median and the width of the distribution are suitable measures used to characterize the flame response. Compared with laminar unstrained calculations, especially very lean flames (