Showing papers by "Alfred Leipertz published in 1998"

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

Abstract: 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.

Structure of locally quenched highly turbulent lean premixed Flames

Abstract: 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.

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

Abstract: 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 (

One-dimensional, time-resolved Raman measurements in a sooting flame made with 355-nm excitation

Abstract: Single-shot vibrational Raman measurements were performed along an 11-mm-long line crossing the reaction zone in a premixed, fuel-rich (phi = 10), laminar methane-air flame by use of a frequency-tripled Nd:YAG laser with a 355-nm emission wavelength. This laser source seems to have advantages relative to KrF excimer lasers as well as to Nd:YAG lasers at 532 nm for hydrocarbon combustion diagnostics. The Raman emissions of all major species (N(2), O(2), CH(4), H(2), CO(2), H(2)O) were detected simultaneously with a spatial resolution of 0.4 mm. By integration over selected spectral intervals, the mole fractions of all species and subsequently the local gas temperatures have been obtained. A comparison of the temperatures that were found with results from filtered Rayleigh experiments showed good agreement, indicating the success of what are to the best of our knowledge the first one-dimensional single-shot Raman measurements in a sooting hydrocarbon flame.

Thermal Diffusivity and Sound Velocity of Toluene Over a Wide Temperature Range

Abstract: The thermal diffusivity and the sound velocity of toluene have been determined in a wide temperature range up to the critical point by dynamic light scattering. Measurements were performed for both the liquid and the vapor phase at saturation conditions. The results obtained corroborate the usefulness of the technique for the determination of thermophysical properties. From the lack of reference data for the thermal diffusivity or other properties from which the thermal diffusivity may be derived in the high-temperature range and from deviations of experimental sound velocity data compared to values derived from the currently established equation of state, the need for further experimental investigations on this important reference fluid was established.

Pure rotational coherent anti-stokes Raman scattering: comparison of evaluation techniques for determining single-shot simultaneous temperature and relative n(2)-o(2) concentration.

Abstract: The accuracy and precision of time-resolved simultaneous temperature and O(2)-concentration measurements in binary N(2)-O(2) mixtures by single-pulse dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS) have been investigated. We present a detailed comparison of the applicability of six evaluation procedures to measurements of air in a temperature range 300-2050 K. Special emphasis is put on the dependence of the results on experimental restrictions and distortions. This comparison includes the least-sum-of-squared-differences fit (LSF) in the frequency space obtained by use of three different kinds of weighting with respect to signal intensity and in Fourier space by use of the complex or the cosine Fourier transformation, both of which permit a great reduction in the number of data points necessary for multidimensional evaluation. Additionally, a cross-correlation technique is tested that, to the best of our knowledge, was not previously applied to pure rotational CARS. We also present the results of measurements directed to the determination of low O(2)-concentration levels that were performed for various binary mixtures (1.0-15.6% O(2)) and for natural air within a temperature range of 300-773 K. A comparison is given for the three evaluation techniques that have proved most promising for the high-temperature investigations, i.e., the constant and the inverse weighted LSF in frequency space and the Fourier analysis technique.

Quantitative temperature and concentration measurement in heterogeneous systems, e.g. injection and combustion systems, using Raman scattering

Abstract: The temperature and concentration measurement involves stimulating Raman signals at a wavelength greater than 310 nm and smaller than 400 nm, whereby the OH and O2-molecules present in the measurement volume (3) do not absorb and hence do not fluoresce. The remaining unpolarized background is separated from the Raman signals by dividing (5) the optical signals into two mutually perpendicular polarization directions and subtracting them from each other.

Measurement of drop size or size distribution in two phase flows, without calibration

Abstract: The method involves separately recording the signal components of light scattered by the drops, polarized perpendicular and parallel to the scattering plane, using two suitable detectors at the same time. By comparing the signal ratio with calculated curves, the drop size and its distribution can be determined.

Dynamic range enhancement for pure rotational coherent anti-Stokes Raman scattering thermometry by use of an optical arrangement with two dye lasers

Abstract: A disadvantage of pure rotational coherent anti-Stokes Raman scattering (CARS) compared with vibrational CARS is the limited dynamic range for temperature measurements. Here an optical configuration is described that overcomes this limitation by the use of two different center-frequency dye lasers in a dual-broadband CARS approach. Its performance is demonstrated for simultaneous pure rotational CARS temperature and relative N(2)-O(2)concentration measurements up to 1950 K at ambient pressure.

Verfahren zur zweidimensional lokal und zeitlich hochaufgelösten Bestimmung der Tropfengröße und Tropfengrößenverteilung in Zweiphasen-Strömungen Method for two-dimensional locally and temporally highly resolved determination of droplet size and droplet size distribution in two-phase flows

Abstract: Verfahren zur kalibrationsfreien Messung der Tropfengrosen und der Tropfengrosenverteilung in Zweiphasen-Stromungen, bei dem linear polarisiertes Licht auf einen Strom von Tropfen gelenkt wird, wobei das Licht, das zu einem den Untersuchungsbereich bildenden Lichtschnitt durch den Strom geformt ist, an den Tropfen in dem Untersuchungsbereich gestreut wird, bei dem ferner das gestreute Licht simultan zweidimensional lokal und zeitlich aufgelost in einer Streuebene hinsichtlich seiner senkrecht und parallel zu der Streuebene polarisierten Signalanteile je Tropfen von zwei Detektoren zum selben Zeitpunkt getrennt voneinander aufgenommen wird und ein Verhaltnis der beiden Signalanteile gebildet wird, bei dem schlieslich durch Vergleich des Signalanteilverhaltnisses mit berechneten Kurven die Tropfengrose und deren Verteilung bestimmt wird, wobei einerseits der Mittelwert von Signalanteilverhaltnissen je Tropfen von allen im Untersuchungsbereich detektierten Tropfen gebildet wird und andererseits ein Verhaltnis von jeweils uber alle Tropfen im Untersuchungsbereich summierten Signalanteilen gebildet wird und aus dem Vergleich dieser beiden unterschiedlich gebildeten Verhaltnisse die Verteilungsbreite... A method for calibration-free measurement of the droplet size and the droplet size distribution in two-phase flows, is directed at the linearly polarized light to a stream of drops, in which the light that is formed into a the examination region forming the light section through the stream to the drop in the examination zone is scattered, further comprising the scattered light simultaneously two-dimensionally locally and temporally resolved in a scattering plane with respect to its perpendicular and parallel to the scattering plane polarized signal components per drop of two detectors is recorded separately at the same time from each other, and a ratio of the two signal components is formed, wherein the finally calculated curves drop size and its distribution is determined by comparing the signal component ratio, on the one hand, the mean of signal component ratios for each drop of all detected in the study area drop is formed and on the other hand, totaled a ratio of over all drop in the examination area signal components is formed and from The comparison of these two ratios differently formed, the distribution width ...