Showing papers by "Alfred Leipertz published in 2004"

Application of 266-nm and 355-nm Nd:YAG laser radiation for the investigation of fuel-rich sooting hydrocarbon flames by Raman scattering

Abstract: We describe the use of linear Raman scattering for the investigation of fuel-rich sooting flames. In comparison, the frequency-tripled and -quadrupled fundamental wavelengths of a Nd:YAG laser have been used as an excitation source for study of the applicability of these laser wavelengths for analysis of sooting flames. The results obtained show that, for the investigation of strongly sooting flames, 266-nm excitation is better than 355-nm excitation. Although the entire fluorescence intensity of polycyclic aromatic hydrocarbons (PAHs) decreases with rising excitation wavelength, there is increased interference with the Raman signals by displacement of the spectral region of the Raman signals toward the fluorescence maximum of the laser-induced fluorescence emissions. Besides the broadband signals of PAHs, narrowband emissions of laser-produced C2 occur in the spectra of sooting flames and affect the Raman signals. These C2 emission bands are completely depolarized and can be separated by polarization-resolved detection. A comparison of the laser-induced fluorescence emissions of an ethylene flame with those of a methane flame shows the same spectral features, but the intensity of the emissions is larger by a factor of 5 for the ethylene fuel. Using 266-nm radiation for Raman signal excitation makes possible measurements in the ethylene flame also.

Gas-phase temperature measurement in the vaporizing spray of a gasoline direct-injection injector by use of pure rotational coherent anti-Stokes Raman scattering.

Abstract: Pure rotational coherent anti-Stokes Raman spectroscopy is applied for quantitative gas-phase temperature measurements in the vaporizing spray of an automotive fuel injector. Interferences from elastically scattered stray light are greatly reduced by use of a polarization technique and spectral filtering in a double monochromator. The applicability of this technique to probing low-temperature sprays is successfully demonstrated.

Determination of primary particle size distributions from time-resolved laser-induced incandescence measurements.

Abstract: For a polydisperse nanoparticle ensemble the evaluation of time-resolved laser-induced incandescence (LII) measurements yields a weighted average value for the primary nanoparticle size. Although this value is sufficient for narrow size distributions, a comprehensive characterization of a particle-evolution process requires the reconstruction of the size distribution. An easy-to-use online approach is presented to evaluate the LII signal regarding higher moments of the distribution. One advantage of this approach is that the size distribution results in a deceleration of the LII signal decay with time after the laser pulse. Therefore LII signal-decay curves are evaluated in two different time intervals after the laser pulse, providing information about the desired distribution parameters that has been tested successfully with experimental curves taken in different soot-formation processes.

Application of an optical pulse stretcher to coherent anti-Stokes Raman spectroscopy.

Abstract: An external optical cavity pulse stretcher for nanosecond-long laser pulses has been applied to coherent anti-Stokes Raman spectroscopy (CARS). An increased signal-to-noise ratio was achieved for both vibrational and pure rotational CARS, while the power density of the laser beams remained constant. Moreover, it was demonstrated that the use of the pulse stretcher also leads to improved precision of the determined temperatures and concentrations as a result of repeated excitation of the dye laser.

Viscosity and Surface Tension of Saturated n-Pentane

Abstract: Light scattering by thermally excited capillary waves on liquid surfaces or interfaces can be used for the investigation of viscoelastic properties of fluids. In this work, the simultaneous determination of surface tension and liquid kinematic viscosity of n-pentane by surface light scattering (SLS) on a gas–liquid interface was carried out. The experiments are based on a heterodyne detection scheme and signal analysis by photon correlation spectroscopy (PCS). Measurements were performed under saturation conditions over a wide temperature range from about 233 to 363 K. For the whole temperature range the total uncertainty of the liquid kinematic viscosity and surface tension is estimated to be better than 1.0 and 1.2, respectively. The results obtained corroborate the reliability of the SLS technique for the determination of thermophysical properties.

Temperature and CO concentration measurements in a partially premixed CH4/air coflowing jet flame using coherent anti-stokes Raman scattering

Abstract: In a partially premixed methane/air flame, temperature and CO concentration measurements were conducted using coherent anti-Stokes Raman spectroscopy. Additionally, qualitative two-dimensional OH laser-induced fluorescence imaging was applied. Measurements were performed at different positions downstream of the burner's exit for equivalence ratios (φ) between 1.7 and 4.0. A distinct double-flame structure was observed. Accurate temperature and CO concentration results were achieved, even under sooting conditions. Temperature measurements close to the burner (5 mm downstream) indicated a temperature peak of 2000 K while further downstream at 10 and 20 mm a high-temperature plateau could be found. The CO concentration increases with decreasing φ starting below a particular threshold value. It was a maximum at the inner flame due to the formation of CO as a result of partial combustion. The maximum CO is found to be greater than that in a diffusion flame. The maximum CO concentration has been found for φ = 1...

Thermophysical Properties of 1,1,1,3,3-Pentafluorobutane (R365mfc)

Abstract: This paper presents an experimental study on various thermophysical properties of a new fluoroalkane, 1,1,1,3,3-pentafluorobutane (R365mfc). The thermal conductivity of R365mfc was measured in the liquid phase near saturation conditions at temperatures between 263 and 333 K using a parallel plate instrument with an uncertainty of less than ±5%. For the measurement of the saturated liquid density between 273 and 353 K, a vibrating tube instrument was used. The uncertainty of the density measurements is less than ±0.1%. In addition, experimental data have been obtained for R365mfc under saturation conditions over a wide temperature range from about 253 to 460 K using light scattering techniques. Light scattering from the bulk fluid has been applied for measuring both the thermal diffusivity and the sound speed in the liquid and vapor phases. Light scattering by surface waves on a horizontal liquid–vapor interface has been used for the simultaneous determination of the surface tension and kinematic viscosity of the liquid phase. With the light scattering techniques, uncertainties of less than ±1.0, ±0.5, ±1.0, and ±1.2% have been achieved for the thermal diffusivity, the sound speed, the kinematic viscosity, and the surface tension, respectively.

Thermophysical Properties of Binary Mixtures of R125 + R143a in Comparison with a Simple Prediction Method

Abstract: The thermal diffusivity and sound speed of binary refrigerant mixtures of R143a (1,1,1-trifluoroethane) and R125 (pentafluoroethane) have been determined for both the saturated liquid and vapor phase using dynamic light scattering (DLS). Measurements were performed for four quite different mixture compositions over a wide temperature range from 293 to 345 K approaching the vapor-liquid critical point. The results obtained corroborate the usefulness of a simple prediction method for the determination of different thermophysical properties of multicomponent mixtures in the two-phase region up to the critical point. Besides the information on the properties for the pure components, the successful application of the prediction method is also based on an exact knowledge of the critical temperature. The composition dependence of the critical temperature has been determined by observation of the vanishing meniscus between liquid and vapor phases. The mixture results are discussed in detail and compared with available literature data.

Method for determining the distribution of particle sizes in a polydisperse particle set

Abstract: A method for determining the distribution of particle sizes in a set of particles from time-resolved measurement of the radiant heat of particles heated over a short period of time. In order to achieve comprehensive characterization of a set of particles, it is necessary to determine the size distribution of said particles, especially primary particles, preferably on line. The inventive method is based on the fact that during the cooling of heated particles, the weighting of the signal quantities of individual particle size categories is modified, as a result of the conduction of heat, whereby a radiant heat signal is formed. Smaller particles have quicker signal delays and thus contribute in a time-delayed manner to the overall signal of the particle collective. The overall signal of a polydisperse particle collective does not decrease in a simply exponential manner. The signal delay is modified over time and increases. On-line evaluation of the time-resolved signal by mathematical adaptation into two or several time periods during cooling produces characteristic signal delays for the various time domains. The higher moments of the particle size distribution can be clearly determined therefrom by making specific assumptions as to distribution function. The invention also relates to on-line analysis or process control of particle synthesis processes during product characterization or analysis of the waste gases of engine combustion processes or other combustion processes.