R. Suntz

Size distributions of nanoscaled particles and gas temperatures from time-resolved laser-induced-incandescence measurements

Laser-induced-incandescence (LII) signal decays are measured in sooting premixed atmospheric and low-pressure flames. Soot particle temperatures are obtained from LII signals measured at two wavelengths. Soot particle size distributions P(r) and flame temperatures T are measured spatially resolved by independent techniques. Heat and mass transfer kinetics of the LII process are determined from measured soot particle temperatures, flame temperatures, and particle sizes. Uncertainties of current LII models are attributed to processes during the absorption of the laser pulse. Implications for LII experiments are made in order to obtain primary soot particle sizes. Soot particle size distributions and flame temperatures are assessed from measured particle temperature decays by use of multi-D nonlinear regression.

Two-dimensional visualization of the flame front in an internal combustion engine by laser-induced fluorescence of OH radicals

Two-dimensional laser-induced fluorescence (2D-LIF) imaging of OH radicals, excited at 308 nm, has been employed to visualize the flame front in an internal combustion engine burning air/propane mixtures. Light sheet thicknesses down to 70 μm have been attained for excitation. Hydroxyl radicals were detected up to pressures of 7.5 bar at engine speeds of 500 rpm. An upper limit of 300 μm for the flame front thickness was obtained from line intensity profiles.

Investigations of the Formation and Oxidation of Soot Inside a Direct Injection Spark Ignition Engine Using Advanced Laser-Techniques

In this work the formation and oxidation of soot inside a direct injection spark ignition engine at different injection and ignition timing was investigated. In order to get two-dimensional data during the expansion stroke, the RAYLIX-technique was applied in the combustion chamber of an optical accessible single cylinder engine. This technique is based on the quasi-simultaneous detection of Rayleigh-scattering, laser-induced incandescence (LII) and extinction which enables simultaneous measurements of temporally and spatially resolved soot concentrations, mean particle radii and number densities. These investigations show that in our test engine the most important source for soot formation during combustion are pool fires, i.e. liquid fuel burning on the top of the piston. These pool fires were observed under almost all experimental conditions.

Influence of exhaust gas recirculation on soot formation in diffusion flames

A recently developed measuring technique for simultaneous imaging of two-dimensional fields of soot volume fractions, particle number densities, and mean particle radii was used to investigate the influence of exhaust gas recirculation on soot formation in laminar and turbulent co-flowing diffusion flames. These investigations are of special concern with respect to the use of exhaust gas recirculation in internal combustion engines. From direct ignition diesel engines, it is known that exhaust gas recirculation results in a decrease in the soot oxidation rate and, therefore, higher soot emission levels. The exhaust gas recirculation in several laminar and turbulent ethyne/nitrogen diffusion flames was simulated by the variation of the chemical composition of the oxidizer mixture in the co-flow. Soot formation was delayed and a lower maximum soot volume fraction was observed, if the ratio of the oxygen/nitrogen mole fractions in the oxidizer mixture of the co-flow decreased. Nevertheless, toward the flame tip, oxidation of soot particles also decelerated under the aformentioned situation, leading to a higher soot emission level. The addition of carbon dioxide to the oxidizer flow also reduced maximum soot volume fractions but did not affect soot oxidation. Moreover, slight temperature changes in the oxidizer co-flow led to significant changes in the observed profiles.

Flame front imaging in an internal-combustion engine simulator by laser-induced fluorescence of acetaldehyde.

Acetaldehyde has been used as a fluorescent dopant for two-dimensional imaging of the flame front in an internalcombustion-engine simulator. The molecule was excited with a XeCl-laser-light sheet at 308 nm, and broadband fluorescence centered at 400 nm was detected. In this way, the flame front could be marked by mapping regions of unburned gas. Also, the intake process into the engine could be followed.

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.

Investigation of flame structure and burning behaviour in an IC engine simulator by 2D-LIF of OH radicals

Turbulent combustion of propane/air mixtures in an internal combustion engine simulator has been studied by 2D-LIF of OH radicals formed in the combustion process. A laser light sheet of thickness 75 μm at 308 nm was used for excitation of OH and the fluorescence imaged onto an image-intensified CCD-camera. From the large number of images recorded, information on the burning behaviour of various flame structures could be obtained. In particular, flame extinction was clearly observed for lean (λ=1.5) mixtures.

Progress in characterization of soot formation by optical methods

A two dimensional in situ optical technique is used to measure absolute soot volume fractions, particle number densities, and mean particle sizes in moderately sooting laminar and turbulent diffusion flames with high spatial and temporal resolution. These data are of special interest for the development and validation of models for the formation and oxidation of soot. The technique (RAYLIX) is based on the simultaneous two dimensional detection of Rayleigh scattering and the laser induced incandescence (LII) in combination with the detection of the integral extinction from one single laser pulse. All signals are induced by a single pulse of a frequency doubled Nd–YAG laser. Besides mean particle sizes it is of special interest to derive information about the particle size distribution by the detection of the temporal decay of the LII signal. Information about the particle size distribution can then be obtained by simulating this decay using a LII model in combination with multidimensional non-linear regression. With this strategy the parameters describing the particle size distribution as well as the temperature of the surrounding gas phase are varied so that the calculated decay of the LII signal is in accordance with the measured one. In addition to these quantities also probability–density functions (PDF), correlation functions and length scales are derived from the soot volume fraction in the turbulent flames. These quantities are of special interest for the modelling of turbulent reacting flows.

Assessment of soot volume fractions from laser-induced incancescence by comparison with extinction measurements in laminar, premixed, flat flames

For the validation of detailed chemical models for soot formation, and for their application to turbulent flames, 2-D measuring techniques are necessary to derive locally resolved soot volume fractions, particle sizes, and number densities. In one-dimensional laminar flames, these particle properties are derived from Rayleigh-scattering/extinction techniques. The extinction technique, as a line-of-sight method, is not appropriate for investigation of three-dimensional, nonstationary, turbulent systems. Therefore, for turbulent flames, other measuring techniques have to be developed that can be employed in combination with, for example, Rayleigh scattering. One technique that has been applied to obtain soot volume fractions in laminar flames is Laser-Induced Incandescence (LII) [1]. The major task in applying this technique is to clarify in which way the relative LII signals should be calibrated to yield absolute soot volume fractions. In this work, the LII technique is investigated systematically in laminar, premixed, ethyne/argon/oxygen flames using a pulsed, high-power Nd:YAG laser. Therefore, the LII signals are compared with soot volume fractions obtained using extinction. The systematic investigation of the LII technique under different detection conditions (camera gate width and gate delay times) shows that there are inconsistencies between calibrated LII signals and soot volume fractions from extinction. The aim of this work is to explain these inconsistencies by a numerical simulation of the LII signal, based on mass and energy balance equations of the heated soot particles for different flame conditions. Additionally, a sensitivity analysis with respect to different parameters of the balance equation is performed. The results indicate that geometric irregularities of the soot particles probably have to be taken into account for correct prediction of soot volume fractions using LII.

Two-dimensional imaging of soot volume fractions, particle number densities, and particle radii in laminar and turbulent diffusion flames

A two-dimensional optical technique has been developed to measure absolute soot volume fractions, particle number densities, and mean particle radii in moderately sooting laminar and turbulent diffusion flames. The RAYLIX technique is based on the two-dimensional detection of Rayleigh scattering and the laser-induced incandescence (LII) signal in combination with the detection of the extinction from one single laser pulse. For this, the beam of a frequency-doubled Nd-YAG laser is split into a low-energy beam and a high-energy beam and then expanded into a light sheet. The high-energy beam is delayed by about 20 ns by means of an optical delay line. The low-energy beam is used for Rayleigh scattering as well as for measuring the extinction. Scattering and extinction are simultaneously detected by a single-imageintensified charge-couple device (CCD) camera. The delayed high-energy beam passes the flame and induces the LII signal, which is detected by a second CCD camera. From these signals, two-dimensional fields of soot volume fractions (fv), particle number densities (Nv), and particle radii (rm) are derived. The profiles of the soot volume fractions, particle number densities, and mean particle radii for laminar flames exhibit maxima in the particle number density at radial positions, where minima in the particle radii were found, whereas the maxima in the soot volume fractions were shifted somewhat toward lower radical distances. This remains the case also for the turbulent flame, indicating that the timescales for soot formation are smaller than the turbulent timescales.