Christian Brackmann

Laser-induced fluorescence of formaldehyde in combustion using third harmonic Nd : YAG laser excitation

Formaldehyde (CH2O) is an important intermediate species in combustion processes and it can through laser-induced fluorescence measurements be used for instantaneous flame front detection. The present study has focussed on the use of the third harmonic of a Nd:YAG laser at 355 nm as excitation wavelength for formaldehyde, and different dimethyl ether (C2H6O) flames were used as sources of formaldehyde in the experiments. The investigations included studies of the overlap between the laser profile and the absorption lines of formaldehyde, saturation effects and the potential occurrence of laser-induced photochemistry. The technique was applied for detection of formaldehyde in an internal combustion engine operated both as a spark ignition engine and as a homogenous charge compression ignition engine.

Thermometry in internal combustion engines via dual-broadband rotational coherent anti-Stokes Raman spectroscopy

Rotational coherent anti-Stokes Raman spectroscopy (CARS) has since the beginning of the 1980s been developed as a non-intrusive tool for temperature measurements in combustion. Since the introduction of the dual-broadband concept in 1986, the quality of the technique has been much improved, and application to practical combustion situations facilitated. Since the first demonstration of its use in spark-ignition engines in 1993, several measurement campaigns in engines have been accomplished. These campaigns concerned temperature measurements in the unburned gas mixture before combustion as part of a larger project with the aim of improving the knowledge on the phenomenon of engine knock. In this paper, the results of this work are reviewed with a focus on the characteristics of the technique and the quality of the evaluated temperatures. Re-evaluations of data using an improved theoretical model are presented and compared with previous results. Moreover, the treatment of large data sets on single shots from spatial regions with conditions varying from unburned to burned gas is discussed. It is demonstrated that dual-broadband rotational CARS probing nitrogen and oxygen has a high potential for thermometry at the conditions in the unburned gas mixture. Merits and limitations of the technique are discussed and the issues treated are, among others, experimental problems, data evaluation, and single-shot temperature accuracy.

Simultaneous vibrational and pure rotational coherent anti-Stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames

The potential of measuring temperature and multiple species concentrations (N2, O2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS spectra, O2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique for probing of additional species are discussed.

Pure rotational coherent anti-Stokes Raman spectroscopy in mixtures of CO and N 2

We present a model for quantitative measurements in binary mixtures of nitrogen and carbon monoxide by the use of dual-broadband rotational coherent anti-Stokes Raman spectroscopy. The model has been compared with experimental rotational coherent anti-Stokes Raman scattering spectra recorded within the temperature range of 294-702 K. Temperatures and concentrations were evaluated by spectral fits using libraries of theoretically calculated spectra. The relative error of the temperature measurements was 1-2%, and the absolute error of the CO concentration measurements was <0.5% for temperatures < or =600 K. For higher temperatures, the gas composition was not chemically stable, and we observed a conversion of CO to CO2. The influence of important spectroscopic parameters such as the anisotropic polarizability and Raman line-broadening coefficients are discussed in terms of concentration measurements. In particular, it is shown that the CO concentration measurement was more accurate if N2-CO and CO-N2 line-broadening coefficients were included in the calculation. The applicability of the model for quantitative flame measurements is demonstrated by measuring CO concentrations in ethylene/air flames.

The effect of knock on the heat transfer in an SI engine : Thermal boundary layer investigation using CARS temperature measurements and heat flux measurements

It is generally accepted that knocking combustion influences the heat transfer in SI engines. However, the effects of heat transfer on the onset of knock is still not clear due to lack of experimental data of the thermal boundary layer close to the combustion chamber wall. This paper presents measurements of the temperature in the thermal boundary layer under knocking and non-knocking conditions. The temperature was measured using dual-broadband rotational Coherent anti-Stokes Raman Spectroscopy (CARS). Simultaneous time-resolved measurements of the cylinder pressure, at three different locations, and the heat flux to the wall were carried out. Optical access to the region near the combustion chamber wall was achieved by using a horseshoe-shaped combustion chamber with windows installed in the rectangular part of the chamber. This arrangement made CARS temperature measurements close to the wall possible and results are presented in the range 0.1-5 mm from the wall. The engine was run with constant fuel flow under near stoichiometric conditions. Knocking and non-knocking conditions were achieved by using different mixtures of n-heptane and iso-octane. Copyright

Real-Time Gas-Phase Imaging over a Pd(110) Catalyst during CO Oxidation by Means of Planar Laser-Induced Fluorescence

The gas composition surrounding a catalytic sample has direct impact on its surface structure, which is essential when in situ investigations of model catalysts are performed. Herein a study of the gas phase close to a Pd(110) surface during CO oxidation under semirealistic conditions is presented. Images of the gas phase, provided by planar laser-induced fluorescence, clearly visualize the formation of a boundary layer with a significantly lower CO partial pressure close to the catalytically active surface, in comparison to the overall concentration as detected by mass spectrometry. The CO partial pressure variation within the boundary layer will have a profound effect on the catalysts’ surface structure and function and needs to be taken into consideration for in situ model catalysis studies.

QUANTITATIVE MEASUREMENTS OF SPECIES AND TEMPERATURE IN A DME-AIR COUNTERFLOW DIFFUSION FLAME USING LASER DIAGNOSTIC METHODS

A diffusion flame of dimethyl ether (DME) and air in a counterflow burner has been investigated experimentally by means of different laser-based methods. Quantitative measurements of temperature, O2 concentration, and OH concentration have been carried out using dual broadband rotational coherent anti-Stokes Raman Spectroscopy (CARS), Rayleigh scattering and laser-induced fluorescence (LIF). Furthermore a qualitative formaldehyde profile has been measured using LIF. The measured profile has been compensated for fluorescence signal quenching and the Boltzmann distribution to obtain a representative qualitative formaldehyde profile for the flame.

Strategies for formaldehyde detection in flames and engines using a single-mode Nd:YAG/OPO laser system.

This paper presents technical developments for the detection of formaldehyde (CH2O) using laser-induced fluorescence. The easily accessible third harmonic of the Nd:YAG laser at 355 nm was used for excitation of formaldehyde. In order to investigate potential background fluorescence, e.g., from large molecules such as polyaromatic hydrocarbons, special attention was paid to investigating the possibility of scanning the wavelength of a single-mode Nd:YAG laser under the gain profile, ∼3 cm−1, on and off resonance. Furthermore, a technique for simultaneous detection of formaldehyde and OH using one laser system is presented. The single-mode Nd: YAG laser at 355 nm in combination with an optical parametric oscillator (OPO) laser tuned to 283 nm was used for simultaneous two-dimensional imaging of both species using one charge-coupled device (CCD) detector equipped with a dual filter image separator. The techniques are demonstrated with measurements in laboratory flames and the combined measurements are also demonstrated in an engine.

Optical and mass spectrometric study of the pyrolysis gas of wood particles

A detailed experimental investigation has been made of the pyrolysis—the first step in biomass combustion—of single birchwood particles. In addition to mass spectrometric and gravimetric analysis, the pyrolysis volatiles were characterized by different optical techniques. Absorption measurements showed a nearly featureless absorption in the ultraviolet spectral region with a continuously stronger absorption for shorter wavelengths. Using different excitation wavelengths, laser-induced fluorescence measurements revealed generally broad spectra in the spectral region from 300–500 nm, which are characteristic spectral signatures for larger hydrocarbons. The optical data were monitored at different times in the pyrolysis process of the particles and compared with the results from the mass spectrometric and gravimetric analysis. The sensitivity of the optical techniques for differentiation between specific molecules was rather low, although formaldehyde could be observed both in absorption and fluorescence spectra. Laser-induced fluorescence measurements were also made for two-dimensional visualization of the pyrolysis volatiles emitted from heated birchwood particles, indicating much higher flows along the fiber direction than across.

Heat Release in the End-Gas Prior to Knock in Lean, Rich and Stoichiometric Mixtures With and Without Egr

SI Engine knock is caused by autoignition in the unburnt part of the mixture (end-gas) ahead of the propagating flame. Autoignition of the end-gas occurs when the temperature and pressure exceeds a critical limit when comparatively slow reactions - releasing moderate amounts of heat - transform into ignition and rapid heat release. In this paper the difference in the heat released in the end-gas - by low temperature chemistry - between lean, rich, stochiometric, and stoichiometric mixtures diluted with cooled EGR was examined by measuring the temperature in the end-gas with Dual Broadband Rotational CARS. The measured temperature history was compared with an isentropic temperature calculated from the cylinder pressure trace. The experimentally obtained values for knock onset were compared with results from a two-zone thermodynamic model including detailed chemistry modelling of the end-gas reactions. (Less)