Per-Erik Bengtsson

Rotational Cars Generation Through A Multiple Four-color Interaction

A novel technique for the generation of single-pulse rotational CARS spectra is presented and demonstrated flows and flames. The technique is based on a multiple four-color interaction, where the rotational in gas with two photons of different frequencies from a broadband dye laser, and by coupling to a energies are excited photon from a frequency-doubled Nd:YAG laser a rotational CARS photon is created. An interesting third feature of the technique is the possibility of simultaneously generating both a rotational and vibrational using a double-folded BOXCARS arrangement. This technique is demonstrated on N2 CARS spectrum molecules.

Laser-induced incandescence for soot particle size measurements in premixed flat flames

Measurements of soot properties by means of laser-induced incandescence (LII) and combined scattering-extinction were performed in well-characterized premixed ethylene-air flames. In particular, the possibility of using LII as a tool for quantitative particle sizing was investigated. Particle sizes were evaluated from the temporal decay of the LII signal combined with heat balance modeling of laser-heated particles, and these sizes were compared with the particle sizes deduced from scattering-extinction measurements based on isotropic sphere theory. The correspondence was good early in the soot-formation process but less good at later stages, possibly because aggregation to clusters began to occur. A critical analysis has been made of how uncertainties in different parameters, both experimental and in the model, affect the evaluated particle sizes for LII. A sensitivity analysis of the LII model identified the ambient-flame temperature as a major source of uncertainty in the evaluated particle size, a conclusion that was supported by an analysis based on temporal LII profiles.

A test of different rotational Raman linewidth models: Accuracy of rotational coherent anti-Stokes Raman scattering thermometry in nitrogen from 295 to 1850 K

Rotational Raman linewidths calculated from three different models have been used in temperature measurements by rotational coherent anti-Stokes Raman scattering (CARS)—a semiclassical ab initio model, the modified exponential energy gap model (MEG), and the energy corrected sudden scaling law (ECS). Experimental rotational CARS spectra were generated, using the dual-broadband approach, in pure nitrogen at atmospheric pressure in a heat pipe in the temperature range from 295 to 1850 K. Below 1500 K, the temperatures evaluated using the ECS linewidths agreed with the heat-pipe temperatures to within 20 K. Above 1500 K, the errors in the evaluated temperatures increased steeply for all linewidth models, reaching errors of several hundreds of Kelvins at 1850 K. This behavior of the evaluated temperature is probably caused by the uncertainty in the values of the rotational Raman linewidths for high rotational states at high temperatures. This work therefore illustrates that rotational CARS can be used for experimentally studying Raman linewidths and in particular their dependence on temperature and rotational quantum number. The influence of different experimental parameters on the evaluated temperatures is discussed, and the spectral synthesis program is presented. The Journal of Chemical Physics is copyrighted by The American Institute of Physics. (Less)

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.

Rotational CARS: a comparison of different techniques with emphasis on accuracy in temperature determination.

Different rotational CARS techniques have been evaluated in terms of single-shot temperature accuracy and signal intensity in room temperature nitrogen and in flames. The different techniques include both dual broadband techniques, using one or two broadband dye lasers, and conventional rotational CARS with different dye lasers. These techniques are also compared with vibrational CARS concerning temperature accuracy and with theoretical predictions. The results indicate that the dual broadband techniques are to be preferred over conventional rotational CARS and also over vibrational CARS at room temperature. At flame temperatures the vibrational CARS technique seems to be the technique yielding highest temperature accuracy. The experimental results are also generally in good agreement with the calculated values.

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.

Vibrational CARS thermometry in sooty flames: Quantitative evaluation of C2 absorption interference☆

The application of nitrogen vibrational CARS thermometry to sooty, premixed, atmospheric pressure flames has been investigated using a Nd:YAG laser based system. It was found that laser-produced C2 radicals strongly absorb part of the fundamental band peak in the CARS spectrum. This was the most severe interference to the CARS signal. A quantitative investigation of temperature errors caused by the C2 absorption effect is presented. The correlation between the absorption interference and the soot volume fraction was examined for different flame conditions. Also, the increase of the nonresonant susceptibility in sooty flame regions is clearly illustrated and its effect on thev evaluated temperature is quantitatively determined. The single-shot temperature standard deviation has also been investigated in flames with different soot loadings. Finally, other interference effects to the CARS signals in sooty flames are described and discussed.

Rotational cars thermometry in sooting flames

Coherent anti-Stokes Raman scattering of pure rotational transitions, rotational CARS, is demonstrated as an efficient method for temperature determination in sooting flames. The dual broadband CARS approach was used to measure temperature profiles in premixed, sooting ethylene flames at atmospheric pressure by probing the nitrogen gas. The recorded spectra were of equally high quality in non-sooting and sooting flames with volume fractions of soot of up to 7 x 10 7 cm3 soot/cm3The advantages of rotational CARS in comparison with several other techniques for the measurement of temperatures in sooting flames, and the general applicability of the technique to different combustion conditions, are discussed. Potential limitations in the application of rotational CARS to sooting flames that are more heavily sooting than the ones investigated in this study, are outlined.

An evaluation of precision and systematic errors in vibrational CARS thermometry

The extraction of temperatures from experimental CARS spectra is discussed and demonstrated in an electrically heated over and a low-pressure flame. The present work is focused, in particular, on how the precision of CARS thermometry is limited by the accuracy with which the experimental parameters are known, on characteristics of the diode-array detector, on saturation effects due to stimulated Raman scattering and on the influence of spectral resolution on precision and accuracy.

Optical Investigation of Laser-produced C2 In Premixed Sooty Ethylene Flames

Abstract Emission from laser-produced C 2 has been studied at different heights in premixed ethyleneoxygennitrogen flames at various fuel-oxidant ratios and laser pulse energies for the wavelengths 266, 355, and 532 nm. At the laser wavelengths 355 and 532 nm, the laser-produced C 2 emission was detected in sooting flames, both in the sootfree reaction zone and in the sooty region, whereas using 266 nm radiation it was observed both in sooting and nonsooting flames. Polyaromatic hydrocarbons were monitored by laser-induced fluorescence and the properties of the soot particles were determined using scattering/extinction measurements.