Frederik Ossler

Temperature measurements of single droplets by use of laser-induced phosphorescence

A novel technique for measuring droplet temperatures has been demonstrated. Laser-induced phosphorescence from thermographic phosphors, seeded to distilled water and iso-octane, was used to measure temperatures of single falling droplets. The phosphors were excited by the fourth and third harmonics of a Nd:YAG laser. The subsequent emission was evaluated by spectral and temporal investigations of the thermographic phosphors Mg4FGeO6:Mn and La2O2S:Eu, respectively. The spectral and the temporal methods permitted temperature measurements of free-falling droplets up to 433 K. Results from both methods, which show an estimated accuracy of better than 1%, are presented.

TWO-DIMENSIONAL SURFACE TEMPERATURE MEASUREMENTS OF BURNING MATERIALS

A new technique for two-dimensional temperature measurements of burning surfaces is presented. Laser-induced phosphorescence from a thermographic phosphor material applied to a surface of investigation was measured with a fast framing camera. The phosphor was excited by the 4th harmomic from a pulsed Nd:YAG operating at 10 Hz. The phosphorescence images measured by eight consecutively gated CCD detectors enable pixel-by-pixel lifetime evaluation of the phosphorescence by interpolating an exponential decay curve to the counts of the corresponding pixel positions of the sequential CCD images. The temperature at each pixel position was evaluated using a calibration procedure of temperature against lifetime. These measurement procedures were used for surface temperature measurements of the evolution of flame spread on low-density fiber boards. The results from experiments showed the possibility of measuring surface temperature during all phases of the flame spread. The total time window used for each two-dimensional temperature measurement was 800 mus to obtain high accuracy and precision at high temperatures, 680-780 K, temperatures characteristic of burning surfaces. The best precision, better than +/-5 K, was obtained at these temperatures. In this region, evaluation by the lifetime method shows a higher sensitivity to temperature than what can be expected from methods based on spectral line intensities. The results of the experiments were in accordance with those reported from previous one-point measurements. In the low-temperature region close to room temperature, the accuracy deteriorated considerably. The results obtained from the two-dimensional imaging experiments are presented and discussed. (Less)

Measurements of the structures of nanoparticles in flames by in situ detection of scattered x-ray radiation

The angular pattern of scattered synchrotron x-ray radiation has been used to measure the composition of molecules and nanometer-sized particles in flames. The measured patterns were compared with patterns obtained from calculations for different species compositions. After ensuring that the calculations could reproduce the experiments for air and for ethylene flames under two different combustion conditions flames under special particle producing conditions were studied. In one case, the patterns showed a strong presence of spherical or fullerenelike structures with very little presence of graphitelike particles and little soot production on a cooling plate. In the other case, under soot producing conditions, the scattering showed a clear presence of nanometer-sized graphite particles. The results show that high concentrations of particles with the size around 1 nm can be produced in flames. These particles either remain free or condense into larger particles, depending on the combustion conditions. This technique opens up for experimental studies of molecular and particle dynamics in combustion processes and other processes where nucleation and structural transformations of particles occur.

Measurements of collisional quenching of hydrogen atoms in an atmospheric-pressure hydrogen oxygen flame by picosecond laser-induced fluorescence

Atomic hydrogen has been probed in a hydrogen/oxygen flame at atmospheric pressure by use of two-photon, picosecond Laser-Induced Fluorescence (LIF). The fluorescence was detected and temporally resolved by a streak camera with a temporal resolution of a few ps which allowed determination of the collisionally quenched lifetime of then = 3 level of atomic hydrogen. The measurements were performed for three flame stoichiometries,φ = 1.4, 2.0 and 3.5, and were spatially resolved with respect to the reaction zone. The evaluated lifetime was found to vary rapidly in the reaction zone from 60 to 105 ps just above and then levelled out at 90 ps throughout the post-flame region. Power dependence measurements of the fluorescence signal indicated the presence of other phenomena, such as saturation, ionization and photodissociation. Since an inverted population is created between then = 3 and then = 2 level, Stimulated Emission (SE) of considerable magnitude occurred and was studied temporally resolved using the streak camera. The LIF and SE signal strengths for individual laser pulses were recorded in order to analyse a possible anti-correlation between the LIF and the SE.

Detection of fluorescent nanoparticles in flame with femtosecond laser-induced fluorescence anisotropy

The mean size of fluorescent nanoparticles produced in a propane flame has been measured with an in-situ technique employing a femtosecond laser to excite the sample and a streak camera for time-resolved detection of the fluorescence. The time profile of the fluorescence anisotropy showed a Gaussian behaviour, typical of free rotor reorientation. By measuring its width, we estimated an average carbon particle diameter of 3.3 nm, thus confirming the existence of combustion produced nanoparticles. The technique proves to be applicable to the study of gas-phase nanoparticles, both in combustion and environmental studies.

Measurements of the effective lifetime of O atoms in atmospheric premixed flames

Abstract The effective lifetimes of the 2p 3 3p 3 P and the 2p 3 3p 5 P states of atomic oxygen have been measured to be 290 ± 80 and 250 ± 80 ps, respectively, in the recombination zone of a H 2 /O 2 flame at atmospheric pressure. Measurements were also performed in different zones of the H 2 /O 2 flame and in the recombination zone of a N 2 O/H 2 flame. Laser pulses with a duration of 35 ps and a wavelength of 225.6 nm excited the atoms by a two-photon absorption process. Lifetimes were deduced from time-resolved recordings of the fluorescence at 845 and 777 nm using a fast photomultiplier tube.

Fluorescence lifetimes of formaldehyde (H2CO) in the Ã1A2→X̃1A1 band system at elevated temperatures and pressures

Fluorescence lifetimes of formaldehyde excited by picosecond laser radiation with a wavelength of 355 nm were determined in nitrogen gas in a cell using time-resolved laser-induced fluorescence spectroscopy. The measurements were conducted at temperatures between 295 and 770 K and pressures up to 10 bar (1 MPa). Detection was broadband in most cases. Temperature and pressure were found to have a quenching effect on the fluorescence. At 295 K and pressures between 1 and 5 bar, decay rates between 0.03 and 0.04 ns(-1) were observed. At 770 K, the decay rates increased from 0.11 to 0.17 ns(-1) as the pressure was raised from 1 to 10 bar. The dependence on pressure was not linear at 1 bar. At 10 bar the linearity is unclear. The dependence on temperature appeared to be exponential.

Surface Temperature Measurement Of Flame Spread Using Thermographic Phosphors

A technique based on remote measurements of surface temperature in connection to fires is presented. Pulsed ultraviolet laser radiation at 266 nm and 7 ns duration was used to excite a thermographic phosphor, Mg4FGeO6: Mn, which was adapted on the surface of the investigated material. The laser-induced emission from the phosphor was recorded. A calibration of the phosphorescence lifetime and spectral properties against temperature allowed surface temperature measurements between 25°C and 500°C. The method was tested and compared with thermocouple measurements on burning materials such as lowdensity fiberboards and polymethylmethan acrylate.

Fluorescence lifetimes of formaldehyde (H2CO) in the band system at elevated temperatures and pressures

Fluorescence lifetimes of formaldehyde excited by picosecond laser radiation with a wavelength of 355 nm were determined in nitrogen gas in a cell using time-resolved laser-induced fluorescence spectroscopy. The measurements were conducted at temperatures between 295 and 770 K and pressures up to 10 bar (1 MPa). Detection was broadband in most cases. Temperature and pressure were found to have a quenching effect on the fluorescence. At 295 K and pressures between 1 and 5 bar, decay rates between 0.03 and 0.04 ns(-1) were observed. At 770 K, the decay rates increased from 0.11 to 0.17 ns(-1) as the pressure was raised from 1 to 10 bar. The dependence on pressure was not linear at 1 bar. At 10 bar the linearity is unclear. The dependence on temperature appeared to be exponential.

Two-dimensional visualization of fluorescence lifetimes by use of a picosecond laser and a streak camera

Two-dimensional distributions of the effective lifetime of the fluorescence emission induced by short-pulsed laser radiation are obtained from two-dimensional images recorded with a streak camera and a charge-coupled device by means of a separation algorithm method (SAM). In theory, the best response with respect to noise is obtained for lifetimes corresponding to a range of pixels of 5-50 in the CCD, that is, 5-50 ps at the fastest streak speed. In experiments the SAM is compared with pure time-resolved measurements, and it is used for two-dimensional lifetime evaluation. The laser-pulse duration is 25 ps, and the lower limit of the lifetime resolution as used in the experiments is estimated to be 200-250 ps. The results demonstrate the possibility of performing pattern recognition independently of the relative distribution of emission intensity between regions of different fluorescence lifetimes. The technique is demonstrated for static objects but can in principle be extended to nonstationary objects if two detectors are used.