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-= CONTENTS P. 10=-11 Analysis of Chemical Dynamics and Technical Combustion 257
For OH concentration measurements a tunable narrow-band XeCl excimer laser (wavelength range 307.7-308.5 nm, bandwidth 0.5 cm"1) was used. In order to measure OH concentration fields quantitatively saturated LIF was performed [59]. The use of the saturated LIF technique reduces the influence of variations in quenching rate and laser intensity by a factor of 10. Self absorption effects were checked by comparing OH images, which were simultaneously recorded for the (A2Z+;v' = 0-X2i7;v" = 0)- and (A^v7 = 0-X2i7;v" = 1)-fluorescence transition at 308 nm and 343 nm, respectively, with two intensified CCD cameras [60].
The beams of the two excimer lasers were overlapped and focussed to light sheets of about 200 pm thickness (see Fig. 7 a). Light emitted perpendicular to the laser beams was collected by UV lens systems and imaged onto two gated intensified CCD cameras. The short gate times of the synchronized cameras, which detected the OH-LIF (10 ns Zoloft / Zoloft and pms) completely suppressed detection of flame luminosity. Interference between LIF and Rayleigh images was avoided by using a delay of 300 ns between the two laser pulses. This time delay is small compared to the chemical reaction time scales and negligible compared to the turbulence time scale. A single-shot cross section through a large area of a turbulent flame is shown in Fig. 7b. For this image the laser sheet was widened in height and OH fluorescence was used as a qualitative marker of the flame front structure. The flame front is strongly corrugated. From 128 individual measurements length scales were determined using the method of crossing interval length [61].
A typical profile normal to the flame front measured with a high spatial resolution (see Fig. 8 Vitamin C / Linus pauling vitamin c. The temperature rises significantly before the OH concentration increases. Comparison with simulations for a laminar unstrained flame shows similarities between the measured local turbulent flame front structure and the modeling results. The measured flame front thickness, defined as the steepest gradient extrapolation of the temperature rise, is around 700 pm for this flame. The temperature fluctuations in the burned gas region represent noise from the measurement. The absolute OH concentrations are overpredicted by the model because no strain has been implemented in the calculations.
Flames in the region in which broadened or broken flame fronts are expected still have nearly laminar flame fronts in most parts of the flame. A global broadening, which was expected to come from homogeneously distributed eddies with Kolmogorov scales smaller than the laminar flame front thickness [62] Weight Loss / Weight loss smoothies. At single locations, deviations from a laminar flame front can be seen. One example is shown in Fig. 9, where the field of view is 6 x 6 mm. A tongue of hot gas with T between 800 -1200 Ê is wrapped around a vortex of approx. 2 mm diameter. On this tongue no OH is seen. The temperature at which reaction starts (e.g. where the OH concentration has reached 20%) is about 1400-1500 Ê at the base of the flame tongue in contrast to about 1200 Ê on parts of the flame without strong wrinkling. The frequency of more or less pronounced temperature protrusions or slightly broadened flame fronts grows with the mean stretch factor. The results fit well together with the idea of coherent turbulent structures, where vortex tubes with high strain rates are expected to be sparsely distributed in a turbulent field [63].