Publicado 6 números por año
ISSN Imprimir: 2150-766X
ISSN En Línea: 2150-7678
Indexed in
GAS TEMPERATURE MEASUREMENTS IN A FLAT FLAME BY TUNABLE DIODE LASER SPECTROSCOPY
SINOPSIS
Due to their very fine spectral linewidth, lower than 10−3 cm−1, tunable infrared diode lasers offer the opportunity to explore the whole absorption line profile of a chemical species, even for very narrow lines existing in low-pressure environments. Therefore, its temperature and absolute density, averaged along the laser beam path, can be calculated, if the molecular properties are known, by using the Beer-Lambert law of radiation absorption. In this investigation, the Tunable Diode Laser Spectroscopy (TDLS) technique was applied to measure the temperature profile in a one-dimensional, low-pressure (5700 Pa), premixed H2/CO/O2/Ar flat flame. For this purpose, a pair of two spectrally adjacent absorption lines of carbon monoxide was used. The concentration profile of this species could then also be deduced from the measurements.
To achieve high accuracy in the temperature measurement, the difference between the values of the lower energy level for the two selected rovibrational transitions should be as high as possible. In order to satisfy this condition, as well as to provide a sufficient absorption in the broad range of temperature investigated (400 - 1400 K), the pair of two adjacent lines 1-2 P(35) and 0-1 P(40) of carbon monoxide, respectively located at 1963.6607 and 1963.7290 cm−1, was chosen. One of the major TDLS problems, the Fabry-Pérot effects due to lens reflection, was avoided by using an optical setup based on metallic mirrors instead of lenses. This optical arrangement allowed a 2 mm measurement spatial resolution along the burner axis. This value was shown to cause a temperature overestimation by only 5 % in the lower and also steeper part of the profile.
The highest measured temperature was 1420 K, at a location of about 2.5 cm above the burner surface. The level of temperature in the experimental profile was lower than in the theoretical profile. This was attributed to the cooling effect caused by both two-dimensional heat transfer and radiation heat losses from the flame. These effects were not accounted for in the theoretical model. The analyzed pair of absorption lines provided accurate temperature measurements (± 3.7 %) in the whole axial flame profile, but the absorption level was unsufficient to allow the measurement of the CO concentration in the lower part of the profile.