The laboratory is also equipped with an advanced Sirah Ring Dye
CW laser (Matisse) system pumped by Spectra Physics MilleniaPrime
532 nm laser and a CW Spectra Physics Wave Train frequency
doubler. The MilleniaPrime CW laser outputs 10.5 W of green
532 nm light that pumps 0.75 g/L Rhodamine B in Ethylene Glycol
solution to obtain a red laser beam of a tunable wavelength around
613 nm. The red output from the CW Matisse dye laser is then frequency
doubled to produce UV light near 306 nm where OH has
good absorption characteristics. Approximately, 1 W of red light is
generated by the Matisse dye laser that produced about 200 mW
of UV light after frequency doubling. In the current experiments,
the laser wavelength is tuned to the center (306.6868 nm) of the
well-characterized R1(5) absorption line in the OH AX (0, 0) band.
During ignition delay time experiments, the laser wavelength is
tuned to the wing of the R1(5) line to avoid signal saturation. A common-mode-rejection
scheme is used here which gives a detection
limit of about 0.2 ppm at 1400 K and 1 atm assuming a minimum
absorbance of 0.1% could be detected. The OH species concentration
are calculated from Beer’s law, I/Io = exp(kOHXOHPL), where I and Io
are the transmitted and incident laser intensities, kOH is the OH
absorption coefficient, XOH is the OH mole fraction, P is the total
pressure (atm), and L is the path length (14 cm). The overall estimated
uncertainty in the measured OH mole fraction (XOH) is
approximately ±3%, mainly due to the uncertainty in temperature
(±0.7%). All data are recorded at a sampling rate of 2.5 MHz using
a high-resolution (14 bit) data acquisition system (National Instruments).
The OH absorption experimental setup is shown in Fig. 1.
Ignition delay times are monitored using two different techniques:
pressure time history and OH absorption near 306 nm.