Optical measurements of atomic oxygen concentration, temperature and nitric oxide production rate in flames.

Abstract

An optical method for measuring nitric oxide (NO) production rates in flames was developed and characterized in a series of steady, one-dimensional, atmospheric-pressure laminar flames of $\rm 0.700 H\sb2/0.199 N\sb2/0.101 CO\sb2$ or 0.700 CH$\sb4$/0.300 N$\sb2$ (by moles) with dry air, with equivalence ratios from 0.79 to 1.27. Oxygen atom concentration, (O), was measured by two-photon laser-induced fluorescence (LIF), temperature was measured by ultraviolet Rayleigh scattering, and nitrogen concentration was calculated from supplied reactant flows; together this information was used to calculate the NO production rate through the thermal (Zel'dovich) mechanism. Measurements by two other techniques were compared with results from the above method. In the first comparison, gas sampling was used to measure axial NO concentration profiles, the slopes of which were multiplied by velocity to obtain total NO production rates. In the second comparison, LIF measurements of hydroxyl radical (OH) were used with equilibrium water concentrations and a partial equilibrium assumption to find (O). Nitric oxide production rates from all three methods agreed reasonably well. Photolytic interference was observed during (O) LIF measurements in all of the flames; this is the major difficulty in applying the optical technique. Photolysis of molecular oxygen in lean flames has been well documented before, but the degree of interference observed in the rich flames suggests that some other molecule is also dissociating; the candidates are OH, CO, CO$\sb2$ and H$\sb2$O. An extrapolative technique for removing the effects of photolysis from (O) LIF measurements worked well in all flames where NO production was significant. Using the optical method to measure NO production rates in turbulent flames will involve a tradeoff among spatial resolution, systematic photolysis error, and random shot noise. With the conventional laser system used in this work, a single pulse with a resolution of 700 $\mu$m measured NO production rates as low as $2\times10\sp{-3}\ \rm gmol/m\sp3$-s with photolysis error less than a factor of two and random shot noise of 35%. By using a double-pulse technique and relaxing the spatial resolution to 2 mm, production rates down to $2\times10\sp{-5}\ \rm gmol/m\sp3$-s can be measured with shot noise of 22%. Extension to two-dimensional imaging will require a multipass cell or very large pulse energy ($\sim$200 mJ at 226 nm).