Absorption diagnostics and modeling investigations of RDX flame structure

Absorption spectroscopy has been applied to low-pressure, self-deflagrating RDX flames in an attempt to refine the database for evaluation of detailed -chemistry combustion models. Semi-empirically determined production rates of reactants from the solid propellant surface together with a detailed gas -phase elementary reaction set were used to develop a model that minimizes the effect of uncertainties in the description of solid-phase processes. Th e spatial profiles of two low-concentration, highly reactive, short-lived d iatomic species, CN and NH, were obtained at pressures varying from 1 to 2 atm in air. Two major species, NO and OH were also profiled using this tech nique. Resultant absorption spectra of the B(2)Sigma(+) - X(2)Sigma(+) tran sitions for CN, A(3)II - X(3)Sigma(-) transitions for NH, and the A(2)Sigma (+) - (XII)-I-2 transitions for NO and OH were least-squares fitted to obta in best values for concentration and/or temperature. Peak CN mole fractions of about 200 ppm are obtained from "snapshot" absorption spectra of RDX bu rning in 1 atm air, mole fractions for NH are about a factor of 2 lower. As the pressure is increased, the CN and NH species peak closer to the combus ting surface and reside over a smaller spatial extent. Peak concentrations drop for these higher pressures, but may be due, at least in part, to limit ations of the spatial resolution of the pressurized absorption experiment c hamber. Results from high-resolution absorption measurements instituted to overcome these limitations reveal CN concentrations of 140 ppm with more co nsistent peak positions, widths, and CN to NH peak ratios. Measurements of the concentration of NO species show a peak concentration of 25 mole-percen t occurring at a position of 0.12 mm above the surface of the propellant. I n the final flame region, measured OH concentrations of 1.7 mole-percent ag ree well with adiabatic calculations performed with the NASA-Lewis thermoch emical equilibrium code. Calculated profiles of NO and OH are in good agree ment with these measurements. Thr model peak positions and relative concent rations of CN and NH are also in good agreement with experiment. During the course of these spectroscopic measurements, burning rates for RDX over a p ressure range of 1 to 2 atm have also been determined. These values range f rom 0.23 mm/s at atmospheric pressure to 0.50 mm/s for 2 atm and are notice ably lower than some of the other published measurements. Calculations stem ming from the proposed model predict a burning rate of 0.29 mm/s and 0.54 m m/s for the same pressure levels. (C) 1999 by The Combustion Institute.