Studying high pressure monopropellant combustion with operando optical spectroscocpy: nitromethane -- a case study

Abstract

High pressure monopropellant combustion mechanisms are traditionally difficult to validate, due to difficulties associated with maintaining stable flames and measuring intermediates in situ. We have recently developed a continuous feed, liquid-propellant strand burner assembly capable of sustaining stable monopropellant flames in inert atmospheres up to 80 bar for more than 30 minutes. The assembly is housed in a high-pressure chamber that has optical access for observing the flame. With an ultimate goal of assessing combustion mechanisms for complex, monopropellant fuel mixtures, the strand burner and chamber were tested using a simple monopropellant, nitromethane (NM) in both inert and oxidizing atmospheres. Detailed NM combustion mechanisms are well developed, although most are based on data from studies at lower pressures (< or =10 bar) where only bipropellant (in air) combustion is feasible. Models provide strong -- but unverified -- evidence that NM combustion mechanisms depend sensitively on pressure and the relevant mechanisms are expected to change depending on whether NM combusts under mono- or bipropellant conditions. Optical emission from NM flames was directed onto a spectrograph/CCD assembly to show a rich collection of well- resolved, rovibrational lines having linewidths on the order of 3-4 wavenumbers (cm -1). Of particular interest were features between 14,500 cm -1 (690 nm) and 13,000 cm -1 (769 nm) assigned to rovibrational transitions from the H2O (3,0,1) vibrational state relaxing to the ground state. Spectra also showed clear evidence of OH* emission from the electronic (A-X) transition. Together with ex situ measurements of combustion exhaust, these data are being used to identify specific NM combustion pathways. The first ever spatially resolved 1D spectral and 2D chemiluminescence images for both intermediates and products allows direct comparison of relative species concentration and allows juxtaposition of experimental data with simulated speciation data for mechanism validation. Furthermore, temperature profiles were calculated for bipropellant flames using 1D spectral images of rotationally resolved OH* (A-X) transitions and compared to simulated speciation data based on previously proposed combustion mechanisms. These results are some of the first experimental data capable of directly evaluating the validity of proposed high-pressure nitromethane combustion mechanisms.