Determination of temperature profiles of self-deflagrating RDX by UV/Visible absorption spectroscopy and fine-wire thermocouples

Temperature profiles of self-deflagrating RDX in a nitrogen environment at pressures up to 0.79 MPa were determined using UV/Visible absorption spectroscopy and fine-wire thermocouples (TCs). The burning surface of RDX was covered by a very dynamic two-phase foam zone, and the thickness of the foam zone decreases very rapidly with an increase of pressure. The temperatures within the foam zone were readily identified in the thermocouple traces, especially at low pressures. Depending upon the pressure, the temperature was found to be around 500-520 K at the bottom surface of the foam zone, and around 590-690 K at the top surface of the foam zone. These temperatures are higher than the RDX melting temperature (477 K at one atmosphere). A relationship between RDX burning surface temperature and pressure was developed for pressures between 1 and 90 atm using data from this study and Zenin's TC data. The final flame temperature of RDX increases with an increase of pressure: 2,950 K at 0.17 MPa, 3,002 K at 0.45 MPa, and 3,204 K at 0.79 MPa, which agree well with chemical equilibrium calculations. The flame height, determined from temperature traces, decreases very rapidly with an increase of pressure. Profiles of OH concentrations were also deduced; however, because of the severe non-one-dimensionality of the flame structure, the deduced OH concentrations in the final-flame region were smaller than equilibrium calculations. A "flow-replenishing" process was deduced from the recorded video images of RDX burning. Due to this process, a planar-regression mode can still occur even when the burning surface is subject to a non-uniform heat flux over its cross section.