Mechanism of HMX combustion in a wide range of pressures

Abstract

Data obtained in the present work and available publications on combustion of cyclotetramethylene tetranitramine (HMX) at different initial temperatures are analyzed. The temperature sensitivity of the HMX burning rate is demonstrated to increase with increasing initial temperature at pressures of 0.1 to 10 MPa, which is typical for combustion of substances with the leading reaction in the condensed phase (c-phase model). Experimental values of the temperature sensitivity of the burning rate in the pressure interval between 0.1 and 1 MPa are higher than the values predicted by the c-phase model, but this fact indicates the transition of the combustion process to another regime rather than the combustion instability in this area. The flame structure of burning HMX with different additives is studied with the help of thin tungstenrhenium thermocouples in the pressure range from 0.025 to 1 MPa. The gas-phase flame is found to ignite in an inductive mode, at least up to a pressure of 1 MPa. The surface temperature is obtained as a function of pressure on the basis of experimental data in a wide range of pressures: ln p = −14,092/T + 21.72 (p in atm). Two possible reasons for the oscillatory regime of HMX combustion observed at atmospheric pressure are proposed: the emergence of resonance phenomena during combustion of an inhomogeneous gas mixture in the tube and the lack of correspondence between the chemical reaction rate in the gas phase at the instant of the resonance and its energy capabilities, which do not allow a necessary HMX gasification rate to be ensured. A mechanism of HMX combustion is proposed, which offers an adequate description in a wide range of pressures up to 10 MPa. The mechanism is based on the leading role of HMX decomposition in the melt at the surface temperature.

Determination of the coefficient of the temperature sensitivity of the burning rate for condensed systems from the heat conduction equation

“…Zel'dovich, neglecting the second (convective) item in compari son with the remaining ones (the so called k phase model). The verification of this assumption was car ried out with the use of experimental (by thermocou ple measurements of temperature distributions [2,4,6] with evaluation of their determination accuracy in the combustion front for a number of propellants) val ues of each of the terms of the last equation. Their comparison was carried out in [3,7].…”

“…As a result, we obtain for nonvolatile infusible sys tems (7) Correspondingly, in the case of volatile melting CSs, the formula has the form (8) To check the obtained dependence of the coeffi cient β on the initial temperature Т 0 and the pressure, experimental data of papers [2,6,8] for combustion of HMX were used. By data [6], q and T s for this matter depend weakly on Т 0 at atmospheric pressure, which completely corresponds to the assumption of their invariability at р = const as a first approximation.…”

“…Figure 1 represents computation results by the approximate kinetic equation obtained on the basis of the Zel'dovich mathematical model for combustion of k phase systems (k phase model) as was done in [2].…”

“…The results of detailed measurements in connec tion with checking the combustion stability criterion for the N powder (single based propellant) in a wide temperature range (from -198°С up to +160°С) at atmospheric pressure are given, for example, in [1,2]. The values of β in [2] were determined at high pres sures in a wide range of Т 0 and low pressures.…”

“…The values of β in [2] were determined at high pres sures in a wide range of Т 0 and low pressures. The aim of the present paper is determination of the analytic dependence of the coefficient β on the thermal properties of energy intensive condensed sys tems (CSs) in the stationary mode of combustion directly by the heat balance equation.…”