The propagation and stability of the decomposition flame of hydrazine

Abstract

Pre-eminent among problems facing the theory of laminar flame propagation is the prediction of the properties of flames from the kinetics of the chemical reactions occurring in them. Such predictions require, inter alia, knowledge of the kinetic order and activation energy of the over-all decomposition reaction under the conditions of the flame. This in turn requires extrapolation of experimental data on reaction kinetics, usually far beyond the range in which their validity is sure. An alternative approach uses measurements made on the laminar flame to derive the form of kinetic equations governing reactions in the flame, and then compares this with the kinetic behavior observed at lower temperatures. This approach is adopted here. The rigorous solution of the flame equations for even an elementary, single-step, reaction requires lengthy calculations and for this reason their excessive chemical complexity hinders the use of this information in testing flame theories, although in the past most information has been derived from studies of hydrocarbon flames. Flames which are chemically much less complicated may be expected to supply suitable data more readily. The most simple flames known are those supported by the exothermic breakdown of a single compound and among these the hydrazine decomposition flame stands out. Hydrazine decomposition produces only nitrogen, hydrogen and ammonia. Thus, chemical analysis is easy and stoichiometry and thermoehemistry are clear cut. The kinetics of the initial step in homogeneous decomposition ( N N bond fission) are known I as a function of temperature. The spontaneous 2,a and spark 4 ignitions have been observed. The gaseous decomposition flame has been stabilized on burners 5, G at atmospheric pressure. The decomposition flame has been burned above the surface of the liquid J. s Furthermore, some theoretical work has also been done on this system 9, 10 in an a t tempt to predict the flame velocity from the kinetics of the initial step in homogeneous decomposition. Two fundamental properties of the hydrazine flame are (1) the dependence of flame speed on pressure from which the over-all kinetic order of reaction may be found, and (2) the dependence of flame speed on temperature from which the over-all activation energy may be found. The primary purpose of this work is to supply this information experimentally, to examine its theoretical implications, and to see how they may be interpreted. Two experimental methods have been used. In the first, flame speeds of gaseous hydrazine have been measured by the closed vessel method. This permits the accurate control of initial pressure and temperature and avoids the hazards of the considerable flow of heated hydrazine necessary for a normal burner technique. The pressure range which can be studied in a glass vessel extends from very tow values up to about 10 cm Hg. In the second method the pressure range between 10 cm Hg and atmospheric has been examined by burning liquid hydrazine in narrow tubes.S. 7 This provided the secondary purpose of this work-to examinc the validity of this simple technique. In tubes, the flame is not adiabatic but subject to strong quenching influences as a result of lateral heat losses. This difficulty has been pointed out by Gray and Kay s but not previously overcome. An at tempt has now been made to allow for these varying heat losses.