Incidents involving metals and oxygen can be costly in terms of damage to both personnel and equipment. To ensure safety through good design, a thorough understanding of the fundamental science involved in the burning of metals is important, which would also be useful in the utilization of the high heats of reaction associated with burning metal for positive applications and the development of alloys. With more than eighty years of knowledge and experience with metal fires, the precise rate-limiting mechanisms and/or processes are still not well characterised. Modelling, in conjunction with experimental results, is a highly useful tool for the analysis of experimental data and the investigation of possible rate-limiting mechanisms and processes. The most common approach to modelling burning metals has been to focus on a single process or mechanism, without an interdependent approach to these processes and mechanisms. A model of the entire system allows the characterization of a burning metal in terms of variables and parameters other than experimental measurements.
The results for the promoted ignition of 3.2-mm-diameter iron rods are the focus of this investigation, because of the relative availability of data sets. Currently the standard method of reporting promoted ignition test results is in terms of the minimum initial conditions where burning of the specimen is self-sustained: threshold pressure or limiting oxygen index; or one of the few measurable parameters: the average regression rate of the melting interface (RRMI) as the solid specimen is incorporated into the molten mass. The investigation of the relationship between the RRMI and possible rate-limiting process(es) or mechanism(s) for the burning iron system, using analysis of experimental and model results, is presented. Cylindrical iron specimens were chosen for investigation, based on the perceived relative simplicity of the burning iron system and the availability of published results. The results for experiments, selected to compliment published experimental data, are presented. Analysis results for RRMI data sets, utilising the high population data sets available using ultrasonic measurement techniques, are used to investigate the trend of the instantaneous RRMI during tests in reduced gravity conditions and over drop-cycles in normal gravity conditions. The relationship between the RRMI and possible rate-limiting steps for the burning iron system is also investigated.
A new model for the burning iron system in reduced gravity and normal gravity conditions is presented. Since the velocity and temperature fields are not known for the internal molten mass, a lumped-parameter approach is used to incorporate the interaction of mass and energy balances and the influences of reaction kinetics and dynamic geometry of the molten mass. A heuristic approach, using a constant RRMI value, is initially used to assess the average response of the model sub-system and to determine improved estimates for model parameters. A predictive RRMI approach is then taken to determine better approximates for the values of adjustable model parameters and to enable the investigation of the instantaneous RRMI. The model is validated using comparison with average experimental values and trends for the RRMI, surface temperatures of the molten mass and temperature and pressure changes associated with the chamber atmosphere during a test. Similar to the experimental approach, the model results are analysed with a particular interest in the relationship between the model RRMI and possible rate-limiting mechanisms and processes.
The results of this work have lead to a better understanding of the burning iron system and the relationship between the RRMI and other system variables and parameters. Specific conclusions reached include:
· The instantaneous RRMI is not constant during a reduced gravity test or over a drop-cycle in normal gravity conditions.
· In reduced gravity conditions, the instantaneous RRMI is dependent on the mode of molten mass motion. Precessional molten mass motion is associated with a higher average RRMI than straight molten mass motion.
· The rate-limiting step for burning iron rods is likely to be oxygen incorporation at the outer surface of the molten mass for lower pressures and oxygen purities, and heat transfer within the molten mass for higher pressures and oxygen purities.
· The rate-limiting mechanism or process may change during a test in reduced gravity conditions or over a drop-cycle in normal gravity conditions, depending on the pressure, oxygen purity, circulation within the molten mass (Marangoni, mechanical stirring or buoyant effects) and dimensions of the molten mass.