The presently used Halons 1301 and 1211 fire extinguishing agents are being phased out due to their role in the catalytic destruction of the stratospheric ozone layer. There is presently a major effort underway within the DoD to find environmentally acceptable Halon replacement compounds. The goal of this research project is to develop a detailed flame chemistry computer model which will be able to predict the relative flame extinguishment properties of new Halon alternative compounds as well as to identify the possible formation of toxic flame products resulting from the use of the agent. This model, once fully verified, will become a very important predictive and cost-saving tool for the RDT&E survivability laboratories for screening new compounds, or mixtures of compounds, and for interpreting results of full-scale testing. This project is a 6.2/6.3a effort since it uses information and data generated from previous basic research in flame inhibition science. It is transitioning into 6.3a as the models are simplified to include the important overall reactions. Ultimately, these reduced/ simplified flame chemistry models are expected to be run on advanced PCs, rather than on the workstations which are presently required for running the full chemistry. The part of this project that covers the near-ir tunable diode laser (TDL) spectroscopy development represents 6.3a work with the intention of transferring this instrumentation technology to the DoD's real-scale testing facilities.
This project continues our very successful state-of-the-art approach which is aimed towards identifying the detailed chemical and physical mechanisms which are responsible for flame extinguishment. This approach consists of an experimental flame program closely coupled with various types of computational modeling efforts. This project has been particularly prolific with a large number of manuscripts having been published or in the process of being published. This includes a book on Halon Replacements: Technology and Science as well as open literature publications.
We are in the middle of transitioning from our previous work on low pressure premixed flames to our current work on counterflow diffusion burners. We are still using the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique for flame profile studies. The detailed chemical flame mechanisms are tested on the basis of agreement with the experimental results. The modeling work typically involves on the order of a thousand elementary chemical reactions as well as nearly 100 flame species. Due to the magnitude of the required computations, this work is typically performed on workstations or larger computers. However, after the complete reaction set is verified, then the number of reactions can be significantly reduced through the use of sensitivity analysis. The detailed kinetic models are based on accurate knowledge of thermodynamic and kinetic properties of the relevant species and reactions. For those reactions where previous data does not exist, an estimate has to be determined through the use of computational chemistry tools such as the BAC-MP4 and Transition State Theory programs. Significant progress continues to be made in both experiments and modeling. We have already obtained a large number of flame structure profiles for methane/oxygen flames doped with various perfluorocarbon (PFC), hydrofluorocarbon (HFC), and iodofluorocarbon (IFC) compounds as well as Halon 1301. Detailed chemistry flame models have been run at 20 Torr for various PFC and HFC compounds at a number of concentrations. Flame model calculations include both freely propagating flames and burner stabilized flames.
Our work has been moving continuously from the more basic research aspects to the more applied work for some time. Besides initiating diffusion flame research, which will give us very important information concerning the interplay between flame kinetics and transport processes, we have also directed our modeling toward the practical application of trying to minimize acid gas (HF) formation for new replacement compounds. In addition, we are exploring a new TDLAS technology based on room temperature near-infrared tunable diode lasers which hold great promise for in-situ and real-time detection of acid gases in large scale testing.
This research continues to be closely coordinated with the survivability organization (TARDEC, Wright-Pat, NRL, others) and the work is coupled with these organizations as our models are developed to predict large-scale testing of individual as well as mixtures of agents. As an example, we have focussed some of our attention towards the agents FE13 and FM200, which are prime candidate replacement compounds of special interest to the Army. Also, we have been working with the Army's T&E fire safety group at the Aberdeen Test Center (ATC) for some time on various issues related to flame extinction, environmental acceptability, and new instrumentation for full-scale testing. As the reduced chemistry computer combustion models are successfully developed so they can be run on advanced PCs, we will transfer this technology to the T&E communities.
The successful execution of this research program will benefit all organizations concerned with survivability of military platforms involved in a fire scenario. The DoD organizations working in this area include TARDEC, ATCOM, Wright-Pat, and NRL. Flame chemistry models that include fluorine, bromine, as well as iodine chemistry will be particularly useful as screening tools for a wide range of candidate fire extinguishing agents, thus leading to significant savings by avoiding unnecessary large-scale testing.