Description of Research Projects

Studies of flame and combustion processes on earth are frequently complicated by the inevitable presence of buoyancy, which not only quantitatively affects the combustion rate but can also severely distort the flame configuration from some degree of symmetry needed for fundamental understanding of the flame structure. In this project we study the structure of perfectly cylindrical and spherical flames obtainable only in the microgravity environments of drop towers, parabolic flights, and space stations.

A flame is a complex system, being governed by the aerodynamics of fluid flows, the molecular transport of heat, mass and momentum, and the kinetics of chemical reactions. In order to quantify and understand its structure, we use various laser based diagnostic techniques to determine the variations of velocity, temperature and species concentrations. These experimental results are then compared with those obtained from detailed numerical simulations. Such a comparison yields crucial information on the dominant physical and chemical processes controlling the flame behavior.

The process of ignition and extinction control the transition between burning and non-burning states of a combustible mixture. Since these processes are highly abrupt events, occurring over very short time intervals or spatial scales, their characteristics are resolved using laser imaging and numerical simulation. Understanding of these phenomena allows improved design and operation of combustors, the identification of additives for enhanced combustion, and improvements in the prevention and control of fire and explosion hazards, for which retarded combustibility is desired.

Incineration is a viable approach for the terminal disposal of hazardous wastes. The two major technical considerations are: (1) Most hazardous wastes are halogenatod hydrocarbons which are incineration resistant, and (2) it is imperative to ensure that the act of incineration does not generate combustion intermediates and products which are even more toxic than the original waste. Our interests in this area are in the development of blending strategies of wastes of different properties for optimum incineration in terms of cost and emissions, and the study of the combustion chemistry of halogenated hydrocarbons.

Droplet and spray processes are central to the operation of a great variety of liquid fueled combustors such as diesel engines, gas turbines, and liquid propellent rockets. Current projects include the droplet combustion of alcohols and slurries, and the dynamics of droplet collision and coalescence. The effects of high pressure on droplet combustion are also of particular interest because most liquid fueled combustors are pressurized.

We are interested in the formulation and combustion of new propulsion fuels with highly strained molecules. We have recently formulated and patented a class of azide based ramjet fuels whose burning rates can be an order of magnitude faster than those of the regular jet fuels. This allows the potential for substantial compaction of, say, the ramjet combustor. New classes of fuels are currently being synthesized.

We are interested in understanding the chemistry of the generation and destruction of such pollutants as soot and oxides of nitrogen in various flame environments, and identifying technological approaches towards their abatement. The formation of soot during the combustion of halogenated hydrocarbons and under high pressure situations are of particular interest.

Most practical combustion devices operate under turbulent conditions. The study is challenging because it couples two very difficult areas: turbulence and chemical kinetics. Under certain situations the turbulent flame can be considered to consist of collections of laminar flamelets and the analysis can be significantly simplified. We are currently formulating turbulent flame theories in which the dynamic behavior of such laminar flamelets in the highly strained turbulent flow field is realistically described.

Combustion approaches have been fruitfully applied to the synthesis of industrial diamonds, fullerenes, superconductors and materials of special properties such as hardness and hightemperature resistivity. In our laboratory we use the Self propagating High temperature Synthesis technique to produce high purity ceramics including the various borides, nitrides, carbides, etc.

The significant advance in scientific computing has led to tremendous progress in combustion research. We are pursuing computational combustion along two directions. The first is to simulate the structure and response of simple flames with detailed chemistry involving many simultaneously occurring reactions characterized by various time scales and multitudes of species. Through such well-controlled simulations the dominant kinetics in the diffusive medium of a flame can be identified. The second focus is on the simulation of the dynamics of flame surfaces in complex flow fields such as turbulent flows, with the ultimate aim of being able to describe the turbulent flame structure including the occurrence of local and global extinction.