Why Study Liquid Fuel Combusion
Prof. Frederick L. Dryer
Dept. of Mechanical and Aerospace Engineering
D-316 Engineering Quadrangle
Princeton University, Princeton, NJ 08544
(609) 258-5206; Fax: (609) 258-1939
e-mail: fldryer@phoenix.princeton.edu
Energy Utilization Efficienty & Environmental Impact
Today, despite efforts to develop and utilize renewable energy sources, 85% of all energy consumed in the United States is derived from the combustion of fossil fuels. In 1994, statistics show that the combustion of liquid petroleum-based fossil fuels accounted for 39% of all energy consumption, and an astounding 97% of energy consumption in the transportation sector. While we continue to rely on liquid petroleum-based fuels as a major energy source in spite of the their finite supply, it is of paramount importance to maximize the efficiency and minimize the environmental impact of the devices which burn these fuels. Development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to reducing the production of green-house gases, particularly CO2, to meeting both local and regional ground-level air quality standards for other emissions, and to assuring that air transportation in the future (particularly high speed transportation) does not result in unwanted degradation in stratospheric ozone levels.
One of the most fundamental distinctions among combustion phenomena is that between premixed flames and diffusion flames. In the former, all reactants are thoroughly mixed prior to the combustion process, while in the latter (non-premixed combustion), the fuel and oxidizer mix as they are consumed. Except for the spark ignition engine, nearly all other designs purposely operate as non-premixed combustion systems, in which liquid fuel is injected as an intermittent or continuous spray into the combustion device. Droplets seldom vaporize and burn individually, but more generally burn as ensembles imbedded in non-premixed combusting fuel-vapor/oxidizer mixtures. The size distribution of the spray droplets, and their rate of vaporization and mixing with the surrounding oxidizer control the overall rate of energy release and the net formation of important combustion emissions such as nitrogen oxides (NOx), hydrocarbons (HC's), and soot.
Improved operation can no longer be achieved (either on reasonable time scale or economically) solely through the empirical engineering development that was effective in early development, particularly when emissions were not a concern. Further optimization requires hardware refinements integrated with control of large numbers of the non-linearly related parameters through on-board micro-processors. The complexity of the problem can only be addressed efficiently by computationally assisted design. The multi-dimensional, time-dependent computational tools required must embody compact sub-models which accurately represent the properties and coupling of fluid dynamics, diffusive processes, heat transfer, and combustion/emissions chemistry. Well defined, uni-dimensional, time-dependent, laminar, non-premixed combustion problems are critical to developing, testing, validating these sub-models.
Fire Safety
Most (unwanted) fire hazards also involve non-pre-mixed combustion. Through practical experience and considerable testing, substantial understanding and computational tools to guide how best to detect, control, and mitigate unwanted fires have already been developed for buildings. New insights on and computational tools which describe fire development in aircraft strategic situations and/or under commercial aircraft upset conditions that involve rupture of fuel systems, both in the air and in crash situations, can further reduce fatalities. These tools depend on improving the definition of the sub-models mentioned above, particularly for ignition and combustion of the aerosol clouds formed by aerodynamic shear and ground-impact. Furthermore, In addition, nearly all of our present understanding is for situations in which changes in gas-density caused by the heat generated in the combustion event result in natural convection. This intimate coupling of combustion with natural convection is absent in space environments due to reduced gravity conditions. The effects of gravity in defining how best to sense and mitigate unwanted fires must be determined in order to prepare ourselves for the future colonization of large structures in space. Combustion experiments in low gravity environments provide new insights and data for developing better fire safety standards on the ground, in the air, and for future space applications.
