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Experimental Studies on Nonpremixed Combustion at Atmospheric and Elevated Pressures

  • Author(s): Gehmlich, Ryan Kyle
  • et al.
Abstract

Experimental and numerical studies are carried out employing the counterflow configuration to advance understanding of nonpremixed combustion of hydrocarbon fuels. The motivation for performing these studies is to increase the knowledge and accuracy of the parameters associated with the transport and chemical-kinetic rate processes of combustion. The counterflow configuration is a very useful tool in elucidating and inferring these parameters for using in numeric or analytical models of real combustion systems. First, a new counterflow burner was constructed for carrying out experiments on high molecular weight hydrocarbon fuels and jet fuels, in particular JP-8, at elevated pressures up to 2.5 MPa. Many of these fuels are liquids at room temperature and pressure. Previously, the U.S. Army Research Office (ARO) funded the design and construction of a High Pressure Combustion Experimental Facility (HPCEF) at the University of California, San Diego. The main pressure chamber with optical access from that project is used, and this new burner is placed inside the chamber. The "extinction top'", or the apparatus used to inject an oxidizing stream onto the fuel surface is also used from the previous work. The new burner is used to measure critical conditions of extinction for hydrocarbon fuels at elevated pressures. In the research previously supported by ARO, experiments were performed at elevated pressures on fuels that are gases at room temperature. Construction of the new liquid pool counterflow burner has extended the scope and quality of that research because it is now possible to characterize combustion of fuels that are liquids at room temperature and atmospheric pressure. An experimental study of nonpremixed combustion of a number of hydrocarbon fuels under moderate pressures is carried out. Fuels and blends used in this study include n-heptane, cyclo-hexane, n- octane, iso-octane, JP-8, Jet-A, and two surrogate blends. Next, experiments and numerical computations are completed to characterize mixtures of dimethyl ether and n-heptane at atmospheric pressures. Dimethyl ether is being studied as an oxygen-rich fuel additive or replacement for diesel fuel in compression-ignition engines due to its high cetane number, negligible global warming potential, it's ability to be produced from multiple sources, and it's high well-to-wheel efficiency. The research focuses on combining the well-validated and detailed LLNL DME mechanism with other hydrocarbon mechanisms to study blends of these fuels. Critical limits of extinction and autoignition of various blends are reported. Using a combined mechanism developed at RWTH Aachen, the extinction limits are very well predicted numerically. A formulation for calculating reactant mass fractions fixing stoichiometric mixture fraction and adiabatic flame temperature is described, which can be easily adapated for two-component blends of fuels with non-unity, unequal Lewis numbers. Experiments and computations both show that dimethyl ether enhances reactivity of blends of dimethyl ether and heptane. Ignition limits for blends are also reported, with numerical predictions overpredicting experimental ignition temperature by approximately 50-70 K, but otherwise predicting ignition temperatures well. Finally, in order to understand the gas-phase combustion characteristics of nitramine monopropellants, a number of subsystems of reactions among the major intermediate products are studied. This work considers the effect of the intermediate product nitrous oxide (N₂O) on the autoignition temperature of ethane (C₂H6). An improved understanding of the combustion taking place in this subsystem is required to model the combustion of nitramines. Here an experimental and computational study is carried out to determine the autoignition temperature of nonpremixed ethane flames with added (N₂O). The oxidi

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