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Dimethyl Ether (DME) is a candidate fuel that has potential to be renewable and advantageous over diesel in terms of combustion and emission characteristics as well as suitable for use in stationary gas turbines. Further, it can be used neat as well as blended with diesel, gasoline or other fuels in conventional and advanced CI engines. The design of various types of engines that use DME as a fuel is greatly dependent on computational simulations which require validated chemical kinetic mechanisms that can reliably mimic the combustion and pollutant formation behavior of DME at physical conditions that are relevant to engines. The objective of this work is to contribute to a better understanding and validation of chemical kinetics of DME, particularly at elevated pressures. This is done by obtaining data for auto-ignition and laminar flame speed of DME, which is subsequently used to assess and refine existing chemical kinetic mechanisms.To this effect, a novel optically accessible experimental facility, called DCF (Dynamic Combustion Facility), is first designed, fabricated, characterized and validated for laminar flame propagation studies. In this facility, the combustible mixture in the reactor cylinder is compressed to elevated pressures and temperatures by controlled motion of the reactor piston through a custom-designed hybrid cylinder arrangement. Spark is initiated after compression in the constant volume spherical chamber, yielding an outward propagating flame which is observed by Schlieren imaging technique. The procedures for data interpretation are developed and the experimental conditions under which piston motion induced temperature non-homogeneity is avoided are delineated. The facility is validated by obtaining data for methane/air flame speed at atmospheric and elevated pressures and comparing with the literature data. Subsequently, flame speed data for DME is obtained over a range of pressures and compared with predictions from recent chemical kinetic mechanism. The phenomenon of autoignition in the low-to-intermediate temperature region is of great practical importance in engines. Advanced combustion engines are based on low temperature combustion regime. Operation at these low temperature strategies is significantly kinetically-influenced by the complex low temperature chemistry of fuels. Therefore, autoignition of DME is investigated at low temperatures (630-785 K) and high pressures (8-38 bar) over a range of equivalence ratios (1-6) using a Rapid Compression Machine (RCM). In addition, the effect of CO2 addition on ignition is investigated to gauge the effect of exhaust gas recirculation. Results show that DME is very reactive and there is significant kinetic activity during the compression stroke. Experiments using CO2 show that there is no kinetic effect of CO2 on ignition delay. The experimental data are compared with simulations from available detailed and skeletal chemical kinetic models. In general, there is good overall agreement and discrepancies are noted at low temperatures. The key reactions are identified through flux and sensitivity analysis.The designed facility (DCF) is a novel approach and will be a substantial contribution to the existing arsenal of experimental facilities in combustion. The innovation can extend the range of experimental studies to higher pressures and temperatures, conditions beyond those attainable in existing facilities. |
