First-stage and overall ignition delay times of dimethyl ether (DME) were measured in a high-pressure shock tube for various mixtures containing high amounts of CO2. The data are available for DME/air mixtures diluted with 40% CO2 as well as for the mixture of DME and oxidizer containing 20.5% O2 and 79.5% CO2. As a reference, data were also collected for mixtures containing the corresponding amounts of N2. Measurements were conducted for pressures of 15, 35, and 50 bar and a range of temperatures (744–1316 K). For the DME/air mixture diluted with CO2, the equivalence ratio was varied in the range of 0.5–2.0. The results demonstrate that CO2 dilution has a strong effect on ignition delay times in the NTC (negative temperature coefficient) region. The kinetic study that was conducted showed that this phenomenon can be attributed to a thermal effect resulting from the high heat capacity of CO2. In low and high temperature ranges, the effect of CO2 is less pronounced. Additionally, a chemical effect was identified. At high temperatures this effect can be attributed mostly to the influence of CO2 on third-body reactions and leads to slight acceleration of ignition. Various chemical-kinetic models available were evaluated with respect to their accuracy in prediction of ignition delay times for mixtures containing DME and large amounts of CO2.

Constant-volume (pressure-gain) combustion cycles show much promise for further increasing the efficiency of modern gas turbines, which in the last decades have begun to reach the boundaries of modern technology in terms of pressure and temperature, as well as the ever more stringent demands on reducing exhaust gas emissions. The thermodynamic model of the gas turbine consists of a compressor with a polytropic efficiency of 90%, a combustor modeled as either a pulse detonation combustor (PDC) or as an isobaric homogeneous reactor, and a turbine, the efficiency of which is calculated using suitable turbine operational maps. A simulation is conducted using the one-dimensional reacting Euler equations to obtain the unsteady PDC outlet parameters for use as turbine inlet conditions. The efficiencies for the Fickett–Jacobs and Joule cycles are then compared. The Fickett–Jacobs cycle shows promise at relatively low compressor pressure ratios, whereas the importance of the harvesting of exhaust gas kinetic energy for the cycle performance is highlighted.

High-pressure shock tube ignition delay data are essential for fuel characterization and for the validation and optimization of chemical-kinetic models. Therefore, it is crucial that realistic measurement conditions are considered in modeling. Furthermore, an accurate uncertainty quantification for experimental data is the basis for evaluation of the predictive reliability of chemical-kinetic models. Several measurement aspects are investigated to improve the interpretation of measurement results: (1) A new approach for integrating the nonideal pressure rise into chemical-kinetic modeling based on a correlation to measurement data is introduced, which enables the determination at each condition and fuel-air mixture individually with minimal effort. (2) A semiempirical model for available test times of reactive mixtures is introduced, which is based on measurement data of nonreactive mixtures. It allows for a priori prediction of test times and provides experimental limits to support the measurement. (3) A literature review shows that different uncertainty sources are considered in ignition delay time uncertainty analysis. A comparative analysis is conducted to investigate the significance of different uncertainty sources for test temperature and ignition delay time. The analysis of ignition delay time uncertainty indicates that for fuels with negative temperature coefficient behavior a comprehensive uncertainty analysis has to be conducted to accurately estimate measurement uncertainty in the intermediate temperature range. Additionally, ignition delay times of dimethyl ether-air mixtures are measured at pressures of 8, 12, and 35 bar and at equivalence ratios of 0.5, 1.0, and 2.0. Furthermore, the data on first-stage ignition delay are rather scarce and have therefore been recorded additionally. The new approach of integrating the nonideal pressure rise into modeling and the comprehensive uncertainty analysis supports the interpretation of measurement data, such that the prediction capabilities of chemical-kinetic models can be evaluated thoroughly. List of abbreviations: DME, dimethyl ether; IDT, ignition delay time; ISW, incident shock wave; NTC, negative temperature coefficient; RSS, root sum of squares This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Advanced combustion concepts based on autoignition of combustible mixture for internal combustion engine and gas turbine applications are sensitive to reactivity gradients arising from temperature inhomogeneities. A reaction front resulting from such reactivity gradient depends on the physicochemical properties of a combustible mixture and can be influenced by fuel tailoring. The present study investigates the effects of blending fuels with and without negative temperature coefficient (NTC) behavior on the temperature sensitivity of ignition delay times. A first set of experimental data on ignition delay times of a ternary equimolar dimethyl ether/hydrogen/methane fuel blend has been collected in a high-pressure shock tube at a pressure of 33 bar in a range of temperatures between 700 and 1150 K under variation of equivalence ratio. The experimental data are used to evaluate the predictability of available detailed reaction mechanisms regarding ignition delay time and its temperature sensitivity. Both experimental and numerical data have been analyzed in the context of autoignitive wave propagation and detonation development in a generic hot spot. Based on a numerical study, the effect of each fuel component on relevant parameters is investigated.

Combustion concepts based on auto-ignition are prone to detonation formation and premature ignition in the presence of local spots with increased reactivity, which can be caused by unavoidable temperature inhomogeneities in real technical applications. In this study, a fuel blend optimization approach using a fuel component with negative temperature coefficient (NTC) behavior (dimethyl ether, DME) and a fuel component without NTC behavior (dimethoxymethane, DMM) is employed to reduce the temperature sensitivity of ignition delay times and therefore reduce the tendency of premature ignition and detonation development. First, ignition delay times and first stage ignition of a DMM/air mixture are measured behind reflected shock waves in a high-pressure shock tube (20 and 35 bar, stoichiometric conditions) to select a suited chemical kinetic model. Using these data and additional literature data, a mechanism is chosen, which is used to numerically optimize the blending ratio of DME and DMM to minimize the temperature sensitivity of ignition delay times in the temperature range between 800 and 900 K for both considered pressures. The two optimized fuel blends are investigated in the shock tube regarding their ignition characteristics as before (20 and 35 bar, stoichiometric conditions). Finally, the impact of pressure, equivalence ratio, and temperature range on the optimization result is investigated numerically.