Low temperature oxidation of cyclohexane: uncertainty of important thermo-chemical properties

The study of the standard formation enthalpy, entropy, and heat capacity for key species relevant to the low-temperature combustion of cyclohexane has been performed by applying the group additivity method of Benson. The properties of 18 Benson groups (8 of them for the first time), and 10 ring correction factors for cyclic species were estimated through different empirical and semi-empirical methods. The method validation proceeded through comparison of predicted values for certain number of newly estimated groups and available literature data derived from quantum chemistry estimations. Further validations of the estimated properties of groups have been provided by comparing estimated properties of test species with data in literature and kinetic databases. Also the standard deviation between prediction and reported values has been evaluated for each validation case. A similar approach has been applied for validation of the estimated ring correction groups. For selected well-studied cyclic molecules the predicted values and the literature data have been compared with each other, and the standard deviations have been also reported. The evaluated properties of the cyclohexane relevant species were also compared with similar ones available in other kinetic models and in databases. At the end the estimated properties have been presented in a tabulated form of NASA polynomial coefficients with extrapolation up to 3500 K

Impact of impurities on liquid methane properties under typical rocket operation conditions

The paper reports the results of the investigation of the influence of small quantities of impurities (nitrogen, ethane, propane, carbon dioxide, and a small number of higher hydrocarbons) on the thermophysical properties of liquified natural gas (LNG). For the first time the two-phase diagrams, critical temperature, pressure, and non-monotone behavior of thermodynamic properties around critical point were calculated, systematically described, and analyzed for selected LNG mixtures (CH4/N2/C2H6/C3H8) with impurities content not higher than 5%. It was shown that qualitative and quantitative composition of impurities impact the thermodynamic properties differently and can change Tcr and Pcr significantly. Areas of retrograde condensation have been identified for mixtures with a C3H8 concentration greater than 2%. The Widom Area (WA) was introduced to characterize a supercritical region with not negligible non-monotonous variations of thermophysical properties. The calculated WAs for mixtures can be larger or smaller for different thermophysical properties relative to purer methane. The obtained results can be useful for rocket engine design in simulations of trans-critical and supercritical reacting flows, mixing, evaporation, combustion, flame stability, thermo-acoustic instabilities, and heat transfer at typical operating conditions. It is also essential for the design of the cooling system of a rocket engine

A modelling study of acetylene oxidation and pyrolysis

This study initiates the gradual upgrade of the DLR reaction database The upgrade plan has two main steps: an optimisation of the C1–C4 oxidation chemistry and a revision of the polyaromatic hydrocarbon (PAH) formation sub-mechanism based thereupon. The present paper reports the main principles applied to model improvements and results obtained for the acetylene (C2H2) oxidation sub-mechanisms. The principle acetylene oxidation reactions have been revised as well as the detailed chemistry of important intermediates, i.e. methylene, ethynyl, vinylperoxy radical and also diacetylene, vinylacetylene and higher diacetylenes, important for PAH formation. The uncertainty intervals of the studied reactions were statistically evaluated, providing general bounds for the performed modifications to reaction rate coefficients. The first stage of the presented update was performed through revision of the thermochemical data and model optimisation on ignition delay data and laminar flame speed data, since they exhibit lower uncertainty in comparison to species profile data. The final model optimisation was obtained through simulations of concentration profiles measured in shock tubes and laminar flames for improvement of the reaction paths and rate coefficients related to acetylene pyrolysis and PAH precursor formation. Approximately 500 data points were analysed. The updated reaction mechanism predicts all simulated experimental data, also not included in the optimisation loop data prom plug flow and jet-stirred reactors, either with good or satisfactory agreement. It was found that the vinylperoxy radical formation and consumption dictate the reaction progress at low temperatures. The performed study clearly determined that acetylene combustion proceeds through the strongly coupled reaction paths of fuel oxidation and PAH precursor formation; the same species are involved in these parallel processes. Therefore, the self-consistent reaction model for acetylene combustion could be obtained only by an optimisation performed on the experimental dataset encompassing both processes