Some Aspects of Time-Reversal in Chemical Kinetics

Chemical kinetics govern the dynamics of chemical systems leading towards chemical equilibrium. There are several general properties of the dynamics of chemical reactions such as the existence of disparate time scales and the fact that most time scales are dissipative. This causes a transient relaxation to lower dimensional attracting manifolds in composition space. In this work, we discuss this behavior and investigate how a time reversal effects this behavior. For this, both macroscopic chemical systems as well as microscopic chemical systems (elementary reactions) are considered

Joint scalar PDF simulations of a bluff-body stabilised flame with the REDIM approach

Transported joint scalar probability density function (PDF) results are presented for ‘Sydney Flame HM3’, a jet type turbulent flame with strong turbulence – chemistry interaction, stabilized behind a bluff body. We apply the novel Reaction-Diffusion Manifold (REDIM) technique, by which a detailed chemistry mechanism is reduced, including diffusion effects. Only N2 and CO2 mass fractions are used as reduced coordinates. A second-moment closure RANS turbulence model is applied. As micro-mixing model, the modified Curl’s coalescence/dispersion (CD) and the Euclidean Minimum Spanning Tree (EMST) models are used. In physical space, agreement between experimental data and simulation results is good up to the neck zone, for the unconditional mean values of velocity, mixture fraction, major and some minor chemical species. Conditional mean profiles in mixture fraction space are also in reasonable agreement with experiments up to the neck zone, though conditional fluctuations tend to be under-predicted. CD generally yields better predictions for the level of fluctuations in mixture fraction space than EMST, but this is partly due to unrealistic particle evolution in composition space. In general, simulations using the REDIM approach for reduction of detailed C2-chemistry confirm earlier findings for micro-mixing model behaviour, obtained with C1-chemistry

Multi compression–expansion process for chemical energy conversion: Transformation of methane to unsaturated hydrocarbons and hydrogen

With the global energy system moving towards renewable energies, there is an increasing demand for flexible conversion processes which can cope with the temporally and locally fluctuating nature of energy supply and energy demand. Promising candidate processes are based on coupled chemical/energy conversion. In this work, the pyrolytic conversion of methane to valuable high-energy content substances like hydrogen and unsaturated hydrocarbons by the compression/expansion process of a piston engine is investigated. In particular, the potential of running this conversion in a multi-compression–expansion (MCE) mode where a gas sample is subject to multiple compression–expansion strokes, is assessed. The methane conversion and target species yields of this multi-compression mode relative to a single compression–expansion mode are assessed. Experimental studies with a rapid compression–expansion machine are used for this. The experiments are complemented by numerical simulations, which help to interpret the experimental findings. We found that both conversion and target species yields can be increased significantly by the multi-compression–expansion processes relative to a single compression–expansion. For instance, at typical engine operation conditions, ten compression–expansion cycles increase the methane conversion by a factor of three to four (from approx. 15 % to 68 %), the hydrogen yield by a factor of five, and the unsaturated hydrocarbon yields by a factor of three, compared to a single compression–expansion process. The results encourage considering a new role for piston-engines as work-to-chemical energy converters, in addition to their conventional heat-engine (chemical energy to work) operation

Numerical Studies on Minimum Ignition Energies in Methane/Air and Isooctane/Air Mixtures

In this study, the dependence of minimum ignition energies (MIE) on ignition geometry, ignition source radius and mixture composition is investigated numerically for methane/air and isooctane/air mixtures. Methane and isooctane are both important hydrocarbon fuels, but differ strongly with respect to their Lewis numbers. Lean isooctane air mixtures have particularly large Lewis numbers. The results show that within the flammability limits, the MIE for both mixtures stays almost constant, and increases rapidly at the limits. The MIEs for both fuels are also similar within the flammability limits. Furthermore, the MIEs of isooctane/air mixtures with a small spherical ignition source increase rapidly for lean mixtures. Here the Lewis number is above unity, and thus, the flame may quench because of flame curvature effects. The observations show a distinct difference between ignition and flame propagation for iso-octane. The minimum energy required for initiating a successful flame propagation can be considerably higher than that required for initiating an ignition in the ignition volume. For iso-octane with a small spherical ignition source, this effect was observed at all equivalence ratios. For iso-octane with cylindrical ignition sources, the phenomenon appeared at lower equivalence ratios only, where the mixture’s Lewis number is large. For methane fuel, the effect was negligible. The results highlight the significance of molecular transport properties on the decision whether or not an ignitable mixture can evolve into a propagating flame