Henri Temianka Correspondence; (maas)

https://digitalcommons.chapman.edu/temianka_correspondence/2336/thumbnail.jp

Henri Temianka Correspondence; (maas)

https://digitalcommons.chapman.edu/temianka_correspondence/2335/thumbnail.jp

Henri Temianka Correspondence; (maas)

https://digitalcommons.chapman.edu/temianka_correspondence/2334/thumbnail.jp

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

Ignition delay times of methane/diethyl ether (DEE) blends measured in a rapid compression machine (RCM)

Diethyl ether (DEE) is an interesting species for combustion for at least two reasons: On the one hand, it is used as a kind of "worst case" reference substance for studies concerned with the prevention of accidental ignition events. On the other hand, it is also a candidate bio-fuel. For this reason, in this work, auto-ignition of two different CH$_{4}$/DEE-mixtures (90/10 and 95/5 mol-% CH$_{4}$/DEE) are studied in a rapid compression machine (RCM). In the RCM, the gas mixture is compressed in a piston-cylinder device up to 20 bar and held under isochoric conditions at top dead center. Autoignition occurs after an ignition delay time (IDT). IDTs are measured for compression temperatures ranging between 515 and 925 K, for both, stoichiometric and fuel-rich mixtures (equivalence ratio $\phi$ = 2). The experimental data are compared to results of simulations involving detailed chemistry, as well as to other fuels investigated in the same RCM (results from literature)

Effects of ozone addition on the kinetics and efficiencies of methane conversion at fuel-rich conditions

Compression–expansion processes have the potential of converting mechanical work to chemical energy at fuel-rich conditions, allowing for the storage of fluctuating renewable energies. In this work, the conversion of methane and natural gas (NG) is investigated for this purpose. A focus is on using ozone as a reaction promoter for the otherwise slow reaction. The kinetics of fuel-rich methane/NG oxidation with ozone addition is investigated experimentally and numerically. To this end, ignition delay times (IDTs) for CH$_4$/O$_2$/O$_3$/Ar and NG/O$_2$/O$_3$/Ar mixtures are measured in a rapid compression machine (RCM). It is shown that a reaction mechanism obtained by simply combining a previously developed mechanism for methane conversion (PolyMech2.0) with an ozone sub-mechanism does not accurately predict IDTs. Sensitivity analyses identify reactions in the methane submechanism that become more important for ignition delay time when ozone is added in comparison to mixtures without O$_3$. The rate coefficients of these reactions are modified within their uncertainty ranges to better match the experimentally obtained IDTs. The resulting kinetic model, named PolyMech 3.0, predicts the IDTs obtained in RCM-experiments well. Analysis reveals a two-fold promoting effect of ozone addition on methane/air ignition: Ozone causes a temperature rise by the reactions associated with its decomposition. Ozone also forms reactive products such as hydrogen and oxygen radicals, which can then promote reactions of the hydrocarbons. Quantitative analysis shows that the latter effect is more pronounced. Using PolyMech 3.0, parametric simulation studies for methane conversion in four-stroke engine cycles are carried out to explore the effects of ozone addition on chemical energy storage and efficiencies of engine-based polygeneration processes. Results show that with ozone addition, methane conversion can take place at high engine speeds, while without ozone, there is nearly zero conversion of fuel rich methane mixtures because of the low reactivity. Therefore, ozone addition allows for reasonable efficiencies across a wider range of operating conditions

Cryogenic orthogonal turning of Ti-6Al-4V – Analysis of nitrogen supply pressure variation and subcooler usage

Cooling of machining operations by liquid nitrogen is a promising approach for reducing cutting temperatures, increasing tool life and improving the workpiece surface integrity. Unfortunately, the cooling fluid tends to evaporate within the supply channel. This induces process variations and hinders the use of nitrogen cooling in commercial applications. In this work, the coolant is applied via the tool’s rake face during orthogonal turning of Ti-6Al-4V. The effect of a nitrogen supply pressure adjustment and a subcooler usage—proposed here for the first time for machining—is analyzed in terms of process forces, tool temperatures and wear patterns, taken dry cutting as a reference. Thereby, reliable cooling strategies are identified for cryogenic cutting