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Three-dimensional (3D) detonation simulations solving the compressible Navier-Stokes equations with detailed chemistry are performed in both square channel and round tube geometries. The simulations are compared with each other and with two-dimensional (2D) channel simulations and round tube experiments of identical mixture and conditions (stoichiometric hydrogen-oxygen with 3000 PPMv ozone at 300 K and 15 kPa) with the goal of understanding the effect of confinement and boundaries on detonation structure. Results show that 3D detonations propagate with highly inhomogeneous blast dynamics, where blasts emerge not only from intersections of two transverse waves (similar to 2D propagation) but also from intersections of many transverse waves (unique to 3D detonations in the confinements tested). Intersections of many transverse waves lead to extreme thermodynamic states and highly overdriven wave velocities, well in excess of those seen in the ZND model and in 2D simulations. 3D simulations in the square tube show highly regular blast latticing, smaller detonation cells, and highly oscillatory velocities when compared to the round tube simulations. Round tube simulations show more spatially non-uniform blast dynamics. The conclusions reached in the current work are found irrespective of numerical grid resolution. [ABSTRACT FROM AUTHOR]

The dependence of detonation cell regularity on mixture gasdynamic and chemical kinetic properties are investigated through hydrogen-oxygen detonations with different diluent and ozone addition. A total of seven mixtures are specifically designed such that the post-shock specific heat ratio γ VN , representing the gasdynamic effect, and the effective activation energy ϵ i , representing the chemistry effect, can be varied independently among these mixtures. Both experiments and two-dimensional (2D) Navier-Stokes simulations are performed. For the ranges of γ VN and ϵ i considered, γ VN is found to moderately affect cell regularity. For mixtures of relatively large γ VN (≥ 1.3), detonation cells become slightly irregular with decreasing γ VN. A clear increase in cell irregularity is found when γ VN is small (< 1.3). In contrast, the impact of ϵ i on cell regularity is more notable: mixtures of large ϵ i produce significantly more irregular cells, corresponding to frequent appearances of transverse detonation structures that dominate the global burning behavior. Finally, ozone is qualitatively and quantitatively shown to regulate the detonation structures in both simulations and experiments tested in this study, and results suggest that trace additives that alter the ϵ i of a mixture can likely be used to tune the mixture for a desired regularity. [ABSTRACT FROM AUTHOR]

Electrochemical carbon dioxide (CO2) conversion to hydrocarbon fuels, such as methane (CH4), offers a promising solution for the long-term and large-scale storage of renewable electricity. To enable this technology, CO2-to-CH4 conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO2 mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH4 selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO2 concentration by combining both dissolved CO2 and in-situ generated CO2 from the bicarbonate. In-situ Cu activation through alternating current operation renders and maintains the catalyst highly selective towards CH4. The combination of these strategies leads to CH4 Faradaic efficiencies of over 70% in a wide current density range (100 – 750 mA cm-2) that is stable for at least 12 h at a current density of 500 mA cm-2. The system also delivers a CH4 concentration of 23.5% in the gas product stream. Electrochemical CO2 conversion to methane offers a promising solution for the large-scale storage of renewable electricity, yet the catalytic selectivity at high current density still needs to be refined. Here the authors report to use both dissolved CO2 and in-situ generated CO2 from bicarbonate to sustain high local CO2 concentration around Cu electrode and thus achieve selective CO2 conversion to methane. [ABSTRACT FROM AUTHOR]

A series of sodium phosphorothioate complexes are shown to have electrochemical properties attractive for sodium-sulfur battery applications across a wide operating temperature range. As cathode materials, they resolve a long-standing issue of cyclic liquid-solid phase transition that causes sluggish reaction kinetics and poor cycling stability in conventional, room-temperature sodium-sulfur batteries. The cathode chemistry yields 80% cyclic retention after 400 cycles at room temperature and a superior lowtemperature performance down to 260 °C. Coupled experimental characterization and density functional theory calculations revealed the complex structures and electrochemical reaction mechanisms. The desirable electrochemical properties are attributed to the ability of the complexes to prevent the formation of solid precipitates over a fairly wide range of voltage. [ABSTRACT FROM AUTHOR]

A geometric model with a low computational complexity capable of simulating detonation behavior in physical systems is proposed. In support of the geometric model development, a series of cylindrical 1D simulations with a variable size initiation kernel are performed in hydrogen-oxygen mixtures. From these 1D simulations a detonation cell stabilization mechanism is identified. The stabilization mechanism is predicated on the size of the gap between the pressure and temperature fronts at the point where the average pressure front velocity along one cell length is equal to the CJ velocity. This gap, in a multidimensional detonation, is the ignition kernel of a subsequent blast, and dictates the formation of the subsequent cell. Serial analysis of blasts in this context leads to a unique stable blast kernel size for any mixture, which, within the uncertainty of the initial kernel state, can predict the experimental cell length for mixtures considered in this study. Using a tabulation of the 1D simulations as an input, a formulation and sample results of the geometric model are shown. The geometric model can reproduce both qualitative and quantitative features of experimental detonation cellular structure. [ABSTRACT FROM AUTHOR]

Ozone as an ignition promoter was used to study the detonation limit behavior in small, round tubes. Experimentally, detonation cellular structures and velocity deficits of fuel-oxygen mixtures with and without 3000 PPM of ozone were measured in tubes of 32 and 6 mm in inner diameter. Both hydrogen-oxygen and methane-oxygen mixtures were tested. The addition of ozone reduces the detonation cell size and extends the transition to spinning detonation to lower initial pressures. Ozone addition, however, produces a negligible effect on velocity deficit far from the detonation limit while providing a measurable extension of the limit. The change in deficit caused by ozone does not correlate with the reduction in cell size, suggesting that the velocity deficit behavior is not solely a function of the detonation structure. By analyzing the velocity deficit data in the d / λ (the ratio of tube diameter to cell width) domain, two regimes of detonation limits are identified: a loss-governed regime where velocity deficiency results from various losses (mass, momentum, and heat) and is insensitive to detonation structure, and a geometry-limited regime determined by the geometric accommodation of the detonation structures. Two velocity deficit models were evaluated against the experimental results: a modified ZND model with heat and momentum losses (HML) and the flow divergence (FD) model. Velocity deficits in the loss-governed regime are well predicted by the HML model but not by the FD model. Velocity deficits in the geometry-limited regime can be correlated with the critical condition of d / λ = 1 / π and the transition point between the two regimes. Ozone addition extends the detonation limit by moving the reactive mixtures away from its geometric detonation limit, and promotes detonation propagation in miniaturized geometries. [ABSTRACT FROM AUTHOR]

Electrochemical carbon dioxide (CO 2 ) conversion to hydrocarbon fuels, such as methane (CH 4 ), offers a promising solution for the long-term and large-scale storage of renewable electricity. To enable this technology, CO 2 -to-CH 4 conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO 2 mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH 4 selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO 2 concentration by combining both dissolved CO 2 and in-situ generated CO 2 from the bicarbonate. In-situ Cu activation through alternating current operation renders and maintains the catalyst highly selective towards CH 4 . The combination of these strategies leads to CH 4 Faradaic efficiencies of over 70% in a wide current density range (100 - 750 mA cm -2 ) that is stable for at least 12 h at a current density of 500 mA cm -2 . The system also delivers a CH 4 concentration of 23.5% in the gas product stream., (© 2023. The Author(s).)