Presence Of Homogeneous Reactions Research Articles

Catalytic characterization in plasma wind tunnels under the influence of gaseous recombination

The catalytic properties of materials used in re-usable thermal protection systems (TPSs) of re-entry vehicles are mainly characterized in plasma wind tunnels. These facilities are adequate to reproduce the thermo-chemical environment found in hypersonic flights. Catalytic recombination coefficients (W) determined empirically for the TPS design process are used to validate catalytic models. Recent experiments in plasma wind tunnels present some contradictions with current catalytic models used in computational fluid dynamics simulations. This work explores the coupling between homogeneous and heterogeneous reactions and its influence on the heat flux measurements, in order to explain such contradictions. For that, three copper calorimeters (at 350 K) are inserted in probes of different sizes, and they are tested in the Plasmatron facility at the von Karman Institute under different chemical regimes, which are modified by the probe diameter. A dimensional analysis of the acquired data based on previous non-equilibrium boundary formulations shows that an empirical recombination coefficient can be falsified by the presence of homogeneous reactions. Then, in the framework for the post-flight analysis of the European Space Agency's intermediate experimental vehicle, the local heat transfer simulations methodology is applied under peak heating conditions. A new probe is designed, manufactured, and tested in the Plasmatron to measure the recombination coefficient on C/SiC under the closest flow conditions with respect to the flight.

Singular transients in the presence of homogeneous reactions at cylindrical wire electrodes: Simulation by the adaptive Huber method for integral equations

Current transients possessing a weak temporal singularity of the t-1/2 type arise in the theory of chronoamperometry and cyclic voltammetry. Simulating them by the method of integral equations (IEs) requires a highly accurate algorithm for computing integrals ∫0tK(t,τ)τ-1/2dτ for any kernel term K(t,τ) present in the IEs. Such an algorithm is presented for the kernel term K(t,τ)=exp[-k(t-τ)]kcylw[ρ(t-τ)1/2] characteristic of IEs arising for diffusion coupled with (pseudo-) first order homogeneous reactions at cylindrical wire electrodes. The formerly described adaptive Huber method, equipped with this algorithm, is applied to three example IEs describing singular transients. These simulations are successful when the parameter k (typically a rate constant of a homogeneous reaction) and the electrode cylindricity parameter ρ are moderately large. For too large k or ρ a divergence of the method was observed, due to inevitable roundoff errors. Substantial amendments to the adaptive Huber method may be necessary in order to overcome this numerical difficulty.

Investigation of the Reaction of Ethanol-Steam Mixtures in a YSZ Electrochemical Reactor Operated in a Fuel Cell Mode

In the present investigation ethanol-water mixtures were directly fed to a yttria stabilized zirconia (YSZ) electrochemical reactor operated in a fuel cell mode and the preliminary results are presented and discussed. Polycrystalline films of platinum (Pt) and silver (Ag) were, respectively, tested as anode catalysts in a wide range of experimental conditions while the cathode was exposed to the atmospheric air. The single direct ethanol solid oxide fuel cell (DE-SOFC) tests were performed in order to investigate separately Pt and Ag’s activities towards ethanol steam reaction and fuel cell performance (Pt‐DESOFC and Ag‐DESOFC). In both cases the products were on-line analyzed by a mass spectrograph, a gas chromatograph and a series of gas analysers under fuel cell mode of operation. The results showed that even at high temperature values (>750°C) the main products were CH3CHO, CH4, CO, H2, and CO2. Furthermore, as expected the single DESOFC performance was improved as the temperature increased. However, a relatively poor fuel cell performance has been obtained in both cases, which could be attributed to the following reasons: the relatively low (for YSZ electrolyte) operation temperature, the presence of homogeneous reactions, and the cell configuration.

Some numerical investigations of the stability of electrochemical digital simulation, particularly as affected by first-order homogeneous reactions

A reported analysis of the stability of some digital simulation methods is investigated by numerical experiments and the results are consistent with the analysis. Traditional stability conditions need to be modified slightly in the presence of homogeneous reactions, though not to a degree that has practical significance. The oscillatory response of the Crank-Nicolson method compared with the Laasonen method can be reduced or eliminated by a preliminary expansion of the time steps, as suggested by Feldberg.

Progress since the 1960s in the treatment of complex mechanisms and concentration overpotential for metal electrodes

A recall of papers, dealing with the general mathematical procedure for impedance treatments of complex mechanisms, is first presented and conditions for a correct diagnosis are discussed through some examples. Improvements of Warburg's impedance, especially for rotating disks, are recalled in particular in the presence of homogeneous reactions and with semiblocked electrodes. Transfer functions obtained when the input and/or the output is not an alternating signal are surveyed.