This is the first of two papers that describe underlying theory and modeling approaches for electrochemical processes in solid-oxide fuel cells (SOFC). SOFC systems, which can operate with hydrocarbon or hydrocarbon-derived fuels, can deliver energy-conversion efficiencies greater than 60%. This technology is likely to emerge as a significant energy-conversion technology. Cell operation depends on the coupled interactions of fluid flow, diffusive transport, thermal chemistry, and electrochemistry. This paper discusses the basic principles of these interactions. The second paper discusses research challenges.
Pyrolysis and oxidation in fuel-rich and fuel-lean mixtures of dimethyl ether (DME) highly diluted with argon were studied behind reflected shock waves in the temperature range 950 - 1900 K at total pressures between 0.8 and 2.9 atm. The study was carried out using the following methods: 1) time-resolved IR-laser absorption at 3.39 μm for DME decay and CH-compound formation rates, 2) time-resolved UV absorption at 216 nm for the CH3 radical formation rate, 3) time-resolved UV absorption at 306.7 nm for the OH radical formation rate, 4) time-resolved IR emission at 4.24 μm for the CO2 formation rate, and 5) a single-pulse technique for product yields. The pyrolysis and oxidation of DME, which were for very wide mixture compositions ranging from highly DME-rich to highly DME-lean, were modeled using a reaction mechanism with 178 reaction steps and 53 species including the most recent sub-mechanisms for formaldehyde, ketene, methane, acetylene, and ethylene oxidation. This and previously reported data were reproduced using this mechanism. The rate constant k5 = 4.5 x 1014 exp (-6.1 kcal/RT) cm3mol-1s-1 of reaction DME + OH → CH3OCH2 + H2O was evaluated.
The dynamic behavior of premixed flames propagating in non-uniform velocity fields has been investigated by two-dimensional unsteady calculations of reactive flows, based on the compressible Navier-Stokes equation including a one-step irreversible chemical reaction. We consider two basic types of phenomena to account for the intrinsic instability of premixed flames, i. e., hydrodynamic and diffusive-thermal effects, and examine the influence of intrinsic instability on the dynamic behavior. A sinusoidal disturbance is superimposed on the velocity field of the unburned gas, and its wavelength is set equal to the critical wavelength at Le = 1.0. The behavior of cellular flames generated by a disturbance and by hydrodynamic and diffusive-thermal effects is numerically simulated. When Le = 1.0, at the beginning of calculations, we observe the cellular flames whose wavelength is equal to that of a disturbance. After that, cells combine together and a large cell appears. The wavelength of a large cell is consistent with the size of cellular flames generated only by intrinsic instability. When Le = 0.5, on the other hand, small cells appear, and the division and combination of cells are observed. This is because that the size of cells due to intrinsic instability is shorter than the wavelength of a disturbance. The burning velocity of cellular flames propagating in non-uniform velocity fields is larger than that of planar flames propagating in uniform, which is due to a disturbance and intrinsic instability. The increment in the burning velocity depends not only on the intensity of a disturbance but on the Lewis number.
Flame propagation experiments of n-octane spray were performed in microgravity to investigate the flame propagation mechanism of volatile fuel sprays. The pre-vaporization occurred due to high volatility before ignition, and the equivalence ratio of fuel vapor and air mixture equaled or exceeded the lean flammability limit. Therefore, the flame could propagate through the n-octane vapor. n-decane spray mixed with small amounts of methane was also used as a fuel to simulate the flame propagation of volatile fuel sprays. The flame speed had a maximum around 6 vol.% of methane in n-decane spray. The aspect of flame propagation changed as the concentration of methane increased, and we could reasonably divide into three flame propagation regimes. In order to explore the effect of droplets in fuel vapor on distortion of the propagating flame front, the behavior of laminar flat flame passing through a suspended fuel droplet was experimentally observed. Results show that the propagating flame was transformed into a convex flame toward the unburned gas and the flame speed increased, when the flame passed through the droplet.