Characteristics of combustion in a narrow channel with a temperature gradient

Characteristics of premixed combustion in a heated channel with an inner diameter smaller than the conventional quenching distance of the employed mixture were investigated experimentally, analytically, and numerically. A cylindrical quartz tube with an inner diameter of 2 mm was used as a model channel. The downstream part of the tube was heated by an external heat source, and hence the temperature gradient in the axial direction was formed in the middle of the tube. Flat and stationary conventional premixed flames were stabilized at a point in this temperature gradient. In addition to these flames, various other flames that exhibit dynamic behaviors such as cyclic oscillatory motions, and repetitive ignition and extinction were also observed experimentally. These flames with large amplitude oscillatory motion might be utilized as a heat source with high speed temporal temperature variations in microsystems for future application. Another stable flame region in extremely low speed criteria at a mixture velocity of 2–3 cm/s was also experimentally confirmed. This flame was inferred to be an example of mild combustion, and it might also be used as a mild heat source for microdevices. The overall stability criteria of these flame regimes were analytically examined, and the detailed structure of each flame on the stable solution branches was numerically examined by employing 1D computation with detailed chemistry. The two results qualitatively agreed with each other and clarified the mechanism of the present various flames and their dynamic characteristics.     

Effect of the settings of electrospray deposition method on the structure and performance of the fuel cell catalyst layer

Catalyst layers of proton exchange membrane fuel cells (PEMFC) are formed by electrospray deposition (ESD) method. The cathode catalyst layers are formed and characterized by varying the settings of the system, such as flow rate, applied voltage and the distance between the capillary and the substrate. The dryness of the aggregates during deposition is evaluated using the Damkhöler number (Da), and the structure of the catalyst layer is observed using SEM, which shows that the catalyst layer is porous when dry and non-porous when insufficiently dry. In the case of insufficient drying, the structure changed significantly depending on the position. Single cell tests show that the maximum power density varies from 105 to 253 mWcm⁻² depending on the settings, even with the same catalyst ink and the same amount of platinum. Electrochemical impedance spectroscopy shows that the charge transfer and mass transport resistances tend to decrease with increase in Da.