Early Stages Of Combustion Process Research Articles

Laser-induced multi-point ignition with a single-shot laser using two conical cavities for hydrogen/air mixture

A multi-point ignition technique with a single-shot laser is proposed and its feasibility has been tested experimentally in a constant volume chamber filled with premixed hydrogen/air mixture. Two conical cavities are installed on combustion chamber walls, which are aligned with laser. The first cavity has an opening near the apex. By directing unfocused laser beam into the first cavity, part of the incident laser energy passes through the opening and irradiates into the second cavity. Simultaneous two-point ignition is achieved at each cavity. A simultaneous three-point ignition technique is based on directing focused laser beam into the two-cavity arrangement, which produces additional ignition at the center of the chamber through laser-induced spark ignition. Advantages of the proposed techniques over single-point ignition techniques are demonstrated through significant reduction in total combustion time. Shadowgraphs of the early stage of combustion process show that hot gaseous jets are ejected from the cavities, resulting in rapid combustion. It is also found that increase in incident laser energy accelerates jet ejection from the cavities.

An experiment of the combustion characteristics with laser-induced spark ignition

The combustion characteristics and minimum ignition energies using laser-induced spark ignition were demonstrated for quiescent methane-air mixtures in an optically-accessible, constant volume combustion chamber. Initial pressure and equivalence ratio as well as spark energy were varied in order to explore the flame behavior with laser-induced spark ignition. Shadowgraphs for the early stages of combustion process showed that the flame kernel becomes separated into two, one of which grows back towards the laser source. Eventually after a short period, the two flame kernels developed into two flame fronts propagating individually, which is unique in laser-induced spark ignition. For a given mixture, lower initial mixture pressure and higher spark energy resulted in shorter flame initiation period and faster flame propagation. The results of minimum ignition energies for laser ignition shows higher values than electric discharge results, however, the difference decreases toward lean and rich flammability limits.

Fate of Volatile-N and Char-N in the Formation Process of N2O and NO during Coal Combustion.

Formation mechanisms of N2O and NO have been experimentally investigated using a temperature-controlled one-dimensional laminar-flow pulverized coal combustion furnace. The temperature range is 800-1100°C including that of usual fluidized bed combustion. The effect of temperature and the separated contribution of Volatile-N and Char-N are clarified using raw coals and their chars carbonized in an actual bubbling fluidized bed combustor. N2O formation and destruction reactions strongly depend on temperature, and N2O almost disappears with the inclease of temperature above 1100°C. The conversion ratios of Volatile-N to N2O and NO are higher than those of Char-N. While the Fast Volatile-N released at the early stage of combustion process is mainly converted to NO, the Slow Valatile-N released at the char combustion stage is selectively converted to N2O.

Separation of the basic factors affecting no formation in pulverized coal combustion

Separated influences of various basic factors influencing NO formation behavior in pulverized coal combustion have been derived through experiments in a one-dimensional laminar furnace and the simplified theoretical analysis of flame structural change around coal particles. The effect of particle size on combustion history of coal particles the influences of oxygen-fuel stoichiometric ratio and particle size on NO formation behaviors in the furnace, and the individual separated effects of the stoichiometric ratio, flame temperature and coal properties on the conversion ratio from fuel N to exhaust NO were clarified. When the particle size is large enough, there appears the region in the early stage of combustion process where the flame zone is established away from the particle surface, while for small particles both volatile matter and fixed carbon burn on the particle surface or in the pores of particles through the combustion process. The NO formation rate in the early stage of combustion process increases with the stoichiometric ratio for large particles and decreases with the particle size. The rate is less affected by the stoichiometric ratio for small particles. The conversion ratio from fuel N to NO decreases with the nitrogen content and increases with the volatile matter content under the excess oxygen condition. The conversion ratio to NO increases with the stoichiometric ratio especially for high volatile coals, and temperature has little effect on it except for fuel rich condition for high volatile coals. These results on the conversion ratio could be well explained by the fractions of gas phase N-species converted to NO, HCN and NH3 from fuel N.