Date of Award

12-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Ke Nguyen

Committee Members

Jeffery Hodgson, David Irick

Abstract

An experimental investigation is conducted to determine the effect of pressure on breakdown threshold energies of methane and air. In addition the effect of pressure and equivalence ratios on minimum ignition energies of methane-air mixtures are also investigated. The pressures for the breakdown threshold energy experiments are varied from 0.02 to 1.17 MPa, and pressures and equivalence ratios for minimum ignition energy experiments are varied from 0.1 to 1.04 MPa and 0.6 to 1.2, respectively. The gas breakdown and the ignition of the methane-air mixture is achieved using a laser-induced spark from a 5.5 ns pulse Q-switched Nd:YAG laser at a wavelength of 1.064 μm. Since ignition is preceded by generation of a spark, thus determining the breakdown threshold energies can provide the insight on the generation of spark required for ignition. It is suggested that the gas that has lower breakdown threshold energy would provide the spark for ignition, and the minimum ignition energy would be close to the breakdown threshold energy of that gas. The breakdown threshold energies measured for methane at 0.02 MPa and 1.17 MPa are 23.23 and 1.9 mJ and for air 0.02 MPa and 1.17 MPa are 28.84 and 2.74 mJ, respectively. The breakdown threshold energies of methane and air are found to be of the same order with breakdown threshold energies of air being a few millijoules higher than those of methane. It is observes that breakdown threshold energies of methane and air is always much larger than the minimum ignition energies of methane-air mixtures, and hence there is no correlation between breakdown threshold energy and minimum ignition energy. The mixture during the minimum ignition energy experiments ignited before the spark was generated. The temperature and pressure in the focused region were extremely high, which ignited the mixture directly or created a rapidly expanding shockwave strong enough to ignite the mixture. The results indicate that at a given equivalence ratio the minimum ignition energy decreases with increasing pressure. Furthermore at a given pressure minimum ignition energy is found to be lowest at stoichiometric and increases as the mixture deviates from stoichiometric. Similar to the breakdown threshold energy, the minimum ignition energy of methane-air mixture is found to be dependent on pressure.

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