A Comparison of Infrared Light Emitting Diodes (IR-LED) versus Infrared

In lean premixed combustion systems, inadequate mixing of the fuel and air, prior to combustion can cause unnecessarily large pollutant emissions. Measuring the extent of mixing of fuel into air is often difficult, since combustion in lean premixed gas turbines takes place at high pressures, often making optical access to the combustion area limited. In addition, the pressure broadening of the molecular absorption lines renders the spectrally narrow line associated with a laser light source less useful. This paper studies some of the problems in determining the extent of mixing of the fuel into air in these lean premixed combustion systems. The focus of this paper is the use of an infrared light emitting diode (IR-LED) to quantitatively measure fuel concentration in a lean premixed gas turbine. The IR-LED emits radiation over a wide wavelength range compared to a laser, meaning that the development of an absorption coefficient to relate the fuel concentration to the absorption of the IR-LED radiation is not as direct as developing the absorption coefficient for the absorption of laser light. Controlled experiments were performed where the pressure, path length and fuel concentration were varied and the effects of these three parameters on the absorption of radiation from the IR-LED were studied. A broad band absorption coefficient was developed relating the absorption of light from the IR-LED to the fuel concentration. This broad band absorption coefficient was found to be in good agreement with calculated coefficient values. Experiments were performed on a lean premixed gas turbine combustor modified for line-of-sight optical access. The concentration profile of this high pressure combustor was found by tomographic reconstruction from line-of-sight absorption measurements using the IR-LED. We demonstrated that the IR-LED can be used for quantitative measurements of the fuel concentration for high pressure systems.