Fast Response Extraction Probe for Measurement of Air-Fuel Ratio Fluctuations in Lean Premixed Combustors FREE

[+] Author Affiliations
Rajiv Mongia, John Torres, Robert Dibble

University of California at Berkeley, Berkeley, CA

Darren Lee, Torger Anderson, William Sowa

United Technologies Research Center, East Hartford, CT

Paper No. 99-GT-277, pp. V002T02A048; 10 pages
  • ASME 1999 International Gas Turbine and Aeroengine Congress and Exhibition
  • Volume 2: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations
  • Indianapolis, Indiana, USA, June 7–10, 1999
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-7859-0
  • Copyright © 1999 by ASME


The extent of air and fuel mixing prior to combustion in lean premixed combustion has been shown to drastically affect combustor performance, both in terms of emissions and stability. Standard extraction probes are often used for measuring the spatial (average over time) distribution of fuel concentration. However, the temporal fluctuations in fuel mole fraction, which are averaged by conventional extraction probes, have been also shown to drastically affect combustor performance. Several methods have been developed to measure the fluctuations in fuel mole fraction both temporally and spatially. These techniques are often difficult or impossible to apply to an operating combustor at high pressures and temperatures.

In this paper, we describe a Fast Response Extraction Probe (FREP) which is capable of measuring temporal mole fraction fluctuations up to frequencies of 1 kHz. A short, capillary tube is inserted into the premixing passage (which is at high pressure) for sampling the flow. The capillary tube is connected to a small gas cell at atmospheric pressure. Light from a 3.39 μm He-Ne laser is passed through the gas cell. By measuring the absorption of laser light, the concentration of hydrocarbons can be determined. The temporal response of the system is dependent on the geometry of the gas cell and sampling tube and the pressure drop through the sampling tube. A model for predicting the time response of the FREP is presented and compared to a laboratory scale experiment. The FREP was also tested in a high-pressure combustion rig at United Technologies Research Center. It is shown that the FREP is capable of measuring fuel-air fluctuations at gas turbine conditions. It is also shown that, at the conditions studied, there is a relationship between pressure oscillations and fuel mole fraction oscillations.

Copyright © 1999 by ASME
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