Study of Lean Blowout Limits and Effects of Near Blowout Oscillations on Flow Field and Heat Transfer on Gas Turbine Combustor
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Modern gas turbine combustors implement lean premixed (LPM) combustion system to reduce the formation of NOx pollutants. LPM technology has advanced to have the ability to produce extremely low level of NOx emissions. The current focus of research on LPM is focused on reducing the NOx emission to much smaller scales, which is mandated because of the stricter regulations and environmental concerns. However, LPM combustors are susceptible to lean blowout (LBO), and other corresponding instabilities as the combustor is operated lean. Therefore, it is essential to understand the LBO limits and dynamics of flow in lean operating conditions. One of the other primary parameters for the improved combustion chamber designs is an accurate characterization of the heat loads on the liner walls in the wide range of operating conditions. Currently, there are very limited studies on the flame side heat transfer in reacting conditions. Current gas turbine combustion technology primarily focuses on burning natural gas as the gas fuel option for industrial systems. However, interest in utilizing additional options due to environmental regulations as well as concerns about energy security have motivated interest in using fuel gases that have blends of Methane, Propane, H2, CO, CO2, and N2. For example, fuel blends of 35%/60% to 55%/35% of CH4/CO2 are typically seen in Landfill gases. Syngas fuels are typically composed primarily of H2, CO, and N2. Gases from anaerobic digestion of sewage, used commonly in wastewater treatment plants, usually have 65�"75% CH4 with the balance being N2. The objective of this study is to understand the LBO limits and the effects of the instabilities that arise (called near blowout oscillations) as the combustor is operated lean. Near blowout oscillations arise as the equivalence ratio is reduced. These oscillations are characterized by continuous blowout and re-ignition events happening at low frequencies. The low-frequency oscillations have very high-pressure amplitude and can potentially damage the liner wall. The impact of the near blowout oscillations on the flow field and heat transfer on the liner walls are studied. To accomplish this, the experiments were conducted at Advanced Propulsion and Power laboratory located at Virginia Tech. A lean premixed, swirl stabilized fuel nozzle designed with central pilot hub was used for the study. Additionally, this work also studies the lean blowout limits with fuel blends of CH4-C3H8, CH4-CO2, and CH4-N2 and also their effect on the stability limits as the pilot fuel percentage was changed. Flow field during near blowout oscillations was studied using planar particle image velocimetry (PIV) and flame shapes and locations during these oscillations was studied by using high-speed imaging of the flame. A statistical tool called proper orthogonal decomposition (POD) was utilized to post-process the PIV data and high-speed imaging data. Heat transfer on the liner walls was studied using a transient IR thermography methodology. The heat transfer on the liner wall during the near blowout instabilities was resolved. LBO limits and near blowout oscillations were characterized by studying the pressure measurements in the primary combustor region. Fluctuating heat loads on the liner walls with the same frequency as that of near blowout instabilities was observed. The magnitude of fluctuation was found to be very high. Phase sorted POD reconstructed flame images demonstrated the location of the flame during near blowout oscillations. Thus, blowout and re-ignition events are resolved from the high-speed flame images. POD reconstructed flow field from the PIV data demonstrated the statistically significant flow structures during near blowout oscillations. A hypothesis for the mechanism of near blowout oscillations was explained based on the measurements and observations made. Lean Blowout limits (LBO) changed when the percentage of pilot and air flow rates was changed. As the pilot percentage increased, LBO limits improved. Results on the study of fuel mixtures demonstrate that the addition of propane, nitrogen and carbon dioxide has minimal effect on when the flame becomes unstable in lean operating conditions. However, on the other hand, the addition of diluent gas showed a potential blowout at higher operating conditions. It was also observed that Wobbe index might not be a good representation for fuels to study the fuel interchangeability in lean operating conditions.
- Doctoral Dissertations