Fluent’s consulting group and the U.S. Federal Energy Technology Center (FETC) are working together to minimize pollution from burning natural gas by optimizing lean premixed combustion (LPM).

LPM burns low quantities of fuel relative to air, which reduces nitrogen oxide emissions, but can also lead to combustion instability and pressure variations that can damage turbines. To pinpoint the cause of combustion instability, Fluent’s Morgantown, West Virginia, consultants and FETC researchers are replicating LPM conditions using FETC’s Dynamic Gas Turbine test rig. This specially-designed test rig provides data on combustion instability with different fuel injector geometries and at varying flow rates of fuel-air mixtures.

LPM combustion is reached when the fuel-air ratio falls below the ideal stoichiometric ratio so that excess air passes through the combustor. The combustion air passes through swirl vanes whose location relative to the fuel nozzle exit can be adjusted. The natural gas fuel mixes with the swirling combustion air downstream of the swirl vanes. Natural gas is injected through six injection holes drilled in each of the eight radial fuel spokes with the holes oriented perpendicular to the mean flow. The resultant fuel-air mixture then enters the water-jacketed combustor through the fuel nozzle exit. A flame is stabilized at the nozzle exit by the step expansion from nozzle exit to combustor and the swirl-induced recirculation zone. Combustion products exit the combustor through a water-cooled constriction.

LPM combustion is more likely to lead to unsteady combustion with the possibility of large-amplitude pressure oscillations that can damage the combustor and turbine. Experimental observations indicate that swirl vane location relative to the fuel nozzle exit can greatly affect the magnitude of pressure oscillations produced by the unsteady flame. Coupling between combustion oscillations and acoustic pressure oscillations can lead to large-amplitude pressure oscillations within the combustor. This interaction between combustion and acoustics is an area of active research in the application of LPM combustion to gas turbines used for power generation.

Experiments conducted using the test rig were converted into three-dimensional models to better study the combustors’ transient behavior. The CRAY T3E, a “supercomputer” housed in the Pittsburgh Supercomputing Center, was used to develop the 3D models. Using the computer’s platform, multiple simulations were performed in a short time frame to study the effects of swirling vane position, nozzle velocity, and fuel-air ratio on flow and combustion.

Preliminary results suggest that careful placement of the swirl vanes enhances combustion stability. Simulation reveals that swirl vane location and flow conditions affect the flame’s shape and orientation within the combustor, which affect the time delay between fuel injection and the point where fuel reacts in the flame. This time delay determines when combustion and pressure oscillations occur, potentially causing turbine problems. Additional study is underway at FETC to measure the time delay in the combustor. Combined with experimental data, these simulations may identify a design approach that will enhance stability.

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