Investigation Of Fuel/Air Mixing And Combustion Characteristics Of A Lean Premixed Burner For Natural Gas Turbine Application

The demand of reducing nitrogen oxides (NOx) and other emissions has been one of the main driving factors for the development of gas turbine combustors. Lean premixed combustion in gas turbine combustors is one of the most widely used methods in turbine industry. When it comes to lean premixed combustion, the fuel/air uniformity has significant influence on premixed flame stability and NOx production. Therefore, numerical simulation and combustion experiments were conducted to investigate the combustion performance of a lean premixed burner which would be utilized in gas turbines. The burner geometry and its influence on fuel/air mixing were also studied to improve the combustion performance. The work is shown as follows:(1) Chemical reaction network model was built by CHEMKIN to get the NOx emissions under different mixing performance and adiabatic flame temperature. The calculated results provided reference for improving fuel/air mixing uniformity. Different methods of evaluating mixing performance were compared and different factors influencing fuel/air uniformity were also investigated, which guided fuel/air mixing optimization work.(2) By numerical approach, the effects of burner geometry on fuel/air mixing were studied. First, the numerical results were compared to experimental results to verify the simulation model. Afterwards, burners with different geometry were modeled to investigate the flow and fuel/air mixing characteristics. It is shown that the air does not entrain the fuel sufficiently when the swirl intensity is over high. Otherwise, the swirling air will not generate much turbulence to enhance mixing if the swirl is weak. Meanwhile, the length of the premixing channel will also influence flow characteristics and mixing effects. Longer channel can help yield more homogeneous mixture but will result in swirl intensity dissipation. In addition, different fuel injection diameters have different injection depths, which have significant effects on mixing. When the injection depth is shallow, the turbulence of swirling air is not fully used to improve fuel/air mixing performance, so the fuel concentrates in the central zone of the premixing channel. With the increasing trend of fuel injection depth, the mixing performance gets better. However, over-penetration results in poorer mixing due to the effect of centrifugal force. Fuel gets "sucked" into the space near the outer wall of the premixing channel, consequently, misses the chance to dissipate in the whole space. Apart from geometry features mentioned above, many other inner structures could also influence mixing performance. At last, geometries with good mixing performance were selected to conduct experiments.(3) Afterwards, combustion experiments were conducted. Combustion field were measured by laser techniques and emissions were detected by gas analyzers. Flame stability and reaction field characteristics of burners with different swirl and fuel injection geometry were compared. The results indicate that better mixing helps achieve more homogeneous reaction zone, less flame disturbation and less NOx emission.The present research combines numerical simulation and experimental study, which promotes understanding of mixing process and combustion characteristics of lean premixed burners. The results of this study could provide reference for the design of the premixed burners utilized in gas turbine applications.