Numerical Analysis on Conceptual Design of Swirl Stabilized Fuel Nozzle for Micro Gas Turbine Using Converge™ Cfd

Recent developments of industry have caused increasing use fossil fuels and accelerating climate changes. Alternative fuels such as clean shale gas and biogas, which are in the spotlight as alternative energy sources for industrial power generation systems. These can reduce CO2 emissions, one of the main source of greenhouse gases, by 50% compared to emissions from coal fired power plants. As a result, studies on gas turbines using methane based fuel including natural gas, have been actively conducted, but NOx generated during combustion phase has been recognized as a main problem. NOx from industrial gas turbine is mainly produced by thermal NOx mechanism, which is caused by hot combustion temperature. Therefore, there is a need to increase fuel-air mixing to lower the local temperature rise in the combustor. For this purpose, the present study introduces the parametric study to increase the fuel mixedness by changing the fuel nozzle geometry.

The starting point of the nozzle concept determination was the perfect mixing concept. The main swirler, which improves fuel-air mixing inside the combustor, was placed in after downstream of nozzle holes, and a swirl support(SS) was installed at the rear part of nozzle tip to assist mixing flow. Based on the derived geometry, the analysis of non-reacting and reacting flow were carried out, and the correlations between mixedness of fuel in the combustor, flow tendency, and NOx emission were investigated

In this study, CFD program, CONVERGE™, was used to analyze a single nozzle with a heat output of 150kWth and TIT(Turbine inlet temperature) of 1200K. The SN(Swirl number of the swirler) was 0.8, having high swirl strength. Geometric change was performed based on the base nozzle. As the first concept, dimple was added to end of the nozzle center body and pin model also installed to handle the mixing length as the second concept. GRI-3.0 mechanism was used to describe combustion behavior. In addition, NOx formation quantity was calculated based on the Zeldovich mechanism, which is well known as the oxidation mechanism of nitrogen describing thermal NOx formation.

The base and dimple nozzle were compared in terms of flow velocity and fuel concentration on the nozzle surface. Compared to the base nozzle, the velocity distribution decreased at the end of the dimple nozzle, owing to vortex which can makes a relatively low velocity area on the wall of the nozzle tip. It is confirmed that areas with high concentration of fuel are created. With dimple nozzle concept, NOx was increased compared to NOx with base nozzle. The velocity and fuel concentration of base and pin 1,2 and 3 nozzle were also compared. In these cased, flow velocity was increased in the swirler area according to the SS installation position. In the case of pin 2, the high-velocity region is more widely distributed. On the other hand, the lower the velocity is, the higher the fuel concentration is.

In the case of the dimple nozzle, NOx emission increased by 0.4ppm compared to the base nozzle. On the contrary, NOx emissions were decreased with swirl support concept(Pin 1,2 and 3) owing it’s better mixing performances. In particular, it was found that the decreased NOx was more than 0.7ppm compared to the base model. Through this, it was implied that the mixedness of fuel and air in the combustor equipped with SS could be improved.

As a result, mixedness can be improved with SS concept, resulting in a reduction in NOx emissions. However, there are still rooms for improvement to optimize with in-depth analysis of pin’s location. In our future work, the fuel mixedness will be deeply investigated under MILD(moderate or intense low oxygen dilution) combustion condition of micro gas turbine which is our final goal of the present study.