Werschnik, Holger (2017)
Aerodynamic impact of swirling Combustor Inflow on endwall heat transfer and the robustness of the film cooling design in an axial turbine.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

The development of new gas turbines and aero engines is dedicated to reduce pollutant emissions in addition to the continuous strive to improve component efficiency and the consumption of fossil fuels. To foster this trend, new combustion concepts have come into play such as lean combustion. Whereas the emission of carbon dioxide can be reduced by lower fuel consumption, the formation of thermal nitrogen oxide can only be hindered by a leaner fuel-to-air mixture: Lower peak temperatures and avoiding a stochiometric concentration in the combustion chamber slow the thermal reaction process responsible for the formation of nitrogen oxides.

Swirl and a recirculation zone are used to stabilize the combustion process and a redistribution of mass flow towards the endwalls occurs. Additionally, a changed temperature profile with reduced peak temperature, but increased temperature near both endwalls due to the reduced injection of dilution air in the combustor approaches the subsequent turbine stage. Associated, positive and negative incidence, high turbulence intensities and increased thermal load to the endwalls challenge the turbine design.

To improve the understanding of the complex aerodynamic and aerothermal interaction, the aerodynamic impact of combustor swirl on the first vane row of a turbine, the nozzle guide vane (NGV), is investigated. The experiments are conducted at the Large Scale Turbine Rig (LSTR) in Darmstadt, which consists of a 1.5-stage axial turbine that is subject to an engine-representative swirl. A combustor simulator is used to vary the inflow to the turbine. Further goals of the investigation are to evaluate the robustness of its endwall film cooling design and to investigate endwall heat transfer and film cooling effectiveness experimentally by using infrared thermography and the auxiliary wall method.

As a reference, axial and low-turbulent inflow to the turbine is investigated. A variation of the coolant mass flow rate highlights the influence on Nusselt numbers and film cooling effectiveness as well as the aerodynamic flow field. An increase of Nusselt numbers by up to 80% is observed with a concurrent increase of the film cooling effectiveness by up to 25%. In a combined analysis a significant heat flux reduction due to film cooling by 30% is achieved. A coolant mass flow rate (MFR) of one yields the greatest benefit. For higher MFR the further improvement of the film cooling effectiveness is counteracted by the even greater increase in heat transfer.

With applied swirl, the flow field changes significantly. Averaged whirl angles of 15 ? to 20 ? and a mass flow redistribution to the endwalls are detected. The NGV exit flow exhibits a dominating influence of swirl on pressure losses instead of the coolant flows as it had been observed for the baseline. For similar settings of the stage parameters, an increase in Nusselt numbers by up to 40% is observed. The film cooling effectiveness is reduced because of the mass flow redistribution. For MFR greater than 1.5, the increase in Nusselt numbers is less decisive and remains at a similar level to the reference case. To achieve the same level of film cooling, the double amount of coolant air is necessary. These general trends are resolved for two clocking positions between swirler and vanes, whereby local differences are observed.

The combined analysis of the thermal parameters shows a local increase of endwall heat flux and a local influence on the coolant injection. The coolant injection is still beneficial in reducing the heat flux for low injection rates, but the local extent varies much more. For higher injection rates above 1.5, only sections of the endwall face an improvement and there is a growing area, where increased heat flux and in consequence higher thermal load is applied in comparison to the reference.

Frage zum Eintrag

Redaktionelle Details anzeigen